An improved method for the analysis of dissolved noble gases in the porewater of unconsolidated sediments

Yama Tomonaga, Matthias S. Brennwald, and Rolf Kipfer

Limnol. Oceanogr. Methods 9:42-49 (2011) | DOI: 10.4319/lom.2010.9.42

ABSTRACT: We developed a new and improved method for the extraction of dissolved noble gases from the porewater of unconsolidated lacustrine sediments. The new method relies on the separation of the porewater from the sediment matrix by centrifugation of the bulk sediment within the closed copper tubes being used as sampling containers. After centrifugation, the porewater liberated from sediment matrix is completely separated from the dewatered sediment by applying and closing a metal clamp between the two phases. After separation, the noble-gas abundances are determined only in the porewater phase by applying the standard protocols used for noble-gas analysis in water samples, which allows simpler, faster and more reliable measurements. The new method overcomes known experimental difficulties encountered by our former method, such as the release of He from sediment grains, and incomplete extrusion of the bulk sediment from the copper tube used as a sample container. It follows that the determination of the He concentrations and isotope ratios in the sediment porewater is less affected by He locally produced in sediment minerals. The new method is less operator-sensitive and the noble-gas extraction efficiency (especially for heavy noble-gas species) has, in general, improved. As only bulk porewater is used for analysis, the new method can easily be adopted by other noble-gas laboratories. Also, the new method can be used to determine the abundance of other dissolved gases such CH4, N2, or CO2.