Earliest Ediacaran speleothems and their implications for terrestrial life after the Marinoan snowball Earth

Abstract

Cryogenian snowball Earth glaciations may have had disastrous impacts on the biosphere, particularly the terrestrial ecosystem. However, how the terrestrial ecosystem responded to and recovered from these glaciations remains poorly understood. Speleothems offer important insights into terrestrial life because their formation is critically dependent on soil CO2 derived from microbial respiration. Here we report the wide distribution of miniature paleo-speleothems from the ∼ 635 Ma Doushantuo cap dolostone in South China and assess their implications for the recovery of terrestrial life after the terminal Cryogenian Marinoan glaciation. The ∼ 3-m-thick cap dolostone was deposited during the initial transgression following deglaciation but subsequently experienced subaerial exposure due to post-glacial rebound, which resulted in the development of extensive sheet-cavities in the cap dolostone and a widespread karstic surface atop the cap dolostone. The sheet-cavities were filled with multiple generations of minerals, including isopachous dolomite interpreted to have formed in the phreatic zone, speleothems consisting of fibrous calcite interpreted to have formed in the vadose zone, as well as later phases (i.e., isopachous radial chalcedony, crystalline quartz, and blocky calcite). The Doushantuo speleothems are millimeter-to-centimeter in size and include gravitational speleothems (stalactites and stalagmites) and non-gravitational speleothems (helictites, moonmilk, botryoids, and flat crusts). Some of them were secondarily silicified by hydrothermal fluids before a renewed transgression in which ∼ 632 Ma shales overlying the cap dolostone were deposited. The wide distribution of Doushantuo speleothems and the preservation of microfossils in these speleothems indicate the presence of an active soil-microbial ecosystem in the earliest Ediacaran Period and the rapid recovery of terrestrial life after the Marinoan snowball Earth.