Inserting the Arctic Ocean into the global late Pleistocene climate/ocean system: The Graal Quest?

The setting of a consensual climate history of the Arctic Ocean spanning the last major climatic cycles is still unachieved despite recent converging views about the chronostratigraphy of marine archives from this ocean. Under both permanent and seasonally-opened sea-ice covers, sedimentary recordings present anomalies, ranging from hiatuses under thick ice-shelf covers, during glacials, to winnowed or mixed sequences generated by deep-density currents, under seasonally freezing sea-ice conditions during interglacials or interstadials. In opposition, short, early, or late-glacial events (e.g., continental ice surging and glacial lake drainage events) may have led to the deposition of relatively thick layers occasionally with reworked material. Accordingly, time interpolation between dated layers and within these layers is misleading, and lateral sediment advection leads mixed microfossils and biomarkers records, thus to biased paleoceanography/paleoclimate inferences. Interglacial as well as glacial sequences are subsequently poorly recorded. Along ridges, erosion of fine particles by sinking brines and deep density-driven current, with redeposition in sheltered/deeper sites, further results in the mixing of fossil populations. This process and its impact on paleoecological reconstructions are well-documented by ¹⁴C records spanning the Holocene-Marine Isotope Stage 3 interval. In several cores raised from central Arctic ridges, for example, a few centimeters of mixed Holocene and Marine Isotope Stage 3 assemblages illustrate this interval. Nonetheless, the positions of the last two interglacials in deep sedimentary cores may be set with some confidence based on the relative decay of sedimentary excesses in U-series isotopes (²³¹Pa vs ²³⁰Th) and detrital feldspar grain OSL-ages. With the complementary support of paleomagnetic records, a tentative outline of the major late Quaternary glacial/interglacial events may be proposed, as illustrated here by records from the Chukchi Sea margin. In this area, high interglacial/interstadial sea-level intervals allowed Pacific Water influx through the shallow Bering Strait, as recorded by radiogenic Nd-isotope excursions and enhanced Si-fluxes (thus high primary productivity). High sea levels also resulted in the flooding of shelves, leading to high manganese fluxes in the deep basins. These provided a cyclostratigraphic tool for the correlation of records throughout the Arctic Ocean, as documented in several studies of the last decades.