Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2019 / Cataloged from PDF version of thesis. / Includes bibliographical references. / Nitrogen (N) species in hydrothermal vent fluids serve as both a nutrient and energy source for the chemosynthetic ecosystems surrounding deep-sea vents. While numerous pathways have been identified in which N-species can be produced and consumed in the context of submarine hydrothermal vent systems, their exact nature has been largely limited to interpretation of variations in concentrations. This thesis applies stable isotope approaches to further constrain the sources and fate of N-species in deep-sea vents across a variety of geological settings. First, I discuss isotope fractionation and reaction kinetics during abiotic reduction of nitrate (NO₃⁻) to ammonium ([sigma]NH₄⁺ = NH₃+NH₄⁺) under hydrothermal conditions. Results of lab experiments conducted at high temperatures and pressures revealed a wide degree of N isotope fractionation as affected by temperature, fluid/rock ratio, and pH-all which exert control over reaction rates. / Moreover, a clear pattern in terms of reaction products can be discerned with the reaction producing [sigma]NH₄⁺ only at high pH, but both [sigma]NH₄⁺ and N₂ at low pH. This challenges previous assumptions that NO₃⁻ is always quantitatively converted to NH₄⁺ during submarine hydrothermal circulation. Next, I report measurements of [sigma]NH₄⁺ concentrations and N isotopic composition ([delta]¹⁵N[subscript NH4]) from vent fluid samples, together with the largest compilation to date of these measurements made from other studies of deep-sea vent systems for comparison. The importance of different processes at sediment-influenced and unsedimented systems are discussed with a focus on how they ultimately yield observed vent [sigma]NH₄⁺ values. / Notable findings include the role that phase separation might play under some conditions and a description of how an unsedimented site from Mid-Cayman Rise with unexpectedly high NH4+ may be uniquely influenced by N₂ reduction to [sigma]NH₄⁺. Lastly, I explore [sigma]NH₄⁺ dynamics in the context of low-temperature vent sites at 9°50'N East Pacific Rise to investigate dynamics of microbially-mediated N transformations. Through both measurements of natural samples, as well as isotopic characterization of N species from incubation experiments and model simulations thereof, an exceptionally high variability observed in [delta]¹⁵N[subscript NH4] values emphasizes the complexity of these microbe-rich systems. / In sum, this thesis highlights the role of microbial processes in low temperature systems, demonstrates a more mechanistic understanding of lesser-understood abiotic N reactions and improves the coverage of available data on deep-sea vent [sigma]NH₄⁺ measurements. / by Chawalit "Net" Charoenpong. / Ph. D. / Ph.D. Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution)