Enhanced acidification of intermediate water in the South China Sea during the Pliocene-Pleistocene transition

Ocean interior acidification is predicted to exacerbate in the future due to persistent emissions of anthropogenic carbon dioxide (CO₂), which may excert disastrous impact on marine ecosystems. However, whether this trend is ubiquitous in the global oceans is not well understood. In this study, we reconstruct the pH changes of intermediate water in the northern South China Sea (SCS) since the Pliocene using bacterial branched glycerol dialkyl glycerol tetraethers. The results indicate a significant decline in pH during the Pliocene-Pleistocene transition, when atmospheric CO₂ was decreasing and thus not conductive to pH drop. We examined the controlling factors and found that weakened vertical mixing between intermediate and deep waters during this period played a crucial role in the decrease of intermediate water pH, rather than the influence by changes in atmospheric CO₂. Our findings highlight the effect of stratification of the ocean interior on the balance of the carbonate system in the SCS, which has been overlooked in modern observations and projections.