The production and fate of nitrogen species in deep-sea hydrothermal environments

Abstract

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 (ΣNH4+ = 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 of which exert control over reaction rates. Moreover, a clear pattern in terms of reaction products can be discerned with the reaction producing ΣNH4+ only at high pH, but both ΣNH4+ and N2 at low pH. This challenges previous assumptions that O3 - is always quantitatively converted to NH4+ during submarine hydrothermal circulation. Next, I report measurements of ΣNH4+ concentrations and N isotopic composition (δ15NNH4) 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 δ15NNH4 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 ΣNH4+. Lastly, I explore ΣNH4+ 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 δ15NNH4 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 ΣNH4+ measurements.