Biogenic Opal‐ to CaCO 3 ‐Dominant Sedimentary Shift in the Ross Sea Sector of the Southern Ocean During the Mid‐Pleistocene Transition

Abstract The Mid‐Pleistocene Transition (MPT) is characterized by global cooling, an increase in ice volume, and a glacial‐interglacial cycle shift from 41 to 100 kyr without any significant change in insolation. Although the role of the Southern Ocean in driving the MPT has been proposed, only a few studies have been conducted in the Ross Sea sector of the Southern Ocean that span the MPT. In particular, our understanding of CaCO 3 variation in the Southern Ocean is limited due to its general absence in Antarctic sediments. We analyzed biogenic opal, CaCO 3 , bulk δ 15 N, and grain size from two cores collected from the Ross Sea continental rise to reconstruct the paleoceanographic record during the Pleistocene including the MPT. We found a shift from diatom‐to CaCO 3 ‐dominant deposition with stepwise increases in bulk δ 15 N values during the MPT. The pre‐MPT interval was characterized by high diatom productivity with no evidence of CaCO 3 preservation under nutrient‐enriched conditions. During the MPT diatom production decreased leading to improved CaCO 3 preservation due to enhanced stratification, as supported by increased bulk δ 15 N values and higher coccolith abundances. Further increases in bulk δ 15 N values indicate that stratification became even more enhanced after the MPT. Based on the foraminiferal fragmentation ratio, we infer that carbonate preservation in the Ross Sea sector of the Southern Ocean improved after the MPT, and that during the late Pleistocene, enhanced preservation extended to deeper water depths.