NATURE GEOSCIENCE | VOL 4 | MAY 2011 | www.nature.com/naturegeoscience 285 T he 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 CO 2 emitted from volcanoes and the weathering of sedimentary rocks departs the fuid Earth in two primary sinks — carbonate minerals, and organic car- bon; the burial of organic carbon can be linked stoichiometrically to fuxes of O 2 to the atmosphere. Owing to biases intrinsic to the sedi- mentary 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 pro- portional organic carbon burial fux (for example, ref. 1), creating time series data by measuring many samples collected in a strati- graphic 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. Te history of the carbon cycle is recorded in carbonate miner- als and organic compounds found in sediments and sedimentary rocks of modern to Archean age 3–10 . 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 fuxes of carbon 2,8,11–15 . Te behaviour of the carbon cycle varies over geologic time, marked by diferent steady states punctuated by brief intervals of anomalous dynamics 16–22 . Tese anomalies ofen signify special events in the history of life, such as the rise of macroscopic and skeletonized ani- mals, as well as mass extinctions 2,8,16,23–26 . Te secular variability of marine carbon isotope ratios in carbon- ate-rich sedimentary successions also provides a basis for global cor- relation 18 . Owing to the lack of robust biostratigraphic constraints, construction of ‘carbon isotope curves’ has seen widespread applica- tion to correlate poorly fossiliferous Precambrian strata, particularly those of the Neoproterozoic era 17 . Carbon isotopic data typically are Enigmatic origin of the largest-known carbon isotope excursion in Earth’s history John P. Grotzinger 1 *, David A. Fike 2 and Woodward W. Fischer 1 Carbonate rocks from the Middle Ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in Earth history. This fnding suggests a dramatic reorganization of Earth’s carbon cycle. Named the Shuram excursion for its original discovery in the Shuram Formation, Oman, the anomaly closely precedes impressive events in evolution, including the rise of large metazoans and the origin of biomineralization in animals. Instead of a true record of the carbon cycle at the time of sedimentation, the carbon isotope signature recorded in the Shuram excursion could be caused by alteration following deposition of the carbonate sediments, a scenario supported by several geochemical indicators. However, such secondary processes are intrinsically local, which makes it difcult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe. Both possibilities are intriguing: if the Shuram excursion preserves a genuine record of ancient seawater chemistry, it refects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern Earth. If, however, it represents secondary alteration during burial of sediments, then marine sediments must have been globally preconditioned in a unique way, to allow ordinary and local processes to produce an extraordinary and widespread response. plotted as a function of stratigraphic position 12,24,27–31 to highlight secular changes in seawater chemistry through time. As the evaluation of Neoproterozoic chemostratigraphy devel- oped, an extraordinary excursion was discovered in the Ediacaran Shuram Formation, Oman 28 . Now known as the ‘Shuram excur- sion’ (SE), it constitutes one of the most impressive carbon isotopic excursions in Earth history 32,33 . Plotting, by rank order, the sign and magnitude of carbon isotope excursions recorded over the past 1,000 million years (Myr) shows that the largest anomalies are nega- tive in sign and found in Neoproterozoic strata (Fig. 1). Te SE is 1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA, 2 Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA. *e-mail: grotz@gps.caltech.edu –15 –10 –5 0 5 10 Neoproterozoic Palaeozoic Mesozoic Cenozoic Rank order Shuram Magnitude (‰) Figure 1 | Historical variability of carbon isotopic composition of sedimentary rocks. Rank order of carbon isotope excursions greater than 2 ‰ over the past 1,000 Myr is shown in terms of sign and magnitude. Negative excursions are common in the Neoproterozoic stratigraphic record. The SE is the largest of these; such a distribution of isotope excursions through time provides clear evidence for large-scale change in the operation of the global carbon cycle over geologic history. Data from ref. 18. REVIEW ARTICLE PUBLISHED ONLINE: 17 APRIL 2011 | DOI: 10.1038/NGEO1138 © 2011 Macmillan Publishers Limited. All rights reserved