Imprint of seasonality changes on fluvio-glacial dynamics across Heinrich Stadial 1 (NE Atlantic Ocean)
Introduction
The last glacial period was accompanied by millennial-scale abrupt climate shifts, portrayed in Greenland ice-cores as rapid transitions from cold atmospheric phases termed Greenland Stadials (GS) to warm atmospheric phases referred to as Greenland Interstadials (GI; e.g., Dansgaard et al., 1993; Rasmussen et al., 2014). Despite their original designation, these climate excursions had an impact across the globe (e.g., Voelker, 2002). In the North Atlantic Ocean, some GS were associated with massive iceberg surges mainly from the Laurentide Ice Sheet (LIS) via the Hudson Strait Ice Stream (e.g., Bond et al., 1992, Bond et al., 1993; Broecker et al., 1992; Broecker, 1994; Hemming, 2004), identified in marine sediments as Ice Rafted Debris (IRD)-enriched layers (e.g., Bond et al., 1993; Broecker, 1994; Heinrich, 1988). These massive iceberg(and thus freshwater) surge events are known as Heinrich Events (HEs including HE1), with their corresponding stadial phases called Heinrich Stadials (HSs including HS1; Barker et al., 2009; Harrison and Sanchez Goñi, 2010). The associated huge freshwater releases resulted in large reductions of the Atlantic Meridional Overturning Circulation (AMOC; e.g., McManus et al., 2004; Stanford et al., 2006, Stanford et al., 2011; Ng et al., 2018; Toucanne et al., 2021). Numerous studies demonstrated that Greenland Iceland and European Ice Sheets were also major contributors to the oceanic disturbances in the North Atlantic Ocean, especially when considering the surge sequencing along time (e.g., Bond et al., 1997, Bond et al., 1999; Grousset et al., 2000, Grousset et al., 2001; Hemming et al., 2000; Hemming, 2004; Knutz et al., 2001, Knutz et al., 2007; Hall et al., 2006; Peck et al., 2006; Nygård et al., 2007; Toucanne et al., 2008; Toucanne et al., 2010; Toucanne et al., 2015).
HS1, including the HE1 layer, occurred at the onset of the last deglaciation (~19–11 ka BP; Clark et al., 2012a), just before the abrupt Bølling-Allerød (B/A) warming event starting at ca. 14.7 ka BP (Rasmussen et al., 2014) and after the Last Glacial Maximum (LGM; Mix et al., 2001). Over the LGM, which was characterized by a large European Ice Sheet (EIS) including the British-Irish (BIIS) and Scandinavian (SIS) Ice Sheets, the ‘Fleuve Manche’ paleoriver (Channel River) was one of the largest river systems that drained western Europe (e.g., Gibbard, 1988; Toucanne et al., 2009, Toucanne et al., 2010, Toucanne et al., 2015). This huge fluvial system included the French, Belgian and British rivers, and the merged German, Polish and Dutch rivers, on the exposed English Channel and North Sea Basin, respectively. Multiproxy studies conducted along the northwestern European margin and especially from the northern Bay of Biscay (e.g., Zaragosi et al., 2001; Auffret et al., 2002; Mojtahid et al., 2005, Mojtahid et al., 2017; Eynaud et al., 2007, Eynaud et al., 2012; Penaud et al., 2009; Toucanne et al., 2009, Toucanne et al., 2010), identified recurrent phases of meltwater inputs at the onset of HS1 (between 18.3 and 17 ka BP). Materialized in sediments as millimeter- to centimeter-scale laminations, they were attributed to the seasonal melting of the EIS and seasonal subsequent freshwater discharge from the ‘Fleuve Manche’ paleoriver. Further works provided new evidence of a differential contribution from ice sheets to the laminated deposit, with a particularly large SIS/Baltic sourced part during the last deglaciation and the HS1 interval (Toucanne et al., 2015). Until now, palynological investigation of this laminated facies (Eynaud, 1999; Eynaud et al., 2007; Eynaud et al., 2012; Auffret et al., 2000; Zaragosi et al., 2001; Penaud et al., 2009) was performed at resolution varying between 70 and 250 years only, due to the strong dilution of palynomorphs in sediments. Such laminated facies, corresponding to exceptionally high sedimentation rates, appear as ideal candidates to increase the temporal resolution of marine records and thus improve our understanding of short-lived fluctuations in the regime of the ‘Fleuve Manche’ paleoriver and associated EIS dynamics.
Our study thus constitutes the first detailed dinocyst study encompassing the HS1 interval in the Bay of Biscay with a special focus on the laminated facies deposited at the onset of HS1. Our main objective was to decipher the set and sequence of events that occurred during this period over the northern Bay of Biscay. Our high-resolution palynological study was conducted on core MD13–3438 and combined with micropaleontological, geochemical and sedimentological analyses available for the twin reference core MD95–2002. Our multiproxy approach led to:
- (1)
the reconstruction of the coupled EIS and ‘Fleuve Manche’ paleoriver dynamics across HS1;
- (2)
the study of high frequency seasonal variability within the laminated deposit, providing the first reconstruction of the sub-centennial climate variability across HS1 in the NE Atlantic;
- (3)
and the characterization of sea surface conditions over the northern Bay of Biscay using dinocyst quantifications (keeping in mind their potentialities and limits in the study area).
Section snippets
Location of the studied core
The Calypso long piston core MD13–3438 (47°27′ N; 8°27′ W; 2180 m water depth; 36 m long) and twin core MD95–2002 (47°27′ N; 8°32′ W; 2174 m water depth) were respectively collected during the VT 133/MERIADZEK (Woerther, 2013) and MD101-IMAGES (Bassinot and Labeyrie, 1996) oceanographic cruises on board the R/V Marion Dufresne (Table 1).
These marine sedimentary archives were retrieved from the Meriadzek Terrace, northern Bay of Biscay, directly off the mouth of the ‘Fleuve Manche’ paleoriver (
Stratigraphy of core MD13–3438
The age model of core MD13–3438 is wedged on the last updated chronostratigraphy of core MD95–2002 (Toucanne et al., 2015). This latter was built with the Clam software (Blaauw, 2010) by integrating 22 AMS-14C dates over the last 40 kyr with additional tie-points: (i) 4 AMS-14C dates tied from the neighbouring cores MD03–2690 and MD03–2692, and (ii) N. pachyderma abundances correlated with the NGRIP δ18O signal. Modern reservoir age correction is estimated to about 352 ± 92 years. Prior to HS1
Palynological results
Based on a cluster analysis run on dinocyst taxa percentages with the Psimpoll program, 9 palynozones were identified in core MD13–3438. We labelled them according to the stratigraphic interval they match: LGM (595–578 cm), HS1-a as the onset of HS1 prior to HE1 – HS1-a being subdivided into 5 sub-palynozones termed HS1-a1 to HS1-a5 (578–330 cm), and HS1-b (330–270 cm) and HS1-c (270–240 cm) as the first and second phase of HE1, respectively. Palynological data are presented and discussed
The last Deglaciation on the northern Bay of Biscay
Our compilation of MD13–3438 multiproxy signals indicative of fluvial discharge, allows us to describe for the first time the consequence of those hydrographic events at sub-centennial scale over the period covering the LGM to the B/A in the northern Bay of Biscay. We superimpose to our new and high-resolution results some of those previously published and established at lower temporal resolution (but longer time-scale) from twin core MD95–2002, (Fig. 5, Fig. 6; Zaragosi et al., 2001), and
Conclusion
The high-resolution palynological investigation of the last deglaciation in core MD13–3438 (northern Bay of Biscay) highlights significant climatic and paleoenvironnmental changes related to both the proximal European Ice Sheets (EIS) and the ‘Fleuve Manche’ paleoriver dynamics. Dinocyst-based quantitative reconstructions provide an evaluation of past hydrographical changes. Seven short-scale sub-phases within the HS1 interval were identified for the first time on the northern Bay of Biscay:
Declaration of Competing Interest
None.
Acknowledgments
This work was supported by the French projects: ANR IDEGLACE, INSU RISCC, INSU ICE-BIO-RAM and by the European Research Council ERC grant ACCLIMATE/n° 339,108. This work results from regional, national and international collaborations, between LGO laboratory (Brest University,), Ifremer (GM), EPOC laboratory (Bordeaux University), LSCE and the Universitat Autònoma of Barcelona. We received funding from the CG29 (Conseil Général du Finistère, 29) and financial support from LGO and EPOC
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