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A 300 m long depth profile of metabolic activity of sulfate‐reducing bacteria in the continental margin sediments of South Australia (ODP Site 1130) derived from inverse reaction‐transport modeling

Reaction‐transport modeling of dissolved species in interstitial water allows for the inversion of transport processes and thus facilitates the detailed investigation of signals which are usually blurred by diffusion and advection. Here I present a case study from the South Australian continental margin (ODP Leg 182, Site 1130) where I use reaction‐transport modeling to derive a depth transect of volumetric sulfate reduction rates. Site 1130, located on the shelf slope in 500 m deep water, is of special interest as an upwelling sulfate‐rich brine allows for an extended sulfate reduction zone which reaches to a depth of at least 300 mbsf. The obtained reduction rates vary from 600 pmol/cm−3 yr−1 at 30 mbsf to 63 pmol/cm−3 yr−1 at 300 mbsf. The depth‐integrated sulfate consumption equals 65 × 10−6 mol/yr cm−2, which is similar to other shelf slope settings without advecting sulfate. This suggests that the primary control on sulfate reduction rates is organic matter reactivity, rather than sulfate availability. However, similar to other ODP Leg 182 sites, the interstitial water chemistry in the upper 30 mbsf is inconsistent with a diffusive/advective transport system. While the actual process causing this remains elusive, pyrite burial rates from this zone suggest that sulfate reduction rates in this zone are at least 60 times higher than those derived from reaction‐transport modeling assuming diffusion and advection alone.