A STATISTICAL ANALYSIS OF THE CARBON ISOTOPE RECORD FROM THE ARCHEAN TO PHANEROZOIC AND IMPLICATIONS FOR THE RISE OF OXYGEN

Organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales. Permanent burial of organic carbon in the crust or mantle oxidizes the surface environment (atmosphere, ocean and biosphere) by removing reduced carbon. It has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout Earth's history, thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen (Schidlowski, 1988; Calling and others, 2001; Holland, 2002; Holland, 2009; Kump and others, 2009; Rothman, 2015). However, the opposite conclusion has been drawn from the same carbon isotope record (Des Marais and others, 1992; Bjerrum and Canfield, 2004). To test these opposing claims, we compiled an updated carbon isotope database and applied both parametric and non-parametric statistical models to the data to quantify trends and mean-level changes in fractional organic carbon burial with associated uncertainties and confidence levels. We first consider a conventional mass-balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon. For this model, statistical analysis implies fractional organic burial has increased over Earth history by a factor of 1.5 relative to organic burial at 3.6 Ga, with the 95 percent confidence interval ranging from factors of 1.2 to 2.0. An increase in organic burial by a factor of 1.2 cannot explain the rise of oxygen, whereas an increase by a factor of 2 could conceivably explain the rise of oxygen. There is, however, a highly significant and well constrained increase in organic burial from the Proterozoic to the Phanerozoic. We also analyze changes in the difference between carbonate and organic carbon isotopic ratios over Earth history. There is a statistically significant increase in this difference from the early to late Archean, possibly caused by increased biological fractionation due to methanotrophic recycling. This transition is consistent with the evolution of oxygenic photosynthesis at 2.8 Ga or earlier. Finally, we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes, specifically ocean crust carbonatization and authigenic carbonates. Because the size of these fluxes has a large, poorly constrained range, our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all. However, it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time, in which case conclusions from the conventional model would be valid.