Earth's Nitrogen and Carbon Cycles

Earth and Planetary Science Letters, 1993

In this paper we discuss the distribution, geochemical cycle, and evolution of CO 2 and N 2 in Earth's degassed mantle, and atmosphere plus continental crust. We estimate the present distribution of CO 2 and N 2 in Earth's degassed mantle based on amounts of He and Ar in the degassed mantle and observed outgassing ratios of CO2/He and N2/Ar at mid-ocean ridges. Estimated CO 2 in present degassed mantle is (1.8_ +9) x 1022 mol, representing (72 + 10)% of total degassable CO2, an amount far higher than fractions previously inferred for noble gases. This strongly suggests that most CO 2 has been recycled from Earth's surface into the degassed mantle through subduction, which is consistent with many recent discussions. For N2, the estimated amount in the present mantle is ~ 2.5 × 1019 mol, representing ~ 12% of total degassable N 2. Recycling of N 2 back to the mantle is also inferred, but on a much smaller scale. A simple model for the outgassing and recycling of CO 2 and N 2 in Earth is presented. Outgassing is assumed to be via melt-vapor partitioning of volatiles. Recycling back into the mantle via subduction is assumed to be proportional to the mass of the volatile component in the crust. This simple model is consistent with all currently available constraints. Difficulties arise from the dependence of the recycling constant on time. Hence, no single evolution history can be obtained for CO 2 based on the available data. Model results tentatively point to a higher CO 2 content on Earth's surface in the Archean and Proterozoic than at present. Important future constraints may come from records in sedimentary rocks, improved understanding of carbonate production, and better modeling of the recycling process.

The nitrogen concentrations [N] and isotopic compositions of ultramafic mantle rocks that represent various dehydration stages and metamorphic conditions during the subduction cycle were investigated to assess the role of such rocks in deep-Earth N cycling. The samples analyzed record low-grade serpentinization on the seafloor and/or in the forearc wedge (low-grade serpentinites from Monte Nero/Italy and Erro Tobbio/Italy) and two successive stages of metamorphic dehydration at increasing pressures and temperatures (high-pressure (HP) serpentinites from Erro Tobbio/Italy and chlorite harzburgites from Cerro del Almirez/Spain) to allow for the determination of dehydration effects in ultramafic rocks on the N budget. In low-grade serpentinites, d 15 N air values (-3.8 to ?3.5 %) and [N] (1.3-4.5 lg/g) are elevated compared to the pristine depleted MORB mantle (d 15 N air * -5 %, [N] = 0.27 ± 0.16 lg/g), indicating input from sedimentary organic sources, at the outer rise during slab bending and/or in the forearc mantle wedge during hydration by slab-derived fluids. Both HP serpentinites and chlorite harzburgites have d 15 N air values and [N] overlapping with low-grade serpentinites, indicating no significant loss of N during metamorphic dehydration and retention of N to depths of 60-70 km. The best estimate for the d 15 N air of ultramafic rocks recycled into the mantle is ?3 ± 2 %. The global N subduction input flux in serpentinized oceanic mantle rocks was calculated as 2.3 9 10 8 mol N 2 /year, assuming a thickness of serpentinized slab mantle of 500 m. This is at least one order of magnitude smaller than the N fluxes calculated for sediments and altered oceanic crust. Calculated global input fluxes for a range of representative subducting sections of unmetamorphosed and HP-metamorphosed slabs, all incorporating serpentinized slab mantle, range from 1.1 9 10 10 to 3.9 9 10 10 mol N 2 /year. The best estimate for the d 15 N air of the subducting slab is ?4 ± 1 %, supporting models that invoke recycling of subducted N in mantle plumes and consistent with general models for the volatile evolution on Earth. Estimates of the efficiency of arc return of subducted N are complicated further by the possibility that mantle wedge hydrated in forearcs, then dragged to beneath volcanic fronts, is capable of conveying significant amounts of N to subarc depths.

Geochimica et Cosmochimica Acta, 2011

Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8°C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6-55 ppm) occurs as bound ammonium (NH 4 þ) substituting for K or Na-Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300°C). In low-strain metagabbros, d 15 N values of +0.8& to +8.1& are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a d 15 N-Cu relationship that does not match the d 15 N-Cu correlation found in low-strain rocks. This d 15 N-Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu-N compounds, possibly of the form Cu(NH 3) 2 2+ , during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same d 15 N-Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system. We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) Â 10 11 g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that $80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via subduction zones is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of $9.6 Â 10 11 g/yr. Assuming a constant flux of subducted N over the Earth's history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.

Chemical Geology, 2022

The development of high-resolution gas source mass spectrometry has permitted entirely new types of measurements of multiply-substituted isotopologues in gas species of geochemical significance. Here, we present recent advances afforded by measurements of 15 N 15 N in natural samples, together with 14 N 14 N and 15 N 14 N. We show that the abundance of the doubly-substituted 15 N 15 N isotopologue in hydrothermal gases, often mixtures of volatiles of widely different origins, allows tracing the provenance of nitrogen. The approach is based on the recent finding that atmospheric N 2 has a substantial enrichment in 15 N 15 N of nearly 20‰ relative to any other source of N 2. This is particularly useful for the study of hydrothermal gases, where characterizing the isotopic composition and provenance of volcanic N 2 is important for a wide range of applications in high-temperature geochemistry, but where air-derived N 2 is unavoidable. In this review, we summarize the evidence that 15 N 15 N is an unambiguous tracer of air contamination. We compare two sets of published 15 N 15 N data acquired on gases from plume and arc volcanoes. We show how different sources of volcanic N 2 may be in plume versus arc environments, and discuss the first-order constraints on the deep N cycle that are provided by the new 15 N 15 N data. Important findings include that the δ 15 N tracer, used alone or in conjunction with N 2 /Ar and N 2 /He ratios, can be surprisingly deceiving. Isotope fractionation of atmospheric nitrogen occurs within hydrothermal systems, resulting in negative δ 15 N values similar to estimates for mantle values, yet with 15 N 15 N values that preclude a mantle origin. The 15 N 15 N data show that the true δ 15 N of volcanic components is positive in arcs but near-zero at the Yellowstone plume. In other words, atmospheric N 2 can mimic mantle δ 15 N, and mantle δ 15 N can look like the value of air. Without 15 N 15 N, the apportioning of mantle and atmospheric N 2 in mixed gases can easily be wrong. With 15 N 15 N, we also determine the true N 2 / 3 He and N 2 / 36 Ar ratios of volcanic components in hydrothermal systems. Results inform our understanding of the deep nitrogen cycle. Plume and arc volcanic endmembers show distinct isotope and elemental ratios, consistent with sub-arc sources being overwhelmed by nearquantitative slab devolatilization, while the Yellowstone plume source is not reflecting volatile subduction.

Earth and Planetary Science Letters, 2008

In order to better characterise mantle CO 2 /Nb-variability, we obtained and compiled major and trace elements, content and isotope composition of both CO 2 and water on two series of mid-ocean ridge basalt (MORB) samples dredged at ∼ 14°N (n = 6) and 34°N (n = 11) on the mid-Atlantic ridge. All samples are carbon-saturated. One, the so-called popping rock 2ΠD43 kept its vesicles, the initial (pre-degassing) C-contents of the 16 other samples being reconstructed from their assumed degassing history. For water, the samples show large variations, from 1300 to 6900 ppm and from 1900 to 7900 ppm with associated δD-values ranging from −55 to −79‰ and from −55 to −88‰ for samples at 14°N and 34°N respectively. For carbon, the inferred initial predegassing contents vary greatly, from 660 to 14,700 ppmCO 2 and from 1400 to 57,600 ppmCO 2 for samples at 14°N and 34°N respectively. Measured Nb-contents range from 4.5 to 29.6 ppm show both good agreement with previously published data and positive correlations with reconstructed initial CO 2-contents. The mean CO 2 /Nb range from ∼ 570 to ∼ 730 at 14°N and 34°N respectively. CO 2 and Nb data for the two undegassed samples available so far (i.e. the popping rock of the present study and the basaltic glasses from the Siqueiros transform fault from the study of Saal et al., 2002) show significant variations in CO 2 /Nb over a factor of 2 and thus questions the constant CO 2 /Nb previously emphasised for these two samples, this view being supported by CO 2 /Nb-ratios of samples whose initial C-contents were reconstructed. For incompatible elements such as Ce, K and including water, a comparison of the geochemical characteristics of transform fault basaltic magmatism with other MORB systems shows magma transform fault magmatism to be unrepresentative of mantle compositions. Assuming a more appropriate average MORB CO 2 /Nb-ratio of ∼ 530 and a mean MORB Nb-content of 3:31 þ3:99 À1:8 , we computed a mantle carbon flux of 2:3 þ2:7 À1:3 × 10 12 mol/yr, a value actually consistent with that derived from C/ 3 He systematics.

Terra Nova, 1992

The C02 atmospheric content has shown large variations over geological times. High contents (up to one order of magnitude more than present-day values) ultimately correspond to discrete episodes of mantle degassing, either juvenile, or subduction-related (carbon recycling). A number of arguments (e.g. the continuous volume increase of carbonate-bearing sediments with time) suggest that, throughout the Earth's history, juvenile C02 has formed a major contribution to the global carbon budget of the Earth.

Chemical Geology, 2022

Plate tectonics is thought to be a major driver of volatile redistribution on Earth. The budget of nitrogen in Earth's mantle has been suggested to be almost entirely surfacederived. Recycling would contribute nitrogen with relatively heavy 15 N/ 14 N isotope ratios to Earth's mantle. This could explain why the Earth's mantle 15 N/ 14 N isotope ratio is substantially higher than both solar gases and chondritic parent bodies akin to enstatite chondrites. Here, published nitrogen isotope data of mid-ocean ridge and ocean island basalts are compiled and used to evaluate the nitrogen subduction hypothesis. Nitrogen isotope ratios are used in conjunction with published N2/ 3 He and K2O/TiO2 ratios on the same basalts. Assuming that 3 He is not recycled, N2/ 3 He ratios are argued to trace nitrogen addition to mantle sources via subduction. Various mantle source enrichments for basalts are tracked with K2O/TiO2 ratios: elevated K2O/TiO2 ratios are assumed to primarily reflect the contributions of recycled components in the basalts mantle sources. The main result of our data compilation is that for most basalts, d 15 N and N2/ 3 He remain constant across a vast range of K2O/TiO2 ratios. Mid-ocean ridge basalts have d 15 N signatures that are lower than air by ~ 4‰ and an average N2/ 3 He ratio of 3.7 (±1.2) x10 6 (95% confidence, n= 30). Published d 15 N and N2/ 3 He are invariant across K2O/TiO2 ratios that vary over a factor of ~ 20. Using estimates of slab K2O/TiO2 and [TiO2], the observed invariant d 15 N and N2/ 3 He may be fit with slabs containing ~0.1 ppm N. A mass balance shows that adding ~10% recycled slabs to the convective mantle only raises the N2/ 3 He by < 5%. Lavas from Iceland, Galapagos and Hawaii have high 3 He/ 4 He and 15 N/ 14 N ratios relative to the convective mantle. Only seven samples show nitrogen isotopic signatures that are unaffected by air contamination, although those samples are poorly characterized for N2/ 3 He. The seven basalts show d 15 N between-2 and 0‰ that do not vary systematically with K2O/TiO2 ratios that vary over a factor of ~ 5. The N2/ 3 He ratios of these seven basalts is unknown, but the high 3 He/ 4 He mantle may be estimated by combining published N2/ 36 Ar to 3 He/ 36 Ar ratios. This yields a N2/ 3 He of 2.3 (±1.2) x 10 6 (1s uncertainty). This is indistinguishable from the MORB estimate of 3.7 (±1.2) x 10 6. Invariant d 15 N across variable degrees of mantle enrichments and MORBlike N2/ 3 He for the high 3 He/ 4 He mantle are not consistent with nitrogen addition to plume sources with elevated 3 He/ 4 He ratios. A d 15 N between-2 and 0‰ for plume sources, only marginally higher than MORB, could be a primordial feature of undegassed mantle reservoirs. Nonetheless, nitrogen subduction may have contributed to a specific array of mantle sources, as revealed by the few published data on basalts with low 3 He/ 4 He ratios. Lavas from the Society plume with low 3 He/ 4 He ratios show an enriched mantle source, and they have elevated d 15 N ≥ +0.5‰ and N2/ 3 He > 10 7. For those, the addition of slabs with concentrations of ~ 0.1 ppm N to a mantle source can account for the integrated dataset. To summarize, the published data suggest that nitrogen subduction may explain a subset of published N isotope data on basalts, but that N recycling has an overall more limited impact on mantle nitrogen than previously thought.

Nature

Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth's accretion versus that subducted from the Earth's surface is unclear 1-6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15 N 15 N isotopologue of N 2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ 15 N (the fractional difference in 15 N/ 14 N from air), N 2 / 36 Ar and N 2 / 3 He. Our results show that negative δ 15 N values observed in gases, previously regarded as indicating a mantle origin for nitrogen 7-10 , in fact represent dominantly air-derived N 2 that experienced 15 N/ 14 N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15 N 15 N data allow extrapolations that characterize mantle endmember δ 15 N, N 2 / 36 Ar and N 2 / 3 He values. We show that the Eifel region has slightly increased δ 15 N and N 2 / 36 Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts 11 , consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ 15 N values substantially greater than that of the convective mantle, resembling surface components 12-15 , its N 2 / 36 Ar and N 2 / 3 He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ 15 N values may both be dominantly primordial features. Differentiated bodies from our Solar System have rocky mantles with 15 N/ 14 N ratios within ±15‰ of modern terrestrial air 16,17. This is true for Earth's convective mantle, which has a δ 15 N value of approximately −5 ± 3‰, based on measurements from diamonds 5,18 and basalts that have been filtered for air contamination 3,11. Conversely, volatilerich chondritic meteorites exhibit highly variable δ 15 N values between −20 ± 11‰ for enstatite chondrites and 48 ± 9‰ for CI carbonaceous chondrites 16,19. The distinct 15 N/ 14 N of rocky mantles relative to the chondrites may reflect inheritance of N from a heterogeneous mixture of chondritic precursors 3. Alternatively, the relatively high 15 N/ 14 N values could be the result of evaporative losses 20 , or equilibrium partitioning of N isotopes between metal cores and rocky mantles 21,22. For Earth, plate tectonics allows for another interpretation 1. Geochemists have suggested that mantle δ 15 N values reflect subduction of nitrogen from the surface. Some of the evidence comes from studies of gases from mantle plumes. On Earth, mantle plumes with high 3 He/ 4 He ratios relative to mid-ocean-ridge basalts (MORBs) result from melting of relatively undegassed portions of the deep mantle 23. Nitrogen data are sparse, but plumes with both high and low 3 He/ 4 He values have δ 15 N values between 0 and +3‰ (refs. 2,4), higher than the values attributed to the convective mantle and similar to both sediments and altered oceanic crust (Extended Data Fig. 1) 12,13,15,24. One hypothesis is that the convective and deep mantle reservoirs both initially had identical but low enstatite chondrite-like δ 15 N values 6. Over geological time, these

Research Square (Research Square), 2021

The early Cenozoic exhibited profound environmental change influenced by plume magmatism, continental breakup, and opening of the North Atlantic Ocean. Global warming culminated in the transient (170 thousand year, kyr) hyperthermal event, the Palaeocene-Eocene thermal maximum (PETM) 56 million years ago (Ma). Although sedimentary methane release has been proposed as a trigger, recent studies have implicated carbon dioxide (CO 2) emissions from the coeval North Atlantic igneous province (NAIP). However, we calculate that volcanic outgassing from mid-ocean ridges and large igneous provinces associated with the NAIP yields only one-fifth of the carbon required to trigger the PETM. Rather, we show that volcanic sequences spanning the rift-to-drift phase of the NAIP exhibit a sudden and ∼220-kyr-long intensification of volcanism coincident with the PETM, and driven by substantial melting of the sub-continental lithospheric mantle (SCLM). Critically, the SCLM is enriched in metasomatic carbonates and is a major carbon reservoir. We propose that the coincidence of the Iceland plume and emerging asthenospheric upwelling disrupted the SCLM and caused massive mobilization of this deep carbon. Our melting models and coupled tectonic-geochemical simulations indicate the release of >10 4 gigatons of carbon, which is sufficient to drive PETM warming. Our model is consistent with anomalous CO 2 fluxes during continental breakup, while also reconciling the deficit of deep carbon required to explain the PETM.