HR: 16:45h
AN: V54B-04    [Abstracts]
TI: Coupled Fe and S Isotope Evidence for Archaean Microbial Fe(III) and Sulphate Reduction
AU: * Archer, C
EM: c.archer@bristol.ac.uk
AF: Royal Holloway University of London, Geology Department , Egham, TW20 0EX United Kingdom
AU: Vance, D
EM: d.vance@bristol.ac.uk
AF: Bristol University, Department of Earth Sciences, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ United Kingdom
AB: The development of transition metal stable isotope geochemistry over recent years, fuelled by advances in analytical technology, has permitted new approaches to established geochemical problems. One such application is the understanding and tracing of early Earth biogeochemistry, with a particular emphasis on the activities of primitive microbial life, where evidence from traditional isotope systems has so far been equivocal. The earliest forms of microbial respiration are thought to involve sulphate or Fe (III) reduction, $^{e.g. 1,2}$ or perhaps both, and as such combined Fe-S isotope systems provide a potentially powerful tool to trace these processes. Here we report a Zn-Fe-S data set from sulphide and pyrite grains from the 2.7 Ga Belingwe Belt, associated with sulphate reducing microbial communities. Analyses of individual mm size sulphide and pyrite grains show large depletions of light Fe isotopes, as well as a large variation in Fe isotope composition with a range of -0.7 to -2.7 \permil. Furthermore these variations are correlated with depletions in light S isotopes measured from the same samples, which range in isotope composition from -3 to -18 \permil $^{3}$. Zn isotopes also show significant positive fractionations, up to 0.8\permil, particularly in organic rich black shales. The most striking feature of our dataset is a tight correlation between sulphur and iron isotopes. This relationship is most readily explained in terms of a reducing sedimentary environment. In modern anoxic sediments dissimilatory bacterial Fe (III) reduction, and further down the sediment column, sulphate reduction produce solubilised light Fe$^{2+}$ and S$^{2-}$, which in solution together immediately react to form isotopically depleted sulphides and ultimately pyrite. Experimental constraints$^{4}$ and measurements of natural pyrite from modern sedimentary settings (S. Severmann, pers comm) demonstrate that the solid sulphide produced is light in Fe. We will present a quantitative diagenetic model which demonstrates that, in a closed system coupled Rayleigh depletion in the light isotopes of both Fe and S by these processes down the sedimentary column could produce a correlated Fe and S dataset. Quantitatively, this process requires the sulphate in the pore water to become significantly depleted, a process that generally does not occur today because of high pore-water sulphate levels$^{5}$ but is much more likely in the Archaean where seawater sulphate levels were probably two orders of magnitude lower. We note that both Fe and sulphate reduction as late as 2.7 Ga is indicated by SSU rRNA phylogenetic models, but our approach has potential applications further back in time where such constraints are lacking. $^{1}$ Y. Shen and R. Buick, Earth Sci. Rev., 2004, {\bf64}, 243-272 $^{2}$ M. Vargas et. al., Nature, 1998, {\bf395}, 65-67 $^{3}$ N.V. Grassineau et al., 2001, Proc. Roy. Soc. London B, {\bf268}, 113-119 $^{4}$ I. Butler et al., 2003, Geochim. Cosmochim. Acta, {\bf67}, A51 $^{5}$ S. Severmann et al, 2004, this volume
DE: 9619 Precambrian
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 0330 Geochemical cycles
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting