Depth distributions of alkalinity, TCO2 and d13CTCO2 at SEATS time-series site in the northern South China Sea

Abstract

In this study, measurements of titration alkalinity (TA), total dissolved carbon dioxide (TCO2), and d13C of TCO2 (d13CTCO2 ) throughout the water column at the SouthEast Asian time-series study (SEATS) site were investigated in order to understand better the fundamental processes controlling their vertical distributions in the South China Sea (SCS). The linear correlations between TA and salinity in the shallow waters, as identified by the mixing line between the surface water and salinity maximum water suggested the predominant control of physical mixing on the variability of TA. In contrast, TCO2 and d13CTCO2 showed the non-conservative behavior in the respective TCO2 and d13CTCO2 vs. salinity plot due to the effect of biological production. A stoichiometric model further showed that the depth profile of NTA ( ¼ TA salinity/35) largely reflects the increase of preformed NTA in the shallow waters, whereas carbonate dissolution was responsible for the continuous increase of NTA in the deep waters. A one-dimensional diffusion–advection model further revealed that the carbonate dissolution could account for 28% of NTCO2 ( ¼ TCO2 salinity/35) increase in deep waters, and the remaining 72% of NTCO2 was from organic decomposition. Calculation of excess TA further showed that it emerged well above the aragonite and calcite saturation depths at 600 and 2500 m, respectively, indicating that some biologically, chemically, and physically-mediated processes must be involved to provide excess TA into the shallow waters. The decrease in d13CTCO2 with depth primarily resulted from organic decomposition. The influence of anthropogenic CO2 throughout the water column was assessed with the carbon chemistry and the isotope-based approach in this study. Both methods obtained nearly the same results in which the signal of anthropogenic CO2 decreased exponentially with depth, and its penetration depth were found to be at 1000 m. The inventory of anthropogenic CO2 in the water column was estimated to be 16.6 mol C m 2 , which was less than that reported in the northwest Pacific at the same latitude, presumably due to the enhanced upwelling in the SCS. Such an anthropogenic CO2 penetration had led to decreases of the saturation levels of aragonite and calcite by 17% and 14%, respectively, in the surface water, and an upward migration of aragonite saturation depth by 100 m since industrial revolution. r 2007 Elsevier Ltd. All rights reserved.