NO EVIDENCE FOR MARINE DISSOLVED ORGANIC CARBON DRIVING CHANGES IN EDIACARAN ECOLOGY AND DIVERSITY

Marine dissolved organic carbon (DOC), the largest pool of reduced carbon in the oceans, plays a critical role in the global carbon cycle and contributes to the regulation of atmospheric oxygen and carbon dioxide concentrations. Despite its importance in global biogeochemical cycling, the long-term evolution of the marine DOC reservoir has historically been poorly constrained. However, significant changes to the size of the marine DOC reservoir through Earth’s history have commonly been invoked as an explanation for major state shifts in ocean chemistry, carbon cycling, and marine ecology inferred from geologic and fossil archives. In particular, the presence of a substantially larger marine DOC reservoir—potentially even 30 times larger than modern seawater levels of dissolved inorganic carbon—has been widely interpreted to have shaped some of the dramatic negative carbon isotope excursions of the Neoproterozoic, such as the upper Ediacaran Shuram excursion, and the ecology and diversity of the Ediacara Biota.

Using a mechanistic carbon cycle model that can accurately reproduce DOC concentrations and gradients in both oxic and anoxic marine environments, we present a revised view of the evolution of the marine DOC reservoir. We demonstrate that—in contrast to long-held views—the overall size of the marine DOC reservoir has likely undergone only minimal variation through Earth’s history, despite major changes in the redox state of the ocean-atmosphere system, global climate state, and in the nature and efficiency of the biological carbon pump. A relatively static marine DOC reservoir through Earth’s history further indicates that it is unlikely that major past changes in marine DOC concentrations were responsible for driving either the large carbon isotope excursions observed in the Neoproterozoic stratigraphic record or changes in Ediacara assemblage diversity. These results additionally cast doubt on previously hypothesized links between marine DOC levels, the rise of eukaryotic algae and the emergence and radiation of early crown-group animals.