A CONTINENTAL FLOOD BASALT DRIVER FOR NEOPROTEROZOIC CLIMATE AND OXYGENATION

Extensive continental flood basalts (CFBs) were emplaced on Rodinia in the early Neoproterozoic as the supercontinent began to break apart. The predominance of mafic provenance during this time is recorded in Nd isotope data on Neoproterozoic shales from Laurentia, Australia, and South China. These data show a high fraction of sediment derived from CFB versus average continental basement leading into the Cryogenian period (f_maf = 0.6–0.9), followed by a nearly stepwise decline afterward. The abrupt decline in f_maf is attributed to the scouring of the continents by global ice sheets, and is mirrored by the seawater strontium isotope record as preserved in carbonate rocks. For ~100 m.y. preceding the Cryogenian, ⁸⁷Sr/⁸⁶Sr ratios varied between 0.7063 and 0.7068, with downturns and inflections linked to LIP emplacement. Strontium isotope ratios then shifted upward in step-wise fashion following both the Sturtian and Marinoan glaciations, reaching 0.7080 by the early Ediacaran, before rising more gradually to >0.7085 during Ediacaran assembly of Gondwana. These shifts reflect paleogeographies increasingly dominated by fresh basement complexes and diminished CFB. The coupled Nd and Sr isotope datasets, as well as published Os isotope data, support a basalt-weathering trigger for Sturtian glaciation and imply that hydrolysis of CFBs exerted a first-order control on early Neoproterozoic seawater chemistry and global carbon cycling. Because CFBs contain ~3 times more phosphorous than average continental basement, intense basalt weathering at this time can help account for the elevated organic carbon burial implied by high carbonate carbon isotope values in the early Neoproterozoic. At the same time, the abundance of reduced iron and sulfur in rapidly weathering CFBs would have limited growth in the atmospheric O₂ inventory while increasing delivery of sulfate to the oceans and ferric oxyhydroxides to marine sediments.