Carbon budget in the East China Sea in spring

Abstract

Results of total dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements made in the East China Sea (ECS) during a geochemical expedition of KEEP (Kuroshio Edge Exchange Processes) program in May of 1996 show that ECS is a CO2 sink during the spring season. The mean diVerence of fCO2 (fugacity of CO2) between the atmosphere and surface water is calculated to be 28 matm, and the resulting net CO2 invasion flux is 2.1 mol m-2 yr-1, which gives about 0.03 GtC/ yr of CO2 uptake in this continental shelf in spring. This study supports the notion that the shelf regions can be a significant CO2 sink. The riverine alkalinity, which discharges into ECS, is estimated to be 1,743 μmol/kg on the basis of a linear relationship between TA and salinity. The observed salinity-normalized alkalinity in ECS is higher than that in the open sea, and this excess alkalinity is estimated to be 42 μmol/kg-1. With the known rate of the Changjiang discharge, this excess TA gives a mean residence time of 1.2 years for the continental shelf water in the ECS. The DIC in the ECS is also found to be higher than that in the open sea. This excess DIC is estimated to be about 76 ± 70 μmol/kg-1, which is equal to a net carbon input to ECS of 3.9 ± 3.6 mol m-2 yr-1. Based on the riverine alkalinity input, the equivalent riverine carbon flux from Changjiang discharge is estimated to be about 1.8 mol m-2 yr-1. With net CO2 invasion flux of 2.1 ± 2.8 mol m-2 yr-1, the remaining 0 ± 4.6 mol m-2 yr-1 could come from remineralization of organic matter derived from biological pump in the shelf or terrestrial sources. Although this preliminary carbon budget implies that gas exchange and riverine input are the main sources of excess carbon in ECS, the contribution of biological carbon flux can not be ruled out because of the large uncertainty associated with these estimates.