HR: 0800h
AN: V51B-0575    [Abstracts]
TI: In-situ Sulphur isotope measurements of Archean microbes by NanoSIMS
AU: * Kilburn, M R
EM: matt.kilburn@uwa.edu.au
AF: University of Western Australia, Centre for Microscopy, Characterisation and Analysis, 35 Stirling Highway, Crawley, WA 6009, Australia
AU: Wacey, D
EM: david.wacey@earth.ox.ac.uk
AF: University of Oxford, Dept. of Earth Science, Parks Road, Oxford, OX1 3PR, United Kingdom
AB: Sulphur isotope signals preserved within ancient sedimentary sulphides can be used to trace the activity of sulphur-cycling microbes and help to enhance our understanding of the sulphur cycle on the early Earth. Sulphides formed through microbial sulphate reduction have characteristically light, and often highly variable δ³⁴S signatures, compared to their source sulphate. Early Archean sedimentary sulphides are, however, rare and show restricted ranges of δ³⁴S compared to younger examples, meaning that the timing of emergence of sulphate-reducing microbes, and the nature of the Archean sulphur-cycle remains controversial. The lack of clear biogeochemical evidence for such a cycle in sedimentary rocks older than ~2700 Ma is puzzling because sulphur is an essential bio-limiting nutrient and sulphur microbes are widely regarded as branching near the base of the tree of life. We here report the first in situ NanoSIMS δ³⁴S data from synsedimentary, micron-sized pyrite grains, sampled from within the ~3430 Ma Strelley Pool Formation of Western Australia. These show maximum fractionations, relative to their source sulphate, of -54 ‰, with a spread of ~75 ‰. This spread is three times greater than fractionations previously reported for the early Archean, but directly comparable with modern marine settings exhibiting a biologically-coupled redox sulphur cycle. This data combined with associated carbonaceous matter and redox sensitive detrital minerals indicates a biologically-coupled redox sulphur cycle in anoxic beach sediments as early as ~3430 Ma.
DE: 0456 Life in extreme environments
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1094 Instruments and techniques
DE: 5225 Early environment of Earth
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting