IRON AND CARBON ISOTOPE EVIDENCE FOR ECOSYSTEM AND ENVIRONMENTAL DIVERSITY IN THE ~2.7 TO 2.5 GA HAMERSLEY BASIN, WESTERN AUSTRALIA

Iron and carbon isotope compositions of Neoarchean (2.7 to 2.5 Ga) marine sedimentary rocks from the Mt. Bruce Supergroup of the Hamersley Basin of the Pilbara Craton in Western Australia reflect the actions of multiple microbial metabolisms, including dissimilatory iron reduction (DIR), as well as abiological processes. Samples of shales, carbonates, and mixed carbonate/shale lithologies were collected from three drill cores recovered from differing regions of the basin; two cores from the center and one from the margin. These cores were temporally correlated using published high precision U-Pb dates.

Shallow water clastic rocks deposited in the center of the basin (the Tumbiana Formation) record δ¹³C values for kerogen that indicate C cycling by various anaerobic metabolisms and record δ⁵⁶Fe values that indicate a lack of Fe redox cycling. Deepwater sediments deposited contemporaneously in both parts of the basin (the Jeerinah Formation) record slightly positive δ⁵⁶Fe values in the relatively shallower and suboxic margin and strongly negative δ⁵⁶Fe values in the deeper euxinic center of the basin. This pattern is consistent with Fe cycling via a basin shuttle, a process powered by DIR, and previously only postulated to exist in Archean basins. Younger shales, co-deposited with iron formation (the Marra Mamba Iron Formation) in the center of the basin, record a shift to near-zero δ⁵⁶Fe values reflecting an Fe budget dominated by hydrothermal and clastic Fe sources. However, time-equivalent, Fe-rich carbonate/shale lithologies deposited on the margin of the basin (the Carawine Dolomite) have δ⁵⁶Fe values that steadily decrease from near-zero to strongly negative values. This environment was very shallow and included locally restricted regions so it is likely that these δ⁵⁶Fe values reflect a mixture of Fe cycled by multiple biological and abiological processes, as well as a basin shuttle, which trapped Fe in both pyrite and ferroan carbonates. Carbonates formed in the photic zone and deposited by turbidity currents in relatively deep water in the center of the basin (the Wittenoom Formation) have δ⁵⁶Fe values which co-vary with Fe concentrations a pattern that indicates a Rayleigh fractionation via oxidation of ferrous to ferric iron was a significant means of Fe cycling in this environment.