Lithium isotope evidence for a plumeworld ocean in the aftermath of the Marinoan snowball Earth

The snowball Earth hypothesis predicts that continental chemical weathering diminished substantially during, but rebounded strongly after, the Marinoan ice age some 635 Mya. Defrosting the planet would result in a plume of fresh glacial meltwater with a different chemical composition from underlying hypersaline seawater, generating both vertical and lateral salinity gradients. Here, we test the plumeworld hypothesis using lithium isotope compositions in the Ediacaran Doushantuo cap dolostone that accumulated in the aftermath of the Marinoan snowball Earth along a proximal-distal (nearshore-offshore) transect in South China. Our data show an overall decreasing δ⁷Li trend with distance from the shoreline, consistent with the variable mixing of a meltwater plume with high δ⁷Li (due to incongruent silicate weathering on the continent) and hypersaline seawater with low δ⁷Li (due to synglacial distillation). The evolution of low δ⁷Li of synglacial seawater, as opposed to the modern oceans with high δ⁷Li, was likely driven by weak continental chemical weathering coupled with strong reverse weathering on the seafloor underneath silica-rich oceans. The spatial pattern of δ⁷Li is also consistent with the development and then collapse of the meltwater plume that occurred at the time scale of cap dolostone accumulation. Therefore, the δ⁷Li data are consistent with the plumeworld hypothesis, considerably reduced chemical weathering on the continent during the Marinoan snowball Earth, and enhanced reverse weathering on the seafloor of Precambrian oceans.