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1. A new model of the Earth system nitrogen cycle: how plates and life affect the atmosphere

   Johnson, Benjamin W.; Goldblatt, Colin (January 2017, AGU Fall Meeting)

   Nitrogen is the main component of Earth’s atmosphere. It plays a key role in the evolution of the biosphere and surface of Earth [1]. There are contrasting views, however, on how N has evolved on the surface of the Earth over time. Some modeling efforts [e.g., 2] indicate a steady-state level of N in the atmosphere over geologic time, while geochemical [e.g., 3], other proxies [e.g., 4], and more recent models [5] indicate the mass of N in the atmosphere can change dramatically over Earth history. This conundrum, and potential solutions to it, present distinct interpretations of the history of Earth, and teleconnections between the surface and interior of the planet have applications to other terrestrial bodies as well. To help investigate this conundrum, we have constructed an Earth-system N cycle box model. To our knowledge, this is the most capable model for addressing evolution of the N reservoirs of Earth through time. The model combines biologic and geologic processes, driven by a mantle cooling history, to more fully describe the N cycle through geologic history. In addition to a full biologic N cycle (fixing, nitrification, denitrification), we also dynamically solve for PO4 through time and we have a prescribed O2 history. Results indicate that the atmosphere of Earth could have experienced major changes in mass over geologic time. Importantly, the amount of N in the atmosphere today appears to be directly related to the total N budget of the silicate Earth. For example, high initial atmospheric mass, suggested as a solution to the Faint Young Sun Paradox [1], is drawn down over time. This supports work that indicates the mantle has significantly more N than the atmosphere does today [6]. Contrastingly, model runs with low total N result in a crash in atmospheric mass. In nearly all model runs the bulk silicate Earth contains the majority of the planet's N. [1] Goldblatt et al. (2009) Nat. Geosci., 2, 891-896. [2] Berner, R. (2006) Geology., 34, 413-415. [3] Barry, P.H. and Hilton (2016) Geochem. Persp. Letters, 2, 148-159. [4] Som, S.M. et al. (2016) Nat. Geosci., 9, 448-451. [5] Stueken et al. (2016) Astrobiology, 16, in press. [6] Johnson et al. (2015) Earth Science Reviews, 148,150-173. 

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2. Lithological Heterogeneities in the Mantle: Origins and Contributions to Magma Genesis (Keynote 3a)

   https://doi.org/10.46427/gold2020.1405  

   Lambart, Sarah; Lang, Otto (January 2020, Goldschmidt 2020)

   null (Ed.)

   The quantification of the mantle heterogeneity and its contribution to magma genesis are crucial for our understanding of the nature of Earth’s geochemical reservoirs and recycling processes. Today, the source of basalts is often envisioned as a heterogeneous mantle that comprises a range of lithological heterogeneities, especially pyroxenites, introduced into the mantle by various geodynamic and magmatic processes. Different chemical parameters have been proposed to trace evidence for the contribution of lithological heterogeneities during magma genesis (e.g., major elements1-2, major element ratios1,3 or logratios4, first row transition element concentrations5-6, iron isotopes7) and several empirical models for partial melting of a heterogeneous mantle have been developed in an attempt to quantify the proportion of pyroxenites in the mantle source of magmas8-11. However, the large range of compositions covered by the potential lithologies present in the mantle makes it challenging to determine a unique proxy for their contributions in the magmas2,12. Additionally, these contributions are controlled by various factors, such as the abundances of the different lithologies in the mantle, their respective melting behaviors (solidus temperatures and melt productivities) and the thermal and melting regimes of the mantle. Finally, interpretations from the magma compositions are complicated by the potential presence of volatiles in the mantle source and/or by modifications experienced by the magmas during their journey through the mantle8,12. This keynote presentation will present examples of challenges that can rise when we try to quantify the nature and abundance of the mantle heterogeneity, the efforts that have been published by various authors, and a few potential research directions that could bring prospects for success. 1- Hauri (1996), Nature 382, 2- Lambart et al. (2013), Lithos 160-161, 3- Hirschmann et al. (2003), Geology 31, 4- Yang et al. (2019), JGR-Solid Earth 124, 5- Sobolev et al (2007), Science 316, 6- Sobolev et al. (2008), Science 321, 7- Williams and Bizimis, EPSL 404, 8- Mallik and Dasgupta (2014), GGG 15, 9- Kimura and Kawabata (2015), GGG 16, 10- Lambart et al. (2016), JGR-Solid Earth 121, 11- Brown and Lesher (2016), GGG 17, 12- Mallik et al. (in press), in: Konter J., Ballmer M, Cottaar S, & Marquardt H. (Eds. ), AGU monograph. 

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3. PP12B-05 and CO2 14C - Constraints on the Deglacial Release of Geologic Carbon Using Atmospheric Records

   RYAN GREEN, MATHIS HAIN (January 2021, AGU Fall Meeting)

   While a reinvigoration of ocean circulation and CO 2 marine geologic carbon release over the last 20,000 years. Much of this evidence points to outgassing is the leading explanation for atmospheric CO rise since the Last Glacial Maximum (LGM), there is also evidence of regions of the mid-depth Pacific Ocean, where multiple radiocarbon (1 4 C) records show anomalously low 14 C/C values, potentially caused by the addition of carbon [1,2]. To better constrain this geologic carbon release hypothesis, we aim to place 14 C-free geologic an upper bound limit on the amount of carbon that may have been added, in addition to the geochemical pathway of that carbon. To do so, we numerical invert a carbon cycle model based on observational atmospheric CO 2 and 14 C records. Given these observational constraints, we use data assimilation techniques and an optimization algorithm to calculate the rate of carbon addition and its alkalinity-to-carbon ratio (R ) over the last 20,000 A/C years. Using the modeled planetary radiocarbon budget calculated in Hain et al. [3], we find observations allow for only ~300 Pg of carbon to be added, as a majority of the deglacial atmospheric 14 C decline is already explained by magnetic field strength changes and ocean circulation changes [3]. However, when we adjust the initial state of the model by increasing C by 75‰ to match the observational C records, we find that observations 14 14 allow for ~3500 Pg of carbon addition with an average R of ~1.4. A/C These results allow for the possibility of a large release of 14C-free geologic carbon, which could provide local and regional 14C anomalies, as the records have in the Pacific [1,2]. As this geological carbon was added with a RA/C of ~1.4, these results also imply that 14C evidence for significant geologic carbon release since the LGM may not be taken as contributing to deglacial CO2 rise, unless there is evidence for significant local acidification and corrosion of seafloor sediments. If the geologic carbon cycle is indeed more dynamic than previously thought, we may also need to rethink the approach to estimate the land/ocean carbon repartitioning from the deglacial stable carbon isotope budget. [1] Rafter et al. (2019), GRL 46(23), 13950–13960. [2] Ronge et al. (2016), Nature Communications 7(1), 11487. [3] Hain et al. (2014), EPSL 394, 198–208. 

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4. Thermodynamic modeling of hydrothermal REE partitioning in critical mineral deposits

   Gysi, A.P.; Hurtig, N.; Water, L.; Migdisov, A.; Dub, P.; Zhu, C. (July 2022, Goldschmidt)

   Critical mineral deposits form through an interplay of magmatic-hydrothermal processes in carbonatites and (per)alkaline systems during their emplacement in the Earth’s crust. Hydrothermal aqueous fluids can lead to the mobilization, transport, and deposition of the rare earth elements (REE) coupled to development of alteration zones at the deposit scale [1]. However, unraveling the underlying processes that affect the solubility of REE in these geologic fluids is a challenge in high temperature and pressure fluids [2]. A holistic approach is key to understand the controls of fluid-rock interaction in mobilizing REE in critical mineral deposits. Through a joint effort, we formed a new U.S. geoscience critical minerals experimental–thermodynamic research hub between New Mexico Tech, Los Alamos National Laboratory and Indiana University. The goal of this project is to conduct frontiers research on the behavior of critical elements in supercritical aqueous fluids by integration of a wide array of high temperature solubility experiments complemented by spectroscopic measurements and molecular dynamic simulations. Here we present current advances to simulate a significant vein paragenesis of barite + fluorite +calcite +bastnäsite-(Ce) observed in many critical mineral deposits. A case study will be presented from the Gallinas Mountains REE-fluorite hydrothermal breccia deposit in New Mexico. Using the GEMS code package [3] and the MINES thermodynamic database (https://geoinfo.nmt.edu/mines-tdb), we highlight our current capabilities and limitations to simulate the behavior of REE in these hydrothermal fluids and minerals. A thermodynamic model is presented to simulate the partitioning of REE between calcite- and fluorite-fluid based on recent and ongoing experimental and thermodynamic work on the synthesis of REE doped minerals [4] and REE speciation in acidic and alkaline fluids. We further show how to integrate multiple experimental datasets and develop new thermodynamic models based on the new research efforts from the research hub and future directions to improve our prediction capabilities of REE complexation in supercritical fluids. [1] Gysi et al. (2016), Econ. Geol. 111, 1241-1276; [2] Migdisov et al. (2016), Chemical Geology 439, 13-42. [3] Kulik et al. (2013), Comput Geosci 17, 1–24. [4] Perry and Gysi (2020), Geochim. Cosmochim. Acta 286, 177-197. 

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5. Using Io's Sulfur Isotope Cycle to Understand the History of Tidal Heating

   https://doi.org/10.1029/2023JE008086  

   Hughes, Ery C; de_Kleer, Katherine; Eiler, John; Nimmo, Francis; Mandt, Kathleen; Hofmann, Amy E (April 2024, Journal of Geophysical Research: Planets)

   Abstract Stable isotope fractionation of sulfur offers a window into Io's tidal heating history, which is difficult to constrain because Io's dynamic atmosphere and high resurfacing rates leave it with a young surface. We constructed a numerical model to describe the fluxes in Io's sulfur cycle using literature constraints on rates and isotopic fractionations of relevant processes. Combining our numerical model with measurements of the³⁴S/³²S ratio in Io's atmosphere, we constrain the rates for the processes that move sulfur between reservoirs and model the evolution of sulfur isotopes over time. Gravitational stratification of SO₂in the upper atmosphere, leading to a decrease in³⁴S/³²S with increasing altitude, is the main cause of sulfur isotopic fractionation associated with loss to space. Efficient recycling of the atmospheric escape residue into the interior is required to explain the³⁴S/³²S enrichment magnitude measured in the modern atmosphere. We hypothesize this recycling occurs by SO₂surface frost burial and SO₂reaction with crustal rocks, which founder into the mantle and/or mix with mantle‐derived magmas as they ascend. Therefore, we predict that magmatic SO₂plumes vented from the mantle to the atmosphere will have lower³⁴S/³²S than the ambient atmosphere, yet are still significantly enriched compared to solar‐system average sulfur. Observations of atmospheric variations in³⁴S/³²S with time and/or location could reveal the average mantle melting rate and hence whether the current tidal heating rate is anomalous compared to Io's long‐term average. Our modeling suggests that tides have heated Io for >1.6 Gyr if Io today is representative of past Io. 

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