Millennial-scale fluctuations of the European Ice Sheet at the end of the last glacial, and their potential impact on global climate
Introduction
A central question of climate sciences is the understanding of the causes of the Pleistocene ice ages, and of the rapid collapse of ice-sheets (i.e., ‘terminations’; see Paillard, 2015 for a thorough review). The emergent ‘termination paradigm’ posits that the necessary condition to drive the earth out of ice ages is the occurrence of a single (Terminations II and IV) or series (Terminations I and III) of multi-millennial climatic oscillations involving variations in the strength of Atlantic Meridional Oceanic Circulation (AMOC) (Barker et al., 2011, Broecker et al., 2010, Cheng et al., 2009, Denton et al., 2010, Ruddiman et al., 1980). These long-lived AMOC slowdowns would have led to prolonged stadial conditions in the Northern Hemisphere (NH), Southern Hemisphere (SH) warming, and CO2 degassing from the Southern Ocean, in turn amplifying global deglacial warming (Barker et al., 2009, Cheng et al., 2009, Denton et al., 2010, Shakun et al., 2012). Thus, the ‘termination paradigm’ implies that the primary condition required to trigger a termination is not solely the magnitude of the boreal insolation change but also a sufficient volume of freshwater released into the North Atlantic that can perennially weaken the AMOC. The only available reservoir for such large volumes of freshwater was the extensive and isostatically-depressed Laurentide (LIS) and European (EIS) ice-sheets that achieved maxima on both sides of the North Atlantic at the end of each ice age. However, the potential sources of such prolonged events of freshwater release and any associated AMOC reduction are still uncertain due to the difficulties to connect continental ice-sheet fluctuations and associated meltwater releases to paleoclimatic and paleoceanographic records (e.g., Broecker, 2006). In the specific case of the last Termination (∼19–10 ka; Clark et al., 2012c), the first prolonged event of AMOC reduction is thought to have occurred between ∼18 and 15 ka (Hall et al., 2006, McManus et al., 2004), corresponding to Heinrich Stadial 1 (HS1). Evidence from the southern LIS margin and the western North Atlantic suggest that the LIS could have provided substantial freshwater during this interval (Clark et al., 2004a, Clark et al., 2007, Clark et al., 2001), as well as during the subsequent prolonged AMOC slowdown that occurred during the Younger Dryas cold event (Broecker et al., 1988, Carlson et al., 2007, Clark et al., 2004a, Clark et al., 2007, Clark et al., 2001). The meltwater contribution of the EIS remains largely unknown in comparison. Substantial hydrographic changes have been reported along the European margin at times of AMOC perturbations including HS1, thus pointing out the possible participation of the EIS to these events (Eynaud et al., 2012, Hall et al., 2011, Hall et al., 2006, Knutz et al., 2007, Lekens et al., 2006, McCabe and Clark, 1998, Peck et al., 2006, Peck et al., 2007, Scourse et al., 2000). However, our understanding remains incomplete since the correlation of EIS fluctuations with these paleoceanographic changes and with well-dated proxy records for AMOC variability only relate to the marine ice-streams and ice-shelves draining into the North Atlantic (e.g., Peck et al., 2006). In contrast, the correlation with the evolution of the major terrestrial ice-streams in the southern EIS (e.g., southern Baltic ice stream complex), known to be very active due to melting bed conditions (Boulton et al., 2001, Boulton et al., 1985), is poorly documented (Lehman et al., 1991, Rinterknecht et al., 2006). In addition, hosing experiments demonstrate that the sensitivity of ocean circulation depends on the location of the freshwater perturbation and that the climate system is very sensitive to freshwater perturbations originating from the European margin (Roche et al., 2010). Finally, just as the LIS, the EIS had reached its maximum extent during the Last Glacial Maximum (LGM, ∼26–19 ka; Clark et al., 2009), making it a potential source of freshwater at the end of the last ice age. This leads to the possibility that the EIS might have played a significant role in the first steps of the last termination.
The EIS, composed of the British-Irish (BIIS) and the Scandinavian (SIS) ice sheets, formed the second largest NH ice mass (Fig. 1). The two regional ice-sheets merged during the last glacial (Bradwell et al., 2008, Carr et al., 2000, Sejrup et al., 2009), covering the North Sea area and leading to the formation of a large river system that drained the western European continent (Gibbard, 1988, Toucanne et al., 2009b, Toucanne et al., 2010). During glacial times, the so-called Channel River routed substantial amounts of meltwater to the North Atlantic (Eynaud et al., 2007, Ménot et al., 2006, Roche et al., 2010, Toucanne et al., 2010, Zaragosi et al., 2001). To explore the potential role of the EIS during the last termination, we investigate the link between the EIS ice-margin fluctuations, Channel River meltwater discharge, and AMOC rate. Our results provides direct evidence that the EIS played a crucial role in the abrupt reorganizations of the global climate system that accompanied the end of the last glacial period.
Section snippets
Material and methods
We focus on core MD95-2002, a sedimentary archive recovered directly off the mouth of the Channel River (Meriadzek Terrace; 2174 m water depth; 47′27°N, 8′32°W) (Fig. 1). Previous studies have shown that core MD95-2002 is suitable for reconstructing the deglacial pulses of meltwater emanating from the EIS (Eynaud et al., 2012, Eynaud et al., 2007, Ménot et al., 2006, Toucanne et al., 2009a, Toucanne et al., 2010, Zaragosi et al., 2001). To decipher the coupling between EIS ice-margin
MD95-2002 core chronology
The chronostratigraphic framework of core MD95-2002 is based on 22 monospecific 14C ages [performed on Globigerina bulloides and Neogloboquadrina pachyderma (left coiling) from the >150 μm fraction] and 4 additional 14C ages from nearby cores MD03-2692 (Trevelyan Escarpment; Eynaud et al., 2007, Mojtahid et al., 2005) and MD03-2690 (Armorican turbidite system; Toucanne et al., 2008) (Fig. 2, Fig. 3, Fig. 4; Table 1). The cores were synchronized by means of their XRF-Ti/Ca records with an
X-ray fluorescence
The XRF Ti/Ca and Fe/Ca ratios in core MD95-2002 show a good correlation (r = 0.975; p < 0.01) all along the sedimentary sequence (Fig. 4, Fig. 5), indicating that the Fe and Ti elements fluctuate together from a common source. Higher (lower) values of both ratios are observed during stadials (interstadials), when sedimentation rates in core MD95-2002 (Fig. 3) and turbidite flux in the deep Bay of Biscay (Toucanne et al., 2012, Toucanne et al., 2008) reach high (low) levels (Fig. 4, Fig. 5).
EIS fluctuations and the MD95-2002 εNd record
The εNd detrital value (−11.1) determined for core MD95-2002 surficial sediments (dated at ca. 1.6 ka) is very similar to those measured in sediment layers from the Mid and Early Holocene (about εNd −11.6) and from the core-top of a nearby sedimentary record (εNd −10.9; KECP-11, see Freslon et al., 2014). Taken together, these data indicate that a εNd value of about −11.2 ± 0.4 (1SD) at site MD95-2002 is representative for the present interglacial, and can be used as an estimate for the Nd
Conclusion
Our Nd isotope record of Channel River meltwater discharges to the North Atlantic provides, after comparison with continental morphostratigraphical evidences and associated glacigenic samples, a continuous and well-dated record for the evolution of the EIS southern margin through the end of the last glacial period and during the deglaciation. Importantly, our results show that the pattern of ice-margin retreat for the SIS is similar in timing to that of the southern LIS margin, with moderate
Acknowledgements
This work was sponsored by the French National Research Agency (ANR) via the ECO-MIST project (#2010-JCJC-609-01). The authors warmly acknowledge those who kindly supplied the continental sediment samples (P. Antoine, F. Buscheers, S. Carr, S. Clerc, C. Dubrulle-Bruneau, J. Ehlers, A. Elverhoi, D.J.A. Evans, M. Frueggard, P.L. Gibbard, A. Groengroeft, H. Hünecke, G. Kowaleska, D. Krzyszkowski, N.K. Larsen, T. Leipe, C. Lüghtens, J.P. Lunkka, C. Mellett, F. Nordblad, C. O'Cofaigh, J. Patzold, D.
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