Sulfate reduction and methane oxidation in continental margin sediments influenced by irrigation (South-East Atlantic off Namibia)

Sulfate reduction rates (SRR) and concentrations of SO42-, H2S, pyrite sulf ur, total sulfur, CH4, and organic carbon were measured with high depth res olution through the entire length of the SO42--zone and well into the CH4-z one at two continental slope stations in the eastern South Atlantic (Bengue la upwelling area). The sediments were characterized by a high organic carb on content of approx. 7.5% at GeoB 3703 and 3.7% at GeoB 3714. At GeoB 3703 SO42- concentrations decreased linearly with depth to about 40 mu M at the sulfate-methane transition zone (SMT) at 3.5 m, while at GeoB 3714, SO42- remained at sea water concentration in the top 2 m of the sediment and then decreased linearly to about 70 mu M at the SMT at 6 m. Direct rate measure ments of SRR ((SO42-)-S-35) showed that the highest SRR occurred within the surface 3-5 cm with peak rates of up to 20 and 7 nmol SO42- cm(-3) day(-1) at GeoB 3703 and GeoB 3714, respectively. SRR decreased quasi-exponentiaIl y with depth at GeoB 3703 and the cumulative SRR over the length of the SO4 2- zone resulted in an areal SRR (SRRarea) of 1114-3493 mu mol m(-2) day(-1 ) (median value: 2221 mu mol m(-2) day(-1)) at GeoB 3703 with more than 80% of the total sulfate reduction proceeding in the top 30 cm sediment. At Ge oB 3714 SRR exhibited more scatter with a cumulative SRRarea of 398-1983 mu mol m(-2) day(-1) (median value: 1251 mu mol m(-2) day(-1)) and with >60% of the total sulfate reduction occurring below a depth of 30 cm due partial ly to a deeply buried zone of sulfate reduction located between 3 and 5 m d epths. SRR peaks were also observed in SMT of both cores, ostensibly associ ated with methane oxidation. but with rates about 10 times lower than at th e surface. Modeled SRR balanced both methane oxidation rates and measured S RR within the SMT, but severely underestimated by up to 89% the total SRRar ea that were obtained from direct measurements. Modeled and measured SRR we re reconciled by including solute transport by irrigation described by a no n-local pore water exchange function (a) which had values of up to 0.3 year (-1) in the top sediment, and decreased exponentially to zero (i.e., no irr igation) at 2-3 meters (i.e., above SMT). These results suggested that co-e xisting sulfate reduction processes and linear SO42--gradients can be maint ained 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 ha d been inhabited. Copyright (C) 2000 Elsevier Science Ltd.