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Shallow gas charged sediments off the Indian west coast: Genesis and distribution

DRS at CSIR-National Institute of Oceanography

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Title Shallow gas charged sediments off the Indian west coast: Genesis and distribution
 
Creator Mazumdar, A.
Peketi, A.
Dewangan, P.
Badesab, F.K.
Ramprasad, T
Ramana, M.V.
Patil, D.J.
Dayal, A.M.
 
Subject shallow gases
gas masking
enhanced reflection
sulfate reduction
 
Description Geophysical and geochemical surveys were carried out off Goa, central west coast of India, to understand the genesis and distribution of shallow gases in marine sediments. Shallow gas charged sediments within the water depths of approx. 15 to 40 m are reported all along the west coast and are characterized by gas masking, high amplitude and reverse polarity reflections in high resolution seismic (sparker) data, attributed to the presence of free gas. These high amplitude reflections (gas front) are observed within a Holocene fine grained, wedge shaped sediment package overlying the maximum flooding surface. The gas front lies between 1.2 and 5 m below the seabed and demarcates the transition from gas in the dissolved phase to bubble phase when the gas concentration exceeds the saturation level. The observed and extrapolated sulfate methane transition zone (SMTZ) lies between 0.7 and 2.25 m below the seabed and it is related to the depth of the gas front. Methane concentration reaches saturation below the SMTZ, and produces a bubble phase which lowers the p-wave velocity and produces high amplitude reflection observed in the seismic data. Depleted carbon isotope ratios as low as -88.9 ppt suggest a microbial origin for the methane in the study area. High sulfate reduction rate, high concentrations of pore water methane, phosphate and ammonium in the shallowest site (Sasu123/1 at 14.7 m water depth) suggest an enhanced availability of reactive organic matter required for microbially mediated biodegradation processes. High gas flux in the shallow waters is also indicated by the shallowing of the gas front and the widening of the gas masked zone towards shore. The present investigations do not testify the origin of Holocene shallow gases either from buried channel deposits or the Pleistocene sediments
 
Date 2009-11-10T11:27:48Z
2009-11-10T11:27:48Z
2009
 
Type Journal Article
 
Identifier Marine Geology, vol.267(1-2); 71-85
http://drs.nio.org/drs/handle/2264/3423
 
Language en
 
Rights An edited version of this paper was published by Elsevier. Copyright [2009] Elsevier
 
Publisher Elsevier