Friday, October 28, 2016

The Strontium Cycle of the NeoProterozoic was Driven by Seemingly Unique Paleogeography


Goddéris et al


The period spanning from 825 to 540 Ma is characterized by major changes in the surficial Earth system. This extraordinary interval starts with the breakup of the Rodinia supercontinent and eruption of a series of large igneous provinces and ends with the assembly of Gondwana, giving rise to the Pan-African orogenies. This paleogeographic reorganization is accompanied by a global climatic cooling, including the paroxysmal Cryogenian “snowball” glacial events. The 87Sr/86Sr of seawater displays a major long-term rise over this interval that is punctuated by episodic, smaller declines and inflections. We use a coupled deep time climate-carbon numerical model to explore the complex role of tectonics and climate on this distinct evolution in seawater 87Sr/86Sr. We show that the modulation of the weathering of the erupted large igneous provinces by continental drift explains the changes in seawater 87Sr/86Sr from 800 to 635 Ma. The subsequent sharp rise in seawater 87Sr/86Sr from 635 to 580 Ma is the result of erosion of radiogenic crust exposed in the Pan-African orogens. Coeval evolution of atmospheric CO2 displays a decrease from about 80 times the pre-industrial level around 800 Ma to 5 times just before the beginning of the Phanerozoic.

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