The evolution of deep ocean chemistry in the Eastern Equatorial Pacific over the last deglaciation

Abstract

It has been shown that the deep Eastern Equatorial Pacific (EEP) region was poorly ventilated during the Last Glacial Maximum (LGM) relative to Holocene values. In addition, surface productivity records from this region have indicated higher export productivity over this period. Together, these findings suggest a more efficient glacial biological pump, which indirectly supports the idea of increased carbon storage in the glacial deep ocean contributing to decreased LGM atmospheric CO2. However, proxies related to dissolved CO2 concentrations are needed in order to assess this proposition more directly. Here we present C. wuellerstorfi B/Ca ratios from ODP1240 measured by LA-IPCMS as a proxy for Δ[CO32-]insitu-sat (and therefore [CO32-]), along with δ13C measurements. Our results show lower [CO32-] and δ13C values during the LGM, which would be consistent with higher respired carbon levels in the deep EEP, driven by both reduced deep-water ventilation and higher export productivity. However, the ∆[CO32-]LGM-Holocene observed at ODP1240 is relatively small, suggesting that a “counteracting” mechanism, such as seafloor carbonate dissolution, also played a role. If so, this mechanism would have increased average ocean alkalinity, allowing even more atmospheric CO2 to be “sequestered” by the ocean. Similar deep-ventilation and ∆[CO32-]LGM-Holocene results have been shown by other records from the Pacific. Therefore, the deep Pacific Ocean very likely sequestered a significant amount of atmospheric CO2 during the LGM, specifically due to a more efficient biological carbon pump, but also to an increase in average ocean alkalinity