Organic matter mineralization in the deep water of the Gotland Basin (Baltic Sea): Rates and oxidant demand

Abstract

Abstract Time-series observations of total CO2 (CT) accumulation in the deep water of the Gotland Basin were initiated in 2003. The measurements were performed four to five times per year and were complemented by the determination of nutrient and O2/H2S concentrations. The major objective of those measurements was to study organic matter (OM) mineralization. This was facilitated during two periods of stagnation, from 2004 to 2006 and from 2007 to 2014, identified on the basis of temperature-salinity diagrams. In a follow-up to our assessment of the data from the first period, we present here an overall picture of CT accumulation in the deep water of the Gotland Basin. CT measurements were performed at five different depths between 150 m and the sediment surface at 235 m. Mineralization rates for three individual sub-layers were obtained from CT mass balances that took into account vertical mixing across the boundaries of the sub-layers. Mixing coefficients were calculated from the mass balances for salinity. The mass balance calculations yielded the source/sink strength of CT, O2, H2S, NO3− and NH4+ for each time interval between measurements. The accumulated release of CT was shown to increase linearly with time and to reflect consistent mean annual mineralization rates of ~2 mol m−2 for both stagnation periods, which indicated the absence of significant decadal variability. A dependency of the mineralization rate on the redox conditions in the overlying water column was not detected. For the first stagnation period, the sequential use of different oxidants, including O2, NO3− and SO42−, was identified. A determination of oxidant consumption with respect to OM mineralization yielded a mean molar ratio of 1.2 oxygen equivalents per carbon atom of OM, a distinct deviation of the OM composition described by classical Redfield stoichiometry.