Abstract: We model nitrogen (N) partitioning in the magma ocean stage and cycling between the surface and mantle through Earth's history, and suggest that N in the present-day mantle may be set by subduction before the development of the modern N cycle.Introduction: On present-day Earth, N cycling between the surface and mantle is largely controlled by biological N fixation and aerobic biological processing. Biological N fixation brings the majority of inorganic N into the modern N cycle. In the oceans, dissolved nitrate is the main form of nitrogen available for life, and dissimilatory denitrification leads to residual nitrate being kinetically enriched in 15N by ~6‰. The isotopically enriched nitrate is then reduced to ammonium and finally trapped in sediments (e.g., Stüeken al. 2016). Though secular subduction of 15N-rich sediments should cause 15N enrichment in the mantle, the mantle N sampled from mid-ocean ridge basalt (MORB) is rather depleted in  15N (~-5‰), which is known as the N isotopic disequilibrium. Previous studies hypothesized that N in the mantle is a primordial component (Cartingy & Marty, 2013; Labidi et al. 2020). In the primordial origin scenario, the isotopic disequilibrium is attributed to atmospheric escape, which enriched the atmosphere with 15N. Another study proposed a recycling origin scenario in which the N isotopic composition of sediments has changed over time (Marty & Dauphas, 2003). Neither of these scenarios has been modeled quantitatively. Here we test the different scenarios by using numerical models coupled with N isotopes.Methods: We developed two sets of models for the origin of observed mantle N isotopic composition: i) the primordial origin and ii) the recycling origin. The results are either accepted or rejected by the comparison to the amounts and isotopic compositions of N in contemporary surface reservoirs and mantle (Johnson & Goldblatt, 2015; Labidi et al. 2020).For the primordial origin scenario, we calculate N partitioning between the atmosphere and mantle upon magma ocean solidification by using a melt-trapping model (Hier-Majumder & Hirschmann, 2017) and the partitioning coefficients between minerals, silicate melt, and the atmosphere (Li et al. 2013; Dalou et al. 2017). We consider the range of oxygen fugacity relevant to Earth's formation. We also estimate the 15N-enrichment effect due to EUV-driven escape (Watson et al. 1981) and solar-wind pick-up (Lichtenegger et al. 2010) to see how much atmospheric N should be removed to reproduce ~+5‰ difference between the atmosphere and mantle.For the recycling origin scenario, we calculate secular N exchange between the surface reservoirs and mantle. Our model is based on that of Labidi et al. (2020). The isotopic fractionation between the atmosphere and subducting sediments is taken to be ~-9‰ and ~+6‰ before and after the Great Oxidation Event at 2.4 Ga, respectively, considering the change from abiotic fixation and anaerobic processing to biological fixation and aerobic processing. We fix the subduction and degassing fluxes on present-day Earth, and their power-law indices as a function of time as parameters. Bulk N partitioning and the isotopic difference between the reservoirs in the initial state are also treated as parameters.Figure 1: Nitrogen partitioning between the atmosphere and mantle at the time of magma ocean solidification (PAN = present-day atmospheric nitrogen). The range of the modeled mantle N content reflects the uncertainty in the oxygen fugacity of the magma ocean.Figure 2: Evolution of masses (left) and 15N/14N ratios (right) in the surface reservoirs (blue) and mantle (red). Curves show accepted models having different initial conditions and fluxes. Gray areas are the estimates for present-day surface reservoirs and mantle.Results: Because N is relatively insoluble in silicate melts, it is mostly partitioned into the atmosphere even when trapping in interstitial melts is considered (Figure 1). Partitioning N into the mantle as much as present-day leads to 100 times excess in PAL N. We found that the excess amount of N can be removed neither by EUV- nor solar-wind-induced loss without excessive 15N enrichment in the atmosphere. Impact erosion by the late veneer bombardment removes atmospheric N without isotopic fractionation up to ~10 bar (Sakuraba et al. 2019), but it may not be sufficient to remove all excess N in the atmosphere.Since the results of our partitioning model suggest that the primordial origin is unlikely, next we tested the recycling scenario in our N cycling model (Figure 2). In our successful runs, the mantle is initially depleted in N, and N in the present-day mantle is a result of higher net subduction flux on early Earth where sedimentary N is depleted in 15N due to abiotic N fixation and anaerobic N processing. The change to modern N cycle is visible in the kink in δ15N evolution, which may provide a way to test our model with evidence from geologic record.Discussion: We suggest that N partitioning between the surface and deep Earth may be set by subduction driven by plate tectonics and partially by biology. This also suggests that the difference of atmospheric N contents between Venus and Earth, the former of which has three times more N in the atmosphere, is caused by their long-term evolution rather than early formation and differentiation processes.Conclusions: We conclude that N in the present-day mantle may be set by subduction before the development of the modern N cycle. Further results for parameter survey and discussion on other geological and geochemical constraints will be shown in our presentation. 
Earth's mantle nitrogen (N) content is comparable to that found in its N-rich atmosphere. Mantle N has been proposed to be primordial or sourced by later subduction, yet its origin has not been elucidated. Here we model N partitioning during the magma ocean stage following planet formation and the subsequent cycling between the surface and mantle over Earth history using argon (Ar) and N isotopes as tracers. The partitioning model, constrained by Ar, shows that only about 10% of the total N content can be trapped in the solidified mantle due to N's low solubility in magma and low partitioning coefficients in minerals in oxidized conditions supported from geophysical and geochemical studies. A possible solution for the primordial origin is that Earth had about 10 times more N at the time of magma ocean solidification. We show that the excess N could be removed by impact erosion during late accretion. The cycling model, constrained by N isotopes, shows that mantle N can originate from efficient N subduction, if the sedimentary N burial rate on early Earth is comparable to that of modern Earth. Such a high N burial rate requires biotic processing. Finally, our model provides a methodology to distinguish the two possible origins with future analysis of the surface and mantle N isotope record.
The Goshogake hydrothermal field in Tohoku, northern Japan, is located on the western flank of the Akita Yakeyama volcano. This N-S elongated field extends over ~20-25 acers and is placed over a tectonic discontinuity that controls the migration of hydrothermal fluids. A variety of surface degassing manifestations can be distinguished including small hydrothermal lakes, clustered bubbling pool fields, active mud erupting gryphons, sulfur-rich fumaroles and localized apparently oil-rich pools. So far it remains unclear if this system and the released gases are purely magmatic or if is also involved the migration of mantle-derived fluids interacting with buried sedimentary deposits. Here we report the result of field observations and fluid samples analyses. The measured temperatures of the active seepage sites range from 33 to 97 °C (in large part higher than 90 °C), while pH is overall low at all sites (i.e. ~2.5). Gas analyses reveal that all the active sites are CO2-dominated with slightly higher CH4 content in the colder and mud seeping sites. While CO2 has a distinct mantle-derived isotopic signature, the origin of methane is still debated. The apparent d13C thermogenic signature of methane, could also be related to an abiotic origin which would be consistent with a CO2-dominated geothermal system. The presence of oil, if confirmed, could be related to shallow-sourced hydrothermal oil (e.g. similar to that described by Didyk and Simoneit (1989), or, more interestingly, could indicate that this active site is part of a petroleum system potentially linked with deeply buried lacustrine sediments that fill a 1-Ma-caldera formed after a ignimbrite eruption. The possible presence of these deposits in the erupted mud has been suggested by Komatsu et al., (2019) based on mineralogical analyses. To further test this hypothesis, we are now conducting multiple analyses on the samples recovered from 6 locations at the Goshogake field, including the isotope analysis of water, gas, oil and mud compositions to unravel the source of these fluids as well as the reactions that took place at this site. If confirmed, Goshogake could potentially be the first example of known sedimentary hosted geothermal system in Japan. We speculate that targeted fieldworks would likely identify other hybrid systems in the country.
Amino acids in carbonaceous chondrites may have seeded the origin of life on Earth and possibly elsewhere. Recently, the return samples from a C-type asteroid Ryugu were found to contain amino acids with a similar distribution to Ivuna-type CI chondrites, suggesting the potential of amino acid abundances as molecular descriptors of parent body geochemistry. However, the chemical mechanisms responsible for the amino acid distributions remain to be elucidated particularly at low temperatures (<50°C). Here, we report that two representative proteinogenic amino acids, aspartic acid and glutamic acid, decompose to β-alanine and γ-aminobutyric acid, respectively, under simulated geoelectrochemical conditions at 25°C. This low-temperature conversion provides a plausible explanation for the enrichment of these two n-ω-amino acids compared to their precursors in heavily aqueously altered CI chondrites and Ryugu’s return samples. The results suggest that these heavily aqueously altered samples originated from the water-rich mantle of their water/rock differentiated parent planetesimals where protein α-amino acids were decomposed.