Friday, December 16, 2016

Was Delayed Intercontinental Seas' Euxinia A Vital for Eukaryotes to Survive During the Statherian PaleoProterozoic?


Spinks et al


Increased flux of sulfate to the oceans in the aftermath of the Great Oxidation Event (GOE) ∼2.4 billion years ago (Ga) caused major changes in seawater chemistry, which eventually contributed to the cessation of iron formation deposition ∼1.8 Ga. It is generally accepted that this engendered heterogeneous stratified redox conditions, with anoxic and sulfidic (euxinic) conditions in shallow open-marine environments and anoxic ferruginous conditions in deeper environments. However, the redox evolution of intracontinental marine basins following the cessation of iron formation deposition remains poorly understood.

Here, we report contrasting paleoredox conditions in two shale units of the lower McArthur Basin, northern Australia, soon after the cessation of iron formation deposition ∼1.84 Ga. Our data shows that the ∼1.78 Ga McDermott Formation was deposited in a sulfur-limited, anoxic shallow-marine environment, whereas the younger ∼1.73 Ga Wollogorang Formation was deposited in a euxinic shallow-marine environment. This implies a delay in the development of euxinia in a shallow intracontinental basin following the onset of euxinia in the open marine realm. Since bioessential metals are sequestered by pyrite deposition under euxinic conditions, protracted low-sulfidic conditions in 1.78 Ga intracontinental shallow environments could have provided vital niches for nitrogen-fixing prokaryotes and eukaryotes. Thus the ability for localized Paleoproterozoic intracontinental basins to remain non-euxinic after the onset of euxinia in shallow open-marine shelves highlights the importance of intracontinental environments to the evolution and diversification of microbial life, perhaps throughout the wider Proterozoic.

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