Precipitation of Marinoan cap carbonate from Mn-enriched seawater

Abstract

Rapid diversification of eukaryotes immediately after the Marinoan Snowball Earth glaciation suggests the possible linkage between the extreme icehouse climate and biological evolution. To unravel how the severe ice-age had triggered biological evolution, it is essential to retrieve the postglacial environmental background. Cap carbonate that directly overlies the glacial deposit represents the earliest chemical deposition after the Snowball Earth glaciation, and thus records the postglacial seawater compositions. In the Yangtze Block, South China, the Nantuo Formation represents the deposition of the Marinoan Snowball Earth glaciation, and the overlying cap carbonates of the basal Doushantuo Formation have nearly identical thickness and similar lithological sequence and widely outcropped from the shelf to basin environments. In this study, we report carbonate associated Fe (Fecarb) and carbonate associated Mn (Mncarb) contents of the Doushantuo (Marinoan) cap carbonate samples collected from 13 outcrop sections and 2 drill cores, spanning from the shelf to basin environments of the Yangtze Block. In addition, we also analyzed Fe and Mn contents in siliciclastic deposits (Feclast and Mnclast) of the upper Nantuo Formation. The Doushantuo cap dolostone have significantly higher Mncarb but comparable Fecarb values, and thus show extremely high Mncarb/Fecarb ratios, compared with dolostone of other ages. This observation suggests precipitation of the Doushantuo cap dolostone from Mn-enriched yet Fe-lean seawater after the Marinoan Snowball Earth glaciation. The Mn-enrichment in the postglacial ocean might be attributed to a continuous hydrothermal Mn2+ flux for ~15 million years during the Snowball Earth. The hydrothermal-derived Mn2+ could be accumulated in the Snowball Earth ocean due to the absence of synglacial carbonate precipitation and the persistent reduced condition during the Snowball Earth glaciation. In addition, the postglacial remobilization of MnO2 from glacial sediments might have been fueled by the recovery of surface ocean productivity, contributing to the postglacial seawater Mn2+ inventory. MnO2 remobilization is supported by the lower Mnclast values of the top of Nantuo Formation. Our study supports the delayed cap carbonate precipitation after the melting of the Marinoan Snowball Earth glaciation, and the postglacial seawater was suboxic and enriched in Mn2+ but not Fe2+ throughout the Yangtze Block. Thus, the Ediacaran ocean oxidation might have predated the evolution of eukaryotes or the marine redox condition during cap carbonate precipitation might have been oxic enough for the evolution of eukaryotes, though there is a slightly delayed appearance of Doushantuo-type fossils in the stratigraphic record.