GEOCHEMISTRY OF VOLATILES RELEASED BY INCIPIENT CONTINENTAL RIFTING AND SUBDUCTION PROCESSES

Volatiles (N2, CO2, and He) are released by volcanism and hydrothermal activity during continental rifting and subduction processes. Analyses of volatile components have been conducted to obtain gas contents (CO2, SO2, H2S, N2, Ar, He, and so on), stable isotope compositions (δ13C, δ15N, and so on), and noble gas isotopes (3He/4He, 40Ar/36Ar, and so on). This dissertation includes four chapters to report new nitrogen isotope fractionation factors of bubbling gases during gas-water transfer at various water temperatures (Chapter 1), first measurements of massive amounts of CO2 released by incipient continental rifting in the Magadi and Natron Basin, East African Rift (Chapter 2), new results of gas chemistry, stable isotopes, and noble gas isotopes of hot springs in the same area as Chapter 2 (Chapter 3), and new nitrogen isotope compositions of springs at the Costa Rican subduction zone (Chapter 4). Chapter 1 (published in Geochemical Journal) is the first experimental work to acquire nitrogen isotope fractionation factors during N2 gas and water transfer at various temperature to examine nitrogen isotope shift for hydrothermal systems. This work reports measured δ15N values of dissolved N2 gas at 5 to 60°C. We obtained δ15N values of 0.91, 0.73, -0.04, and -0.42‰ at 5, 20, 40, and 60°C, respectively. Nitrogen isotope fractionation depending on temperature is more significant than previously published results, showing an isotopic \u27crossover\u27 at 40°C. A kinetic incorporation of 14N into water is enhanced by rising temperatures could explain the steep temperature dependence. In hydrothermal systems, small negative δ15N values could be attributed to kinetic fractionation between dissolved N2 and N2 in air. Chapter 2 (published in Nature Geoscience) is the first estimation of diffuse CO2 degassing (“Tectonic Degassing”) along faults which are away from active volcanic centers in the East African Rift. We used results of diffuse soil CO2 measurements combined with carbon isotopic compositions to quantify the flux of CO2 and constrain CO2 sources. This study reports that 4.05 mega tons per year of mantle-derived CO2 is released by faults penetrating the lower crust in the Magadi-Natron Basin. Extrapolated CO2 flux (71±33 mega tons per year) of the entire Eastern rift (~3,000 km long from Afar to Mozambique) is comparable to CO2 emission from the entire mid-ocean ridge system (53-97 mega tons per year). Therefore, widespread continental rifting and super-continent breakup could result in massive and long-term CO2 emissions, contributing prolonged greenhouse conditions likely during the Cretaceous. Chapter 3 (published in Journal of Volcanology and Geothermal Research) reports new results of gas compositions, stable isotopes (O, H, N, and C), and noble gas isotopes (He and Ar) of hot spring samples from the Magadi and Natron basin in the East African Rift (EAR). In dissolved gases, CO2 is the most abundant deep and shallow sources are mixed based on the N2-He-Ar abundances. δ18O and δD values of the springs waters indicates that the local meteoric water is dominant with minor evaporation. Most of δ15N and δ13C values and 3He/4He ratios suggest that Subcontinental Lithospheric Mantle (SCLM) is the major mantle source. The 4He flux values, significantly greater than the reported mean of global continental flux values, imply that elevated mantle 4He flux is due to magmatism and related heating, and crustal 4He is released by fracturing of old rocks in the Tanzanian craton and Mozambique belt. SCLM-derived volatiles can be ascribed to that a relatively small volume of lithospheric mantle has been replaced by asthenosphere during incipient rifting (Ma). Chapter 4 (in preparation) shows new results of gas compositions, nitrogen, and helium isotopes of springs at forearc and arc front areas in Costa Rica. Nitrogen isotope compositions (9-11N°) with less pelagic sediment contribution compared to further north (Guatemala and Nicaragua) result in insufficient nitrogen output to the atmosphere. This work supports the subduction erosion model in conjunction with seamount subduction. The overlying forearc crustal materials incorporated into the Costa Rican subduction zone dilute sediment-derived nitrogen signals. These results support the deep recycling of N into the deep mantle

Electric current fluctuations in mesoscopic systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1996.Includes bibliographical references (p. 115-119).by Hyunwoo Lee.Ph.D

When to Signal? The Contextual Conditions for Career-Motivated User Contributions in Online Collaboration Communities

This paper examines the contextual conditions for users’ career concern as a motivational driver of contributions in online collaboration communities. On the data of user-level activities from a computer programming-related online Q&A community (Stack Overflow), merged with job-market data for software-developer, we find robust evidence of a positive association between individual users’ career concern and their contributions. More important, we find that this positive relationship is further strengthened through the contextual conditions: the number of vacancies in the job market, the expected salaries from these jobs, and the transparency in the flow of career-related information within the community. We contribute to the literature on motivation in online collaboration communities. Our study thus offers insight into how career concern can be effectively utilized to motivate contributors in these communities. Our findings also foreshadow a possible paradigm change by characterizing online collaboration communities as institutions of career concern and skill signaling

When to Signal? Contingencies for Career-Motivated Contributions in Online Collaboration Communities

Online collaboration communities are increasingly taking on new roles beyond knowledge creation and exchange, especially the role of a skill-signaling channel for career-motivated community members. This paper examines the contingency effects of job-market conditions for career-motivated knowledge contributions in online collaboration communities. From the data of individual-level activities in a computer programming-related online Q&A community (Stack Overflow), merged with job-market data for software developers, we find robust evidence of a positive association between community members’ career motivations and their knowledge contributions. More importantly, we find that this positive relationship is strengthened by job-market conditions: the number of vacancies in the job market, the expected salaries from these jobs, and the transparency in the flow of career-related information between the community and external recruiters. We contribute to the motivation literature in online collaboration communities by identifying and substantiating the role of contextual factors in mobilizing members’ career motivation. Our study thus offers novel insight into how career motivation can be effectively utilized to motivate contributors in these communities. Our findings also point to a possible paradigm change by characterizing online collaboration communities as emerging institutions for career motivation and skill signaling