Enhanced Deep‐Water Oxygenation in the South China Sea Driven by Middle Miocene Climate Cooling

Abstract The middle Miocene represents a significant climatic transition period within the Cenozoic era. However, ocean redox conditions during this period remain poorly understood. Here, we present Fe speciation and redox‐sensitive element enrichment factors (EF) for bulk sediments from Site 1146 in the northern South China Sea (SCS) to constrain deep‐water redox changes in the region during the middle Miocene. From the Miocene Climatic Optimum (MCO) to the Middle Miocene Climate Transition (MMCT), Mn EF was consistently greater than 1 and gradually increased, while U EF remained less than 1 and showed a decreasing trend. These results indicate that deep water in the northern SCS contained dissolved oxygen and the degree of oxygenation increased throughout the middle Miocene. In addition, the ratio of high reactive Fe to total Fe (Fe HR /Fe T ) was consistently lower than the mean value of oxic continental margin sediments (0.28) and gradually increased from the MCO to the MMCT, while the ratio of pyrite Fe to high reactive Fe (Fe py /Fe HR ) gradually decreased. This result indicates that the reductive mobilization of sedimentary Fe was weakened and the deep water changed from a dysoxic condition to an oxic condition. The increased oxygenation of deep water in the SCS during the cooler MMCT compared to the warmer MCO is not only related to the reduced oxygen consumption for organic matter remineralization, but is also influenced by the increased oxygen supply associated with the enhancement of Antarctic Circumpolar Deep Water.