THE SR ISOTOPIC EVOLUTION OF THE OCEANS DURING THE CAMBRIAN EXPLOSION

The Late Precambrian and Cambrian were periods of marked global change in the biosphere with the appearance and rapid radiation of skeletali zed metazoans. However, the changes in atmosphere and oceanic environm ents that may have accompanied this evolutionary episode remain largel y unresolved. In this contribution carbonate samples from Lower Cambri an sections in eastern Siberia are used to construct a composite Sr is otope curve across the Nemakit-Daldynian-Tommotian boundary (the forme r Precambrian-Cambrian boundary in Siberia). The results show a signif icant fall in seawater Sr-87/Sr-86 ratios across this interval. This d ecrease is placed in context by the construction of a new seawater Sr curve for the late Neoproterozoic to early Ordovician, compiled from t hese data and those available in previous publications. The ocean-atmo sphere interactions recorded by the marked variation in seawater Sr im mediately preceding and during the Cambrian potentially record short-t erm (15-20 Ma) hydrothermal events punctuating an overall increase in terrestrial weathering rates, although further research is needed to c onfirm this conclusion.