An Assessment of the Precambrian/Cambrian Transition Events on the Basis of Carbon Isotope Records Paul Aharon 1. 2 and T. C. Liew 1 I Max-Planck lnstitut ftir Chemie, Saarstrassc 2:\, W-6500 MailE, rRG C On sabbatical lea\c from the Departmcnt of Geology & Geophysics, Louisiana State University. Baton Rouge. LA 70803. USA Abstract Renewed interest in the isotope events at the Pre- cambrian/Cambrian (PC/C) transition has led to a recent proliferation of high resolution b 13 C records acquired from sedimentary carbonate sections that are stratigraphically continuous. These b 13 C records gen- erally 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 b 13 C excursions have considerable potential as chron- ostratigraphic markers which are independent of the controversial biostratigraphic definitions currently used to delineate the PC/C boundary. The relation between b 13 C 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 b 13 C 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 dolo- mite phases are significantly more depleted in 18 0 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 rel- ationship observed between the 15 18 0 and b 13 C com- positions of all dolomite phases does not offer evidence of primary dolomite precipitation from Pre- cambrian sea water, as recently proposed by Tucker, but attests to the conspicuous absence of soil-derived CO 2 prior to the advent of organic soils. 1 Introduction Whereas the transition from the Precambrian Eon to the last 500 Ma of Earth history has long attracted the attention of geologists, only recently is its intricate nature becoming unraveled. Profound biotic changes, consisting of the emergence and subsequent extinction of the soft-bodied (Ediacaran) fauna, metazoan ex- plosive radiation, and the emergence of fauna with mineralized skeletons, figure prominently among those events which have occurred during the transition from the Precambrian to the Cambrian over a time interval of ca. 100 Ma (Fig. 1). Several models have attempted to establish a cause and effect relationship between the documented biotic events at the Precambrian/Cambrian (PC;C) bound- Major Events Vendian Cambrian i i L IMlu I Carbon Isotope Offset 1 ------- .> 6= 3%0 ---- Phosphogenesis 2 -?-?.. ? Biomineralization 3 -.- _7_7 Metazoan Radiation 4 -== - Ediacoran I soft - bodied) fauna 5 -? a. ?- Pre - EdiQ(oran biota biota (acritarchs, etc.) 6 high - - Y"Y , Sea level 7 700 650 600 550 500 Myrs Fig. 1. An "inventory" of major events during the Pre- cambrian-Cambrian transition which left their imprint in the geologic record. Sources for the compilation are: 1 Schidlowski et al. (1975); Veizer and Hoefs (1976); 2 Brasier (1980); Cook and Shergold (1984); Yanshin and Zharkov (1986);3.4.5 Cowie and Glaessner (1975); Brasier (1982); McMenamin (1987): 6 Knoll and Butterfield (1989); 7 Matthews and Cowie (1979); Bjorlykke (1982) M. Schidlo\\ski et aL (Eds.) Early Organic Eyolution: Implicatiom for Mineral and Energy Resources ( Springer-Verlag Berlin Heidelberg 1992