Stable carbon isotopes in pore waters of coastal marine sediments (The Gulf of Trieste, N Adriatic)

Nives Ogrinc,^a* and Jadran Faganeli^b

^a Dept. Environ. Sci., “Jozef Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia

^b Marine Biological Station, National Institute of Biology, Fornače 41, 6330 Piran, Slovenia

Abstract

The sources of dissolved inorganic carbon (DIC) were determined in pore waters of coastal marine sediments in the Gulf of Trieste, Northern Adriatic Sea, using stable carbon isotopes. The negative correlation between concentration and isotopic composition of DIC indicates that the oxidation of organic carbon, C_org, is an important process in the CO₂ formation. A diffusion-reaction-exchange model that accurately described the profiles of DIC concentrations at this site could not be used to reproduce the observed isotopic profiles. The values of d¹³C_DIC added to the sediment pore waters calculated using a mixing model for non-methanogenic sediments were between -3.8 and –13.7‰, significantly greater than d¹³C of C_org (-21‰). Dissolution of CaCO₃ was estimated to contribute 35-57% to DIC in winter and late spring and 14% in late summer when the degradation of organic matter was the highest. Another source of ¹³C-enriched carbon in pore waters might be diffusion of bottom-water CO₃^2– ion into the sediments, or the diffusion of ¹³C-depleted H₂CO₃* from pore waters to bottom water.