Global Environmental Catastrophe at About 2300 Ma

Abstract

It was found that the sedimentary REE patterns had changed at 2300 Ma at the southern margin of North China Craton (Chen and Zhao, 1997), resulting from the variation in oxygen fugacity of the hydrosphere and atmosphere. Since this discovery, we have studied comprehensively the changes in surficial environment at this time, and obtained a series of inspiring achievements which encourages us to exchange our research works and ideas with colleagues. Therefore, this paper attempts to display the evidence for a catastrophe at about 2300 Ma. Data shown in Table 1 support the suggestion that the geological environment (including sedimentary sphere, biosphere, hydrosphere and atmosphere) underwent a global catastrophe caused by extraterrestrial factors at about 2300 Ma. The catastrophe resulted in an obvious weakening of volcanism, the formation of an O2-rich atmosphere, a leap in palaeontological evolution, a sharp decrease in climatic temperature, and some other subsequent changes in rocks, ore deposits and element distributions. Our summary of these changes supports the points of view of Frakes (1979), Kasting (1993) and Schidlowski (1983). They had also argued that the atmosphere became oxidic from anoxic during the Palaeoproterozoic, especially around 2300 Ma. Now the time for environmental change is tentatively determined at 2300 Ma, the proof is as follows: (1) the strata over the boundary have not yet acquired convincing isotopic ages more than 2300 Ma; (2) the upper age limit of deposition for the Witwatersrand supergroup before the catastrophe is 2340 Ma (Rb-Sr isochron dating) (Frakes, 1979), and the Rb-Sr isochron age of the tuff interbeds in the Elliot Lake Group in the bottom of Huronian supergroup is 2330 Ma, they both abound with Rand-type Au-U deposits, so the catastrophe probably took place after 2330 Ma. The Cobalt Group, the topmost strata of the Huronian Supergroup, containing red beds, evaporates and carbonate rocks, showing clear negative sedimentary Eu-anomalies, was obviously formed after the catastrophe and dated at 2288+77 Ma (Rb-Sr isochron age for whole rocks) (Mossman and Harron, 1983), which indicates that the catastrophe took place before 2288 Ma. Hence, the catastrophe might occur between 2330 Ma and 2288 Ma, i.e. 2300_+ 30 Ma.