Lithological Controls on Aqueous Phosphorus on Ocean-covered Exoplanets

On Earth, weathering of subaerial continental crust is necessary for the delivery of phosphorus (P)—a critical nutrient for life—to the oceans. However, P inputs, and therefore the potential for nutrient limitation on biological productivity, must be different on terrestrial planets completely covered in oceans. Absent subaerial continents, the sole source of P can only be the submarine weathering of oceanic crust. Because nonvolatile dissolved species in the ocean must ultimately be sourced from reactions between seawater and the seafloor, the chemical composition of the oceanic crust is necessarily a primary control on ocean chemistry. Here we use geochemical modeling to explore the relationship between seafloor lithology and aqueous P availability. We find that the concentration of dissolved inorganic phosphorus (DIP) is coupled to the amount of calcium (Ca) in the submarine weathering system. More specifically, higher Ca decreases the solubility of the calcium phosphate mineral fluorapatite, which decreases DIP upon precipitation. Calcium also affects fluorapatite solubility via the precipitation of Ca-bearing carbonate minerals. For example, a decrease in calcite formation—and consequent increase in fluorapatite formation—leads to lower DIP. Because ocean crust Ca abundance controls aqueous Ca, we demonstrate the importance of considering lithological controls on ocean chemistry when assessing the habitability of ocean-covered planets. Lastly, we make first-order predictions that suggest that the range in metallicity of nearby stars—and the inferred resulting exoplanet compositions—could strongly modulate the potential for these stars to host planets with P-rich oceans.