HR: 1340h
AN: B33B-0263    [Abstracts]
TI: Diethers enriched in $^{13}$C suggest carbon-limitation at the Lost City Hydrothermal Field
AU: * Bradley, A S
EM: bradleya@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Hayes, J M
EM: jhayes@whoi.edu
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, 266 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Summons, R E
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: Active and inactive carbonate vent structures from the Lost City Hydrothermal Field (LCHF) contain up to 0.6% organic carbon including diverse lipids. Values of $\delta$$^{13}$C for total organic carbon (TOC) range from -18.7$\permil$ vs. VPDB at the active, high-temperature vent known as "The Beehive" ($90\deg$C), to -3.1$\permil$ at Marker 7 (active, $70\deg$C). Samples with relatively high levels of $^{13}$C also contained high amounts of isoprenoidal and nonisoprenoidal diethers. Samples more depleted in $^{13}$C lacked or contained low amounts of these diethers. The correlation between high $^{13}$C and abundant diethers is supported by compound-specific isotopic analyses. Archaeal and bacterial diethers are enriched in $^{13}$C relative to photosynthetically derived marine carbon. The biomarkers {\it sn}-2 hydroxyarchaeol, {\it sn}-3 hydroxyarchaeol, and dihydroxyarchaeol - considered diagnostic for methane-cycling archaea - had $\delta$ values ranging from -8.5 to +4.8$\permil$. Phylogenetic data confirms the presence at these vents of a single group of methanogens, related to the Methanosarcinales (Schrenk et al., 2004). Diethers with non-isoprenoidal alkyl chains are also present, are of presumed bacterial origin, and may indicated the presence of sulfate-reducing bacteria. Values of $\delta$ for these compounds range from -7.3 to +1.0$\permil$. At the Beehive vent, diether lipids are absent and the TOC is depleted in $^{13}$C. Coexistence of isotopically similar hydroxyarchaeols and nonisoprenoidal glycerol diethers is typical of marine, cold-seep environments at which concentrations of H$_{2}$ are low and methane is oxidized anaerobically. At the LCHF, however, concentrations of H$_{2}$ in pore waters reach 15 mM (Proskurowski et al., 2003). This H$_{2}$, produced by serpentinization reactions, drives production (rather than oxidation) of methane. Simultaneously, sulfate-reducing bacteria can flourish as carbon-fixing autotrophs. Under such conditions, carbon may be the limiting substrate, its nearly complete consumption accounting for the enrichment of $^{13}$C in biomass. Proskurowski, G., Lilley, M.D., Olsen, E.J., and Larson, B. (2003) Preliminary geochemistry of volatile species from active vents at the Lost City Hydrothermal Field. Eos, Trans. American Geophysical Union 84(46): Fall Meeting Suppl. Abstract \#B12A-0770 Shrenk, M.O., Kelley, D.S., Bolton, S.A., and Baross, J.A. (2004) Low archaeal diversity linked to sub-seafloor geochemical processes at the Lost City Hydrothermal Field, Mid-Atlantic Ridge. Environmental Microbiology 6(10) 1086-95.
DE: 4832 Hydrothermal systems
DE: 4850 Organic marine chemistry
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting