Abstract
Preservation of seawater samples was tested for total inorganic carbon (∑CO2), stable carbon isotope (δ13C), and radiocarbon (14C) applications using foil bags and storage by refrigeration and freezing. The aim was to preserve representative samples with minimal storage effects but without using toxic methods such as mercuric chloride poisoning. Hydrolysis of samples to CO2 was based on existing methods. Results of IAEA-C2 standard used with deionized water stored in the foil bags showed complete reaction yields, 14C results within 2σ of the consensus value, and δ13C that were internally consistent, indicating that there were no procedural effects associated with the foil bags. 14C results were statistically indistinguishable across the storage times, for frozen and refrigerated seawater samples from a coastal site, Elie Ness, Fife, UK. The scatter of ∑CO2 concentrations and δ13C was within scatter observed in other studies for lake- and seawater samples preserved by acidification or using mercuric chloride. However, both ∑CO2 and δ13C were less variable for frozen samples compared with refrigerated samples. The foil bags are lighter, safer to transport, and similar in cost to glass bottles and allow sample collection in the field and transfer to the hydrolysis vessel without exposure of the sample to atmosphere. Storage of seawater samples in the foil bags was considered a reliable, alternative method to poisoning for ∑CO2, δ13C, and 14C, and freezing the samples is recommended for storage time beyond a week.
Highlights
Carbon isotope measurements of inorganic carbon in freshwater and marine samples are used for examining hydrological patterns, carbon sources and cycling, and the relationship between the atmosphere and aquatic environment
Preservation of seawater samples was tested for total inorganic carbon ( CO2), stable carbon isotope ( 13C), and radiocarbon (14C) applications using foil bags and storage by refrigeration and freezing
Results of IAEA-C2 standard used with deionized water stored in the foil bags showed complete reaction yields, 14C results within 2 of the consensus value, and 13C that were internally consistent, indicating that there were no procedural effects associated with the foil bags. 14C results were statistically indistinguishable across the storage times, for frozen and refrigerated seawater samples from a coastal site, Elie Ness, Fife, UK
Summary
Carbon isotope measurements of inorganic carbon in freshwater and marine samples are used for examining hydrological patterns, carbon sources and cycling, and the relationship between the atmosphere and aquatic environment. Bard et al 1987; Schlosser et al 1988) and subsequent large-scale 14C analytical programs such as the World Ocean Circulation Experiment (WOCE, McNichol et al 1994) involved sample preservation with mercuric chloride While this compound inhibits biological activity and reliably preserves the carbon isotope ratios and concentrations of inorganic carbon even after long-term storage (McNichol et al 1994), its toxicity and potential hazard to the environment have stimulated the search for alternative preservation methods. Lake-water samples stored in 12-mL septa-covered Exetainers® were acidified with phosphoric acid in the field and retained their original 13C values after 6 months of storage (Taipale and Sonninen 2009) Where both 13C and 14C are to be analyzed, a larger volume of sam-
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