Links between intermediate ocean circulation and cryosphere dynamics during Heinrich Stadials in the NE Atlantic: a foraminiferal perspective

Understanding the interaction between ocean circulation and ice sheet dynamics is fundamental to study the rapid Quaternary climate changes that punctuate major glacial-interglacial periods. Compared to the surface and deep compartments of the Atlantic Meridional Overturning Circulation (AMOC), intermediate water depths during key time periods, such as Heinrich Stadials (HSs), remain poorly documented, especially in the Northeast Atlantic.

In this study, we use benthic foraminiferal assemblage data from an upper slope sediment core from the Northern Bay of Biscay to reconstruct paleoenvironmental and paleohydrological changes at ~1000m water depth, from ~35 to 14 kyr cal BP. Our results show a strong response of benthic communities to hydrodynamic changes (related to AMOC) and to instabilities of the European Ice Sheet during the last three HSs. Benthic foraminifera provide species-specific responses to the induced physico-chemical changes, in coherence with the various geochemical and sedimentological proxies documented in the area. The three HSs are characterized by the low abundance of species indicative of high-energy environments (Cibicides lobatulus and Trifarina angulosa) and the simultaneous presence of Cibicidoides pachyderma (meso-oligotrophic species) and Globobulimina spp. (anoxia-tolerant species).   This species composition suggests a slowing of the intermediate circulation during the three HSs. Nevertheless, HS1 is very distinct from HS2 and HS3 by the high presence of high-organic flux indicator species (Cassidulina carinata and Bolivina spp.) during its early phase (Early HS1). This result confirms that EIS meltwaters were much less charged in organic material derived from the continent during HS2 and HS3 than during HS1 due to the scarcer vegetation cover and partially frozen soils. Finally, benthic foraminifera depict clearly the rapid "re-ventilation" during Mid-HS2, corresponding to a response to regional glacial instabilities.