Flood Basalts and Neoproterozoic Glaciation

Abstract:

Large igneous provinces (LIPs), which are commonly associated with supercontinental break-up, are the product of the emplacement of >10⁶ km³ of mafic rocks in less than a few million years. LIP magmatism, in particular continental flood basalt (CFB) volcanism, perturbs global climate on shorter time scales through the radiative effects of degassed SO₂ and CO₂. On longer time scales, CFBs alter climate through the effect of the high weatherabilty of mafic rocks (5–10 times greater than average continental crust) on global silicate weathering. A link between flood basalt weathering, Rodinia break-up, and Neoproterozoic snowball glaciation has been postulated. Here we present a new compilation of Nd isotope data on Neoproterozoic mudstones from Laurentia, Australia, and South China along with a new seawater strontium isotope record from well preserved carbonates that support this hypothesis. These datasets are consistent with an outsized role of basalt weathering on the global silicate weathering budget during the second half of the Tonian period (~850 to 725 Ma). Along with Os isotope data, they also suggest that an additional pulse of basalt weathering at the end of the Tonian may have initiated the Sturtian snowball glaciation. CFBs have relatively high concentrations of phosphorous. Hence, the drawdown in atmospheric CO₂ required to trigger the Sturtian snowball Earth was likely accomplished through a combination of increased silicate weathering rates and enhanced biological productivity driven by greater nutrient supply to the oceans. CFBs were also the likely source of the iron in Neoproterozoic iron formation (IF), all significant occurrences of which are restricted to Sturtian-aged glacial successions. Dramatic declines in εNd following the Cryogenian snowball glaciations are mirrored by stepwise increases in ⁸⁷Sr/⁸⁶Sr, reflecting the scouring of the continents by global ice sheets. This continental resurfacing removed the extensive basalt carapace as well as infertile regolith which had developed in the interior of the long-lived supercontinent and eliminated the unique driving mechanism for Cryogenian snowball glaciation.