C- O- S- and Sr-Isotope Variations in the Carbonate Successions of Siberian Platform and Events on the Precambrian/Cambrian Boundary

B. G. Pokrovsky Geological Institute of RAS, Pyzhevsky per.,7, 109017 Moscow, Russia

pokrov@ginran.msk su

The classic, fossil-rich sequences of Vendian-Cambrian transition beds are located in the Siberian Platform. Isotope study of them was begun by Magaritz et al. (1986), who found a strong increase of d13C values from -4.8 to 3.6 (” PDB) in the Ust'Yudoma formation of the Dvortsy section, Aldan river. Now, isotope date from the North (Kotuykan river), South-West (Vanavara region) and Far South-East (Uchur and Gonam rivers) of the Siberian Platform are also available (Pokrovsky and Vinogradov, 1991; Pokrovsky and Missarzhevsky, 1993; Vinogradov et al., 1994; Pokrovsky, 1996).

The carbon isotope variations in all studied sections are similar. The most significant element of d13C curves is strong negative anomaly, with d13C values decrease to -5.8” in Kotuykan and to -9.1” in the Vanavara sections. The anomaly coincide with the base of the Manykayan (Nemakit-Daldynian) Stage - the first stage of the Cambrian and marks extremely important event in the Earth's history - occurrence of the oldest skeletal animals (Missarzhevsky, 1989). During the Manykayan Stage the d13C values increase in 9.3 ” in the Kotuykan, 11.9 ” in the Vanavara, 8.4 ” in the Aldan and 5.7 ” in the Uchur sections, mounting to maximum (1.2..3.6”) near the lower boundary of the Tommotian Stage. The d34S values in anhydrites layered with carbonates in the Vanavara sections increase from 30...31 to 35...38 (” MS) contemporaneously with the fall of d13C just below of the Manykayan Stage, showing a significant activisation of Corg - sulphate oxidation - reduction processes.

There is no strontium isotope evidences for change of tectonic activity during theLate Vendian - Early Cambrian: initial (87Sr/86Sr)0 ratios in carbonates and anhydrites from Kotuykan and Vanavara sections display a time-independent variation from 0.70799 to 0.71140, explicable by diagenetic migration of Sr from terrigeneous deposits, and in the less altered samples are withing the limits: 0.70799 - 0.70854. The oxygen isotope curves within the transition sequences contain however a characteristic ledge - the fall of d18O values from 27-28 to 24-25 (” SMOW), which is difficult to explane by diagenesis, because it has identical patterns in sections distant more, than 1000 km and any d13C -d18O correlation is absent. The oxygen isotope fall can be explained by a replacement of evaporite basins by normally marine on the regional and warming/deglasiation on the global scale. The fall of d18O is not synchronously, but always after the fall of d13C, showing a connection of climatic and glacio-eustatic events with global carbon cycle.

It can be suggested, that the lowering of 13C in sedimentary carbonates and corresponding lowering of 13C in CO2 atmosphere at the Precambrian/Cambrian boundary reflect an increase in the rate of oxidation of organic matter accumulated at the Late Riphean - Early Vendian, when rate of Corg barial was unusually high (Kaufman et al., 1993). Black shales, oil and gas pools are common in the Late Riphean and Vendian of Siberia, as well as high 13C carbonates, being evidences of high rate Corg barial. Important role in the Corg oxidation could play diagenetic processes in the evaporitic basins occupied large areas at the InphraCambrian time. Those basins, with lowest 13C carbonates registered (Vanavara), were probably those "windows" through which light CO2 penetrated into the atmosphere. It should be evident, that negative d13C excursion signals an increase in CO2 contents of the atmosphere leading to global environmental changes, among which the warming, intensification of chemical weathering and enrichment of seawater by Ca and P are most important. Thus isotope data do not contradict the hypothesis (Kazmierczak et al., 1985) suggesting that mineral skeleton occurs previously as a reaction of organisms to oversaturation of seawater by calcium carbonate.


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