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We collected bottom water samples and unconsolidated sediment samples for noble-gas analysis in the water column as well as in the pore water phase along the Nankai Trough to assess the sources and...

Highlights: • Clay dehydration water expelled from buried sediments drives mud volcanism. • Rise of fluids mediated by crustal-scale strike-slip faults cross-cutting wedge. • On active accretionary wedge, petroleum accumulations were dismantled in Neogene. • 4He enrichment and δ13C-CH4 ~−50‰ in fluids reflect an open hydrocarbon system. • Petroleum pools remain on shallow margin. Microbial gas vented out of active wedge. Abstract: A geochemical study of the composition of hydrocarbon gases and helium isotopes (3He/4He) in fluids from Mud Volcanoes (MVs) located on and out of the active accretionary wedge of the Gulf of Cadiz (GoC) provides information on fluid sources and migrations in deeply buried sediments. The GoC is a tectonically active segment of the Africa-Iberia plate boundary occluded beneath the thick sediments of an accretionary wedge dissected by crustal-scale strike-slip faults. Initially built during the Miocene Gibraltar Arc subduction, the wedge has since developed toward the W-NW in an oblique convergent setting. Interstitial water expelled from clays undergoing diagenesis in buried sediments drives mud volcanism on the wedge, with MVs located along strike-slip faults mediating fluid ascent. The large excess of radiogenic helium (4He) in all GoC fluids agrees with a clay mineral dehydration source of water. Hydrocarbon gases from all deepwater MVs bear methane having similar stable carbon isotope compositions of ~−50‰VPDB whether fluids are highly enriched in methane relative to heavier homologues (C2+) or not (Methane / (Ethane + Propane) ~10 to 10,000). We suggest that methane with −50‰VPDB was largely diffused out of early generating source rocks, and became dissolved in the water expelled by the buried sediments. Consistently, low 3He/4He ratios suggest an open hydrocarbon system: Petroleum accumulations and 3He dissolved in the original sedimentary pore water have mostly escaped into the water column during the major Late Neogene compressional events. At present, some MVs vent CH4-rich fluids from dewatering sediments, while other structures located on active thrusts additionally vent C2+-rich gases generated by active Cretaceous source intervals. By contrast, evaporitic seals preserved petroleum accumulations on the shallow Moroccan Margin, while the westernmost MVs located out of the accretionary wedge vent microbial gas.

The Nirano mud volcano is located in the western sector of the Modena Apennine margin (Italy). It represents one of the most spectacular phenomena of sedimentary volcanism in the entire Italian territory and is among the largest in Europe. Here numerous aligned gryphon clusters and seeping pools constantly burst gas and mud inside a morphological depression. Besides the obvious surface expressions of these emission spots, until now the type and amount of gas released in the rest of the large Nirano caldera zone remained unknown. An extensive geochemical soil gas survey (O2, N2, CO2, CH4, 222Rn, He, H2, and light hydrocarbons) and exhalation fluxes (CO2 and CH4), was carried out inside the mud volcano field with the aim of identifying soil degassing distribution, and to estimate the micro- and macro-seepage budget for both CO2 and CH4. Soil gas data highlight the presence of two zones characterized by high concentrations and flux values. These enhanced seepage zones are located in the SW and NE sectors of the mud volcano suggesting that the enhanced gas emissions present in the peripheral zones, are controlled by caldera collapse structures. The most significant CO2 flux (up to 91 g m−2 d−1) and 222Rn anomalies are located in the central part of the crater in correspondence of a morphological escarpment. Here we infer the presence of a buried tectonic system of collapsed terraces that facilitate fluids degassing. In contrast, CH4 fluxes show a scattered distribution and low values (mean 221 mg m−2 d−1). Overall the CH4 degassing budget is low (27.09 t km−2 y−1) when compared with other Italian mud volcanoes. This could be related to a relative low emission activity during the period of the geochemical survey and to a more homogeneous dilution of surface distribution of the emission point-s. Chemical and isotopical composition of the gas discharged from the active gryphons is methane-dominated and the thermogenic signature (ranging from −41 to −47‰) suggests a deep reservoir source. This conclusion is supported by noble-gas measurements (He, Ne, Ar, Kr, Xe) conducted in the pore water phase of the emitted mud, indicating a secondary gas exchange occurring at a depth of a few kilometers. The geochemical anomalies found in this study, successfully predicted the occurrence of new degassing phenomena towards the NE sector of the caldera. Indeed recently (i.e. after the survey data acquisition) new manifestations of mud and gas emissions appeared in the north-eastern edge of the caldera.

Highlights • Noble gas analysis of sediment porewater close to hydrothermal vent system. • Origin of porewater is solely Guaymas Basin bottom water. • Transport of hydrothermal fluids is limited to narrow pathways. • Temporal variability of He isotopic composition recorded in porewater. Abstract We present noble gas concentrations determined in pore water of deep-sea sediments close to a recently discovered hydrothermal vent site, consisting of a mound structure and several black smokers, located in the northern Guaymas Basin, Gulf of California. Noble gases were used as tracers to identify the origin of fluids within the sediment pore space and to gain insight into transport dynamics of hydrothermal fluids in this region. Our data suggest that Guaymas Basin bottom water is the only source of pore water in the pelagic sediment body close to the hydrothermal vent field. In particular, there is no evidence of any direct (diffusive or advective) transport of hydrothermal fluids through the deep-sea sediments surrounding the black smoker system. This finding implies that at this black smoker site hydrothermal fluids are transported upwards from the fluid source in very narrow pathways below the smokers. Thus, the fluids are only injected into the ocean directly through the chimneys of the black smokers and no additional emission from the surrounding sediment takes place. Helium isotope data show that during a more active phase of the vent field in the past (supposedly representing the early onset of the black smokers 5–6 kyrs ago), bottom water with a different isotopic signature was incorporated into the sediment column.

Carbon capture and storage (CCS) may play a significant role in reducing greenhouse gas emissions. Noble gases are potential tracers to monitor subsurface CO2 storage sites and verify their containment. Naturally occurring noble gases have been used successfully to refute alleged CO2 leakage in the past. We present results from several sampling campaigns at two Norwegian CO2 capture facilities, the demonstration plant Technology Centre Mongstad (TCM) and the natural gas processing plant with CO2 capture and storage on Melkøya. The gas streams in the capture plants were monitored with a combination of on-site mass spectrometry and subsequently analysed discrete samples. This allows us to define the factors controlling noble gas concentrations in captured CO2, to monitor temporal variation of noble gas concentrations and finally evaluate the potential to use noble gases as inherent environmental tracers for labelling CO2 in storage reservoirs. At both sites, CO2 is captured using amine gas treatment. Noble gas concentrations in the gas streams were observed to decrease by several orders of magnitude during the processing. Isotopic ratios are air-like for CO2 captured after natural gas combustion at TCM and natural gas-like for CO2 captured from natural gas processing on Melkøya. Further, we detected a solubility trend caused by the amine solvent at TCM with higher solubility for heavier noble gases. We find that the relative concentrations of noble gases in the captured CO2 are defined by the gas from which the CO2 is captured and the design of the amine gas treatment process. Both factors were observed to cause temporal variation in the captured CO2. Using mixing and noble gas partitioning calculations we show that the significant depletion in noble gas concentrations, together with degassing of noble gas enriched formation water, means that the injected CO2 will inherit the noble gas signature of the storage formation, even following the injection of significant CO2 volumes. Any CO2 leaked from the storage formation is thus likely to have a crustal noble gas signature, characteristic of the storage site, which can be targeted for monitoring. © 2020 The Author(s), International Journal of Greenhouse Gas Control, 106, ISSN:1750-5836, ISSN:1878-0148

Bentonite is a common backfill material used in Engineered Barrier Systems (EBS) for radioactive waste disposal. Its thermal, hydraulic, and chemical properties have been extensively studied. However, empirical evidence on the interactions with the free gas phase present in its pore space under conditions close to those prevailing in deep geological repositories (i.e., exposure to multiple gas species at temperature and humidity conditions close to those of freshly backfilled EBS) is scarce. This limits the prediction/assessment of the evolution of the gas composition in EBS. Here we present the outcome of laboratory experiments targeting such gas-bentonite interactions using two different types of bentonite (Italian bentonite; Wyoming bentonite) being exposed to atmospheric air in sealed stainless-steel vessels. The results indicate that gas sorption on bentonite can occur at repository-like conditions and is likely to play a significant role in the overall gas dynamics in the backfill of EBS. Drying of both bentonites was shown to partly re-activate their CO2 sorption capacity during a subsequent re-hydration phase. Furthermore, the grinding of Wyoming bentonite resulted in the generation of new pyrite surfaces that significantly enhanced O2 removal from the free gas phase by oxidation. Our initial investigations call for more systematic and extensive series of laboratory experiments in order to quantitatively constrain the different processes involved in gas-bentonite interactions., Applied Geochemistry, 138, ISSN:0883-2927, ISSN:1872-9134

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., MethodsX, 7, ISSN:2215-0161

In this work we report tritium concentrations determined in seawater samples collected offshore of Fukushima in the northwestern Pacific Ocean, immediately after the Fukushima Dai-ichi Nuclear Power Plant (F1-NPP) accident. We found surface seawater to have high concentrations of tritium (3H) and cesium-137 (137Cs) with respect to the concentrations expected for the investigated region. Tritium concentrations were up to six times the pre-accident level. However, these concentrations were found to be relatively low compared to those of 137Cs and 129I. This is most likely because of the very low production ratio of tritium to 137Cs in the F1-NPP, and the inherently high background concentration of tritium in these environmental waters (mainly ascribed to global fallout from nuclear weapons tests conducted during the 1960s). The tritium distribution in surface seawater showed higher concentrations close to the shore and lower concentrations offshore. Higher tritium concentration areas had spread both northward and southward from the F1-NPP along the coast, indicating that large amounts of tritium were carried by coastal currents. A positive correlation was found to exist between 3H and 137Cs concentrations in the seawater. The calculated 3H/137Cs ratio was similar to the production ratio of these isotopes reported for the broken reactors in the F1-NPP, which indicates that both radionuclides might have originated from the F1-NPP. Direct emission of tritium to the ocean from the F1-NPP was estimated to be approximately 0.05 PBq immediately after the accident, which is much smaller than the total inventory in the environment.

Tunnel drilling provides a unique opportunity to sample and study deep groundwaters that are otherwise difficult to access. Understanding deep groundwater flow is of primary importance in assessing the possible impacts of tunnelling on hydrogeological systems. During this study, water was sampled for noble-gas analysis from tunnel inflows in the AlpTransit Ceneri Base Tunnel (Canton Ticino, southern Switzerland), which passes through an area mainly characterized by metamorphic rocks (gneiss). Furthermore, water was sampled from springs located in the same geological environment. Based on the measurement of noble-gas concentrations and isotope ratios, tritium concentrations, the stable isotope composition of hydrogen ( δ 2 H) and oxygen ( δ 18 O), and the concentrations of major ions in the water, a conceptual hydrogeological model was established for this case study that allowed the most probable origin of the groundwaters sampled at different locations to be determined. The measured abundances of 3 He, 4 He, and 20 Ne allow the geochemical characterization of old groundwaters strongly enriched in terrigenic helium of crustal origin and the identification of mixing with water that circulates preferentially through cataclastic structures. Noble-gas concentrations and isotope ratios as well as tritium are useful proxies for the characterization of faults that may be critical for tunnel drilling because of their active hydrogeological role and their influence on the mechanics of the rocks.

The commonly observed heterogeneous and erratic emissions of methane from lacustrine sediments are difficult to quantify. The combined analysis of methane and noble gas concentrations in the pore water opens a novel route to analyse and quantify such emissions and was used in this work to explore the gas dynamics in the sediments of two lakes with different hydro-geochemical settings. In the hypertrophic Lake Rot the microbial activity in the sediment pore water results in a virtually continuous gas bubble formation and emission. By determining noble gas concentration gradients and the maximum methane supersaturation, the minimum methane emission from the sediment was quantified. The gas evolution in the pore water of the sediments of artificial Lake Lungern is strongly affected by annual lake level variations of up to 20 m. Noble gas concentrations allow past gas dynamics at different lake levels to be traced back. At “hot spots” of bubble emission microbial methane from deeper sediments was released in response to the decreasing hydrostatic pressure, i.e. man-induced lake level drop. Within zones of the sediment that seasonally fall dry, the air supply to the sediment was inferred from the noble gas excess observed in the pore water, which is due to dissolving air bubbles which entered the sediment during the dry period. These case studies demonstrate the potential of the combined analysis of noble gas and CH4 concentrations for exploring the gas dynamics in lacustrine sediments. The feasibility of this method under different conditions calls for an application in other aquatic environments, such as wetlands and oceans.

This report describes spatial and temporal variations of helium, carbon, and nitrogen isotopes measured in CO2-rich bubbling gases from natural springs associated with Mt. Ontake (central Honshu, Japan) from November 1981 through September 2015. During the entire period, the 3He/4He ratio decreased concomitantly with increasing distance of the sampling site from the central cone. In contrast, the CO2/3He ratios and the δ13C values of CO2 increased with distance. These spatial trends became more pronounced after the September 27, 2014 Ontake eruption, suggesting reactivation of the volcano plumbing system with enhanced emission of magmatic volatiles, although the spatial trend of nitrogen isotopes disappeared. The 3He/4He ratios of the most proximal site to the central cone remained constant until 2000, apparently increasing from June 2003 through October 2014. They became constant soon after the eruption until September 2015. The ten-year 3He enhancement might have been a precursor of the 2014 Ontake eruption. In contrast, δ13C values of CO2 at the same site remained constant through this period. The lack of δ13C anomaly might be attributable to 1) negligibly small amounts of magmatic CO2 introduced into the source of hot springs compared to ambient CO2 in the air-saturated aquifer, or 2) close resemblance of the carbon signature of magmatic CO2 to that of ambient CO2. Therefore its addition cannot change the overall δ13C value of the sampled hot springs. At distal sampling sites, no measurable change of helium, carbon, or nitrogen isotopes was observed in relation to the 2014 Ontake eruption, suggesting that the effect of this eruption on the Ontake hydrothermal system was geographically localized.

A sediment core has been sampled at a spatial resolution of about two centimeters for noble-gas analysis of the sediment pore water. This high-resolution sampling allowed an excess of dissolved atmospheric gases to be identified in the pore water at a sediment depth of 40 cm. Sedimentological evidence indicates that the observed gas excess is related to a past mass movement that trapped and conserved the gas surplus for more than a century. The measured noble-gas concentrations support the hypothesis that, depending on the sediment texture or particular structures in the sediment, the transport of noble gases in the pore space can be significantly attenuated compared to molecular diffusion in bulk water. The atmospheric noble-gas abundance in the pore water allows the deposition of sediments originally located near the air/water interface to be identified. Although the nature of the physical processes that limit the fluid transport in the pore space could not be fully assessed, our observations indicate that noble-gas concentration profiles can be archived for unexpectedly long times in specific sedimentary sequences. This finding represents a significant step forward in the interpretation of noble-gas concentrations in sediment pore waters in terms of past environmental conditions.

In closed-basin lakes, sediment porewater salinity can potentially be used as a conservative tracer to reconstruct past fluctuations in lake level. However, until now, porewater salinity profiles did not allow quantitative estimates of past lake-level changes because, in contrast to the oceans, significant salinity changes (e.g., local concentration minima and maxima) had never been observed in lacustrine sediments. Here we show that the salinity measured in the sediment pore water of Lake Van (Turkey) allows straightforward reconstruction of two major transgressions and a major regression that occurred during the last 250 ka. We observed strong changes in the vertical salinity profiles of the pore water of the uppermost 100 m of the sediments in Lake Van. As the salinity balance of Lake Van is almost at steady-state, these salinity changes indicate major lake-level changes in the past. In line with previous studies on lake terraces and with seismic and sedimentological surveys, we identify two major transgressions of up to +105 m with respect to the current lake level at about 135 ka BP and 248 ka BP starting at the onset of the two previous interglacials (MIS5e and MIS7), and a major regression of about −200 m at about 30 ka BP during the last ice age.

Methane emission from the geosphere is generally characterized by a radiocarbon-free signature and might preserve information on the deep carbon cycle on Earth. Here we report a clear relationship between the origin of methane-rich natural gases and the geodynamic setting of the West Pacific convergent plate boundary. Natural gases in the frontal arc basin (South Kanto gas fields, Northeast Japan) show a typical microbial signature with light carbon isotopes, high CH4/C2H6 and CH4/3He ratios. In the Akita-Niigata region – which corresponds to the slope stretching from the volcanic-arc to the back-arc –a thermogenic signature characterize the gases, with prevalence of heavy carbon isotopes, low CH4/C2H6 and CH4/3He ratios. Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH4/C2H6 ratios and low CH4/3He ratios. On the other hand, those from the Tokara Islands situated on the volcanic front of Southwest Japan show the heaviest carbon isotopes, middle CH4/C2H6 ratios and the lowest CH4/3He ratios. The observed geochemical signatures of natural gases are clearly explained by a mixing of microbial, thermogenic and abiotic methane. An increasing contribution of abiotic methane towards more tectonically active regions of the plate boundary is suggested., 論文

The structural evolution of Lake Van Basin, eastern Turkey, was reconstructed based on seismic reflection profiles through the sedimentary fill as well as from newly acquired multibeam echosounder data. The major sub-basins (Tatvan Basin and Northern Basin) of Lake Van, bound by NE-trending faults with normal components, formed during the past similar to 600 ka probably due to extensional tectonics resulting from lithospheric thinning and mantle upwelling related to the westward escape of Anatolia. Rapid extension and subsidence during early lake formation led to the opening of the two sub-basins. Two major, still active volcanoes (Nemrut and Suphan) grew close to the lake basins approximately synchronously, their explosive deposits making up > 20 % of the drilled upper 220 m of the ca. 550-m-thick sedimentary fill. During basin development, extension and subsidence alternated with compressional periods, particularly between similar to 340 and 290 ka and sometime before similar to 14 ka, when normal fault movements reversed and gentle anticlines formed as a result of inversion. The similar to 14 ka event was accompanied by widespread uplift and erosion along the northeastern margin of the lake, and substantial erosion took place on the crests of the folds. A series of closely spaced eruptions of Suphan volcano occurred synchronously suggesting a causal relationship. Compression is still prevalent inside and around Lake Van as evidenced by recent faults offsetting the lake floor and by recent devastating earthquakes along their onshore continuations. New, high-resolution bathymetry data from Lake Van reveal the morphology of the Northern Ridge and provide strong evidence for ongoing transpression on a dextral strike-slip fault as documented by the occurrence of several pop-up structures along the ridge.

Fossil long-chain alkenones have been used for several decades to reconstruct past ocean surface water temperatures and gained recent interest as a paleotemperature proxy for continental lake settings. However, factors besides temperature can affect alkenone distributions in haptophyte algae, and alkenone compositions can differ between haptophyte species. Alkenone-biosynthesizing haptophyte algae are genetically much more diverse in lakes than in the marine realm, and species-level variations in alkenone compositions could have implications for alkenone paleothermometry. Here, we performed a paired analysis of alkenone distributions and haptophyte species compositions using ancient DNA in up to 270 ka-old sediments of Lake Van in Turkey to reveal a possible species-effect on fossil alkenone distributions and paleotemperature estimates. The same predominant haptophyte in Lake Van today prevailed also since the last similar to 100 ka. However, a calibration of alkenone paleotemperature especially in the oldest analyzed intervals is complicated due to a more complex haptophyte species composition predominated by a haptophyte (LVHap_6), which is phylogenetically different from sequences recovered from currently existing lakes including Lake Van and from haptophyte species existing in culture. The predominance of LVHap_6 coincided with the presence of alkenone MeC38:3 and relatively high MeC37:3/4 (2.4) and Me08:4/5 ratios (3.0). Uk37 index values in the sediment core over the last 270 ka reflect relative changes in past temperature and are additionally linked to haptophyte species composition. A sustained period of high salinity, as indicated by pore-water salinity measurements, could potentially have triggered the succession of haptophytes as sources of alkenones in Lake Van. 0 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

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.

Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. The 3He flux and methane flux in the investigated area are estimated to be (0.99–2.6) × 104 atoms/cm2/sec and 6–60 t/yr, respectively. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.

There is conclusive evidence that the methods most commonly used to sample methane (CH4) dissolved in the pore water of lake sediments produce results that are likely to be affected by gas loss or gas exchange with the atmosphere. To determine the in situ amount of CH4 per unit mass of pore water in sediments, we developed and validated a new method that combines techniques developed for noble-gas analysis in pore waters with a standard headspace technique to quantify the CH4 present in the pore space in dissolved and gaseous form. The method was tested at two sites: Lake Lungern, where CH4 concentrations were close to saturation; and Lake Rotsee, where CH4 concentrations are known to exceed saturation and where CH4 bubble formation and gas ebullition are commonly observed. We demonstrate that the new method, in contrast to the available methods, more reliably captures the total amount of CH4 per unit mass of pore water consisting of both dissolved and free CH4 (i.e., gas bubbles) in the pore space of the sediment.

International Continental Scientific Drilling Program (ICDP) drilled a complete succession of the lacustrine sediment sequence deposited during the last ~500,000 years in Lake Van, Eastern Anatolia (Turkey). Based on a detailed seismic site survey, two sites at a water depth of up to 360 m were drilled in summer 2010, and cores were retrieved from sub-lake-floor depths of 140 m (Northern Basin) and 220 m (Ahlat Ridge). To obtain a complete sedimentary section, the two sites were multiple-cored in order to investigate the paleoclimate history of a sensitive semi-arid region between theBlack, Caspian, and Mediterranean seas. Further scientific goals of the PALEOVAN project are the reconstruction of earthquake activity, as well as the temporal, spatial, and compositional evolution of volcanism as reflected in the deposition of tephra layers. The sediments host organic matter from different sources and hence composition, which will be unravelled using biomarkers. Pathways for migration of continental and mantle-derived noble gases will be analyzed in pore waters. Preliminary 40Ar/39Ar single crystal dating of tephra layers and pollen analyses suggest that the AhlatRidge record encompasses more than half a million years of paleoclimate and volcanic/geodynamic history, providing the longest continental record in the entire Near East to date.

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 3 He / 4 He 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 ( 3 He / 4 He ∼ 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 ( 3 He / 4 He ∼ 10 - 5 ) .

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 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-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.

Gas exchange between seepage water and soil air within the unsaturated and quasi-saturated zones is fundamentally different from gas exchange between water and gas across a free boundary layer, e.g., in lakes or rivers. In addition to the atmospheric equilibrium fraction, most groundwater samples contain an excess of dissolved atmospheric gases which is called “excess air”. Excess air in groundwater is not only of crucial importance for the interpretation of gaseous environmental tracer data, but also for other aspects of groundwater hydrology, e.g., for oxygen availability in bio-remediation and in connection with changes in transport dynamics caused by the presence of entrapped air bubbles. Whereas atmospheric solubility equilibrium is controlled mainly by local soil temperature, the excess air component is characterized by the (hydrostatic) pressure acting on entrapped air bubbles within the quasi-saturated zone. Here we present the results of preliminary field experiments in which we investigated gas exchange and excess air formation in natural porous media. The experimental data suggest that the formation of excess air depends significantly on soil properties and on infiltration mechanisms. Excess air was produced by the partial dissolution of entrapped air bubbles during a sprinkling experiment in fine-grained sediments, whereas similar experiments conducted in coarse sand and gravel did not lead to the formation of excess air in the infiltrating water. Furthermore, the experiments revealed that the noble gas temperatures determined from noble gases dissolved in seepage water at different depths are identical to the corresponding in situ soil temperatures. This finding is important for all applications of noble gases as a paleotemperature indicator in groundwater since these applications are always based on the assumption that the noble gas temperature is identical to the (past) soil temperature.

Although the naturally occurring atmospheric noble gases He, Ne, Ar, Kr, and Xe possess great potential as tracers for studying gas exchange in living beings, no direct analytical technique exists for simultaneously determining the absolute concentrations of these noble gases in body fluids in vivo. In this study, using human blood as an example, the absolute concentrations of all stable atmospheric noble gases were measured simultaneously by combining and adapting two analytical methods recently developed for geochemical research purposes. The partition coefficients determined between blood and air, and between blood plasma and red blood cells, agree with values from the literature. While the noble-gas concentrations in the plasma agree rather well with the expected solubility equilibrium concentrations for air-saturated water, the red blood cells are characterized by a distinct supersaturation pattern, in which the gas excess increases in proportion to the atomic mass of the noble-gas species, indicating adsorption on to the red blood cells. This study shows that the absolute concentrations of noble gases in body fluids can be easily measured using geochemical techniques that rely only on standard materials and equipment, and for which the underlying concepts are already well established in the field of noble-gas geochemistry. ISSN:1932-6203

Sediment samples acquired in 2010 from the long cores of the International Continental Scientific Drilling Program (ICDP) PaleoVan drilling project on Lake Van for noble-gas analysis in the pore water allow determination of the local terrestrial He-gradient as a function of depth within a sediment column of more than 200 m. These measurements yield first insights into the physical transport mechanisms of terrigenic He through the uppermost part of unconsolidated lacustrine sediments overlying the continental crust. In line with our previous work on the spatial distribution of the terrigenic He release into Lake Van, we identify a high He concentration gradient in the uppermost 10 m of the sediment column. The He concentration gradient decreases below this depth down to approx. 160 m following in general the expectations of the modelling of radiogenic He production and transport in a sediment column with homogeneous fluid transport properties. Overall the in-situ radiogenic He production due to the decay of U and Th in the mineral phases of the sediments accounts for about 80% of the He accumulation. At approx. 190 m we observe a very high He concentration immediately below a large lithological unit characterised by strong deformations. We speculate that this local enrichment is the result of the lower effective diffusivities in the pore space that relate to the abrupt depositional history of this deformed unit. This particular lithological unit seems to act as a barrier that limits the transport of solutes in the pore space and hence might “trap” information on the past geochemical conditions in the pore water of Lake Van. The dissolved concentrations of atmospheric noble gases in the pore waters of the ICDP PaleoVan cores are used to geochemically reconstruct salinity on the time scale of 0–55 ka BP. Higher salinities in the pore water at a depth of about 20 m suggest a significantly lower lake level of Lake Van in the past.

In well-studied aquatic systems such as surface waters and groundwater, noble gases are used extensively as natural tracers to reconstruct palaeoenvironmental conditions, to study transport and mixing, and to identify the geochemical origin of geogenic fluids. It has been suggested that less well-studied aquatic systems such as the porewaters of lacustrine and oceanic sediments and the fluid inclusions present in stalagmites might also be suitable as noble gas archives for environmental studies, but until recently the lack of adequate experimental techniques had hindered the development of noble gas geochemistry in these systems. This chapter reviews recent technical advances in this field and describes the scientific applications that these advances have made possible. The porewaters of lacustrine and oceanic sediments are now well established as noble gas archives in studies of temperature, salinity and mixing conditions that prevailed in the overlying water body in the past, as well as in studies of the transport and origin of solutes and pore fluids in the sediment. The geochemistry of noble gases in stalagmite fluid inclusions is still in the early stages of development. However, the results available to date suggest that stalagmite fluid inclusions have great potential as a noble gas archive in reconstructing palaeoclimatic conditions near caves with suitable stalagmites.

Analysis of multi-channel seismic reflection and chirp data from Lake Van (eastern Turkey) reveals various shallow gas indicators including seismic chimneys, enhanced reflections, bright spots, mud volcanoes, pockmarks, and acoustic blanking. The enhanced reflections, suggesting the presence of free gas, are most dominant and observed at more than 200 locations. They are characterized by very-high amplitude reflections and occur in both deep and shallow sedimentary sections. Some enhanced reflections are accompanied by very subtle seafloor expressions such as mounds, which may suggest active venting activity. Seismic chimneys or columnar zones of amplitude blanking have been observed in much of the surveyed area. Seismic chimneys in the study area cannot be associated with any known faults that would act as migration pathways for deep fluids. This suggests that the observed structures in Lake Van sediments allow the preferential emission of gases which might be for a large share of biogenic origin. The acoustic blanking, characterized by transparent or chaotic seismic facies, is seen in the eastern part of the lake. The lakeward edge of the acoustic blanking largely coincides with the 100 m water depth contour, indicating that (past) changes of the hydrostatic pressure may be responsible for the distribution of these anomalies. Mound-like features, interpreted as mud volcanoes, occur in a few locations. The presence of these features may suggest active gas emission. Very strong amplitude anomalies or bright spots with negative polarity, indicating gas-charged zones, are also seen in a number of locations. Pockmarks are observed only in the northeastern part of the study area. The scarce occurrence of pockmarks in the study area might be ascribed to a higher permeability of the lake sediments or to the absence of the substrate/reservoir providing the critical mass of gases necessary to produce such features. Turbidites, tephra layers, and deltaic deposits have the potential to provide ideal conditions to allow the sediments to act as a gas reservoir. (c) 2013 Elsevier Ltd. All rights reserved.

[1] Eutrophic Lake Hallwil (Switzerland) is equipped with an artificial aeration system to prevent anoxic conditions developing in the deep water during stratification in summer. The aeration system consists of diffusers releasing oxygen-rich gas containing noble gases into the deep water at the bottom of the lake. The deep water is strongly supersaturated with He, Ne, and Ar, while Kr and Xe are present at concentrations corresponding to their respective atmospheric equilibria. The observed noble gas excesses are related to the operation of the aeration system and to the composition of the injected aeration gas. We show how noble gas data were used successfully to estimate the fraction of the injected aeration gas that effectively remains dissolved in the water body. In particular, as the physical properties of Ar (e.g., atomic mass, solubility, diffusion coefficient) are similar to those of oxygen, the measured noble gas concentrations allow the efficiency of the aeration system to be quantified.

[1] Changes in the hydrological regime of the saline closed basin Lake Van, a large, deep lake in eastern Turkey, resulted in a lake level increase by about 2 m between 1988 and 1995, followed by a 1.5 m decrease until 2003 and a relatively constant lake level thereafter. Based on measurements of transient tracers (sulfur hexafluoride, CFC-12, 3H, 3He, 4He, Ne), dissolved oxygen, light transmission, conductivity-temperature-depth profiles, and thermistor data, we investigate the implications associated with lake level fluctuations for deep-water renewal and oxygenation. Our data suggest that deep-water renewal was significantly reduced in Lake Van between 1990 and 2005. This change in mixing conditions resulted in the formation of a more than 100 m thick anoxic deep-water body below 325 m depth. Apparently, the freshwater inflows responsible for the lake level rise between 1988 and 1995 decreased the salinity of the surface water sufficiently that the generation of density plumes during winter cooling was substantially reduced compared to that in the years before the lake level rise. Significant renewal and oxygenation of the deep water did not occur until at least 2005, although by 2003 the lake level was back to almost the same level as in 1988. This study suggests that short-term changes in the hydrological regime, resulting in lake level changes of a couple of meters, can lead to significant and long-lasting changes in deep-water renewal and oxic conditions in deep saline lakes.

Sediments of Lake Van, Turkey, preserve one of the most complete records of continental climate change in the Near East since the Middle Pleistocene. We used seismic reflection profiles to infer past changes in lake level and discuss potential causes related to changes in climate, volcanism, and regional tectonics since the formation of the lake ca. 600 ka ago. Lake Van's water level ranged by as much as 600 m during the past 600 ka. Five major lowstands occurred, at 600, 365-340, 290-230, 150-130 and 30-14 ka. During Stage A, between about 600 and 230 ka, lake level changed dramatically, by hundreds of meters, but phases of low and high stands were separated by long time intervals. Changes in the lake level were more frequent during the past 230 ka, but less dramatic, on the order of a few tens of meters. We identified period B1 as a time of stepwise transgressions between 230 and 150 ka, followed by a short regression between ca. 150 and 130 ka. Lake level rose stepwise during period B2, until 30 ka. During the past 30 ka, a regression and a final transgression occurred, each lasting about 15 ka. The major lowstand periods in Lake Van occurred during glacial periods, suggesting climatic control on water level changes (i.e. greatly reduced precipitation led to lower lake levels). Although climate forcing was the dominant cause for dramatic water level changes in Lake Van, volcanic and tectonic forcing factors may have contributed as well. For instance, the number of distinct tephra layers, some several meters thick, increases dramatically in the uppermost 100 m of the sediment record (i.e. the past 230 ka), an interval that coincides largely with low-magnitude lake level fluctuations. Tectonic activity, highlighted by extensional and/or compressional faults across the basin margins, probably also affected the lake level of Lake Van in the past.