(Georgia Institute of Technology, 2023-05-05)
Kim, Grace
The mechanisms responsible for lowering atmospheric CO2 levels during glaciation have yet to be constrained, but the deep ocean is the most likely reservoir of CO2 drawdown. Deep ocean carbon export and storage are suggested to have increased cyclically during glacial periods due to greater biological pump efficiency and ocean stratification, and poor ventilation. Increased respired carbon in the Eastern Equatorial Pacific Ocean (EEP) would be evident with depleted dissolved oxygen content, but there is insufficient paleo-oxygen data in this region. This study uses a benthic foraminifera Δδ13C proxy to provide quantitative assessments of changes in oxygen concentration between the Holocene and LGM. The proxy relies on the empirical relationship between bottom water oxygen and the carbon isotope gradient between the sediment-water interface and oxic-anoxic interface preserved in benthic foraminifera. Cibicidoides wuellerstorfi and Globobulimina spp. are benthic foraminifera that preferentially reside at these interfaces and record δ13C at equilibrium with bottom water and pore water dissolved inorganic carbon, respectively. The findings of this paper provided oxygen concentrations in the Holocene and LGM that indicate a more depleted bottom water oxygen content and higher mid-depth oxygen concentrations during the last glacial period. This suggests increased carbon storage, poorer ventilation, and greater water mass stratification and supports the respired carbon deepening hypothesis and corresponds to oxygen trends of qualitative paleo-oxygen proxies in the EEP.