Lithium isotope constraints on the plumeworld hypothesis for the Marinoan Snowball Earth

Abstract

The Snowball Earth hypothesis predicts that continental chemical weathering was curtailed substantially during but rebounded strongly after the Marinoan ice age some 635 million years ago. Defrosting the planet would result in a plume of fresh glacial meltwater with a different chemical composition than underlying hypersaline seawater, generating an onshore-offshore geochemical gradient. Here we test the plumeworld hypothesis using lithium isotope abundances in the basal Ediacaran Doushantuo cap dolostone that accumulated in the aftermath of the Marinoan snowball Earth along an onshore-offshore transect in South China. Critical to this analysis is whether the cap dolostone formed simultaneously at different water depths, or whether it accumulated sequentially in shallow marine environments as sea level rose in the glacial aftermath. Our data shows an overall decreasing δ7Li trend with distance from the shoreline, which is consistent with either variable mixing of a meltwater plume with high δ7Li and hypersaline seawater with low δ7Li under the isochronous accumulation scenario, or with the progressive dominance of the meltwater plume under the diachronous end member, wherein cap dolostone accumulation accompanied sea level rise. In contrast to the modern oceans with high δ7Li, our geochemical model explains the evolution of low δ7Li of syn-glacial silica-rich seawater through enhanced silicate reverse weathering.