PP21A-2218
CLUMPED ISOTOPE VALUES FROM THE DOUSHANTUO FORMATION OF SOUTH CHINA: EVALUATION OF HYDROTHERMAL INFLUENCE, DISEQUILIBRIA AND DIAGENETIC EFFECTS
Tuesday, 15 December 2015
Poster Hall (Moscone South)
Ian S. Foster1, Maoyan Zhu2, Miao Lu2, Thomas Bristow3, Magali Bonifacie4 and Aradhna Tripati1,5, (1)Laboratoire Domaines Océaniques, Institut Universitaire Européen de la Mer, Plouzané, France, (2)State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China, (3)NASA Ames Research Center, Moffett Field, CA, United States, (4)CNRS, Paris Cedex 16, France, (5)University of California Los Angeles, Planetary and Space Sciences, Department of Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, Los Angeles, CA, United States
Abstract:
The Ediacaran Doushantuo Formation (635 – 551 Ma) of southern China is a phosphate-dolostone-black shale sequence following the Marinoan “Snowball Earth” episode that represents an important period in Earth history. It contains abundant phosphate-preserved microfossils, and extremely low carbon isotope values in the cap dolostone unit that have been interpreted to reflect formation in a methane seep environment [1]. Previous clumped isotope analysis of 13C-depleted carbonate veins from the basal Doushantuo samples have been interpreted to reflect hydrothermally-derived thermogenic methane oxidation [2], however recent work on modern and ancient cold seep samples suggests clumped isotope signatures in these environments are influenced by disequilibria effects [3] and are vulnerable to post-depositional diagenesis via internal reordering at temperatures exceeding ~ 100 – 120 ˚C [4]. Here we present initial data from the cap-carbonates overlying the Nantuo diamictite. Our analysis includes a variety of micro-facies from the cap-carbonate including pure calcite and micrite, with a range of carbonate carbon isotopic values. Data presented here will be used to attempt to determine if the samples exhibit disequilibria effects such as those observed in modern cold seep environments, as well as to evaluate the role of hydrothermal activity in the Doushantuo Formation. [1] Jiang, G., Kennedy, M.J., Christie-Blick, N., 2003. Stable isotope evidence for methane seeps in Neoproterozoic postglacial cap carbonates. Nature 426, 822-826. [2] Bristow, T.F., Bonifacie, M., Derkowski, A., Eiler, J.M., Grotzinger, J.P., 2011. A hydrothermal origin for isotopically anomalous cap dolostone cements from south China. Nature 474, 68-72. [3] Loyd, S., Sample, J.C., Orphan, V.J., Marlow, J., Eagle, R., Tripati, A.K., 2012. Clumped isotope analyses of cold seep carbonates: Insights into formation environment and mechanisms. Abstract B51G-0639 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec. [4] Henkes, G.A., Passey, B.H., Grossman, E.L., Shenton, B.J., Pérez-Huerta, A., Yancey, E.L., 2014. Temperature limits for preservation of primary calcite clumped isotope paleotemperatures. Geochemica et Cosmochimica Acta 139, 362-382.
