VOFAS: Carbon isotopic compositions of volatile fatty acids as proxies for biogeochemical processes in the deep marine biosphere (IODP Exp. 301)

Abstract

By demonstrating the existence of an extensive deep marine biosphere the Ocean Drilling Program has posed some fundamental questions for biogeochemistry, namely: (1) How can bacterial populations survive on buried, recalcitrant organic material for millions of years? (2) Is acetate of biogenic or thermogenic origin an alternative substrate for theses organisms? (3) Do chemoautotrophic microbes act as primary producers at great sediment depth?

This project was driven by the hypothesis that carbon-isotopic investigations of volatile fatty acids (VFA), most importantly acetate, will shed more light on the cycling of carbon within the deep biosphere. VFAs are key intermediates in microbially mediated processes. Their abundance and isotopic composition are controlled by a balance of biological sinks and sources. Here we aimed to decipher the information encoded in the carbon isotopic signatures of VFAs with the aim to develop a biogeochemical proxy that aids in the identification of in situ diagenetic processes in the deep biosphere. Our study seeked to fill the vast gap in knowledge on the extent of and underlying processes causing the isotopic variability of VFAs in marine sediments and elucidate the processes that cause extremely high acetate concentrations in some deeply buried sediments.

We investigated pore-waters from IODP Expedition 301 (Juan de Fuca Ridge Flank) as well as pore-waters from a range of sediments with various biogeochemical conditions to test the following working hypothesis: The isotopic signatures of VFAs exhibit a great range and are in a certain, yet unknown manner linked to the predominant biogeochemical processes in the sediment horizon. With complementary laboratory studies, we simulated the presumed temperature-dependent biological production of acetate at great burial depth in order to characterize and track this pool isotopically.

Publications

Lever, M.A., Heuer, V.B., Morono, Y., Masui, N., Schmidt, F., Alperin, M.J., Inagaki, F., Hinrichs, K.-U., Teske, A. (2010) Acetogenesis in deep subseafloor sediments of the Juan de Fuca Ridge flank: a synthesis of geochemical, thermodynamic, and gene-based evidence. Geomicrobiology Journal, 27, 183-211. doi: 10.1080/01490450903456681

Heuer V., Elvert M., Tille S., Krummen M., Prieto Mollar X., Hmelo L.R., Hinrichs K.-U. (2006) Online d13C analysis of volatile fatty acids in sediment/porewater systems by liquid chromatography-isotope ratio-mass spectrometry. Limnology and Oceanography: Methods, 4, 346-357.

Technical Reports

Fisher, A.T., Urabe, T., Klaus, A., and the Expedition 301 Scientists (2005) Proceedings of the Integrated Ocean Drilling Program, Volume 301. College Station TX (Integrated Ocean Drilling Program Management International, Inc.) doi: 10.2204/iodp.proc.301.101.2005

Fisher, A.T., Urabe, T., Klaus, A., and the IODP Expedition 301 Scientists (2005) IODP Expedition 301 installs three borehole crustal observatories, prepares for three-dimensional, cross-hole experiments in the northeastern Pacific Ocean. Sci. Drill., 1:6–11. doi:10.2204/iodp.sd.1.01.2005

Shipboard Scientific Party (2004). Juan de Fuca hydrogeology: the hydrogeologic architecture of basaltic oceanic crust: compartmentalization, anisotropy, microbiology, and crustal-scale properties on the eastern flank of Juan de Fuca Ridge, eastern Pacific Ocean. IODP Prel. Rept., 301. doi:10.2204/iodp.pr.301.2004