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 skeletalized metazoans. However, the changes in atmosphere and oceanic environments that may have accompanied this evolutionary episode remain largely unresolved. In this contribution carbonate samples from Lower Cambrian sections in eastern Siberia are used to construct a composite Sr isotope curve across the Nemakit-Daldynian-Tommotian boundary (the former Precambrian-Cambrian boundary in Siberia). The results show a significant fall in seawater Sr-87/Sr-86 ratios across this interval. This decrease is placed in context by the construction of a new seawater Sr curve for the late Neoproterozoic to early Ordovician, compiled from these data and those available in previous publications. The ocean-atmosphere interactions recorded by the marked variation in seawater Sr immediately preceding and during the Cambrian potentially record short-term (15-20 Ma) hydrothermal events punctuating an overall increase in terrestrial weathering rates, although further research is needed to confirm this conclusion.