Figure 4 provides a conceptual view of Arctic paleoceanography regimes over the last 1.5 Myr, which helps explain the abrupt shift in Arctic climate at the MBE, post-MBE thermal maxima events, Atlantic Water and underlying AIW and, more broadly, the evolution of Arctic cryosphere and climate. Figure 4a shows the modern regime with perennial sea ice in the central Arctic and seasonal sea ice along the margins. Examining the last 50 kyr, AIW temperature maxima were centered on the Younger Dryas (12 ka), and Heinrich events H1, H2 and H3 (17 ka, 24 ka, 32 ka, respectively, Figs. 2c,4b)6. These appear to be related to collapse and renewal of AMOC during Heinrich Events in the North Atlantic19,20.

Figure 4 Conceptual view of Arctic paleoceanography regimes over the last 1.5 Myr. (a) Modern-day and post-MBE interglacial regime. Perennial sea-ice characterizes the central Arctic Ocean, with seasonal sea-ice at the ocean margins. Cold, low salinity Polar Surface Layer (purple) underlies sea-ice, and is separated from the warmer Atlantic Layer (orange) by a halocline. Productivity (green arrow) is locally enhanced at the ice margin. Faunal and geochemical records indicate low intermediate-depth temperatures, higher local productivity, and the presence of sea-ice. (b) TME/stadial regime. Extensive ice shelves and/or freshwater input drive a deepened Polar Surface Layer and Atlantic Layer, associated with higher intermediate-depth temperatures, low local productivity, and low sea-ice in our records. (c) Pre-MBE interglacial conditions. Sea-ice is likely absent outside of the Central Arctic, with higher productivity shifted to the area of seasonal sea-ice. Our records indicate lower intermediate-depth temperatures, low local productivity, and absent sea-ice. Full size image

Prior to 50 ka, the largest Arctic TME-3 around 210–205 ka, occurred during a strong stadial within MIS 7 (MIS 7.4) when sea level21, Antarctic temperature17 and atmospheric CO 2 concentrations22 all fell sharply. TME-4 and TME-2 occurred at 300 to 290 ka, during the MIS 9-MIS 8 transition, and 106 to 92 ka, during MIS 5.4-MIS 5.2, respectively. Between 390 and 365 ka, there was a small but consistent increase in AIW temperature during the aforementioned MIS 11-to-MIS 10 transition. Although the precise ages of thermal maxima in AIW are uncertain due to low sedimentation rates, all TMEs, including those of the last 50 kyr, seem to occur during periods of climatic transition between interglacial and glacial periods.

It has been known for some time that the largest manifestations of climatic transition at the MBE were interglacial CO 2 levels and deep-sea temperature23. Most other continuous proxy records spanning the MBE (see Extended Data)–deep-sea temperature, ice volume, sea level, global sea-surface temperature, East Asian Monsoon strength–show progressive change in the amplitude of orbital cycles through the MBE interval. In contrast, our data suggest that the Arctic Ocean (Fig. 4c), specifically sea ice [and perhaps also ice shelves], productivity and intermediate water temperature, shifted over several tens of thousands of years beginning ~400 ka. The AIW temperature shift at the MBE is, in turn, likely a reflection of changes in the strength of inflowing Atlantic water and AMOC.

The occurrence (absence) of TMEs after (prior to) ~ 400 ka reflects an enhanced sensitivity of Arctic climate, cryosphere and ocean circulation to relatively small differences in external forcing from insolation18 and higher post-MBE CO 2 concentrations during the last five interglacial periods (280 ppmv post-MBE versus 240 ppmv pre-MBE)23,24. One factor that may explain this shift is that post-MBE northern hemisphere ice sheets and Arctic Ocean ice shelves were larger than those prior to the MBE. Growing evidence shows that the Arctic Ocean has been partially covered with thick (up to 1-km) ice shelves and sea-ice cover during recent glacial maxima (Extended Data).

If as our results suggest, the 40-ppmv higher post-MBE interglacial CO 2 concentrations produced conditions conducive to large-scale ocean-ice changes within the Arctic Ocean, there may be implications for the global carbon cycle. For example, Bouttes et al.25 conducted carbon-cycle modeling of the MBE finding the magnitude of the pre and post-MBE CO 2 difference cannot be accounted for by insolation, glacial ice sheet extent, or terrestrial carbon storage. Thus the Arctic Ocean may have contributed to the MBE CO 2 change via temperature and sea-ice changes discussed above. We speculate that the Arctic Ocean system would act as a relative carbon sink during warm periods and a carbon source during cool periods. During pre-MBE interglacials, there would be greater CO 2 and organic carbon storage in the Arctic Ocean Arctic Ocean and sediments, respectively, due in part to colder AIW temperatures and higher integrated surface Arctic productivity due to less perennial sea ice. In contrast, during post-MBE interglacial periods, warmer AIW temperatures and lower integrated surface Arctic productivity and increased sea ice would result in less CO 2 and organic carbon production/storage in Arctic Ocean. Although the Arctic Ocean’s small size limits the possible magnitude of carbon storage, this differential Arctic Ocean carbon storage during interglacials would nonetheless complement Southern Ocean changes in deep ventilation26 and polar front position27 proposed to account for higher post-MBE interglacial CO 2 concentrations. It is unclear why Polycope spp. and associated sea-ice margin species were extremely rare in the pre-MBE Arctic but it is likely due to the very different summer sea-ice conditions and competition with the diverse pre-MBE ostracodes (Extended data).

Southern Hemisphere ocean-atmosphere-sea ice processes are critical for understanding the MBE, specifically the idea that there is a bipolar seesaw operating between Northern and Southern Hemispheres on millennial timescales explaining warmer interglacial conditions in the Southern Hemisphere. Barker et al. (2011)28 demonstrated that abrupt millennial-scale AMOC variability characterized the last 800 ka, albeit without the large amplitude shift seen in our Arctic records. Holden et al.29 proposed a role for decreased stability of the West Antarctic Ice Sheet following the MBE, leading to AMOC slowdown during deglacials. Thus, it is possible that ice sheet/ice shelf instability characterized both hemispheres providing the necessary non-linear dynamics to explain large amplitude temperature events in the Arctic Ocean. However, establishing details of the timing of post-MBE suborbital events – especially the relationship between bottom temperature, sea ice and productivity during stadial and interstadial periods - requires better sediment core resolution in the Arctic. Nonetheless, the large shift in Arctic land ice, ice shelves and sea ice at the MBE, suggests an amplification of Arctic climate sensitivity related to higher interglacial CO 2 concentrations and associated feedbacks involving ice shelves and ice sheets, Heinrich-like events, AMOC-forced Arctic Ocean temperature oscillations, and deeper submergence of Atlantic water in the central Arctic Basin.

All data presented in this manuscript have been deposited in the National Climatic Data Center (NCDC) (https://www.ncdc.noaa.gov/data-access/paleoclimatology-data).