Magnesium isotope evidence for enhanced crustal reworking as a cause for the Ediacarian mass extinction

The initiation of the Phanerozoic eon was accompanied by a unique (bio-) geochemical revolution in Earth´s history, leading to the rise of metazoan life. Concomitant continent re-organization and collision is associated with enhanced continental reworking and mixing of water masses through changing global ocean currents, affecting the weathering of rocks and consequently the ocean nutrient balance. A causal relationship between these geological events and timing of the biologic revolution, however, remains unclear. In this study, we investigate phosphatic shallow water sediments from Kazakhstan, key deposits from the Precambrian-Cambrian (Pc-C) boundary. We show that the temporal response in ocean chemistry consequent to crustal reworking is remarkably fast. A rapid increase of the stable Mg and radiogenic Sr isotope signatures in seawater indicates a swift and severe response in ocean chemistry to enhanced crustal reworking. Our data strongly suggest that the oceans at the Pc-C boundary, although rapidly recovering through ocean spreading buffering, were strongly affected by this continent re-organization. A direct link between the unique response of ocean chemistry to Gondwana assembly at the Pc-C boundary and the marked change in oceanic fauna at this time seems most plausible. Accordingly, the rise of metazoan life during the early Cambrian following the Ediacaran mass extinction was a fortuitous circumstance at a critical stage in the evolution of life.