GLOBAL EVENTS ACROSS THE MESOPROTEROZOIC-NEOPROTEROZOIC BOUNDARY: C AND SR ISOTOPIC EVIDENCE FROM SIBERIA

Abstract

Thick, unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western Siberia preserve a record of Middle Riphean to Early Upper Riphean sedimentary environments and geochemistry. Consistent with data from other continents, our studies in the Uchur–Maya region in southeastern Siberia and the Turukhansk Uplift in northwestern Siberia suggest a first-order shift in δ13C from values near 0‰ in the early Mesoproterozoic to values near +3.5‰ after about 1300 Ma. Over this same interval, primary 87Sr/86Sr values decrease from >0.7060 to <0.7053. Combining lithologic, biostratigraphic, and geochemical data sets with available geochronologic constraints, we present a refined correlation between these two key Proterozoic successions in Siberia and add this dataset to a growing body of C and Sr isotopic data from this time interval. Carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first-order, ∼3.5‰ positive shift ca. 1250–1300 Ma, approximately coeval with the onset of active margin activity that predates the main phase of Rodinia assembly. Sr isotopic data may also be interpreted within the context of the evolving Mesoproterozoic tectonic regime. Available data suggest that no dramatic rise in 87Sr/86Sr heralds the main phase of Rodinia assembly in the terminal Mesoproterozoic, suggesting that significant juvenile crust was involved in mountain building, that relative hydrothermal flux from mid-ocean ridges remained high throughout the assembly of Rodinia and/or that increased continental runoff related to intense erosion of Grenvillian mountain belts terminated shortly after orogeny.