Changes in northeast Atlantic hydrology during Termination 1: Insights from Celtic margin's benthic foraminifera

Quaternary Science Reviews 175 (2017) 45e59

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Changes in northeast Atlantic hydrology during Termination 1: Insights from Celtic margin's benthic foraminifera M. Mojtahid a, *, S. Toucanne b, R. Fentimen a, 1, C. Barras a, S. Le Houedec a, G. Soulet c, J.-F. Bourillet b, E. Michel d a

LPG-BIAF UMR-CNRS 6112, University of Angers, UFR Sciences, 2 bd Lavoisier 49045, Angers Cedex 01, France IFREMER, UR G
eosciences Marines, Laboratoire G
eophysique et Enregistrements S
edimentaires, BP70, 29280 Plouzan
e, France Department of Geography, Durham University, South Road, DH1 3LE, United Kingdom d ^t.12, 91198 Gif-sur-Yvette, France Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL), Domaine du CNRS, Ba b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 June 2017 Received in revised form 12 September 2017 Accepted 12 September 2017

Using benthic foraminiferal-based proxies in sediments from the Celtic margin, we provide a well-dated record across the last deglaciation of the Channel River dynamics and its potential impact on the hydrology of intermediate water masses along the European margin. Our results describe three main periods: 1) During the Last Glacial Maximum, and before ~21 ka BP, the predominance of meso-oligotrophic species suggests well oxygenated water masses. After ~21 ka BP, increasing proportions of eutrophic species related to enhanced riverine supply occurs concomitantly with early warming in Greenland airtemperatures; 2) A thick laminated deposit, occurring during a 1500-years long period of seasonal melting of the European Ice Sheet (EIS), is associated with early Heinrich Stadial 1 period (~18.2e16.7 ka BP). The benthic proxies describe low salinity episodes, cold temperatures, severe dysoxia and eutrophic conditions on the sea floor, perhaps evidence for cascading of turbid meltwaters; 3) During late HS1 (~16.7e14.7 ka BP), conditions on the Celtic margin's seafloor changed drastically and faunas indicate oligotrophic conditions as a result of the ceasing of EIS meltwater discharges. While surface waters were cold due to Laurentide Ice Sheet (LIS) icebergs releases, increasing benthic Mg/Ca ratios reveal a progressive warming of intermediate water masses whereas oxygen proxies indicate overall well oxygenated conditions. In addition to the well known effect of EIS meltwaters on surface waters in the Celtic margin, our benthic record documents a pronounced impact on intermediate water depths during HS1, which coincided with major AMOC disruptions. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Quaternary Paleoceanography North Atlantic Micropaleontology Foraminiferal assemblages Channel River Deglaciation Elemental ratios Stable isotopes AMOC

1. Introduction The last deglaciation (Termination 1) began in the northern hemisphere some 19 ka ago (~19e10 ka; Clark et al., 2012), and marks the transition from the Late Glacial Maximum (LGM) (23e18 ka after Mix et al., 2001; ~26.5e19 ka after Clark et al., 2009) to the Holocene (11.7 ka to the present; Rasmussen et al., 2006). During the LGM, expansive ice sheets covered large areas of the Northern Hemisphere: the Laurentide Ice Sheet (LIS) over northern North America, and the European Ice Sheet (EIS) including the

* Corresponding author. E-mail address: [email protected] (M. Mojtahid). 1 Present address: Department of Geosciences, University of Fribourg, Chemin du
e 6, 1700 Fribourg, Switzerland. Muse https://doi.org/10.1016/j.quascirev.2017.09.003 0277-3791/© 2017 Elsevier Ltd. All rights reserved.

Scandinavian (SIS) and British-Irish Ice Sheets (BIIS) over Europe (see Ehlers and Gibbard, 2004 for a thorough review). While these ice sheets were retreating during Termination 1, they delivered massive quantities of icebergs and meltwaters into the North Atlantic (Clark et al., 2001; Knutti et al., 2004; Denton et al., 2010; Toucanne et al., 2015 among others). This resulted in the slowing down of the Atlantic Meridional Overturning Circulation (AMOC) for several millennia, triggering two cold (stadials) events between ~18.2 and 14.7 ka (Heinrich Stadial 1; HS1), and between ~12.7 and 11.7 ka (Younger Dryas event; YD) (e.g., McManus et al., 2004; Hall et al., 2006; Carlson et al., 2007; Roberts et al., 2010; Lynch-Stieglitz et al., 2014). Amongst the scientific community, the LIS is widely recognized as the major meltwater contributor to the North Atlantic during the last deglaciation (e.g., Broecker et al., 1988; Thornalley et al., 2010). In comparison, the role of EIS meltwaters in disturbing the AMOC is still poorly documented (e.g., Peck et al.,

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2006, 2007; Toucanne et al., 2009a). During Pleistocene glacial lowstands, the Channel River was flowing at the present-day location of the English Channel (Fig. 1a), and draining EIS meltwaters towards the North Atlantic Ocean (Gibbard, 1988; Bourillet et al., 2003; Mojtahid et al., 2005; Eynaud
not et al., 2006; Toucanne et al., 2009b, 2010). Lately, et al., 2007; Me by using neodymium isotopic composition of sediments located off the Channel River mouth, Toucanne et al. (2015) emphasized a strong impact of the southern EIS on the reduction of the AMOC during the last deglaciation. As importantly, the greatest contribution of the EIS to the North Atlantic freshwater budget was suggested during the first part of Heinrich Stadial 1 from ~18.2 to 16.7 ka (referred to in the following as “early HS1”). This resulted locally in the deposition of a thick laminated sediment layer (Zaragosi et al., 2001a; Auffret et al., 2002). The LIS contributed significantly to the second part of HS1 (referred to as “late HS1”; i.e. Heinrich event 1 sensu stricto) from ~16.7 to 14.7 ka resulting in the deposition of ice-rafted debris (IRD) rich sediments in the North Atlantic all along the northwestern European margin (Grousset et al., 1993; Scourse et al., 2009). The present study focuses on marine sediment core MD99-2328 (942 m water depth) collected directly off the Channel River mouth (Fig. 1). Core MD99-2328 covers the past ~23 ka with exceptionally high sedimentation rates of ~260 cm/ka across the LGM and HS1. Our study discusses benthic foraminiferal assemblages, oxygen and carbon stable isotopes and elemental ratios measured on their carbonate tests. In general, the standing stock, composition, and geochemical signature of benthic foraminifera observed at a specific time interval and location result from the interaction between biological, hydrological (e.g., thermohaline circulation, oxygenation), and ecological factors (e.g., food availability). Each species might respond differently to one or more of these parameters depending on its ecological requirements. The aims of this study is: i) to provide new insights to better constrain the complex interactions between north hemisphere climate and Channel River

activity, and ii) to better apprehend the impact of EIS meltwaters release on the regional intermediate waters characteristics, especially when knowing that these water masses played a major role in the dynamics of the last deglaciations (Alvarez-Solas et al., 2010; Marcott et al., 2011). 2. Study area 2.1. Geography and paleogeography Core MD99-2328 was collected from the continental slope of the northern Bay of Biscay (the Celtic margin) on the Brenot Spur at 942 m water depth (Fig. 1). During glacial periods, two main geographically distinct sediment sources were feeding the Celtic margin (e.g., Scourse and Furze, 2001; Auffret et al., 2002): i) the Irish Sea draining a large part of the BIIS meltwaters, and ii) the Channel River. Using neodymium tracers, Toucanne et al. (2015) showed that the Channel River drained a large part of EIS meltwaters and was the major source of sediments to the Celtic margin during glacial lowstands (Fig. 1a). Further downstream, due to a direct connection with the shelf edge (~200 m water depth at present; Toucanne et al., 2012), the Channel River supplied sediments to the deep-sea through a complex network of submarine canyons (Zaragosi et al., 2001b; Bourillet et al., 2006). 2.2. Present and past oceanic circulation A detailed description of the present-day oceanographic setting of the mid-latitude northeast Atlantic Ocean is given by van Aken (2000a, 2000b). In the upper ~600 m of the water column, the Celtic margin is under the influence of the North Atlantic open ocean surface circulation (Eastern North Atlantic Water; Pollard and Pu, 1985). Between ~600 m and 1300 m water depths, a branch of the Mediterranean Outflow Water (MOW) is present. Below the MOW and until about 2000 m depth, Labrador Sea

Fig. 1. a, b) Figure modified from Toucanne et al. (2015): a) location of core MD99-2328 (this study) and paleogeography of Western Europe showing the maximum extension of the European Ice Sheet (EIS) including the Scandinavian (SIS), British-Irish Ice Sheets (BIIS), the Irish Sea Ice Stream (ISIS) and the Channel Paleoriver hydrographic network during the
not et al., 2006; Eynaud et al., 2012; Toucanne et al., 2015) and MD01-2461 (Peck et al., 2006) is Last Glacial Maximum (LGM). The location of the nearby cores MD95-2002 (Me indicated; b) a simplified pattern of Atlantic Ocean circulation, with the warm saline waters of the North Atlantic current (red arrows) and the return flow pathway of the deep waters (blue arrows). White arrows indicate the main supply sources of freshwater to the North Atlantic. The white cross indicates the locations of cores MD99-2328 and MD952002, whilst the yellow cross indicates the location of core OCE326-GGC5 (McManus et al., 2004); c) Morphobathymetric 3D block of the Brenot Spur (Bourillet and Loubrieu, 1995) performed using the submarine mapping software CARAIBES (IFREMER). Water depth spans from 200 m (red) to 4000 m (blue). The white cross indicates the location of core MD99-2328. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Waters are found. Deeper, the cold and low salinity North Atlantic Deep Waters (NADW) flow. During the LGM and Termination 1, the AMOC underwent major changes. Several studies suggest that the NADW was replaced by a nutrient-poor, cold and relatively fresh water mass, the Glacial North Atlantic Intermediate Water (GNAIW), which was dominant above 2000 m water depth, and between 60 and 20
N (e.g., Oppo and Lehman, 1993; Lynch-Stieglitz et al., 2007; Gebbie, 2014; Marson et al., 2015). GNAIW was overlying a more nutrient-rich water mass below ~2000 m, the glacial analog of Antarctic Bottom Water (AABW). During HS1, McManus et al. (2004) suggested a significant reduction in the strength of the AMOC. Since then, more recent studies showed a more complicated pattern. From the northwestern Atlantic, Roberts et al. (2010) and Hoogakker et al. (2016) emphasized that any freshwater perturbation caused by HS1 could have only affected the shallow overturning in the North Atlantic. In the northeastern Atlantic, other studies showed that intermediate overturning cell shoaled and slowed but remained active during HS1, and concluded that a significant AMOC characterized the entire deglaciation at intermediate depths (Gherardi et al., 2009; Bradtmiller et al., 2014). Other authors suggested a see-saw pattern in the North Atlantic during Heinrich events between the two main Antarctic water masses, where deep waters showing increased contribution of Antarctic Bottom Waters (AABW), while intermediate depths showing decreased contribution of Antarctic Intermediate Waters (AAIW) (e.g., Piotrowski et al., 2005; Huang et al., 2014). On the opposite, Rickaby and Elderfield (2005) and Thornalley et al. (2011) suggested that AAIW may have penetrated into the high-latitude North Atlantic during HS1 and YD and have competed with GNAIW for occupation of the zone between ~1000 and 2500 m. After the YD, the AMOC accelerated, similarly to today's configuration (Gherardi et al., 2009). When the NADW spread back, Antarctic waters retracted and were constrained to near bottom depths (e.g., Marson et al., 2015). Regarding the MOW, its outflow volume was reduced during glacial sea level lowstands, and therefore restricted to the southern Iberian margin (e.g., Rohling and Bryden, 1994; Rogerson et al., 2012). 3. Material and methods Piston core MD99-2328 (48
04.62 N, 09
30.35 W, 23.9 m length), was recovered during the INTERPOLE MD99-114/IMAGESV cruise with the CALYPSO corer onboard the R/V Marion-Dufresne (Labeyrie, 1999) (Fig. 1).

47

3.2. Age model The age model is based on XRF-Ti/Ca synchronization between core MD99-2328 and the well dated nearby core MD95-2002 (Fig. 1a; Fig. 2a) (Toucanne et al., 2015). The age scale obtained using this method was tested against five 14C dates (obtained on monospecific planktonic foraminifera) at Beta Analytic (USA) (Table 1). 14C ages were first corrected for reservoir age and then calibrated to calendar age using the IntCal13 calibration curve (Reimer et al., 2013). Because the regional reservoir age has changed through time (e.g., Stern and Lisiecki, 2013), all 14C ages (in our core and in core MD95-2002) were corrected accordingly for each stratigraphic unit: prior to HS1 and during the Holocene, reservoir age was almost constant and centered on ~400 ± 200 14C years. During HS1, the Bølling-Allerød (BA) and the YD, average reservoir ages were 970, 680 and 875 14C years with a typical uncertainty of 200 14C years (see Toucanne et al., 2015 for details; Table 1). Both XRF-Ti/Ca-based age model and 14C dates are in an overall good agreement (Fig. 2b) and the difference could be due to some reservoir age spatial variability. Nonetheless, in order to keep the best chronological fit with the well-studied and dated MD952002 (Fig. 1a) for comparisons and interpretations of paleoenvironmental changes, we chose to use only the XRF-Ti/Ca based age model. To be entirely confident in the robustness of our XRF-Ti/Cabased age model, this latter was further tested against percentages of the polar planktonic taxon Neogloboquadrina pachyderma obtained in both MD99-2328 and MD95-2002 cores (Fig. 2c). Neogloboquadrina pachyderma is a morphotype which today dominates the polar environments of the North Atlantic Ocean. High percentages of this species in the sediments of the Bay of Biscay are associated with the incursion of polar water masses (Auffret et al., 1996) and/or the local input of cold meltwaters (Eynaud et al., 2007). Both N. pachyderma curves show a clear concordance when applying our age model (Fig. 2c). All ages presented in this paper are expressed in calendar before present (cal BP). Note that we specifically focus our study on sediment older than 16 ka. From this time onwards, the sedimentation in the deep Bay of Biscay was strongly affected by the erosion of the shelf deposits in response to the significant sea-level rise and the embayment of the English Channel which led to poorly preserved and reworked Holocene sediments in the area (e.g., Bourillet et al., 2003; Toucanne et al., 2012). This explains the offset between the most recent Holocene (core top) 14C date (at ~3.5 ka) with our final age model (Fig. 2b). Nonetheless, the presence of non-fossilizing agglutinated benthic species in the most recent sediments informs us that our top core is modern.

3.1. XRF and grain size analyses 3.3. Foraminiferal assemblage analyses X-Ray images were obtained at EPOC (Bordeaux, France) with a SCOPIX image processing tool (Migeon et al., 1999). The bulk intensity of major elements (calcium (Ca) and titanium (Ti)) was measured using Avaatech X-Ray Fluorescence (XRF) core scanner at 1 cm-resolution at IFREMER (Brest, France). XRF data were measured with a 10 s account time, by setting the voltage to 10 kV (no filter) and intensity to 600 mA. CaCO3-free grain size analyses were performed every 10 cm using a Coulter LS200 laser microgranulometer (IFREMER Brest, France). Sediment decarbonatation was performed with 1 M hydrochloric acid. Particle Size Distribution D50 (i.e., the medium value of the particle size distribution) and sortable silt mean size (SS) (i.e., the mean of the 10e63 mm grain-size range of McCave et al. (1995)) were computed. Because of the very low biogenic siliceous flux in the Celtic margin and as opal is almost entirely dissolved on the seabed (<1%; Hall and Mccave, 1998; McCave et al., 2001), silicates were not removed for sortable silt measurements.

Core MD99-2328 was subsampled every 20e40 cm (87 samples; ~30e310 years resolution for glacial times and ~200e3300 years for the Holocene). All subsamples were washed through a 150 mm sieve. Samples were split (>150 mm), using a microsplitter when necessary. At least 200 specimens were picked out and identified from a single split. Relative abundances (percentages) of benthic (or planktonic) species were estimated from the total of benthic (or planktonic) foraminiferal assemblages. Error bars for relative abundances were computed using the binomial standard pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi error pð1  pÞ=n (Buzas, 1990; Fatela and Taborda, 2002), where p is the species proportion estimate (number of counted individuals for a given species/n). Foraminifer diversity was quantified using the Shannon index (entropy, H) using PAST software (PAleontological STatistics; Version 2.14; Hammer et al., 2001). Error bars representing 95% confidence interval were computed with a bootstrap procedure via PAST software.

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Fig. 2. a) Stratigraphic correlation of core MD99-2328 (black line) with core MD95-2002 (grey line) based on the XRF-Ti/Ca ratio (Zaragosi et al., 2006); b) In red, the final age model based on the XRF-Ti/Ca ratio correlation (a). In blue, the calibrated 14C ages and associated error range (2s; grey shaded area; see Table 1). YD: Younger Dryas; HS1: Heinrich Stadial 1; LGM: Last Glacial Maximum; SAR: Sediment accumulation rates. The depth range of the five dominating facies shown in Fig. 2d is indicated; c) Test of the robustness of the age model with the percentages of the polar taxon N. pachyderma in core MD99-2328 (black line) with core MD95-2002 (grey line); d) X-ray photographs showing the different observed facies. Each image corresponds to a 20 cm section. Facies 1: 2024e2004, Facies 2: 1387e1407 cm, Facies 3: 700e720 cm, Facies 4: 321e341 cm, and Facies 5: 266e286 cm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 1 AMS radiocarbon ages of core MD99-2328. G. bull: Globigerina bulloides. N. pch: Neogloboquadrina pachyderma. error (1s)

Reservoir correctiona (14C errora yr) (1s)

14

C age corrected for reservoirb (14C yr BP)

errorc (1s)

Calendar age ranged (yr BP, 2s)

G. bull 3510

30

400

200

3110

202

2837e3728

G. bull 10090

40

400

200

9690

204

10489-11758

N. pch

11160

30

875

200

10285

202

11324-12598

N. pch

11370

30

875

200

10495

202

11693-12750

N. pch

11340

30

875

200

10465

202

11611-12736

Depth (cm)

Lab. Number

Species

10

Beta438722 Beta438723 Beta438724 Beta438725 Beta438726

90 140 170 200 a b c d

14

C age (yr BP)

Reservoir correction infered from Stern et Lisieki (2013). Corrected 14C ages are obtained by subtracting the reservoir correction to the original 14C age. Errors associated to the corrected 14C were propagated through the quadratic sum. Corrected 14C ages were then calibrated using the atmospheric calibration curve IntCal13 (Reimer et al., 2013).

We interpret the abundance of Elphidium spp. and Cibicides lobatulus as indicators of transport from the upper inner shelf (see details in paragraph 5.1.). In order to estimate oxygen content variation, we used benthic foraminiferal assemblages after discarding these transported species, following the method proposed by Schmiedl et al. (2003) using the formula: (OH/ (OH þ LO) þ Div)  0.5, with OH ¼ relative abundance of high oxygen indicators (Cibicides pachyderma, Gyroidina orbicularis, Hanzawaia boueana, Lenticulina spp., Pyrgo spp., Quinqueloculina spp., and Sigmoilopsis schlumbergeri), LO ¼ relative abundance of low oxygen indicators (Bolivina spp., Bulimina spp., Cassidulina carinata, Chilostomella oolina, Globobulimina spp., Melonis barleeanus, Nonionella turgida, Praeglobobulimina ovata, Trifarina spp., and Uvigerina spp.) and Div ¼ normalized benthic foraminifera diversity. Diversities were normalized relative to the maximum H value. Finally, the term was multiplied by 0.5 in order to distinguish between anoxic (minimum value ¼ 0) and oxic (maximum value ¼ 1) conditions. The species included in the two ecological groups (OH, and LO) were adapted to this particular study based on the good knowledge of their ecological niches in the Bay of Biscay (e.g., Fontanier, 2003; Duchemin et al., 2008; Mojtahid et al., 2010).

3.4. Oxygen and carbon stable isotopes Every 5e40 cm (for planktonic species), and every 5e210 cm (for benthic species), stable oxygen and carbon isotope analyses were performed on one planktonic species Neogloboquadrina pachyderma, and depending on their abundances, on seven benthic species Cibicides pachyderma, C. lobatulus, C. wuellestorfi, C. kullenbergi, Uvigerina peregrina, and Globobobulimina affinis. About 10 tests of N. pachyderma were hand-picked from the 200e250 mm fraction, 1 to 2 tests for C. pachyderma/wuellestorfi/kullenbergi from the >150 mm fraction, 1 test of U. peregrina and G. affinis from the 315e450 mm fraction, and 1 to 10 tests for C. lobatulus from the 150e250 mm fraction. They were then cleaned in a methanol ultrasonic bath for a few seconds then roasted under vacuum at 380
C for 45 min to remove organic matter, prior to isotopic analyses (Duplessy, 1978). The d18O and d13C (expressed in ‰ VPDB) were measured at LSCE on a GV Isoprime mass-spectrometer coupled with a Carbo Prep preparation h line for benthic species (~1 test) and on OPTIMA with a common acid bath for N. pachyderma (~10 tests). The external reproducibility (1s) of NBS19 is ± 0.05‰ and 0.06‰ for d13C and d18O respectively, measured NBS18 d18O is 23.3 ± 0.2‰ VPDB. The reproducibility for N. pachyderma is 0.10‰ and 0.21‰ for d18O and d13C respectively.

3.5. LA-ICPMS elemental ratios For LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) analyses, several specimens of the benthic species Cassidulina carinata were picked out from the >150 mm fraction every ~50 cm on average between 23.1 and 11.6 ka (total of 47 subsamples). Ten specimens per sediment subsample on average were measured with one to two measurements per specimen in the central chambers. The picked foraminifer tests were first rinsed with milli-Q water (1e2 rinses). Methanol (CH3OH) was added (100 mL par tube), and mixed. After a few minutes of settling, the supernatant methanol was removed. The tests were then rinsed two times with milli-Q water and dried for at least 24 h in a drying stove (50
C). Foraminiferal Mg/Ca, Mn/Ca, and Sr/Ca were measured using LA-ICP-MS performed at LPG-Nantes (UMR CNRS 6112, France). Isotopes used for single chamber element quantification were 24Mg, 88Sr, 55Mn, 43Ca, 44Ca, and their relative natural abundances. All laser spots were 65e85 mm in diameter, repetition rate was 6 Hz and laser energy density was set at 1 Jcm2. Elemental analyses were calibrated against NIST612 glass, using value from Jochum et al. (2011). Time resolved signals were selected for integration, background subtracted, and internally standardized to 43Ca. Glass standard was ablated at a higher energy density (5 Jcm2). Using different ablation energies for glass and calcite was previously shown not to affect the analyses (Wit et al., 2010). The external reproducibility for standard NIST was 2s < 106 (n ¼ 221) for all E/Ca ratios (Relative Standard Deviation <1.2%). Standard-sample bracketing was used to correct for instrumental isotopic fractionation. On encountering surficial clay contamination (indicated by Al and Fe peaks) the data integration interval was adjusted to exclude the Al and Fe enrichment. Integration windows that separate the calcitic signal from background and detection of any contaminants at the test surface were done using designated software (GLITTER). Elemental ratios with respect to Ca were based on the average of each ablation profile. For each lithological layer data were processed with a 2s outlier rejection to remove outliers and then were averaged. The resulting depth profile was smoothed with a 3-point moving average. 4. Results 4.1. Sedimentological description The visual description of core MD99-2328 together with grainsize measurements allowed the identification of five sedimentary facies (Fig. 2d). Facies 1 consists of mottled silty clays to clayey silts with intense bioturbation. This facies was mainly found in the early

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record from ~2350 to 1650 cm (~23.1e20.6 ka BP) and from ~1020 to 910 cm (~18.6e18.2 ka BP). Facies 1 is interpreted as contourite deposits, i.e. sediments affected by along slope bottom currents (e.g., Rebesco et al., 2014). Facies 2, which is transitional between Facies 1 and Facies 3, consists of slightly bioturbated silty clays with some laminations. This facies was mainly dominant from ~1650 to 1025 cm corresponding to the time period from 20.6 to 18.6 ka BP. Facies 3 consists of millimeter to centimeter-scale layers of siltyclays alternating with millimeter-scale layers of silty-clays rich in anchor-ice IRDs (Zaragosi et al., 2001a; Toucanne et al., 2009a) that were only present between ~930 and 388 cm (~18.2e16.7 ka BP). Facies 3 is typical of hyperpycnal flow deposits from the cascading of meltwater turbid plumes (e.g., Mojtahid et al., 2005; Toucanne et al., 2012). From ~388 to 235 cm (~16.7e16.0 ka BP), Facies 4 consists of IRD-rich silty clays. This facies is interpreted as resulting from the release of ice-rafted icebergs particles after melting. After a hiatus of ~3.4 ka, Facies 5 consisting of highly bioturbated clayey silts characterized the top 230 cm of the core. Facies 5 resembles Facies 1 (although the latter having a finer grain size) and is also interpreted as contourite deposits. 4.2. Diversity and relative abundances Highest benthic Shannon diversity (H) (>2.5) occurred during the Holocene and the YD. Average values of about 2.0 were found during the LGM (from 23.1 to 18.2 ka BP). Low H diversity (<2.0) marked HS1 from 18 to 16.3 ka BP (Fig. 3b). Highest relative abundances (>80%) of the planktonic species N. pachyderma were found during late HS1 period between ~16.7 and 16.0 ka BP (Fig. 3a). Values of ~40e80% were found during early HS1 (~18.2e16.7 ka BP), and during the LGM at ~19e18.5 ka BP and ~23e22 ka BP. The rest of the record was characterized by values lower than 20% (Fig. 3a). Major benthic foraminiferal species found in core MD99-2328 are presented in Fig. 3(cem), and illustrated in Plate 1. Cassidulina carinata accounted for an average of 25% of the total benthic foraminiferal assemblage with maximum abundances (>40%) between ~16.5 and 17 ka BP and minimum values (<10%) around 16.2e16.5 ka and ~22.3 ka (Fig. 3c). For the rest of the glacial record, C. carinata occurred at 20e40% whereas during the Holocene and YD, it accounted for about 20% (Fig. 3c). Elphidium spp. (encompassing the various morphotypes of E. excavatum and E. albiumbilicatum) showed maximum abundances (up to 71%) during late HS1 and minimum values during early HS1 and at ~22.1 ka BP. In the rest of the record, Elphidium spp. accounted for about 20% (Fig. 3d). Highest abundances of Sigmoilopsis schlumbergeri (>20%), which accounts for 7.5% on average of the total assemblage, occurred between 22.5 and 21 ka BP (Fig. 3d). Except for early HS1 when it attained episodically ~20%, S. schlumbergeri occurred with very low abundances in the rest of the record (Fig. 3e). Bolivinids (comprising B. spathulata and B. albatrossi) (4.6% on average) were nearly absent (<2%) until a first occurrence at ~18.6 ka BP (~24%). They reached 40% of the assemblage during early HS1 and disappeared again during late HS1 (Fig. 3f). After 12 ka BP, Bolivina spp. occurred with less than 7% of the total assemblage (Fig. 3f). Cibicides pachyderma (2.5% on average) was present with very low percentages along the record except around 22 ka, 16 ka and some short periods from 19 to 17.5 ka BP when it exceeded 5% (Fig. 3g). Cibicides lobatulus (7.9% on average) was present almost continuously at 5e20% except during late HS1 period when it nearly disappeared (Fig. 3h). Quinqueloculina spp. (comprising Q. seminula and Quinqueloculina sp. 1) occurred at 5.8% on average until 17 ka BP, after which they totally disappeared. Pyrgo spp. (comprising

Fig. 3. a) Percentages of the planktonic species N. pachyderma; b) Diversity index (Shannon H). (cem) Relative abundances (%) of the most representative benthic foraminiferal species (>5% in at least one sample). The main events are indicated in light to dark grey. YD: Younger Dryas; HS1: Heinrich Stadial 1; IRD: Ice Rafted Detritus; LGM: Last Glacial Maximum. Attention has to be paid to the different time scaling before and after 16 ka. R3, R4 and R5 events are in reference to Toucanne et al. (2015).

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P. comata and P. subsphaerica) (2.6% on average) occurred at lower percentages (2.6% on average) until 18 ka BP and totally disappeared after (Fig. 3i). Melonis barleeanus was nearly absent across the glacial period whereas it dominated the fauna (20e30%) during the Holocene (Fig. 3j). The deep infaunal Globobulimina spp. (comprising G. affinis and G. pyrula var. pseudospinescens; Plate 1) was present nearly exclusively during early HS1 (Fig. 3k). Bulimina spp. (comprising B. aculeata and B. elongata) were rare and occurred with >5% only during the LGM (Fig. 3l), whereas Nonionella turgida was nearly absent along the whole record apart around 16.1 ka BP when it reached 20% of the total fauna (Fig. 3m).

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4.3. Stable oxygen and carbon isotopes Stable oxygen isotope values of N. pachyderma (d18ON. pachyderma) showed four main phases throughout the record (Fig. 4a): (1) overall stable values around 3.1‰ during the LGM with two short periods recording heavier values (~3.5‰) at ~22 and 19 ka BP; (2) a shift to values as heavy as ~3.8‰ from ~18.2 to 16.7 ka BP; (3) a general decrease to reach ~3.0‰ from ~16.7 to 16.0 ka BP; and (4) a general increase from ~2.6‰ to ~3.4‰ after 12 ka BP. Stable oxygen isotope values measured on the six benthic foraminiferal species showed large differences in terms of absolute values (Fig. 4b). This feature is known to be partly due to the “vital effect” that prohibits some species from calcifying in a complete equilibrium with seawater (see reviews in Ravelo and Hillaire-Marcel, 1999; Hoogakker et al., 2010). Cibicides lobatulus registered the lightest values (~1.9‰). The other Cibicidids varied from ~3.6‰ in the early record to 2.0e2.8‰ in the Holocene. Uvigerina peregrina varied around 4.3‰ whereas Globobulimina affinis registered the heaviest values of ~4.7‰ (Fig. 4b). Continuous d18O benthic record could only be obtained for C. pachyderma showing three main phases: (1) overall stable values around 3.8‰ during the LGM; (2) an overall progressive decrease from 18.2 to 16.0 ka to reach 2.6‰. Three peculiar peaks of much lighter values stand out from this general trend from 18.2 to 17.2 ka BP; and (3) overall stable values after 12 ka BP at 2.8‰. Stable carbon isotope values of N. pachyderma (d13CN. pachyderma) showed three main phases throughout record (Fig. 4c): (1) overall light values around 0.8‰ during the LGM with high variability. A specific period around 22 ka BP registered a maximum of about 0.2‰; (2) overall stable values at ~ -0.2‰ during HS1; and (3) overall stable values at ~0.0‰ after 12 ka BP. Stable carbon isotope values of the Cibicidids showed a stable trend along the record with values of ~0.4e0.8‰ (Fig. 4d). d13CU. peregrina were overall constant during the LGM with values of ~ 0.2‰. Globobulimina affinis registered the lowest d13C values and showed a significant decrease from ~ 0.7‰ during the LGM to ~ 2.3‰ during early HS1 (Fig. 4d).

4.4. LA-ICPMS elemental ratios

Fig. 4. Oxygen and carbon stable isotopes measured on the planktonic species Negloboquadrina pachyderma (a; c) and the benthic species Cibicides lobatulus, Cibicides wuellestorfi, Cibicides Kullenbergi, Cibicides pachyderma, Uvigerina peregrina, and Globobulimina affinis (b; d). Values of d18O (a; b) and values of d13C N. pachyderma (c) are in inverse order for an easier reading of environmental interpretations. Lines represent a three-point moving average. The main events and climatic phases are reported similarly to Fig. 3.

Mg/Ca ratios measured on Cassidulina carinata showed three main phases (Fig. 5): (1) an overall decreasing trend during the LGM from ~2.5 to 1.2 mmol mol1. Two short-time periods centered around 22.0 and 18.6 ka BP stood out from the general trend and towards lower Mg/Ca values; (2) overall stable low values of ~1.0 mmol mol1 during early HS1; and (3) an overall increasing trend during late HS1 period up to ~ 2.5 mmol mol1 with a peculiar peak at the start. Mn/CaC. carinata showed also three main phases throughout the record (Fig. 5): (1) overall stable low values during the LGM at ~0.03 mmol mol1; (2) this was followed by an increase up to ~0.08 mmol mol1; and (3) lower values of ~0.05 mmol mol1 during late HS1 period. Sr/CaC. carinata ratios were generally stable during the LGM and early HS1 with values close to 1.15 mmol mol1 and decreased significantly during late HS1 period to reach values around 1.06 mmol mol1 (Fig. 5). Since there is no calibration to date of these elemental ratios for Cassidulina carinata, we cannot quantify the different parameters of which they are the proxies. Therefore, the most significant increasing and decreasing trends will be discussed and interpreted qualitatively.

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Fig. 5. LA-ICPMS Mg/Ca, Mn/Ca, and Sr/Ca ratios in mmol mol1 measured on the benthic species Cassidulina carinata. Lines represent a three-point moving average. Error bars correspond to 2s standard deviation (n ¼ 10 on average). The main events and climatic phases are reported similarly to Fig. 3.

5. Discussion 5.1. Impact of Channel River discharges on the benthic foraminiferal community during the last deglaciation in the southern celtic margin Modern benthic foraminiferal faunas from the Celtic margin were first documented by Weston (1985) and studied in details at several stations close to the MD99-2328 site by Duros et al. (2011, 2012). Along our record, C. carinata and Elphidium spp. were highly dominant. We consider C. carinata as being autochthonous because it is a shallow infaunal species that inhabits nowadays the Celtic margin at water depths ranging from ~300 to 1000 m (Duros et al., 2011). As the growth and reproduction of this species can be favoured by input of continental organic matter (e.g., Fontanier et al., 2003; Duchemin et al., 2008; Garcia et al., 2013), its overall continuous presence during the LGM and Termination 1 can be related to organic input from the Channel River. Concurrent high occurrences of this taxon with high C/N ratios further confirms this
not et al., 2006) (Fig. 6i; j). On the other hand, ecological trend (Me Elphidium spp. are typical nearshore shallow-water species (e.g., Horton, 1999; Mojtahid et al., 2016) that are found in our samples with generally etched and broken test walls (Plate 1). In the modern Celtic margin, Duros et al. (2012) found Elphidium specimens almost exclusively in the dead assemblages and concluded that these were allochthonous, and regularly imported to the study area by bottom currents, gravity flows, in suspension and/or attached to floating algae. That said, when this species reaches 70% of the total assemblage during late HS1 (Fig. 3d), it cannot be discarded that a change in the environmental regime (e.g., circulation, productivity) may have enabled Elphidium species to expand their depth range. For instance, Corliss and Emerson (1990) found living specimens of E. excavatum ranging from shallow water to 3000 m depth. Cibicides lobatulus was also dominant downcore whereas it is totally absent from the modern fauna in the Celtic Margin (Duros et al., 2011).

€ nfeld, Although C. lobatulus can be found alive in deep settings (Scho 1997), this species is mainly known for its epifaunal/epiphytic life style, attached to different substrates from coastal shelf environments with dynamic conditions (e.g., Linke and Lutze, 1993; Bolliet et al., 2014). Because of the significant positive correlation between relative abundances of C. lobatulus and Elphidium spp. (except during late HS1 period), we can assume that C. lobatulus was also transported to our study site, most likely attached to floating algae (the Brenot Spur being sheltered from gravity flow mechanisms). This hypothesis is also corroborated with the much lighter d18O values (Fig. 4b) that suggest that C. lobatulus had calcified at higher temperatures (or lower salinities), in shallower environments. The high proportion of transported species during the LGM compared to the top core sediments is likely a result of the increased proximity between the Channel River's mouth and the continental shelf during sea-level lowstands (Toucanne et al., 2012, Fig. 1a). Other species were intermittently present downcore with high percentages. For instance, Bolivina spp., which dominated the fauna mainly during early HS1 (Fig. 3f), are known for their opportunistic behaviour and their ecological preference for phytodetritusenriched sediments (Schmiedl et al., 1997; Duros et al., 2011). The periodic presence of the Bolivinids might indicate episodic input of fresh organic matter. Cibicides pachyderma and Sigmoilopis schlumbergeri were positively correlated along the record suggesting similar ecological requirements. Cibicides pachyderma is usually found in meso-oligotrophic open-slope environments, including the Bay of Biscay and the Celtic margin, with good oxygenation (Schmiedl et al., 2000; Mojtahid et al., 2010; Duros et al., 2012). In the Bay of Biscay, S. schlumbergeri is found alive around 800e1000 m water depth together with C. pachyderma (kullenbergi) (Fontanier et al., 2006; Mojtahid et al., 2010). We assume that high percentages of both species in our record indicate well ventilated bottom waters and meso-oligotrophic settings. The miliolids are generally rare or absent in oxygen-deficient environments because they are more sensitive to oxygen stress than other species (Nolet and Corliss, 1990; Murray, 2006). Amongst the dominant miliolids, Pyrgo spp. totally disappeared during the whole HS1 (Fig. 3i), probably indicating low oxygenation whereas Quinqueloculina spp. only disappeared during late HS1 (Fig. 6). The presence of the deep infaunal species Globobulimina spp. during early HS1 (Fig. 3k) and in relatively high proportions (up to 20%), is indicative of organically-enriched and/or oxygen-depleted benthic environments (e.g., Jorissen, 1999; Schmiedl et al., 2003; Mojtahid et al., 2010), which is a priori not a favorable environment for Quinqueloculina spp. In the Celtic margin, Q. seminula lives at >1000 m deep stations characterized by well ventilated water masses (Duros et al., 2011). However, it was also described as an early colonizer after sedimentary disturbance such as ash layer deposits (Hess and Kuhnt, 1996) or episodic gravity flow (Duros et al., 2012) like the hyperpycnal flow leading to the deposition of laminated Facies 3 during early HS1 (Zaragosi et al., 2001a; Mojtahid et al., 2005; Eynaud et al., 2007; Toucanne et al., 2009a). 5.2. The climatic periods recorded in the Celtic margin from a benthic foraminiferal standpoint 5.2.1. The last glacial maximum (23e18.2 ka BP) The definition of the global term “LGM” used here is that recommended by the EPILOG project at ‘Chronozone Level 2’. Namely the LGM corresponds to the period between ~23 and 18 cal ka (Mix et al., 2001), i.e., to the end of HS2 and the start of HS1. During this period, long considered as stable and cold, our data from the Celtic margin show a rather complex hydrological architecture corroborating the earlier work of Peck et al. (2006, 2007) north of our study area.

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5.2.1.1. General features. During the LGM, surface and deep water temperatures in the Celtic Margin were generally mild. This is shown in our data by the low percentages of the polar planktonic taxon N. pachyderma, and the relatively high to moderate Mg/ CaC. carinata values (temperature proxy) (Fig. 6a; e). Although there is no Mg/Ca-temperature calibration specifically for C. carinata, there is good evidence that Mg/Ca incorporated in Cassidulina genus, including the very close morphologically Cassidulina neoteretis, is
ttir et al., mainly temperature-dependant (Izuka, 1988; Kristj
ansdo 2007) similarly to many benthic species (e.g., Rosenthal et al., 1997; Lear et al., 2000; Toyofuku et al., 2000, 2011; Elderfield et al., 2006). The relatively light values of d18ON. pachyderma (Fig. 6b) indicate either warm or low saline surface waters. The comparison with sea surface salinities reconstructed from the nearby core MD95-2002 (Eynaud et al., 2012) (Fig. 6c), point out to a major control of temperature, in coherency with the high percentages of N. pachyderma (Fig. 6a). The d18ON. pachyderma record is furthermore consistent with that published by Peck et al. (2006) in core MD01-2461 off Ireland, indicating a larger scale forcing (Fig. 6b). In the subtropical northeast Atlantic (off Portugal), alkenone unsaturation ratios-SSTs described a rather mild LGM (SSTs ~13
C; Bard et al., 2000). Zaragosi et al. (2001a) and Mojtahid et al. (2005) hypothesized episodic intensifications of the Northern Atlantic Current to explain the several warm events found during the LGM in the Bay of Biscay. On the other hand, the fairly coarse “sortable silt” mean grain size (SS) (Fig. 6k) indicates rather active near-bottom current flow (McCave and Hall, 2006). Because contouritic Facies 1 is dominant during the LGM (Fig. 2d), SS are more likely representing changes in the mean flow of the slope current rather than other near bottom flow controls (e.g., storms, eddies, internal waves) (McCave et al., 1995). Furthermore, our assemblage-based oxygen index (Fig. 6f) suggests well ventilated water masses (i.e., GNAIW), consistently with low Mn/CaC. carinata ratio that can be used as an oxygen proxy. Indeed, Mn2þ is remobilized from the solid phase to dissolved Mn2þ in oxygen depleted bottom waters and in pore waters due to the oxidation of organic matter with depth in the sediment (e.g., Froelich et al., 1979). Thus, high Mn/Ca ratios are indicative of hypoxic conditions whereas low Mn/Ca ratios would indicate well oxygenated conditions (e.g., Ní Fhlaithearta et al., 2010; Groeneveld and Filipsson, 2013). Hence, by suggesting well oxygenated intermediate water masses of mild temperatures, our data support the thesis of active northward heat transport during the LGM resulting from wind intensification and the subsequent active AMOC (Fig. 6h) (Hewitt et al., 2001; McManus et al., 2004; Lynch-Stieglitz et al., 2007; Gherardi et al., 2009).

5.2.1.2. Specific features. The LGM-period after ~21 ka BP was clearly marked by an increase in riverine sediment supply as shown by the less bioturbated sedimentary Facies 2, and increasing

Fig. 6. Summary figure of the main results from this study and comparison to intervals/episodes defined from other proxies and by other authors at the regional to global scale. a) Percentages of Neogloboquadrina pachyderma in inverse order; b) d18O of Neogloboquadrina pachyderma in inverse order (3-pt moving average) from this

study (in black) and from the study of Peck et al. (2006) in core MD01-2461 (in yellow); c) Sea surface salinities derived from dinocysts in core MD95-2002 in inverse order (Eynaud et al., 2012); d) d18O of Cibicides pachyderma (3-pt moving average) and the percentages of Bolivina spp.; e) Mg/Ca measured on Cassidulina carinata (3-pt moving average); f) Assemblage-based oxygen proxy; g) Mn/Ca measured on Cassidulina carinata in inverse order (the black line: 3-pt moving average); h) Proxy for AMOC strength: 231Pa/230Th ratio at sites OCE326-GGC5 (full line; McManus et al., 2004) and site DAPC2 (Dashed line; Hall et al., 2006; Gherardi et al., 2009); i) Percentages of Cassidulina carinata and Bolivina spp.; j) Total organic carbon-to-nitrogen
not et al., 2006) and BIT index at site MD95ratios (C/N) at site MD95-2002 (Me
not et al., 2006), OM: organic matter; k) Sedimentary D50 and the mean 2002 (Me sortable silts (this study); Coarse Lithic Grains (CLGs) including Ice Rafted Detritus (IRDs) concentrations at site MD95-2002 (Eynaud et al., 2007). The main events and climatic phases are reported similarly to Fig. 3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Plate 1. SEM images of the dominant benthic species: 1. Cassidulina carinata; 2. Elphidium excavatum; 3. Elphidium albiumbilicatum; 4. Elphidium excavatum (morphotype 2); 5. Elphidium excavatum (morphotype 3); 6. Sigmoilopsis schlumbergeri; 7. Bolivina spathulata; 8. Bolivina spathulata f. alata; 9. Bolivina albatrossi; 10. Cibicides pachyderma; 11. Cibicides lobatulus; 12. Quinqueloculina seminula; 13. Quinqueloculina sp. 1; 14. Pyrgo subsphaerica; 15. Melonis barleeanus; 16. Globobulimina affinis; 17. Globobulimina pyrula var. pseudospinescens; 18. Nonionella turgida; 19. Bulimina marginata. Scale bar ¼ 100 mm.

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proportions of C. carinata (Fig. 6i) at site MD99-2328, and high BITindex and C/N ratio at site MD95-2002 indicating an early and
not et al., 2006) (Fig. 6j). drastic reactivation of European rivers (Me The period between ~21 and 18 ka BP was characterized by two episodes of substantial EIS recession (R3 (~22.5e21.3 ka) and R4 (~20.3e18.7 ka); Toucanne et al., 2015) coinciding with an early warming in the northern hemisphere, well recorded in marine and
nchez-Gon ~ i et al., 2000; continental sequences (Bard et al., 2000; Sa Combourieu Nebout et al., 2002). These EIS recession events and the simultaneous release of meltwaters to the North Atlantic coincided with an increase in the proportions and d18O of N. pachyderma at site MD99-2328, indicating a cooling of surface waters (Fig. 6a; b). This cooling was also perceptible in bottom waters Mg/Ca signature (Fig. 6e). This shows that the release of meltwaters during EIS recession events occurring in the course of its LGM position impacted surface and intermediate water masses in the Celtic margin. 5.2.2. Heinrich Stadial 1 (HS1: 18.2e14.7 ka BP) 5.2.2.1. Early HS1 (18.2e16.7 ka BP). At site MD99-2328, early HS1 was marked by a 575 cm-thick fine-grained laminated sediment deposit with few IRDs. This sediment layer is a result of the last paroxysmal phase of EIS deglaciation; the laminae are strongly supposed to result from seasonal EIS melting dynamics (Zaragosi et al., 2001a; Mojtahid et al., 2005; Eynaud et al., 2007; Toucanne et al., 2009a). Toucanne et al. (2015) defined this event as R5 meltwater-event (sourced from south SIS), which preceded LIS iceberg outbursts (late HS1) by some 1500 years (Fig. 6). During this time-period, our data show cold surface and deep water temperatures (Fig. 6a; b; e), simultaneously to massive dis
not et al., 2006; Eynaud charges of EIS meltwaters (Fig. 6c; j) (Me et al., 2012). Until now, most studies gave evidence of a clear impact of EIS meltwaters on surface water temperatures and salinities in the study area (Zaragosi et al., 2001a; Mojtahid et al., 2005; Eynaud et al., 2007), but none evidenced such an impact on intermediate water masses (i.e., low Mg/Ca recording low temperatures) (Fig. 6e). Furthermore, it is interesting to note the deviation of benthic d18OC. pachyderma signature (up to 2‰ difference) from its usual glacial-interglacial transition pattern towards much lighter values (Fig. 6d). Because bottom water temperatures remained constant during this event (Mg/Ca ~1.07 mmol/mol; Fig. 6e), this isotopic lightening can only be due to an input of meltwaters that freshened significantly (three major d18O peaks) bottom waters (by ~3e4 units psu). The fact that this deviation was not recorded in the d18O of the deep infaunal Globobulimina spp (Fig. 4b) can be explained by the seasonality in EIS melting. Cibicides pachyderma and Globobulimina spp. have nowadays rather contrasting ecological preferences in the Bay of Biscay (Mojtahid et al., 2010). Cibicides pachyderma might have profited from the mesotrophic conditions of early spring melting whereas Globobulimina spp. would have benefited from late spring and summer eutrophic conditions. Thus, Cibicides pachyderma may have recorded the cascading of cold and turbid meltwaters (hyperpycnal flow). This deep hyperpycnal flow could also explain why we record the influence of freshwater only in deep d18O signature. Such a phenomenon was observed in the Gulf of Mexico during deglacial Mississippi freshwater discharges (Aharon, 2006). In the meantime, assemblage-based oxygen index, Mn/ CaC. carinata (Fig. 6f and g), and the overall low biodiversity (Fig. 3b) indicate oxygen depleted bottom waters. The massive Channel River discharges (Fig. 6j) were synchronous with strong decreases in both the rate of deep-water formation and the strength of the AMOC (McManus et al., 2004, Fig. 6h). Via modeling, Roche et al. (2010) and Eldevik et al. (2014) emphasized the non-negligible role of EIS meltwater discharges in the weakening of deep ocean

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convection. Concomitantly, riverine input enhanced the trophic state of benthic environment as shown by the predominance of opportunistic species (C. carinata, and Bolivina spp.; Fig. 6i). The frequency peaks of Bolivina spp. during this time period seemed to be simultaneous with benthic d18O “anomaly” (Fig. 6d). It is therefore possible that this species had an opportunistic response to the introduction of organic matter via the hyperpycnal meltwater flow. Surprisingly however, the increasing difference during early HS1 between the d13C signature of the deep infaunal species G. affinis and the shallow infaunal species C. pachyderma (Dd13CG. affinis-C. pachyderma; Fig. 4d) seems to contradict all other proxies. According to McCorkle and Emerson (1988) and more recently Hoogakker et al. (2015), high Dd13Cinfaunal-epifaunal is indicative of deeper oxygen penetration into the sediment and therefore well oxygenated bottom waters. Although this is true in deep ocean settings, in coastal settings, this relation might be hampered by local parameters such as the seasonality characterizing our study area during early HS1. Assuming that both species did not grow during the same season, the negative d13CGlobobulimina excursion could be explained by depleted d13CDIC of porewaters due to high respiration rates of organic matter during eutrophic periods (late spring and summer). This phenomenon was probably further accentuated by the input of light d13C terrestrial carbon. 5.2.2.2. Late HS1 (16.7 e 14.7 ka BP). During late HS1 (i.e. Heinrich event 1 sensu stricto), conditions on the Celtic margin's seafloor have changed drastically. Although our age model includes a reservoir age of 970 years during HS1, this value may have varied between as little as 800 years and up to 2500 years (Waelbroeck et al., 2001; Thornalley et al., 2011; Stern and Lisiecki, 2013), so the timing of events within HS1 is uncertain especially when comparing our data to records other than MD95-2002. In our record, the change from previously laminated sediments (Facies 3) to fine-grained sediments with no laminae (Facies 4) and the decrease in sedimentation rates (Fig. 2b) prove that the activity of the Channel River decreased strongly (Zaragosi et al., 2001a; Eynaud et al., 2007, 2012; Toucanne et al., 2010). This is further confirmed by the decrease of BIT-index (Fig. 6j) and the change in meltwater source at site MD95-2002 (Toucanne et al., 2015). The high abundance of IRDs (also at site MD95-2002; Fig. 6k) shows that icerafting was the main sedimentary mechanism with icebergs originating mainly from the LIS (Grousset et al., 1993). The observed progressive increase in grain size and SS during this time-period (not only due to the presence of IRDs) may indicate a strengthening of bottom water flow (i.e., slope current) (Fig. 6k). Increased bottom current speeds after HS1 have previously been inferred from records of sortable silt obtained north of our study area from the Feni Drift (Praetorius et al., 2008) and south of the Rockall Bank (McCave et al., 1995). This may further suggest that the sedimentary hiatus from 16 to 12.4 ka BP is mainly the result of erosion from this slope current. However, high SS values could also result from the drastic decrease in the input of fine material from the Channel River. The high percentages of N. pachyderma indicated a southern shift of the polar front resulting in very cold surface waters (Fig. 6a) while decreasing surface water d18O values were probably linked with freshwater releases from the melting of LIS icebergs (Fig. 6b). In a general context of a significant slowdown of the AMOC (Fig. 6h) (e.g., McManus et al., 2004), Gherardi et al. (2009) emphasized that during HS1, a persistent but slower overturning circulation was present down to 2000 m depth in the North Atlantic. Therefore, our study area was perhaps bathed by these intermediate waters from northern origin which were cold and poorly ventilated (Thornalley et al., 2015). Our deep-sea oxygen and temperature proxies seem to argue against this hypothesis: increasing benthic Mg/Ca indicating

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a progressive warming of intermediate water masses while oxygen proxies indicating well ventilated bottom waters. Hence, and with a lot of caution regarding the limitations of our proxies, our data seem to describe relatively warm, well ventilated and active slope current during late HS1. Taking into account the lack of literature tackling the distribution of water masses in the study area during late HS1, and based on the modern hydrology of the Bay of Biscay and the North Atlantic, the two most plausible origins of such warm and well ventilated current would be: i) the warm and saline MOW reaching high latitudes. Indeed, there is evidence of intensified € nfeld and Zahn, 2000; Herna
ndezMOW during glacials (Scho Molina et al., 2006; Voelker et al., 2006). However, it is unlikely that it reached our latitudes due to the low outflow volume at Gibraltar Strait (Rohling and Bryden, 1994; Zahn et al., 1997; Rogerson et al., 2012); ii) the influence of Antarctic waters at ~900 m depth in the Bay of Biscay. This assumption is still debated. Rickaby and Elderfield (2005) and Thornalley et al. (2011) argued in favor of the presence of water masses of southern origin ventilating the Atlantic as far north as the South Iceland Rise during abrupt cooling events of the last Termination. The radiocarbon evidence of Thornalley et al. (2011) has since been attributed to the Nordic Seas overflows (Thornalley et al., 2015). Crocker et al. (2016) argue that the mid-depth northeast Atlantic was never dominantly ventilated by southern-sourced waters, although they specified that increased contribution of radiogenic southern-sourced waters mixing with northern-sourced waters cannot be completely ruled out. In our record and although foraminiferal Sr/Ca proxy is still very poorly investigated, the significantly low Sr/CaC. carinata values during HS1 (Fig. 5), which plot within the published ranges of Sr/Ca shell abundances (Yu et al., 2014; Allen et al., 2016), might indicate a change in water mass chemistry. The work of Yu et al. (2014) evidenced a significant control (positive correlation) of deep water D 2 2 [CO2 3 ] ([CO3 ]-[CO3 ]sat) on benthic foraminiferal Sr/Ca during the last glacial-interglacial cycle. Since northern and southern water masses can clearly be differentiated by their seawater [CO2 3 ] (Key et al., 2004; Yu et al., 2008), it is therefore possible that the low Sr/Ca indicates a probable contribution of southern-origin waters. Of course, this is a speculative hypothesis that requires more robust data. Further investigations in the area are needed in order to confirm the potential of benthic Sr/Ca in tracing water sources during glacial times in addition to other novel proxies (e.g., ƐNd on foraminifera; Tachikawa et al., 2014). The low numbers of autochthonous benthic faunas with a dominance of C. pachyderma (Fig. 3) indicated oligotrophic conditions on the seafloor, consistently with the ceasing of the Channel River discharges (Fig. 6j). Low biological productivity has already been observed for Heinrich events in the study area and in the
not et al., 2006). The Iberian margin (Pailler and Bard, 2002; Me peculiar pattern during this period is the large dominance of the allochthonous Elphidium spp. (up to ~70% of the total assemblage). Also, this is the only time where C. lobatulus and Elphidium spp. were not correlated suggesting that Elphidium specimens found during late HS1 originated from a different source. It has been reported that benthic foraminifera can be incorporated in icebergs (e.g., LIS icebergs) as a result of ice grounding on the seafloor or the incorporation of suspended sediment on the base of an iceberg by freezing (Dieckmann et al., 1987). Additionally, Elphidium spp., unlike C. lobatulus, are abundant in intertidal areas of the nearby French and English margins that are subject to freezing under dry and cold climate and undergo transport of material by ice. 6. Conclusions Over the past ~23 ka, EIS meltwater discharges directly affected surface and deep water characteristics in the Celtic margin, hence

controlling the development of benthic foraminiferal fauna. During the LGM, and before ~21 ka BP, our data indicated mesooligotrophic conditions and mild and well oxygenated bottomwaters in relation to an active slope current. Increasing proportions of C. carinata after ~21 ka BP marked an increase in the Channel River’s sediment supply. During early HS1, benthic proxies indicated a general eutrophication and a severe bottom water dysoxia due to intensified meltwaters discharges marking the paroxysmal phase of EIS deglaciation. The significant episodic isotopic lightening of benthic d18O most likely recorded the cascading of turbid EIS meltwaters. The latter resulted in cold, episodically low saline and dysoxic intermediate water masses. During late HS1, conditions on the Celtic margin's seafloor changed drastically and foraminiferal composition indicated oligotrophic conditions due to the ceasing of Channel River discharges. Foraminiferal assemblage was nearly monospecific with the predominance of the allochthonous Elphidium spp. which were probably brought to the study area by LIS icebergs and/or near-shore ice. Meanwhile, increasing benthic Mg/Ca ratio indicated a progressive warming of intermediate water masses. In the general context of a near shutdown of the AMOC and the consequent limited formation of the GNAIW, our data could describe an intensified slope current. Therefore, in addition to the well-known effect of EIS meltwaters on surface waters in the Celtic margin, our benthic record evidenced a pronounced impact on intermediate water depths as well. This may further corroborate the role of EIS meltwaters in disturbing the hydrology of the northeast Atlantic across the last deglaciation. Acknowledgments We would like to thank the editor and the two anonymous reviewers for their pertinent comments that helped to improve considerably the manuscript. A 5 month MSc fellowship was funded for Robin Fentimen by the “Region Pays de Loire” via the “Jeune Equipe” project (piloted by Mary Elliot; LPG-Nantes, France). We are thankful to “Labex MER” (Brest, France) for funding 14C dating and isotope measurements. We would like to thank the SCIAM platform (University of Angers) for SEM images. We are thankful to Sebastien Zaragosi and Joel St Paul (EPOC, Bordeaux, France) for providing us
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