The Sr, C and O isotopic evolution of Neoproterozoic seawater

Sr and C isotopic data obtained on stratigraphic suites of well-preserved marine limestone from Siberia, Namibia, Canada, Svalbard and East Greenland provide a relatively detailed first-order record of isotopic variation in seawater through the late Neoproterozoic Era. This data is used to revise the 87Sr/86Sr and δ13C curves of this important interval, during which several discrete global ice ages occurred and the first macroscopic animals evolved. Through this time, the lowest 87Sr/86Sr values (ca. 0.7056) characterize the interval between about 750–800 Ma and have been interpreted to reflect a major hydrothermal event. From 750 to 600 Ma, the Sr isotope values oscillate between highs and lows, ranging between 0.7063 and 0.7074. Between 600 Ma and the Early Cambrian (ca. 535 Ma), 87Sr/86Sr values rise sharply from 0.7063 to 0.7087. This is thought to reflect enhanced continental input to the oceans associated with a Pan-African continental collision. This small subset of limestone samples (dolomites dominate the Neoproterozoic record) shows the δ13C curve rises from values close to 0 prior to 800 Ma to about +6‰ at 750 Ma and about +8‰ for the time between 600 and 730 Ma. During the time between 600 and 542 Ma, the highest values are about +4‰ (higher values in each interval are preserved in little-altered dolomites). Strong positive-to-negative excursions to values of −5‰ are associated with both Vendian glaciations estimated at about 575 and 590 Ma and with Sturtian glaciations estimated at about 720 and 740 Ma. In strong contrast, based on our view of least altered samples, there are no distinct changes in 87Sr/86Sr across Neoproterozoic glacial intervals. The duration of these global refrigeration events is a subject of considerable debate. However, consideration of Sr residence times based on elemental partitioning, and the relationship between δ13C and 87Sr/86Sr variations, suggest that these negative carbon isotope excursions would have lasted at least 350,000 years and no more than about one million years, assuming modern diagenetic fluxes of Sr to the oceans and total absence of continental fluxes.