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

Mingyu Zhao,Shuang Zhang,Christopher T. Reinhard,Lidya G. Tarhan,Noah Planavsky

doi:10.1038/s41467-020-15673-3

Mingyu Zhao, Shuang Zhang + Show 3 more

Open Access

https://doi.org/10.1038/s41467-020-15673-3

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Abstract

The marine phosphorus cycle plays a critical role in controlling the extent of global primary productivity and thus atmospheric pO2 on geologic time scales. However, previous attempts to model carbon–phosphorus-oxygen feedbacks have neglected key parameters that could shape the global P cycle. Here we present new diagenetic models to fully parameterize marine P burial. We have also coupled this diagenetic framework to a global carbon cycle model. We find that seawater calcium concentration, by strongly influencing carbonate fluorapatite (CFA) formation, is a key factor controlling global phosphorus cycling, and therefore plays a critical role in shaping the global oxygen cycle. A compilation of Cenozoic deep-sea sedimentary phosphorus speciation data provides empirical support for the idea that CFA formation is strongly influenced by marine Ca concentrations. Therefore, we propose a previously overlooked coupling between Phanerozoic tectonic cycles, the major-element composition of seawater, the marine phosphorus cycle, and atmospheric pO2.

Highlights

The marine phosphorus cycle plays a critical role in controlling the extent of global primary productivity and atmospheric pO2 on geologic time scales
The kinetics of carbonate fluorapatite (CFA) formation are typically simplified for use in diagenetic models; commonly, CFA formation is parameterized as a linear relationship with dissolved phosphate, and the roles played by other key CFA components such as Ca2+, CO32−, and F− are largely overlooked
This exercise indicates that an increase in seawater Ca concentrations will drive a significant increase in CFA formation (Fig. 1 and Supplementary Figure 1), decreasing phosphate diffusion (i.e., P recycling) from the sediment pile back to seawater

Summary

Results and discussion

Model evidence for a Ca control on marine P burial. To explore the idea that Ca may exercise a major control on P cycling, we first built a simple diagenetic model that includes only organic matter, organic P, Ca2+, CO32−, PO43−, F−, and CFA (see Methods). In contrast to the simplified rate law used in previous studies[5,8,10], this model includes a rate law that describes the formation rate of CFA as a function of its saturation state, which is consistent with experimental observations[15,16] This exercise indicates that an increase in seawater Ca concentrations will drive a significant increase in CFA formation (Fig. 1 and Supplementary Figure 1), decreasing phosphate diffusion (i.e., P recycling) from the sediment pile back to seawater. A compilation of deep-sea P speciation data from the past 80 million years provides empirical support for the effect of seawater dissolved Ca concentrations on P burial and bolster the case that this Ca-driven feedback strongly impacts the global ocean-atmosphere system These results provide a new view of the processes linking marine elemental cycles, tectonics and atmospheric pO2, and suggest that the major ion composition of seawater has been an important driver of biospheric change and atmospheric evolution throughout Earth’s history

Methods

Rl ð1Þ

Code availability