Global tectonics and oxygenation events drove the Earth-scale phosphorus cycle
Introduction
Phosphorus (P) is crucial to all organisms (Benitez-Nelson, 2000; Ruttenberg, 2014; Reinhard et al., 2017). It has no stable gas phases in the atmosphere, and ecosystems must rely on the aqueous phase to transfer it, unlike other biological nutrients such as nitrogen and carbon (Schlesinger and Bernhardt, 2013). For most of Earth's history, the ocean has been the primary site for P-related (bio)chemical reactions (Benitez-Nelson, 2000; Guilbaud et al., 2020). Based on the systematical studies of surface P cycle, especially the source and sink of P in the ocean, the following understandings have been obtained (Ruttenberg, 2014; Zerkle, 2018): (1) P is the biolimiting nutrient for global primary productivity on long time scales (>20 ka) after an electron donor-limited biosphere; (2) P is released from rock-forming minerals during weathering, and the most input of P to the marine biosphere relies on riverine delivery from continents; (3) P exists mainly as Porg and as P adsorbed to Fe oxides or hydroxides (following called as Fe oxides) in the ocean.
The P source of early oceans transitioned from extraterrestrial meteorites and seafloor weathering to continental weathering (Cox et al., 2018). The surface P cycle connecting the continent and the ocean was dominant from the late Archean, and ensuing continental weathering became the primary P source (Reinhard et al., 2017; Hao et al., 2020). Previous studies have shown that the surface P cycle is affected by many factors, including weathering, redox conditions, and the presence of Fe oxides (Bjerrum and Canfield, 2002; Horton, 2015; Jones et al., 2015). For the input of P, more consideration is given to the impact of weathering and scale of source rocks while ignoring the change of rock P concentrations, resulting in an inaccurate estimate of active P variation. In addition, the multi-layer P cycle on Earth scale includes both transportations from the deep Earth to the surface and from the surface into the deep. The former is related to magmatism (Horton, 2015; Cox et al., 2018), while the latter is associated with subduction, implying the impact of tectonics (Ma et al., 2022a; Ma et al., 2022b).
P is incompatible during partial melting of the mantle and magma differentiation until apatite is saturated (Horton, 2015; Cox et al., 2018). Therefore, continental crust contains higher P concentration of ~570 ppm (Rudnick and Gao, 2014) than the primitive mantle, although locally metasomatized mantle may have P up to ~4000 ppm (Watson, 1980). P in rivers and oceans today is mainly derived from igneous fluorapatite and marine sedimentary carbonate fluorapatite, while fluorapatite might not actually be the dominant P host in Archean igneous rocks or apatite-unsaturated rocks generally (Walton et al., 2021). However, almost all P in surface ecosystems ultimately originates from the weathering of igneous rocks regardless of mineral-specific weathering pathways (Horton, 2015). Thus, the first-order control of P in marine sediments is magmatism (Cox et al., 2018). In igneous rocks, volcanic rocks exposed to the surface are more likely to enter the biosphere through weathering than deep-buried intrusive rocks that require tectonic uplift. This process results in rapid release of P from volcanic rocks, which immediately affects the biosphere (Hartmann et al., 2014). Therefore, it is necessary to quantify the secular variation of continental igneous P and explore its mechanisms for understanding the P cycle and thus impact on biological evolution on Earth scale. Here, we study the variation of P in continental igneous and marginal sedimentary rocks (deposited in shallow marine environments) to document the controlling effect of tectonism and surface environment on the Earth-scale P cycle and the interaction between the surface and deep Earth.
Section snippets
Data preparation
We collect geochemical data on Archean, Proterozoic, and Phanerozoic igneous rocks, including major and trace elements, from the freely accessible EarthChem (https://Earthchem.org/) rock repository, as well as literature published. The data structure includes ages, longitude-latitude coordinates, lithology, and data sources. The overall trends observed in geochemical datasets may be affected by upstream biases related to sampling and analytical focus, regardless of dataset size and statistical
Results
The SiO2 content of igneous rocks varies widely. Generally, early igneous rocks have mafic composition with SiO2 contents lower than the late ones (Fig. 1). The P concentration of these igneous rocks increases with the decrease of rock age. In contrast, this negative relationship in volcanic rocks is more evident than in intrusive rocks (Fig. 2). The uncertainty of P concentration in intrusive rocks is more prominent than in volcanic rocks and shows a broader range. The fluctuation of P
Earth's cooling increased igneous P concentration
The solubility of P in mafic magmas is higher than in felsic magmas (Watson, 1980; Green and Watson, 1982), making mafic magmas more efficient in transporting P. Since volcanic rocks undergo a simple magmatic process relative to intrusive rocks, they retain more geochemical signatures about melting and magmatic formation (Horton, 2015). Compared with the Proterozoic and Phanerozoic volcanic rocks, Archean mafic volcanic rocks are very low in P2O5 contents and show a remarkably different
Conclusions
Based on the study of P cycle on a global scale, we reveal the effects of tectonics and surface oxygenation events on the P concentration in igneous rocks and marginal sedimentary rocks and draw the following main conclusions.
- (1)
The P concentration in igneous rocks, represented by mafic volcanic rocks, fluctuates and gradually rises over geological history, resulting from the incompatibility of P in the magmatic processes and Earth's cooling.
- (2)
During the supercontinent cycles, thick lithosphere and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We greatly appreciate the critical and thoughtful comments from two anonymous reviewers, which helped improve this paper's quality. Special thanks go to editor Christina Yan Wang for efficiently handling and improving the presentation. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41725014 and 42288201).
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