An Assessment of the Precambrian/Cambrian Transition Events on the Basis of Carbon Isotope Records
Renewed interest in the isotope events at the Precambrian/Cambrian (PC/C) transition has led to a recent proliferation of high resolution δ13C records acquired from sedimentary carbonate sections that are stratigraphically continuous. These δ13C records generally show a bimodal distribution of values, with a 13C-enriched mode in the end-Precambrian and a sharp transition to a 13C-depleted mode at, or slightly above, the inferred PC/C boundary. The time-bound δ13C excursions have considerable potential as chronostratigraphic markers which are independent of the controversial biostratigraphic definitions currently used to delineate the PC/C boundary. The relation between δ13C shifts and oceanic fertility changes is reasonably well known but the circumstances under which these changes have occurred at the PC/C boundary are unclear. Using quantitative arguments derived from a simplified carbon cycle model, an ocean stratification event is proposed to have occurred during the latest Vendian followed by a turnover and a return to a ventilated ocean in the lowest Cambrian. The exact relationship between the changing rates of ocean ventilation and the seemingly contemporaneous faunal turnovers remains to be explored.
An understanding of the process of dolomitization is essential for the correct interpretation of the δ13C records because of the predominance of dolomites at the end Precambrian. Using the Lesser Himalaya section as a case study, it is shown that samples contain at least two phases of dolomitization and only the early phase contains the seawater imprint. Late dolomite phases are significantly more depleted in 18O and 13C and more enriched in radiogenic 87Sr relative to the early dolomite phases, and preserve the imprint of hot fluids of crustal provenance which advected through the sediments during burial. The linear relationship observed between the δ18O and δ13C compositions of all dolomite phases does not offer evidence of primary dolomite precipitation from Precambrian sea water, as recently proposed by Tucker,but attests to the conspicuous absence of soil-derived CO2 prior to the advent of organic soils.
KeywordsCarbon Isotope Lower Cambrian Ocean Anoxic Event Positive Excursion Stratify Ocean
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