Atlantic Water distribution in the central and eastern Arctic Ocean during past interglacials

We present planktic foraminiferal and planktic stable isotope records from central and eastern Arctic Ocean sediment cores with a particular attention on the development of the structure of upper water masses during two past interglacials (here termed IG3 and IG2), in comparison to the present Holocene (IG1). The age of interglacials IG2 and IG3 is currently under discussion. While the "classic" age model based on Jakobsson et al. (2000, Geology) would relate them to marine isotope (sub)stages (MIS) 5e and 5a, latest work (e.g., Song et al., 2023, Earth Sci. Rev.; Razmjooei et al., 2023, Quat. Sci. Rev.) would assign ages of MIS 11, 9, 7 or 5.

Stable oxygen and carbon isotopes from polar planktic foraminifers Neogloboquadrina pachyderma give clues on their habitat within the upper water column which today is characterized by an ice-covered low-saline cold surface layer, underlain by high-saline warm Atlantic Water. Sediments from IG2 and IG3 containing also subpolar planktic foraminifers Turborotalita quinqueloba show oxygen isotope values of close to modern ones, indicating a similar water mass structure as today, with a transition level between freshwater-rich and Atlantic Water. Carbon isotope values are lower and may point at a higher bioproductivity due to less sea ice and a decomposition of carbon in the upper waters. Interestingly, in the sediments underneath, which are barren in T. quinqueloba but contain abundant N. pachyderma, both oxygen and carbon isotopes are significantly higher. These data can be interpreted as evidence of a strongly stratified water column, a deeper habitat of the foraminifers, a strong subsurface advection of Atlantic Water and more sea ice during the early phases of IG2 and IG3. In cases, due to a lack of carbonate microfossils this interval is not represented in all analyzed cores. We assume that near-surface salinities were below the tolerance limit of planktic foraminifers in the very early parts of IG2 and IG3, probably due to a strong influence of meltwater from disintegrating ice sheets on northern Eurasia in the preceding glacial stages. Our results reveal a two-step development of conditions in the central Arctic during previous warm intervals. In the first part, the uppermost water column (including the habitat depth of T. quinqueloba) always had very low salinties due to freshwater discharge from ice sheets on the continents. Only in the second part Atlantic Water was shoaling and allowed the occupation by shallow-dwelling T. quinqueloba. Data from the Kara Sea continental margin suggest that upper water conditions in the eastern Arctic remained under strong freshwater influence, at least throughout IG2.