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Energies 2015, 8(3), 1561-1583; doi:10.3390/en8031561

Deep Sediment-Sourced Methane Contribution to Shallow Sediment Organic Carbon: Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico

1
Naval Research Laboratory, Washington, DC 20735, USA
2
Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA
3
Department of Geology, University of Maryland, College Park, MD 20742, USA
4
Oceanography Department, US Naval Academy, Annapolis, MD 21402, USA
Current Address: Department of Physical and Environmental Sciences, Texas A&M University—Corpus Christi, Corpus Christi, TX 78412, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Enrico Sciubba
Received: 23 August 2014 / Accepted: 11 February 2015 / Published: 18 February 2015
(This article belongs to the Special Issue Coastal Ocean Natural Gas Hydrate 2014)
View Full-Text   |   Download PDF [2484 KB, uploaded 17 March 2015]   |  

Abstract

Coastal methane hydrate deposits are globally abundant. There is a need to understand the deep sediment sourced methane energy contribution to shallow sediment carbon relative to terrestrial sources and phytoplankton. Shallow sediment and porewater samples were collected from Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico near a seafloor mound feature identified in geophysical surveys as an elevated bottom seismic reflection. Geochemical data revealed off-mound methane diffusion and active fluid advection on-mound. Gas composition (average methane/ethane ratio ~11,000) and isotope ratios of methane on the mound (average δ13CCH4(g) = −71.2‰; D14CCH4(g) = −961‰) indicate a deep sediment, microbial source. Depleted sediment organic carbon values on mound (δ13CSOC = −25.8‰; D14CSOC = −930‰) relative to off-mound (δ13CSOC = −22.5‰; D14CSOC = −629‰) suggest deep sourced ancient carbon is incorporated into shallow sediment organic matter. Porewater and sediment data indicate inorganic carbon fixed during anaerobic oxidation of methane is a dominant contributor to on-mound shallow sediment organic carbon cycling. A simple stable carbon isotope mass balance suggests carbon fixation of dissolved inorganic carbon (DIC) associated with anaerobic oxidation of hydrate-sourced CH4 contributes up to 85% of shallow sediment organic carbon. View Full-Text
Keywords: methane; advection; geochemistry; carbon isotopes; sediment carbon methane; advection; geochemistry; carbon isotopes; sediment carbon
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Coffin, R.B.; Osburn, C.L.; Plummer, R.E.; Smith, J.P.; Rose, P.S.; Grabowski, K.S. Deep Sediment-Sourced Methane Contribution to Shallow Sediment Organic Carbon: Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico. Energies 2015, 8, 1561-1583.

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