Deep Arctic Ocean warming during the last glacial cycle

Abstract

In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean 1-7 have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations 8-10 is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11 kyr ago, the central Arctic Basin from 1,000 to 2,500 m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1-2 • C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming. Our study of deep Arctic Ocean temperature variability during the last glacial-interglacial cycle focuses on sediment cores from Arctic submarine ridges (Lomonosov, Gakkel and Mendeleev), Nansen and Makarov abyssal plains, Yermak Plateau and Morris Jesup Rise, Laptev Sea Slope, Chukchi Shelf and the Iceland Plateau in the central Nordic seas (Greenland, Norwegian and Iceland seas; Fig. 1a and Supplementary Information). Modern Arctic water masses in these regions (Fig. 1b) consist of cold, low-salinity water from the Polar Mixed Layer that is separated from the underlying warm Atlantic Layer by a strong halocline. Atlantic water enters the Arctic Basin in two branches, one through the Fram Strait and the other through the Barents Sea 11,12. The inflowing Atlantic water is entrained with water formed along the margins to form Arctic Intermediate Water (AIW), which lies above Eurasian and Amerasian Basin Deep Water (Fig. 1b). The results presented here show that the central Arctic Basin at depths occupied by today's AIW and upper Eurasian Basin Deep Water and Amerasian Basin Deep Water experienced temperature variability during the last glacial period and, to a lesser degree, the Holocene interglacial, signifying large changes in circulation in Arctic and subarctic seas and variability in halocline depth.

Major impacts of climate change on deep-sea benthic ecosystems

“…Similar fluctuations in OMZ intensity have occurred during the Dansgaard-Oeschger (D-O) events (millennial-scale abrupt climate oscillations) during the last glacial period Schmittner et al, 2007). Bottom-water temperature and current velocities have also fluctuated in relation to decadal-millennial scale climatic changes during the last de-glaciation and Holocene (Bianchi and McCave, 1999;Marchitto and deMenocal, 2003;Farmer et al, 2011;Cronin et al, 2012). Finally, deep-sea bottom temperatures have also exhibited systematic glacial-low, interglacial-high patterns during the Plio-Pleistocene (Dwyer et al, 1995;Martin et al, 2002;Sosdian and Rosenthal, 2009;Elderfield et al, 2010).…”

A warm and poorly ventilated deep Arctic Mediterranean during the last glacial period

“…5 and SOM). A 2-4°C warmer-than-modern, intermediate-to-deep AM has been inferred during MIS 3, caused by a deep inflow of Atlantic water (24,35). During MIS 2, when we infer an isolated deep AM, it is likely that geothermal heating contributed to warming the deep AM.…”

The interaction between sea ice and salinity-dominated ocean circulation: implications for halocline stability and rapid changes of sea ice cover

“…2b). Thus, in the limit of small freshwater input, the low salinity surface layer becomes very deep, implying that the upper limit of the warmer Atlantic water layer is displaced to greater depths; a feature which may have relevance for the glacial Arctic Ocean stratification (Jakobsson et al 2010;Cronin et al 2012). Nilsson and Walin (2010) showed that the salinity-dominated circulation does not have a solution below a threshold value of F riv .…”