A novel method for concurrent measurements of dissolved inorganic carbon concentration and its carbon isotope composition δ13C

The concentration of dissolved inorganic carbon ([DIC]) and its stable carbon isotopic composition (δ¹³C) in the surface ocean are key to studying the important processes in the carbon cycle, e.g., photosynthesis and respiration, calcification, water mass mixing, and, in particularly, the Suess effect as well as the penetration of anthropogenic carbon into the surface ocean and the subsequent ocean acidification. Real-time, shipboard measurements of these properties are highly desired. Here we present a new method that concurrently measures [DIC] and δ¹³C of DIC in the surface ocean. The method couples sample acidification and delivery techniques (Apollo Scitech) with a cavity ring-down spectrometer (CRDS, Picarro), and works automatically to analyzer samples at the throughput of 8 minutes/sample. In each sampling cycle, a syringe pump withdraws a fixed volume of phosphoric acid and seawater sample, and injects them slowly into a reaction chamber where they were mixed. In the meantime, the CO₂ evolved from the acidified sample is purged by a CO₂-free gas flow into a CRDS carbon isotope analyzer for measurements of the CO₂ concentration and δ¹³C-CO₂. The concurrent injection, acidification, and purging yield a broad, flat peak of CO₂ which is precisely and frequently measured by the CRDS analyzer. [DIC] and δ¹³C can then be calculated by integrating the concentration and δ¹³C of the CO₂ peak. The precision of the [DIC] and δ¹³C is <0.15% (3 mmol kg^-1 when [DIC] = 2000 μmol kg^-1), and <0.1‰, relatively. The system is automated to run continuously onboard a research vessel as well as discrete samples in a lab environment.