Ph.D. Thesis Defense: Boda Li
Student: Boda Li
Department: Earth, Environmental and Planetary Sciences
Defense Date: Thursday, September 2, 2021
Time: 1:00 p.m.
Thesis Title: Tracing oxygen cycling in the ocean by dissolved oxygen multi-isotopologues
Understanding the distribution and cycling of O2 is one of the core scientific questions in oceanography. The distribution of dissolved oxygen is influenced by physical, biochemical, and abiotic chemical processes. However, determining the relative importance of each mechanism in different environments remains difficult. This thesis utilizes the dissolved oxygen multiple isotopologues (16O16O, 16O17O, 16O18O, 17O18O, 18O18O), which participate biogeochemical cycling, advection, and diffusion as new direct tracers to constrain these processes.
The theme of my first project is the oxygen production in the surface ocean, with a particular focus on the isotopic effects of gas exchange when the mixed layer is out of solubility equilibrium. Here we measured the kinetic and equilibrium fractionation factors for the four rare O2 isotopologues 16O17O, 16O18O, 17O18O, 18O18O relative to 16O16O in air-water gas transfer experiments. Moreover, we evaluate the possible effects of the updated fractionation factors on multiple isotopologue-based gross oxygen production (GOP) estimates and connect the observed air-water kinetic fractionation factors to dissolved-phase diffusive isotopic fractionation.
Next, this thesis shifts the focus to the subsurface ocean, where uncertainties in transport flux and isotopic fractionation factors have long confounded the interpretation of the isotopic composition of dissolved oxygen. Therefore, we investigated the systematics of oxygen isotopologues in the subsurface Pacific using new data and a 2-D isotopologues-enabled isopycnal reaction-transport model. We measured multiple O2 isotopologues in the northeast Pacific, and compared them to previously published data. We find that transport and respiration rates constrained by O2 concentrations in the oligotrophic Pacific yield good measurement-model agreement across all O2 isotopologues when using a recently reported set of respiratory isotopologue fractionation factors. Estimated respiration rates range from 0.5 to 2.7 μmol/kg/yr for various isopycnal surfaces consistent with those previously inferred for the Atlantic.
Lastly, this thesis attempts to address the mystery of puzzlingly low 17O/16O ratios observed for some subsurface low-oxygen samples. We resampled and remeasured the dissolved oxygen isotopologues at the San Pedro Ocean Time Series (SPOT) site, where low 17O/16O ratios were reported in previous studies. We find that this low ratio is caused by a typical analytical artifact, pressure baseline effect (PBE), which exhibits a non-linear behavior with the fraction of inert gas present in the analyte and can cause an overestimation of GOP at SPOT. A correction on these previously reported low 17O/16O ratios were applied to the GOP estimation model, in which we find that calculated GOP at SPOT can be overestimated by ~50 mmol O2/(m2d). Therefore, we underscore the necessity of a pre-purification process for insert gas removal during dissolved oxygen isotope measurements to avoid this potential PBE artifact.