MACROSTRATIGRAPHIC CONSTRAINTS ON THE GLOBAL CARBON CYCLE

The global carbon cycle plays a critical role in maintaining an equable climate on Earth. Thus, studying its operation in deep time is a cornerstone in our understanding of the co-evolution of life and the surface environment. Carbon isotopic measurements (δ¹³C) of marine carbonates are used as proxies for understanding the ancient carbon cycle. When high fidelity δ¹³C time-series can be constructed, and isotopic changes in contemporaneous dissolved inorganic carbon (DIC) are inferred, these records typically are interpreted as reflecting the relative importance of two canonical modes of carbon burial: (1) organic matter and (2) carbonate minerals. In this framework, temporal changes in δ¹³C result from changes in the fraction of total carbon burial as reduced carbon, which is linked directly to the redox state of Earth’s surface. Recent work, however, has challenged this paradigm, by suggesting that authigenic carbonate, precipitated during early diagenesis, may be an important ‘third sink’ for carbon that is itself sensitive to changes in atmospheric oxygen concentrations. If this model is correct, our understanding of the carbon cycle and global redox budgets needs to be redressed at a fundamental level. Testing this model is, however, challenging, as authigenic carbonate can be disseminated over a large volume of sediment. Here we use Macrostrat, a database of 23,841 rock units distributed among 949 geographic regions in North America, to help constrain the potential magnitude of the authigenic carbonate burial effect on the global carbon cycle. We use a newly developed continuous time age model for rock units in Macrostrat to assess continental patterns of sedimentation in the Phanerozoic with greater temporal precision than previous studies. Because sedimentary lithologies vary in their potential for fostering authigenic carbonate formation, our analysis focuses on quantifying the North American burial flux of individual sedimentary lithotypes. Across a range of reasonable parameters for weight % authigenic carbonate and values of its δ¹³C, the predicted impact on the δ¹³C of DIC is considerable in comparison to the overall variability in Phanerozoic δ¹³C records. Incorporating measurements of authigenic carbonates into Macrostrat will further improve the accuracy and precision of model predictions.