BALANCE AND IMBALANCE IN MAJOR ELEMENT CYCLES (Invited Presentation)

Earth’s long-term carbon cycle reflects a balance between inputs from volcanic/metamorphic degassing and weathering of sedimentary rocks and outputs through burial of carbonate rock and organic matter. Although the alkalinity removed during carbonate burial is typically thought to be replenished by weathering of silicate minerals, generation of sulfuric acid during pyrite oxidation counteracts silicate weathering by removing alkalinity from the ocean-atmosphere system. Together with additional proton sources and sinks, these alkalinity fluxes modulate the relationship between carbon fluxes and atmospheric carbon dioxide. While some existing models have considered carbon and alkalinity fluxes together when reconstructing carbon dioxide over geologic time, they often do so by embedding hypothesized feedback relationships. Such an approach may be limiting, however, given our incomplete knowledge of biogeochemical feedbacks and how those relationships have varied through time. That is, uncertainty in how the long-term carbon cycle is stabilized influences our interpretation of geochemical proxies and reconstructions of paleoenvironmental conditions. In this work, we explored controls on the stability of Earth’s carbon cycle using an idealized mathematical model of the ocean-atmosphere system that tracked carbon and alkalinity fluxes without making assumptions about positive or negative feedbacks. Our analysis identified sets of mutually canceling processes, called closed loops, that can operate at arbitrary magnitudes without influencing carbon dioxide. These loops included both simple sets of forward and reverse processes, such as evaporite formation and dissolution, as well as more complex interactions such as that between metamorphism, weathering, and carbonate formation. At steady-state, this model predicted that changes in the relative output of organic carbon and carbonate can be decoupled from the carbon isotopic composition of the ocean, and do not strictly necessitate changes in the redox state of the ocean-atmosphere system. These results naturally gave rise to timescale-dependence in both the organic carbon and pyrite sulfur burial fractions, and have broad implications for interpreting the geologic record of carbon and sulfur isotope ratios in sedimentary materials.