Carbon isotopes in recent planktonic foraminifera: A record of anthropogenic CO 2 invasion of the surface ocean

Abstract

The release of isotopically light anthropogenic CO 2 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 δ 13C ( Δδ 13C) in Eastern Atlantic surface waters as a result of the invasion of anthropogenic CO 2. 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 δ 13C 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 Δδ 13C values of −0.53‰ to −0.62‰, which is similar to other estimates from shallow water coral and sponge records for the decrease in the δ 13C 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 δ 13C 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 Δδ 13C values (0.2‰) which are substantially lower than their surface-dwelling counterparts. This reduction in Δδ 13C with increasing depth habitat within the mixed layer may be related to limited vertical penetration of anthropogenic CO 2 as a result of the strong upwelling which is observed in the Eastern Atlantic along the Northwest African margin.