Description: Dansgaard–Oeschger oscillations constitute one of the most enigmatic features of the last glacial cycle. Their cold atmospheric phases have been commonly associated with cold sea-surface temperatures and expansion of sea ice in the North Atlantic and adjacent seas. Here, based on dinocyst analyses from the 48–30 ka interval of four sediment cores from the northern Northeast Atlantic and southern Norwegian Sea, we provide direct and quantitative evidence of a regional paradoxical seesaw pattern: cold Greenland and North Atlantic phases coincide with warmer sea-surface conditions and shorter seasonal sea-ice cover durations in the Norwegian Sea as compared to warm phases. Combined with additional palaeorecords and multi-model hosing simulations, our results suggest that during cold Greenland phases, reduced Atlantic meridional overturning circulation and cold North Atlantic sea-surface conditions were accompanied by the subsurface propagation of warm Atlantic waters that reemerged in the Nordic Seas and provided moisture towards Greenland summit.

Description: Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.

Description: © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Waelbroeck, C., Lougheed, B. C., Riveiros, N. V., Missiaen, L., Pedro, J., Dokken, T., Hajdas, I., Wacker, L., Abbott, P., Dumoulin, J., Thil, F., Eynaud, F., Rossignol, L., Fersi, W., Albuquerque, A. L., Arz, H., Austin, W. E. N., Came, R., Carlson, A. E., Collins, J. A., Dennielou, B., Desprat, S., Dickson, A., Elliot, M., Farmer, C., Giraudeau, J., Gottschalk, J., Henderiks, J., Hughen, K., Jung, S., Knutz, P., Lebreiro, S., Lund, D. C., Lynch-Stieglitz, J., Malaize, B., Marchitto, T., Martinez-Mendez, G., Mollenhauer, G., Naughton, F., Nave, S., Nuernberg, D., Oppo, D., Peck, V., Peeters, F. J. C., Penaud, A., Portilho-Ramos, R. d. C., Repschlaeger, J., Roberts, J., Ruehlemann, C., Salgueiro, E., Goni, M. F. S., Schonfeld, J., Scussolini, P., Skinner, L. C., Skonieczny, C., Thornalley, D., Toucanne, S., Van Rooij, D., Vidal, L., Voelker, A. H. L., Wary, M., Weldeab, S., & Ziegler, M. Consistently dated Atlantic sediment cores over the last 40 thousand years. Scientific Data, 6, (2019): 165, doi:10.1038/s41597-019-0173-8.

Description: Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.

Description: The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013 Grant agreement n° 339108). New 14C dates for cores EW9209-1JPC and V29-202 were funded by NSF OCE grants to DWO. FN, ES and AV acknowledge FCT funding support through project UID/Multi/04326/2019. We thank T. Garlan and P. Guyomard for having given us access to cores from the Service Hydrographique et Océanographique de la Marine. We acknowledge N. Smialkowski for help with formatting the data into text files, and L. Mauclair, L. Leroy and G. Isguder for the picking of numerous foraminifer samples for radiocarbon dating. We are grateful to S. Obrochta, E. Cortijo, E. Michel, F. Bassinot, J.C. Duplessy, and L. Labeyrie for advice and fruitful discussions. This paper is LSCE contribution 6572.

Description: The last glacial millennial climatic events (i.e. Dansgaard-Oeschger and Heinrich events) constitute outstanding case studies of coupled atmosphere-ocean-cryosphere interactions. Here, we investigate the evolution of sea-surface and subsurface conditions, in terms of temperature, salinity and sea ice cover, at very high-resolution (mean resolution between 55 and 155 years depending on proxies) during the 35-41 ka cal BP interval covering three Dansgaard-Oeschger cycles and including Heinrich event 4, in a new unpublished marine record, i.e. the MD99-2285 core (62.69°N; -3.57°E). We use a large panel of complementary tools, which notably includes dinocyst-derived sea-ice cover duration quantifications. The high temporal resolution and multiproxy approach of this work allows us to identify the sequence of processes and to assess ocean-cryosphere interactions occurring during these periodic ice-sheet collapse events. Our results evidence a paradoxical hydrological scheme where (i) Greenland interstadials are marked by a homogeneous and cold upper water column, with intensive winter sea ice formation and summer sea ice melting, and (ii) Greenland and Heinrich stadials are characterized by a very warm and low saline surface layer with iceberg calving and reduced sea ice formation, separated by a strong halocline from a less warm and saltier subsurface layer. Our work also suggests that this stadial surface/subsurface warming started before massive iceberg release, in relation with warm Atlantic water advection. These findings thus support the theory that upper ocean warming might have triggered European ice-sheet destabilization. Besides, previous paleoceanographic studies conducted along the Atlantic inflow pathways close to the edge of European ice-sheets suggest that such a feature might have occurred in this whole area. Nonetheless, additional high resolution paleoreconstructions are required to confirm such a regional scheme.

Description: The last glacial period was punctuated by abrupt climatic events with extrema known as Heinrich and Dansgaard-Oeschger events. These millennial events have been the subject of many paleoreconstructions and model experiments in the past decades, but yet the hydrological processes involved remain elusive. In the present work, high-resolution analyses were conducted on the 12-42 ka BP section of core MD99-2281 retrieved southwest of the Faeroe Islands, and combined with analyses conducted in two previous studies (Zumaque et al., 2012; Caulle et al., 2013). Such a multiproxy approach, coupling micropaleontological, geochemical and sedimentological analyses, allows us to track surface, subsurface, and deep hydrological processes occurring during these rapid climatic changes. Records indicate that the coldest episodes of the studied period (Greenland stadials and Heinrich stadials) were characterized by a strong stratification of surface waters. This surface stratification seems to have played a key role in the dynamics of subsurface and deep-water masses. Indeed, periods of high surface stratification are marked by a coupling of subsurface and deep circulations which sharply weaken at the beginning of stadials, while surface conditions progressively deteriorate throughout these cold episodes; conversely, periods of decreasing surface stratification (Greenland interstadials) are characterized by a coupling of surface and deep hydrological processes, with progressively milder surface conditions and gradual intensification of the deep circulation, while the vigor of the subsurface northward Atlantic flow remains constantly high. Our results also reveal different and atypical hydrological signatures during Heinrich stadials (HSs): while HS1 and HS4 exhibit a "usual" scheme with reduced overturning circulation, a relatively active North Atlantic circulation seems to have prevailed during HS2, and HS3 seems to have experienced a re-intensification of this circulation during the middle of the event. Our findings thus bring valuable information to better understand hydrological processes occurring in a key area during the abrupt climatic shifts of the last glacial period.

Description: The past two million years of eastern African climate variability is currently poorly constrained, despite interest in understanding its assumed role in early human evolution1,2,3,4. Rare palaeoclimate records from northeastern Africa suggest progressively drier conditions2,5 or a stable hydroclimate6. By contrast, records from Lake Malawi in tropical southeastern Africa reveal a trend of a progressively wetter climate over the past 1.3 million years7,8. The climatic forcings that controlled these past hydrological changes are also a matter of debate. Some studies suggest a dominant local insolation forcing on hydrological changes9,10,11, whereas others infer a potential influence of sea surface temperature changes in the Indian Ocean8,12,13. Here we show that the hydroclimate in southeastern Africa (20–25° S) is controlled by interplay between low-latitude insolation forcing (precession and eccentricity) and changes in ice volume at high latitudes. Our results are based on a multiple-proxy reconstruction of hydrological changes in the Limpopo River catchment, combined with a reconstruction of sea surface temperature in the southwestern Indian Ocean for the past 2.14 million years. We find a long-term aridification in the Limpopo catchment between around 1 and 0.6 million years ago, opposite to the hydroclimatic evolution suggested by records from Lake Malawi. Our results, together with evidence of wetting at Lake Malawi, imply that the rainbelt contracted toward the Equator in response to increased ice volume at high latitudes. By reducing the extent of woodland or wetlands in terrestrial ecosystems, the observed changes in the hydroclimate of southeastern Africa—both in terms of its long-term state and marked precessional variability—could have had a role in the evolution of early hominins, particularly in the extinction of Paranthropus robustus.