Paper No. 1
Presentation Time: 8:00 AM
THE SHURAM EXCURSION: ENIGMATIC CARBON CYCLING
GROTZINGER, John, Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, FISCHER, Woodward W., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 and FIKE, David A., Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, grotz@gps.caltech.edu
A profound negative excursion in the carbon isotopic composition of sedimentary rocks is recorded in middle Ediacaran-age strata globally. The excursion was first recognized in the Shuram Formation, Sultanate of Oman, by Burns and Matter (1993), who interpreted it as a primary feature of the carbon cycle based on its widespread distribution throughout Oman. Subsequently, similar magnitude excursions were found in South Australia, the western US, south China, and Siberia. These sections have globally dispersed paleogeographic locations, but robust correlation is hindered by limited geochronologic data. The defining attributes of the excursion are its large magnitude (>15 permil); stratigraphic ordination between Marinoan post-glacial cap carbonates (630 Ma) and the Precambrian-Cambrian boundary (542 Ma), likely pre-dating the appearance of Ediacaran macrobiota; expression as a single, continuous fall and rise; the substantial thickness of strata over which it is recorded, often hundreds of meters; and development in wave- and storm-dominated outer shoreface facies interbedded with oxidized, organic-lean red siltstones.
The global biogeochemical significance of the Shuram excursion has been the subject of intensifying discussion, including speculation about a causal relationship to the rise of Ediacaran organisms. A further complication arises because many chemostratigraphic sections show a correlation between oxygen and carbon isotopic variations; this pattern has invited additional hypotheses wherein the Shuram excursion arises from processes that are fundamentally diagenetic in origin. In this talk we will review the suite of geological observations that must be satisfied by a good hypothesis. Currently no hypothesis adequately explains all of the features of the chemostratigraphic pattern. The Shuram excursion is possibly unique in Earth history, and future work will be well served to consider and devise tests for multiple hypotheses simultaneously.
