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H fractionation of organic hydrogen
A572
Goldschmidt Conference Abstracts 2006
Equilibrium D/H fractionation of organic hydrogen
The isotopic expression of Fe shuttling in modern and ancient euxinic sediments: Implications for the rise of oxygen
A.L. SESSIONS, Y. WANG Department of Geological and Planetary Sciences, California Institute of Technology, USA ([email protected]; ywjojo@ gps.caltech.edu) Interpretations of D/H data from ancient organic molecules must generally address the possibility of isotopic exchange with ambient water. A lack of systematic changes in dD of organic molecules with age is commonly used to infer the lack of exchange, as are large differences in the D/H ratios of organic molecule and water. Implicit in this argument is the assumption that organic molecules are far from isotopic equilibrium with environmental water. Yet there are currently no experimental data to buttress the assumption. The goal of our current study is to experimentally measure the relevant equilibrium fractionation factors in model organic compounds. Rapid isomerization of keto and enol forms of ketones under acidic or basic catalysis produces rapid equilibration of a-H on timescales of days, facilitating experimental measurements. A cyclic ketone, cyclohexanone, was incubated with four waters of varying D/H ratio under acidic or basic conditions, at 25, 50, or 70 °C, and for up to 3 weeks. Exchange was quantified by measuring substrate dD values by GC-IRMS at discrete time intervals. Isotopic equilibrium was confirmed via attainment of constant dD values, and was approached from both directions. The equilibrium fractionation factor (a) for a-carbonyl H can be calculated from the slope of the correlation line between dD(substrate) and dD(water) at equilibrium. At 25 and 50 °C, a is found to be 0.832 ± 0.016 and 0.811 ± 0.015, respectively. The increase in fractionation with increasing temperature is unexpected, but also reproducible. Preliminary exchange experiments under acid catalysis show that the equilibrium fractionation is 10& larger compared to base catalysis at the same temperature, while the exchange half-live is about two orders of magnitude longer when [H+] is equivalent to [OH ] in base-catalyzed experiments. The uncertainty in our estimate of fractionation factors is about one order of magnitude lower than previous estimates based on spectroscopic data. The measured equilibrium fractionations are similar to the range of dD values observed in n-alkyl lipids of living organisms. If our results are representative of methylenic H, then most n-alkyl lipids are close to H-isotopic equilibrium with environmental waters. We suggest that the biosynthetic fractionation of hydrogen in n-alkyl lipids is probably affected by the rapid equilibration of ketonic a-H in the acetogenic pathway. Regardless, our results imply that constant dD values over time for these lipids cannot be used as de facto evidence for a lack of exchange. doi:10.1016/j.gca.2006.06.1059
S. SEVERMANN1, T.W. LYONS1, Y. DUAN2, A. ANBAR2, G. GORDON2, J. MCMANUS3 1
Department of Earth Sciences, University of California, Riverside, CA, USA ([email protected]) 2 Department of Geological Sciences, Arizona State University, Tempe, AZ, USA 3 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA Modern sediments that are deposited beneath a sulfidic water column, such as in the Black Sea, are typically enriched in reactive iron relative to average oxially or suboxically deposited sediments. Similar iron enrichments are observed in sedimentary rocks from ancient euxinic (anoxic and sulfidic) basins. The mechanism commonly proposed for this iron enrichment is shuttling of benthic iron from suboxic shelf sediments to the deep basin, where it is sequestered quantitatively during Fe-sulfide precipitation. In this study we investigate how iron isotopes, in combination with traditional paleoredox proxies, can be used to identify this benthic iron source and to recognize ancient euxinia. Low d56Fe values in porewaters from suboxic sediments indicate that sedimentary iron flux has a characteristically light (< 2.0&) iron isotope composition (Severmann et al., 2006). These low d56Fe values are a consequence of the continuous iron redox-recycling in suboxic continental shelf sediments. Consistent with shelf-to-basin iron transport, modern and ancient euxinic sediments show light bulk iron isotope compositions that are significantly decreased relative to average oxic weathering products. Low d56Fe values of sedimentary pyrites and bulk sediments throughout the Archean (Rouxel et al., 2005; Yamaguchi et al., 2005) may indicate that a similar iron-shuttle could have operated during that time. Our interpretations implies that shallow sediments in the Archean ocean underwent redox-cycling in order to generate an isotopically light benthic iron flux. The isotopic expression of the iron enrichment mechanism could therefore provide important insights into the evolution of the iron cycle and the redox balance of the atmosphere-ocean system in the early Earth.
References Rouxel, O., Bekker, A., Edwards, K., 2005. Science 307, 1088–1091. Severmann, S., Johnson, C.M., Beard, B.L., McManus, J., 2006. Geochim. Cosmochim. Acta 70, 2006–2022. Yamaguchi, K.E., Johnson, C.M., Beard, B.L., Ohmoto, H., 2005. Chem. Geol. 218, 135–169. doi:10.1016/j.gca.2006.06.1060
Goldschmidt Conference Abstracts 2006
Equilibrium D/H fractionation of organic hydrogen
The isotopic expression of Fe shuttling in modern and ancient euxinic sediments: Implications for the rise of oxygen
A.L. SESSIONS, Y. WANG Department of Geological and Planetary Sciences, California Institute of Technology, USA ([email protected]; ywjojo@ gps.caltech.edu) Interpretations of D/H data from ancient organic molecules must generally address the possibility of isotopic exchange with ambient water. A lack of systematic changes in dD of organic molecules with age is commonly used to infer the lack of exchange, as are large differences in the D/H ratios of organic molecule and water. Implicit in this argument is the assumption that organic molecules are far from isotopic equilibrium with environmental water. Yet there are currently no experimental data to buttress the assumption. The goal of our current study is to experimentally measure the relevant equilibrium fractionation factors in model organic compounds. Rapid isomerization of keto and enol forms of ketones under acidic or basic catalysis produces rapid equilibration of a-H on timescales of days, facilitating experimental measurements. A cyclic ketone, cyclohexanone, was incubated with four waters of varying D/H ratio under acidic or basic conditions, at 25, 50, or 70 °C, and for up to 3 weeks. Exchange was quantified by measuring substrate dD values by GC-IRMS at discrete time intervals. Isotopic equilibrium was confirmed via attainment of constant dD values, and was approached from both directions. The equilibrium fractionation factor (a) for a-carbonyl H can be calculated from the slope of the correlation line between dD(substrate) and dD(water) at equilibrium. At 25 and 50 °C, a is found to be 0.832 ± 0.016 and 0.811 ± 0.015, respectively. The increase in fractionation with increasing temperature is unexpected, but also reproducible. Preliminary exchange experiments under acid catalysis show that the equilibrium fractionation is 10& larger compared to base catalysis at the same temperature, while the exchange half-live is about two orders of magnitude longer when [H+] is equivalent to [OH ] in base-catalyzed experiments. The uncertainty in our estimate of fractionation factors is about one order of magnitude lower than previous estimates based on spectroscopic data. The measured equilibrium fractionations are similar to the range of dD values observed in n-alkyl lipids of living organisms. If our results are representative of methylenic H, then most n-alkyl lipids are close to H-isotopic equilibrium with environmental waters. We suggest that the biosynthetic fractionation of hydrogen in n-alkyl lipids is probably affected by the rapid equilibration of ketonic a-H in the acetogenic pathway. Regardless, our results imply that constant dD values over time for these lipids cannot be used as de facto evidence for a lack of exchange. doi:10.1016/j.gca.2006.06.1059
S. SEVERMANN1, T.W. LYONS1, Y. DUAN2, A. ANBAR2, G. GORDON2, J. MCMANUS3 1
Department of Earth Sciences, University of California, Riverside, CA, USA ([email protected]) 2 Department of Geological Sciences, Arizona State University, Tempe, AZ, USA 3 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA Modern sediments that are deposited beneath a sulfidic water column, such as in the Black Sea, are typically enriched in reactive iron relative to average oxially or suboxically deposited sediments. Similar iron enrichments are observed in sedimentary rocks from ancient euxinic (anoxic and sulfidic) basins. The mechanism commonly proposed for this iron enrichment is shuttling of benthic iron from suboxic shelf sediments to the deep basin, where it is sequestered quantitatively during Fe-sulfide precipitation. In this study we investigate how iron isotopes, in combination with traditional paleoredox proxies, can be used to identify this benthic iron source and to recognize ancient euxinia. Low d56Fe values in porewaters from suboxic sediments indicate that sedimentary iron flux has a characteristically light (< 2.0&) iron isotope composition (Severmann et al., 2006). These low d56Fe values are a consequence of the continuous iron redox-recycling in suboxic continental shelf sediments. Consistent with shelf-to-basin iron transport, modern and ancient euxinic sediments show light bulk iron isotope compositions that are significantly decreased relative to average oxic weathering products. Low d56Fe values of sedimentary pyrites and bulk sediments throughout the Archean (Rouxel et al., 2005; Yamaguchi et al., 2005) may indicate that a similar iron-shuttle could have operated during that time. Our interpretations implies that shallow sediments in the Archean ocean underwent redox-cycling in order to generate an isotopically light benthic iron flux. The isotopic expression of the iron enrichment mechanism could therefore provide important insights into the evolution of the iron cycle and the redox balance of the atmosphere-ocean system in the early Earth.
References Rouxel, O., Bekker, A., Edwards, K., 2005. Science 307, 1088–1091. Severmann, S., Johnson, C.M., Beard, B.L., McManus, J., 2006. Geochim. Cosmochim. Acta 70, 2006–2022. Yamaguchi, K.E., Johnson, C.M., Beard, B.L., Ohmoto, H., 2005. Chem. Geol. 218, 135–169. doi:10.1016/j.gca.2006.06.1060