Carbon isotope records from extant Caribbean and south Pacific sponges: Evolution of delta C-13 in surface water DIC

Stable isotope records of demosponges from the Caribbean and Coral Sea are described for the purpose of studying the influence of fossil fuel CO2 on the carbon isotopic composition of dissolved inorganic carbon (DIC) in surface water. The slow-growing sponges precipitate calcium carbonate in isotopic equilibrium with ambient sea water and are used to detect changes in delta(13)C(DIC) from pre-industrial times (early 19th century) to the present. We observed similar shapes and ranges in delta(13)C curves measured on Caribbean specimens collected from water depths of 25, 84 and 91 m as well as a specimen collected in shallow waters off New Caledonia. The records reveal a highly significant correlation with atmospheric delta(13)C(CO2). delta(13)C(DIC) values for Caribbean and Coral Sea surface waters were calculated using the delta(13)C sponge records. While delta(13)C of atmospheric CO2 decreased by about 1.4 parts per thousand from the early 19th century to 1990, delta(13)C(DIC) of Caribbean and Coral Sea surface waters decreased by 0.9 +/- 0.2 parts per thousand and 0.7 +/- 0.3 parts per thousand, respectively. No isotopic equilibrium between surface water DIC and atmospheric CO2 was observed, either during the pre-industrial steady state or during the last 100 years. The lower amount of depletion in the surface water delta(13)C(DIC) With respect to the atmospheric anthropogenic signal is explained by the dilution of the surface waters by biologically altered subsurface water DIC. The lower delta(13)C decrease in the Coral Sea points to a stronger influence of the subsurface water source compared to the Caribbean.