PALEO-SPELEOTHEMS AS EVIDENCE FOR WIDESPREAD KARSTIFICATION IN THE AFTERMATH OF THE MARINOAN DEGLACIATION (635 MA) IN SOUTH CHINA

The Cryogenian Period (720–635 Ma) is bookended by the Sturtian and Marinoan glaciations, and represents a geological interval of catastrophic climate events. However, paleontological and biomarker data indicate that marine life persisted through and diversified after the Cryogenian Period. Similarly, geochemical data provide tentative evidence that terrestrial life may have thrived in the early Ediacaran Period, immediately after the Marinoan glaciation. However, geological evidence for terrestrial ecosystem in the early Ediacaran Period is poorly documented. Speleothem formation is critically influenced by ground water interactions with soil-microbial ecosystems, thus paleo-speleothems are valuable archives of paleo-climatic changes as well as terrestrial ecosystems. Here, we describe six types of paleo-speleothems, including stalactites, stalagmites, helictites, moonmilk, flat coatings, and botryoidal crusts, from sheet-cracks in the basal Ediacaran cap dolostone (~635 Ma) of the Doushantuo Formation at multiple localities in South China. These paleo-speleothems are characterized with alternating organic-rich and organic-poor laminae. Some of these paleo-speleothems are silicified and preserve chalcedony fabrics; these are interpreted as low-temperature silicified calcareous speleothems. These silicified paleo-speleothems can be further distinguished into several genetic types based on the inferred influence of dripping water, capillary water, and evaporative water. Our petrographic analysis shows that the Doushantuo sheet-cracks were sequentially filled with isopachous dolomite, calcareous speleothems, chalcedony (which also replaces some speleothems), and quartz, which are inferred to have formed during the uplift, karstification, and subsequent low-temperature hydrothermal processes. The wide distribution of ~635 Ma paleo-speleothems in South China indicates pervasive post-glacial karstification and terrestrial weathering, perhaps facilitated by an active soil-microbial ecosystem.