The stable isotope composition of metals (Mo and Fe) in ancient

stromatolites as paleo-environmental indicators.

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Ocean chemistry has changed dramatically over geological time. One of the first transitions in marine redox conditions occurred around 2.4 Ga and is coincident to first permenent rise in atmospheric oxygen concentration (Farquhar et al., 2007). This event, termed the “Great Oxidation Event” (GOE), is believed to be close to the radiation of the oxygenic photosynthetic organisms, the primary sources of dioxygen (O2). Even if pO2 in the atmosphere immediately following the GOE was not high enough to oxygenate the deep ocean, it was sufficient to maintain an oxic surface layer in the ocean throughout the Proterozoic (ca. 1.8 Ga - 0.54 Ga). However, transient whiffs or oases O2 production are also suggested to have occurred before the onset of the GOE (eq. Lyons et al., 2014 or Lalonde & Konhauser 2015). In order to understand the timing of the GOE and the presence of these fleeting oxic episodes, I am investigating carbonates bracketing the GOE using novel geochemical redox proxies. The development of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) has enabled the routine measurement of metal isotope compositions. It has been demonstrated that carbonates record the Mo and Fe isotopic composition of the water from which they precipitated, at global and local scales, respectively. My work is focused on Archean stromatolite samples tin an effort o better under understand the earliest biologic control on O2 production, the accumulation in the ocean and atmosphere, and the expression of this O2 in surface weathering.

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Advisor: Stefan Lalonde (Laboratoire Géosciences Océans, Brest): perso page

Supervisor: Kurt Konhauser (Univiversité d'Alberta, Canada): perso page

Co-supervisor: Jacques Deverchere (Laboratoire Géosciences Océans, Brest): perso page

Farquhar, J., Peters, M., Johnston, D. T., Strauss, H., Masterson, A., Wiechert, U., & Kaufman, A. J. (2007). Isotopic evidence for Mesoarchaean anoxia and changing atmospheric sulphur chemistry. Nature, 449(7163), 706.

Lyons, T. W., Reinhard, C. T., & Planavsky, N. J. (2014). The rise of oxygen in Earth’s early ocean and atmosphere. Nature, 506(7488), 307.

Lalonde, S. V., & Konhauser, K. O. (2015). Benthic perspective on Earth’s oldest evidence for oxygenic photosynthesis. Proceedings of the National Academy of Sciences, 112(4), 995-1000.