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An international inter‑comparison of carbon isotope measurements of dissolved inorganic carbon in seawater

Posted by Liz Kerrigan & Lin Cheng on January 25, 2019 in News
CERC.OCEAN co-authors (left-right) Dr. Doug Wallace, Claire Normandeau, and Lin Cheng in front of the Picarro G2201-i analyzer for d13C-DIC analysis coupled with an Apollo acidification system (photo by Liz Kerrigan)
CERC.OCEAN co-authors (left-right) Dr. Doug Wallace, Claire Normandeau, and Lin Cheng in front of the Picarro G2201-i analyzer for d13C-DIC analysis coupled with an Apollo acidification system (photo by Liz Kerrigan)

In early January 2019, Lin Cheng, Claire Normandeau, and Dr. Doug Wallace of the CERC.OCEAN team, among other international colleagues, published a new study in Limnology and Oceanography: Methods, highlighting their novel inter-comparison of carbon isotope measurements of dissolved inorganic carbon (δ13C-DIC) in seawater. δ13C-DIC is a useful tracer for estimating anthropogenic carbon accumulation in the ocean. However, the overall accuracy and inter-comparability of data derived from different laboratories across the world are not well known.

For the first time, our group organized an inter-comparison study for this parameter, which included 16 laboratories from across the world (see locations on map below), to ensure the comparability of δ13C-DIC measurements in the future. First, deep water samples were collected during the 2017 GO-SHIP A02 trans-Atlantic research cruise onboard the R/V Celtic Explorer (read more about CERC.OCEAN’s involvement, and the other research conducted onboard, HERE) from the northeastern basin of the Atlantic Ocean at 4000 m depth (location identified in blue on the map below). A CRM for oceanic CO2 measurements and deep seawater samples were distributed to the participating labs in 8 countries all over the world.

One major conclusion from this study was that δ13C-DIC results measured on new laser-based optical spectroscopy methods such as Isotope Ratio Infrared Spectrometer (IRIS) and cavity ring-down spectroscopy (CRDS) were comparable to the more established, and currently more common, isotope-ratio mass spectrometry (IRMS) method. This implies that the use of different methods and instrumentation does not lead to major systematic differences between laboratories, which is encouraging, as δ13C-DIC measurements will likely increase in the future, as more portable instruments (such as CRDS and IRIS) make these measurements easier and easier, both in the lab and at sea. While this study found that the required (based on recommendations from the Global Ocean Observing System) measurement precision and accuracy of δ13C-DIC measurements might not be achieved with current analysis and standardization approaches, it also found that the development of a CRM for δ13C-DIC analysis might significantly improve data quality.

Overall, this study highlights the inter-comparibility of methods and the need to ensure accurate and precise between-lab agreement, likely with δ13C-DIC-specific CRMs, as more frequent sampling occurs worldwide, which will help to accurately constrain this important indicator of changing ocean carbon cycle processes in the future.

The full-text article can be accessed HERE.

Cheng, L.Normandeau, C., Bowden, R., Doucett, R., Gallagher, B., Gillikin, D.P., Kumamoto, Y., McKay, J.L., Middlestead, P., Ninnemann, U., Nothaft, D., Dubinina, E.O., Quay, P., Reverdin, G., Shirai, K., Mørkved, P.T., Theiling, B.P., van Geldern, R., and Wallace, D.W.R. 2019. An international intercomparison of stable carbon isotope composition measurements of dissolved inorganic carbon in seawater. Limnology and Oceanography: Methodshttps://doi.org/10.1002/lom3.10300

Locations of participating labs (black), with the A02 deepwater sample location in blue, and the location of the CERC.OCEAN lab, who spearheaded this project, in red.