Shuram excursion

Abstract

The Shuram excursion represents the greatest negative carbon isotopic excursion in earth history, and provides an important chemostratigraphic marker horizon of global extent. The excursion is linked to the second great oxygenation event in earth history, an oxygen crisis that resulted in a transition from sulfidic oceans to a marine realm rich in sulfate. The Shuram excursion (560–550 Ma) is represented in Sonora, México by the Clemente oolite of the Clemente Formation. Ediacaran fossils (such as the Clemente biota of Unit 4 of the Clemente Formation) occur in rocks deposited below the excursion. The age of the Clemente Ediacaran biota thus falls between 550 and 560 Ma. In spite of the fact that the Sturtian glaciation apparently triggered the earliest known mass extinction on earth (the Tindir Mass Extinction), several lines of evidence suggest that the biosphere controlled the timing of and the onset of the Late Proterozoic glaciations, and that it also controlled the timing of the melting of the ice. Furthermore, it appears that the biosphere itself influenced the timing of the appearance of the Ediacaran biota. Whereas snowball earth events lurched suddenly from very cold (tillites) to very hot (cap carbonates) climate, the sequence going from the Gaskiers glacial event (c. 580 million years ago) to the Shuram was part of a wild climatic gyration where the earth went from hot (intense granite weathering at high latitudes) to cold (Gaskiers glaciation) to hot (Shuram event). The Shuram is the greatest negative carbon isotopic excursion in earth history, possibly because this is the moment in earth history when the burrowing animals assert themselves in a geochemical sense, and by remobilizing sea floor carbon, forestall a major glaciation.