Climate fluctuations during the Holocene in NW Iberia: High and low latitude linkages

Continental Shelf Research 30 (2010) 1487–1496

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Climate fluctuations during the Holocene in NW Iberia: High and low latitude linkages L.D. Pena a,n, G. France´s b, P. Diz b,c, M. Esparza d, J.O. Grimalt d, M.A. Nombela b, I. Alejo b a

Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA ´n do Territorio, Facultade de Ciencias do Mar, Universidade de Vigo, As Lagoas-Marcosende, s/n, E-36310 Vigo, Spain ˜as e Ordenacio Departamento de Xeociencias Marin c ´rio Nacional de Energia e Geologia, Estrada da Portela, Zambujal, Amadora 2721-866, Portugal Unidade de Geologia Marinha, Laborato d Department of Environmental Chemistry, Institute of Chemical and Environmental Research (CSIC), Jordi Girona, 18, 08034 Barcelona, Spain b

a r t i c l e in fo

abstract

Article history: Received 18 December 2009 Received in revised form 4 May 2010 Accepted 12 May 2010 Available online 1 June 2010

High resolution benthic foraminiferal stable isotopes (d18O, d13C) and molecular biomarkers in the sediments are used here to infer rapid climatic changes for the last 8200 years in the Rı´a de Muros (NW Iberian Margin). Benthic foraminiferal d18O and d13C potentially register migrations in the position of the hydrographic front formed between two different intermediate water masses: Eastern North Atlantic Central Water of subpolar origin (ENACWsp) and subtropical origin (ENACWst). The molecular biomarkers in the sediment show a strong coupling between continental organic matter inputs and negative d13C values in benthic foraminifera. The rapid centennial and millennial events registered in these records have been compared with two well known North Atlantic Holocene records from the subtropical Atlantic sea surface temperatures (SST) anomalies off Cape Blanc, NW Africa and the subpolar Atlantic (Hematite Stained Grains percentage, subpolar North Atlantic). Comparison supports a strong link between high- and low-latitude climatic perturbations at centennial–millennial time scales during the Holocene. Spectral analyses also points to a pole-to-equator propagation of the socalled 1500 yr cycles. Our results demonstrate that during the Holocene, the NW Iberian Margin has undergone a series of rapid events which are likely triggered at high latitudes in the North Atlantic and are rapidly propagated towards lower latitudes. Conceivably, the propagation of these rapid climatic changes involves a shift in atmospheric and oceanic circulatory systems. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Paleoceanography Isotopes Molecular biomarkers Benthic foraminifera 1500 yr cycles Spectral analysis

1. Introduction In recent years, researchers have documented the existence of a series of recurring millennial-scale climatic and oceanographic events during the Holocene in the North Atlantic (Bond et al., 1997, 2001; Bianchi and McCave, 1999; Keigwin and Boyle, 2000; Giraudeau et al., 2001; Andrews et al., 2003; Marchitto and deMenocal, 2003; Oppo et al., 2003; Solignac et al., 2004) and Subtropical Atlantic (deMenocal et al., 2000a, b; Keigwin and Boyle, 2000) with a mean periodicity of ca. 15007500 years. This sub-Milankovitch climate variability has also been recognized in pollen records, lacustrine deposits (Campbell et al., 1998; Viau et al., 2002; Hu et al., 2003) and speleothems (McDermott et al., 2001). Although increasing evidences of these quasi-periodic cycles during the Holocene have arisen from records around the world, there is still considerable debate regarding their pacing (Schulz and Paul, 2002), driving force (Bond et al., 2001) as well as the actual mechanism propagating these Holocene climatic

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perturbations into the atmosphere–ocean system. It has also been argued that subtle alterations in the Atlantic meridional overturning circulation (AMOC) may have played a significant role in the amplification of these millennial-scale events due to changes in meridional heat transport (Marchitto and deMenocal, 2003; Oppo et al., 2003) and that weakening of the thermohaline circulation (THC) might have triggered and/or amplified Late Holocene cold episodes (i.e. the Little Ice Age) in the North Atlantic (Keigwin and Boyle, 2000). Such evidences render even more important the need to obtain better paleo-records that permit to resolve accurately these high frequency climatic oscillations. Shallow marine areas and continental shelves have a great potential as high resolution paleo-archives, yet little attention has thus far been directed to these areas due to the difficulty to discriminate between local and global climatic signals in the past. In this sense, the northwestern Iberian Margin has proved to be particularly sensitive to rapid climate changes during the late Holocene, both hydrographical (Diz et al., 2002; A´lvarez et al., 2005; Gonza´lez-A´lvarez et al., 2005) and atmospheric changes (Martı´nez-Cortizas et al., 1999; Desprat et al., 2003). However, whether these climatic changes are local or a response to a global

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Table 1 AMS radiocarbon dated levels from core EUGC-3B. All the analyses were performed at the AMS Laboratory, Institute of Physics and Astronomy (University of Aarhus, Denmark). Raw radiocarbon 14C ages were d13C corrected and calibrated to calendar ages using the Calib 5.01 program (Stuiver and Reimer, 1993) and the MARINE04 calibration data (Hughen et al., 2004). Sample (cm)

Sample type

EUGC-3B (1) EUGC-3B (57) EUGC-3B (93) EUGC-3B (178) EUGC-3B (238) EUGC-3B (321) EUGC-3B (407)

Benth. foram. (Nonion commune) Bivalve shell (Venus nux) Bivalve shell (Myrtea spinifera) Bivalve shell (Myrtea spinifera) Bivalve shell (Venus nux) Bivalve shell (Myrtea spinifera) Bivalve shell (Lucinoma borealis)

14

C Age (BP)

920 7 65 12297 39 15757 40 26237 45 33557 55 4805 7 50 7610 7 60

forcing is still a matter of debate (Diz et al., 2002; A´lvarez et al., 2005; Gonza´lez-A´lvarez et al., 2005). Therefore, more efforts need to be directed towards these particularly sensitive regions, in order to obtain high resolution Holocene records at centennial time scales. In this study we analysed the stable oxygen and carbon (d18O, 13 d C) isotopic composition of benthic foraminifera from a sediment core located in the Rı´a de Muros (NW Iberian Peninsula), a shallow marine environment with relatively high sedimentation rates. The gravity core EUGC-3B records the last 8200 calendar years before present (cal. yr BP, Table 1) and is located in the vicinity of the Finisterre Front, a hydrographic front between two components of the Eastern North Atlantic Central Water (subpolar and subtropical end-members, ENACWsp and ENACWst, respectively) and thus it is a potential area for recording past oceanographic fluctuations at centennial and millennial time scales. In order to establish the linkages with general and widespread climatic patterns in the North Atlantic during the Holocene, we compare our record with previously published North Atlantic Holocene records which are known to reflect centennial–millennial-scale variability: SST anomaly off Cape Blanc, ODP site 658C (deMenocal et al., 2000b) and hematite stained grains percentage (HSG), VM 29-191 (Bond et al., 1997, 1999). The SST anomaly at ODP site 658C is constructed from planktonic foraminifera assemblages and is an indicator of how cold or warm the SSTs off Cape Blanc were during summer and winter times along the Holocene compared to the modern SSTs (deMenocal et al., 2000b). On the other hand, the HSG percentage shows the relative abundance of terrigenous materials transported by icebergs to the ocean during relatively cold episodes in the Holocene (Bond et al., 1997, 1999). Both records, therefore, are sensitive gauges for climatic and oceanographic fluctuations during the Holocene in the North Atlantic.

2. Study area and regional oceanography The Rı´a de Muros is the northernmost of the so-called Rı´as Baixas, a set of funnel-shaped shallow incised valleys that distinguish the NW Iberian coasts (Fig. 1a–c). They are under the direct influence of shelf winds that modify the typical two-layer residual circulation pattern characteristic of partially mixed estuaries (A´lvarez-Salgado et al., 2000). During the upwelling favourable season (April–September) cool and nutrient-rich intermediate waters (mostly ENACW) are advected into the Rı´as Baixas increasing the primary productivity in the surface waters as well as the flux of organic carbon to the sediment and the adjacent continental shelf (A´lvarez Salgado et al., 2000). As a result, during relatively strong or persistent upwelling periods, a large fraction of the organic materials produced within the Rı´as is exported to the continental shelf, thus contributing to the rapid aging of the upwelled ENACW (A´lvarez-Salgado et al., 2000). Conversely, during the rest of the year, downwelling-favourable

Calibrated age7 2 std. dev. [435–638 cal. BP] (536 cal. BP) [682–879 cal. BP] (780 cal. BP) [1035–1246 cal. BP] (1140 cal. BP) [2154–2430 cal. BP] (2292 cal. BP) [3057–3357 cal. BP] (3207 cal. BP) [4922–5266 cal. BP] (5094 cal. BP) [7938–8194 cal. BP] (8066 cal. BP)

d13C (%) V-PDB  2.53  1.97  0.84  0.14  0.04 0.17 2.23

southerly winds prevail and the circulation pattern reverses accumulating surface waters into the outer areas of Rı´as Baixas and preventing the entrance of ENACW. Downwelling periods are characterized by lower water renewal rates, consumption of nutrients and in situ organic matter degradation (A´lvarez-Salgado et al., 2000). The Galician continental shelf is hydrographically characterized by the presence of two different intermediate water masses that converge off Cape Finisterre (Fraga, 1991; Pe´rez et al., 1993, Fig. 1d, e): subtropical and subpolar ENACW. ENACWst can be identified on a Temperature/Salinity plot as a straight line ranging from 13.13 1C, 35.80 psu (practical salinity units) to 18.50 1C and 36.75 psu (Fiu´za, 1984) (Fig. 1d, e). ENACWst is formed near 351N along the Azores Front as a result of the subduction of large water volumes due to local evaporation processes and the subsequent winter surface cooling (Fiu´za, 1984). After water mass formation, ENACWst is advected eastward by the Azores Current to be finally split into two different branches: one spreads southward to northwestern Africa and the other flows northwards reaching the occidental margin of the Iberian Peninsula more aged and progressively less oxygenated (Pe´rez et al., 2001, Fig. 1f). As a consequence, ENACWst is relatively less oxygenated than ENACWsp at the area of convergence between both water masses at the Finisterre Front (see Fig. 1f). The ENACWsp is relatively colder and less saline than the ENACWst and can be identified on a T–S plot as a straight line between 10.0 1C, 35.40 psu and 12.20 1C, 35.66 psu (Fig. 1d, e). This water mass is formed in the Northeast Atlantic along the 461N parallel (Celtic Sea) due to winter cooling and deep convection (McCartney and Talley, 1982; Brambilla et al., 2008). These hydrographical models (Fiu´za, 1984) have been subsequently confirmed by different authors (Rı´os et al., 1992; A´lvarez-Salgado et al., 1993, 1997, 2003; Castro et al., 1994; Pe´rez et al., 2001; Peliz et al., 2002). The convergence of these two water masses forms a subsurficial hydrographic front with the ENACWst to the south and the ENACWsp to the north (Fraga, 1991; Pe´rez et al., 1993) (Fig. 1d, e). The regional position of this front varies at annual/interannual time scales, with northward and southward migrations depending on regional climatic and oceanographic oscillations and thus alternating the upwelling of either ENACWst or ENACWsp (A´lvarez-Salgado et al., 1993, 1997; Pe´rez et al., 2001). It is reasonable to assume that this mechanism could have also operated in a similar fashion on longer time scales (centennial to millennial), with either ENACWst and/or ENACWsp upwelling accordingly to general northward or southward shifts of hydrographical fronts in the North Atlantic.

3. Material and methods 3.1. Stable isotopes analyses Gravity core EUGC-3B (421 45.1050 N, 91 02.2310 W; 412 cm long, 38 m water depth) was recovered within the framework of the EU HOLSMEER project during the 2001 BIO Mytilus cruise. The

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Fig. 1. (a) Map showing the location of core site EUGC-3B (421450 N, 091020 W, 38 m water depth) and the two North Atlantic core sites selected for comparison, VM 29-191 (541160 N, 161470 W, 2370 m water depth, Bond et al., 1997) and ODP Site 658C (201450 N, 181350 W, 2263 m water depth, Cape Blanc, Mauritania, deMenocal et al., 2000b). (b) Schematic representation of the subsurface hydrographic front formed between ENACWsp and ENACWst (Fraga, 1991) over the continental shelf in the vicinity of Cape Finisterre and close to the location of core EUGC-3B. (c) Detailed bathymetric chart of the Rı´a de Muros (NW Spain) and core site EUGC-3B. (d, e) Hydrographic section of the western Iberian Margin illustrating the mean annual temperature and salinity fields that characterize ENACWsp and ENACWst water masses (Conkright et al., 2002). Isotherms and isohalines plotted as defined by Fiu´za (1984). MOW: Mediterranean Outflow Water. The red star shows the location of core EUGC-3B between the two water masses. (f) Triple plot of depth (m), oxygen concentration (ml/l) and salinity (psu) from western Iberia Margin. Note that for a constant depth, ENACWsp is relatively more oxygenated than ENACWst (Conkright et al., 2002). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

upper part of the core (2.5–0 m) was sampled at 2 cm intervals and the lower part (4.1–2.5 m) at 3–4 cm intervals. As a general sampling routine, the outer rind of the core was removed to prevent from contamination. Bulk sediment samples were oven dried, weighed and soaked into a disaggregating solution and then wet sieved (4125 mm). About 15–25 specimens of the benthic foraminifera Nonion commune (d’Orbigny) were picked from the 4125 mm fraction for stable isotopic (d18O, d13C) measurements. Only well preserved individuals (without clear signals of test alteration and/or partial dissolution) were selected for these analysis. All samples were first sonicated in distilled water in

order to remove potentially attached clays from the foraminiferal tests. The high abundance of N. commune along the core (Lebreiro et al., 2006) together with its shallow infaunal preferential habitat (Diz and France´s, 2008) makes this species an excellent candidate for paleoceanographic reconstructions. Stable carbon and oxygen isotopic measurements were carried out at MARUM, University of Bremen (Geosciences Faculty) using a Finnigan MAT 251 mass spectrometer coupled online with a Kiel automated carbonate preparation device. Long term reproducibility was calculated using an internal standard (Solnhofen Limestone) routinely measured over several months ( 70.07% for d18O and 70.05%

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for d13C). All the isotopic data were referred to the Vienna Pee Dee belemnite standard (V-PDB scale). 3.2. Molecular biomarkers The analytical methodology for the measurement of molecular biomarkers consists of several steps (Cacho et al., 1999). First, the sediment samples (  2 g) were freeze-dried and ground in a mortar to achieve homogeneity. Then, after addition of an internal standard mixture (n-nonadecan-1-ol, n-hexatriocontane and ntetracontane) the sedimentary lipids were extracted from the sediments in an ultrasonic bath with dichloromethane. The extracted lipids were then hydrolyzed with 6% potassium hydroxide in methanol for the elimination of wax ester interferences. Finally, the extracts were derivatized with bis(trimethylsilyl) trifluoroacetamide before the instrumental analysis (see Cacho et al., 1999 and references therein for further details). The analyses were performed at the Institute of Environmental Assessment and Water Research (Barcelona) on a Varian gas chromatograph Model 3400 equipped with a septum programmable injector and a flame ionization detector. Concentrations of C37 alkenones ([C37:2 +C37:3]), n-alkanes (n-nonacosane) and n-alcohols (n-hexacosanol) were estimated by quantification of the internal standard n-hexatriocontane. The average reproducibility of these concentrations is better than 10%. The C37 alkenones ([C37:2 + C37:3]) are synthesized mostly by Haptophyceae algae (cocolitophorids) and, therefore, are produced in the water column (Brassell et al., 1986). On the other hand, the nalkanes (n-nonacosane) and n-alcohols (n-hexacosanol) are constituents of the epicular waxes in the leaves from high plants, hence having a distinctive terrestrial origin (Cacho et al., 2000).

we can examine possible changes in the nature of most important periodicities all along the time period of selected records. For this purpose we have selected a complex Morlet wavelet which offers great frequency resolution but is slightly less efficient in ascertaining time localization. Results are shown as 2D contours of spectral variance (variance normalized CWS) in both time and frequency domains.

4. Results 4.1. Stable isotopes results

The age model was established by means of accelerator mass spectrometry (AMS) radiocarbon dating of seven levels (Table 1) at the AMS Laboratory, Institute of Physics and Astronomy (University of Aarhus, Denmark). One benthic foraminiferal sample and 6 well preserved bivalve shells (e.g. unbroken, both valves still articulated, delicate structures preserved) were selected for 14C AMS dating. All species are characterized by very shallow infaunal habitat preferences (Table 1). Raw radiocarbon 14 C ages were calibrated to calendar ages with the Calib 5.01 software (Stuiver and Reimer, 1993) using the MARINE04 calibration curve (Hughen et al., 2004). No local reservoir correction (DR) was applied to the raw 14C ages. Radiocarbon dates and calibrated ages (with 72s ranges) are listed in Table 1. All the ages in this paper are expressed in calendar years before present (cal. yr BP) unless otherwise specified. The age model was calculated by linear interpolation between the calibrated ages of the dated levels resulting in average sedimentation rates of 49 cm/kyr with intervals of up to 137 cm/kyr. The age framework obtained ranges from 8200 cal. yr BP at the core bottom to 500 cal. yr BP at the core top, probably indicating some sediment loss at the core top upon retrieval.

The benthic foraminifera stable isotopes (d18O, d13C) results obtained for core EUGC-3B are shown in Fig. 2. Overall, the d18O variability is relatively small compared to the d13C variability (Fig. 2a, b). Benthic d18O values range from 1.60% to 2.15% and show three distinctive intervals that can be differentiated (Fig. 2a). Between 8200 and 7300 cal. yr BP the d18O values increase rapidly from 1.60% to 1.99%. After this short interval, the d18O record attains more constant values within the 7300 and 4000 cal. yr BP period. The last period (4000–500 cal. yr BP) starts with a new abrupt change in the d18O values (from 1.90% to 1.75%). After this decrease, the d18O record comes into a new relatively steady period without significant variability until the upper part of the core (Fig. 2a). A simple overview of the d18O record points towards the existence of at least two different periods or ‘‘modes’’ in the area. Both periods evidence different isotopic signatures, the former with relatively higher d18O values and the latter with lower d18O values. The transition between both modes happens at around 4000 cal. yr BP and can also be identified in the d13C record (see below). Benthic foraminiferal d13C shows a larger range of variation between  0.70% and  4.45% (Fig. 2b). This record also illustrates two distinctive intervals: early Holocene (8200– 4200 cal. yr BP) and late Holocene (4200–500 cal. yr BP). The early Holocene section is characterized by a generally decreasing trend in the d13C values (Fig. 2b) that occurs in three major steps. The first step (8200–7300 cal yr BP) depicts a shift in the d13C values from  0.30% to 1.60% and is followed by a relatively stable plateau (Fig. 2b). The second isotopic pulse (7300–6300 cal. yr BP) also took place as an initial depletion in the d13C values (from  1.60% to  2.58%) and the subsequent plateau. The last step follows a similar pattern with d13C values dropping from  2.58% to  3.86% (Fig. 2b). An interesting feature is that these d13C isotopic plateaus usually occur with relatively lighter d18O values (Fig. 2a, b). The late Holocene section is characterized by generally low d13C values which are punctuated by rapid positive d13C excursions (Fig. 2b). In the first of these rapid events (3700 2900 cal. yr BP), d13C values increase by 2.5% immediately after the d13C minimum (  4.45%). Afterwards, low d13C values are registered again in the record, presenting a slightly increasing trend towards more positive values from this point to the core top. Positive excursions also occur at 1800 and 1000 cal. yr BP (Fig. 2b).

3.4. Spectral analysis

4.2. Molecular biomarkers results

We have performed a continuous wavelet spectrum (CWS) analysis on the three selected data-sets: NW Iberia benthic d13C (EUGC-3B), SST anomaly off Cape Blanc (Hole 658C) and Hematite Stained Grains percentage in the subpolar North Atlantic (HSG, VM 29-191) (Fig. 1a). The CWS is a multi-resolution time– frequency technique that is useful for exploring data known to be non-stationary (Torrence and Compo, 1998). With this approach

The measured abundance in the EUGC-3B sediments of C37 alkenones ([C37:2 + C37:3]), n-alkanes (n-nonacosane) and n-alcohols (n-hexacosanol) is illustrated in Fig. 2c–e. The three measured biomarkers, both from terrestrial (n-alkanes, n-alcohols) or marine provenance (alkenones) clearly show an increasing concentration trend from 8200 up to about 4200 cal. yr BP, coinciding with the decreasing trend in the d13C isotopic

3.3. Age model of core EUGC-3B

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Fig. 2. Compilation of data from core EUGC-3B. Age model based on 7 AMS radiocarbon dates (see Table 1 for details). Black triangles correspond with calibrated AMS ages and horizontal lines are estimated age errors (see text for details). (a, b) Benthic foraminifera (Nonion commune) d18O and d13C records from core EUGC-3B. (c, d) Molecular biomarkers n-alkanes (n-nonacosane) and n-alcohols (n-hexacosanol) abundances as a proxy for continental organic matter inputs. (e) C37 alkenones ([C37:2 +C37:3]) abundances as a proxy for marine productivity. (f) Modelled sea level reconstruction for the last 9 kyr (Siddall et al., 2004). Vertical yellow bars indicate positive d13C excursions.

composition. Such an increase in the concentration of organic compounds is coeval with the final stage of global sea level rise (Siddall et al., 2004) (Fig. 2f). Following this period there are a series of high and low organic matter events (particularly significant in the terrestrial biomarkers that are synchronous with the rapid d13C excursions registered in the benthic foraminifera; Fig. 2). For every biomarker concentration maximum there is an associated d13C minimum, indicating a very tight coupling between both records. It is also noteworthy that the concentrations of terrestrial biomarkers (n-alkanes, n-alcohols) is significantly higher than the marine biomarkers (alkenones), pointing towards a strong terrestrial control in the Rı´a de Muros paleoceanography.

5. Discussion The Holocene variability of benthic foraminiferal stable isotopes in the Rı´a de Muros suggests that this area has undergone a set of major hydrographic changes during the last 8200 years. There are at least two different mechanisms that control the variability in the d13C record at site EUGC-3B. The general decreasing trend shown by the d13C record from ca. 8200 to 4200 cal. yr BP is interpreted to be caused by local changes in sea level, which induce changes in the supply of organic carbon to the seafloor as well as changes in water column ventilation rates (Fig. 2). The organic carbon is 12C enriched, therefore an increased supply will shift d13C towards lower values. In general, sea level

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was located about 20 m below present sea level at about 8200 cal. yr BP (Dias et al., 2000; Siddall et al., 2004). As the sea level rises in the Rı´a de Muros, the primary production of the surface waters progressively increases as well as the organic carbon flux to the sea floor, therefore explaining the continuous decrease of benthic d13C values. From 4200 cal. yr BP onwards, there is a smooth decrease in sea level that is also accompanied by an overall small increasing trend in the d13C (Fig. 2). This transition is also synchronous with a major shift in the benthic foraminifera d18O values at 4200 cal. yr BP that has been interpreted as a change in the predominance of the water mass that was primarily upwelling into the Rı´a de Muros (see below). The d13C record is punctuated by at least six rapid millennialscaled events of high d13C values (Fig. 2). These events can be explained by two complementary explanations. The first one is that the d13C enrichments are linked to a decreased influence from the continental runoff into the Rı´a de Muros as corroborated by the close relation between d13C enrichments and decreased concentrations of the n-alkanes and n-alcohols records. At every positive d13C excursion (e.g. 500 and 3000 yr) there is a reduced abundance in the terrestrial biomarkers concentration, thus indicating that the primary source of organic matter to the sediments is controlled by continental inputs into this semi-enclosed basin (Fig. 1). Additionally, there is also a strong coupling between marine and terrestrial biomarkers (alkenones) which suggests that the continental inputs also played an important role in controlling the primary productivity in the Rı´a de Muros during the Holocene (Fig. 2c–e). This control could have been exerted by nutrients inputs from the continents during periods of increased runoff. An alternative and/or complementary interpretation is that the variation of d13C through the Holocene is related to the type of water mass upwelling into the Rı´a de Muros. Relatively ‘old’ water masses present low d13C ratios due to the respiration processes that consume oxygen and release metabolic 12CO2, whereas relatively ‘young’ water masses typically show higher d13C ratios and higher oxygen concentrations. According to this interpretation, positive d13C events can be related to the upwelling of the relatively more oxygenated ENACWsp into the Rı´a de Muros. Upwelling of this water mass can induce both more ventilation at the sea floor at this shallow site and an increase in the outwelling (export to the shelf) of the organic matter produced is situ or incorporated from the continental runoff at this site. According to this model, positive d13C values indicate more ventilation and less in situ organic matter remineralization coinciding with periods of ENACWsp upwelling. Conversely, more negative d13C values account for reduced ventilation rates and enhanced in situ organic matter degradation (i.e. decrease of the outwelling). These periods could be also related with the advection of the poorly oxygenated ENACWst into the Rı´a de Muros. Fig. 1f clearly shows how at a certain water depth ENACWsp is always better ventilated than its subtropical counterpart. We want to stress here that both interpretations (atmospheric vs. oceanic) are not incompatible, rather, they are complementary and it is very likely that both the atmospheric and oceanic processes involved in this rapid centennial changes were taking place simultaneously in the North Atlantic. The centennial and millennial-scale variability of the benthic d18O record does not illustrate major hydrographic changes in the Rı´a de Muros during the last 8200 years. This could be partially due to the opposite effects that salinity and temperature have in the d18O. Higher (lower) temperatures induce lighter (heavier) d18O values whereas salinity presents the opposite effect, with higher (lower) salinities showing heavier (lighter) d18O values. ENACWst is a relatively warm and salty water mass whereas ENACWsp is colder and fresher, therefore having both compensating effects on the d18O that reduce the variability at EUGC-3B. Nevertheless, there is a

significant shift in the d18O values at around 4200 cal. yr BP (Fig. 2a) coinciding with the d13C minimum that can be a result of a hydrographic reorganization in the water masses affecting the Rı´a de Muros. The change form heavier d18O values (8200–4200 cal. yr BP) to slightly lighter values (4200–500 cal. yr BP) could be interpreted as a change in the water mass that was primarily upwelling into the Rı´a de Muros. According to that interpretation, the predominant water mass upwelling in the Rı´a de Muros during the Early Holocene was ENACWst (with relatively heavier d18Osw values), whereas during the Late Holocene the ENACWsp (lighter d18Osw values) (LeGrande and Schmidt, 2006) entered more frequently into the Rı´a de Muros. This shift from relatively warm and saltier to relatively cold and fresher waters is certainly in accordance with a general cooling trend throughout the Holocene as reported in many different paleo-records in the North Atlantic region (Marchal et al., 2002). Interestingly, periods characterized by the presence of ENACWsp and strong upwelling events in the Rı´a de Muros (more positive benthic d13C values) illustrate a remarkable similarity with the cold SST anomalies registered at ODP site 658C off Cape Blanc (deMenocal et al., 2000b) (Fig. 3b). These SST anomalies in the subtropical Atlantic have been interpreted to be induced either by a southward advection of colder subpolar waters and/or by an intensification of large upwelling events that typically occur in this area (deMenocal et al., 2000b). Moreover, it has also been proposed that these cooling events in the subtropical North Atlantic are directly linked to the variations in the abundances of ice rafted debris found in the subpolar North Atlantic sediments (Bond et al., 1997; deMenocal et al., 2000b) (Fig. 3a, b). Such a strong climatic linking between the subpolar and subtropical Atlantic supports the interpretation of colder water masses and enhanced upwelling events in the Rı´a de Muros (i.e. increased influence of ENACWsp) during these rapid cold spells in the North Atlantic. This seems to be the more plausible explanation as both locations (Rı´a the Muros and Cape Blanc) are directly under the influence of the western North Atlantic Subpolar–Subtropical Gyre (i.e. Canary Current, aka Portugal Current). Therefore, any alteration in this regional oceanic system (e.g. intensification of northerly winds and/or southward displacement of the hydrographical fronts) should be recorded almost simultaneously in both areas. There are strong evidences that the North Atlantic polar front migrated southwards to a more zonal position during the Last Glacial Maximum (Keffer et al., 1988; Broccoli et al., 2006), therefore it is likely that minor cooling events in the North Atlantic could have induced similar southward displacements of the subpolar–subtropical gyre and the associated hydrographical fronts so affecting the wind and surface circulation patterns in the Rı´a de Muros. Indeed, increased influence of ENACWsp into the Rı´a de Muros requires a southward displacement of the ENACWsp/ENACWst front. Further support to this hypothesis arises from the terrestrial biomarkers record in the Rı´a de Muros. At present, NW Iberia is under the direct influence of the North Atlantic storm tracks which deliver large amounts of rainfall to the area. The Rı´a de Muros acts like a catchments basin due to its incised valley topography and therefore delivers great amounts of rainfall as runoff that washes away and incorporates terrestrial organic compounds that eventually make its way into the sediments. Thus any change in the relative position of the storm tracks pathway will certainly result in variations in the runoff and hence in the abundance of terrestrial biomarkers incorporated into the sediments. In this context, Fig. 3 clearly illustrates the strong coupling between the EUGC-3B n-alcohol record and both the benthic d13C in the Rı´a de Muros and the SST anomalies in the subtropical Atlantic. Cold SST anomalies in the subtropics (southward shift in the atmospheric fronts) are recorded in the Rı´a de Muros as reduced concentrations of terrestrial sediment compounds which

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Fig. 3. Comparison of the EUGC-3B benthic foraminiferal d13C and terrestrial biomarkers records (a–d) with two other North Atlantic records: Sea Surface Temperature anomalies for the warm and cold seasons from ODP Site 658C (201450 N, 181350 W, 2263 m water depth, Cape Blanc, Mauritania, deMenocal et al., 2000b) and hematite stained grains percentage (%HSG) at core VM 29-191 (541160 N, 161470 W, 2370 m water depth, Bond et al., 1997). Calibrated radiocarbon ages and uncertainty bars are shown for each record. Yellow bands illustrate the relationships between cold periods in the subtropical Atlantic and periods of predominant ENACWsp at the NW Iberian Margin and higher percentages of HSG in the subpolar North Atlantic. Bold numbers refer to the Holocene cold events (Bond et al., 1997). (e) Detrended EUGC-3B d13C record and Gaussian band-pass filter on the 1500 yr band (15 yr window). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

are interpreted as decreased runoff (less rainfall) due to a southward shift in the mean position of the storm tracks. Remarkably, a southward shift in the atmospheric fronts would have as a result more northerly winds on the Rı´a de Muros and the associated continental shelf, thus inducing an intensification of the upwelling systems that would have as a result more subpolar ENACW advected into the Rı´a and would be in agreement with the southward migration of the oceanic fronts proposed previously. Based on these results we can hypothesize that these Holocene cold spells are propagated southward from the North Atlantic region through small shifts in the general atmospheric and oceanic circulation patterns. This is supported by the fact that

for the full suite of Holocene cooling events, there is no systematic temporal offset between subpolar and subtropical Atlantic within the calibrated radiocarbon chronologies (deMenocal et al., 2000b). It could also be argued that our temporal framework is not robust enough as to state that cold events in the North Atlantic are synchronous with the isotopic changes in the Rı´a de Muros. However, and despite the age model limitations, the timing of positive d13C excursions in core EUGC-3B shows a significant correlation (r2 ¼0.794, p¼0.001) with the timing of the subtropical Atlantic SST anomalies (Fig. 3). Therefore, it is reasonable to conclude that the propagation of such rapid events from the North Atlantic towards southern latitudes could have been nearly

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instantaneous, at least in terms of centennial–millennial time scales. These relatively short-lived perturbations of the climatic system involved large-scale oceanic and atmospheric reorganizations that were accomplished within centuries or perhaps decades. There is a general agreement that subpolar and mid-latitude records all over the Atlantic region have been influenced by persistent climate variability during the Holocene with a periodicity around 1500 yr (O’Brien et al., 1995; Bond et al., 1997, 1999; Bianchi and McCave, 1999). It is hence possible to determine the persistence of these cycles and the relative influence of global climatic cycles versus local and/or more regional influences. The continuous wavelet time–frequency spectrums (Fig. 4a–c) illustrate that there is a generally pervasive pattern in the three time series (North Atlantic, Rı´a de Muros and subtropical Atlantic) with a mean periodicity of ca. 1500 yr, although the relative significance of the spectral signatures vary between the different records. At site EUGC-3B, a Gaussian band-pass filter on the detrended d13C record (with a mean periodicity of 1500 yr and window of ca. 15 yr) shows a clear modulation on the 1500 yr band (Fig. 3e). In a similar fashion, the subtropical SST anomalies present a spectral periodicity in the range of 1500 yr, although the significance of these periodicity is rather diminished (Fig. 4c). Provided that the temporal resolution of this record should be enough to resolve centennial and millennial-scale variability changes, it is plausible that the influence of the northern 1500 yr climatic cycle gets muted on its way towards the tropical regions. Also, another possibility could be that subtropical SST anomalies are under the influence of regional hydrographic changes. In this regard, ODP site 658C is under the influence of the African monsoon, which in turn is mainly controlled by insolation changes (deMenocal et al., 2000a; Kuhlmann et al., 2004). Hence, during time periods of strong subtropical monsoon the subpolar signal might be weakened to some extent, or perhaps completely erased from the sedimentary records. It has been widely recognized that the Holocene records of low latitudes have been strongly forced by monsoon processes (Neff et al., 2001; Fleitmann et al., 2003; Russell and Johnson, 2005). The EUGC-3B d13C record (located half-way between the subpolar and subtropical North Atlantic) is strongly affected by the 1500 yr cycles (Fig. 4b). It is likely, therefore, that the primary source of the 1500 yr periodicity is located somewhere at the Northern Hemisphere high latitudes and that this climatic signal is rapidly propagated southward where its effects are weakened or perhaps dampened by low latitude climatic processes (e.g. African monsoon). Recent findings in the Alboran Sea (Western Mediterranean Sea) further support this hypothesis (Moreno et al., 2005). These authors have clearly established a dissimilarity in the spectral patterns between marine-based proxies (biomarkers and SSTs presenting a significant 1500 yr periodicity) and land-based proxies (i.e. Saharian dust input without significant 1500 yr periodicity) (Moreno et al., 2005).

6. Conclusions The analyses of benthic foraminiferal stable isotopes and molecular biomarkers in the sediments of gravity core EUGC-3B have allowed the reconstruction of the Holocene paleoceanographic history of the NW Iberian Margin. Benthic foraminiferal oxygen and carbon isotopes register the predominant water mass advection and/or upwelling into the Rı´a de Muros for the last 8200 cal. yr BP. Oxygen isotopes indicate that the Early Holocene (8200– 4200 cal. yr BP) was mainly influenced by ENACWst whereas the Late Holocene (4000–500 cal. yr BP) was affected by ENACWsp.

Fig. 4. Multiple spectral analyses of the selected records. Linear trends were removed in every data-set before running spectral analysis in order to reduce red noise (i.e. background power decreases with increasing frequencies). (a–c) Continuous wavelet spectrum (CWS) illustrating the spectral variance through time in the three selected records. Colour scales show the different spectral variances intensities for each plot. Vertical axes represent periods (yr) computed at every single time step of the time series. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Benthic foraminifera carbon isotopes show a larger range of variability between positive and very negative values. Negative d13C episodes correspond with periods of increased continental

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runoff that brings large amounts of terrestrial compounds into the sediments. These periods could be related to increased rainfall associated with the presence of the storm tracks over NW Iberia. Six major benthic foraminiferal positive d13C isotopic events were registered for the last 8200 years indicating the presence of cold and better ventilated ENACWsp as well as reduced terrestrial organic carbon export into the sediments. We have interpreted these isotopic excursions as southward migrations of the atmospheric and oceanic fronts associated with cold spells in the North Atlantic. There is a clear link between these centennial changes in the Rı´a de Muros and more widespread Holocene centennial– millennial time scale climatic perturbations at high and low latitudes in the North Atlantic region. A spectral analysis of the benthic foraminifera d13C record in the Rı´a de Muros indicates that these events present a recurrent 1500 yr periodicity throughout the Holocene. It is suggested that these rapid events are likely triggered at high latitudes and then rapidly propagated towards lower latitudes by means of southwards shifts of atmospheric and oceanic circulatory systems.

Acknowledgements The authors acknowledge to the EU project HOLSMEER: Holocene Shallow Marine Environments of Europe (EVK2-CT-200000060) for providing samples and financial support, GRACCIE (Consolider-Ingenio, CDS 2007-00067); CONTOURIBER (CTM 2008-06399-C04-01/MAR) y Proxecto 08MDS036000PR (Xunta de Galicia). L.P. and P.D. also thank the EU Acces to Research Infrastructures Paleostudies Program (Geosciences Faculty, Bremen University, Germany) for providing us with analytical facilities, and to F. Lamy for all the help provided. L.P. acknowledges a fellowship from the Comer Abrupt Climate Change Foundation (USA). Thoughtful comments and discussion provided by P.G. Mortyn are greatly thanked. Also thanks to Antje Volker and an anonymous reviewer for very insightful comments that helped to improve the quality of this paper. This is LDEO contribution 7352.

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