Deep water dissolution in Marine Isotope Stage 3 from the northern South China Sea

Abstract:

The production, transport, deposition, and dissolution of carbonate profoundly implicate the global carbon cycle affect the inventory and distribution of dissolved organic carbon (DIC) and alkalinity (ALK), which drive atmospheric CO₂ change on glacial-interglacial timescale. the process may provide significant clues for improved understanding of the mechanisms that control the global climate system. In this study, we calculate and analyze the foraminiferal dissolution index (FDX) and the fragmentation ratios of planktonic foraminifera over 60-25 ka based on samples from 17924 and ODP 1144 in the northeastern South China Sea (SCS) to reconstruct the deep water carbonate dissolution during Marine Isotope Stage 3 (MIS 3). Result shows that the dissolution of carbonate increases gradually at 17924 but keeps stable at ODP 1144. The changes of FDX coincidence with that of fragmentation ratios at 17924 and ODP 1144 suggest both indexes can be used as reliable dissolving proxies of planktonic foraminifera. Comparing FDX and fragmentation ratios at both sites, we find the FDX and fragmentation ratios at 17924 are higher than those at 1144, indicating that carbonate dissolution is intenser in 17924 core during MIS 3. The increasing total percentage of both N. dutertrei and G. bulloides during MIS 3 reveals the rising primary productivity that may lead to deep water [CO₃^2-] decrease. The slow down of thermohaline circulation may increase deep water residence time and accelerate carbonate dissolution. In addition, the covering of ice caps, iron supply and increased surface-water stratification also contribute to atmosphere CO₂ depletion and [CO₃^2-] decrease in deep water. In the meanwhile, regression result from colder temperature increases the input of ALK and DIC to the deep ocean and deepens the carbonate saturation depth, which makes the deep water [CO₃^2-] rise. In ODP Site 1144, the decrease in [CO₃^2-] caused by more CO₂ restored in deep water is equal to the increase in [CO₃^2-] because of regression, so dissolution keeps steady. However, [CO₃^2-] is probably more strongly controlled by regression, the decrease in [CO₃^2-] result from more CO₂ restored in deep water overwhelms the increase in [CO₃^2-] due to regression at 17924, so more carbonate dissolved from MIS 3 to the Last Glacial Maximum (LGM).