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

Moderate elevated vertical methane (CH 4 ) 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 CH 4 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 CH 4 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 (δ 13 C) 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 δ 13 C for pore water and sediment inorganic carbon were 13 C-depleted (−28.7‰ and −7.9‰, respectively) relative to all core subsamples (−19.9‰ and −2.4‰, respectively) suggesting localized anaerobic CH 4 oxidation and precipitation of authigenic carbonates. Through the transect there was low contribution from anaerobic oxidation of CH 4 to organic carbon pools; for all cores δ 13 C values of pore water dissolved organic carbon and sediment organic carbon averaged −24.4‰ and −22.1‰, respectively. Anaerobic oxidation of CH 4 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 CH 4 and phytodetrital carbon sources across the ridge and show a low methane contribution to organic carbon.

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