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

Abstract

In recent years, the analysis of noble gases in meteoric waters has become a powerful tool for studying mixing dynamics in lakes, oceans, and groundwaters, and for reconstructing past climate conditions (Kipfer et al. 2002;Schlosser and Winckler 2002). Noble-gas concentrations in sediment porewater have been shown to reflect past noble-gas concentrations in the overlying water body (Brennwald et al. 2003(Brennwald et al. , 2004, and can provide information on transport and gasexchange processes within the sediment (Brennwald et al. 2005;Strassmann et al. 2005;Holzner et al. 2008).Until now, the method developed by Brennwald et al. (2003) was the only method available for the quantitative analysis of the concentrations of noble gases dissolved in the porewater of lacustrine sediments. This method was successfully used to study the noble-gas abundance in the sediments in various lakes (Brennwald et al. 2003(Brennwald et al. , 2004(Brennwald et al. , 2005Strassmann et al. 2005), and in the Black Sea (Holzner et al. 2008). Gas extraction is achieved by connecting the bulk sediment sample container to an evacuated extraction vessel. The sample container is then rapidly heated to 150°C, which results in a pressure increase in the sample container. The bulk sediment is then extruded explosively into the extraction vessel ("blowout"), and the sediment grains are dispersed over the interior surface of the extraction vessel. The porewater is liberated from the sediment matrix and is degassed in the extraction vessel. Chaduteau et al. (2007) recently proposed an extraction method based on flushing the sediment samples with degassed water. However this method was only applied to determine the concentrations of He isotopes, requires a dedicated extraction apparatus, and is time-consuming.Although the analytical protocol of Brennwald et al. (2003) AbstractWe 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 noblegas abundances are determined only in the porewater phase by applying the standard protocols used for noblegas 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-...

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Microbial abundance in lacustrine sediments: a case study from Lake Van, Turkey

“…These WRCs were originally taken for noble gas analyses (Tomonaga et al 2011b). Sampling for noble gases requires transferring the sediment into gas-tight copper tubes without any atmospheric contact (Tomonaga et al 2011a). This was achieved by hydraulically squeezing a WRC that was fitted with a gas-tight sleeve connected to the copper tube.…”

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“…A 1.03‐m‐long sediment core taken by a gravity corer was sampled for noble‐gas analysis using the method of Brennwald et al (). Bulk sediment was transferred from the plastic liner containing the core into small copper tubes which were closed air‐tight with two special metal clamps (Brennwald et al ; Tomonaga et al ).…”

“…To fill a larger number of samplers, more bulk‐sediment volume is required. During the transfer of the sediment into the copper tube samplers, two pistons push the core material out of the liner simultaneously from both sides (Brennwald et al ; Tomonaga et al ). As the uppermost part of the sediment column normally shows a strong porosity gradient (i.e., the porosity decreases with depth), and as sediment of higher porosity often flows more easily than sediment of lower porosity, the upper samplers tend to be filled with sediment before the bottom samplers.…”