Unsteady Gypsum Cycling as a Driver of Cryptic Perturbations in the Ancient Carbon Cycle

Earth's sedimentary record preserves large, intermittent deposits of evaporitic gypsum (CaSO 4 ·2H 2 O) that invoke the possibility of rapid changes in past seawater chemistry. In particular, strong imbalances between gypsum weathering and burial may drive changes in Earth's carbon cycle because calcium is a component of both gypsum and calcite/aragonite (CaCO 3 ). Here we use a box model of the coupled calcium‐sulfur‐carbon cycles to simulate gypsum cycling in the late Permian and more broadly across a range of Phanerozoic conditions. We find that large gypsum weathering or burial events can produce long‐lived (∼10 7 year) excursions in seawater alkalinity, dissolved inorganic carbon (DIC) and pH without affecting atmospheric pCO 2 . For example, late Permian gypsum burial drives increases in DIC and alkalinity roughly commensurate with the effects of a permanent 50% increase in CO 2 degassing, despite negligible change in pCO 2 . These excursions may represent a novel mechanism for generating long‐lived perturbations in Earth's ancient carbon cycle; they may also be relevant to pH‐based reconstructions of pCO 2 . The excursions documented here result from two interacting gypsum cycle drivers: tectonic forcing, which disrupts seawater carbon over <10 7 ‐year timescales, and the gypsum saturation state of seawater (Ω gypsum ), which stabilizes and restores the seawater carbon system toward baseline over >10 7 ‐year timescales. We argue that a gypsum burial flux influenced by Ω gypsum is consistent with the Phanerozoic geochemical record and has likely stabilized the seawater carbon system since the Neoproterozoic buildup of seawater sulfate.