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Changes in the formation of dense water in the Arctic Ocean and Nordic Seas (the 'Arctic Mediterranean', AM) likely contributed to the altered climate of the last glacial period. We examine past changes in AM circulation by reconstructing 14 C ventilation ages of the deep Nordic Seas over the last 30,000 years. Our results show that the deep glacial AM was extremely poorly ventilated (ventilation ages of up to 10,000 years). Subsequent episodic overflow of aged water into the mid-depth North Atlantic occurred during deglaciation. Proxy data also suggest the deep glacial AM was ~2-3°C warmer than modern; deglacial mixing of the deep AM with the upper ocean thus potentially contributed to melting sea-ice and icebergs, as well as proximal terminal icesheet margins.One Sentence Summary: New proxy data reveals the extremely poor ventilation of a warmer, glacial deep Arctic Mediterranean, which overflowed into the Northeast Atlantic during the last deglaciation. Main Text:The Atlantic meridional overturning circulation (AMOC) plays an important role in Earth's climate because it redistributes ocean heat and helps control the storage of carbon in the deep ocean. The primary northern hemisphere sources of dense water supplied to the AMOC are produced in the Arctic Mediterranean (1). Here, warm surface waters from the Atlantic flow northwards and circulate around the AM via several different pathways, gradually cooling, thereby releasing heat to the atmosphere, and becoming denser. Much of this water mass transformation is thought to occur in the Nordic Seas via intermediate and deep open ocean convection, with a smaller contribution from the Arctic that also involves the addition of dense waters from brine-2 enhanced shelf water production (1, 2). The dense water produced by these processes overflows the Greenland-Scotland Ridge (GSR), ultimately forming lower North Atlantic Deep Water (NADW), as part of the deep southward return flow of the AMOC.Because of the northward heat transfer associated with the flow of warm surface water to convection sites, changes in deep water formation in the North Atlantic and Nordic Seas are thought to be associated with the altered climate of the last glacial maximum (LGM) and the abrupt climate events of the last deglaciation (~19 to 7 thousand years ago, ka), such as the northern hemisphere cold intervals Heinrich Stadial 1 (HS1) and the Younger Dryas (YD) (3-5), which affected global climate (6,7). In this study we investigate circulation changes over the past 30 ka in the deep Norwegian Sea (and by inference, the broader AM) by reconstructing radiocarbon ventilation age and deep ocean temperature. Our results reveal an absence of deep convection within the AM throughout much of the last glacial and deglaciation and instead suggest the presence of a relatively warm and extremely poorly ventilated water mass in the glacial deep AM that subsequently overflowed southward into the North Atlantic during the deglaciation.North Atlantic radiocarbon reconstructions. Several studies...

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[1] In-depth analysis of planktic foraminiferal census data paired with d18 O records of specific indicator species provides new insight into the surface ocean evolution of the northeast Atlantic during the previous interglacial warm period (oxygen isotope stage (OIS) 5e). Full interglacial conditions existed at the study site for a maximum of only 8 kyr, between 125 and 117 ka. Highest sea surface temperatures (SSTs) occurred during early OIS 5e concomitant with high summer insolation but after the main phase of ice sheet melting of the preceding glaciation (Saalian). This early peak SST interval is marked by the appearance of tropical-subtropical species and lasted for 4 kyr until 121 ka, as corroborated by a major change in planktic d18 O. Relative stability in global ice volume continued for another 3-4 kyr before SSTs dropped further toward the next stadial. During early OIS 5e the situation of the surface water vertical structure appears to have been different from the early Holocene. For OIS 5e it is therefore suggested that the particular melting history of late Saalian ice had a long-lasting and profound effect on both postdeglacial surface water mass configuration in the North Atlantic and heat-moisture transfer into Europe.Citation: Bauch, H. A., and E. S. Kandiano (2007), Evidence for early warming and cooling in North Atlantic surface waters during the last interglacial, Paleoceanography, 22, PA1201,

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.