A kinetic-model approach to quantify the effect of mass transport deposits on pore water profiles in the Krishna–Godavari Basin, Bay of Bengal

Abstract

Mass transport deposits (MTDs) in the Krishna–Godavari basin from the eastern Indian margin are common sedimentary features over the modern continental slope. Quantitative understanding of the scale and age of these MTDs is relevant to understanding their triggering mechanisms and environmental feedbacks, as well as for interpreting pore water chemical profiles. Pore water profiles in sediments recovered during the 2006 Indian National Gas Hydrate Project (NGHP-01) expedition suggest that MTDs are present at seven of the ten sites cored in the Krishna–Godavari basin. Kinetic modeling of the S-shaped pore water sulfate and ammonium profiles from these sites provides quantitative estimates of the individual MTDs thickness, time elapsed after the event, rate of organic matter-fueled sulfate reduction, and time required to reach a new steady state. Model results suggest that the MTDs at the seven study sites are 8–25 m thick and are 300–1600 years old. Within the MTD sections, the organic matter-fueled sulfate reduction rates are 130–1200 mmol/m2/yr and the time needed to reach a new steady state ranges from 2000 to 3800 years. In comparison to depth-integrated sulfate reduction rates estimated in other regions, our estimates are relatively high reflecting the much thicker sulfate reduction zone that results from the MTDs. A positive correlation is observed between water depth and the MTD activity index defined as thickness of MTD package divided by its emplacement age. This correlation suggests the close relationship between MTD activity and water depth which agrees with previous studies of MTDs in this region and elsewhere. We caution against using the pore water data from sites experiencing significant MTDs to infer metabolic processes and to quantify steady-state reaction rates as our model results confirm the remarkable influence of transient MTDs on pore water profiles.