Geobiology of the late Paleoproterozoic Duck Creek Formation, Western Australia

14 ________________________________________________________________________ 15 The ca. 1.8 Ga Duck Creek Formation, Western Australia, preserves 1000 m of 16 carbonates and minor iron formation that accumulated along a late Paleoproterozoic 17 ocean margin. Two upward-deepening stratigraphic packages are preserved, each 18 characterized by peritidal precipitates at the base and iron formation and carbonate 19 turbidites in its upper part. Consistent with recent studies of Neoarchean basins, carbon 20 isotope ratios of Duck Creek carbonates show no evidence for a strong isotopic depth 21 gradient, but carbonate minerals in iron formations can be markedly depleted in 13 C. In 22 contrast, oxygen isotopes covary strongly with depth; ! 18 O values as positive as 2‰ 23 VPDB in peritidal facies systematically decline to values of -6 to -16‰ in basinal rocks, 24 * Corresponding author: [email protected] 1 Department of Earth and Planetary Sciences, Harvard University 2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA USA 3 Department of Organismic and Evolutionary Biology, Harvard University 4 The Agouron Institute 5 Australian Centre for Astrobiology, University of New South Wales, Australia 6 Massachusetts Institute of Technology 7 Jet Propulsion Laboratory 8 Chevron Corp. 9 Curtin University of Technology, Australia