Temperature dependence of organic matter remineralization in deeply-buried marine sediments

Assuming that the temperature dependence of sediment organic matter remineralization can be described by the Arrhenius equation, organic matter that is highly refractory at seafloor temperatures (similar to 2-3 degrees C) should become more reactive at sediment depths of several hundred meters due to burial and heating by the natural geothermal gradient. Results obtained using a coupled non-linear reactive-transport model support this suggestion. For deeply-buried marine sediments (i.e., those found 100s of meters below the seafloor) model results predict the occurrence of a deep zone of methanogenesis that is separated by a relatively thick region in which methane diffuses upwards to be oxidized by downward diffusing sulfate. This depth zonation of biogeochemical processes in deeply-buried sediments is in sharp contrast to that observed in nearshore marine sediments where sulfate reduction and methanogenesis generally occur in much closer vertical proximity. Model results presented here also provide a simple mechanistic explanation for the occurrence of linear pore water sulfate profiles that are common in many continental margin sediments. Linear sulfate gradients are often taken as an indirect indicator of gas hydrate occurrence although results presented here suggest that this does not necessarily always have to be the case. Application of the model to gas hydrate-containing sediments on the Blake Ridge yields results that are in good agreement with sediment and pore water data, and microbial rate studies in these sediments. Model results also suggest that in situ methanogenesis in and/or near the gas hydrate stability zone may play an important role in supplying the methane found in Blake Ridge gas hydrates. (C) 2011 Elsevier B.V. All rights reserved.