Deep-water carbonate dissolution in the northern South China Sea during Marine Isotope Stage 3

The production, transportation, deposition, and dissolution of carbonate profoundly form part of the global carbon cycle and affect the amount and distribution of dissolved inorganic carbon (DIC) and alkalinity (ALK), which drive atmospheric CO2 changes during glacial/interglacial cycles. These processes may provide significant clues for better 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 for the 60–25 ka B.P. time-span, based on samples from Core 17924 and ODP Site 1144 in the northeastern South China Sea (SCS), so as to reconstruct the deep-water carbonate dissolution during Marine Isotope Stage 3 (MIS 3). Our analysis shows that the dissolution of carbonate increases gradually in Core 17924, whereas it remains stable at ODP Site 1144. This difference is caused by the deep-sea carbonate ion concentration ([CO32−])([CO32−]) that affected the dissolution in Core 17924 where the depth of 3440 m is below the saturation horizon. However, the depth of ODP Site 1144 is 2037 m, which is above the lysocline where the water is always saturated with calcium carbonate; the dissolution is therefore less dependent of chemical changes of the seawater. The combined effect of the productivity and the deep-water chemical evolution may decrease deep-water [CO32−][CO32−] and accelerate carbonate dissolution. The fall of the sea-level increased the input of DIC and ALK to the deep ocean and deepened the carbonate saturation depth, which caused an increase of the deep-water [CO32−][CO32−]. The elevated [CO32−][CO32−] partially neutralized the reduced [CO32−][CO32−] contributed by remineralization of organic matter and slowdown of thermohaline. These consequently are the fundamental reasons for the difference in dissolution rate between these two sites.