pH rise in intermediate water of the South China Sea during the last deglaciation

Abstract

During the last deglaciation, the reduction of deep-ocean carbon storage in the western Pacific has been considered a contributing factor to the increase in atmospheric pCO₂. However, there is limited understanding of how inorganic carbon in intermediate water changed and responded to water column ventilation. This study utilized marine-produced lipid biomarkers to reconstruct temperature and pH in the intermediate waters of the South China Sea (SCS). Our results show a decrease in temperature and an increase in pH of intermediate waters of both the northern and southern SCS during the last deglaciation. The temperature decrease is likely due to the enhanced mixing of intermediate and deep waters induced by stronger ventilation, with a stronger mixing intensity in the southern SCS contributing to a greater temperature drop. In contrast, the increase in pH is only partly consistent with the proposed enhanced mixing of intermediate and deep waters during the last deglaciation. A reduced organic carbon pump may also have played an important role, particularly in the northern SCS. Overall, our findings indicate a significant rise in pH in the intermediate waters of the SCS during the last glaciation, pointing to reduced carbon storage, which likely have contributed to the increase in atmospheric CO₂ during this period.