Development of a high-precision seawater DIC isotope analysis system

Julia L Dohner, Mariela K Brooks, Bill Paplawsky, Andreas J Andersson and Ralph F Keeling, Scripps Institution of Oceanography, La Jolla, CA, United States
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.