Environmental Insights into Paleobiological Patterns of the Cambro-Ordovician Port au Port and St. George Groups, Western Newfoundland  

Marine ecosystems experienced significant changes during the Cambro-Ordovician interval.  The Cambrian Explosion established new ecosystem complexity and an increase in marine invertebrate diversity, yet the Middle Cambrian–Lower Ordovician interval is characterized by low abundance of skeletal organisms, such as reef builders.  Recent work on the carbonate-clastic strata of the Cambro-Ordovician Port au Port and St.  George Groups, western Newfoundland, has focused on quantifying skeletal abundances and analyzing geochemical proxies as a way to provide the environmental framework for the observed biological pattern.  Measurements of δ ¹³C_carbonate and δ ³⁴S_{(sulfate and pyrite)} reveal a possible link between environmental fluctuations and the abundance of skeletal organisms.  The geochemical data from Middle-Upper Cambrian strata show systematic shifts in δ ³⁴S_sulfate of >15‰ over relatively short stratigraphic distances (10 m, likely <1 Ma), low average δ ³⁴S_{sulfate-pyrite} (ca.  23‰) and a general positive coupling between δ ¹³C_carbonate and δ ³⁴S_sulfate.  In combination, these results indicate that Middle to Late Cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to isotopic variability than it was in either terminal Neoproterozoic or Cenozoic oceans. 

The skeletal abundances from the same Cambro-Ordovician sections were examined alongside the geochemical data.  Carbonate facies were analyzed in thin section to determine the proportion of carbonate that was skeletal.  In general, skeletal abundance is low during the Middle and Late Cambrian and shows a small increase in the Early Ordovician.  The major radiation of carbonate skeletal organisms occurs in the second half of the Early Ordovician and Middle Ordovician in western Newfoundland, indicating that environmental conditions were favourable for diversification of skeleton-secreters.  Future work on Lower and Middle Ordovician strata exposed in western Newfoundland will determine if the radiation of skeletal organisms occurs concurrently with stabilization of the carbon and sulfur cycles and/or the disappearance of anoxia.