Sulfate reduction and methane oxidation in continental margin sediments influenced by irrigation (South-East Atlantic off Namibia)
Sulfate reduction rates (SRR) and concentrations of SO 42-, H 2S, pyrite sulfur, total sulfur, CH 4, and organic carbon were measured with high depth resolution through the entire length of the SO 42--zone and well into the CH 4-zone at two continental slope stations in the eastern South Atlantic (Benguela upwelling area). The sediments were characterized by a high organic carbon content of approx. 7.5% at GeoB 3703 and 3.7% at GeoB 3714. At GeoB 3703 SO 42- concentrations decreased linearly with depth to about 40 μM at the sulfate-methane transition zone (SMT) at 3.5 m, while at GeoB 3714, SO 42- remained at sea water concentration in the top 2 m of the sediment and then decreased linearly to about 70 μM at the SMT at 6 m. Direct rate measurements of SRR ( 35SO 42-) showed that the highest SRR occurred within the surface 3-5 cm with peak rates of up to 20 and 7 nmol SO 42- cm -3 day -1 at GeoB 3703 and GeoB 3714, respectively. SRR decreased quasi-exponentially with depth at GeoB 3703 and the cumulative SRR over the length of the SO 42- zone resulted in an areal SRR (SRR area) of 1114-3493 μmol m -2 day -1 (median value: 2221 μmol m -2 day -1) at GeoB 3703 with more than 80% of the total sulfate reduction proceeding in the top 30 cm sediment. At GeoB 3714 SRR exhibited more scatter with a cumulative SRR area of 398-1983 μmol m -2 day -1 (median value: 1251 μmol m -2 day -1) and with >60% of the total sulfate reduction occurring below a depth of 30 cm due partially to a deeply buried zone of sulfate reduction located between 3 and 5 m depths. SRR peaks were also observed in SMT of both cores, ostensibly associated with methane oxidation, but with rates about 10 times lower than at the surface. Modeled SRR balanced both methane oxidation rates and measured SRR within the SMT, but severely underestimated by up to 89% the total SRR area that were obtained from direct measurements. Modeled and measured SRR were reconciled by including solute transport by irrigation described by a non-local pore water exchange function (α) which had values of up to 0.3 year -1 in the top sediment, and decreased exponentially to zero (i.e., no irrigation) at 2-3 meters (i.e., above SMT). These results suggested that co-existing sulfate reduction processes and linear SO 42--gradients can be maintained by a non-local transport mechanism such as irrigation, by which pore water in tubes or burrows is exchanged with bottom waters by activities of tube-dwelling animals, or some similar physical transport phenomenon (i.e., bubble ebullition). Further support for an irrigation mechanism was found in the observations of open tubes of up to 8 mm (ID) at depths down to 6 m, which also contained fecal pellets, indicating that these tubes were or had been inhabited.