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-atmosphere carbon reservoir has resulted in a decrease in the carbon isotopic composition of the surface ocean. In this paper we examine differences between the carbon isotope composition of planktonic foraminifera from sediment traps with older late Holocene material to estimate the 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 that foraminifera recovered in sediment traps are representitive of foraminifera deposited in the sediments. However, carbon isotopes recorded in planktonic foraminifera from recent trap material reveal a decrease in the delta(13)C composition of all species relative to the core-top samples, although the magnitude of the decrease varies significantly between species. Surface-dwelling foraminifera (G. ruber, G. bulloides) 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 water coral and sponge records for the decrease in the delta(13)C composition of dissolved organic carbon in surface waters which has taken place since the start of the anthropogenic emissions. The magnitude of decrease in the delta(13)C of surface waters suggested by these records is also close to that predicted by global carbon cycle models. Deeper, thermocline dwelling foraminifera such as G. inflata however, show Delta delta(13)C values (0.2 parts per thousand) which are substantially lower than their surface-dwelling counterparts. This reduction in Delta delta(13)C with increasing depth habitat within the mixed layer may be related to limited vertical penetration of anthropogenic CO2 as a result of the strong upwelling which is observed in the Eastern Atlantic along the Northwest African margin.