Carbon Isotope Composition of Caribbean Sea Surface Waters: Response to the Uptake of Anthropogenic CO2

The burning of fossil fuels and deforestation have significantly increased atmospheric CO₂ levels, from ~280 ppm prior to the industrial revolution to the present value of ~390 ppm. Suess (1955) was the first to show that the carbon isotopic composition of the atmosphere is changing in response to the anthropogenic input of radiocarbon-dead, ¹³C depleted CO₂ from fossil fuel combustion. While biological processes may be the primary mechanism controlling the δ¹³C of marine carbon, air-sea gas exchange should allow for the transfer of the so-called ¹³C Suess effect (Keeling, 1979) from the atmosphere to the surface ocean. The first measurements of the δ¹³C of dissolved inorganic carbon of ocean surface waters were made in 1970 (Kroopnick, 1974) and serve as a baseline for assessing how the carbon isotopic composition of the oceans have changed in response to the invasion of fossil fuel CO₂ over the last 40 years. However, in order to gain a longer and more complete picture of the marine δ¹³C Suess effect, we must rely on indirect measures of changes in surface water δ¹³C, most notably those preserved in carbonate secreting marine organisms. The Cariaco Basin is one of the rare locations where one can directly compare sediment records with modern instrumental data. Here we report a high resolution planktonic foraminiferal δ¹³C record from the basin for the last 300 years that clearly resolves the timing and magnitude of the marine ¹³C Suess effect associated with the oceanic uptake of anthropogenically derived CO₂. Cariaco Basin sediment trap and upper-most box core sediment δ¹³C match both the trend and magnitude of observed δ¹³C changes in atmospheric CO₂ over the last 15 years. The longer sediment record suggests the marine Suess effect to be -0.75 % from pre-industrial values, with most of the change occurring since 1950, coincident with the rapid rise in atmospheric CO₂ noted in ice core and instrumental data. If the current anthropogenic CO₂ emission trend continues, extrapolating our marine δ¹³C rate curve into the future suggests that the rate of marine δ13C change caused by anthropogenic CO₂ will increase to -0.10 % yr^-1 by the end of this century, an increase of more than an order of magnitude from 1950 values.