Climate and vegetation changes in coastal ecosystems during the Middle Pleniglacial and the early Holocene: Two multi-proxy, high-resolution records from Ría de Vigo (NW Iberia)

Accepted Manuscript Climate and vegetation changes in coastal ecosystems during the Middle Pleniglacial and the early Holocene: Two multi-proxy, high-resolution records from Ría de Vigo (NW Iberia)

Iria García-Moreiras, Cristina Delgado, Natalia MartínezCarreño, Soledad García-Gil, Castor Muñoz Sobrino PII: DOI: Reference:

S0921-8181(18)30358-8 https://doi.org/10.1016/j.gloplacha.2019.02.015 GLOBAL 2923

To appear in:

Global and Planetary Change

Received date: Revised date: Accepted date:

27 June 2018 26 February 2019 28 February 2019

Please cite this article as: I. García-Moreiras, C. Delgado, N. Martínez-Carreño, et al., Climate and vegetation changes in coastal ecosystems during the Middle Pleniglacial and the early Holocene: Two multi-proxy, high-resolution records from Ría de Vigo (NW Iberia), Global and Planetary Change, https://doi.org/10.1016/j.gloplacha.2019.02.015

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ACCEPTED MANUSCRIPT

Climate and vegetation changes in coastal ecosystems during the Middle Pleniglacial and the early Holocene: Two multi-proxy, high-resolution records from Ría de Vigo (NW Iberia) García-Moreiras, Iria1,2*; Delgado, Cristina3; Martínez-Carreño, Natalia1,4, García-Gil,

ECIMAT, Marine Science Station of Toralla (University of Vigo), Illa de Toralla s/n, E-36331 Vigo,

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Soledad1,4; Muñoz Sobrino, Castor1,2

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Spain.

Department of Plant Biology and Soil Sciences, Sciences faculty, University of Vigo, E-36310 Vigo,

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Spain.

Department of Ecology and Animal Biology, Sciences faculty, University of Vigo, E-36330 Vigo, Spain.

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Department of Marine Geosciences, Sciences faculty, University of Vigo, E-36310 Vigo, Spain.

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E-mail: [email protected].

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* Corresponding author: García-Moreiras, Iria.

Postal Address: Laboratorio de Palinología, Dept. Bioloxía Vegetal y Ciencias del Suelo, Facultad de Biología, Campus Lagoas-Marcosende s/n, Universidad de Vigo E-36310 (España).

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Telephone: +34 (9)-86 812598

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Keywords: pollen, dinoflagellate cysts, diatom frustules, lithostratigraphy, palaeoclimatology, sea level change, MIS-3, early Holocene, Southwestern Europe.

Abstract

New multi-proxy analyses were performed on two sedimentary sections from shallow marine ecosystems of Ría de Vigo (NW Iberia) to study the effects of the MIS-3 and early Holocene environmental variability. High-resolution data (microfossils, sedimentary facies, and geochemistry) allowed performing a comprehensive reconstruction of the main environmental changes (climate, vegetation, hydrology and sea level) that occurred during part of the MIS-3 1

ACCEPTED MANUSCRIPT period (57.0-38.8 cal ka BP) and the early Holocene (11.2-7.0 cal ka BP). The chronology is supported by isotopic dating and the correlation of pollen data with other regional palaeoclimatic records. Several phases characterised by increasing pollen representation of deciduous Quercus alternate with others marked by increasing representation of heliophytes that may reflect the succession of Interstadials (GI) and Stadials (GS) described in Greenland ice cores.

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In addition, dinoflagellate cysts and diatoms reflect conditions in the marine environment (SST and productivity). New palynological data confirms that coastal ecosystems of Atlantic Iberia

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were sensitive to the main climatic oscillations affecting the North Atlantic. It also suggests that

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pinewoods, juniper communities and mesophilous deciduous forests with Carpinus betulus L. (which shows exceptionally high pollen abundances for the MIS-3) persisted in the

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surroundings of Ría de Vigo until ~7500 a BP. For the first time in this region, we describe the

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effects of an abrupt episode of cooling that may correspond to Bond cycle 7 (10.5 ka event).

1. Introduction

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Galician rias are submerged unglaciated river valleys located on the coast of NW Iberia

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(Fig. 1A), an area that is very sensitive to climate and oceanographic oscillations affecting the North Atlantic region (e.g., deMenocal et al., 2000). The rias hold highly productive and diverse

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habitats that provide valuable ecological and socio-economical services (Figueiras et al., 2002); however, their vegetation and hydrology are very vulnerable to natural and anthropogenic

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disturbances. In particular, changes in relative sea level directly impact coastal habitats (Muñoz Sobrino et al., 2012, 2014). Palynological analyses of stratigraphic samples obtained from highly climate-sensitive environments such as coastal sediments constitute a basic tool for palaeoecological reconstructions (Gómez-Orellana et al., 2007; Costas et al., 2009; Muñoz Sobrino et al., 2012, 2016; Ellegaard et al., 2017; etc.). In shallow marine basins (i.e. NW Iberian rias), pollen assemblages are assumed to accurately reflect changes in the regional vegetation cover, as they

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ACCEPTED MANUSCRIPT are largely influenced by inputs from the river catchments (Chmura et al., 1999; GarcíaMoreiras et al., 2015). Additionally, dinoflagellate cysts and diatom frustules are useful sources of data for reconstructing past hydrological changes (Bao et al., 2007; Pospelova et al., 2008, Cermeño et al., 2012; Zonneveld et al., 2013; etc.). Dinoflagellates and diatoms are among the main

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contributors to phytoplankton and primary productivity in marine environments and, in Ría de

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Vigo, their abundances and composition are closely related to upwelling and fluvial dynamics (Fraga et al., 1988; Tilstone et al., 2000; Crespo et al., 2006). Therefore, dinoflagellate cysts and

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diatom frustules preserved in sediments are very useful to infer the past hydrological conditions — i.e., temperature, salinity and nutrient availability — occurring at the time of their production

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(e.g., Ellegaard et al., 2017; Taffs et al., 2017; García-Moreiras et al., 2018; Sáez et al., 2018).

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During the MIS-3 period, the succession of cold (Greenland Stadials, GS) and warm (Greenland Interstadials, GI) phases that were associated with D-O cycles (Dansgaard et al.,

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1993) had important impacts on the oceanography and coastal sedimentation of the Atlantic

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margin of Iberia (e.g., Lebreiro et al., 2009; Plaza-Morlote et al., 2017). Isotopic records from marine sediments reveal global millennial-scale sea-level oscillations during the Pleniglacial (Arz et al., 2007). The decrease in temperatures that was characteristic of GS is usually related

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to sea-level lowstands that promoted more intense erosive processes on the littoral margin and

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increasing sedimentation rates on the continental slope (e.g., Mena et al., 2010). The intensity of these changes varied within different GS, being greater during those stadials related to Heinrich events (Hemming, 2004; Lebreiro et al., 2009). Subsequently, early Holocene warming was accompanied by a fast sea-level rise, with increasing erosion events in some coastal areas of Atlantic Iberia (e.g. Arribas et al., 2010). The intense climatic variability during the Middle Pleniglacial and the early Holocene affected land vegetation as it has been recorded in a number of long pollen sequences from continental Europe (e.g., Moreno et al., 2014). Terrestrial pollen records from NW Iberia do not

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ACCEPTED MANUSCRIPT cover periods before the Late Glacial, i.e. >17 cal ka BP (Allen et al., 1996; Gómez-Orellana et al., 1998; Muñoz Sobrino et al., 2001, 2007, 2013; Iriarte-Chiapusso et al., 2016; Moreno et al., 2011; López Merino et al., 2012), with few exceptions (Gómez-Orellana et al., 2007, 2012). The impact of the climate on the regional vegetation may also be reconstructed by using deep-marine sediments from the Atlantic margin of Iberia. However, most of the available

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marine sequences (e.g., Sánchez Goñi et al., 2000, 2008; Roucoux et al., 2005; Penaud et al.,

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2011; Salgueiro et al., 2014; Eynaud et al., 2016) rarely have enough resolution to reflect rapid climatic shifts, and those recording the MIS-3 and the early Holocene are scarce (e.g.,

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Boessenkool et al., 2001; Turon et al., 2003; Naughton et al., 2007, 2015). Besides, there are some reconstructions of sea surface temperatures (SST), salinities (SSS) and productivities

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(SSP) from dinoflagellate cysts in deep sediments at the Iberian Atlantic margin (Boessenkool,

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et al., 2000; Turon et al., 2003; Roucoux et al., 2005; Naughton et al., 2007, 2015; Penaud et al., 2011; Eynaud et al., 2016; Datema et al., 2017). A clear lack of information exists about the responses of coastal ecosystems to the MIS-3, the MIS-2 and the early Holocene climatic

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oscillations. Specifically, concerning some short relapses related to the Greenland substadials or

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Bond cycles (Rasmussen et al., 2014), some of which have been already described in other sedimentary systems from the region (e.g., Muñoz Sobrino et al., 2013; Iriarte-Chiapusso et al.,

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2016).

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In littoral areas, climatic and relative sea level (RSL) fluctuations, as well as variations in the river sediment supply, primarily control sedimentary depositional patterns (Vis et al., 2008). The coastal ecosystems of the rias may have been very sensitive to the intense environmental variability of the MIS-3 and the early Holocene. During periods of cooling and RSL decrease —or deceleration in the rate of RSL rise— such as GS, shore progadation and the expansion of supratidal habitats are expected, as the result of higher sediment supply and changes in the accommodation space (e.g. Baeteman, 1999). On the other hand, during periods of warming and RSL rise, marine and tidal transgression is expected, together with: 1) the landward displacement of coastal habitats, 2) the development of riverine and other continental 4

ACCEPTED MANUSCRIPT wetlands due to an increase of the accommodation space, and 3) the expansion of deciduous mesophilous forests, as it occurred in other areas of NW Iberia (e.g. Gómez-Orellana et al., 2007, 2012; Iriarte-Chiapusso et al., 2016). This work has been designed to contribute to filling the gaps in knowledge that exist from the Middle Pleniglacial to the early Holocene in coastal areas of Atlantic Iberia and, more

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specifically, to study the influence of North Atlantic climatic oscillations on coastal ecosystems.

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Two new stratigraphic units covering part of the MIS-3 and the Holocene period have been recently obtained (see Martínez-Carreño and García-Gil, 2017; Muñoz Sobrino et al., 2018)

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from shallow marine environments of Ría de Vigo (NW Iberia). Here, we intend to use their contents of organic-walled and siliceous microfossils combined with seismic data to infer the

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most relevant environmental fluctuations — including climatic and RSL changes — and to

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study the responses of the local marine productivity and the regional vegetation to that environmental variability.

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2. Site

2.1. Geography and oceanography

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Ría de Vigo (area = 175 km2) is the southernmost of the Galician Rias Baixas (Fig. 1A). It is a funnel-like shape flooded valley that extends > 35 km in length and < 15 km in width.

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The Rande Strait (a narrow channel 600 m across and 1.5 km long) connects the outer ria with the innermost part of the estuary, San Simón Bay (Fig. 1B). The Cíes Islands partially block the connection with the open sea, affecting the sedimentation and the hydrology inside the ria (Fig. 1B). The water depths vary within 53 m in the outer ria to < 7 m in San Simón Bay. Here, the tidal range has an average of 2.2 m (Nombela et al., 1995). The total catchment area of Ría de Vigo comprises approximately 709 km2 (Pérez-Arlucea et al., 2000), and the Verdugo-Oitavén and Alvedosa Rivers are the main fluvial systems (Fig. 1B). A marked horizontal salinity gradient exists, increasing from the inner part of the ria (~ 31-32 psu) towards the outer part (36 5

ACCEPTED MANUSCRIPT psu). The modern sea surface temperature (SST) is ~15ºC, averaging 19-20ºC in summer and 11-12ºC in winter (Pérez-Arlucea et al., 2007). The study area is situated at the northern margin of the NW African coastal upwelling system and is under the influence of the Eastern North Atlantic Central Water (ENACW) (Fig. 2). Surface offshore western Iberian waters are dominated by the Portugal Current System

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(PCS) flowing towards the Equator in summer and towards the Arctic in winter (Sprangers et

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(ENACWst) and subpolar (ENACWsp) waters (Fig. 2B).

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al., 2004; Otero et al., 2008). Two branches of the ENACW flow under the PCS: the subtropical

The ria behaves like an extension of the shelf rather than a typical well-mixed estuary,

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and stratified waters generally dominate throughout the year (Villacieros-Robineau et al., 2013). In summer, the Azores high lies closer to the Galician margin, favouring northerly winds and

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promoting the outflow of surface waters and the compensating inflow of cold nutrient-rich ENACWsp, which is upwelled into the ria (Fig. 2B). During the downwelling season (typically

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in winter, when southerly winds prevail), the circulation pattern is reversed, particularly in the

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outer and middle zones of the ria. Water column mixing is more intense in fall, during the transition from the upwelling to the downwelling season (Álvarez-Salgado et al., 2000). Upwelling events bring nutrient-rich and relatively cold waters to the ria that generally enhance

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phytoplankton growth (Fraga et al., 1988; Crespo et al., 2006). Diatoms dominate

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phytoplankton communities in the upwelling season (spring-summer), whereas abundances of dinoflagellates increase over the summer and reach maximum values in fall, at the end of the upwelling season (Fraga et al., 1988; Tilstone et al., 2000; Crespo et al., 2006).

2.2. Climate and vegetation Rías Baixas (Fig. 1) are affected by a submediterranean variation in the oceanic climate, with mild temperatures and abundant rainfall but with a certain degree of drought in summer. The mean annual air temperature in Ría de Vigo is 14.8 ºC, and the mean annual precipitation is ~1690 mm (Ninyerola et al., 2005). 6

ACCEPTED MANUSCRIPT Currently, most of the basin is densely human-populated and consists of a complex mosaic of urban soil, forest plantations (mainly Eucalyptus spp. and Pinus spp.) and shrubs (Erica spp., Calluna vulgaris (L.) Hull., Ulex spp., etc.). In addition, scarce stands of deciduous oak forests with Quercus robur L. and riparian woodlands (mainly formed by Alnus glutinosa (L.) Gaertn., Corylus avellana L., Fraxinus angustifolia Vahl. and Salix spp.) are found in some

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less populated areas. The fluvio-marine transition area encompasses very diverse and productive habitats that include estuarine-deltaic complexes, muddy intertidal flats, sandy intertidal flats,

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beaches, and marshes (Ramil-Rego et al., 2008; Muñoz Sobrino et al., 2016). The altitudinal

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gradient from the subtidal area to the upper levels of the coastal wetlands shows the typical vegetation zonation in marshland environments (Sánchez et al., 1998), with Zostera spp.

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appearing in subtidal environments, mostly at the innermost part of the ria (San Simón Bay; Fig. 1B). Saline-tolerant species dominate the lowest levels of the salt marsh, where Spartina

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maritima (Curtis) Fernald and different species of Chenopodiaceae are common. Juncus maritimus Lam. and other less salinity-tolerant Cyperaceae and Poaceae species usually inhabit

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intermediate marsh levels (e.g., García-Moreiras et al., 2015). Coastal swamps with riparian

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forests extend to the upper fringe of the marsh, where there are a low tidal influence and periodic freshwater inundations (Amigo et al., 2004).

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Dunes, coastal flatlands and rocky habitats are also common in the area. Embryonic, shifting or fixed dunes may be typically inhabited by psammophytes such as Elymus farctus

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(Viv.) Runemark ex Melderis, Ammophila arenaria (L.) Link., Artemisia crithmifolia L., Othanthus maritimus Hoffmanns. and Link, Scrophularia frutescens L., Pancratium maritimum L., Eryngium maritimum L., Helichrysum picardii Boiss. and Reuter, Iberis procumbens Lange, Silene littorea Brott. subsp. littorea, etc. Coastal flatlands and rocky habitats are widely colonized by shrubs with Erica spp., Ulex spp. Cytisus spp. and diverse species of Poaceae and Cistaceae (Pulgar Sañudo, 2004; Ramil-Rego et al., 2008).

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ACCEPTED MANUSCRIPT 3. Materials and methods Two vibro-cores have been recovered at water depths of 18 m (B5 core) and 30 m (MVR-3 core) in the middle (42º 15′ 44′′ N / 8º 42′ 21′′ W) and in the outer (42° 14′ 04″ N / 8° 49′ 07″ W) parts of the ria, respectively (Fig. 1B). The cores were collected on board the B.O. Mytilus in June 2011 (B5) and July 2012 (MVR-3). The total length of the cores was 333 cm

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(B5) and 279 cm (MVR-3). Detailed lithostratigraphic descriptions of complete cores and their

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seismic stratigraphic context can be found in Martínez-Carreño (2015) and Martínez-Carreño and García-Gil (2017). Different volumes and sampling intervals were used for grain size,

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geochemical and microfossil analyses, depending on the facies distribution and desired

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resolution.

The sections B5 at 333-144 cm depth and MVR-3 at 279-104 cm depth have been

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previously discussed in Muñoz Sobrino et al. (2018), but only some selected pollen data and preliminary interpretations regarding the chronologies and major environmental changes were

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included. Here, the two records are presented within a more detailed study corresponding to the

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MIS-3 and early Holocene sections of both cores. Our study focuses on these periods because 1) we aimed at contributing to filling the gaps in knowledge that exist concerning the environmental variability of the Middle Pleniglacial and the early Holocene in coastal areas of

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Atlantic Iberia, and 2) a hiatus probably occurs at the top section of the B5 core (~140 cm; see

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Figs. 3A and 4A), after the early Holocene (see García Moreiras, 2017). This work includes a number of high-resolution (centennial to millennial) multi-proxy analyses (chronology, geochemistry, pollen, dinoflagellate cysts and diatoms) that enabled building more accurate chronologies and a more comprehensive interpretation of the paleoenvironmental changes in the area.

3.1. Grain size and geochemical analyses Grain size distributions of the complete B5 and MVR-3 cores were determined at 10 cm intervals. H2O2 and (NaPO3)6 were added to the sediment to remove the organic matter and to 8

ACCEPTED MANUSCRIPT disperse the clay. The suspension was then sieved to separate the different fractions (large fragments of shells were excluded to avoid data biasing). The sandy fraction was dried at 60ºC and determined by a standard dry-sieving procedure. The distribution of clay and silt was determined using a Micromeritics SediGraph 5100. Total Nitrogen (TN) and Total Inorganic Carbon (TIC) contents were determined at 20 cm intervals by combustion with a Carlo Erba-

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EA1108 elemental analyser. Total Organic Carbon (TOC) was analysed using a ThermoFinnigan FlashEA 1112 analyser after being acidified with 30% HCl. Geochemical

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analyses have not been performed in the 333-295 cm depth section of B5. N and C values are

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expressed as percentages of dry weight (wt%) (Fig. 3).

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3.2. Chronologies

A number of potential error sources can affect the level of precision of the radiocarbon

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dates obtained from marine detritus and bulk sediment samples (Table 1). First, the variable carbon sources that contribute to bulk sediments can be related to large uncertainties of the 14C chronology obtained from such samples (Lowe and Walker, 2000). Moreover, in a coastal site

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with spatiotemporally varying upwelling, the marine reservoir values are expected to be variable

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too (Lougheed et al., 2017). In the Atlantic Iberian margin, local reservoirs greatly fluctuated during the Holocene (Soares et al., 2009), and the same may be expected for earlier periods.

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Hence, calibration of dates from marine detritus (Table 1) could involve important errors, and

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the resolution of the proposed event chronologies (Fig.7) must be interpreted carefully. Fourteen radiometric dates were obtained along the B5 and MVR-3 cores (Table 1). Radiocarbon dates (Beta Analytic Laboratory, Florida, USA) were obtained from shells, plant remains and bulk sediment using AMS Standard dating methods. Dates on marine samples were calibrated by using the MARINE13.14C calibration curve (Reimer et al., 2013) and applying a local marine reservoir correction of σR = -7±90 (Reimer and Reimer, 2001) that corresponds to the nearest point in Stuiver et al. (1986-2018). Such correction has been useful to date shells found in late Holocene sediments from Ría the Vigo (e.g. Muñoz Sobrino et al. 2007, 2014). Dates on terrestrial remains were calibrated by using the INTCAL13.14C calibration curve 9

ACCEPTED MANUSCRIPT (Reimer et al., 2013). Finally, a mixed curve MARINE/INTCAL13.14C (50:50) was used for calibration of dates on bulk sediments (see Muñoz Sobrino et al., 2014). Additionally, two ages (B5-7 and B5-8; Table 1) were taken from the 210Pb chronology built for the upper section (22-0 cm) of the B5 core. These two (sub)modern age-control points allowed a better construction of the Late Holocene chronology in the B5 core. For details and

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predicted sedimentation rates see García-Moreiras et al. (2018). Furthermore, eight pollen-

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inferred dates (Table 2) were used to construct age-depth models. They have been proposed by biostratigraphic correlation with other well-dated climatic/pollen events that have been

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previously described in the region. The software CLAM 2.2. (Blaauw, 2010) was used to

interpolation (Figs. 4AB).

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3.3. Pollen, dinoflagellate cysts and NPP

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calibrate the dates and to construct the age-depth models, by fitting smooth splines or by linear

Organic microfossils have been analysed along the MIS-3 and early Holocene

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sequences of the B5 and MVR-3 cores, which correspond to sections 333-144 cm of B5 and

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279-104 cm of MVR-3. Subsamples (3-6 cm3 of fresh sediment) for pollen, dinoflagellate cysts and non-pollen palynomorphs (NPP) analyses were taken at 1, 3 or 5 cm intervals, depending on the sedimentation rates and were processed using standard methods (Moore et al., 1991;

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Mertens et al., 2009). A total of 86 samples (42 from B5 and 44 from MVR-3) were dried

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(80ºC), spiked with Lycopodium spores for concentration and accumulation rate estimates, treated with HCl and HF (room temperature), and sieved to remove coarse (> 120 µm) and fine (< 10 µm) materials. No acetolysis or oxidation was used for pollen or dinocyst extractions in order to prevent the corrosion of some cyst types (see Zonneveld et al., 2008; Eynaud et al., 2016). Palynomorphs were identified and counted using a light microscope (LM) Nikon Eclipse 50i at 400x and 600x magnifications (1000x for critical determinations). The total pollen sum ranged between 202 and 470 (276 on average). Pollen aggregates found within the 10

ACCEPTED MANUSCRIPT MVR-3 sequence were noted down (Fig. 6A). A minimum of 100 dinoflagellate cysts was counted per sample, except in a few cases where concentrations were very low. Total dinoflagellate cysts sums are shown (Figs. 5 and 6) to assess percentage reliability. Some relevant palynomorphs (LM) are shown in Plate 1, and a list including all the pollen, dinocyst and NPP types identified in this study can be seen in Supplementary Tables 1 and 2.

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The identification and nomenclature of pollen types are mainly based on Moore et al.

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(1991). The identification and nomenclature of dinoflagellate cysts and NPP mainly follow

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Zonneveld and Pospelova (2015), van Geel et al. (2011), Medeanic (2006) and Gelorini (2011). Quercus pollen species were not systematically segregated in all the B5 and MVR-3

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samples because of their variable degree of preservation, but most of the grains identified belong to the Quercus robur-type of van Benthem et al. (1984) (see Plate 1, Image d). However,

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we assume that some grains of Q. ilex and Q. suber might be included in the Quercus pollen curves. Due to some common difficulties with the identification of spiny brown cysts (see Radi

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et al., 2013), unidentifiable spiny brown cysts that were < 30 µm were grouped as “small spiny

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brown cysts” or SSB. On the other hand, “Smooth Brown Cysts” or SBC include Brigantedinium species and unidentified smooth round brown cysts (probably Protoperidinium

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americanum and others) with similar morphologies and obscure archeopyle. The percentages and accumulation rates (microfossils·cm-2·yr-1) of pollen, NPP and

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dinoflagellate cyst types were calculated in each sample using TILIA software v. 1.17.16 (Grimm, 1990-2011). The percentages of all pollen types and fern and bryophyte spores (onwards, pollen s.l.) were calculated on the basis of the total pollen sum (TPS). AP (Arboreal Pollen) refers to the sum of all tree pollen types. Percentages of NPP were calculated on the basis of a sum that included pollen s.l. and NPP. Finally, percentages of dinoflagellate cysts were calculated considering the total dinoflagellate cysts sum (TDS). The ratio of dinoflagellate cysts to pollen and spores (D/P ratio with values between 0 and 1) is the inversed ratio used by McCarthy and Mudie (1998). Percentages of total heterotrophic vs autotrophic cysts are also

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ACCEPTED MANUSCRIPT represented (Figs. 5C and 6C). Pollen and dinoflagellate cyst diagrams were independently zoned after applying a stratigraphically constrained cluster analysis on square root transformed percentages. The CONISS application of TILIA software was used, applying the Edwards and Cavalli-Sforza's chord distance method (Grimm, 1987).

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3.4. Diatoms We also studied the diatom contents in 29 samples that were taken each 5 cm from the

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B5 section at 333-150 cm depth. Approximately 0.2-0.3 g of sediment were collected per

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sample, dried and prepared by standard procedures (Renberg, 1990). Each sample was cleaned with 4-6 cm3 of 65% HNO3 and 65% K2Cr2O7 at room temperature for 24-48 hours. Afterwards,

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the samples were repeatedly centrifuged (1500 rpm) and rinsed with distilled water to remove oxidation by-products. Finally, permanent slides were mounted using Naphrax®. Two slides (22

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x 22 mm) of each sample were examined for diatom content using LM (Leitz Biomed 20 EB) and a 1000× immersion objective (NA 1.32). Diatoms were identified to the lowest taxonomical

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level mainly according to Krammer and Lange-Bertalot (1991, 1999ab, 2000), Lange-Bertalot

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(1993, 2001), Hasle and Syvertsen (1996), Witkowski et al. (2000), Levkov et al. (2010) and Trobajo et al. (2013). LM photographs (Plate 2) were taken using an Olympus DP70 camera

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4. Results

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attached to a Zeiss Axioplan 2.

4.1. Lithostratigraphies The B5 basal sedimentary package (facies 1 or f1, 333-295 cm) consists of muddy sands with parallel lamination and high organic content, including small fragmentsof wood, leaves, and bryophytes. This facies is overlaid by a ~60 cm thick layer (f 2, 295-235 cm) of siliciclastic sands with scarce shell remains. Towards the top of f2, decreases in TOC (from 3 to 0.4 wt%) content and TOC/TN ratio (from 31 to 19) are observed. The third sedimentary package (f3, 235-144 cm) consists of a bioclastic shell matrix supported by sand, with an erosive basal 12

ACCEPTED MANUSCRIPT surface (Fig. 3). F3 is characterised by low TOC and TN concentrations and relatively high TOC/TN values ranging between 13 and 32. The upper package (f4, 144-0 cm) comprises a fining-upward succession from bioclastic sand (144-105 cm) to muddy sand/sandy mud (105-0 cm), with an increase in plant remains from the bottom to the top. This youngest package is characterised by increases in TOC and TN contents and a decrease in the TOC/TN ratio (9-15).

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The lowest section of the MVR-3 core (f1, 279-147 cm) is comprised of medium silt and

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interbedded layers of coarse silt with clear parallel lamination. This sedimentary package includes small gastropods (~1-2 mm in length), mainly associated with the coarser fraction, and

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frequent plant remains. It is also characterised by very scarce bioclastic remains and low TIC values (< 1 wt%). The TOC and TN values are higher at the basal levels (~2 wt% and 0.12,

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respectively), but they diminish progressively from 220 cm to the top of the facies (f 1). The

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TOC/TN ratio shows values ranging between 15 and 17. Between 147 cm and 137 cm (f2), the sediment is mainly muddy and contains a small fraction of coarse bioclastic fragments. The youngest sedimentary package (f3, 137-0 cm) is a fining-upward succession consisting of a basal

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bed (137-110 cm) of bioclastic gravel with a sandy siliciclastic matrix. This package is overlaid

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by several beds (110-60 cm) of bioclastic/siliciclastic sand that changes to sandy mud towards the top (60-0 cm). The facies of the basal bed (f3, 137-110 cm) is characterised by low TOC

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concentrations (< 0.2 wt%) and high TIC content, with values of TOC/TN close to 11.5. The upper part of f3 (110-0 cm) shows more variable values of TOC and TIC, ranging between 0.3-2

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wt% and 1.2-4 wt%, respectively, whereas TOC/TN ratio varies between 10 and 17.

4.2. Chronologies

A seismic unconformity occurs at ~235 cm in the B5 core and ~147 cm in the MVR-3 core (Martínez-Carreño, 2015; Martínez-Carreño and García-Gil, 2017). It represents a discordant contact that coincides with a noticeable change in the lithological facies (Figs. 3AB), the occurrence of anomalous changes in the pollen curves and abrupt declines in the pollen

13

ACCEPTED MANUSCRIPT accumulation rates (Figs. 5 and 6). Hence, it may indicate the occurrence of sedimentary hiatuses in both records (Fig. 4). The chronology of the B5 core was constructed using six radiometric dates (including two dates estimated from

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Pb-based sedimentation rate in the uppermost levels) and four

pollen-inferred dates (Tables 1 and 2). One of the available radiocarbon dates was rejected as

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stratigraphically incongruent (B5-1; Table 1), and another (B5-5) was rejected because it was

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inconsistent with the interpretation of the palynological record and its correlation with the MVR-3 sequence (Fig. 7). Two independent models have been obtained for the B5 sections

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333-235 cm (MIS-3) and 235-0 cm (Holocene) (see red models; Fig. 4A). Additionally, alternative MIS-3 and Holocene models were performed by using radiometric dates only, to

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explore other possibilities (see blue models in Fig. 4a), like the presence of another hiatus at the

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lithological change observed at ~140 cm depth (Fig. 3A). Finally, to build the chronology of the MVR-3 core (Fig. 4B), a total of four radiocarbon dates and six pollen-inferred dates have been considered (Tables 1 and 2). Two independent models have been obtained for MVR-3 sections

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at 279-147 cm (MIS-3) and 147-0 cm (Holocene) (Fig. 4B).

4.3. Microfossil analyses

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Palynological analyses for the MIS-3 and early Holocene sections of B5 (333-144 cm

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depth) and MVR-3 (279-104 cm depth) cores are presented in Figs. 5 and 6. In total, 71 pollen, 41 NPP, 11 dinoflagellate cyst types, and 35 diatom taxa have been identified in the B5 record. In addition, a total of 80 pollen, 28 NPP, and 12 dinoflagellate cyst types have been identified in the MVR-3 record. All identified palynomorph types are shown in Supplementary Tables 1 and 2.

4.3.1. B5 pollen and NPP record Pollen analyses reveal high accumulation rates (250-1400 grains·cm-2·yr-1) at the bottom levels (333-295 cm), but they strongly decrease between 260 and 235 cm depth (0.5-150 14

ACCEPTED MANUSCRIPT grains·cm-2·yr-1). Values increase again at 235-215 cm depth (< 330 grains·cm-2·yr-1). At the top of the record (215-144 cm depth), low values are observed (15-150 grains·cm-2·yr-1) Four main local pollen zones (B5/LPZ) have been identified according to CONISS analysis (Fig. 5A).

B5/LPZ-1; 333-270 cm (~51.4-42.2 cal ka BP). — This zone is dominated by tree

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pollen (AP > 50%), showing notable percentages of Alnus (1.5-5%), Betula (4-15%), Carpinus betulus (2-11%), Corylus (3-9%) and Quercus (5-25%). Pinus percentages usually represent >

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25% of the TPS, but they noticeable decline towards the top of the zone. The presence of

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Ericaceae (< 17%), Poaceae (< 15%) and fungal remains (< 25% of the total microfossils identified) is also remarkable. Nevertheless, their abundances fluctuate throughout the zone.

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Fungal remains are relatively abundant (> 10%) in subzone B5/LPZ-1a (333-313 cm), but their percentages decline in the subsequent B5/LPZ-2b (313-302 cm), which is also characterised by

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a relative minimum of Quercus (5%) and peaks of Ericaceae (17%) and Ranunculus-type (13%). A third subzone (B5/LPZ-1c; 302-270 cm) is defined by increases in AP and Asteraceae

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and decreasing values of Pinus.

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B5/LPZ-2; 270-235 cm (~42.2-38.8 cal ka BP). — The regional recovery of deciduous forests (mainly Quercus) is represented by this zone, with very low Pinus abundances and AP

this zone.

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reaching its maximal values (> 70%) at approximately ~40.1 cal ka BP. D/P ratio increases in

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B5/LPZ-3; 235-162 cm (~9.3-7.9 cal ka BP). — Quercus (~20-35%) dominates the tree pollen, whereas Pinus shows values of < 15%, and other trees are rare. In comparison with lower levels, shrub and herb types are more abundant in B5/LPZ-3, especially Ericaceae, Ulextype, Asteraceae (Tubuliflorae), Chenopodiaceae and Trilete-type. In subzone B5/LPZ-3a (235197 cm), AP decreases, and fungal remains increase. Subzone B5/LPZ-3b (197-162 cm) shows higher Quercus percentages and much lower abundances of fungal remains. In addition, percentages of Alnus, Cyperaceae, Isoetes, Botryococcus spp. and Chlorococcales increase from B5/LPZ-3b towards the top of the record. 15

ACCEPTED MANUSCRIPT B5/LPZ-4; 162-144 cm (~7.9-6.7 cal ka BP). — Quercus decreases in this zone, although it is still the dominant tree (15-30%). On the other hand, the representations of hygrophilous species (riparian trees and Isoetes), Asteraceae, Poaceae and fungal remains considerably increase.

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4.3.2. B5 dinoflagellate cyst record Cysts from autotrophic taxa dominate the B5 record (~90-100%), and Lingulodinium

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machaerophorum (35-90%) and Spiniferites spp. (5-60%) are the most abundant species. Cyst

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richness and diversity are low: the number of taxa identified oscillates between 2 and 6, and the Shannon-Wiener index is 0.4-1.1 (Fig. 5C). At the bottom of the sequence (~333-300 cm), cyst

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accumulation rates range from 60 to 530 cysts·cm-2·yr-1. They notably decrease above ~300 cm depth, with very low values at 255-230 cm and 200-170 cm depth (~0.1-20 cysts·cm-2·yr-1).

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Values slightly increase (40-120 cysts·cm-2·yr-1) at 230-215 cm and at 165-144 cm. We grouped the samples into five local dinoflagellate cyst zones (B5/LDZ) according to CONISS analysis.

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B5/LDZ-1; 333-315 cm (~51.4-48.2 cal ka BP). — This first zone is characterised by

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the dominance of L. machaerophorum (35-70%) and Spiniferites spp., the percentages of which are high at the bottom of the core (~50%) The presence of B. tepikiense (< 6%) and SBC (< 5%)

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is also remarkable.

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B5/LDZ-2; 315-302 cm (~48.2-45.4 cal ka BP). — Values of Spiniferites spp. and SBC decrease, whereas B. tepikiense increases, showing a relative maximum (~15%) at 305 cm. B5/LDZ-3; 302-228 cm (~45.4-38.8 and ~9.3-9.2 cal ka BP). — This zone has been divided into three subzones: B5/LDZ-3a (302-294 cm), B5/LDZ-3b (294-242 cm) and B5/LDZ3c (242-228 cm). Spiniferites spp. and L. machaeorophorum are the most abundant types, showing several peaks that alternate between them (e.g., in B3/LDZ-3b, a peak in L. machaerophorum is followed by a peak in Spiniferites spp.). Spiniferites spp. (20-50%) and D/P

16

ACCEPTED MANUSCRIPT (> 0.3) increase in B5/LDZ-3b. Finally, B5/LDZ-3c is characterised by increasing values of heterotrophic cysts and a peak in L. machaerophroum. B5/LDZ-4; 228-202 cm (~9.3-8.8 cal ka BP). — This zone is characterised by a new increase of Spiniferites spp., higher proportions of SBC and other heterotrophic cysts, and very

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low (< 0.1) D/P values. L. machaerophorum increases from ~218 cm upwards. B5/LDZ-5; 202-144 cm (~8.8-6.7 cal ka BP). — Low D/P values persist in subzone

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B5/LDZ-5a (202-172 cm), which also shows increasing values of Spiniferites spp.,

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Operculodinium centrocarpum, Impagidinium spp., and a number of heterotrophic cysts (Selenopemphix quanta and SBC). The uppermost subzone B5/LDZ-5b (170-144 cm) shows

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notably higher abundances of L. machaerophorm (> 80%) and D/P (> 0.5).

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4.3.3. B5 diatom record

Diatoms had a variable degree of preservation within the B5 section at 333-150 cm

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depth (Table 3), but frustules were completely absent in most of the samples corresponding to

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the coarser sediment (Fig. 3). In the rest of the studied samples, the resulting total counts were too low to be quantitatively representative. Identified species were noted when applicable (Table 3). Levels between 327-261 cm yielded the most continuous diatom record, which

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generally consisted of diatom assemblages from freshwater with moderate electrolyte (i.e.,

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relatively low salinity) content such as Rhoicosphenia abbreviata (Agardh) Lange-Bertalot. Notably, Navicula capitatoradiata Germain appears at 316-317 cm depth, suggesting (Witkowski et al., 2000) an increase in the electrolyte content (i.e., increasing salinity). Some other fragments of large diatoms with brackish and marine affinities (Plate 2) were also identified at the levels below (> 321 cm).

4.3.4 MVR-3 pollen and NPP record Pollen accumulation rates are high (500-2500 grains·cm-2·yr-1) below 147 cm depth. They considerably decrease upwards, with minimum values (< 100 grains·cm-2·yr-1) recorded 17

ACCEPTED MANUSCRIPT between 147 and 120 cm. Accumulation rates increase again (< 400 grains·cm-2·yr-1) towards the top of the sequence (120-104 cm). Six pollen zones (MVR-3/LPZ) have been identified after CONISS analysis (Fig. 6A). MVR-3/LPZ-1; 279-232 cm (~57.0-52.4 cal ka BP). — AP varies between 45% and 65%, with Pinus (35-55%) being the dominant tree. The contributions of Ericaceae (10-20%)

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and Poaceae (5-13%) are also remarkable. Two subzones can be recognised, MVR-3/LPZ-1a

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(279-257 cm) and MVR-3/LPZ-1b (257-232 cm); the latter shows higher proportions of

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Carpinus betulus, Betula, Chenopodiaceae and Asteraceae (Liguliflorae).

MVR-3/LPZ-2; 232-192 cm (~52.4-48.4 cal ka BP). — It is characterised by increases

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in Quercus (< 5%), Asteraceae (Tubuliflorae) (2-7%) and Blechnum spicant/Polypodiaceae spores (< 2%). Subzone MVR-3/ZPL-2a (232-212 cm) shows a relative maximum of Poaceae

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(~12.5%), and subzone MVR-3/LPZ-2b (212-192 cm) shows a decrease in pollen from riparian trees, with increasing values of Ericaceae and a final maximum of Betula (~10.8%).

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MVR-3/LPZ-3; 192-147 cm (~48.4-43.8 cal ka BP). — This zone is mainly

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characterised by very high percentages of Pinus (also pollen aggregates). Three subzones can be recognised: the oldest (MVR-3/LPZ-3a; 192-172 cm) shows the decrease in Quercus (~1.4%), a

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relative maximum of Pinus (~70%), and increases in Ericaceae and Artemisia. The subsequent MVR-3/LPZ-3b (172-158 cm) reflects a slight recovery of deciduous mesophilous woodlands,

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with increases in Quercus, Corylus, and Blechnum spicant/Polypodiaceae. Finally, subzone MVR-3/LPZ-3c (158-147 cm) shows a new increase in Pinus. MVR-3/LPZ-4; 147-137 cm (~11.2-10.6 cal ka BP). — It represents an abrupt increase in Quercus (~22%) together with higher abundances of Ulex-type, some aquatics/hygrophytes (Isoetes-type and Ranunculus-type), freshwater/brackish algae, fungal remains and foraminiferal linings. MVR-3/LPZ-5; 137-114 cm (~10.6-8.0 cal ka BP). — This zone shows strong fluctuations in pollen curves but generally reflects a noticeable Quercus decline and the increase 18

ACCEPTED MANUSCRIPT in some heliophilous/cryophilous communities (i.e., Pinus, Ericaceae, and Poaceae). Numerous pollen aggregates of Pinus and Poaceae have been found in this zone. Three pollen subzones may be described: the lowest MVR-3/LPZ-5a (137-133 cm) is characterised by a major Quercus pollen decline and increases in Betula, Pinus, Asteraceae (Tubuliflorae) and Poaceae. Subzone MVR-3/LPZ-5b (133-120 cm) shows increases in Quercus and Polypodiaceae/Blechnum

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spicant, but at 125-128 cm depth, it also includes new peaks of Pinus (~50%) and Ericaceae (> 15%). Finally, subzone MVR-3/LPZ-5c (120-114 cm) shows new relative maxima of Pinus

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(~60%) and Ericaceae (~15%) and the decline in deciduous trees.

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MVR-3/LPZ-6; 114-104 cm (~8.0-7.0 cal ka BP). — The last interval reflects notable increases in Alnus and Quercus and the decline in cold tolerant tree species such as Betula,

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Pinus and Carpinus betulus. Representation of herbs and shrubs notably diminish, whereas

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aquatic/hygrophilous herbs generally increase. In addition, higher values of fern spores, fungal remains, freshwater/brackish algae and foraminiferal linings can also be observed.

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4.3.5 MVR-3 dinoflagellate cyst record

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Autotrophic taxa represent most of the TDS (> 75%) in the MVR-3 record, and Bitectatodinium tepikiense (< 80%), Lingulodinium machaerophorum (5-70%) and Spiniferites

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spp. (10-60%) are the most common taxa. Between 147 cm and 104 cm, cysts from heterotrophic dinoflagellates increase (> 20%). Cyst richness and diversity are low, with 3-9

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taxa identified per sample and the Shannon-Wiener index ranging between 0.6 and 1.8 (Fig. 6C). Cyst accumulation rates show great fluctuations along the MVR-3 record, with values ranging from 5 to 900 cysts·cm-2·yr-1. Values noticeably decline at 147-117 cm depth (< 10 cysts·cm-2·yr-1), but they increase again (~200 cysts·cm-2·yr-1) towards the top of the sequence (< 110 cm). Samples have been grouped into six local dinoflagellate cyst zones (MVR-3/LDZ) according to CONISS analysis (Fig. 6C). MVR-3/LDZ-1; 279-240 cm (~57.0-51.0 cal ka BP).— B. tepikiense (20-70%), Spiniferites spp. (10-40%) and L. machaerophorum (15-40%) are the main cyst types in this 19

ACCEPTED MANUSCRIPT zone, which is also characterized by low D/P values (< 0.2). B. tepikiense (< 65%) increases in subzone MVR-3/LDZ-1a (279-257 cm) and declines in MVR-3/LDZ-1b (257-240 cm). MVR-3/LDZ-2; 240-172 cm (~51.0-45.4 cal ka BP). — It is characterised by high B. tepikiense proportions and usually higher D/P (> 0.2). However, abundances of the main cyst types greatly fluctuate throughout the zone. A relative maximum in B. tepikiense (> 70%) is

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observed at the base of subzone MVR-3/LPZ-2a (230-205 cm). MVR-3/LDZ-2b (205-172 cm)

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shows increasing percentages of Spiniferites spp. and decreasing D/P values.

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MVR-3/LDZ-3; 172-147 cm (~45.4-43.8 cal ka BP). — This zone reveals a decline in B. tepikiense, with the expansion of Spiniferites spp. (> 45%) and very low proportions of

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heterotrophic cysts (< 5%).

MVR-3/LDZ-4; 147-137 cm (~11.2-10.6 cal ka BP). — Spiniferites spp. reaches its

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maximum (> 65%), and B. tepikiense greatly decreases. In addition, increases in the proportions of SBC and other heterotrophic cysts are observed.

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MVR-3/LDZ-5; 137-117 cm (~10.6-8.1 cal ka BP). —Cyst percentages and D/P ratio

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values markedly fluctuate throughout this zone, but B. tepikiense peaks (50%) at ~120 cm depth, coinciding with an abrupt decrease in D/P values.

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MVR-3/LDZ-6; 117-104 cm (~8.1-7.0 cal ka BP). — It shows conspicuous decreases in B. tepikiense and Spiniferites spp. and increases in the proportions of L. machaerophorum and

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O. centrocarpum. In addition, D/P (> 0.3) and proportions of heterotrophic cysts (> 20%) — mainly Q. concreta, S. quanta, SCB and SSC (cf. Echinidinium spp.) — reach their maximum values for the entire record. Heterotrophic cysts decrease again towards the top of the zone.

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ACCEPTED MANUSCRIPT 5. Discussion 5.1. Cronostratigraphies 5.1.1. Presence of hiatuses The hiatuses identified at ~235 cm in the B5 core and ~147 cm in the MVR-3 core

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represent a gap of more than 25,000 years that would extend from ~38.8 to 11.2 cal ka BP (Fig. 4). The lack of sediments during that time interval is related to the dramatic relative sea level

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(RSL) occurred during the Last Glacial Maximum (LGM) (~23-19 cal ka BP; Hughes et al.,

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2013), when the RSL was -120 m below the present sea level (bpsl). Displacement of the coastline and river mouths towards the adjacent shelf, with the concomitant change of the base-

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level, caused the incision of the fluvial network and favoured the transport of terrigenous material towards deeper areas (Lantzsch et al., 2010). That period was characterized by

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important changes in the fluvial network, with channel avulsion and formation of new channel incision (e.g., Fagherazzi et al., 2004; Vis et al., 2008), but also in the sedimentary patterns,

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with the erosion of older deposits and prevailing conditions of sedimentary bypass across the

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rias (Martínez-Carreño and García-Gil, 2017). Those changes in sedimentary patterns, controlled by RSL and climate oscillations would be responsible of the bad preservation of the

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sedimentary deposits in cores B5 and MVR-3 in that time interval (Martínez-Carreño, 2015). Multi-proxy data on sections 279-147 cm of the MVR-3 core and 333-235 cm of the B5

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core suggest that sedimentation could be continuous. The decrease of palynological accumulation rates from 300 to 235 cm depth in the B5 record (Fig. 5) is gradual, and no major changes in the lithological facies indicates any important interruption of the sedimentation (Fig. 3A). However, both cores are close to the margins of the ria and the area was expected to be very sensitive to RSL and climatic changes; therefore, the presence of multiple minor hiatuses (particularly regarding the MIS-3 sections) cannot be completely discarded.

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ACCEPTED MANUSCRIPT 5.1.2. Alignment of the B5 and MVR-3 sequences Due to inherent age-range limitations of 14C dating, few radiocarbon ages were available for some sections (i.e., early Holocene for the B5 core and MIS-3 for MVR-3) (Table 1). Nevertheless, the alignment of B5 and MVR-3 records by pollen correlation (Fig. 7) aided in establishing a relative chronology for these levels.

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Correlation between MIS-3 sequences (Fig. 7) was possible using the available

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radiocarbon dates (Table 1) and some environmental inferences (from proxies studied) that enable the identification of the GS-13 in both records. This event (GS-13 or Heinrich Stadial 5;

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~48-47 cal ka BP; Rasmunssen et al., 2014) can be recognised in both B5 and MVR-3 records, and two absolute dates support its chronology. Uncertainty ranges of 14C dates B5-2 (2σ: 44.3-

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46.7 cal ka BP) and MVR-3-3 (σ: 44.7-47.1 cal ka BP) overlap (Table 1), and indicate that the

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sediments dated were probably deposited between the end of GS-13 and the GI-12c. Date B5-2 coincides (Fig. 5) with a phase of increasing representation of Quercus and L. machaerophorum, which probably indicates warmer conditions (Muñoz Sobrino et al., 2014),

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since high percentages of L. machaerophorum typically occur in eutrophic and stratified coastal

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waters, highly influenced by river inputs (Leroy et al., 2013; García-Moreiras et al., 2015; Donders et al., 2018). This warming phase occurs just after another phase of marked AP decline

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and increase of the water cooling indicator species B. tepikiense (e.g., Penaud et al., 2011; Eynaud et al., 2016). On the other hand, date MVR-3-3 coincides with the end of a phase

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characterised by lower D/P and Quercus, and increasing Pinus values (Fig. 6) that probably indicate another phase of cooling. Then, most likely, B5-2 is dating the GI-12c, and the MVR-3 the end of the colder GS-13 (Table 1). Finally, B5-3 (2σ: 42.8-44.4 cal ka BP) may be dating the GS-12 (~43.4-44.2 cal ka BP; Rasmunssen et al., 2014) as it coincides with changes in the pollen curves that can be related to a phase of cooling, i.e. decreasing values of D/P and A/P, and decline of Quercus representation (Fig. 7). On the other hand, for the alignment of the early Holocene sequences (Fig. 7), we considered the available radiocarbon ages (Table 1) as well as the D/P dynamics. A pronounced 22

ACCEPTED MANUSCRIPT D/P increase is observed in the B5 sequence from 175 to 165 cm depth (from ~0.05 to 0.6; Fig. 5). Similarly, a ten-fold increase in D/P values is observed in the MVR-3 at 120-115 cm (from ~0.04 to 0.4; Fig. 6). This event, which may be indicating in both records a substantial increase in marine sedimentation due to an RSL rise (Machado et al., 2018), occurs at < 8.2 cal ka BP in the MVR-3 (Fig. 6). This interpretation seems incongruent with the B5-5 radiocarbon date that

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provides an age of 9.3-8.7 cal ka BP (165.5 cm) to that sharp D/P increase observed in the B5

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record (Fig. 5).

5.1.3. Age-depth models

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C dates (blue curves in Fig. 4A), and considering a hiatus at ~144 cm depth. These

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using

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In the B5 core, alternative and conservative age-depth models were primarily based on

chronologies were little useful because they showed enormous uncertainties and did not fit well

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with the pollen stratigraphy (Fig. 5). More precise chronologies are proposed by establishing four relative dates (ii, iii, vi and vii; Table 2 and Fig. 4A) from the interpretation of the pollen stratigraphy and its correlation with a number of regional palaeoclimatic phases (Fig. 7). The

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latter chronologies are those which are drawn in the pollen diagrams (Fig. 5). Similarly, feasible

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chronologies for the MVR-3 core were constructed by combining the available radiocarbon dates with some pollen-inferred dates (Table 2 and Fig. 4B).

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The tie-points used for the MIS-3 chronologies (Table 2) correspond to the onsets of

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GS-15.1 (~54.8 cal ka BP), GI-14a (51.2 cal ka BP), GS-13 (~48.2 cal ka BP) and GS-12 (~44.2 cal ka BP), as dated in the NGRIP sequence (Rasmunssen et al., 2014) after applying the Greenland Ice Core Chronology 2005 (GICC05) (Svensson et al., 2008). These colder events are detected in the pollen records (Figs. 5 and 6) by increases in cryophilous and heliophilous taxa (e.g., Pinus, Ericaceae, Poaceae, etc.), decreases in Quercus and AP accumulation rates and, usually, D/P decreases (Fig. 7). The presence of outliers (date B5-5; Table 1 and Fig. 4A) as a result of material reworking might be a common issue in sedimentary records that were deposited during a highly 23

ACCEPTED MANUSCRIPT dynamic phase of rapid sea level rise and coarse material deposition (Fig. 3A). However, given the similarity of D/P dynamics in both the B5 and MVR-3 sequences (Fig. 7), we can estimate a relative chronology to the B5 early Holocene sequence by correlating the two D/P curves (Figs. 5 and 6) and rejecting the B5-5 radiocarbon age. Therefore, to build a reliable age model for the B5 early Holocene record, we used the maximal D/P value as a tie-point, which has been

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correlated with a similar D/P increase dated to 7.4 cal ka BP in the MVR-3 record (Fig. 4B). Note that we are assuming certain degree of variation in the age of the D/P peaks, as the B5 site

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was shallower than the more external MVR-3 site (Martínez-Carreño, 2015) and some delay

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(decadal/centennial) probably existed in getting similar fully marine conditions at both sites after the regional sea-level rose.

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The chronostratigraphical interpretation of the upper sediments of B5 (235-140 cm) and

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its correlation with MVR-3 is coherent with the B5-4 radiocarbon age (9.0-9.4 cal ka BP; Table 1) and with the identification of the cold 9.3 ka event at 235-215 cm depth. This event is represented in the B5 palynological record by the expansion of shrubs and the decrease in L.

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machaerophorum, which may indicate cooler waters (Fig. 5AC; Leroy et al., 2013). In addition, the onsets of the abrupt climatic 9.3 ka and 8.2 ka events have been used to refine the early Holocene chronologies. In the MVR-3 sequence (Fig. 6AB), these events are

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clearly recognised (at ~127 and ~118 cm depth) by increases in the representation of

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heliophilous vegetation (mainly Pinus and Ericaceae) and decrease in Quercus and D/P (Fig. 6). At ~174 cm in the B5 record, a similar decrease in D/P coinciding with the increases in Ericaceae and Spiniferites spp. (Fig. 5) may be related to the start of the cold 8.2 ka event. The effects of these two fast relapses on the vegetation of NW Iberia have been dated to 9.4-9.2 cal ka BP and 8.3-8.1 cal ka BP, respectively (Muñoz Sobrino et al., 2005; Iriarte-Chiapusso et al., 2016). In summary, the proposed age-depth models (Figs. 4AB) suggest that the basal section of the MVR-3 (279-147 cm) was deposited at ~57.0-43.8 cal ka BP and that of the B5 (333-235

24

ACCEPTED MANUSCRIPT cm) at ~51.4-38.8 cal ka BP (MIS-3 period). The upper levels of MVR-3 (147-0 cm) may have been deposited from 11.2 cal ka BP and those of the B5 core (235-0 cm) during the last 9.3 cal ka BP (Holocene).

5.2. The period 57.0-38.8 cal ka BP (MIS-3) in Ría de Vigo

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5.2.1. RSL and sedimentation conditions

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The lowest beds of cores B5 (333-270 cm) and MVR-3 (279-147 cm) are characterized by 1) prevailing deposition of fine sediments, 2) high organic contents with high TOC/TN ratios

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–particularly in the B5 core, where ratios are always >15 with values of 30 in its basal facies-

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(Figs. 3ab), 3) low D/P ratios and 4) remarkable palynomorph accumulation rates (Figs. 5 and 6). These results agree with the occurrence of shallow, low-energetic depositional environments

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(e.g. Machado et al., 2018) characterised by the predominance of terrestrial organic matter and good preservation conditions for organic microfossils during the period 57.0-42.2 cal ka BP. The RSL would be significantly lower than today (see Muñoz Sobrino et al., 2018), in

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accordance with RSL reconstructions performed by Martínez-Carreño (2015) from seismic and

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sedimentary data (Fig. 8A). Nevertheless, D/P > 0.2 (see B5-record; Fig. 5C) may indicate relevant marine sedimentation in fluvio-marine transition environments (Czarnecki et al., 2014).

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Two pre-Holocene stages of slightly higher sea level and probably warmer conditions

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can be recognised at ~52.5-49.7 cal ka BP and 42.2-39.8 cal ka BP, suggesting that they may be related to the succession of some interstadials recorded in Greenland ice-cores, i.e., GI-14, GI10 and GI-9 (Fig. 7). These stages are characterised by higher D/P ratios and lower representations of Pinus communities, which could preferably occupy lowland coastal habitats. During these warmer stages, the pollen representation (percentages) of the vegetation from more distant sources in the fluvial basin (i.e. Betula, Carpinus-type, Corylus and Quercus) can increase (Figs 5A and 6A), since these sources get closer to the sedimentary point as the RSL rise. Moreover, this second stage (42.2-39.8 cal ka BP) coincides in the B5 record with marked decreases in pollen and cyst accumulation rates (270-235 cm; Fig. 5BC), TOC values (Fig. 3A) 25

ACCEPTED MANUSCRIPT and fungal remains (Fig. 5A). In our view, this must be related to a change in the sedimentary environments and a decay of the preservation conditions, probably due to a slight RSL rise and higher fluvial sediment supplies within 42.2-39.8 cal ka BP -which also agree with the increase of sediment grain-size (Fig. 3A)- rather than to a decrease in the vegetation or marine productivities.

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During the common period recorded by both sequences (~51.4-43.8 cal. ka BP), fluvial

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influence was probably higher at the B5 site than at the MVR-3, which is coherent with its inner position within the ria. This, according to the interpretations of some authors (e.g. Vilas et al.,

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2005; Arribas et al., 2010), would be supported by several lines of evidence, namely 1) the higher content of lithogenic sands at this point (B5) compared to MVR-3 (Fig. 3), 2) the greater

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evidence of more proximal sources in the fluvial basin (Alnus, Corylus, Apiaceae, Ranunculus-

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type, fern spores, etc.) in the B5 record (Fig. 5A) and 3) higher abundances of L. machaerophorum (Fig. 5C), which is a dinoflagellate cyst species with great affinity for highly fluvial-influenced stratified waters (e.g., Bringué et al., 2013; Leroy et al., 2013; García-

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Moreiras et al., 2015).

In addition, the preponderance of freshwater epiphytic and benthic diatom species in the B5 record (333-150 cm; Table 3) such as Cocconeis euglypta, C. pseudolineata, C. scutellum,

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Gomphonema parvulum, Rhoicosphenia abbreviata and Achnanthidium minutissimum would

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indicate important fluvial inputs at this site during the period 51.4-38.8 cal ka BP (Sáez et al., 2018). The poor occurrence of brackish/marine diatoms in the bottom B5 record (Table 3) also highlights the lack of a water-column depth able to maintain highly productive communities of marine diatoms (e.g., Sáez et al., 2018). Nevertheless, the diversity of brackish/marine diatoms increases at ~51.4-47.0 cal ka BP (333-316 cm depth; Table 3). The deepest sample includes Paralia sulcata, a species that is currently associated with nutrient-rich and high-salinity waters (McQuoid and Nordberg, 2003; Lewis, 2011) and that may be observed in the middle part of Ría de Vigo during periods of sediment re-suspension and high turbidity (Bernárdez, 2007). Thus, the lowest B5 levels (51.4-46.8 cal ka BP) could have been accumulated under weaker 26

ACCEPTED MANUSCRIPT freshwater influence than those accumulated at this site during the subsequent 46.8-38.8 cal ka BP period (Fig. 5). This change could have been mostly driven by less precipitation and increasing evaporation rather than a phase of significant RSL rise (see D/P in Figs. 5A and 6A).

5.2.2. Coastal vegetation Noticeable Pinus values were found in the B5 record, especially before 43 cal ka BP

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(Fig. 5A). Pinus pollen also occurs at very high abundances in the MVR-3 record (Fig. 6AB),

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especially at 190-147 cm, where percentages of 50-70% (TPS), accumulation rates between 250 and 1250 grains·cm-2·yr-1 and concentrations between 0.3·105 and 105 pollen grains·cm-3 are

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observed. In addition, the presence of pollen aggregates in some samples (Fig. 6A) may also

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indicate the occurrence of coastal pinewoods in the close surrounding area. This taxon may be overrepresented in the pollen record because of its high pollen production and dispersion

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capabilities (Broström et al., 2008; García-Moreiras et al., 2015). Nevertheless, Pinus abundances in the MVR-3 record are higher than those recorded in the subtidal sediments of Ría de Vigo after 1930 CE, when modern afforestation with pines occurred (López Torre et al.,

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2009). Modern Pinus pollen percentages in Ría de Vigo sediments vary between 30% and 80%

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(TPS), its accumulation rates in the last decades were between 500 and 3000 grains·cm-2·yr-1 and its concentrations range between 0.1·105 and 0.3·105 pollen grains·cm-3 (Muñoz Sobrino et

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al., 2012, 2014, 2016; García-Moreiras et al., 2015).

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The occurrence of coastal pinewoods in Ría de Vigo during the MIS-3 is consistent with the findings of some macrofossils of Pinus sylvestris/P. nigra at 35-20 cal ka BP on sandy soils from North Portugal (e.g., Granja et al., 2008) and in the Tejo estuary at > 6-5 cal ka BP (Gómez-Orellana et al., 2014). Moreover, the existence of dunes and sandy habitats during the MIS-3 in the Ría de Vigo area may also be supported by other pollen indicators such as Poaceae, Chenopodiaceae, Asteraceae, Brassicaceae, etc. (Fig. 6AB). In particular, the presence of Artemisia, Juniperus-type and Ephedra distachya-type (Plate 1), which consistently appear in the MVR-3 record, may indicate the occurrence of shrub and herb communities associated with stable dunes (e.g., Costa et al., 2000; Picchi, 2008; European Commission, 2013). 27

ACCEPTED MANUSCRIPT New pollen evidence also indicates that during the period 57.0-38.8 cal ka BP the surrounding uplands of Ría de Vigo held deciduous forests with Quercus, Carpinus betulus, Betula, Corylus, Alnus, Fraxinus, Rhamnus, Salix, Sambucus, etc. (Figs. 5A and 6A and Supplementary Table 1). Our results denote the highest Carpinus betulus pollen abundances ever described in the Iberian Peninsula for the MIS-3 or the early Holocene, with values of <

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12.7% and 70 grains·cm-2·yr-1 (and concentrations of 11600 pollen grains·cm-3) for the period 57.0-38.8 cal ka BP (Figs. 5 and 6). B5 site was closely related (Fig. 8A) to the fluvial network

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connecting the more favourable biotopes for the development of ravine, alluvial and hardwood

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floodplain forests (Muñoz Sobrino et al., 2018), which would explain their higher representation in the B5 pollen record, compared to MVR-3. Pollen from Tilia, Ulmus, and Fagus, are present

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in a few samples from B5 at very low values (< 0.8%). Together with Carpinus, these taxa are present in current hardwood forests of slopes, screes and ravines from wide areas of Europe

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(European Commission, 2013). Our results suggest that the surroundings of Ría de Vigo held habitats that acted as a refuge for many of these climate-sensitive species during the MIS-3.

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5.2.3. Coastal waters

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The dominance of the autotrophic cysts L. machaerophorum and its high accumulation rates between ~51.4 and 38.8 cal ka BP in the B5 record (Fig. 5C) may indicate that, in the

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innermost area of the paleo-estuary, relatively warm, well-stratified and nutrient-rich waters prevailed (Sprangers, 2004; Leroy et al., 2013; Zonneveld et al., 2013), primarily due to high

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river discharges (Ribeiro and Amorin, 2008; Donders et al., 2018). Cysts from heterotrophic dinoflagellates are generally related to high nutrient inputs from upwelling and high diatom productivity (Pospelova et al., 2008; Bringué et al., 2013; Zonneveld et al., 2013; Ellegaard et al., 2017; Elshanawany and Zonneveld, 2016). Their scarcity in the B5 record agrees with the very low diatom abundances observed (Table 3) and may reflect that dinoflagellates were the main contributors to the phytoplankton productivity, mainly favoured by strong stratification and shallow conditions.

28

ACCEPTED MANUSCRIPT For the same period, L. machaerophorum is much less abundant in the MVR-3 record, whereas Bitectatodinium tepikiense occurs at relatively high abundances (20-80%; Fig. 6C). The distribution area of this species comprises temperate to subpolar environments from the North Atlantic, with the highest concentrations recorded in the North Sea, the eastern Canada margin and the Argentinean subtropical margin, and particularly (< 10%) in areas with high seasonality

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and well-ventilated bottom waters (Zonneveld et al., 2013). Hence, increases of B. tepikiense at the outer part of the ria (Figs. 5C and 6C), can be related to an unstable hydrology and the

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influence of cold waters from the continental slope (Sánchez-Goñi et al., 2000; Turon et al.,

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2003; Zonneveld et al., 2013; Eynaud et al., 2016) between ~57.0 and 43.8 cal ka BP.

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5.2.4. Effects of climatic oscillations on terrestrial and marine ecosystems The B5 and MVR-3 records reveal the occurrence of a number of pollen events during

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the period 57.0-38.8 cal ka BP that may be related to the GS/GI succession described for the North Atlantic (Rassmussen et al., 2014). In particular, different phases of Quercus retreat and

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Poaceae expansion (blue bands in Figs. 5 and 6) may correlate with the succession of a series of

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stadials defined in Greenland (Fig. 7). Those phases are also represented by decreases in AP (both percentages and accumulation rates) and increases in other heliophilous and cryo-tolerant species such as Pinus and Poaceae. Thus, they may reflect the regional occurrence of episodes

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of cooling that would also favour the regional development of coastal dune ecosystems (Figs.

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5A and 6A). Furthermore, coastal vegetation may have been affected by changes in depositional processes due to RSL variations. Shore progadation as a consequence of RSL drops and/or higher sediment supply in relation to the accommodation space may be linked to the expansion of some supratidal habitats (i.e. coastal meadows and shrublands, marshes and dunes), which representation increase in the sedimentary sequence (Allen, 2000; Muñoz Sobrino et al., 2014). In Ría de Vigo, as recorded in the B5 palynological record (Fig. 5), the most intense of these cooling events may have occurred at ~48-46 cal ka BP; hence, it was related to the GS-13 (Fig. 7). It is characterised by a marked decline in mesophilous forests (minimum values of

29

ACCEPTED MANUSCRIPT Quercus in both records) and the decrease in D/P (Figs. 5 and 6), which indicates higher contribution of terrestrial materials, probably related to a lowering or stabilization of the RSL (Muñoz Sobrino et al., 2012; Leroy et al., 2013). The intensification of erosive processes in the catchment area and terrestrial matter deposited in the ria by runoff during colder phases (when these processes would be promoted by the decline of the tree canopy and changes in the

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precipitation and storm intensities) can also affect the D/P ratio.

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Increases in B. Tepikiense (percentages and accumulation rates) are also observed at ~48-46 cal ka BP in both records (Figs. 5AC and 6AC), probably indicatinglower SST (e.g.,

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Penaud et al., 2011; Eynaud et al., 2016). Accordingly, Bard (2003) and de Abreu et al. (2003) reconstructed decreases of ~ 3ºC SST on the Iberian margin during the Heinrich Stadial-5 (Fig.

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7). Besides, a decrease of total cyst accumulation rates and heterotrophic cyst proportions is

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detected during the GS-13 in Ría de Vigo that may be indicating lower phytoplanktonic productivities. This could be explained by the cooler conditions but also by a decrease in

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inputs or upwelling weakening.

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nutrient availability (e.g., Bringué et al., 2013; Ellegaard et al., 2017) as a result of lower river

5.3. The period 11.2-7.0 cal ka BP (early Holocene) in Ría de Vigo

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5.3.1. RSL rise and changes in coastal ecosystems

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Lithological, geochemical and palynological analyses on the early Holocene sections of B5 (235-144 cm) and MVR-3 (147-10 cm) reveal important modifications in the 1) depositional environments, 2) the climate and 3) the marine productivity of Ría de Vigo, if compared to the lower MIS-3 sedimentary record.Less-organic nearshore bioclastic sands and gravels constitute Holocene sediments in both sequences (Fig. 3), with low TOC/TN values and high D/P ratios and foraminiferal lining contents (Figs. 5 and 6). These data reflect that higher marine contributions and more energetic environments of sedimentation existed at both points during the early Holocene. Moreover, a substantial decrease of palynomorph accumulation rates would be related to lower preservation conditions and climate instability. These interpretations are 30

ACCEPTED MANUSCRIPT consistent with the global deglaciation and subsequent sea level rise (Smith et al., 2011). Relevant climatic/oceanographic changes affected this region in the early Holocene (García-Gil et al., 2011; Muñoz Sobrino et al., 2012), when the RSL rose until at least -30 m (~9.0 cal ka BP; Fig. 8D) (Arribas et al., 2010; Martínez-Carreño, 2015; Martínez-Carreño and García-Gil, 2017).

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The conspicuous increase of D/P detected at < 8.0 cal ka BP is linked to a RSL rise. An

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important depositional change occurs at B5 and MVR-3 sites, coinciding with important changes in the pollen curves. The representation of the basin vegetation in the sediments may

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have been affected by modifications in the depositional and transport patterns of sedimentation. The pollen signal (Figs. 5 and 6) changes from a more local source area

— with high

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representation of coastal pinewoods, shrubs and meadows (usually high abundances of Pinus, — with increasing

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Poaceae, and Ericaceae) — towards a broader catchment signal

representation of Quercus and riparian forests — as the sea floods the basin.

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During the Late Glacial/early Holocene transition, it has been estimated that July air

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temperatures increased ~6 ºC in NW Iberia (Muñoz Sobrino et al., 2013) and that the regional climate became more oceanic (Muñoz Sobrino et al., 2018). Milder climatic conditions and higher moisture allowed a greater development of meso-thermophilous forests in NW Iberia

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(Iriarte-Chiapusso et al., 2016). In Ría de Vigo, the B5 record reflects the predominance of

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Quercus-dominated forests during the period ~9.3-7.0 cal ka BP (Fig. 5A), whereas the MVR-3 record indicates that the expansion of oak forests in the region started at least at ~11.2 cal ka BP (Fig. 6A). It may be surprising that other mesic trees, such as Alnus and Corylus, show very low percentages in the early Holocene B5 record (Fig. 5A), particularly when comparing it with the MIS-3 record. This may be explained by the development of continental wetlands in the closest areas, which representation in the pollen record increase and may be attenuating the regional woodlands pollen signal. The representation of Atlantic shrubs (Ulex-type and Ericaceae), ferns (Polypodiaceae/B.

spicant,

Monolete-type,

Trilete-type,

etc.),

and

continental

aquatic/hygrophilous ecosystems with Isoetes spp., Ranunculus spp. and Cyperaceae 31

ACCEPTED MANUSCRIPT importantly increase in the early Holocene (Figs. 5A and 6A). These taxa include some species that are typical of oligotrophic environments related to ponds and freshwater courses, but also others that typically colonize humid soils and temporarily inundated habitats related to riverine forests, meadows, Sphagnum peat bogs or Atlantic shrubs (Castroviejo, 1986-2012; Romero et al., 2004; Ramil-Rego et al., 2008). The expansion of these habitats in Ría de Vigo during the

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early Holocene could be the result of the combination of groundwater table rise due to the climate change and the modification of the littoral configuration and the depositional patterns as

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the result of the RSL rise. Marine transgression is usually linked to an increase of the

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accommodation space and accumulation of sediments on the supratidal area (Allen, 2000; Blum and Törnqvist, 2000; Muñoz Sobrino et al., 2014). Thus, it could favour the establishment of

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marsh and other hygrophilous plant communities in the closest emerged area (Figs. 5A and 6A).

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Carpinus betulus noticeably declines towards the tops of both B5 and MVR-3 sequences (Figs. 5A and 6A). Currently, C. betulus L. is absent from most of the Iberian Peninsula (Sikkema et al., 2016), except for a few relict populations that still survive in the

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Bidasoa valley in the southern Pyrenees (Aizpuru Oiharbide and Catalán Rodríguez, 1984). Our

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data support that C. betulus persisted in the area until at least ~7.5 cal ka BP (Figs. 5A and 6A). Other deciduous types such as Betula, which could colonise the same biotopes as C. betulus

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also suffered an important regression in the early Holocene. The change to a warmer and moister climate combined with the RSL rise could have contributed to the retreat of coastal

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floodplains and other biotopes that were previously occupied by these subhumid cold-tolerant tree species and therefore to the regional disappearance of Carpinus betulus (Muñoz Sobrino et al., 2018).

Regarding marine conditions in Ría de Vigo during the interval 11.2-6.7 cal ka BP, increases in cysts from heterotrophic dinoflagellates in the MVR-3 record (Fig. 6C) may reflect higher diatom productivity and upwelling influence (e.g., Bringué et al., 2013; Ellegaard et al., 2017). Increasing abundances of Spiniferites spp. (Fig. 6C) in detriment of brackish species such as L. machaerophorum also suggest a deeper water column and stronger marine influence 32

ACCEPTED MANUSCRIPT at that point (Muñoz Sobrino et al., 2012; García-Moreiras et al., 2015). The very low accumulation rates of total dinoflagellate cysts — and those of pollen as well — at the core site (Figs. 5C and 6C) can be better explained by worse preservation and deposition conditions, in a highly energetic environment that would have not favoured the deposition of fine materials (Fig. 3), rather than by changes in the phytoplankton — and pollen — productivity. In addition,

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heterotrophic dinoflagellates have lower cyst production capabilities compared to those of

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5.3.2. Abrupt climatic events during the early Holocene

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Lingulodinium polyedrum (Figueroa and Bravo, 2005).

During the early Holocene, the climate of NW Iberia was characterised by the

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occurrence of a number of rapid relapses that affected vegetation dynamics and lake productivity (e.g., Muñoz Sobrino et al., 2013; Iriarte-Chiapusso et al., 2016). Some of them

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may also be identified in the new sequences from Ría de Vigo (Fig. 7). The expansion of deciduous Quercus during the early Holocene in the surroundings of

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Ría de Vigo was interrupted by at least three brief episodes of cooling, respectively dated to

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~10.5 cal ka BP, ~9.3 cal ka BP and ~8.2 cal ka BP (Fig. 7). Usually, these phases are characterised in the pollen records by the retreat of Quercus and the simultaneous expansion of

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some heliophytes (e.g., Pinus, Ericaceae, Poaceae, etc.). We proposed that they may be associated with Bond cycles 7, 6 and 5 (Bond et al., 1997, 2001) (Fig. 7), the last two of which

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are clearly related to the 9.3 and 8.2 ka cold events described in Greenland isotopic records (Rasmussen et al., 2014) and in different sequences from NW Iberia (e.g., Muñoz Sobrino et al., 2005, 2013; Iriarte-Chiapusso et al., 2016). Nevertheless, the regional effects of Bond cycle 7 (10.5 ka cold event) on the coastal ecosystems from Northern Iberia are described for the first time in this study. Higher percentages of B. tepikiense are observed in the MVR-3 record, with a peak at ~8.2 cal ka BP. This can be interpreted (e.g., Eynaud et al., 2016) as indicative of turbulence and stronger influence of the Polar Front (and lower SST) in Ría de Vigo. A decrease of 1-2 ºC 33

ACCEPTED MANUSCRIPT in SST has been detected on the Lisbon continental slope for a period which duration and chronology are compatible with the 8.2 ka event (Rodrigues et al., 2009). The MVR-3 record suggests that the 8.2 ka event had a stronger impact on ecosystems than other early Holocene abrupt events, according to the observations in other ecotonal situations from inner NW Iberia (Muñoz Sobrino et al., 2007, 2013; Iriarte-Chiapusso et al., 2016). The 8.2 ka the Ría de Vigo is

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characterized by a brief (~250 years) but intense decline of deciduous forests (Quercus, Alnus, Corylus, and Betula) and a marked decline in D/P and foraminiferal linings (Figs. 5A and 6A),

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that indicate lower marine sedimentation, possibly related to a RSL drop or stabilization. A

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stronger local signal (i.e. coastal vegetation with Pinus, shrubs and Poaceae) in the pollen record (Figs. 5A and 6A) during such events of cooling and RSL stabilization is expected.

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Accordingly, preliminary interpretations of multiproxy data from Ría de Ferrol (Fig. 1; Cartelle et al., 2016) and Ría de Arousa (García-Moreiras, 2017) suggest that several minor variations in

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the rates of RSL rise occurred in most of the passive Galician Atlantic coast during the early

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Holocene.

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6. Conclusions

Multi-proxy analyses performed on the lower sedimentary facies of cores B5 and

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MVR-3 from Ría de Vigo (NW Iberia) allowed the development of a comprehensive reconstruction of the environmental changes (vegetation, hydrology, and sea level) that occurred

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in the basin during part of the MIS-3 (~57.0-38.8 ka BP) and the first stages of the Holocene (~11.2-6.7 cal ka BP). New palynological data confirm that the ecosystems of Ría de Vigo were sensitive to the main North Atlantic climatic oscillations (GS/GI), but also to short climatic relapses during the early Holocene. Pollen records suggest the presence of coastal Pinus stands and deciduous mesophilous woodlands that survived in this region during the cold relapses and expanded during the warmer phases. GS-13 was the coldest phase recorded within the MIS-3, marked by a strong decline in AP and the increase in the cold-indicator species Bitectatodinium tepikiense in both cores. 34

ACCEPTED MANUSCRIPT Remarkably high abundances (< 12.7% and < 11600 pollen grains·cm-3) of Carpinus betulus pollen are observed for the period 57.0-38.8 cal ka BP. Besides, we confirm its presence in the area in the early Holocene and until ~7.5 cal ka BP. Its decline could be the result of the RSL rise and the climate change that lead to the retreatment of favourable biotopes within the early Holocene.

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Profound changes in both terrestrial and marine ecosystems are detected between MIS-3

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and early Holocene environments, which may be linked to marine and tidal transgression. Expansion of continental wetlands and Quercus-dominated forests are clearly reflected in the

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pollen record. The pollen signal changes from a more local source area

—with high

representation of coastal pinewoods, shrubs and meadows (usually high abundances of Pinus,

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Poaceae, and Ericaceae)— towards a broader catchment signal with increasing representation of

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Quercus and riparian forests as the sea floods the basin. Moreover, more marine and highly energetic conditions with stronger stratification and higher autotrophic cyst productivities

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characterised the early Holocene environments.

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Regional cooling events 8.2 and 9.3 ka caused short interruptions of the Quercus expansion and can be related to lower SST in the ria (increases of Bitectatodinium tepikiense). Besides, our data reflect the effects of another abrupt event of cooling (10.5 ka) that may

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correspond to Bond cycle 7 and that had not been previously described in NW Iberia. New

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palaeoenvironmental data highlights the great sensitivity of the studied coastal ecosystems to climate and sea level changes and contribute to assess the past environmental heterogeneity of the region.

Acknowledgements: This work was supported by the Spanish Ministry of Education and Science [CGL201233584 (co-financed with ERDF funds)] and the Xunta de Galicia [GRC 2015/020]. Iria GarcíaMoreiras was supported by the predoctoral fellowship from Xunta de Galicia [PRE/2013/404],

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ACCEPTED MANUSCRIPT Natalia Martínez-Carreño by the FPI-MCIIN research program [BES-2010-037268], and

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Cristina Delgado by the postdoctoral fellowship from Xunta de Galicia [IC2-2014].

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ACCEPTED MANUSCRIPT Tables: Mean depth (cm)

Material

δ13C (‰)

Radiocarbon age (a BP)

Calibration Curve (Reimer et al., 2013)

Calibrated age (cal a BP)

Comments

Pb-2

0.5

Bulk sediment

-

-

-

-59

From 210Pb analysis

Pb-1

30.5

Bulk sediment

-

-

-

122

From 210Pb analysis

B5-6

72.5

Shell

+0.2

1440±40

MARINE13.14C

791-1215*

B5-5

165.5

Shell

-0.2

8420±40

MARINE13.14C

8696-9285*

B5-4

225.5

Shell

+0.4

8540±40

MARINE13.14C

8956-9420*

B5-3

284.5

Plant remains

-27.8

39840±490

INTCAL13.14C

42,788-44,415*

a

GS-12

a

GI-12c

PT

Label (Core name-date ner)

303.5

Plant remains

-27.3

42,050±670

INTCAL13.14C

44,331-46,675*

B5-1

328.5

Plant remains

-26.7

35,950±340

INTCAL13.14C

39,872-41,330*

MVR-3-6

95

Shell

-2.4

5700±30

MARINE13.14C

5901-6280*

MVR-3-5

138

Shell

+1.4

9830±50

MARINE13.14C

10,517-11,056*

MVR-3-4

146.5

Shell

-0.1

10370±40

MARINE13.14C

11,174-11,837*

MVR-3-3

179.5

Bulk sediment

-25.4

42,640±650

MARINE/INTC AL13.14C (50:50)

44,672-47,051*

MVR-3-2

190.5

Bulk sediment

-25.8

>43,500

MARINE/INTC AL13.14C (50:50)

MVR-3-1

224

Shell

-2.3

>43,500

MARINE13.14C

D

MA

NU

SC

RI

B5-2

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Table 1 Radiometric dates obtained from the B5 and MVR-3 cores. *Two sigma confidence intervals used in age-models. ** Dates B5-1 and B5-5 have been considered outliers: B5-1 resulted stratigraphically erratic compared to the other two radiocarbon dates available in this same facies (B5-2

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and B5-3), and B5-5 resulted inconsistent after correlating B5 and MVR-3 sequences by palynological evidence (see discussion). (a) Proposed correspondences with the chronostratigraphic scheme of

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Rasmussen et al. (2014) and based on the local pollen stratigraphies (see discussion).

Label (Core name-date ner)

Core

Mean depth (cm)

Age (cal a BP)

viii

MVR-3

0.5

-60

vii

B5

153

7400

Increase of D/P ratio, which has been dated to 7.4 cal ka in MVR-3 (Fig. 4B).

MVR-3

118 8300

Decrease of Quercus and D/P ratio. Start of 8.2 ka event (Muñoz Sobrino et al., 2005; Rasmussen et al., 2014).

9400

Decrease of Quercus, increase of Pinus and Ericaceae. Start of 9.3 ka event (Rasmussen et al., 2014; Iriarte-Chiapusso et al., 2016).

vi

v

B5

174

MVR-3

127

Pollen criteria — Climatic event — References Surface sample

46

**Outlier

**Outlier

a

end of GS-13

Out of the dating range Out of the dating range

ACCEPTED MANUSCRIPT iv

MVR-3

150

B5

314

44,200

48,200

iii

Decrease of AP and D/P, increase of Pinus. Start of GS-12 (Rasmussen et al., 2014). Increase of Pinus and B. tepikiense and decrease of AP (particularly Quercus and Corylus) and D/P. Start of GS-13 (Rasmussen et al., 2014).

MVR-3

192

ii

B5

332

51,200

Increase of AP and Quercus percentages and influxes, and decrease of Ericaceae/Corema and Poaceae. Start of GI-14a (Rasmussen et al., 2014).

i

MVR-3

257

54,800

Decrease of AP influx and D/P ratio, increase of Pinus and Poaceae representation. Start of GS-15.1 (Rasmussen et al., 2014).

Table 2 Age control points and pollen criteria considered to establish the relative dates that have been

PT

used for building the B5 and MVR-3 chronologies (Fig. 4). The stratigraphic locations of the age control

RI

points represent the onset of several climatic events; they were established considering the interpretation of the pollen sequence and its correlation with other published pollen and paleoclimate records, as

SC

discussed in the text. In the chronological models, uncertainties of ±10 years were established for the relative ages corresponding to the Late Holocene (viii), ±100 years for the ages attributed to the

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Early/Mid Holocene (v, vi and vii) and ± 1000 years for the ages corresponding to the MIS-3 period (i to iv). These uncertainties were estimated taking into account the resolution of the sequence and the relative

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error of alignment with the corresponding climatic phases, as well as the uncertainty of the chronology of

Diatom species

Ecology

PT E

Depth (cm) 155-156

D

the events themselves, described in the references.

ø ø

164-165

ø

175-176

ø

185-186

ø

195-196

ø

200-201

ø

210-211

Planotidium frequentissimum, Gomphonema parvulum, reichardtiana, Fragilaria sp., Rhoicosphenia abbreviata.

266-267

271-272 276-277

AC

220-221 230-231 240-241 245-246 250-251 255-256 261-262

CE

160-161

Navicula

Ø Ø Ø Navicula cryptotenelloides Ø Ø Achnanthidium minutissimum, A. atomoides, Cocconeis euglypta. Achnanthidium minutissimum, A. atomoides, Amphora pediculus, Navicula cryptotenelloides, Nitzschia dissipata, Rhoicosphenia abbreviata, A. atomoides, Cymbella sp., Sellaphora pupula, Nitzschia rectiformis. Cocconeis euglypta, Gomphonema sp., Achnanthidium minutissimum, A. pyrenaicum, Navicula tripunctata, Eolimna minima, Amphora pediculus, Adlafia bryophila, Nitzschia sp., Gomphonema tergestinum. Achnanthidium pyrenaicum, Cocconeis euglypta

47

Freshwater with moderate electrolyte content

Freshwater

Freshwater Freshwater with moderate electrolyte content Freshwater Freshwater

ACCEPTED MANUSCRIPT

306-307 311-312 316-317

321-322

326-327

Freshwater Freshwater Freshwater Freshwater with moderate electrolyte content Freshwater Freshwater with moderate or high electrolyte content Freshwater/Brackish waters Freshwater/Brackish waters Brackish/Marine waters

NU

331-333

Freshwater

PT

296-297 301-302

RI

291-292

Cocconeis euglypta, Cocconeis pseudolineata Ø Nitzschia palea, Planothidium frequentissimum, Amphora pediculus, Navicula cryptotenelloides, Nitzschia amphibia, Navicula tripunctata. Achnanthidium atomoides, Achnanthidium minutissimum. Navicula sp., Cyclotella meneghiniana, Nitzschia sp., Nitzschia inconspicua. Rhoicosphenia abbreviata, A. minutissimum, Navicula cryptotenelloides, Cyclotella meneghiniana, Eolimna minima, Brachysira sp., Cocconeis pediculus, Gomphonema sp. Nitzschia amphibia, Gomphonema parvulum, Cocconeis euglypta. Diatoma vulgaris, Nitzschia sp., Achnanthidium pyrenaicum, Rhoicosphenia abbreviata, Navicula sp., Nitzschia palea, Achnanthidium minutissimum, Cocconeis euglypta, Cymbella sp., A. atomoides, Cocconeis pediculus, Navicula capitatoradiata, Eunotia minor, Gomphonema sp. Gomphonema sp., Cyclotella meneghiniana, Cocconeis euglypta, Cocconeis pediculus, Navicula lanceolata, Stephanodiscus sp. with fragments of unidentifiable brackish diatoms Cyclotella meneghiniana, Cocconeis euglypta, Diploneis sp., Cocconeis scutellum, Achnanthidium atomoides and Stephanodiscus sp. and fragments of brackish diatoms. Fragments of brackish and marine diatoms: Diploneis spp., Opephora spp., Paralia sulcata, Psammodiscus nitidus, etc.

SC

281-282 286-287

Table 3 Results of diatom analyses in the 150-333 cm sequence of the B5 core. Data is shown as absence

MA

(ø) and presence (taxa found is specified) for each sample.

List of captions:

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Figures

Fig. 1 Study area: A) Location of Ría de Vigo at the Atlantic margin of Galicia (NW Iberia), and B) Ría de Vigo and the position of studied cores (B5 and MVR-3). Bathymetric contour is in meters

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(cartographic data are from http://mapas.xunta.gal/produtos-cartograficos/capas-six/hidrografia).

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Fig. 2 Schematic representations of: A) the system of currents offshore western Iberia, and B) the vertical distribution of marine waters in front of the Galician coast, modified from Sprangers et al. (2004). ENACW: Eastern North Atlantic Central Water and its subtropical (ENACWst) and subpolar (ENACWsp) branches. PCS: Portugal Current System. MW: Mediterranean Water. LSW: lower saline Labrador Sea Water. NADW: North Atlantic Deep Water.

Fig. 3 Lithostratigraphy, texture and geochemical analyses of the B5 (A) and MVR-3 (B) cores. The facies associations (sediment packages) according to Martínez-Carreño (2015) and Martínez Carreño and García-Gil (2017) are also represented.

48

ACCEPTED MANUSCRIPT Fig. 4 Age-depth models of B5 (A) and MVR-3 (B) cores obtained using CLAM 2.2. in R software (Blaauw, 2010). Black lines represent the radiometric dates (Table 1) and grey lines represent the polleninferred ages (Table 2) (length of lines corresponds to the 95% confidence intervals); those considered as outliers (and not included in the model) are marked with a red cross. Red dashed lines limit the 95% confidence intervals of the estimated ages. GOD = Goodness of fit (-log, lower is better).

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Fig. 5 Abundances of: A, B) selected pollen and NPP types, and C) dinocyst taxa, in the B5 section at 333-144 cm. Blue bands mark the main episodes of AP decline (except Pinus) that generally coincide

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with D/P decline (colder phases) and that have been correlated with Greenland Stadials (GS) and early

SC

Holocene cold events (8.2 and 9.3 ka events; Rasmunssen et al., 2014). Grey shading represents x10 exaggeration of the values. Clustering results, local pollen (LPZ) and dinoflagellate cyst (LDZ) zones are

NU

shown on the right. The chronology corresponds to the age-depth model presented in Fig. 4A (red models). Names in bold represent categories that are the sum of more than one type of palynomorph (see Supplementary Table 1). Riparian trees include Alnus, Salix, Fraxinus, and Corylus. Freshwater and

Pseudoschizaea spp. and Spirogyra spp.

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brackish algae spores include taxa with mainly continental origin: Mougeotia spp., Pediastrum spp.,

D

Fig. 6 Abundances of: A, B) selected pollen and NPP types, and C) dinocyst taxa in the MVR-3 section at

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279-104 cm depth. Blue bands mark the main episodes of AP decline (except Pinus) that generally coincide with D/P declines (colder phases) and have been correlated with Greenland Stadials (GS) and

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early Holocene cold events (8.2, 9.3 and 10.5 ka events; Rasmunssen et al., 2014). Grey shading represents x10 exaggeration of the values. Clustering results, local pollen (LPZ) and dinoflagellate cyst

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(LDZ) zones are shown on the right. The chronology corresponds to the age-depth model presented in Fig. 4B. Names in bold represent categories that are the sum of more than one type of palynomorph (see Supplementary Table 1). Riparian trees include Alnus, Salix, Fraxinus, and Corylus. Freshwater and brackish algae spores include taxa with mainly continental origin: Mougeotia spp., Pseudoschizaea spp., Pediastrum spp. and Spirogyra spp.

Fig. 7 A) Alignment of the B5 (dashed lines) and MVR-3 (continuous lines) pollen sequences for the early Holocene and MIS-3 periods, B) interpretation of colder stages and phases of relative sea level (RSL) highstand, and C) proposed correlation with the palaeoclimatic records of Greenland (ice-core isotopic records; NGRIP dating group, 2006), annual SST on the Atlantic margin of Iberia (MIS-3; Bard,

49

ACCEPTED MANUSCRIPT 2003) and warm/cold SST variations on the subtropical Atlantic (early Holocene; deMenocal et al., 2000). Correlation with Bond events, defined as decreases (colder stages) in percentages of petrologic tracers (haematite-stained grains) from stacked ocean cores MC52-V29191-MC21-GGC22 that reflect variations in North Atlantic ice-rafted debris (IRD) (Bond et al., 2001), is also represented for the early Holocene.

Fig. 8 RSL reconstruction in Ría de Vigo at several times during the Last Glacial-Interglacial Transition

PT

(LGIT) performed from seismic and sedimentary data (Dias et al., 2000; García-García et al., 2005; Martínez-Carreño, 2015; this paper). In figure D (early Holocene), the reconstructed palaeoisobath line of

RI

30 m below the present sea level is shown.

SC

Plates:

NU

Plate 1 Light micrographs of some relevant pollen, NPP and dinoflagellate cysts identified in B5 and MVR-3 cores. a) Carpinus betulus; b, c) Fagus; d) deciduous Quercus, cf. Quercus robur; e) Ephedra

MA

distachya-type; f) Juniperus-type; g) aggregate of Asteraceae pollen (Liguliflorae); h) Type 1174 — Rosellinia spp.; i) Type 113 — Sporormiella sp.; j) Type1066 — Delitschia spp.; k) Pseudoschizaea spp.; l) Operculodinium centrocarpum sensu Wall & Dale 1966; m) Selenopemphix quanta; n) Bitectatodinium

PT E

D

tepikiense; o) Brigantedinium spp. (included in smooth brown cysts, SBC). Scale bars = 10 µm.

Plate 2 Light micrographs of selected diatoms identified in the B5 core: a) Navicula tripunctata (O.F. Müller) Bory; b) Gomphonema tergestinum (Grunow in Van Heurck) Schmidt; c) Caloneis bacillum

CE

(Grunow) Cleve; d) Gomphonema truncatum Ehrenberg; e), f) Cocconeis pediculus Ehrenberg; g), h) Cocconeis euglypta Ehrenberg; i) Achnanthidium atomoides Monnier, Lange-Bertalot, and Ector; j)

AC

Gomphonema aff. olivaceum (Hornemann) Brébisson; k) Rhoicosphenia abbreviata (C.Agardh) LangeBertalot; l) Gomphonema sp.; m) Opephora spp.; n) Nitzschia fonticola Grunow in Van Heurck; o) Nitzschia sp.; p) Paralia sulcata (Ehrenberg) Cleve; q) Psammodiscus nititus (Gregory) Round & Mann.; r) Diploneis spp.

50

ACCEPTED MANUSCRIPT Highlights: MIS-3 and early Holocene environmental variability is poorly studied at NW Iberia High-resolution, multi-proxy data were obtained from two sedimentary sections Palynological data reflect climate effects on marine productivity and vegetation Ría de Vigo was sensitive to the succession of GS/GI and other rapid climatic changes

AC

CE

PT E

D

MA

NU

SC

RI

PT

The effects of 10.5 ka event (Bond-7) are described for the first time in the region

51

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8