A portable and autonomous mass spectrometric system for on-site environmental gas analysis

We developed a portable mass spectrometric system (“miniRuedi”) for quantificaton of the partial pressures of He, Ne (in dry gas), Ar, Kr, N2, O2, CO2, and CH4 in gaseous and aqueous matrices in environmental systems with an analytical uncertainty of 1−3%. The miniRuedi does not require any purification or other preparation of the sampled gases and therefore allows maintenance- free and autonomous operation. The apparatus is most suitable for on-site gas analysis during field work and at remote locations due to its small size (60 cm × 40 cm × 14 cm), low weight (13 kg), and low power consumption (50 W). The gases are continuously sampled and transferred through a capillary pressure reduction system into a vacuum chamber, where they are analyzed using a quadrupole mass spectrometer with a time resolution of ≲1 min. The low gas consumption rate (<0.1 mL/min) minimizes interference with the natural mass balance of gases in environmental systems, and allows the unbiased quantification of dissolved-gas concentrations in water by gas/water equilibration using membrane contractors (gasequilibrium membrane-inlet mass spectrometry, GE-MIMS). The performance of the miniRuedi is demonstrated in laboratory and field tests, and its utility is illustrated in field applications related to soil-gas formation, lake/atmosphere gas exchange, and seafloor gas emanations

Noble gas based temperature reconstruction on a Swiss stalagmite from the last glacial–interglacial transition and its comparison with other climate records

Here we present the results of a first application of a “Combined Vacuum Crushing and Sieving (CVCS)” system to determine past (cave / soil) temperatures from dissolved noble gas concentrations in stalagmite samples grown under ‘cold’ climatic conditions (e.g. close to freezing point of water) during the last glacial-interglacial transition. To establish noble gas temperatures (NGTs) also for stalagmites grown in cold regions, we applied the CVCS system to samples from stalagmite M2 precipitated in the Milandre Cave, located in the Swiss Jura Mountains. The investigated stalagmite M2 covers the Allerød – Younger Dryas – Holocene transitions. Noble gas temperatures are determined by using a new algorithm based on noble gas and water abundances and not from concentrations. Noble gas results indicate annual mean temperatures in the Milandre Cave were 2.2 ± 1.8 °C during the late stages of the Allerød, then dropping to 〖0 〗_((-))^( +) 2.6 °C at the onset of the Younger Dryas. Such temperatures indicate conditions near to the freezing point of water during the first part of the Younger Dryas. During the last part of the Younger Dryas, the temperature increased to 6.3 ± 2.3 °C. No early Holocene temperature could be determined due the non-detectable water abundances in these samples, however one late Holocene sample indicates a cave temperature of 8.7 ± 2.7 °C which is close to the present day annual mean temperature. NGTs estimated for the Allerød – Younger Dryas – Holocene are in good agreement with paleo-temperature reconstructions from geochemical and biological proxies in lake sediments. The observed deviations between the different paleo-temperature reconstructions are minor if the according temperatures are rescaled to annual mean temperatures and are primarily attributed to the chronological tuning of the different records. As in other stalagmites, NGT reconstructions of the recently precipitated stalagmite (‘young’) samples again are biased, most likely due to diffusive gas loss during sample processing. We speculate that a reduced retentivity of noble gases during experimental sample processing is a general feature of recently precipitated stalagmite fabrics. Therefore, the recently precipitated stalagmite samples do not allow the reliable NGT determination given the currently available experimental methods. Nevertheless, this study makes the case that noble gas thermometry can be applied to stalagmites for physically based paleo-temperature reconstruction, also for stalagmites grown during cold climatic conditions

Lake surface cooling drives littoral-pelagic exchange of dissolved gases

The extent of littoral influence on lake gas dynamics remains debated in the aquatic science community due to the lack of direct quantification of lateral gas transport. The prevalent assumption of diffusive horizontal transport in gas budgets fails to explain anomalies observed in pelagic gas concentrations. Here, we demonstrate through high-frequency measurements in a eutrophic lake that daily convective horizontal circulation generates littoral-pelagic advective gas fluxes one order of magnitude larger than typical horizontal fluxes used in gas budgets. These lateral fluxes are sufficient to redistribute gases at the basin-scale and generate concentration anomalies reported in other lakes. Our observations also contrast the hypothesis of pure, nocturnal littoral-to-pelagic exchange by showing that convective circulation transports gases such as oxygen and methane toward both the pelagic and littoral zones during the daytime. This study challenges the traditional pelagic-centered models of aquatic systems by showing that convective circulation represents a fundamental lateral transport mechanism to be integrated into gas budgets. Cooling-induced horizontal circulation redistributes gases daily between littoral and pelagic lake waters under calm conditions

A combined vacuum crushing and sieving (CVCS) system designed to determine noble gas paleotemperatures from stalagmite samples

This paper presents a novel extraction device for water and noble gases from speleothem samples for noble gas paleotemperature determination. The “combined vacuum crushing and sieving (CVCS) system” was designed to reduce the atmospheric noble gas contents from air inclusions in speleothem samples by up to 2 orders of magnitude without adsorbing atmospheric noble gases onto the freshly produced grain surfaces, a process that had often hampered noble gas temperature (NGT) determination in the past. We also present the results from first performance tests of the CVCS system processing stalagmite samples grown at a known temperature. This temperature is reliably reproduced by the NGTs derived from Ar, Kr, and Xe extracted from the samples. The CVCS system is, therefore, suitable for routine determinations of accurate NGTs. In combination with stalagmite dating, these NGTs will allow reconstructing past regional temperature evolutions, and also support the interpretation of the often complex stable isotope records preserved in the stalagmites' calcite

Spatial distribution and flux of terrigenic He dissolved in the sediment pore water of Lake Van (Turkey)

In this study, the largest ever carried out to measure noble gases in the pore water of unconsolidated sediments in lakes, the emission of terrigenic He through the sediment column of Lake Van was successfully mapped on the local scale. The main input of He to the water body occurs at the borders of a deep basin within the lake, which is probably the remains of a collapsed caldera. The 3He/4He3He/4He ratio identifies the He injected into the sedimentary column of Lake Van as a mixture of He released from a mantle source and radiogenic He of crustal origin (3He/4He∼2.6-4.1×10-6)(3He/4He∼2.6-4.1×10-6). During passage through the pore space, terrigenic He seems to be further enriched in radiogenic He that is most likely produced in the sediment column. In fact, two distinct trends in isotopic composition can be distinguished in the He injected from the lake basement into the sediments. One of these characterizes samples from the shallow water, the other characterizes samples from the deep basin. However, both of these trends are related to the same source of terrigenic He. The He fluxes determined seem to be characteristic of each sampling location and might be considered as a proxy for the fluid permeability of the deep sediment column. These new findings provide insight into the process of fluid transport within the sediments and into the process of formation of the lake basin. Moreover, the isotopic signature of the He that emanates into the water column of Lake Van is strongly affected by the mixing conditions prevailing in the overlying water body. This fact misled previous studies to interpret the terrigenic He in Lake Van as being solely of mantle origin (3He/4He∼10-5)(3He/4He∼10-5)

Deconvolution and compensation of mass spectrometric overlap interferences with the miniRUEDI portable mass spectrometer

The miniRUEDI is a portable mass spectrometer system designed for on-site analysis of gases in the environment during field work and at remote locations. For many gas species (e.g., He, Ar, Kr, N-2 , O-2 , CO2) the ion-current peak-heights measured with the mass spectrometer can usually be calibrated in terms of the partial pressures by simple peak-height comparison relative to a gas standard with well known partial pressures. However, depending on the composition of the analysed gases, the ion currents measured at certain m/z ratios may result from overlapping signals of multiple species (for example CH4, O-2 and N-2 at m/z = 15 and 16; or Ne, Ar and H2O at m/z = 20 ). Here, we present a method extension to the existing miniRUEDI peak-height comparison in order to resolve such overlap interferences: We developed and tested a data processing procedure for accurate deconvolution and compensation of such mass-spectrometric overlap interferences. The method was incorporated into the miniRUEDI open-source software (ruediPy). The method substantially improves the analytical accuracy in situations where mass-spectrometric interferences cannot be avoided. (C) 2020 The Author(s). Published by Elsevier B.V.ISSN:2215-016

Using helium and other noble gases in ocean sediments to characterize active methane seepage off the coast of New Zealand

Recently developed analytical techniques to determine the abundances of noble gases in sediment pore water allow noble-gas concentrations and isotope ratios to be measured easily and routinely in lacustrine sediments. We applied these techniques for the first time to ocean sediments to investigate an active cold methane seepage system located in the South Pacific off the coast of the North Island of New Zealand using 3He/4He ratios determined in the sediment pore water. The results show that more 3He-rich fluids are released in the vicinity of the Pacific–Australian subduction zone than at the forearc stations located closer to the New Zealand coast. However, the He isotope signature in the sediment column indicates that only a minor part of the He emanating from deeper strata originates from a depleted mantle source. Hence, most He in the pore water is produced locally by the radioactive decay of U and Th in the sediment minerals or in the underlying crustal rocks. Such an occurrence of isotopically heavy crustal He also suggests that the source of the largest fraction of methane is a near-surface geochemical reservoir. This finding is in line with a previous δ13C study in the water column which concluded that the emanating methane is most likely of biological origin and is formed in the upper few meters of the sediment column. Moreover, the prevalence of isotopically heavy He agrees well with the outcome of other previous studies on island arc systems which indicate that the forearc regions are characterized by crustal He emission, whereas the volcanic arc region is characterized by the presence of mantle He associated with rising magma