Analysis of dissolved noble gases in the porewater of lacustrine sediments

Here we present a new method for the sampling and quantitative extraction of dissolved He, Ne, Ar, Kr, and Xe from lake sediment samples leading to determinations of porewater noble gas concentration profiles and the isotopic ratios 3He/4He, 20Ne/22Ne, and 40Ar/36Ar. Bulk sediment is transferred from a sediment core into standard Cu sample tubes without exposure to the atmosphere or other gas reservoirs. The noble gases are then extracted from the porewater by degassing the sediment in an evacuated extraction vessel and analyzed following standard mass spectrometric procedures. In tests of the new method using 0.8 to 1.4 m long sediment cores from two Swiss lakes, analytical uncertainties were only slightly greater than those of standard water samples. The majority of porewater noble gas concentrations and isotopic ratios were found to correspond closely to those measured in the overlying lake water. Because these values reflect water temperature and salinity during atmospheric equilibration at the lake surface, historical conditions are expected to be archived further downcore in the sediment porewater. This method therefore has great potential for paleolimnological reconstructions. The formation of methane bubbles in anoxic sediment layers is one process that may alter gas distributions. However because the lighter noble gases are most sensitive to degassing effects, noble gas data can be used to detect this process. In addition, noble gas data can yield information on the transport processes occurring in the sediment pore space and on the input of water or gas to the sediment from external sources. Acknowledgments We thank D. M. Livingstone (EAWAG) and two anonymous reviewers for their helpful comments and editing assistance. This work was funded by the Swiss Science Foundation (SNF 2000-061498.00 and 2068191.02). Limnol. Oceanogr.: Methods 1, 2003, 51–62 © 2003, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS Until now, only a few published studies have explored dissolved noble gases in lake sediment porewaters. The aim of these studies was to characterize the transport processes in the sediment pore space; hence these studies focused on the light noble gases, which are influenced only weakly by variations in temperature and salinity because their solubilities do not depend strongly on these variables. Peeper methods have been used to sample dissolved He (Stephenson et al. 1994), but these methods have several disadvantages. Peepers need to be left at the sampling site for He equilibration with the surrounding porewater for several days or weeks (Dyck and Da Silva 1981). Because of their lower diffusivity, the heavier noble gases would require even longer equilibration times. Also, the peeper used by Stephenson et al. (1994) is not suitable for sampling at water depths exceeding 55 m because the membranes of the gas-filled peeper chambers collapse under the hydrostatic pressure exerted. Furthermore, peeper methods seem to be inappropriate to quantitatively capture poorly soluble species, as indicated by comparing CH4 concentrations obtained from peepers and squeezing techniques (B. Wehrli pers. comm. unref.). Alternatively, an in situ sampler (Barnes 1973) has been applied in the analysis of dissolved He, Ne, Ar, and Kr in marine sediments to estimate the He flux through the sediment/water interface (Barnes and Bieri 1976). This nonstandard equipment is complicated to operate, and gas leakage may occur from the samples. Torres et al. (1995) and Winckler (1998) determined He, Ne, Ar, Kr, and Xe concentrations using a “WSTP tool” (Barnes 1979, 1988), an enhanced version of the above in situ sampler. It appears that gas bubbles may form in the system during sampling. Hence, exchange of the dissolved sample gases with the gas bubbles affects the noble gas concentrations in the sample in an uncontrollable manner. Also, contamination with the drilling fluid or the water used to fill the WSTP tool may occur (Winckler 1998). To overcome the shortcomings of the previous methods, we developed a new experimental method for the sampling and extraction of dissolved noble gases in sediment porewater. Lake sediment porewater concentrations of Ne, Ar, Kr, Xe, and in some cases He, can be measured with an accuracy comparable to that attained in the analysis of noble gases in lake water and groundwater. The sampling procedures described here allow the noble gas samples to be prepared rapidly in the field. Both sampling and analysis are routine processes based on standard equipment used in sedimentology and for the analysis of noble gases in water samples. Materials and procedures Sediment sampling for noble gas analysis—Our method for extracting noble gases was designed for sediment cores collected in transparent plastic tubes of 62 mm outer diameter and 1.5 mm liner thickness using either a gravity corer or a UWITEC sampling system. The UWITEC system consists of a sediment corer operated from a small floating platform that allows the iterative collection of overlapping sediment sequences of 3 m length down to depths of 20 m or more in the sediment (Melles et al. 1994). After recovery of the sediment core, about 30 g of bulk sediment is sampled from the desired sediment depths. Similar to noble gas sampling in water (Kipfer 1991; Beyerle et al. 2000), Cu tubes of 30 cm in length and ≈1 cm outer diameter are used as sample containers. The Cu tubes are connected to the sediment core via Swagelock-fittings (SS-600-1-6BT) that have been modified with an additional tube (length 20 mm, diameter 8 mm) to penetrate the sediment core (Fig. 1). These fittings are mounted onto the sediment core by drilling holes 15 mm in diameter into the plastic liner (sampling ports). Threads are cut into the liner to screw-mount the fittings. To minimize the risk of contaminating the sediment with air during drilling, the sampling ports can be drilled before sediment coring. In this case, the ports are covered by adhesive tape during coring. The sediment sample is transferred from the sediment liner into the Cu tubes immediately after collecting the sediment core, using the squeezer setup shown in Fig. 1. The Cu tubes are flushed with sediment several times to remove residual air and to expel the sediment fraction, which may have exchanged noble gases with the atmosphere while mounting the Cu tubes. The average sediment displacement observed in the liner due to squeezing is about 2.5 cm per sample. On this length scale, no significant concentration variations are expected because of diffusion. Hence, consecutively collected samples from the same ports can be treated as replicate samples. The samples are closed and sealed by pinching off the Cu tubes at both ends using the same standard clamps as those used for water samples (Kipfer 1991; Beyerle et al. 2000). This procedure allows the sampling of intact sediment cores and avoids air contamination and gas stripping during sampling. Extraction and analysis of dissolved noble gases—The methods used to extract noble gases dissolved in lake water, seawater, or groundwater (Bayer et al. 1989; Beyerle et al. 2000) cannot be Brennwald et al. Noble gas analysis in sediment porewater 52 Fig. 1. Sediment squeezer and sampling set-up. The squeezer length can be adjusted for sediment cores with lengths of up to 2 m. Longer cores need to be split up into shorter sections. The Cu tubes are attached to the liner by means of Swagelock fittings that have been modified with an additional tube to penetrate the sediment core.