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Energies 2014, 7(8), 5332-5356; doi:10.3390/en7085332

Contribution of Vertical Methane Flux to Shallow Sediment Carbon Pools across Porangahau Ridge, New Zealand

1
Naval Research Laboratory, Washington, DC 20375, USA
2
Department of Environmental Science and Policy, George Mason University, Fairfax, VA 22030, USA
3
United States Naval Academy, Annapolis, MD 21402, USA
4
Naval Research Laboratory, National Research Council, Washington, DC 20375, USA
5
Department of Geology, University of Maryland College Park, College Park, MD 20742, USA
6
Hawaii Natural Energy Institute, University of Hawaii, Honolulu, HI 96822, USA
7
School of Environment, University of Auckland, Auckland 1142, New Zealand
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.
Received: 12 June 2014 / Revised: 4 August 2014 / Accepted: 11 August 2014 / Published: 18 August 2014
(This article belongs to the Special Issue Coastal Ocean Natural Gas Hydrate 2014)
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Abstract

Moderate elevated vertical methane (CH4) flux is associated with sediment accretion and raised fluid expulsion at the Hikurangi subduction margin, located along the northeast coast of New Zealand. This focused CH4 flux contributes to the cycling of inorganic and organic carbon in solid phase sediment and pore water. Along a 7 km offshore transect across the Porangahau Ridge, vertical CH4 flux rates range from 11.4 mmol·m−2·a−1 off the ridge to 82.6 mmol·m−2·a−1 at the ridge base. Stable carbon isotope ratios (δ13C) in pore water and sediment were variable across the ridge suggesting close proximity of heterogeneous carbon sources. Methane stable carbon isotope ratios ranging from −107.9‰ to −60.5‰ and a C1:C2 of 3000 indicate a microbial, or biogenic, source. Near ridge, average δ13C for pore water and sediment inorganic carbon were 13C-depleted (−28.7‰ and −7.9‰, respectively) relative to all core subsamples (−19.9‰ and −2.4‰, respectively) suggesting localized anaerobic CH4 oxidation and precipitation of authigenic carbonates. Through the transect there was low contribution from anaerobic oxidation of CH4 to organic carbon pools; for all cores δ13C values of pore water dissolved organic carbon and sediment organic carbon averaged −24.4‰ and −22.1‰, respectively. Anaerobic oxidation of CH4 contributed to pore water and sediment organic carbon near the ridge as evidenced by carbon isotope values as low as to −42.8‰ and −24.7‰, respectively. Carbon concentration and isotope analyses distinguished contributions from CH4 and phytodetrital carbon sources across the ridge and show a low methane contribution to organic carbon. View Full-Text
Keywords: coastal sediment; carbon cycling; methane; phytodetritus; convergence forcing; stable isotopes coastal sediment; carbon cycling; methane; phytodetritus; convergence forcing; stable isotopes
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Coffin, R.B.; Hamdan, L.J.; Smith, J.P.; Rose, P.S.; Plummer, R.E.; Yoza, B.; Pecher, I.; Montgomery, M.T. Contribution of Vertical Methane Flux to Shallow Sediment Carbon Pools across Porangahau Ridge, New Zealand. Energies 2014, 7, 5332-5356.

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