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

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 O-2 level, predominant oceanic anoxia and sluggish evolution of eukaryotes. The Boring Billion also witnessed the quiescence of orogenesis and a long-lived supercontinent Columbia (similar to 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 ironreducing 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.

Automated summary

The Boring Billion, a period in Earth's middle age characterized by stagnant evolutionary processes, low primary productivity in the oceans, and limited terrestrial phosphorus input, played a crucial role in shaping the biogeochemical cycles of that time. The behavior of phosphorus speciation in continental weathering and the phosphorus cycle in the ocean were key factors influencing the availability of phosphorus for marine organisms and the burial of organic carbon.