Résumé
The evolution of the ocean, atmosphere and biosphere throughout Earth's history has impacted on the biogeochemistry of some key trace metals that are of particular importance in regulating the exchange between Earth's reservoirs. Several geochemical proxies exhibit isotopic shifts that have been linked to major changes in the oxygenation levels of the ancient oceans during the Great Oxygenation Event (GOE) between 2.45 and 2.2 Ga and the Neoproterozoic Oxygenation Event at ca. 0.6 Ga. Studies of the modern marine biogeochemical cycle of the transition metal Cadmium have shown that stable Cd isotope fractionation is mainly driven by biological uptake of light Cd into marine phytoplankton in surface waters leaving behind the seawater enriched in the heavy Cd isotopes. Here we use of the potential of this novel proxy to trace ancient biological productivity which remains an enigma, particularly during the early stages of Earth history. The Turee Creek Group in the Hamersley Basin, Australia, provides a continuous stratigraphic sedimentary section covering the GOE and at least two glacial events, offering a unique opportunity to examine the changes that took place during these periods and possibly constrain the evolution, timing and onset of oxygenic photosynthesis. Stable Cd isotope data were obtained on samples from the Boolgeeda Iron Fm. (BIFs), the siliciclastic and carbonate successions of Kungara (including the Meteorite Bore Member) and the Kazputt Fm., using a double spike technique by TIMS (ThermoFisher Triton) and Cd concentrations were determined by isotope dilution. The Boolgeeda BIFs have generally low Cd concentrations varying between 8 and 50 ppb, with two major excursions marked by an increase in Cd content, reaching similar levels to those in the overlying Kungarra Fm. (> or =150 ppb). These variations are associated with a large range in epsilon (super 112/110) Cd values (-2 to +2), with the most negative values typically found in the organic and Cd-rich shales and siltstones of the Kungara and Kazput formations. This suggests that a significant portion of the Cd present is organically bound, as also supported by the relationship with delta (super 13) C (sub org) and TOC. These data will serve to assess the size of the ancient biomass during the GOE in view of our understanding of the modern Cd isotope biogeochemical cycling in the oceans.