CONSTRAINING THE REDOX STATE OF THE PROTEROZOIC DEEP OCEANS USING THE MO ISOTOPE SYSTEMATICS OF EUXINIC BLACK SHALES

Tracking changes in the redox state of the oceans over geological time using Mo isotopes in euxinic black shales is fundamental to understanding the evolution of eukaryotic and animal life. Initial efforts to study the Mo isotope systematics of Proterozoic shales focused on the Velkerri and Wollogorang Fms. (McArthur Basin, northern Australia; G.L. Arnold et al. Science 304, 87-90, 2004). Six new samples of the uppermost organic-rich interval of the Velkerri Fm. (Re-Os shale age: 1361 ± 21 Ma) overlap in their δ^97/95Mo isotope composition (average: 0.72 ± 0.10‰, 2σ, relative to RochMo2 standard) and have high and invariant DOP (0.90-0.92) and [Mo] (105-119 ppm). Because the Velkerri Fm. was connected to the open ocean, the δ^97/95Mo should reflect global seawater δ^97/95Mo. Velkerri δ^97/95Mo is intermediate between present-day seawater (~ 1.6‰) and riverine Mo (~ 0‰) suggesting decreased uptake of Mo by Mn oxides as a result of a ca. 1360 Ma Proterozoic deep ocean that was more reducing relative to the present-day.

Black shales of the ca. 1730 Ma Wollogorang Fm. show a large range in δ^97/95Mo (0.57 ± 0.33‰, n=9, [Mo] = 41-58 ppm) and DOP (0.57-0.92). No correlations are observed between these parameters. The Re-Os systematics of these samples have been altered by hydrothermal fluid flow. Consequently, the variable δ^97/95Mo suggests that such alteration modestly affects Mo isotopes. Alternatively, Mo uptake from seawater at this location was not always quantitative (consistent with DOP < 0.75) resulting in a net Mo isotope fractionation in the lighter samples. In this case, only the heaviest δ^97/95Mo value (~ 0.8‰) observed may reflect that of contemporaneous seawater. Those values are consistent with those observed in the Velkerri Fm.

The 635-551 Ma Doushantuo Fm. (South China) contains the oldest known animal fossils. Four of five samples from the ca. 551 Ma Miaohe Member yielded anomalously low δ^97/95Mo of 0.11 to -0.37‰ but high DOP (0.95-0.96) and [Mo] (57-355 ppm). If the Doushantuo sea was connected to the open ocean, then the Mo isotope data imply a totally reducing global ocean at 551 Ma. Geological evidence for such an event is weak. Alternatively, the Miaohe Member was deposited in an isolated basin or semi-restricted basin where seawater δ^97/95Mo was overprinted by an unusually high-[Mo] riverine source.