The phosphorus cycle and biological pump in Earth’s middle age: Reappraisal of the “Boring Billion”

Abstract

<p indent="0mm">The Earth’s middle age (1.8–0.8 billion years ago, Ga) is collectively coiled the “Boring Billion”, referring to the invariant carbonate carbon isotope curve, persistently low atmospheric O₂ level, predominant oceanic anoxia and sluggish evolution of eukaryotes. The “Boring Billion” also witnessed the quiescence of orogenesis and a long-lived supercontinent Columbia (~1.7–1.3 Ga). It is widely accepted that the terrestrial phosphorus (P) input ultimately controls the long-term ocean primary productivity and organic carbon burial, and accordingly the weakened mountain-building might have reduced terrestrial P input, in turn limiting organic matter production in the surface ocean and leading to rather inactive biogeochemical cycles in Earth’s middle age. However, this scenario overlooks different behaviors of complex P speciation in continental weathering and P cycle in the ocean. Not all P from continental weathering is bio-available, and not all seawater P is eventually buried with organic matter. Therefore, it is essential to revisit P speciation and P cycle in the “Boring Billion”. On the one hand, apatite (the primary insoluble P minerals) is not completely dissolved in continental weathering, and dissolved P could be scavenged by Fe-oxides or precipitate as authigenic phosphate minerals, further reducing the bio-availability of terrestrial P input. On the other hand, seawater P is variably removed from the ocean inventory via inorganic P burial associated with Fe redox cycles or organic P burial coupled with the preservation of organic carbon. In detail, seawater P would be transported to sediment with FeOOH precipitation and sinking of particulate organic matter (POM). Further reduction of FeOOH by iron-reducing microbes (IRM) and organic matter decomposition in sediments release P into porewater, which either precipitates as authigenic carbonate-fluorapatite or diffuses back to seawater. The intensity of Fe-redox cycle controls the inorganic P sink and determines the availability of P in seawater. The marine P cycle is recorded in the P speciation of sediments/sedimentary rocks, including organic P, Fe bounded P, authigenic P in the form of carbonate-fluorapatite and detrital P in the form of apatite. The fraction of detrital P with respect to total P is determined by continental weathering, while the fraction of organic P relative to total active P (i.e., organic P + Fe bounded P + authigenic P) is related to the marine P cycle. We speculate that low primary productivity in the “Boring Billion” could be attributed to: (1) Low erosion rate in continents due to the quiescence of mountain-building, (2) low degree of P activation in the weathering process due to the absence of land plants and biological weathering, and (3) high inorganic P burial in the ocean as the consequence of active Fe redox cycle. The “Boring Billion” was ended by the reactivation of tectonics that elevated terrestrial P input and/or ocean oxygenation that reduced inorganic burial of seawater P. This interpretation is supported by the available P speciation data showing high fraction of detrital P in early Neoproterozoic sedimentary rocks. In addition, the marine P cycle is also controlled by the nature of biological pump. To sustain a P-C cycle balance, the prokaryote-dominated biology pump in the “Boring Billion” was characterized by the high instantaneous primary productivity and fast decomposition in the water column, favoring the development of ferruginous (anoxic and Fe²⁺-rich) ocean that promoted inorganic P burial. In contrast, the low C burial efficiency and high rate of organic matter decomposition of prokaryote-dominated biological pump would make microbial carbon pump (MCP) play a more important role. Thus, the ocean of the “Boring Billion” was featured by the high production of resistant dissolved organic carbon (RDOC) via active MCP, which might have played a key role in modulating the global biogeochemical cycles and the redox landscape in Earth’s middle age.