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A significantly steeper methane gradient at the SMT would imply a depth to the top of gas hydrate occurrence that is significantly shallower than that observed at Site U On the other hand, the methane gradient could be somewhat greater than the 2. Therefore, we conservatively estimate that at Site U AOM consumes at least about half of the sulfate in the pore water.
In this section we quantify the effect of different sedimentation rates and different SMT depths to determine if the widely observed absence of methane in the SRZ generally implies that AOM is active. This question is worth exploring because a high enough sedimentation rate could result in low methane concentrations in the SRZ even in the absence of the methane sink due to AOM [ Berner , ]. The methane gradient at the seafloor is very close to the gradient at the base of the SRZ Figure 5.
If the rate of pore fluid burial measured by the sedimentation rate is high enough, it will overcome diffusion, resulting in little or no methane in the SRZ [ Berner , ]. Conversely, a low sedimentation rate results in a small Pe , a methane gradient at the seafloor close to that at the base of the SRZ, and hence methane present in the SRZ.
To get a methane profile approaching that observed at Site U without active AOM, the Peclet number must be raised to at least 5 Figure 9c , which requires an unrealistic sedimentation rate that is more than seventy times that estimated at Site U As the Peclet number is a dimensionless quantity that depends both on the depth of the SMT and the sedimentation rate, the results of Figure 9 can be extended to a range of SMT depths and sedimentation rates Figure It will be only when the upward flux of methane balances the downward flux of sulfate [ Borowski et al. Also, the observation of a linear sulfate gradient from the seafloor to the SMT does not by itself imply that AOM is the dominant sulfate reduction process, as proposed by Borowski et al.
Knowing the fraction of sulfate that serves as the terminal electron acceptor for either the anaerobic oxidation of methane AOM or organoclastic sulfate reduction OSR is important for estimating how much methane may be consumed by AOM and establishing whether sulfate gradients may be indicators of gas hydrate in the underlying sedimentary system.