HR: 16:20h
AN: B14A-02    [Abstracts]
TI: Branched Alkanes with Quaternary Carbon Centers (BAQCs): Biomarkers for Ancient Microbes or Recent Microbial Activity?
AU: * Bradley, A S
EM: bradleya@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Love, G D
EM: glove@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Brocks, J J
EM: jochen.brocks@anu.edu.au
AF: Australian National University, Research School of Earth Sciences, Building 61, Mills Road, Canberra, ACT 0200 Australia
AU: Sievert, S M
EM: ssievert@whoi.edu
AF: Woods Hole Oceanographic Institution, Biology Department, Mail Stop 33, Woods Hole, MA 02543 United States
AU: Summons, R E
EM: rsummons@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AB: Branched alkanes with quaternary carbon centers (BAQCs) are enigmatic molecules with presumed biogenic origin but without an identified microbial source. These compounds may have high preservation potential, and have been identified in diverse Phanerozoic and Precambrian sediments (Kenig et al., 2005) where their distribution suggests association with strong paleoredox gradients. This has led previous authors to conclude that the source organism for these compounds was likely to be non-photosynthetic, autotrophic, and thermophilic, and suggest their association with sulfide or iron oxidizing bacteria. We show that BAQCs are produced in modern environments by contemporary chemolithotrophs, and that when encountered in drill cores any syngenetic interpretation of these molecules as must be made cautiously. BAQCs have been detected in organic-lean drill cuttings of early Cambrian rocks from the South Oman South Basin which appear to have been contaminated with modern bacterial growth. Predominant series were 5,5-diethylalkanes with carbon numbers from C17 to C25 and odd over even (o/e) predominance. Cores of the 1.64 Ga Barney Creek formation were found to have abundant 5,5-diethylalkanes concentrated on core exteriors showing evidence of extensive pyrite oxidation. BAQCs were absent from the core interiors. An acid 90°C sulfidic mudpot in the Mud Volcano region of Yellowstone National Park was found to contain several series of BAQCs. The predominant series were 5,5-diethylalkanes with carbon chain lengths from C13 to C25 displaying o/e predominance. Other series included 3-ethyl, 3-methylalkanes C14 to C24 with e/o predominance, 6,6-diethylalkanes with chain lengths from C12 to C22 with e/o predominance and 5-butyl, 5-ethyl alkanes from C13 to C25 with o/e predominance. The common aspect to these occurrences of BAQCs is their prevalence at sites of present day sulfide oxidation, be it hydrothermal or weathering of pyrite. We are investigating the possibility that the presence of these compounds in drill cores represent modern or recent microbial activity on the core surface, and that in these cases they are not ancient biomarkers. We are testing this hypothesis using a combination of radiocarbon analysis and microbiological techniques. F. Kenig et al., Organic Geochemistry 36, 117 (2005).
DE: 0424 Biosignatures and proxies
DE: 0444 Evolutionary geobiology
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
SC: Biogeosciences [B]
MN: Fall Meeting 2005