Development of a high-precision seawater DIC isotope analysis system

The ratio of ¹³C to ¹²C (denoted δ¹³C) in dissolved inorganic carbon (DIC) can serve as a tracer for a variety marine biogeochemical processes including ocean uptake of anthropogenic CO₂, net community production, and coastal mixing events. Unlike DIC, δ¹³C may provide information on sources of local carbon and which organisms are manipulating local carbon pools. However, seawater δ¹³C measurements are sparse because of the high time costs of seawater isotopic analyses. Lowering this analytical barrier will expand the usage of this tracer, furthering our understanding of marine biogeochemistry in a changing ocean. Here we describe the development of a laboratory system for fast, high-precision measurement of δ¹³C and DIC concentrations in seawater. Seawater samples are acidified and passed through a counter-flow membrane “contactor” to extract CO₂ gas from the liquid sample. The contactor consists of an inner CO₂-permeable membrane, through which the sample flows, and outer stainless steel tubing through which CO₂-free carrier gas flows in the opposite direction. The counterflow of liquid sample and carrier gas separated by a CO₂-permeable membrane is designed to strip the liquid sample of nearly all CO₂. The carrier and CO₂ gas mixture is then split and fed to a Picarro 2401 analyzer to measure [DIC] and an Aerodyne Research Inc. continuous-wave quantum cascade laser spectrometer to measure δ¹³C. The expected sample run time is on the order of 10 minutes, a major improvement over the current processing rate of ~2.5 hours/sample for δ¹³C in the Scripps CO₂ Group. The described method uses ~20 mL of seawater to achieve a target precision of ±2 μmol kg^-1 for [DIC] and ±0.015‰ for δ¹³C; the existing system, using double the sample volume, targets ±0.75 μmol kg^-1 for [DIC] and ±0.015‰ for δ¹³C. As a first application this system will be used to measure the spatial and temporal variability of δ¹³C of DIC in seawater samples collected in coral reefs in Okinawa, Japan.