Himalayan magnesite records abrupt cyanobacterial growth that plausibly triggered the Neoproterozoic Oxygenation Event

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The widespread glaciation (∼750–580 Ma) of the late Neoproterozoic Snowball Earth and the subsequent (∼630–551 Ma) oxygenation of the oceans and atmosphere were critical factors in microbial evolution and the following ∼540–520 Ma explosion of complex life forms in the Cambrian. However, the underlying mechanisms that link these processes are still unclear due to poorly preserved fossil records and the tectonic destruction of the paleo-oceans, limiting our understanding of the Neoproterozoic Earth, a key time when unicellular life evolved into complex forms. Droplets of Neoproterozoic oceanwater and freshwater trapped in Himalayan crystalline magnesite (MgCO3) show that sedimentary basins were deprived of calcium for an extended period during the Snowball Earth glaciation, probably due to low riverine input of the dissolved products of weathering. Protracted precipitation of calcite-dolomite in a closed basin, now exposed in Kumaun Himalayas, Uttarakhand (India), decreased the Ca/Mg ratio in the seawater. This fall in the Ca/Mg ratio—initiated magnesite precipitation and increased the basin's oligotrophy (nutrition deficiency). The slow-growing photosynthetic cyanobacterial stromatolites could thrive in such oligotrophic conditions. The same is reflected in the field, i.e., the appearance of stromatolites towards the end of dolomite formation and their population expansion in the overlying magnesite beds. Continued magnesite precipitation in a closed basin, driven by positive feedback, resulted in high photosynthetic oxygen production, conceivably triggering the Neoproterozoic Oxygenation Event and stimulating Cambrian Explosion.