Magnesium isotope evidence for enhanced crustal reworking in lowermost Cambrian sedimentary rocks (Kazakhstan)

Abstract

The transition from the Proterozoic to the Phanerozoic Eon was accompanied by the rise of metazoan life, a key and unique biogeochemical milestone in Earth's history. Concomitant continental re-organization and collision were associated with enhanced continental reworking and changes in global ocean currents, with profound impacts on continental weathering rates, riverine run-off and associated changes in the ocean nutrient budget. The causal relationship between the geological re-organisation of continents and the biologic evolution of marine life, however, remains elusive. In this study, we investigate phosphatic shallow-water sedimentary successions from Kazakhstan, which host key marker horizons from the Precambrian-Cambrian (Pc-C) boundary. We show that a rapid change (over ca. 3 Myrs) towards heavier stable Mg isotope compositions of the phosphatic sedimentary deposits in Kazakhstan, which we consider representative for contemporaneous ocean chemistry, co-varies with changes in radiogenic Sr isotope signatures. We propose that ocean chemistry at the Pc-C boundary, represented through this co-variation, was strongly affected by continental re-organization and associated weathering, which, in analogy, would have affected ocean nutrient levels. A rapid reversal of isotope compositions towards signatures similar to those prior to the isotope excursion likely reflects the fading influence of weathering and a buffering of water-rock interactions during oceanic spreading. We conclude that, based on the positive, coupled 87Sr/86Sr- δ26Mg isotope spike in ocean water chemistry, a link between Gondwana re-organization at the Pc-C boundary and the marked change in marine fauna seems plausible.