Sulfate reduction rates in Peru Margin sediments: From 1 cm to 100 m below seafloor

The Peruvian margin is one of the biologically most productive regions in the world ocean. The oxygen minimum zone impinges on the seabed over the outer continental shelf and across the upper slope to about 700 m water depth. Due to the partial or complete anoxia, microbial sulfate reduction is a predominant, terminal pathway of organic matter oxidation in the sediment. Sulfate reduction rates (SRR) were measured by 35 S-radiotracer technique in multiple sediment cores retrieved by multicorer to 0.3 m depth, by gravity corer to 5 m depth, and by advanced piston corer (ODP, Ocean Drilling Program) to > 100 m depth. Hydrostatic pressure, back-reactions and experimental handling effects were considered and found to have small effect on measured rates. Therefore, the 35 S-SRR yielded discrete data on organic carbon degradability from 1 cm to 100 m sediment depth in the Peru Margin. For all three coring intervals, SRR dropped with depth according to a power law. The mean SRR in the sulfatic zones of the sediment dropped more than a million-fold, from > 100 nmol SO 4 2- cm −3 d -1 at 1 cm depth to < 10 -4 nmol SO 4 2- cm −3 d -1 in million-year-old sediment at 100 m depth. Even in the deep methanic zones of ODP cores active sulfate reduction was detected with rates of 10 -7 -10 -4 nmol SO 4 2- cm −3 d -1 and a sulfate turnover time of up to a million years. Half of the entire sulfate reduction in the continental shelf and upper slope sediments off Peru down to 100 mbsf took place in the top 5 cm. Such an extreme skewing towards the sediment surface is not detectable by diffusion–reaction modeling of sulfate. Nevertheless, the 35 S-SRR matched the estimated flux of sulfate and burial rate of organic carbon in ODP cores beneath 1.5 m sediment depth. The depth-integrated areal SRR in the upper 0.2 m peaked at 85 m water depth with a high rate of 12 mmol SO 4 2- m −2 d -1 and dropped 40-fold to 0.3 mmol SO 4 2- m −2 d -1 at 800 m, at which depth nitrate reduction expectedly dominates. The mean turnover time of the total organic carbon pool in the top 0.05 m of sediment increased nearly 100-fold, from < 50 years on the inner shelf to a few thousand years on the upper slope. Beneath the seafloor, it increased steeply to 30 million years at 100 m sediment depth.