Enigmatic origin of the largest-known carbon isotope excursion in Earth's history

285 The global carbon cycle is the biogeochemical engine at the heart of acid–base and redox processes in the oceans and atmosphere. It constitutes the most fundamental way in which the biosphere shapes the chemistry of our planet. Studying the behaviour of the carbon cycle during times past, however, presents unique challenges. On geological timescales, the CO2 emitted from volcanoes and the weathering of sedimentary rocks departs the fluid Earth in two primary sinks — carbonate minerals, and organic carbon; the burial of organic carbon can be linked stoichiometrically to fluxes of O2 to the atmosphere. Owing to biases intrinsic to the sedimentary record, it is not possible to directly measure the amount of organic carbon buried as a function of time. Instead, geobiologists use carbon isotope ratios in carbonate rocks to constrain the proportional organic carbon burial flux (for example, ref. 1), creating time series data by measuring many samples collected in a stratigraphic section according to height (for example, ref. 2). Ultimately, carbon isotope ratios can provide a measure of the global carbon cycle at the geological instant of sedimentation. The history of the carbon cycle is recorded in carbonate minerals and organic compounds found in sediments and sedimentary rocks of modern to Archean age3–10. Over the past several decades studies of the chemostratigraphic variation of isotopic carbon have enabled reconstructions of ancient seawater composition including the variation of alkalinity, oxygen, and fluxes of carbon2,8,11–15. The behaviour of the carbon cycle varies over geologic time, marked by different steady states punctuated by brief intervals of anomalous dynamics16–22. These anomalies often signify special events in the history of life, such as the rise of macroscopic and skeletonized animals, as well as mass extinctions2,8,16,23–26. The secular variability of marine carbon isotope ratios in carbonate-rich sedimentary successions also provides a basis for global correlation18. Owing to the lack of robust biostratigraphic constraints, construction of ‘carbon isotope curves’ has seen widespread application to correlate poorly fossiliferous Precambrian strata, particularly those of the Neoproterozoic era17. Carbon isotopic data typically are Enigmatic origin of the largest-known carbon isotope excursion in Earth’s history