Increase in methane flux and anaerobic oxidation of methane with iron and manganese oxides recorded in methane-derived carbonate nodules in the eastern margin of the Sea of Japan
Abstract
We studied the evolution of pore water geochemistry recorded in methane-derived carbonate nodules containing aggregates of aragonite. We used two samples from a gas hydrate area in the eastern margin of the Sea of Japan, where an increase in methane flux due to sea level drop depressurization during the last glacial epoch was expected. One sample has ellipsoidal and dike-like aggregates where aragonite needles occur radially and as opposed bands, respectively. The other has a belt-like aggregate in which a microcrystalline zone and needle aragonite zone border on each other. For the dike-like and belt-like aggregates, the carbon isotope composition (δ13C) of aragonite is approximately -20 ‰VPDB, which is similar to the δ13C of dissolved inorganic carbon (DIC) currently found in the sulfate-methane transition (SMT). Because changes in the δ13C of DIC within the SMT is improbable at the sampling site, the aragonites are believed to have precipitated around the paleo SMT and record the pore water geochemistry of the paleo SMT. In the ellipsoidal aggregate, the aragonite δ13C gradually changes from -4 ‰ VPDB at the ellipsoid center to -12 ‰ at the rim and -23 ‰VPDB in the surrounding matrix. Because depth profile of DIC δ13C has negative peak at the SMT and the δ13C of DIC becomes higher with distance from the SMT, this δ13C trend indicates a gradual change in SMT depth relative to the ellipsoidal aggregate in sediments. Laser ablation ICP-MS traverses (a few hundred μm interval) revealed gradually increasing barium concentration from rim to core in the ellipsoidal aggregate and fluctuations in rare earth elements (REEs) and manganese (Mn) abundances. These variations are consistent with a scenario of increasing methane flux and ensuing microbial anaerobic oxidation of methane (AOM) with iron (Fe) and Mn oxides and abiotic reduction of Mn oxide with AOM-derived Fe2+. The ellipsoidal aggregate thus preserves the evolution of pore water geochemistry caused by increased methane flux. Traverses across the dike-like and belt-like aggregates show no Mn peak and have approximately one order of magnitude lower REE concentrations than the ellipsoidal aggregate. The SMTs which geochemistry was recorded in these aggregates was distant from Fe- and Mn-cycling depths or in a steady state with respect to Fe and Mn redox. Gradual depletion of REEs in the older part of the belt-like aggregate is a record of the transition toward a redox steady state.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS43A1801H
- Keywords:
-
- 0404 BIOGEOSCIENCES / Anoxic and hypoxic environments;
- 1050 GEOCHEMISTRY / Marine geochemistry