Interpretations of major climatic and biological events in Earth history are, in large part, derived from the stable carbon isotope records of carbonate rocks and sedimentary organic matter1,2. Neoproterozoic carbonate records contain unusual and large negative isotopic anomalies within long periods (10–100 million years) characterized by δ13C in carbonate (δ13Ccarb) enriched to more than +5 per mil. Classically, δ13Ccarb is interpreted as a metric of the relative fraction of carbon buried as organic matter in marine sediments2,3,4, which can be linked to oxygen accumulation through the stoichiometry of primary production3,5. If a change in the isotopic composition of marine dissolved inorganic carbon is responsible for these excursions, it is expected that records of δ13Ccarb and δ13C in organic carbon (δ13Corg) will covary, offset by the fractionation imparted by primary production5. The documentation of several Neoproterozoic δ13Ccarb excursions that are decoupled from δ13Corg, however, indicates that other mechanisms6,7,8 may account for these excursions. Here we present δ13C data from Mongolia, northwest Canada and Namibia that capture multiple large-amplitude (over 10 per mil) negative carbon isotope anomalies, and use these data in a new quantitative mixing model to examine the behaviour of the Neoproterozoic carbon cycle. We find that carbonate and organic carbon isotope data from Mongolia and Canada are tightly coupled through multiple δ13Ccarb excursions, quantitatively ruling out previously suggested alternative explanations, such as diagenesis7,8 or the presence and terminal oxidation of a large marine dissolved organic carbon reservoir6. Our data from Namibia, which do not record isotopic covariance, can be explained by simple mixing with a detrital flux of organic matter. We thus interpret δ13Ccarb anomalies as recording a primary perturbation to the surface carbon cycle. This interpretation requires the revisiting of models linking drastic isotope excursions to deep ocean oxygenation and the opening of environments capable of supporting animals9,10,11.
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Laboratory assistance was provided by G. Eischeid, E. Northrop, E. Kennedy, T. O’Brien, A. Breus and A. Masterson. We thank G. Halverson, A. Bradley, E. Tziperman and P. Huybers for discussions and comments. We thank the Yukon Geological Survey, the NSF (grant number EAR-IF 0949227 to D.T.J.), KINSC (Haverford College), Henry and Wendy Breck (to D.P.S.), ESEP (Canadian Institute for Advanced Research, to P.F.H.), Harvard University and NASA NAI (D.T.J. and F.A.M.) for funding.
This file contains Supplementary Text, Data, Methods and Materials, Supplementary Figures 1-8 with legends, Supplementary Table 1 and Supplementary References.