Neoarchean carbonates as archives of paleo-redox conditions on early Earth: Insights from metal isotope analyses of the Tumbiana Formation, Pilbara, WA

Farkas, Juraj1; Klaebe, Robert1, Scarabotti, Liam1, Stormberg, Jessica2, Spinks, Sam2

1Metal Isotope Group (MIG), Department of Earth Sciences, University of Adelaide, SA, Australia, 2CSIRO, Division of Mineral Resources, Perth, WA, Australia

The redox conditions of the earth’s surface environments are intimately linked to past changes in the atmospheric O2 levels, and thus to the long-term evolution of photosynthetic life on our planet. It is generally believed and accepted that Archean Eon is characterised by extremely low levels of atmospheric O2 concentrations with predominantly anoxic / euxinic marine and terrestrial environments; where the first significant rise in atmospheric O2 levels is documented much later during the Paleo-Proterozoic period at around 2.4 to 2.1 billion years (Byr) ago, the latter also called the Great Oxidation Event (GOE). This study aims to further constrain paleo-redox conditions during Neoarchean times by analysing selected metal isotope tracers and elemental concentrations in carbonate-rich samples from the Tumbiana Formation of the Fortescue Group in the Northern Pilbara Craton in WA, dated at ~2.7 Byr, which thus represent unique archives of paleo-environmental and redox conditions on early Earth. Here we analysed stable chromium isotopes (d53/52Cr variations), coupled with REEs and elemental Zn/Fe ratios, to infer past changes in redox conditions during the deposition of the Tumbiana Formation; and radiogenic strontium isotopes (87Sr/86Sr ratios) were used to test marine versus continental / lacustrine origin of the studied Neoarchean carbonates from the Fortescue Group. Interestingly, acquired results strongly support non-marine and thus likely lacustrine origin of the Tumbiana Formation carbonates; and more importantly our pilot data from redox-sensitive proxies (d53/52Cr and Zn/Fe ratios) point to an active redox cycling of Cr isotopes during the deposition of the Tumbiana Formation at ~2.7 Byr, thus challenging the prevalent views of strictly anoxic conditions during the Neoarchean. In addition, the above geochemical/isotope evidence for an active redox cycling (i.e., oxidation-reduction) of chromium and iron is documented in horizons with abundant stromatolitic carbonates, perhaps suggesting that local production of O2 via photosynthesizing microbial communities in restricted lacustrine settings could be partly responsible for the observed paleo-redox signals recorded in d53/52Cr and Zn/Fe tracers in carbonates from the Tumbiana Formation. We will discuss broader implications of the above findings for paleo-redox studies in deep times, including possibilities that predominantly anoxic and O2 depleted Archean earth’s surface environments could also harbor localised ‘oxygenated oasis’ in very specific depositional settings.


Biography

My research interest is focused on the application of metal isotopes and novel analytical techniques to low-temperature geochemistry, including earth system evolution studies, and isotope tracing of metals in near-surface environments. I completed my PhD at Uni Ottawa, Postdoctoral training at Harvard, and recently established Metal Isotope Group at UniAdelaide. 

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