Carbon isotope records from extant Caribbean and South Pacific sponges: Evolution of δ13C in surface water DIC

Abstract

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 δ13CDIC from pre-industrial times (early 19th century) to the present. We observed similar shapes and ranges in δ13C 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 δ13CCO2. δ13CDIC values for Caribbean and Coral Sea surface waters were calculated using the δ13C sponge records. While δ13C of atmospheric CO2 decreased by about 1.4‰ from the early 19th century to 1990, δ13CDIC of Caribbean and Coral Sea surface waters decreased by 0.9±0.2‰ and 0.7±0.3‰, 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 δ13CDIC with respect to the atmospheric anthropogenic signal is explained by the dilution of the surface waters by biologically altered subsurface water DIC. The lower δ13C decrease in the Coral Sea points to a stronger influence of the subsurface water source compared to the Caribbean.