Cryogenian cap carbonate models: a review and critical assessment

The Cryogenian Period spans two major glaciations, the Sturtian Ice Age (similar to 720-660 Ma) and the Marinoan Ice Age (similar to 650-635 Ma), the termination of each of which was associated with a unique type of cap carbonate deposit. Cap carbonates are significant in providing a record of as-yet not fully understood ocean-chemical changes during the deglaciations following Snowball Earth events and in serving as readily recognizable event beds useful in global stratigraphic correlation of Neoproterozoic successions. Debate regarding the formation of cap carbonates has focused on three key issues: (1) alkalinity sources, (2) abiotic (chemical) versus biotic (microbial) precipitation, and (3) formation rates. Multiple hypotheses regarding cap carbonate formation have been advanced, including the weathering alkalinity model, the oceanic overturn model, the gas hydrate destabilization model, the plumeworld model, the starvation sediment model, the enhanced microbial activity model, and the calcareous loess model. We evaluated these models by considering their proposed solutions to the key issues above in the context of a global compilation of location, thickness, carbonate C-isotope, and paleomagnetic data for Cryogenian cap carbonates. Cap carbonates were probably produced through a combination of chemical and microbial processes in a strongly stratified deglacial ocean with a low-salinity lid. Correlation of delta C-13(carb) profiles shows that cap carbonate precipitation began synchronously but terminated diachronously at a global scale. Cap carbonates are markedly thicker in low-paleolatitude regions, suggesting greater alkalinity production and/or more rapid carbonate precipitation in those regions, which favors alkalinity production through continental weathering rather than through oceanic upwelling or methane oxidation. Calculation of alkalinity production rates based on a range of cap carbonate masses and formation intervals shows that minimum masses (similar to 2.2 x 10(21) g, i.e., comprising only known continental deposits) could have been produced through intense continental weathering at short timescales (10(3)-10(4) yr), validating rapid deposition models. Longer timescales of cap carbonate formation (10(5)-10(6) yr) are not precluded by these calculations but are hard to reconcile with physical evidence of rapid accumulation and near-complete lack of terrigenous clastic impurities. Alkalinity production rate calculations also show that maximum cap carbonate masses (to similar to 14.4 x 10(21) g; i.e., assuming unproven deep-ocean cap carbonate deposits) are probably unrealistic, requiring more alkalinity than could be generated even through a combination of mechanisms including continental weathering, deep-ocean microbial sulfate reduction, and methane release.