Determination of organic carbon remineralization in near-shore marine sediments (Gulf of Trieste, Northern Adriatic) using stable carbon isotopes

The fate of metabolizable Corg was studied and the Corg budget was constructed in surficial sediments of the Gulf of Trieste (Northern Adriatic), characterized by high carbonate content, bioturbation and benthic microalgal productivity. Isotopic composition of DIC benthic fluxes (δ13CJ-DIC values) measured in flux chambers over the year was used to differentiate between Corg respiration and carbonate dissolution. The δ13CJ-DIC values determined at the sediment-water interface, using laboratory and in situ incubated benthic chambers, indicated that DIC benthic fluxes originate principally from the degradation of sedimentary organic matter. The contribution of carbonate dissolution to DIC benthic fluxes was important during the summer period, up to 1/4 of DIC, when the oxidation of sulfides formed strong sulfuric acid. Summer metabolic dissolution of carbonates in these carbonate-supersaturated bottom waters was supported also by benthic Ca effluxes. In other periods, the possible adsorption of hydration species on carbonate surfaces led to Ca benthic influxes. It was found that the recycling efficiency estimated from DIC fluxes and Corg burial flux, was high, >90%. Comparison between DIC benthic fluxes and O2 consumption revealed similar values. Considering sulfate reduction and S burial in these sediments, about 2/3 of DIC production is derived from sulfate reduction, and about 2/3 of O2 consumption could occur in sulfide oxidation to sulfate.