A Global Record of Surface Earth Oxygenation from Sedimentary V/Sc

Abstract:

One of the simplest methods to investigate changes in oxidation potential in geologic environments is to examine a pair of otherwise similar elements with dramatically different redox chemistry. The first-row transition metals Sc and V represent one such pair, with generally similar mass, radius, and ionization energies up to the trivalent redox state. However, as a d³ transition metal, Sc is limited to a maximum oxidation state of 3+, while d⁵ vanadium may be further oxidized to form tetravalent and pentavalent ions. Due to the greater incompatibility of pentavalent (relative to trivalent) metal cations in silicate melts, V/Sc ratio has been applied to the silicate rock record as a tracer of magma redox – indicating roughly constant mantle oxygen fugacity over Earth history^1,2. As a result, the solid earth represents a boundary condition with constant V/Sc observed over geologic time¹. However, oxidation state also affects the weathering, solubility, and coordination behaviour of V and Sc, resulting in a potential redox signal in the sedimentary V/Sc record.

We apply the weighted bootstrap resampling approach of [2] to a geochemical dataset of ~87,000 sedimentary whole-rock analyses from the EarthChem portal³. The resulting trends reveal a 4 Gyr record of V/Sc redox chemistry, with the great oxidation event represented as an upward step ca. 2.5 Ga, and Phanerozoic variation suggestive of low oxygen in the early Paleozoic, increasing to twin maxima circa 270 Ma and 130 Ma, remarkably similar to that predicted by the COPSE model⁴, and broadly congruent with the Phanerozoic biochar record⁵.

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