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Instrumental observations of subsurface ocean warming imply that ocean heat uptake has slowed 20th-century surface warming. We present high-resolution records from subpolar North Atlantic sediments that are consistent with instrumental observations of surface and deep warming/freshening and in addition reconstruct the surface-deep relation of the last 1200 years. Sites from ~1300 meters and deeper suggest an ~0.5 degrees celsius cooling across the Medieval Climate Anomaly to Little Ice Age transition that began ~1350 ± 50 common era (CE), whereas surface records suggest asynchronous cooling onset spanning ~600 years. These data suggest that ocean circulation integrates surface variability that is transmitted rapidly to depth by the Atlantic Meridional Ocean Circulation, implying that the ocean moderated Earth’s surface temperature throughout the last millennium as it does today.

The distribution of dissolved iodine in seawater is sensitive to multiple biogeochemical cycles, including those of nitrogen and oxygen. The iodine-to-calcium ratio (I/Ca) of marine carbonates, such as bulk carbonate or foraminifera, has emerged as a potential proxy for changes in past seawater oxygenation. However, the utility of the I/Ca proxy in deep-sea corals, natural archives of seawater chemistry with wide spatial coverage and radiometric dating potential, remains unexplored. Here, we present the first I/Ca data obtained from modern deep-sea corals, specifically scleractinian and bamboo corals, collected from the Atlantic, Eastern Pacific, and Southern Oceans, encompassing a wide range of seawater oxygen concentrations (10–280 μmol/kg). In contrast to thermodynamic predictions, we observe higher I/Ca ratios in aragonitic corals (scleractinian) compared to calcitic corals (bamboo). This observation suggests a strong biological control during iodate incorporation into deep-sea coral skeletons. For the majority of scleractinian corals, I/Ca exhibits a covariation with local seawater iodate concentrations, which is closely related to seawater oxygen content. Scleractinian corals also exhibit notably lower I/Ca below a seawater oxygen threshold of approximately 160 μmol/kg. In contrast, no significant differences in I/Ca are found among bamboo corals across the range of oxygen concentrations encountered (15–240 μmol/kg). In the North Atlantic, several hydrographic factors, such as temperature and/or salinity, may additionally affect coral I/Ca. Our results highlight the potential of I/Ca ratios in deep-sea scleractinian corals to serve as an indicator of past seawater iodate concentrations, providing valuable insights into historical seawater oxygen levels.

Reconstructing the strength and depth boundary of oxygen minimum zones (OMZs) in the glacial ocean advances our understanding of how OMZs respond to climate changes. While many efforts have inferred better oxygenation of the glacial Arabian Sea OMZ from qualitative indices, oxygenation and vertical extent of the glacial OMZ is not well quantified. Here we present glacial‐Holocene oxygen reconstructions in a depth transect of Arabian Sea cores ranging from 600 to 3,650 m water depths. We estimate glacial oxygen concentrations using benthic foraminiferal surface porosity and benthic carbon isotope gradient reconstructions. Compared to the modern Arabian Sea, glacial oxygen concentrations were approximately 10–15 μmol/kg higher in the shallow OMZ (<1,000 m), and 5–80 μmol/kg lower at greater depths (1,500–3,650 m). Our results suggest that the OMZ in the glacial Arabian Sea was slightly better oxygenated but remained in the upper 1,000 m. We propose that the small increase in oxygenation of the Arabian Sea OMZ during the last glacial period was due to weaker upper ocean stratification induced by stronger winter monsoon winds coupled with an increase in oxygen solubility due to lower temperatures, counteracting the effects of more oxygen consumption resulting from higher primary productivity. Large‐scale changes in ocean circulation may have also contributed to better ventilation of the glacial Arabian Sea OMZ.