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Abstract 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. 

Abstract Oxic pelagic clays are an important component of seafloor sediment that may hold valuable information about past ocean chemistry due to their affinity for and accumulation of biogeochemically important metals. We present a new approach to calculating site‐specific sedimentation rates (SRs) by comparing authigenic sediment thorium isotope compositions (²³⁰Th/²³²Th) to seawater dissolved²³⁰Th/²³²Th in a suite of deep (>3,000 m) pelagic core sites. We extracted the authigenic sediment fraction using an HHAc leach protocol, which major element chemistry (Al, Mn, Fe, Ti) suggested was less affected by lithogenic contamination than the HCl leach. Four different methods were tested for extracting the appropriate initial²³⁰Th/²³²Th from seawater: using either the nearest water column station (methods 1 and 2) or a regionally averaged profile (methods 3 and 4) and using either the bottommost profile measurement (methods 1 and 3) or linear regression of the profile and extrapolation to the seafloor (methods 2 and 4). Method 3 outperformed the other methods in reconstructing previously published SRs from pelagic clays in the North Pacific. The new thorium‐based SRs were then combined with estimates from the total sediment thickness on ocean crust and non‐lithogenic cobalt accumulation to determine the best estimates for SRs of oxic pelagic clays. The Pacific has the lowest SR (median 0.28 cm/kyr), while the Atlantic is higher (median 0.46 cm/kyr) and the Indian Ocean is highest (median 0.75 cm/kyr). These new estimates are consistent with the expected spatial patterns of sedimentation, but they revise the absolute SR values downward from available gridded SR maps.