River-to-ocean chemical flux has significantly varied over the past 500 million years
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Peer-reviewed literature documents significant long-term variations in riverine chemical and nutrient fluxes into oceans, such as phosphorus transport, across multimillion-year timescales.
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.
Secular variations in the major ion chemistry and isotopic composition of seawater on multimillion-year time scales are well documented, but the causes of these changes are debated. Fluid inclusions in marine halite indicate that the Li concentration in seawater [Li<sup>+</sup>]<sub>SW</sub> declined sevenfold over the past 150 million years (Ma) from ~184 μmol/kg H<sub>2</sub>O at 150 Ma ago to 27 μmol/kg H<sub>2</sub>O today. Modeling of the lithium geochemical cycle shows that the decrease in [Li<sup>+</sup>]<sub>SW</sub> was controlled chiefly by long-term decreases in ocean crust production rates and mid-ocean ridge and ridge flank hydrothermal fluxes without requiring changes in continental weathering fluxes. The decrease in [Li<sup>+</sup>]<sub>SW</sub> parallels the 150 Ma increase in seawater Mg<sup>2+</sup>/Ca<sup>2+</sup> and <sup>87</sup>Sr/<sup>86</sup>Sr, and the change from calcite to aragonite seas, KCl to MgSO<sub>4</sub> evaporites, and greenhouse to icehouse climates, all of which point to the importance of plate tectonic activity in regulating the composition of Earth's hydrosphere and atmosphere.
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