Constrain Paleo pO2 Using One Dimensional Reactive Transport Chromium/Iron Weathering Model

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Xu, Pengxiao
Reinhard, Christopher T.
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Quantitatively constraining atmospheric oxygen level (pO2) in the Proterozoic has been a long time struggle in the study of paleoclimate and biogeochemistry. An accurate reconstruction would benefit our understanding towards Earth’s early history and complex life evolution. The oxidative weathering of (Cr) and iron (Fe) in soil column is controlled ultimately by oxygen , and thus the oxidation and retention of Cr and Fe are typically linked to pO2, making it a promising approach to estimating pO2 in Earth’s history. However, existing models of Cr mobilization during weathering as a function of atmospheric pO2 are oversimplified, and a surprisingly limited amount of work has been done exploring the dynamics of Fe oxidation/retention in a reaction-transport framework that allows for the explicit representation of redox fronts in the weathering realm. Here, we develop a simple two-phase reaction-transport model to explore the impact of atmospheric pO2 on the oxidative mobilization of Cr and the potential for retention of Fe during weathering. The model is meant to represent the key processes regulating Cr, Mn, and Fe redox cycling in a weathering environment forming from an idealized igneous basalt protolith in a way that is simple, transparent, and computationally tractable. Because many key parameters are uncertain, we subject the model to a stochastic resampling routine with the goal of identifying the overarching controls on relative Cr mobilization as dissolved Cr(VI) and retention of Fe as Fe(III) across a wide range of parameter space. Results suggest that atmospheric pO2 must be extremely low in order to prevent significant Cr(VI) release and allow for significant Fe loss during weathering, likely well below ~1% of the present atmospheric level (PAL). This value supports the idea that the emergence of complex life was delayed by low pO2 in the Proterozoic, given the minimum oxygen level to support these forms of life is ~3×10-3 PAL.
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