LIMITS OF THE ISOTOPIC RECORD: THE IMPORTANCE OF RESERVOIR SIZE IN INTERPRETING GEOCHEMICAL EVENTS IN THE PROTEROZOIC

High-amplitude variation observed in the stable isotope records of carbon and sulfur, particularly during the Proterozoic Eon, has fundamentally challenged our understanding with respect to both the behavior of isotopic systems and the evolution of the Earth’s biosphere. In the last decade, this intellectual challenge has been met with exceptional interest from the scientific community and a nearly unprecedented focus on the collection of additional isotopic data. This explosion of data and ideas has been a true joy—not only is current work broadly consistent with the long-held scientific perspective that the Earth’s biosphere evolved from initially low pO2, high pCO2 conditions to low pCO2, high O2 conditions by the end of the Proterozoic, but there is a real excitement in being able to add depth to a system whose complexity has long been underestimated. It has become clear that the story of atmosphere-ocean evolution is one in which critical attention to detail is a prerequisite to understanding.

Yet, the cautionary words of Pierre-Simon Laplace (1749-1827) have never been more relevant: “the weight of evidence for an extraordinary claim must be proportioned to its strangeness.” For instance, the short time frames over which Proterozoic carbon and sulfur isotope variation is observed requires that non-steady state behavior be considered on equal terms with the more standard steady state behavior. Under these conditions, reservoir size becomes a critical limitation on our interpretations of the available data. When the reservoir size is large with respect to input-output fluxes, isotopic compositions are effectively buffered to marine isotopic values, thereby limiting our ability to record even potentially large changes in input-output fluxes. By contrast, when the reservoir size is small with respect to input-output fluxes, reliable signals may be indistinguishable from background, again limiting our ability to accurately interpret isotopic signals. Ultimately, the most effective interpretations of Proterozoic biospheric evolution will result from the combination of multiple geochemical proxies that are interpreted within the context of detailed facies and stratigraphic models, and independently evaluated in terms of source material and potential for diagenetic alteration.