Porewater dissolved organic and inorganic carbon in relation to methane occurrence in sediments of the Gdańsk Basin (southern Baltic Sea)

The impact of methane occurrence on carbon forms dissolved in pore waters was investigated in the present study. The cores were collected at three geochemically different stations in the Gdańsk Basin area (southern Baltic Sea). Sediments were analyzed for methane, total particulate carbon (TPC) and particulate organic carbon (POC). In pore waters extracted using Rhizon samplers the following parameters were measured: sulfate, total dissolved sulfide (TDS), dissolved inorganic (DIC) and organic carbon (DOC). Concentrations of DIC and DOC in pore water profiles were up to 5 times higher at stations with methane compared to the station without methane in shallow sediments. Average fluxes of DIC and DOC were calculated for three separate sediment layers at each station (surface layer - benthic flux, flux in sulfate-methane transition (SMT) or in intermediate layer and flux below SMT/intermediate). Benthic DIC and DOC fluxes were highest at one of the stations with shallow SMT (7-20 cm): 8.71 mmol m−2 d−1 and max. 1.48 mmol m−2 d−1, respectively. Additionally, we found strong dependences between DIC and DOC in sediments of stations with methane (R2 = 0.98). The relationship between methane and DOC in sediment profiles was described by exponential regression (R2 = 0.83). The contribution of particulate inorganic carbon (PIC) in TPC was significantly higher at the methane station with highest DIC concentration and fluxes.