Planktonic foraminiferal δ¹³C records from Southern Ocean sediment traps: New estimates of the oceanic Suess effect

Abstract

The carbon isotopic composition is measured for three species of planktonic foraminifera (Globigerina bulloides, Globorotalia inflata and Neogloboquadrina pachyderma (s.)) from Southern Ocean sediment traps. The sediment traps represent the annual flux of foraminifera in Subtropical to Polar Frontal environments from the western Pacific/Southern Australia sector. Comparison between the seasonal δ13C composition of the foraminifera and estimated δ13C of dissolved inorganic carbon (DIC) allows disequilibrium effects to be determined. Disequilibrium exhibits a latitudinal trend, with greatest offsets from equilibrium at lower latitudes. This effect causes a north to south increase in foraminiferal δ13C , while the δ13CDIC displays a decrease across these latitudes. Disequilibrium in G. bulloides can be accounted for by changes in temperature. The relationship between disequilibrium and temperature which we derive in this field study is consistent with the laboratory relationship of Bemis et al. [2000]. Corrected δ13C for G. bulloides is closely correlated to seasonal changes in nutrients at each site, indicating the utility of G. bulloides δ13C as a nutrient tracer in Southern Ocean environments. Comparison between flux-weighted sediment trap values and nearby core tops indicates a modern depletion in δ13C , which we attribute to the oceanic Suess effect. The imprint of this effect on the foraminiferal isotopes provides further evidence for the equilibration between surface waters and the atmosphere in the Subantarctic Zone.