Sea level and climate control the Arctic Ocean halocline over the last 35,000 years

Abstract:

Salinity stratification of the upper Arctic Ocean, termed the halocline, isolates Arctic sea ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. The strength of the halocline impacts Arctic biological productivity and sea ice extent, both of which have changed rapidly in recent decades. The halocline has strengthened in the western Arctic and weakened in the central and eastern Arctic; whether these trends will persist is unknown. Here we report foraminifera-bound nitrogen isotope records since 35,000 years ago (35 ka) in three sediment cores from the western and central Arctic Ocean. We interpret these records in terms of changes in the source of the nutrient nitrate (and thus water) to the subsurface halocline waters, and the degree of nitrate consumption in overlying surface waters, the latter reflecting halocline strength. Up to 11 ka, the Arctic Ocean was dominated by Atlantic-sourced nitrate and incomplete nitrate consumption, indicating a weaker halocline than today. At 11 ka, there is a clear isotopic signal of Pacific-sourced nitrate associated with the opening of the Bering Strait. Simultaneously, nitrate consumption rose, indicating that the halocline strengthened. In the central Arctic, nitrate consumption declined modestly after 5 ka alongside reduced summer insolation and increased sea ice cover. These changes reveal that the strong modern Arctic halocline relies on Bering Strait inflow, and that, secondarily, regional warming tends to strengthen the halocline. Our results predict that the observed weakening of the central Arctic halocline today, termed “atlantification”, may not continue.