“中年地球”的磷循环与生物泵:再谈“沉寂的十亿年”

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

海洋初级生产力受到主量营养元素磷(P)和氮(N)的共同约束.其中P在基础的生物化学反应中起到不可替代的作用[1],包括组成遗传物质(DNA)、参与能量传输(adenosine triphosphate,ATP)、形成结构支撑(磷脂双分子层与磷酸盐骨骼)等;而N则是蛋白质氨基酸的主要成分.海洋中生物可利用的N(氨和硝酸根)主要来自生物的固氮作用;而P的供给主要依靠陆源输入,即大陆风化作用产生的磷酸根通过河流过程输入.因此,P被认为是地质历史时期海洋初级生产力的主要限制因素[2].

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;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 Fe OOH precipitation and sinking of particulate organic matter(POM).Further reduction of Fe OOH 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.