Constraining the diagenesis of the Puga cap carbonate from U–Pb in‐situ dating of seafloor crystal fans, southern Amazonian craton, Brazil

Abstract

Massive cap carbonates punctuate the aftermath of two long lived global glaciations (Sturtian: ca. 717-659 Ma; and Marinoan: ca. 650/639-635 Ma;Hoffman et al., 2017). It is predicted that their precipitation resulted from alkalinity oversaturation of seawater due to rapid global ice melting and weathering of exposed continental shelves (Hoffman & Schrag, 2002). Oxygenation of the ocean in the recovery from this disturbed state may have promoted the earliest diversification of animals (Sahoo et al., 2012). Nevertheless, due to lack of fossil content and lithologies suitable for precise dating, the chronostratigraphy of these successions is poorly defined.Marinoan cap carbonates are recognized in more than 10 paleocontinents by well-developed decameter-thick cap dolostone and limestone with typical anomalous lithofacies and geochemistry (δ 13 C carb ca. −7 to −3‰ and 87 Sr/ 87 Sr carb ca. 0.7071-0.7073; et al., 2010). Their synchronicity is tested by U-Pb zircon ages (ca. 635 Ma) from volcanic ash layers within deglacial deposits from Australia, China, and Africa (Hoffman et al., 2017). HalversonNevertheless, timescale for cap carbonates deposition is unknown.Carbon cycling under ultra-high pCO 2 conditions would have led to "fast" carbonate precipitation (10 3 -10 4 years, Higgins & Schrag, 2003;Yu et al., 2018), however multiple paleomagnetic reversals in the cap dolostone implies 10 5 -10 6 years of deposition (Trindade et al., 2003).