The role of calcium in regulating marine phosphorus burial and atmospheric oxygenation

This study investigates how seawater calcium concentrations regulate marine phosphorus burial and, in turn, atmospheric oxygen levels over geologic time. Building on the central role of phosphorus in limiting marine primary productivity and controlling redox feedbacks, the authors hypothesize that Ca exerts a first-order control on the formation of carbonate fluorapatite (CFA), the dominant long-term marine P sink. They develop diagenetic models of sedimentary P cycling and couple them to a global carbon-phosphorus-oxygen mass balance framework to quantify the impact of Ca-driven changes in CFA precipitation on global P availability, organic carbon burial, and atmospheric pO2. The work addresses gaps in prior models that did not fully parameterize CFA kinetics and key environmental controls.

Prior work established P as a key regulator of marine productivity and redox (e.g., Van Cappellen & Ingall) and highlighted roles for iron-bound P, bioturbation, and water-column O2 in P burial. Earlier diagenetic models often treated CFA precipitation with simplified linear rate laws tied to phosphate excess and did not explicitly include Ca as a dynamic control. Experimental studies, however, show apatite growth rates depend on saturation state, implicating activities of Ca, PO4, F, and CO3. Geological proxies document large Phanerozoic swings in seawater Ca and Mg/Ca, suggesting potential for major effects on CFA formation and P burial that have been underexplored in global C-P-O models.

The authors built two diagenetic models and coupled outputs to a global carbon-phosphorus-oxygen cycle model. 1) Basic 1D reactive-transport diagenetic model: Components included two solids (organic matter and carbonate fluorapatite, CFA) and four solutes ([Ca2+], dissolved inorganic carbon (DIC), dissolved inorganic phosphate (DIP), and F−). Transport was modeled with molecular diffusion adjusted for tortuosity, solute and solid advection including compaction, and depth-dependent porosity. Organic matter decomposition followed a reactive continuum approach. CFA precipitation kinetics were parameterized as a first-order function of saturation state (QCFA), explicitly depending on activities of Ca2+, Na+, Mg2+, PO4 3−, CO3 2−, and F−, with an equilibrium constant KCFA tied to carbonate activity. CFA dissolution under undersaturation (QCFA<1) was neglected due to high insolubility in marine sediments. 2) Extended 1D multicomponent diagenetic model (SEDCHEM): This model incorporated coupled C, N, P, S, Fe, and Mn cycles with 15 solutes and 21 solids. It included biodiffusion and nonlocal bioirrigation, depth attenuation of mixing, and comprehensive reaction networks: multi-pathway organic matter remineralization (aerobic, denitrification, Mn/Fe reduction, sulfate reduction, methanogenesis) using Monod kinetics; a multi-G representation of organic matter reactivity with depth-varying C/P via partitioning into two pools with distinct C/P; detailed Fe cycling including multiple Fe hydroxide reactivity classes, magnetite, and a silicate Fe proxy (biotite), with Fe-bound P scavenging/release; vivianite precipitation parameterized with Michaelis–Menten kinetics for Fe2+ and PO4; and CFA precipitation using the saturation-state kinetic law. pH was simulated via a total proton balance (TP) approach as an implicit differential variable, avoiding explicit total alkalinity; adsorption was treated as reversible linear equilibrium for key ions (e.g., Fe2+), with instantaneous equilibrium between dissolved and adsorbed phases, and associated proton transfer accounted for. 3) Coupled global C-P-O model: The diagenetic model outputs (P burial efficiencies and CFA burial fluxes) were used to drive a global mass-balance model modified from Van Cappellen & Ingall. Look-up tables of CFA burial were generated for shallow and deep settings across ranges of bottom-water O2 and organic carbon fluxes (JOC), interpolated at 1 Myr time steps. Organic carbon burial and weathering were parameterized as functions of atmospheric O2 following COPSE. The organic C/P of burial was allowed to vary with seawater O2 through functions relating rP factors to [O2]sw. Sensitivity analyses explored effects of bottom-water oxygen, seawater Ca, JOC, bioturbation intensity and depth, and other seawater constituents (Mg, SO4, DIC, pH, F) on CFA burial, P burial efficiency, and Corg/Preac.

- Seawater Ca exerts a major control on CFA formation and total marine P burial. Increasing [Ca] strongly raises CFA saturation and precipitation, enhancing P burial efficiency and reducing recycling. - Model sensitivity shows sizeable roles for bottom-water O2, organic matter loading, and bioturbation on P burial, but Ca has a dominant additional effect. Mg and bottom-water pH have comparatively weak impacts on CFA burial within tested ranges. - Empirical validation: A compilation of deep-sea P speciation data (Pacific and Atlantic) indicates relatively constant CFA burial from ~80 to ~40 Ma followed by a gradual decrease, coincident with proxy-inferred declines in seawater Ca beginning ~40 Ma. This supports a Ca control on CFA burial. - Coupled C-P-O modeling indicates that higher seawater Ca increases CFA precipitation, lowers seawater P inventory, and decreases the burial ratio Corg/Preac; at constant P input this reduces atmospheric pO2. - Quantitatively, increasing seawater Ca from 10 mM to 20 mM can drive a >50% decline in atmospheric oxygen, from ~21% to ~10% by volume, with transient feedbacks in marine P reservoirs via changes in Fe-bound P burial as O2 adjusts. - Geological implications: Low seawater Ca during the Carboniferous–Permian would have suppressed CFA formation and favored high atmospheric pO2, consistent with proxies; elevated Ca in the early Cambrian aligns with proposed ocean deoxygenation episodes.

The findings substantiate the hypothesis that seawater Ca governs CFA formation and thus long-term P burial, providing a powerful external control on the ocean–atmosphere redox state. By explicitly linking Ca-driven changes in CFA saturation to P burial efficiency, the diagenetic models capture how Ca modulates nutrient recycling and the Corg/Preac burial ratio—key determinants of organic carbon burial and atmospheric oxygen. The observed Cenozoic co-variation between deep-sea CFA burial and proxy-derived seawater Ca lends empirical support, bolstering the case that major-ion seawater chemistry, influenced by tectonics and long-term geochemical cycling, couples to the global P cycle and atmospheric pO2. The results align with independent redox proxy records suggesting high pO2 during low-Ca intervals (e.g., late Paleozoic) and deoxygenation during high-Ca episodes (e.g., early Cambrian), suggesting Ca may have been a significant driver of Phanerozoic redox evolution alongside bioturbation and climate-driven feedbacks.

This study introduces a fully parameterized diagenetic framework for marine P burial that incorporates saturation-state kinetics for CFA and couples it to a global C-P-O model. It demonstrates that seawater Ca concentration is a primary regulator of CFA formation and total P burial, thereby exerting strong control on atmospheric oxygenation over geologic time. Empirical deep-sea P speciation trends over the past 80 Myr are consistent with model predictions and proxy records of seawater Ca, supporting a Ca-driven feedback linking tectonics, seawater chemistry, nutrient cycling, and atmospheric pO2. Future work should refine CFA thermodynamics and stoichiometry in sediments, improve constraints on the dependence of KCFA on carbonate chemistry, quantify spatial heterogeneity (shelf vs. deep sea) through expanded sediment datasets, and explore interactions with evolving bioturbation and iron cycling under varying redox states.

- CFA dissolution under undersaturation (QCFA<1) is not included, assuming high insolubility; this could bias results where CFA may dissolve. - The thermodynamic treatment of CFA includes uncertainties: the relationship between KCFA and carbonate activity and the variable CO2 content of CFA are not fully constrained, which may affect pH/CO3 dependencies though not the core Ca–QCFA link. - Adsorption modeling is simplified; NH4+ and Mn2+ adsorption were omitted (based on limited impact for fitted cases), potentially affecting pH and trace-metal dynamics in other settings. - Organic matter stoichiometry and the partitioning into two pools with differing C/P are parameterized to fit profiles and may vary across environments. - The global C-P-O model is deliberately simple; it uses lookup tables and parameterizations (e.g., COPSE-style O2 dependence) and assumes constant P input in key scenarios, which may not capture full Earth system complexity. - Spatial heterogeneity (basin-scale variability in O2, JOC, and mixing) is reduced to shallow vs. deep look-up tables, which may limit generalizability.