CHEMICAL WEATHERING DURING THE MARINOAN SNOWBALL EARTH

Termination of the terminal Cryogenian Marinoan glaciation (ca. 654–635 Ma) was immediately followed by the diversification of eukaryotes, suggesting the possible linkage between the global glaciation and biological evolution. The onset of the Marinoan glaciation was attributed to intense chemical weathering that significantly lowered atmospheric CO₂ level (pCO₂) during the interglacial interval, whereas its termination resulted from extremely high pCO₂, which might have triggered intense chemical weathering and global precipitation of cap carbonate. Thus, the coming and going of the global glaciation might directly relate to chemical weathering, and retrieve the intensity of chemical weathering during the Marinoan glaciation would provide the key evidence to test the snowball Earth hypothesis.

In this study, we measured Mg isotopes of siliciclastic component (δ²⁶Mg) and organic carbon isotopes (δ¹³C_org) of the Datangpo and Nantuo formations, which represent the interglacial and the Marinoan glacial deposits in South China, respectively. Both δ²⁶Mg and δ¹³C_org show gradual increasing trends from the upper Datangpo Formation to the lower diamictite unit of the Nantuo Formation, suggesting a counterintuitive increase of weathering intensity. In the lower sandstone/siltstone unit of the Nantuo Formation, δ²⁶Mg decreases and shows little variation, whereas δ¹³C_org keeps in constant. Further decline of δ²⁶Mg in the upper diamictite unit with invariant δ¹³C_org imply extremely low intensity of chemical weathering and inactive biogeochemical cycle during the global glaciation. A positive excursion in δ²⁶Mg followed by a sharp decline within the upper sandstone/siltstone unit may be attributed to extremely high weathering rate in the initial meltdown of snowball Earth and a decline of weathering rate due to a drawdown in pCO₂ after the intense chemical weathering.

Our data indicate that the hydrological and biogeochemical cycles were still active during the deposition of the lower Nantuo Formation, and thus the ocean might not be completely frozen in the early stage of glaciation. Global frozen might have occurred during the deposition of the upper diamictite unit. These suggest that the onset of the Marinoan glaciation was a gradual process, but the meltdown of the snowball Earth was abrupt instead.