Today was a beautiful, sunny day in Stony Brook: just right for a doctoral hooding! It was an honor to share this day with my wonderful PhD student, Xi Lu. She is smart, kind, funny, and a special friend who will happily dodge four hurricanes with you on a research vessel… and still be ready to dodge some more! (July is coming…) Thank you, Lucy, for all that you have done and all that you have taught me.
Xi Lu will formally defend her dissertation at 9:30 am on June 17:
Photochemical mineralization of marine refractory dissolved organic carbon (RDOC): method evaluation and optimization
By Xi Lu
Abstract: Despite its importance in the global carbon cycle, the budget of marine refractory dissolved organic carbon (RDOC) is out of balance. To constrain the flux of RDOC photochemical mineralization, which is presumably the largest sink of marine RDOC, measurements of both the production rate and radiocarbon (14C) isotopic composition of dissolved inorganic carbon (DIC) photochemically generated from marine DOC (DIChν) are required. In this study, methods for precisely achieving these measurements were evaluated and optimized with error propagations and Monte Carlo simulations. The Moderate DI13C Isotope Enrichment (MoDIE) method by Powers et al. represents the least invasive approach for precise quantification of DIChν, where the ambient DIC pool is isotopically enriched and subsequently “diluted” by the natural isotope-abundance DIChν. The decrease in the bulk DIC isotopic signature after irradiation thus indicates the amount of DIChν produced. This study evaluated the analytical uncertainties of the MoDIE method, and optimized its experimental designs for the most precise DIChν measurements. Based on the optimized MoDIE method, the study tested the feasibility of a dual isotopic (13C and 14C) dilution method to determine both the quantity and the 14C signature of DIChν. Alas, even with the optimized experimental conditions, these isotopic enrichment/dilution methods are still practically impeded by their sensitivities to precise sample manipulations. A well-designed photochemical reactor combined with a well-calibrated vacuum line may be the preferred approach for determining the RDOC photochemical mineralization rate under near-natural conditions. The DIChν collected in a vacuum line is typically quantified with manometry. Its precision depends on the precise measurements of pressure (P), temperature (T), and manometer volume (V). While P and T uncertainties depend on instrument choice and environmental stability, volume uncertainties depend on their method of measurement and are often overlooked. This study elucidated the optimum procedures for measuring V by two common applications of Boyle’s Law: cryogenic transfers and serial gas expansions. The optimized determination of V reduces the minimum achievable relative uncertainties of moles-of-gas to ±0.0026 ~ 0.0027. This optimized manometry improves the precision of marine RDOC photochemical mineralization rate measurements, and therefore our ability to constrain the RDOC budget.