How the Surface Seawater of the South China Sea Exchanged CO2 with the Atmosphere over the Last Glacial Cycle?

Atmospheric CO₂ content, changing synchronously with temperature over recent glacial cycles, is considered as one of the most important mechanisms regulating climate change. The ocean is the dominant force driving atmospheric CO₂ changes in glacial cycles. The CO₂ sink of global coastal seas currently comprise 21% of the net sea-air CO₂ flux of the global ocean, therefore coastal seas play important role in adjusting atmospheric CO₂. The South China Sea (SCS), one of the largest coastal seas separating Asia from the Pacific, currently acts as a source of atmospheric CO₂ due to high seawater temperature and intense vertical mixing bringing CO₂ of deep sea to the surface.

We measured B/Ca ratios in planktonic foraminifers, Globigerinoides ruber and Pulleniatina obliquiloculata, from MD05-2896 located in the southern SCS to reconstruct surface water pH (pHsw) and thermocline water pH (pHtw) and then calculate pCO₂ of surface water (pCO₂sw) and pCO₂ of thermocline water (pCO₂tw) over the last glacial cycle. Additionally, Mg/Ca ratios and δ¹⁸O were measured in G.ruber and P. obliquiloculata to reconstruct seawater temperature and salinity. The difference of pCO₂ between surface water and atmosphere (ΔpCO₂sw-atm) were positive during Holocene and from MIS5.1 to MIS5.4, implying that the southern SCS was the source of atmospheric CO₂. The Holocene result coincides with the modern observation. During the last glacial period, the southern SCS became the sink of atmospheric CO₂, indicated from the negative ΔpCO₂sw-atm values.

We also discovered that ΔpCO₂sw-atm, the difference of pCO₂ between thermocline and surface water (ΔpCO₂tw-sw) and thermocline water temperature (TWT) have similar change trend, presenting obvious 20,000-year precession cycle. Therefore, we regard TWT as one of the dominant elements effecting the SCS to absorb or release CO₂. When TWT were lower during glacial time, the mixed layer was able to dissolve more CO₂, with larger ΔpCO₂tw-sw, and the surface water tended to absorb CO₂ from the atmosphere. Besides, weakened deep ventilation involving vertical mixing and advection in the SCS during the last glacial period was also beneficial for the southern SCS to become the sink of atmospheric CO₂.