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Nitrogen (N) dominates Earth's atmosphere (78% N2) but occurs in trace abundances in silicate minerals, making it a sensitive tracer of recycled surface materials into the mantle. The mechanisms controlling N transfer between terrestrial reservoirs remain uncertain because low N abundances in mineral‐hosted fluid inclusions (FIs) are difficult to measure. Using new techniques, we analyzed N and He isotope compositions and abundances in olivine‐ and pyroxene‐hosted FIs from arc volcanoes in Southern Chile, Cascadia, Central America, and the Southern Marianas. These measurements enable an estimate of the global flux of N outgassing from arcs (4.0 × 1010 mol/yr). This suggests that Earth is currently in a state of net N ingassing, with roughly half of subducted N returned to the mantle. Additionally, the N outgassing flux of individual arcs correlates with the thickness of subducting pelagic sediment, suggesting that N cycling in the modern solid Earth is largely controlled by sediment subduction. Plain Language Summary Nitrogen (N) largely behaves like an inert gas, and so it is substantially more concentrated at Earth's surface than in Earth's deep interior. Over geologic time, N can be transported between the solid Earth and the surface, and its concentration can change in both of these settings. Volcanic gases transport N from the interior to the surface, while some surface N returns into the solid Earth via plate subduction. Here, we present measurements of N and helium (He) gas trapped within crystals in volcanic rocks to determine how much N is transported to the surface through volcanism associated with plate subduction. We find that the amount of N returning to the surface through volcanism is less than estimates of how much N is transported into the solid Earth, suggesting that, overall, N is being returned to the planet's deep interior. Additionally, we observe that the amount of oceanic sediment that is subducted correlates with the amount of N that comes out of volcanoes, making it the primary carrier of N into the solid Earth. Key Points Arc lavas yield fluxes of 4.0 × 1010 mol N/yr, similar to estimates from volcanic arc gases, likely resulting in net mantle ingassing of N Nitrogen isotopes and N‐He mixing models highlight that small contributions of sediment dominate volcanic arc N budgets Subducted sediment thickness correlates with N2/3He ratios, and likely controls arc N fluxes rather than slab parameters or thermal state

The behavior of sulfur (S) in volcanic arcs is of fundamental importance to a wide range of geologically and societally relevant processes. Despite this, the influence of subduction on the sulfur content of primitive arc magmas is poorly understood. This is partially because sulfur behavior in arc magmas is challenging to characterize; sulfur is present in two different valence states within silicate melts, and partitions into multiple phases (sulfide, anhydrite, silicate melt, gas). In this work, I address these challenges through the study of high-Mg melt inclusions. The first portion of this dissertation carries out a detailed investigation on the behavior of sulfur within six mafic cinder cones in the southern Cascade Arc. This investigation focuses on the close connections between magma sulfur content, oxidation state, and metal content. Comparisons of δ34S, S and Fe valence state, and melt S content measured within the same suite of inclusions demonstrate that the influence of oxidized slab-derived sulfur in the sub-arc mantle is linked to increases in the oxidation state of arc magmas. In the second portion of this work, applying this framework to metal concentrations in Lassen magmas shows that Pb is dominantly derived from the subducting slab, whereas Mo, Zn, W, and Sn show no clear subduction-related enrichments. Variations in Cu contents between cinder cones are controlled by a combination of ambient mantle heterogeneity and modern subduction influence. In the third portion of this dissertation, I consider arc magma sulfur behavior in a global context through a compilation of published melt inclusion data spanning 115 volcanoes and 29 arc segments. Correlations between S and Cl in these melt inclusions indicate that S is partially derived from the subducting slab. Using models of sulfur content at sulfide saturation, I show that 90% of arc magmas have sulfur contents requiring conditions more oxidizing than the QFM (quartz-fayalite-magnetite) buffer. Collectively, these three chapters demonstrate that subduction strongly influences the sulfur content of arc magmas, and that the transfer of sulfur from the subducting slab into the mantle source of arc magmas can be an important control on both the oxidation state and metal content of primitive arc magmas. This dissertation includes previously published and unpublished co-authored material.

The objective of this study was to characterize and compare individual and sexual network characteristics of Black, White, and Latino young men who have sex with men (YMSM) as potential drivers of racial disparities in HIV. Egocentric network interviews were conducted with 175 diverse YMSM who described 837 sex partners within 167 sexual-active egos. Sexual partner alter attributes were summarized by ego. Descriptives of ego demographics, sexual partner demographics, and network characteristics were calculated by race of the ego and compared. No racial differences were found in individual engagement in HIV risk behaviors or concurrent sexual partnership. Racial differences were found in partner characteristics, including female gender, non-gay sexual orientations, older age, and residence in a high HIV prevalence neighborhood. Racial differences in relationship characteristics included type of relationships (i.e., main partner) and strength of relationships. Network characteristics also showed differences, including sexual network density and assortativity by race. Most racial differences were in the direction of effects that would tend to increase HIV incidence among Black YMSM. These data suggest that racial disparities in HIV may be driven and/or maintained by a combination of racial differences in partner characteristics, assortativity by race, and increased sexual network density, rather than differences in individual’s HIV risk behaviors.

The objectives of this study were to: (1) describe the quantity and quality of social support networks of Latino immigrants living in a new receiving environment, and (2) determine the role such networks play in their HIV/STI risk behaviors, including substance use. Double incentivized convenience sampling was used to collect egocentric social support network data on 144 Latino immigrants. Latent class analysis was used for data reduction and to identify items best suited to measure quality and quantity of social support. Moderate and high quantity and quality of social support were protective of HIV/STI sexual risk behavior compared to low quantity and quality of support, after adjustment for gender, years in New Orleans and residing with family. Neither measure of social support was associated with binge drinking. The findings suggest that increased quantity and quality of social support decrease HIV/STI sexual risk behaviors but do not influence binge drinking. Interventions that improve the quantity and quality of social support are needed for Latino immigrants.

Young men who have sex with men are at increased risk for HIV. Research with older men and high-risk populations suggests that network dynamics may contribute to the spread of infectious disease and HIV, but little is known about the sexual networks of young men who have sex with men. Utilizing a unique dataset, this study presents novel descriptive data about the sexual networks of racially diverse 17- to 23-year-old young men who have sex with men. Additionally, individual, partner, and network characteristics of these young men who have sex with men were examined as potential drivers of HIV, STI, and unprotected intercourse. Results indicated several partner- and network-level factors associated with HIV and associated outcomes.