The Shuram excursion: A response to climate extremes at the dawn of animal life

Abstract

The Ediacaran-aged Shuram excursion was the last and largest of the Neoproterozoic negative carbon isotope anomalies. Recognized in stratigraphic successions around the globe, it precedes diverse evidence for macroscopic, multicellular life, and follows the Cryogenian global glaciations and Ediacaran Gaskiers glaciation. Hypotheses for the cause of the Shuram excursion can be broadly grouped into those that argue for post-depositional diagenetic alteration of the carbon isotope record and those that argue the extremely low δ¹³C values reflect a primary perturbation to the carbon cycle. Given the timing and magnitude of this event, distinguishing between these disparate hypotheses, or combining them, is critical for reconstructing the environmental conditions under which complex life evolved on Earth. We test specific predictions of each model using a range of stratigraphic observations and micro- and macro-analytical techniques. We find that the type sections in Oman where the Shuram excursion was first described are well preserved and contain a range of features difficult to reconcile with a post-depositional origin. However, many salient features are consistent with an extreme warming event coupled to a carbon cycle perturbation, analogous to the Paleocene-Eocene Thermal Maximum (PETM), and increased middle Ediacaran volcanism. We propose that cooling associated with the recovery was critical for origination rates of macroscopic soft-bodied organisms.

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