Over the past three million years, Earth's climate oscillated between warmer interglacials with reduced terrestrial ice volume and cooler glacials with expanded polar ice sheets. These climate cycles, as reflected in benthic foraminiferal oxygen isotopes, transitioned from dominantly 41-kyr to 100-kyr periodicities during the mid-Pleistocene (1,250 to 700 ka). Because orbital forcing did not shift at this time, the ultimate cause of this mid-Pleistocene transition (MPT) remains enigmatic. Here we present foraminiferal trace element (B/Ca, Cd/Ca) and Nd isotope data that demonstrate a tight linkage between Atlantic Ocean meridional overturning circulation and deep-ocean carbon storage across the MPT. Specifically, between 950 and 900 ka, carbonate ion saturation decreased by 30 µmol/kg and phosphate concentration increased by 0.5 µmol/kg coincident with a 20% reduction of North Atlantic Deep Water contribution to the abyssal South Atlantic. These results demonstrate that the glacial deep Atlantic carbon inventory increased by approximately 50 gigatons during the transition to 100-kyr glacial cycles. We suggest that the coincidence of our observations with evidence for increased terrestrial ice volume reflects how weaker overturning circulation and Southern Ocean biogeochemical feedbacks facilitated deep ocean carbon storage, which lowered atmospheric pCO 2 and thereby enabled expanded terrestrial ice volume at the MPT. Cyclic glaciations are the primary feature of Earth's climate since the late Pliocene and occur at periodicities linked to variations in solar insolation 1. However, the dominant periodicity of glaciations transitioned from 41-kyr to 100-kyr during the mid-Pleistocene without concomitant changes in external insolation forcing 2-5. It has been

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.

Arctic Ocean temperatures influence ecosystems, sea ice, species diversity, biogeochemical cycling, seafloor methane stability, deep-sea circulation, and CO2 cycling. Today’s Arctic Ocean and surrounding regions are undergoing climatic changes often attributed to “Arctic amplification” – that is, amplified warming in Arctic regions due to sea-ice loss and other processes, relative to global mean temperature. However, the long-term evolution of Arctic amplification is poorly constrained due to lack of continuous sediment proxy records of Arctic Ocean temperature, sea ice cover and circulation. Here we present reconstructions of Arctic Ocean intermediate depth water (AIW) temperatures and sea-ice cover spanning the last ~ 1.5 million years (Ma) of orbitally-paced glacial/interglacial cycles (GIC). Using Mg/Ca paleothermometry of the ostracode Krithe and sea-ice planktic and benthic indicator species, we suggest that the Mid-Brunhes Event (MBE), a major climate transition ~ 400–350 ka, involved fundamental changes in AIW temperature and sea-ice variability. Enhanced Arctic amplification at the MBE suggests a major climate threshold was reached at ~ 400 ka involving Atlantic Meridional Overturning Circulation (AMOC), inflowing warm Atlantic Layer water, ice sheet, sea-ice and ice-shelf feedbacks, and sensitivity to higher post-MBE interglacial CO2 concentrations.

[1] The reconstruction of deep-sea bottom water temperature (BWT) is important to assess the ocean's response to and role in orbital-and millennial-scale climate change. Deep-sea paleothermometry employs magnesium to calcium (Mg/Ca) ratios in calcitic benthic microfaunas (foraminifera, ostracodes) as a primary proxy method. Mg/Ca paleothermometry may, however, be complicated by bottom water carbonate ion chemistry, which might affect Mg/Ca ratios in shells. To address temperature and carbonate ion influence on Mg/Ca ratios, we studied Mg/Ca ratios in the benthic ostracode genus Krithe in the North Atlantic and Arctic oceans using a 686-specimen core top collection, including 412 previously unpublished analyses. Mg/Ca ratios are positively correlated to temperature in multiple species from the North Atlantic [BWT = (0.885 Â Mg/Ca) À 5.69, r 2 = 0.73] and for K. glacialis in the Arctic Ocean and Nordic Seas [BWT = (0.439 Â Mg/Ca) À 5.14, r 2 = 0.50], consistent with previously published calibrations. We found no evidence for a relationship between Krithe Mg/Ca and carbonate ion saturation in the North Atlantic Ocean, Nordic Seas, and Arctic Ocean, supporting the use of Krithe Mg/Ca for reconstructing past BWT.

We reconstructed subsurface (∼200–400 m) ocean temperature and sea‐ice cover in the Canada Basin, western Arctic Ocean from foraminiferal δ18O, ostracode Mg/Ca ratios, and dinocyst assemblages from two sediment core records covering the last 8000 years. Results show mean temperature varied from −1 to 0.5°C and −0.5 to 1.5°C at 203 and 369 m water depths, respectively. Centennial‐scale warm periods in subsurface temperature records correspond to reductions in summer sea‐ice cover inferred from dinocyst assemblages around 6.5 ka, 3.5 ka, 1.8 ka and during the 15th century Common Era. These changes may reflect centennial changes in the temperature and/or strength of inflowing Atlantic Layer water originating in the eastern Arctic Ocean. By comparison, the 0.5 to 0.7°C warm temperature anomaly identified in oceanographic records from the Atlantic Layer of the Canada Basin exceeded reconstructed Atlantic Layer temperatures for the last 1200 years by about 0.5°C.

No abstract

The ocean is currently absorbing excess carbon from anthropogenic emissions, leading to reduced seawater-pH (termed 'ocean acidification'). Instrumental records of ocean acidification are unavailable from well-ventilated areas of the deep ocean, necessitating proxy records to improve spatio-temporal understanding on the rate and magnitude of deep ocean acidification. Here we investigate boron, carbon, and oxygen isotopes on live-collected deep-sea bamboo corals (genus Keratoisis) from a pH tot range of 7.5-8.1. These analyses are used to explore the potential for using bamboo coral skeletons as archives of past deep-sea pH and to trace anthropogenic acidification in the subsurface North Atlantic Ocean (850-2000 m water depth). Boron isotope ratios of the most recently secreted calcite of bamboo coral skeletons are close to the calculated isotopic composition of borate anion in seawater (d 11 B borate) for North Atlantic corals, and 1-2& higher than d 11 B borate for Pacific corals. Within individual coral skeletons, carbon and oxygen isotopes correlate positively and linearly, a feature associated with vital effects during coral calcification. d 11 B variability of 0.5-2& is observed within single specimens, which exceeds the expected anthropogenic trend in modern North Atlantic corals. d 11 B values are generally elevated in Pacific corals relative to d 11 B borate , which may reflect pH-driven physiological processes aiding coral calcification in environments unfavorable for calcite precipitation. Elevated d 11 B values are also observed proximal to the central axis in multiple Atlantic and Pacific specimens, relative to d 11 B borate , which might reflect ontogenetic variability in calcification rates. Although the observed boron isotope variability is too large to resolve the present anthropogenic ocean acidification signal at the studied depths in the North Atlantic ($0.03-0.07 pH units), pH changes P0.1 units might still be reconstructed using d 11 B measurements in bamboo corals.

Salinity-driven density stratification of the upper Arctic Ocean isolates sea-ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. Recently, stratification has strengthened in the western Arctic but has weakened in the eastern Arctic; it is unknown if these trends will continue. Here we present foraminifera-bound nitrogen isotopes from Arctic Ocean sediments since 35,000 years ago to reconstruct past changes in nutrient sources and the degree of nutrient consumption in surface waters, the latter reflecting stratification. During the last ice age and early deglaciation, the Arctic was dominated by Atlantic-sourced nitrate and incomplete nitrate consumption, indicating weaker stratification. Starting at 11,000 years ago in the western Arctic, there is a clear isotopic signal of Pacific-sourced nitrate and complete nitrate consumption associated with the flooding of the Bering Strait. These changes reveal that the strong stratification of the western Arctic relies on low-salinity inflow through the Bering Strait. In the central Arctic, nitrate consumption was complete during the early Holocene, then declined after 5,000 years ago as summer insolation decreased. This sequence suggests that precipitation and riverine freshwater fluxes control the stratification of the central Arctic Ocean. Based on these findings, ongoing warming will cause strong stratification to expand into the central Arctic, slowing the nutrient supply to surface waters and thus limiting future phytoplankton productivity.