Evaluating the relationship between the carbon and sulfur cycles in the later Cambrian ocean: An example from the Port au Port Group, western Newfoundland, Canada

We present a high-resolution delta S-34 (sulfate and pyrite) and delta C-13(carbonate) record from the Middle-Upper Cambrian Port au Port Group, a mixed carbonate-siliciclastic succession exposed in western Newfoundland, Canada. The results illustrate systematic delta(34)(sulfate) shifts of > 15 parts per thousand over relatively short stratigraphic intervals (10 m, likely 1 m.y.), low average Delta S-34(sulfate-pyrite) (ca. 23 parts per thousand) and a generally positive coupling between changes in delta C-13(carbonate) and delta S-34(sulfate). Together, these results indicate that Middle to Late Cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to change than it was in either terminal Neoproterozoic or Cenozoic oceans. However, a simple carbon (C) and sulfur (S) isotope box model of the Late Cambrian ocean illustrates that low sulfate concentrations alone fail to account for the > 15 parts per thousand delta S-34(sulfate) shifts recognized in Port au Port strata. Such large shifts can be generated only if fluctuating oceanic redox is invoked; marine anoxia forces reduced C/S burial and elevated Delta S-34, driving larger delta S-34 changes per mole of organic carbon buried. The conclusion that later Cambrian oceans featured both low sulfate levels and widespread subsurface anoxia supports hypotheses that link fluctuating marine redox conditions in the delayed recovery of skeletal animals and metazoan reefs from late Early Cambrian extinction. (c) 2009 Elsevier B.V. All rights reserved.