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Ian Campbell, The Australian National University (Australia)
Charlotte Allen, The Australian National University (Australia)
Rick Squire, Monash University (Australia)
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The rise and fall of ediacaran biota during the late Neoproterozoic period represent the beginnings of what was arguably the most-radical biological episode in Earth's history: the sudden appearance and explosive radiation of animals. Previously, we proposed that a dramatic flux of nutrients into Earth's oceans was crucial to this event. The nutrients were generated by the prolonged and intense erosion of a gigantic mountain ranged that we dubbed the Transgondwanan Supermountains. We now suggest the same episode of catastrophic erosion also triggered a marked increase in the production rate of O2 in the upper ocean. Therefore, extreme rates of continental erosion provided Earth with nutrient- and O2-rich upper oceans that represented the 'elixir of life' for ediacaran biota and their descendants. In fact, all major episodes of mountain building in Earth's history have influenced biological innovation in the same way, although the elixir generated in the late Neoproterozoic to Early Cambrian was by far the most powerful.
The crucial event that influenced the rise of animals was rapid erosion of the 8,000 km long and up to 1,500 km wide Transgondwanan Supermountains, which formed as a result of East-West Gondwana convergence between ca. 650 and 515 Ma. Intense long-lived erosion of the Supermountains introduced a massive flux of sedimentary rocks into the Ediacaran and Cambrian oceans, elevating the 87Sr/86Sr values in seawater to the highest levels in Earth's history. Coincident with this was a dramatic increase in the flux of Ca, bicarbonate ion, Fe and P into the oceans. Calcium and bicarbonate ion are needed for skeletons, Fe to facilitate the electron transfer reactions essential for N-fixing bacteria, and P is an essential component of DNA. As a consequence, primitive life forms (especially algae) underwent an unprecedented expansion and provided abundant food for higher life forms. This led first to the rise of ediacaran biota from ca. 575 Ma and then later to the Cambrian radiation. The three-fold increase in Ca in seawater between ca. 543 and 515 Ma resulted in increased CaCO3 supersaturation that led, in turn, to the first hard skeletal animals at ca. 545 Ma and thus the demise of the soft-bodied ediacaran biota. Although nutrients were clearly important in this process, O2 was crucial too.
We suggest that orogenic events associated with supercontinent assembly have produced step increases in Earth's atmospheric O2. Rapid weathering of common rock-forming minerals was ensured by increased relief and the release of CO2 into the atmosphere by the metamorphism of carbonates. The increase in photosynthetic activity due to the flourishing of algae led to a marked increase in the production of O2. During these periods the organic C was buried, preventing it back reacting with O2. Therefore, oxygen generated by the algae was released into the upper ocean where it was most-effective in supporting the rise of ediacaran biota.
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