A model for the decrease in amplitude of carbon isotope excursions across the Phanerozoic

Abstract

The geological cycling of carbon ties together the ocean-atmosphere carbon pool, Earth9s biosphere, and Earth9s sedimentary reservoirs. Perturbations to this coupled system are recorded in the carbon-isotopic (δ^13^C) composition of marine carbonates. Large amplitude δ^13^C excursions are typically treated as individual events and interpreted accordingly. However, a recent compilation of Phanerozoic carbon isotopic data reveals that δ^13^C excursions are a ubiquitous feature of the geologic record, and thus should be considered in concert. Analysis indicates that Phanerozoic carbon isotope excursions, as a whole, have characteristic durations of 0.5 to 10 M.yr. and exhibit declining amplitude over time. These commonalities suggest a shared underlying control. Here we demonstrate that sinusoidal modulation of the sensitivity of organic carbon and phosphate burial in a simple numerical model of the geologic carbon cycle results in large, asymmetric δ^13^C oscillations that exhibit their largest amplitudes in the 0.5 to 10 M.yr. period range. As anoxia is known to strongly modulate the C:P burial ratio of organic matter in sediments, we propose that sea-level oscillations were the primary source of sinusoidal modulation for the geologic carbon cycle, and that their degree of influence on the carbon cycle was determined by the state of oxygenation of bottom waters overlying the continental shelves. When oxygen minimum zones (OMZs) were large, shallow, and prone to expansion, sea-level changes would have had the capacity to drive large changes in the areal extent of OMZs in contact with the sea-floor, resulting in strong leverage on the burial sensitivity of organic carbon and phosphate, and thus on δ^13^C. Progressive oxygenation of the oceans, which was facilitated by biological innovations, resulted in a decline in the amplitude of δ^13^C excursions over the Phanerozoic, and the biogeochemical stabilization of the Earth System.