Noble gas enrichments in porewater of estuarine sediments and their effect on the estimation of net denitrification rates

The concentration and the isotopic ratios of noble gases He, Ne, Ar, Kr and Xe were measured in porewater trapped in shallow sediments of the estuary of the St-Lawrence River, Québec, Canada. The gases are atmospheric in origin but most samples have gas concentrations 1.7-28 times higher than those expected in solution in water at equilibrium with the atmosphere. Elemental fractionation of heavier noble gases Kr and Xe compared to Ar strongly suggests that noble gases were adsorbed on sediments or organic matter and then desorbed into porewaters due to depressurization, as collected samples were brought to the surface. Atmospheric Ar in porewater is used as a reference to measure the N₂-fluxes at the water-sediment interface. Ignoring the Ar enrichments observed in porewater could lead to a severe underestimation of the denitrification rate in oceans and estuaries.     

Noble gas study of HIMU and EM ocean island basalts in the Polynesian region

Noble gas isotopes of HIMU and EM ocean island basalts from the Cook-Austral and Society Islands were investigated to constrain their origins. Separated olivine and clinopyroxene (cpx) phenocrysts were used for noble gas analyses. Since samples are relatively old, obtained from the oceanic area and showing chemical zoning in cpx phenocrysts, several tests on sample preparation and gas extraction methods were performed. First, by comparing heating and crushing methods, it has been confirmed that the crushing method is suitable to obtain inherent magmatic noble gases without radiogenic and cosmogenic components which were yielded after eruption, especially for He and Ne analyses. Second, noble gas compositions in the core and the rim of cpx phenocrysts were measured to evaluate the zoning effect on noble gases. The result has been that noble gas concentrations and He and Ne isotope ratios are different between them. The enrichment of noble gases in the rim compared to the core is probably due to fractional crystallization. Difference of He and Ne isotope ratios is explained by cosmogenic effect, and isotope ratios of the trapped component seem to be similar between the rim and the core. Third, leaching test reveals no systematic differences in noble gas compositions between leached and unleached samples.³He/⁴He ratios of HIMU samples in the Cook-Austral Islands are uniform irrespective of phenocryst type (olivine and cpx) and age of samples (10–18 Ma), and lower (average 6.8 R_A) than those of the Pacific MORB. On the other hand, ³He/⁴He of EM samples in the Cook-Austral Islands are similar to MORB values. EM samples in the Society Islands show rather higher ³He/⁴He than MORB. Ne, Kr and Xe isotope ratios are almost atmospheric within analytical uncertainties. ⁴⁰Ar/³⁶Ar are not so high as those of MORB. Anomalous noble gas abundance pattern such as He and Ne depletion and Kr and Xe enrichment relative to atmospheric abundances was observed. Furthermore, Ne/Ar and Kr/Ar show correlation with some trace elemental ratios like La/Yb.Lower ³He/⁴He of HIMU than MORB values requires relatively high time-integrated (U + Th)/³He for the HIMU source, which suggests that the HIMU source was produced from recycled materials which had been once located near the Earth’s surface. Moreover, extreme noble gas abundance pattern and strong correlation of Ne/Ar and Kr/Ar with La/Yb indicate that the HIMU endmember is highly depleted in light noble gases and enriched in heavy noble gases. Such feature is not common to mantle materials and is rather similar to the noble gas abundance patterns of the old oceanic crust and sediment, which supports the model that the HIMU source originates from subducted oceanic crust and/or sediment.If the HIMU source corresponds to the oceanic crust which subducted at 1–2 Ga as suggested by Pb isotope studies, however, the characteristic ³He/⁴He of HIMU (6.8 R_A) would be too high because radiogenic ⁴He produced by U and Th decay should dramatically decrease ³He/⁴He. To overcome this problem, the He open system model is introduced which includes the effects of ⁴He production and diffusion between the HIMU source material and the surrounding mantle. This model favors that the HIMU source resides in the upper mantle, rather than in the lower mantle. Furthermore, this model predicts the thickness of the HIMU source to be in the order of 1 km.In contrast to low and uniform ³He/⁴He character of HIMU, ³He/⁴He of EM are rather variable. Entrainment of upper mantle material and/or a less-degassed component are required to explain the observed ³He/⁴He of EM in the Polynesian area. Participation of the less-degassed component would be related to the “superplume” below the Polynesian region.     

Geochemical and anthropogenic enrichments of Mo in sediments from perennially oxic and seasonally anoxic lakes in Eastern Canada

We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, ²¹⁰Pb and ¹³⁷Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition.The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment-water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1-2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment-water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm³ mol⁻¹ s⁻¹.Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment-water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3-16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo.     

Sequestration mechanisms and anthropogenic inputs of rhenium in sediments from Eastern Canada lakes

The concentrations of Re, as well as those of several other geochemical variables, were measured in dated sediment cores and in porewater samples from four lacustrine basins in Eastern Canada: one, perennially oxic, located 40 km from Québec City and three, seasonally anoxic, located within 25 km of non-ferrous metal smelters. The drainage basins of these lakes are uninhabited and have not been affected by human activity or wildfires. All of the depth profiles of dissolved Re indicate: higher Re concentrations in the water overlying the sediment than in the porewater; diffusion of Re across the sediment-water interface; a progressive decrease in porewater Re concentrations to reach minimum values of ∼0.5 pM within a 10-cm sediment depth interval. Modeling of these Re porewater profiles with a one-dimensional transport-reaction equation indicates that Re is removed from porewater within this depth interval. Based on thermodynamic predictions of Re speciation and of saturation states and on comparison of these predictions with sulfide porewater profiles, we infer that Re is removed from porewater by precipitation of rheniite (ReS_2(s)). The rate constant for the formation of ReS_2(s) in sediments is estimated from the modeling exercise to be 0.51 ± 0.64 × 10⁻²¹ mol cm⁻³ s⁻¹. Accumulation of sedimentary Re shows a strong authigenic component, as in anoxic marine sediments. Sharp increases in solid-phase Re during the last century are attributed to atmospheric deposition of anthropogenic Re deriving from coal burning and nearby smelter emissions.     

Noble-gas evidence for geothermal activity in a karstic terrain: Rocky Mountains,Canada.

Geothermally heated fluids are identified as a component in warm (up to 54°C) springs in the southern Canadian Rocky Mountains. High concentrations of radiogenic He (10^?3 ccSTP/cc gas) and atmospheric Ne, Ar, Kr and Xe in the gases that vigorously bubble at the Fairmont Hot Spring, assign the latter to be a “drowned” fumarole, fed by geothermal steam. Up to 75% depletions in the atmospheric noble gases in several warm springs indicate contributions of residual geothermal water. On the other hand, in a few cases noble gases were found in excess (W) over the expected concentrations in airequilibrated recharge water. The observed “reversed” pattern of w_nc >W_Ar >W_kr >W_xe is interpreted as excess air, incorporated during recharge through karstic conduits. The mixing ratios of geothermal and karstic waters can be deduced.     

Controls on uranium distribution in lake sediments

Uranium geochemistry has been investigated in three acid lakes located on the Canadian Shield and one circumneutral lake in the Appalachian Region of Eastern Canada. In all Shield lakes, dissolved U concentrations were higher in the porewater than in the overlying water. In one of them, whose hypolimnion is perennially oxic, U released to porewater at depths of Fe remobilization was removed from the porewater at depths of Fe oxyhydroxides precipitation; these similarities in the U and Fe profiles indicate that part of the U becomes associated to Fe oxyhydroxides. The dissolved U and Fe profiles in the other two Shield lakes, whose hypolimnions were anoxic when sampled, did not show any significant recycling of these elements in the vicinity of the sediment-water interface and both elements diffused from the sediment to the overlying water. In contrast, in the Appalachian Lake, dissolved U concentrations were higher in the overlying water than in porewater, strongly decreased at the vicinity of the sediment-water interface and then remained relatively constant with sediment depth. Diagenetic modeling of the porewater U profiles, assuming steady-state, reveals that authigenic U always represented ?3% of the total U concentration in the sediments of all lakes. This observation indicates that diagenetic reactions involving U are not quantitatively important and that most of the U was delivered to the sediments at our study sites as particulate U and not through diffusion across the sediment-water interface, as is seen in continental margin sediments. Comparison of the U:C_org and U:Fe molar ratios in diagenetic material collected across the sediment-water interface with Teflon sheets and in surface sediments (0-0.5 cm) of the lake having a perennially oxic hypolimnion suggest that solid phase U was mainly bound to organic matter originating from the watershed; a strong statistical correlation between sediment non-lithogenic U and C_org in the Appalachian Lake supports this contention. Thermodynamic calculations of saturation states suggest that dissolved U was not removed from porewater through precipitation of UO_2(s), U₃O_7(s) and U₃O_8(s) as previously proposed in the literature.     

Intercomparison of tritium and noble gases analyses, 3H/3He ages and derived parameters excess air and recharge temperature

Groundwater age dating with the tritium–helium (³H/³He) method has become a powerful tool for hydrogeologists. The uncertainty of the apparent ³H/³He age depends on the analytical precision of the ³H measurement and the uncertainty of the tritiogenic ³He component. The goal of this study, as part of the groundwater age-dating interlaboratory comparison exercise, was to quantify the analytical uncertainty of the ³H and noble gas measurements and to assess whether they meet the requirements for ³H/³He dating and noble gas paleotemperature reconstruction.Samples for the groundwater dating intercomparison exercise were collected on 1 February, 2012, from three previously studied wells in the Paris Basin (France). Fourteen laboratories participated in the intercomparison for tritium analyses and ten laboratories participated in the noble gas intercomparison. Not all laboratories analyzed samples from every borehole.The reproducibility of the tritium measurements was 13.5%. The reproducibility of the ³He/⁴He ratio and ⁴He, Ne, Ar, Kr and Xe concentrations was 1.4%, 1.8%, 1.5%, 2.2%, 2.9%, and 2.4% respectively.The uncertainty of the tritium and noble gas measurements results in a typical ³H/³He age precision of better than 2.5 years in this case. However, the measurement uncertainties for the noble gas concentrations are insufficient to distinguish the appropriate excess air model if the measured helium concentration is not included. While the analytical uncertainty introduces an unavoidable source of uncertainty in the ³H/³He apparent age estimate, other sources of uncertainty are often much greater and less well defined than the analytical uncertainty.     

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/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). 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).     

Noble gases in formation fluids from deep sedimentary basins: a review

Atmospheric Ne, Ar, Kr and Xe are observed in fluids occurring in deep basins. Modifications of their abundance patterns reveal modes of recharge and brine formation, phase separations during boiling, and association of natural gas with water or oil. Radiogenic ⁴He and ⁴⁰Ar serve as age indicators of entrapped fluids, effective over a significant portion of the geological time scale. Simultaneous application of δ¹³C, atmospheric noble gases and radiogenic ⁴He and ⁴⁰Ar, is recommended to identify: (a) recent bacterial natural gas formation, not accompanied by oil, and (b) natural gas formed along with oil in a mature source rock.     

Noble gases as proxies of mean ocean temperature: sensitivity studies using a climate model of reduced complexity

Past global mean ocean temperature may be reconstructed from measurements of atmospheric noble gas concentrations in ice core bubbles. Assuming conservation of noble gases in the atmosphere-ocean system, the total concentration within the ocean mostly depends on solubility which itself is temperature dependent. Therefore, the colder the ocean, the more gas can be dissolved and the less remains in the atmosphere. Here, the characteristics of this novel paleoclimatic proxy are explored by implementing krypton, xenon, argon, and N₂ into a reduced-complexity climate model. The relationship between noble gas concentrations and global mean ocean temperature is investigated and their sensitivities to changes in ocean volume, ocean salinity, sea-level pressure and geothermal heat flux are quantified. We conclude that atmospheric noble gas concentrations are suitable proxies of global mean ocean temperature. Changes in ocean volume need to be considered when reconstructing ocean temperatures from noble gases. Calibration curves are provided to translate ice-core measurements of krypton, xenon, and argon into a global mean ocean temperature change. Simulated noble gas-to-nitrogen ratios for the last glacial maximum are δKr_atm = ?1.10‰, δXe_atm = ?3.25‰, and δAr_atm = ?0.29‰. The uncertainty of the krypton calibration curve due to uncertainties of the ocean saturation concentrations is estimated to be ±0.3 °C. An additional ±0.3 °C uncertainty must be added for the last deglaciation and up to ±0.4 °C for earlier transitions due to age-scale uncertainties in the sea-level reconstructions. Finally, the fingerprint of idealized Dansgaard-Oeschger events in the atmospheric krypton-to-nitrogen ratio is presented. A δKr_atm change of up to 0.34‰ is simulated for a 2 kyr Dansgaard-Oeschger event, and a change of up to 0.48‰ is simulated for a 4 kyr event.     

Modern carbon burial in Lake Qinghai,China

The quantification of carbon burial in lake sediments, and carbon fluxes derived from different origins are crucial to understand modern lacustrine carbon budgets, and to assess the role of lakes in the global carbon cycle. In this study, we estimated carbon burial in the sediment of Lake Qinghai, the largest inland lake in China, and the carbon fluxes derived from different origins. We find that: (1) The organic carbon burial rate in lake sediment is approximately 7.23 g m⁻² a⁻¹, which is comparable to rates documented in many large lakes worldwide. We determined that the flux of riverine particulate organic carbon (POC) is approximately 10 times higher than that of dissolved organic carbon (DOC). Organic matter in lake sediments is primarily derived from POC in lake water, of which approximately 80% is of terrestrial origin. (2) The inorganic carbon burial rate in lake sediment is slightly higher than that of organic carbon. The flux of riverine dissolved inorganic carbon (DIC) is approximately 20 times that of DOC, and more than 70% of the riverine DIC is drawn directly and/or indirectly from atmospheric CO₂. (3) Both DIC and DOC are concentrated in lake water, suggesting that the lake serves as a sink for both organic and inorganic carbon over long term timescales. (4) Our analysis suggests that the carbon burial rates in Lake Qinghai would be much higher in warmer climatic periods than in cold ones, implying a growing role in the global carbon cycle under a continued global warming scenario.     

Noble gas evidence for two fluids in the Baca (Valles Caldera) geothermal reservoir

Noble gas elemental and isotopic abundances were measured in steam from four wells in the Baca geothermal reservoir located in the Valles Caldera, New Mexico. The ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratio and noble gas elemental abundances relative to ³⁶Ar are all strongly correlated with 1/³⁶Ar, the inverse of the argon content. Ratios of (α,n)-produced ²¹Ne1 and radiogenic ⁴⁰Ar1 to total ⁴He (dominantly radiogenic) are nearly constant at 2.1 × 10?8 and 0.20, respectively. The ${}^3\text{He}{}^4\text{He}$ ratio covers a restricted range of 3.9 to 4.8 times atmospheric. The high ³He content of the gas indicates the presence of a helium component ultimately derived from the mantle. Kr and Xe isotopic compositions are close to atmospheric; excess ¹²⁹Xe1 is <0.25% of the total ¹²⁹Xe.The high degree of linear correlation among the various noble gas results strongly suggests that the Baca reservoir contains two distinct fluids that are produced in varying proportions from individual wells. The noble gases in fluid A (~2900 mg/1 C1) are air-like, but with lighter gases and isotopes preferentially enriched. The fluid A ³⁶Ar content is low, only 13% that of 10°C air-saturated water (ASW). The second fluid, B (~ 1700 mg/1 C1), is the dominant carrier of the radiogenic and mantle-derived gases. The heavier non-radiogenic gases are preferentially enriched in fluid B, and its ³⁶Ar content is very low, only 5–7% ASW. The source of the noble gases in fluid A is tentatively ascribed to leaching of the relatively young (<1.4 m.y.) volcanic Bandelier Tuff. The radiogenic gases and mantle-derived helium in fluid B suggest a deeper source, possibly including gases escaping from a magma.     

Improving noble gas based paleoclimate reconstruction and groundwater dating using Ne/Ne ratios

The interpretation of noble gas concentrations in groundwater with respect to recharge temperature and fractionated excess gas leads to different results on paleo-climatic conditions and on residence times depending on the choice of the gas partitioning model. Two fractionation models for the gas excess are in use, one assuming partial re-equilibration of groundwater supersaturated by excess air (PR-model, Stute et al., 1995), the other assuming closed-system equilibration of groundwater with entrapped air (CE-model, Aeschbach-Hertig et al., 2000). In the example of the Continental Terminal aquifers in Niger, PR- and CE- model are both consistent with the data on elemental noble gas concentrations (Ne, Ar, Kr, and Xe). Only by including the isotope ratio ²⁰Ne/²²Ne it can be demonstrated that the PR-model has to be rejected and the CE-model should be applied to the data. In dating applications ³He of atmospheric origin (³He_atm) required to calculate ³H-³He water ages is commonly estimated from the Ne excess presuming that gas excess is unfractionated air (UA-model). Including in addition to the Ne concentration the ²⁰Ne/²²Ne ratio and the concentration of Ar enables a rigorous distinction between PR-, CE- and UA-model and a reliable determination of ³He_atm and of ³H-³He water ages.     

Assessment of branched GDGTs as temperature proxies in sedimentary records from several small lakes in southwestern Greenland

This study of five small (<3.0 ha) lakes in southwestern Greenland examines the veracity of branched glycerol dialkyl glycerol tetraethers (br GDGTs) as a temperature proxy in lacustrine systems. The proximity (<5 km) of the lakes suggests that their temperature history, and thus their br GDGT records, should be similar. Distributions of br GDGTs in (i) surface sediments from all five lakes, (ii) ¹⁴C-dated sediment cores from two lakes (Upper and Lower EVV Lakes) and (iii) soil samples from the area surrounding the lakes were examined. The temporal records of br GDGT-based temperature for the two cores exhibited both similarities and major discrepancies. The differences between the paleotemperature records for the two lakes suggest that br GDGTs are not solely soil-derived, reflecting air temperature, but also indicate an additional br GDGT contribution from another source. Among the broader suite of lake sediments, there was a strong correlation (R² 0.987) between br GDGT-based surface sediment temperatures and measured summer bottom water temperatures for the four lakes with hypoxic/anoxic bottom waters, including Upper EVV Lake. The correlation suggests production of br GDGTs by anaerobic bacteria within the bottom water and/or sediment–water interface, reflecting environmental temperature for the individual lakes and augmenting the uniform, soil-derived signal. Hence, assessment of br GDGTs in Greenland lake sediments provides evidence for their origin from anaerobic autochthonous bacteria and indicates that interpretation of lacustrine br GDGT-based paleotemperature records requires contextual knowledge of individual lake systems and potential source(s) of sedimentary br GDGTs.     

The tritium-helium-3 method and its application to groundwater dating by the example of the Kirovsk mine region,Murmansk oblast

The measurement of ³H, ³He, ⁴He, and ²⁰Ne concentrations in waters at the Tsentralny pumping station (southern Khibiny massif, Kola Peninsula) showed that they are a mixture of young (>90%) and old (<10%) waters. The excess noble gas component from the young water is caused by the dissolution of air bubbles trapped during recharge in the unsaturated zone. The ³H-³He(³H) age of the young water is 21 ± 1.5 yr. The U-Th-4He age of the old water is about 50 ka. The high concentrations of helium and some toxic elements (e.g., aluminum) in this old water are caused by dissolution of the alkaline rocks of the Khibiny massif as a result of water-rock interaction.     

Diagenetic cycling of arsenic in the sediments of eutrophic Baldeggersee,Switzerland

Sedimentation and benthic release of As was determined in Baldeggersee, a eutrophic lake in central Switzerland. Sediment traps recorded As sedimentation during 1994, including a flood event in spring. Diagenetic processes were studied using porewater profiles at the sediment–water interface and in deeper sediment strata deposited in the mesotrophic lake (before 1885). Sediment cores were used to calculate the accumulation and to construct the balance of sedimentation and remobilisation. The results showed that the lake sediment acts as an efficient sink for As. Only 22% of the particulate As flux reaching the sediment surface was remobilised at the sediment–water interface. The As accumulation in the recent varved section of the eutrophic lake was 40 mg As m⁻² a⁻¹. Iron reduction in older sediment caused a remobilisation of 1.2 mg As m⁻² a⁻¹. This upward flux from the deeper sediment was quantitatively immobilised in the recent sulfidic sediments. The flood event in spring contributed about 34% of the yearly sediment load and led to distinct peak profiles of dissolved As in the porewater. This evidence for rapid remobilisation disappeared within months.     

辽宁宽甸新生代火山岩和地幔包体的稀有气体同位素组成及其演化意义

通过对宽甸新生代碱性玄武岩、地幔包体及辉石巨晶的稀有气体同位素组成的分析，认为不同岩性稀有气体含量的差异反映了岩浆作用过程中轻、重稀有气体的分馏特性，较轻的稀有气体(He、Ne)比较重的稀有气体(Kr、xe)具有更高的活动性和不相容性；该地区上地幔源区具有典型的MORB型源区特征，以辉石巨晶为代表；地幔包体的^3He／^4He值较低，可能是地幔流体交代作用造成的；大陆碱性玄武岩具有与大洋玄武岩截然不同的He同位素组成，反映了大陆区地幔岩浆上升过程中受到了陆壳物质混染。地幔源区^40Ar／^36Ar值为350左右，二辉橄榄岩和碱性火山岩的^40Ar／^36Ar值比大气略高，可能有大气组分的混合。部分样品中有^21Ne、^22Ne、^129Xe、^134Xe和^136Xe相对于大气的过剩现象，是核成因造成的。     

Trace metal deposition and mobility in the sediments of two lakes near Sudbury,Ontario

The accumulation and mobility of Fe, Mn, Al, Cu, Ni and Pb in the sediments of two lakes (Clearwater, pH 4.5; and McFarlane, pH 7.5) near Sudbury, Ontario have been investigated. The Al, Cu and Ni concentrations are expectedly relatively high in the overlying waters of Clearwater Lake and much lower for Al and Cu in McFarlane Lake. The low trace metal concentrations found in the anoxic porewaters of Clearwater Lake could be explained by a sharp increase in porewater pH concomitant with SO₄² reduction and H₂S production within the first 1–2 cm of the sediments, which has conceivably led to the precipitation of mineral phases such as AL(OH)₃, NiS, and CuS. In both lakes, Fe concentrations in anoxic porewaters appear to be controlled by FeS and/or FeCO₃ formation. Solubility calculations also indicate MnCO₃ precipitation in McFarlane Lake. In Clearwater Lake, however, both porewater and total Mn were relatively low, a possible result of the continuous loss of Mn(II) through the acidic interface. It is suggested that upwardly decreasing total Mn profiles resulting from the removal of Mn from the top sediment layers under acidic conditions may constitute a reliable symptom of recent lake acidification.The downward diffusion of AI, Cu and Ni from the overlying water to the sediments has been estimated from their concentration gradients at the interface and compared to their total accumulation rates in the sediments. In both lakes the diffusion of Al is negligible compared to its accumulation rate. However, diffusion accounts for 24–52% of the accumulation of Cu in the sediments of Clearwater Lake, but appears negligible in McFarlane Lake. The downward diffusive flux of Ni is important and may explain 76–161% of the estimated Ni accumulation rate in Clearwater Lake, and 59% in McFarlane Lake. The porewater Cu and Ni profiles suggest that the subsurface sedimentary trace metal peaks observed in Clearwater Lake (as in other acid lakes) may not be caused by sediment leaching or by a recent reduction in sedimentation but may have a diagenetic origin instead. Diffusion to the sediments thus appears to be an important and previously overlooked trace metal deposition mechanism, particularly in acid lakes.     

Noble gas tracing of groundwater/coalbed methane interaction in the San Juan Basin, USA

The San Juan Basin natural gas field, located in northwestern New Mexico and southwestern Colorado in the USA, is a case-type coalbed methane system. Groundwater is thought to play a key role in both biogenic methane generation and the CO₂ sequestration potential of coalbed systems. We show here how noble gases can be used to construct a physical model that describes the interaction between the groundwater system and the produced gas. We collected 28 gas samples from producing wells in the artesian overpressured high production region of the basin together with 8 gas samples from the underpressured low production zone as a control. Stable isotope and major species determination clearly characterize the gas in the high production region as dominantly biogenic in origin, and the underpressured low producing region as having a significant admix of thermogenic coal gas. ³He/⁴He ratios increase from 0.0836R_a at the basin margin to 0.318R_a towards the center, indicating a clear but small mantle He signature in all gases. Coherent fractionation of water-derived ²⁰Ne/³⁶Ar and crustal ⁴He/⁴⁰Ar* are explained by a simple Rayleigh fractionation model of open system groundwater degassing. Low ²⁰Ne concentrations compared to the model predicted values are accounted for by dilution of the groundwater-associated gas by desorbed coalbed methane. This Rayleigh fractionation and dilution model together with the gas production history allows us to quantify the amount of water involved in gas production at each well. The quantified water volumes in both underpressured and overpressured zones range from 1.7 × 10³ m³ to 4.2 × 10⁵ m³, with no clear distinction between over- and underpressured production zones. These results conclusively show that the volume of groundwater seen by coal does not play a role in determining the volume of methane produced by secondary biodegradation of these coalbeds. There is no requirement of continuous groundwater flow for renewing the microbes or nutrient components. We furthermore observe strong mass related isotopic fractionation of ²⁰Ne/²²Ne and ³⁸Ar/³⁶Ar isotopic ratios. This can be explained by a noble gas concentration gradient in the groundwater during gas production, which causes diffusive partial re-equilibration of the noble gas isotopes. It is important for the study of other systems in which extensive groundwater degassing may have occurred to recognize that severe isotopic fractionation of air-derived noble gases can occur when such concentration gradients are established during gas production. Excess air-derived Xe and Kr in our samples are shown to be related to the diluting coalbed methane and can only be accounted for if Xe and Kr are preferentially and volumetrically trapped within the coal matrix and released during biodegradation to form CH₄.     

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

Low ¹⁴C activities in groundwater of the confined part of the Aquia aquifer in southeastern Maryland suggest that most of this water infiltrated at least 30,000 years ago. However, radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange, possibly with secondary calcite deposits in the formation, leading to overestimated ¹⁴C ages. Whereas the geochemistry of the Aquia aquifer complicates the application of the widely used ¹⁴C dating method, the accumulation of radiogenic He seems to provide a viable alternative for establishing a chronology. The quasi-linear increase of He concentrations with flow distance observed in the Aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic He. U and Th concentrations in Aquia sand were measured in order to determine the accumulation rate of ⁴He with sufficient confidence to establish a He time scale.Concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration. These noble gas temperatures (NGTs) clearly show the presence of water that infiltrated under much cooler conditions than at present. NGTs are correlated with chloride concentrations, corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal. In contrast, the stable isotope ratios δ¹⁸0 and δD do not provide a clear record of past climatic changes in the Aquia aquifer and the correlation between NGTs and stable isotope ratios is weak. The NGT record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum (LGM) were (9.0 ± 0.6) °C colder than during the Holocene. This difference is slightly lower than estimates derived from pollen data for this region, but considerably larger than the rather uniform cooling of about 5°C indicated by noble gas studies in more southern locations of North America. The larger cooling is ascribed to the influence of the Laurentide ice sheet, which at its maximum extension came as close as 250 km to our study site.