Changes in Deep Water Oxygenation in the Philipean Sea Over the Past 24 Million Years: Insights from IODP Site U1438B

The oxygen concentration of oceanic deep waters in the Pacific and atmospheric carbon dioxide (pCO₂) have been found to be intrinsically linked during glacial-interglacial cycles through processes such as organic carbon remineralization and the storage of dissolved inorganic carbon in the deep ocean. However, the persistence of this link over the Neogene is unclear. Here, we present a reconstruction of oxygenation history over the past 24 million years at IODP Site U1438B (4700.5 m water depth) in the Philippian Sea, where the bottom waters are mainly originated from the Lower Circumpolar Deep Water (LCDW). The employed proxies for oxygenation are the Alcohol Preservation Index (API) and U/Ba ratio. Our results reveal an overall trend of decreased bottom water oxygenation since the early Miocene, which aligns with the long-term decline in atmospheric pCO₂. Notably, we identify a period of significant decrease of oxygenation from 16–12 million years ago (Ma), concurrent with the most significant CO₂ decline in the Neogene centered at the East Antarctic ice sheet (EAIS) expansion. We propose that this significant decreases of oxygenation, indicative of increase in deep-ocean respired carbon pool, was caused by weakened LCDW ventilation due to the EAIS expansion. Interestingly, during the Arctic polar ice sheet expansion at the Pliocene-Pleistocene transition at around 3-2 Ma, LCDW oxygenation also weakened significantly, although the LCDW was unlikely affected by the hypothesized sub-Arctic deep water. Nevertheless, our results support an intrinsic connection between the deep Pacific respired carbon pool and atmospheric CO₂ that has existed since the early Miocene.