Carbon isotopic disequilibrium between seawater and air in the coastal Northern South China Sea over the past century

Abstract Six coastal sediment cores collected from the Northern South China Sea were dated by 210 Pb and analyzed for stable carbon and oxygen isotope composition of planktonic foraminifer Globigerinoides ruber (δ 13 C Gr and δ 18 O Gr ). Three of these cores were located east of Hainan Island and the other three off the Pearl River Estuary (PRE). Surface seawater δ 18 O and dissolved inorganic carbon (DIC) δ 13 C were identified as the dominant factors controlling downcore variations of δ 18 O Gr and δ 13 C Gr , respectively. Results of δ 13 C Gr were then used to study surface water δ 13 C DIC and its relation to δ 13 C of atmospheric CO 2 (δ 13 C atm ) over the past century. Downcore records showed rather constant δ 13 C Gr in cores off Hainan Island, but moderate decreases of δ 13 C Gr , at rates between −0.006‰ and −0.009‰ per year, in cores off the PRE. Isotopic disequilibrium between δ 13 C DIC and δ 13 C atm was observed, with δ 13 C DIC apparently higher than expected at equilibrium with δ 13 C atm except at the site closest to the PRE. The relatively steady δ 13 C Gr values east of Hainan Island were explained by balanced vertical mixing and biological pump, whereas the moderate δ 13 C Gr decreases with time off the PRE were attributable to fluvial input of terrestrial carbon.