Maximum continental freeboard and topographic relief during Gondwana assembly likely triggered the Cambrian explosion

Plate tectonics and supercontinent cycles are the first-order drivers of Earth’s environmental evolution, helping to establish and maintain a habitable surface. The middle Neoproterozoic to Cambrian period recorded dramatic transitions towards Earth’s present-day environment, including the Neoproterozoic Oxidation Event (NOE) and the Cambrian explosion. Key tectonic factors that contributed to changes in Earth’s surface systems during this period, however, are largely confined to conceptual models. We provide deep time quantitative constraints on these changes through documenting the scale of global orogens and modeling continental freeboard conditions using whole-rock geochemical data. The results indicate that the Gondwana assembly formed a 9000 km long orogenic system with a global mean crustal thickness of over 55 km, comparable to a mean elevation of approximately 3 km above sea-level, comparable to that of the modern Alpine-Himalayan system. This resulted from establishment of the Earth’s contemporary plate tectonic regime and associated thermal state that allowed whole plate continental deep-subduction. Modeled continental exposure peaked during Gondwana assembly at 31 % of Earth’s surface area, exceeding that for both preceding and subsequent time frames. This indicates maximum continental freeboard and resultant subaerial exposure. The combination of significant lateral extent, high topographic relief and extensive low-latitude distribution of the Gondwana’s collisional orogenic belts resulted in maximum weathering and erosion intensity, supplying an exceptionally high-level sediment flux to the ocean and corresponding with high seawater Sr isotope and phosphorous (P) input. This profoundly changed seawater compositions and enhanced marine productivity, likely triggering NOE and providing environmental conditions conducive for the Cambrian explosion. This research is funded by the NSFC grants (42322208), the National Key R&D Program of China (grants 2022YFF0802700 and 2023YFF0803604).

Keywords: topography, crustal thickness, mountain elevation, subaerial continental crust, Gondwana assembly, the Cambrian explosion

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