HR: 1340h
AN: B43B-0163    [Abstracts]
TI: Methane Production In Forearc Sediments At The Costa Rican Convergent Margin
AU: * Cardace, D
EM: dcardace@wustl.edu
AF: Washington University in St. Louis, 1 Brookings Drive, Box 1169, St. Louis, MO 63130 United States
AU: Morris, J D
EM: jmorris@levee.wustl.edu
AF: Washington University in St. Louis, 1 Brookings Drive, Box 1169, St. Louis, MO 63130 United States
AU: Peacock, A
EM: apeacock@utk.edu
AF: University of Tennessee Knoxville, Center for Biomarker Analysis, 10515 Research Drive Ste 300, Knoxville, TN 37932 United States
AU: White, D C
EM: Dwhite1@utk.edu
AF: University of Tennessee Knoxville, Center for Biomarker Analysis, 10515 Research Drive Ste 300, Knoxville, TN 37932 United States
AB: Plate tectonics creates suitable habitats for deep biosphere organisms, affecting the distribution of biological communities on Earth. Subduction zones, where crustal materials return to the planetary interior through plate convergence, expose active microbial communities in subducting seafloor sediments to a fresh chemical inventory as diagenesis, metamorphic reactions, and tectonically-induced fluid flow alter sediments and surrounding porewaters. The plate interface (the decollement) experiences persistent geochemical flux of light hydrocarbon- and metal-bearing fluids from depth. This project (1) examines the habitability of the decollement zone at the Costa Rican convergent margin from a geochemical perspective, (2) uses lipid biomarkers to describe biomass distribution in sediment samples adjacent to and within the decollement, and (3) cites methanogenesis as a likely metabolic strategy employed by the resident microbial community. Sterile plugs of sediment were recovered from cores taken during Leg 205 of the Ocean Drilling Program, in the Middle America Trench off Costa Rica. Samples are from the incoming carbonate section of Site 1253 at 370-437 meters below seafloor (mbsf), in the forearc sedimentary wedge at Site 1255 at 134-145 mbsf, and around an upper fault (153-220 mbsf) and in the decollement zone (305-366 mbsf) at Site 1254. Drilling mud and fluid were sampled to monitor potential microbial contamination. Samples were immediately frozen at -80§C. Prior to analysis, samples were freeze-dried in preparation for serial extraction of DNA and lipids. DNA was identified by fluorometry in 13 of 26 samples tested. The DNA was screened for methanogens by real time polymerase chain reaction (PCR), employing ME1 and ME2 primers that amplify a 0.75-kb region of the alpha-subunit gene for methyl coenzyme M reductase (MCR). Methanogen-specific genes were detected in DNA extracted from one Site 1253 sample (at 436.9 mbsf in the basal carbonates) and four Site 1254 samples (at 161.2 and 197.4 mbsf in the upper prism, and at 326.1 and 330.3 mbsf in the decollement). Analysis of ester-linked phospholipid fatty acids (PLFA) in the samples was carried out via capillary gas chromatography with mass spectrometry (GC/MS) in positive ion electron impact mode. PLFA were detected in 8 of the 26 samples, documenting eubacterial presence in those samples. At Site 1254, the maximum PLFA content was observed in the decollement interval with documented occurrence of hydrocarbons (up to 10 carbon atom chains) in gas and fluids sampled shipboard during Leg 205, implying significant deeply sourced fluid flux in that microenvironment. At Site 1253, PLFA were detected in the sample from the base of the carbonate section only. At Site 1255, the highest overall PLFA content was found in the shallow sedimentary wedge. This is the first analysis of microbial community structure in the deep subsurface near the Costa Rican trench, and one of few studies focused on the deep biosphere associated with convergent margins. Documented viable methanogenic populations exist within the decollement and sedimentary wedge fault zone, both sites of deeply sourced hydrocarbon-bearing and chemically distinct fluid flow. Microbial consortia in these settings may convert thermogenic hydrocarbons into methane and carbon dioxide, enhancing the release of these gases associated with subduction.
DE: 4851 Oxidation/reduction reactions
DE: 3022 Marine sediments--processes and transport
DE: 1030 Geochemical cycles (0330)
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting