CARBON ISOTOPES IN RECENT PLANKTONIC-FORAMINIFERA - A RECORD OF ANTHROPOGENIC CO2 INVASION OF THE SURFACE OCEAN

The release of isotopically light anthropogenic CO2 into the ocean-atm osphere carbon reservoir has resulted in a decrease in the carbon isot opic composition of the surface ocean. In this paper we examine differ ences between the carbon isotope composition of planktonic foraminifer a from sediment traps with older late Holocene material to estimate th e change in delta(13)C (Delta delta(13)C) in Eastern Atlantic surface waters as a result of the invasion of anthropogenic CO2. Using stable oxygen isotopes we demonstrate that average trap values for a variety of species are more or less identical to core-top values, suggesting t hat foraminifera recovered in sediment traps are representitive of for aminifera deposited in the sediments. However, carbon isotopes recorde d in planktonic foraminifera from recent trap material reveal a decrea se in the delta(13)C composition of all species relative to the core-t op samples, although the magnitude of the decrease varies significantl y between species. Surface-dwelling foraminifera (G. ruber, G. bulloid es) show Delta delta(13)C values of -0.53 parts per thousand to -0.62 parts per thousand, which is similar to other estimates from shallow w ater coral and sponge records for the decrease in the delta(13)C compo sition of dissolved organic carbon in surface waters which has taken p lace since the start of the anthropogenic emissions. The magnitude of decrease in the delta(13)C of surface waters suggested by these record s is also close to that predicted by global carbon cycle models. Deepe r, thermocline dwelling foraminifera such as G. inflata however, show Delta delta(13)C values (0.2 parts per thousand) which are substantial ly lower than their surface-dwelling counterparts. This reduction in D elta delta(13)C with increasing depth habitat within the mixed layer m ay be related to limited vertical penetration of anthropogenic CO2 as a result of the strong upwelling which is observed in the Eastern Atla ntic along the Northwest African margin.