The Sr, C and O isotopic evolution of Neoproterozoic seawater

Sr and C isotopic data obtained on stratigraphic suites of well-preserved m arine limestone from Siberia, Namibia, Canada, Svalbard and East Greenland provide a relatively detailed first-order record of isotopic variation in s eawater through the late Neoproterozoic Era. This data is used to revise th e Sr-87/Sr-86 and delta(13)C curves of this important interval, during whic h several discrete global ice ages occurred and the first macroscopic anima ls evolved. Through this time, the lowest Sr-87/Sr-86 values (ca. 0.7056) c haracterize 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), Sr-87/Sr-86 values ris e sharply from 0.7063 to 0.7087. This is thought to reflect enhanced contin ental input to the oceans associated with a Pan-African continental collisi on. This small subset of limestone samples (dolomites dominate the Neoprote rozoic record) shows the delta(13)C curve rises from values close to 0 prio r to 800 Ma to about + 6 parts per thousand at 750 Ma and about +8 parts pe r thousand for the time between 600 and 730 Ma. During the time between 600 and 542 Ma, the highest values are about +4 parts per thousand (higher val ues in each interval are preserved in little-altered dolomites). Strong pos itive-to-negative excursions to values of -5 parts per thousand are associa ted with both Vendian glaciations estimated at about 575 and 590 Ma and wit h Sturtian glaciations estimated at about 720 and 740 Ma. In strong contras t, based on our view of least altered samples, there are no distinct change s in Sr-87/Sr-86 across Neoproterozoic glacial intervals. The duration of t hese global refrigeration events is a subject of considerable debate. Howev er, consideration of Sr residence times based on elemental partitioning, an d the relationship between delta(13)C and Sr-87/Sr-86 variations, suggest t hat these negative carbon isotope excursions would have lasted at least 350 ,000 years and no more than about one million years, assuming modem diagene tic fluxes of Sr to the oceans and total absence of continental fluxes. (C) 1999 Elsevier Science B.V. All rights reserved.