查看更多>>摘要：Recent advances indicate that the amount of carbon released by gradual degassing from the mantle needs to be revised upwards,whereas the carbon supplied by plumes may have been overestimated in the past.Variations in rock types and oxidation state may be very local and exert strong influences on carbon storage and release mechanisms.Deep subduction may be prevented by diapirism in thick sedimentary packages,whereas carbonates in thinner sequences maybe subducted.Carbonates stored in the mantle transition zone will melt when they heat up,recognized by coupled stable isotope systems(e.g.Mg,Zn,Ca).There is no single'mantle oxygen fugacity,particularly in the thermal boundary layer(TBL)and lowermost lithosphere,where very local mixtures of rock types coexist.Carbonate-rich melts from either subduction or melting of the uppermost asthenosphere trap carbon by redox freezing or as carbonate-rich dykes in this zone.Deeply derived,reduced melts may form further diamond reservoirs,recognized as polycrystalline diamonds associated with websteritic silicate minerals.Carbon is released by either edge-driven convection,which tears sections of the TBL and lower lithosphere down so that they melt by a mixture of heating and oxidation,or by lateral advection of solids beneath rifts.Both mechanisms operate at steps in lithosphere thickness and result in carbonate-rich melts,explaining the spatial association of craton edges and carbonate-rich magmatism.High-pressure experiments on individual rock types,and increasingly on reactions between rocks and melts,are fine-tuning our understanding of processes and turning up unexpected results that are not seen in studies of single rocks.Future research should concentrate on elucidating local variations and integrating these with the interpretation of geophysical signals.Global concepts such as average sediment compositions and a uniform mantle oxidation state are not appropriate for small-scale processes;an increased focus on local variations will help to refine carbon budget models.
查看更多>>摘要：Plate tectonics plays an essential role in the redistribution of life-essential volatile elements between Earth's interior and surface,whereby our planet has been well tuned to maintain enduring habitability over much of its history.Here we present an overview of deep carbon recycling in the regime of modern plate tectonics,with a special focus on convergent plate margins for assessing global carbon mass balance.The up-to-date flux compilation implies an approximate balance between deep carbon outflux and subduction carbon influx within uncertainty but remarkably limited return of carbon to convecting mantle.If correct,carbon would gradually accumulate in the lithosphere over time by(ⅰ)massive subsurface carbon storage occurring primarily in continental lithosphere from convergent margins to continental interior and(ⅱ)persistent surface carbon sinks to seafloors sustained by high-flux deep CO2 emissions to the atmosphere.Further assessment of global carbon mass balance requires updates on fluxes of subduction-driven carbon recycling paths and reduction in uncertainty of deep carbon outflux.From a global plate tectonics point of view,we particularly emphasize that continental reworking is an important mechanism for remobilizing geologically sequestered carbon in continental crust and sub-continental lithospheric mantle.In light of recent advances,future research is suggested to focus on a better understanding of the reservoirs,fluxes,mechanisms,and climatic effects of deep carbon recycling following an integrated methodology of observation,experiment,and numerical modeling,with the aim of decoding the self-regulating Earth system and its habitability from the deep carbon recycling perspective.
查看更多>>摘要：Nitrogen is a vital element for life on Earth.Its cycling between the surface(atmosphere+crust)and the mantle has a profound influence on the atmosphere and climate.However,our understanding of the origin and evolution of Earth's nitrogen is still incomplete.This review presents an overview of the current understanding of Earth's nitrogen budget and the isotope composition of different reservoirs,laboratory constraints on deep nitrogen geochemistry,and our understanding of the origin of Earth's nitrogen and the deep nitrogen cycle through plate subduction and volcanism.The Earth may have acquired its nitrogen heterogeneously during the main accretion phase,initially from reduced,enstatite-chondrite-like impactors,and subsequently from increasingly oxidized impactors and minimal CI-chondrite-like materials.Like Earth's surface,the mantle and core are also significant nitrogen reservoirs.The nitrogen abundance and isotope composition of these three reservoirs may have been fundamentally established during the main accretion phase and have been insignificantly modified afterwards by the deep nitrogen cycle,although there is a net nitrogen ingassing into Earth's mantle in modern subduction zones.However,it is estimated that the early atmosphere of Earth may have contained～1.4 times the present-day atmospheric nitrogen(PAN),with～0.4 PAN being sequestered into the crust via biotic nitrogen fixation.In order to gain a better understanding of the origin and evolution of Earth's nitrogen,directions for future research are suggested.
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