Evaluation of the moderate DI¹³C isotope enrichment method for measuring photochemical mineralization of marine dissolved organic carbon

The moderate DI¹³C isotope enrichment (MoDIE) method by Powers et al. (2017) is a promising method to precisely measure the photochemical mineralization of dissolved organic carbon (DOC) in water samples without dramatically altering a sample's pH or organic carbon pool. Here, we evaluated the analytical uncertainties of the MoDIE method and used Monte Carlo simulations to optimize the experimental design for the most precise measurements of dissolved inorganic carbon (DIC) that is produced photochemically (DIC_hν). Analytically, we recommend calculating yields of DIC_hv with an exact expression of conservation of mass that intrinsically reduces error and uncertainty. Methodologically, the overall uncertainty and detection limit of the MoDIE method can be significantly reduced by partially stripping away the original DIC pool, enriching the residual DIC with more DI¹³C, and increasing the yields of DIC_hv via longer irradiation. Instrumentally, more precise measurements of enriched δ¹³C values before and after irradiation are needed to further improve the precision of DIC_hν concentration determinations. Higher precision DIC_hv measurements via the optimized MoDIE method can improve our understanding of the photochemical mineralization of DOC and thus the budget of marine DOC. The optimizations and detection limits reported here will become more refined as measurements and associated uncertainties from future MoDIE experiments become available.