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

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 o r g , is an important process in the CO 2 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 δ 1 3 C D I C 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 δ 1 3 C of C o r g (-21‰). Dissolution of CaCO 3 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 1 3 C-enriched carbon in pore waters might be diffusion of bottom-water CO 3 2- ion into the sediments, or the diffusion of 1 3 C-depleted H 2 CO 3 * from pore waters to bottom water.