The Production and Fate of Nitrogen Species in Deep-Sea Hydrothermal Environments

Chawalit Charoenpong, Ph.D., 2019
Scott Wankel, Advisor

This work explores different processes relevant to nitrogen (N) transformations at deep-sea vents. First, I discuss the isotope fractionation and reaction kinetics during abiotic reduction of nitrate (NO3-) to ammonium (SNH4+) under hydrothermal conditions.  Both aspects of the reaction were affected by temperature, fluid/rock ratio, and pH. Moreover, two distinct patterns in the reaction products can be discerned with the reaction producing NH4+ only at high pH, but both SNH4+ and N2 at low pH. Next, I report measurements of NH4+ concentrations and N isotopic composition (d15NNH4)from vent fluid samples, highlighting the importance of different processes operating at sediment-influenced and unsedimented systems. Lastly, I explore SNH4+ 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 d15NNH4 values emphasizes the complexity of these microbe-rich systems. In summary, 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 SNH4+ measurements.