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1 CSIRO Division of Water Resources, GPO Box 1666, Canberra ACT, 2601, Australia
2 BMR Division of Continental Geology, GPO Box 378, Canberra ACT, 2601, Australia
In the Late Proterozoic the world’s oceans changed from being relatively oxic and well-mixed, into to a less mixed and more stagnant system. This resulted in the accumulation of massive amounts of organic matter and pyrite in anoxic sediments on the sea floor, and the enrichment of P in the anoxic deep ocean waters. Except for relatively short periods of increased turnover and better ventilation during times of Late Proterozoic glaciation, the stagnant ocean system appeared to be the ‘normal’ condition. A Cambrian seawater 87Sr/86Sr curve is presented, and it is concluded that continental rather than magmatic inputs were the major influence on the Sr isotopic composition of the latest Proterozoic and Cambrian oceans. Significant rises in atmospheric O2 levels must have accompanied the periods of greatly enhanced organic matter burial. A return to a stable more oxic ocean system occurred around the Proterozoic/Cambrian boundary, at which time rifting of the supercontinent(s) created a large number of epicontinental seas at low latitudes, enabling deep P-rich ocean waters to be moved into shallow-water environments. While increasing pO2 levels during the latest Proterozoic may have been largely responsible for the Cambrian ‘radiation’ event, the increase in pCO2 levels at the beginning of the Cambrian, as the oceans became more oxic, may have been responsible for the acquisition of mineralized skeletal structures by soft-bodied organisms.
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  M. D. Brasier Towards a carbon isotope stratigraphy of the Cambrian System: potential of the Great Basin succession Geological Society, London, Special Publications, 1993; 70: 341 - 350. [Abstract] [PDF]
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