Evolution of the crustal phosphorus reservoir

Abstract

The release of phosphorus (P) from crustal rocks during weathering plays a key role in determining the size of Earth's biosphere, yet the concentration of P in crustal rocks over time remains controversial. Here, we combine spatial, temporal, and chemical measurements of preserved rocks to reconstruct the lithological and chemical evolution of Earth's continental crust. We identify a threefold increase in average crustal P concentrations across the Neoproterozoic-Phanerozoic boundary (600 to 400 million years), showing that preferential biomass burial on shelves acted to progressively concentrate P within continental crust. Rapid compositional change was made possible by massive removal of ancient P-poor rock and deposition of young P-rich sediment during an episode of enhanced global erosion. Subsequent weathering of newly P-rich crust led to increased riverine P fluxes to the ocean. Our results suggest that global erosion coupled to sedimentary P-enrichment forged a markedly nutrient-rich crust at the dawn of the Phanerozoic.

Fig. 1.. Trends in rock-type raw volume abundance and relative proportion of cumulative volume over Earth’s history.

(A) Igneous rock volumes show little variation throughout the Archean-Neoproterozoic but a rapid increase in the Phanerozoic. (B) Siliciclastic rock volumes show a steep increase from 3000 to 2400 Ma, no increase from 2400 to 600 Ma, and then rapidly increase across the Neoproterozoic-Phanerozoic boundary. (C) Carbonate volume increases linearly across most of Earth’s history and rapidly across the Neoproterozoic-Phanerozoic boundary. (D) Metasedimentary rocks increase from 3000 to 2400 Ma, show no increase from 2400 to 600 Ma, and then increase sharply in the Phanerozoic before sharply decreasing toward the present day. (E) Proportion of sedimentary to igneous rocks in weatherable crust over time, showing increase in all sedimentary rock types relative to igneous rocks from 3000 to 2400 Ma, no trend from 2200 to 600 Ma, and a second sharp increase from 600 to 400 Ma. The fraction of cumulative volume of units at a given point in time (t) is calculated for units of given rock type (R_i, km³) within preserved weatherable crust as $\frac{\sum R_i(\mathrm{age}\ge t)}{\sum R_i(\mathrm{age}\ge t)+\dots \sum R_j(\mathrm{age}\ge t)}$. One SD error (inherited from uncertainty in the age and volume of individual units) is shaded with hatched fill [cross-hatched in (E)]. Mean values are plotted in darker shade.

Fig. 2.. Nutrient concentrations of average weatherable crust over Earth’s history.

(A) Nickel contents decline sharply in the end Archean due to the disappearance of Ni-rich komatiite and rise of Ni-poor sediments and igneous rocks. (B) Phosphorus concentrations are stable across the Archean-Proterozoic boundary but increase sharply in the Neoproterozoic-Phanerozoic. (C) Molybdenum concentrations increase during sediment accumulation but only after progressive deep ocean oxygenation. (D to F) Rates of change in crustal composition for each element. SE uncertainties from resampling are shaded in (A) to (C).

Fig. 3.. Mass balance constraints on an evolving P concentration of weatherable continental crust.

(A) Formulation of mass balance calculation, which considers crust-mantle recycling rates and P content of subducted sediments. (B) Schematic illustration of the mass balance test performed, showing the volume of subducted crust needed to satisfy mass balance for two contrasting P ratios between preserved crust—for which Macrostrat delivers observed values over time, with some uncertainty—and subducted material. (C) Ratio of [P] in recycled material relative to bulk crust (ϕ) imposes a constraint on the fraction of eroded material that may be preserved within weatherable crust (solution to Eq. 4; see Materials and Methods). Low values of ϕ, which correspond to strong P enrichments of preserved crust, demand high volumes of recycled material and thus low volumes of preserved material. w-CC, weatherable continental crust. (D) Crust-mantle recycling rates required across a range of values of ϕ. Each line is superimposed, with lines where ϕ = 1 extending the furthest. These values are compared to average crust-mantle recycling rates and those inferred across the Great Unconformity (24, 25).

Fig. 4.. Consequences of weatherable crust evolution for the P cycle.

(A) Phosphorus fluxes relative to modern day (49) over Earth’s history, given land emergence rate of Flament et al. (58). See fig. S5 and S6 for a rapid land emergence scenario. (B) Total P fluxes for land only and land plus seafloor scenarios. If seafloor weathering is assumed not to have contributed bioavailable P to the oceans, then P fluxes may have increased by up to an order of magnitude from 3000 to 2000 Ma. Otherwise and at least from 2000 Ma onward, P fluxes are stable until the Cambrian boundary, after which a step increase occurs.