Photosynthetic fractionation of carbon: A biological driver for the Neoproterozoic Shuram carbon isotope excursion

• The Shuram excursion can be explained by a change in photosynthetic fractionation. • This mechanism does not require unbalanced oxidation of organic carbon. • A fractionation change could be driven by weathering or by the evolution of animals. • Fractionation changes occur in response to a range of environmental drivers. • Fractionation change is not likely to be the sole driver of the Shuram excursion. The Neoproterozoic Shuram excursion (574 to 567 Ma) is the single largest carbon isotope excursion in the past two billion years, yet its cause is still unclear. Previous suggestions have been contentious: suggestions of a diagenetic origin for the excursion cannot explain coeval shifts in organic carbon isotopes, but suggestions that the excursion represents a dramatic perturbation to the carbon cycle have faced criticism on the basis of the considerable flux of oxidants required to drive the observed change. Here, we present a third potential contributing factor: a change in the carbon isotope fractionation of photosynthesis. We investigate the explanatory power of this mechanism using a carbon box model, with comparison to empirical isotopic measurements from the Doushantuo Formation in China and the Shuram and Buah formations in Oman. Our results suggest that a ∼20 ‰ decrease in photosynthetic carbon isotope fractionation over the course of the Shuram excursion is sufficient to drive observed changes in both carbonate and organic carbon isotope ratios. Our modelling shows that this driver frequently outperforms other potential drivers of the Shuram excursion in terms of fit of model results to empirical data. Therefore, we suggest that a change in carbon isotope fractionation may have contributed to the Shuram excursion. However, this mechanism does not explain other contemporaneous phenomena, such as apparent changes in marine sulphur chemistry. Consequently, we suggest that a change in fractionation is unlikely to have been a sole driver of the excursion. A change in fractionation could have arisen either due to changes in the abiotic environment, which could have influenced producer physiology or partitioned carbonate and organic carbon formation, or as a consequence of the evolution of the first macroscopic animal communities in the Ediacaran, which could have modified plankton communities through feeding pressure, impacting the abiotic environment on a global scale.