Measuring the photochemical production of carbon dioxide from marine dissolved organic matter by pool isotope exchange

CO₂ is the major known product of solar photolysis of marine dissolved organic matter (DOM). Measuring the rate of this globally significant process is hindered by low rates per unit volume, high background CO₂ in seawater, and ubiquitous contamination. Current methods utilize CO ₂-free seawater matrices, possibly introducing artifacts. Alternatively, pool isotope exchange (PIE) replaces most of the sample's DI ¹²C with DI¹³C at natural pH and temperature, so that ¹²CO₂ from DOM photooxidation elevates ¹²CO₂/¹³CO₂ ratios in irradiated samples compared to dark controls. ¹²CO₂/ ¹³CO₂ ratios are then measured using a modified GC-IRMS. The minimum detectable concentration change (three standard deviations) is 300 nmol DI¹²C/kg. Methods for minimizing contamination while exchanging, transferring, sealing, and irradiating samples, and for recovering and purifying CO₂ are presented. Results from PIE agree within uncertainties with those from CO₂-free coastal seawater, suggesting that both methods apply to river-dominated coastal waters. However, photooxidation in the open ocean, which likely dominates the global flux despite lower rates per unit volume, involves DOM that differs from coastal DOM, so that coastal agreement cannot validate open-ocean studies. Major advantages of PIE are use of nearly unperturbed seawater matrices, potential to incubate samples in situ to obtain depth-integrated rates directly, and potential to use larger samples to measure open-ocean waters.