Geology: Submarine volcanism along shallow ridges did not drive Cryogenian cap carbonate formation

The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high CaCO3 saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea-level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite — a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea’s passive continental margins, and by assessing Neoproterozoic reconstructions of continents. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations around sea level are rare and have anomalous crustal thicknesses > 14 km that only occur proximal to Large Igneous Provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths < 200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found it to be insignificant along the outer Vøring Plateau — an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates, and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.