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 fue l 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 wate r 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 cor relation 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, del ta(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, respe ctively. No isotopic equilibrium between surface water DIC and atmosph eric CO2 was observed, either during the pre-industrial steady state o r during the last 100 years. The lower amount of depletion in the surf ace water delta(13)C(DIC) With respect to the atmospheric anthropogeni c signal is explained by the dilution of the surface waters by biologi cally altered subsurface water DIC. The lower delta(13)C decrease in t he Coral Sea points to a stronger influence of the subsurface water so urce compared to the Caribbean.