Shallow Microbial Recycling of Deep-Sourced Carbon in Gulf of Cadiz Mud Volcanoes

Abstract

Based upon the molecular and isotopic composition of hydrocarbons it has been proposed that the source of CH 4 in Gulf ofCadiz mud volcanoes (MV) is a mixture of deep sourced thermogenic CH 4 and shallow biogenic CH 4 . We directly investigated this possibility by comparing porewater CH 4 concentrations and their δ 13 C values with the potential for Archaeal methanogenesis in Gulf of Cadiz mud volcano (MV) sediments (Captain Arutyunov, Bonjardim, Ginsburg and Porto) using 14 C-rate measurements. The CH 4 has a deep sourced thermogenic origin (δ 13 C ∼ −49‰) but becomes 13 C-depleted in and beneath the zone of anaerobic oxidation of methane (AOM) where the rates of hydrogenotrophic methanogenesis increase. Thus we infer that a portion of AOMproduced CO 2 is being recycled to CH 4 by methanogens yielding further 13 C-depleted CH 4 , which might be misinterpreted as indicative of a fully shallow biogenic origin for this gas. Production of H 2 is related to compositional changes in sedimentary organic matter, or to upward flux of substrate-enriched fluids. In contrast to other MVs in the Gulf of Cadiz, Ginsburg MV fluids are enriched in SO 2− 4 and contain very high concentrations of acetate (2478 µM below 150 cmbsf); however, the high levels of acetate did not stimulate methanogenesis but instead were oxidized to CO 2 coupled to sulphate reduction. Both anaerobic oxidation of thermogenic CH 4 linked to shallow methanogenesis and fluid geochemistry control the recycling of deep-sourced carbon at Gulf of Cadiz MVs, impacting near-surface δ 13 C-CH 4 values.

Spatial variation in fluid flow and geochemical fluxes across the sediment–seawater interface at the Carlos Ribeiro mud volcano (Gulf of Cadiz)

“…Some studies, based on isotope measurements, have suggested that the hydrocarbons may, in part, be derived from a shallow bacterial source (Stadnitskaia et al, 2006). However, a recent study has indicated that isotope signatures may be affected by partial recycling of thermogenic CH 4 in shallow sediments by AOM-related methanogenic archaea (Nuzzo et al, 2008).…”

Societal need for improved understanding of climate change, anthropogenic impacts, and geo-hazard warning drive development of ocean observatories in European Seas

“…Synoptically monitoring these communities along with changes in biogeochemistry, fluid flux rates, and geophysical motions should increase understanding of how chemosynthetically-driven systems utilise reducing compounds like sulphur or methane to provide energy for carbon fixation instead of using light energy. Methanotrophic microbes, for instance, act as an important sink for methane released at the seafloor (Boetius et al 2000, Boetius and Suess 2004, Niemann et al 2006a, Nuzzo et al 2008, Lichtschlag et al 2010. Chemosynthetic habitats can range from superheated waters with chemical concentrations lethal to most life (Van Dover et al 2002) to more benign areas where methane seeps from sediments and supports life (Paull et al 1985) often with important dependences on symbiotic relationships (e.g.…”

Influence of the carbon isotopic composition of methane and the proportion of methane-derived bicarbonate on the 13C/12C ratio of dissolved inorganic carbon at the sulfate–methane transition in the Joetsu Basin area, eastern margin of the Sea of Japan

“…d 13 C I.W.-CH4 , which is not significantly influenced by degassing during core recovery (Wallace et al, 2000), generally produces a concave profile below the SMT but becomes asymptotic at depth (e.g., Borowski et al, 1997;Lorenson et al, 2008;Paull et al, 2000;Ussler and Paull, 2008). In addition to methanogenesis with methane-derived HCO 3 À , reversible AOM has been proposed as the cause of the large d 13 C change immediately below the SMT (e.g., Borowski et al, 1997;Nuzzo et al, 2008;Pape et al, 2010;Pohlman et al, 2008;Yoshinaga et al, 2014). d 13 C DIC depth profiles show a negative peak at the SMT because of the inheritance of 13 C-depleted carbon from methane into HCO 3 À (e.g., Borowski et al, 1997;Coffin et al, 2008;Paull et al, 2000;Ussler and Paull, 2008).…”