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- Hinrichs Lab - ACS-PRF
Hinrichs Lab - ACS-PRF
Molecular-isotopic studies on the biogeochemistry of volatile fatty acids in marine sediments
| Duration: | May 2004 - August 2007 |
| Funding: | American Chemical Society - Petroleum Research Fund (ACS-PRF) (Project PRF #41192-AC2) |
| Principal Investigator(s): | Kai-Uwe Hinrichs |
| Involved scientists in the Hinrichs Lab: | Verena Heuer |
| Partners: | Prof. J.M. Hayes (Formerly Woods Hole Oceanographic Institution, Woods Hole, MA, USA), Dr. M. Krummen (ThermoElectron, Bremen), Prof. S.B. Joye (The University of Georgia, Ga, USA) Dr. M. Krüger (Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover), Prof. K. Küsel (Friedrich-Schiller-Universität, Jena) |
Abstract
Volatile fatty acids (VFAs), most importantly acetate, are key intermediates in microbially mediated diagenetic processes in sediments. The abundance and isotopic composition of acetate and other VFAs are controlled by a balance of biological sinks and sources. With this projected we seeked to decipher the information encoded in the carbon isotopic composition of VFAs with the aim to develop a biogeochemical proxy that aids in the identification of the dominant diagenetic processes in marine sedimentary microbial ecosystems in which other biogeochemical techniques are at the limit of their applicability.
With this study we started to fill the vast gap in knowledge on isotopic variability of acetate and other VFAs in marine sediments. To relate the expected isotope variability to processes, we studied the isotopic composition of VFAs in a variety of different well-defined biogeochemical regimes in sediments. In addition, we wanted to come to a better understanding of the mechanistic details leading to extremely high acetate concentrations that have been encountered in some deeply buried, methane-laden sedimentary environments. Complementary heating experiments of marine sediments were conducted to simulate the presumed temperature-dependent biological production of acetate at great burial depth and characterize this pool isotopically. Controlled laboratory-based microcosm studies served to amplify different biogeochemical processes separately in order to study their impact on the isotopic compositions of VFAs.
We successfully applied a novel online liquid-chromatography-isotope ratio-mass spectrometry method that is suitable for routine carbon isotopic analysis of VFAs. In an initial survey, we documented a great isotopic variability in acetate. Analysis of pore-waters in sediments and of fluids from sediment incubations showed that d13C values of acetate ranged from -85 to -5‰ and was linked to the geochemical regime (Heuer et al., 2006). We found (a) 13C-enriched acetate (-5‰) where acetoclastic methanogenesis was an important sink; (b) d13C-values close to -20‰, i.e., resembling d13C-values of sedimentary organic matter, where fermentative acetate production prevailed; (c) d13C-acetate between -60 and -40‰ and a ~20 to ~30‰ depletion relative to dissolved inorganic carbon (DIC) and sedimentary organic carbon (TOC) in sediments from the Gulf of Mexico where high hydrogen levels created favorable conditions for autotrophic acetogenesis; and (d) strongly 13C-depleted acetate in pore-waters at an active cold seep pointing to either direct or indirect involvement of anaerobic methane oxidation in the formation of acetate. In conclusion, the observed variability supports our hypothesis that carbon isotopic compositions of VFAs are useful proxies for biogeochemical processes occurring in situ.
Volatile fatty acids (VFAs), most importantly acetate, are key intermediates in microbially mediated diagenetic processes in sediments. The abundance and isotopic composition of acetate and other VFAs are controlled by a balance of biological sinks and sources. With this projected we seeked to decipher the information encoded in the carbon isotopic composition of VFAs with the aim to develop a biogeochemical proxy that aids in the identification of the dominant diagenetic processes in marine sedimentary microbial ecosystems in which other biogeochemical techniques are at the limit of their applicability.
With this study we started to fill the vast gap in knowledge on isotopic variability of acetate and other VFAs in marine sediments. To relate the expected isotope variability to processes, we studied the isotopic composition of VFAs in a variety of different well-defined biogeochemical regimes in sediments. In addition, we wanted to come to a better understanding of the mechanistic details leading to extremely high acetate concentrations that have been encountered in some deeply buried, methane-laden sedimentary environments. Complementary heating experiments of marine sediments were conducted to simulate the presumed temperature-dependent biological production of acetate at great burial depth and characterize this pool isotopically. Controlled laboratory-based microcosm studies served to amplify different biogeochemical processes separately in order to study their impact on the isotopic compositions of VFAs.
We successfully applied a novel online liquid-chromatography-isotope ratio-mass spectrometry method that is suitable for routine carbon isotopic analysis of VFAs. In an initial survey, we documented a great isotopic variability in acetate. Analysis of pore-waters in sediments and of fluids from sediment incubations showed that d13C values of acetate ranged from -85 to -5‰ and was linked to the geochemical regime (Heuer et al., 2006). We found (a) 13C-enriched acetate (-5‰) where acetoclastic methanogenesis was an important sink; (b) d13C-values close to -20‰, i.e., resembling d13C-values of sedimentary organic matter, where fermentative acetate production prevailed; (c) d13C-acetate between -60 and -40‰ and a ~20 to ~30‰ depletion relative to dissolved inorganic carbon (DIC) and sedimentary organic carbon (TOC) in sediments from the Gulf of Mexico where high hydrogen levels created favorable conditions for autotrophic acetogenesis; and (d) strongly 13C-depleted acetate in pore-waters at an active cold seep pointing to either direct or indirect involvement of anaerobic methane oxidation in the formation of acetate. In conclusion, the observed variability supports our hypothesis that carbon isotopic compositions of VFAs are useful proxies for biogeochemical processes occurring in situ.
Application of inhibitors and substrates in sediment incubation experiments confirmed that autotrophic acetogenesis results in highly 13C depleted acetate (Heuer et al., 2010).
Carbon isotopic analysis of anoxic brines in the Gulf of Mexico revealed that 13C-depleted acetate from autotrophic acetogenesis contributes significantly to the overall acetate pool in natural systems as well (Joye et al. 2008). Acetate was as light as -40‰ in a stratified brine pool where autotrophic acetogenesis was a key process in the presence of µmolar-levels of dissolved hydrogen. Surprisingly, hydrogenotrophic methanogenesis was below detection in this system though thermodynamic conditions would have been favorable and acetoclastic methanogenesis prevailed.
Carbon isotopic analysis of anoxic brines in the Gulf of Mexico revealed that 13C-depleted acetate from autotrophic acetogenesis contributes significantly to the overall acetate pool in natural systems as well (Joye et al. 2008). Acetate was as light as -40‰ in a stratified brine pool where autotrophic acetogenesis was a key process in the presence of µmolar-levels of dissolved hydrogen. Surprisingly, hydrogenotrophic methanogenesis was below detection in this system though thermodynamic conditions would have been favorable and acetoclastic methanogenesis prevailed.