Abstract
Although phosphate is an essential macronutrient for marine biota, critical to our understanding of marine productivity, biogeochemistry, and evolution, its long-timescale geologic history is poorly constrained. We constrain weathering-derived fluxes and seawater concentrations of phosphate throughout the Phanerozoic (541 Ma to present), by developing a model for the coupled, long-term biogeochemical cycles of phosphate, carbon, oxygen, and calcium. We find that the relative contribution of continental and seafloor weathering to the total weathering rate exerts a first-order control on ocean productivity, through a previously uninvestigated mechanism. Specifically, continental weathering is a source of the limiting nutrient phosphate, but seafloor weathering is not. As a result, times in Earth history in which seafloor weathering constitutes a large fraction of the total weathering rate (e.g., the early Paleozoic and Mesozoic), are also times in which phosphate delivery to the ocean is relatively low. A lower concentration of phosphate in seawater likely affected primary productivity, oceanic and atmospheric oxygen concentrations, with possible implications for the evolution of marine fauna over Earth history.
Weathering controls on the Phanerozoic phosphate cycle