Friday, June 28, 2013

Profound, Chronologically Correllated Deep Time Events on the Surface of the Earth

Secular changes at the Earth's surface; evidence from palaeosols, some sedimentary rocks, and palaeoclimatic perturbations of the Proterozoic Eon


1. Grant M. Young (a)


a. Department of Earth Sciences, Western University, London, Ontario, Canada N6A 5B7


Secular changes in surficial processes and products are closely linked to plate tectonics, atmospheric composition, solar evolution and climate. Most siliciclastic sediments and rocks are derived from weathering profiles rather than directly from older rocks. Many palaeosols older than ~ 2.2 Ga show depletion in Fe(T), whereas in younger palaeosols, and modern soils, Fe is virtually immobile. This is thought to reflect the accumulation of free oxygen in the Earth's atmosphere, as is the ‘disappearance’ of iron formations after about 1.8 Ga. The temporal distribution of mature siliciclastic sedimentary rocks containing detrital pyrite and uraninite, and the subsequent appearance of fluvial and shallow marine red beds provide compelling evidence for oxygenation in early Palaeoproterozoic times. During periods of supercontinentality, especially at low palaeolatitudes, enhanced weathering of exposed rock surfaces led to production of thick, extensive quartzarenites, some of which are associated with glaciogenic rocks. Intense weathering during periods of supercontinentality would have decreased atmospheric CO2, leading to extensive glaciations near the beginning and end of the Proterozoic Eon, and initiating a feedback loop that resulted in strong climatic oscillations until continental break-up brought stability to the climatic regime. It has been proposed that atmospheric oxygenation took place during warm climatic episodes between these glaciations when abundant nutrients were flushed into the oceans, stimulating unprecedented cyanobacterial blooms. Although many other supercontinents have been proposed, these unusual climatic conditions probably reflect rare critical relationships among solar luminosity, atmospheric composition and palaeolatitudinal distribution of continental lithosphere. Reappearance of iron formations associated with some Neoproterozoic glaciations may be explained by hydrothermal activity in semi-isolated rift basins.

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