Isotopic characteristics of precipitation in Slovenia and Croatia: Comparison of continental and maritime stations

The stable isotopic composition of hydrogen and oxygen (δ²H and δ¹⁸O) and tritium activity (³H) were monitored in monthly precipitation at two continental stations (Ljubljana, Zagreb) and six stations along the eastern Adriatic coasts of Slovenia and Croatia in the period 2001–2003. Mean air temperatures and amount of precipitation were also recorded.

Distinct differences in both meteorological and isotopic data between the continental and maritime stations were observed. Seasonal variations in δ¹⁸O are smaller at the maritime stations than at the continental ones due to smaller seasonal temperature variations. A good correlation between δ¹⁸O and δ²H was obtained for each station, and the local meteoric water lines are close to the Global Meteoric Water Line, with a decreasing trend of slope for the south-Adriatic stations. Good correlations between δ¹⁸O in monthly precipitation and mean monthly air temperature were observed at all stations. The slope of δ¹⁸O vs. T varied between 0.37‰ °C⁻¹ and 0.15‰ °C⁻¹. Mean ³H activity and seasonal variation of ³H activity are smaller at maritime stations than at continental ones. Additionally, ³H activity decreases in the NW–SE direction of the Adriatic coast.

The study of spatial variations over this relatively small area rich in geographical and climatic diversities showed the complexity of the isotopic composition of precipitation and the isotopic data obtained for eight stations, most of them in the karstic area along the Adriatic coast, and gave valuable information for regional hydrological investigations and modelling of isotope variability over the Mediterranean basin.     

An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B

DSDP Hole 504B is the deepest basement hole in the oceanic crust, penetrating through a 571.5 m pillow section, a 209 m lithologic transition zone, and 295 m into a sheeted dike complex. An oxygen isotopic profile through the upper crust at Site 504 is similar to that in many ophiolite complexes, where the extrusive section is enriched in¹⁸O relative to unaltered basalts, and the dike section is variably depleted and enriched. Basalts in the pillow section at Site 504 haveδ¹⁸O values generally ranging from +6.1 to +8.5‰ SMOW(mean= +7.0‰), although minor zeolite-rich samples range up to 12.7‰. Rocks depleted in¹⁸O appear abruptly at 624 m sub-basement in the lithologic transition from 100% pillows to 100% dikes, coinciding with the appearance of greenschist facies minerals in the rocks. Whole-rock values range to as low as +3.6‰, but the mean values for the lithologic transition zone and dike section are +5.8 and +5.4‰, respectively.

Oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone. The pillow section reacted with seawater at low temperatures (near 0°C up to a maximum of around 150°C) and relatively high water/rock mass ratios (10–100); water/rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section. The transition zone and dikes were altered at much higher temperatures (up to about 350°C) and generally low water/rock mass ratios ( 1), and hydrothermal fluids probably contained mantle-derived CO₂. Mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient ( 2.5°C/m) across a 70 m interval at the top of the lithologic transition zone. Progressive reaction during axial hydrothermal metamorphism and later off-axis alteration led to the formation of albite- and Ca-zeolite-rich alteration halos around fractures. This enhanced the effects of cooling and¹⁸O enrichment of fluids, resulting in local increases inδ¹⁸O of rocks which had been previously depleted in¹⁸O during prior axial metamorphism.     

Correlated chemical and isotopic zoning in carbonates in the martian meteorite ALH84001

The meteorite ALH84001, a sample of the ancient martian crust, contains small quantities (1%) of strongly chemically zoned carbonate. High spatial resolution (10 μm) ion microprobe analyses show that the chemical zoning is strongly correlated with variations in oxygen isotope ratios. Early formed Ca,Fe-rich cores have δ¹⁸O 7‰ increasing to 22‰ SMOW in the more Mg-rich outer cores and magnesite rims. Isolated areas of ankerite appear to be isotopically lighter with δ¹⁸O 1‰. The large range in δ¹⁸O requires a significant range in either fluid isotopic composition, or temperature, or both, in the course of the deposition sequence. Our data are inconsistent with formation of the zoned carbonates by closed system Rayleigh fractionation. There is no unique interpretation of the oxygen data, but the recent observation of existence of Δ¹⁷O excesses in the carbonate appears to rule out models which involve high temperature isotopic exchange with silicate. Comparison with terrestrial analogues suggests that ALH84001 carbonates formed in a hydrothermal system with T<400°C, and which, at least in the early stages of formation, may have involved water with δ¹⁸O < 0‰ SMOW. The later stages of deposition probably occurred at temperatures below 150°C, a conclusion which does not preclude the co-existence of thermophilic bacteria; temperatures during earlier stages of deposition are less likely to have been hospitable to bacteria.     

Hydrochemical and stable isotope evidence for the extent and nature of the effective Chalk aquifer of north Norfolk, UK

In eastern England the Chalk aquifer is covered by extensive Pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer. In this study, the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions (δ¹⁸O and δ²H) are interpreted to reveal the extent and nature of the effective Chalk aquifer of north Norfolk. It is found that the Chalk aquifer can be divided into an upper region of fresh groundwater, with a Cl concentration of typically less than 100 mg l⁻¹, and a lower region of increasingly saline water. The transition between the two regions is approximately 50 m below sea-level, and results in an effective aquifer thickness of 50–60 m in the west of the area, but less than 25 m where the Eocene London Clay boundary is met in the east of the area. Hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the Chalk. Where the Chalk is confined by low-permeability Chalky Boulder Clay, isotopically depleted groundwater (δ¹⁸O less than −7.5‰) is present, in contrast to those areas of unconfined Chalk where glacial deposits are thin or absent (δ¹⁸O about −7.0‰). The isotopically depleted groundwater is evidence for groundwater recharge during the late Pleistocene under conditions when mean surface air temperatures are estimated to have been 4.5°C cooler than at the present day, and suggests long groundwater residence times in the confined aquifer. Elevated molar Mg:Ca ratios of more than 0.2 resulting from progressive rock-water interaction in the confined aquifer also indicate long residence times. A conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater, occupying areas where confined groundwater dates from the late Pleistocene, is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body.     

Stable Isotopes (δ18O, δ2H) of Pore Waters in Clay-Rich Aquitards: A Comparison and Evaluation of Measurement Techniques

Pore water collected from piezometers installed in a thick clay-rich till were used to compare and evaluate four techniques for obtaining δD and δ¹⁸O values in these media. The techniques included mechanical squeezing, centrifugation, azeotropic distillation, and a direct soil-water equilibration technique. Direct CO₂-core equilibration yielded sufficiently accurate and reproducible δ¹⁸O results of pore water in clay-rich tills. In addition, this method eliminated the need for labor-intensive complete extraction of water from the geologic media. Mechanical squeezing and centrifugation produced results similar to direct equilibration. However, both of these methods exhibited a greater degree of variability and were laborious and more time consuming. Small differences in δ¹⁸O values between piezometer water and equilibrated, squeezed, and centhfuged samples suggested that each method collected different fractions of the clay-water reservoir. Although these subtle differences were not conclusive, they did suggest the presence of weakly bound water and highlighted the difference between these three techniques for determining the stable isotopic composition of pore water in clay-rich aquitards. Azeotropic distillation produced a high level of discrepancy in δD andδ¹⁸O results compared to the other methods. Incomplete extraction was considered the most probable cause of this error. The results of this study suggested that direct equilibration is the best method for determining detailed δD and δ¹⁸O values of pore water in clay-rich aquitards.     

Isotope variations in a Sierra Nevada snowpack and their relation to meltwater

Isotopic variations in melting snow are poorly understood. We made weekly measurements at the Central Sierra Snow Laboratory, California, of snow temperature, density, water equivalent and liquid water volume to examine how physical changes within the snowpack govern meltwater δ¹⁸O. Snowpack samples were extracted at 0.1 m intervals from ground level to the top of the snowpack profile between December 1991 and April 1992. Approximately 800 mm of precipitation fell during the study period with δ¹⁸O values between −21.35 and −4.25‰. Corresponding snowpack δ¹⁸O ranged from −22.25 to −6.25‰. The coefficient of variation of δ¹⁸O in snowpack levels decreased from −0.37 to −0.07 from winter to spring, indicating isotopic snowpack homogenization. Meltwater δ¹⁸O ranged from −15.30 to −8.05‰, with variations of up to 2.95‰ observed within a single snowmelt episode, highlighting the need for frequent sampling. Early snowmelt originated in the lower snowpack with higher δ¹⁸O through ground heat flux and rainfall. After the snowpack became isothermal, infiltrating snowmelt displaced the higher δ¹⁸O liquid in the lower snowpack through a piston flow process. Fractionation analysis using a two-component mixing model on the isothermal snowpack indicated that δ¹⁸O in the initial and final half of major snowmelt was 1.30‰ lower and 1.45‰ higher, respectively, than the value from simple mixing. Mean snowpack δ¹⁸O on individual profiling days showed a steady increase from −15.15 to −12.05‰ due to removal of lower δ¹⁸O snowmelt and addition of higher δ¹⁸O rainfall. Results suggest that direct sampling of snowmelt and snow cores should be undertaken to quantify tracer input compositions adequately. The snowmelt sequence also suggests that regimes of early lower δ¹⁸O and later higher δ¹⁸O melt may be modeled and used in catchment tracing studies.     

Oxygen isotope geochemistry of lavas from an oceanic to continental arc transition, Kermadec–Hikurangi margin, SW Pacific

New oxygen isotope data are presented for submarine lavas erupted close to the transition between the oceanic Kermadec island arc and the continental Taupo Volcanic Zone, New Zealand. Volcanic glasses display δ¹⁸O values ranging from +5.65‰ to +5.83‰, clinopyroxenes range from +5.23‰ to +5.78‰ and olivines range from +4.83‰ to +5.47‰. Coexisting glass and phenocrysts in the lavas are in isotopic equilibrium, with one exception. Oxygen isotope ratios of back-arc lavas erupted through oceanic crust are indistinguishable from mid-ocean ridge basalts or lavas erupted in nearby back-arc settings. Although lavas from the arc front display elevated oxygen isotope ratios, the magnitude of ¹⁸O-enrichment is too great to result from recycling of subducted material alone. A single back-arc lava erupted through continental crust is also relatively ¹⁸O-rich suggesting that the most likely origin for the high δ¹⁸O signature is limited amounts of interaction between continental crust and melts derived from a mantle wedge that has been variably fluxed by recycled oxygen. The results of modelling open system behaviour in this volcanic system highlight the need for strong controls on the composition of local contaminants. Application of ‘average' crustal lithologies, as in other volcanic provinces, may lead to erroneous conclusions regarding the involvement of local basement.     

Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along strike architecture of the Himalayan system

High-precision in-situ ion microprobe (SIMS) oxygen isotope analysis of zircons from two diorite intrusions associated with the late Caledonian Lochnagar pluton in Scotland has revealed large differences in the degree of heterogeneity in zircon δ¹⁸O between the diorites. Zircon crystals from the Cul nan Gad diorite (CnG) show a unimodal distribution of oxygen isotope values (δ¹⁸O = 6.0 ± 0.6‰ (2σ)) and no or only minor grain-scale variation. Those from the Allt Darrarie diorite (AD1) show a large range in δ¹⁸O and an apparent bimodal distribution with modes of 6.6 ± 0.4‰ and 7.3 ± 0.4‰. Variations of up to 1.2‰ occur between and within grains; both an increase and decrease in δ¹⁸O with zircon growth has been observed. The δ¹⁸O composition of growing zircon can only change if open-system processes affect the magma composition, i.e. if material of contrasting δ¹⁸O composition is added to the magma. The variability in AD1 is interpreted to represent a cryptic record of magma mixing. A ‘deep crustal hot zone’ is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks. The overall small number of zircons with mantle-like δ¹⁸O values (5.3 ± 0.6‰ (2σ)) in the Lochnagar diorites is largely the product of crustal differentiation rather than crustal growth.

The δ¹⁸O of quartz from the CnG and AD1 diorites shows only minor variation (CnG: 10.9 ± 0.5‰ (2σ), AD1: 11.7 ± 0.6‰ (2σ)) within single populations, with no evidence of mixing. Quartz–zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids, and in case of the AD1 diorite after the inferred magma mixing from a homogenised, higher δ¹⁸O melt.

High-precision SIMS oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early-stage magma mixing and constraining the compositions and origins of the component magmas. A combination of zircon, quartz and whole-rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration.     

Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates

The modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl-bearing silicate minerals. The order of ¹⁸O enrichment obtained in common rock-forming minerals is: pyrophyllite > kaolinite > tourmaline talc > prehnite topaz > illite > phengite > lepidolite muscovite staurolite > epidote > glaucophane > serpentine chlorite > tremolite > hornblende > phlogopite biotite > humite > norbergite > ilvaite. Hydroxyl-bearing silicates are enriched in ¹⁸O relative to hydroxyl groups but depleted in ¹⁸O relative to anhydrous counterparts.

Three sets of self-consistent fractionation factors: between quartz and the hydroxyl-bearing silicate minerals, between calcite and the silicate minerals, and between the silicate minerals and water, have been calculated for a temperature range of 0–1200°C. The fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous, metamorphic and sedimentary petrology. They can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest. The difference in oxygen isotope composition between the hydroxyl-bearing mineral and the OH group is quantitatively demonstrated to be temperature dependent and, therefore, can be used as a single-mineral geothermometer.     

Stable isotope values of Costa Rican surface waters

Stable isotope data of surface waters from the humid tropics in general, and Costa Rica in particular, are scarce. To improve our understanding of the spatial distribution of stable isotopes in surface waters, we measured δ¹⁸O and δD in river and lake (n=63) and precipitation (n=3) samples from Costa Rica. We also present data from the IAEA/WMO isotopes in precipitation network as context for our study. Surface water isotope values do not strongly correlate with elevation, stream head elevation, stream length, distance from Caribbean Sea, or estimated mean annual precipitation for the country as a whole. However, the data show distinct regional trends. The δ¹⁸O and δD values downwind of mountain ranges are inversely related to the altitude of the ranges the air masses traverse. In the lee of the high Talamanca Range, δ¹⁸O values are 6–8‰ lower, while in the lee of the lower Tilarán Range δ¹⁸O values are 2–3‰ lower than upwind sites along the Caribbean Slope. An altitude effect of −1.4‰ δ¹⁸O/km is present on the Pacific slope of southern Costa Rica, equivalent to a temperature effect of −0.3‰/°C. The Nicoya and Osa Peninsulas have higher values than upwind sites, suggesting input of Pacific-sourced moisture, evaporative enrichment, or decreased condensation temperatures. Elevated and increasing d-excess values inland along the Nicaragua Trough suggest a recycled component may be an important contributor to the water budget. These data provide preliminary stable isotope information for Costa Rica, and will benefit paleoclimatic research in the region. More detailed studies would be beneficial to our understanding of the controls on stable isotope composition of tropical waters.     

Groundwater recharge and palaeoclimate in the Sirte and Kufra basins, Libya

Stable and radio-isotope results (C, H, O) for groundwaters from the Sirte and northern Kufra basins are used to determine the recharge history during the Holocene and late Pleistocene. Radiocarbon ages have been corrected on the basis of their stable carbon isotope ratios and on environmental samples from the areas, and two groups may be recognised: (1) low ¹⁴C activity groundwaters (13000–34000 yr. BP) with δ ¹³C-5.6 to −11.7‰; and (2) higher ¹⁴C activity groundwaters (5000–7800 yr. BP) enriched in ¹³C up to δ ¹³C = −3.2‰. There is a general correlation of age with depth.

A well defined freshwater (< 50 mg/l Cl⁻) channel can be traced within the aquifer for some 130 km through the region, which is considered to represent recharge from a former wadi. This water with an age of ± 7800 yr. BP is chemically and isotopically distinct from the regional groundwaters and provides direct evidence of a significant recharge event during the Holocene.

The stable isotope (O and H) composition of groundwater from the Kufra and Sirte basins are all related by an evaporative line with slope δ D = 4.5δ ¹³O − 35 with an intercept on the meteoric line of -11‰. This suggests a recharge source continuing into the Holocene from air masses, analogous to current heavy monsoon rain derived from south of the Sahara. The spatial and temporal distribution of groundwaters in relation to the evaporative line suggests a progressive change in character of the recharge which is controlled by a shift towards strongly convective rainfall during the Holocene.

The direct hydrogeological and geochemical evidence supports climatic models proposed by several workers in which discrete humid episodes during the Holocene are inferred.     

Stable hydrogen and oxygen isotope studies of rainfall and streamwaters for two contrasting holm oak areas of Catalonia, northeastern Spain

Results are presented of a study of stable hydrogen and oxygen isotopes in rainfall and streamwaters for the Montseny and Prades areas in northeastern Spain: results cover the full year of 1991. The isotopic pattern for rainfall is similar for both areas: there is a wide range in isotopic contents and the results show a strong, near-linear trend, δ²H = 7.9 × δ¹⁸O + 9.8 (N = 59; r² = 0.952), the ‘local meteoric line’. There is slight curvature to the data which may be related to the sources of water vapour forming the rainfall. Within the streams, the isotopic variability is much less than that of the rainfall although the data lie on, or very near to, the meteoric line. Data for detailed collections during storm events show more scatter than those collected regularly on a fortnightly basis. The event data show a linear feature that conforms to the local meteoric line. These results indicate that: (1) the main supply of water to the stream stormflow comes from water stored in the catchment prior to the event; (2) waters of more than one isotopic composition reside within the catchment and are transferable to the stream during storm events; (3) the main process of water transfer from the catchment back to the atmosphere comes from transpiration by the trees and (possibly) complete evaporation from the near-surface soil horizons and the tree canopy; (4) the isotopic technique cannot be used for quantitative hydrograph separation in this instance — at least two water types can be present within the catchment at any given time.     

Open-system degassing of sulfur from Krakatau 1883 magma

We present the first sulfur and oxygen isotopic data for tephra from the catastrophic 1883 eruption of Krakatau. Sulfur isotopic ratios in unaltered Krakatau tephra erupted August 26–27, 1883 are markedly enriched in ³⁴S relative to mantle sulfur. High δ³⁴S values of +6.3 to +16.4‰ can best be explained by open-system or multi-stage degassing of SO₂ from the oxidized rhyodacitic and gray dacitic magmas with ³⁴S enrichment of SO²⁻₄ remaining in the melt. Lower whole-rock δ³⁴S values of +2.6‰ and +4.0‰ in two oxidized gray dacitic samples indicate more primitive subarc mantle sulfur in the 1883 magma chamber. Initial δ³⁴S of the rhyodacitic magma was probably in the +1.5‰ to +4.0‰ range and similar to δ³⁴S values measured in arc volcanic rocks from the Mariana Arc.     

Glacial–interglacial changes in Subantarctic sea surface temperature and δO-water using foraminiferal Mg

Laboratory culturing experiments with living Globigerina bulloides indicate that Mg/Ca is primarily a function of seawater temperature and suggest that Mg/Ca of fossil specimens is an effective paleotemperature proxy. Using culturing results and a core-top Neogloboquadrina pachyderma calibration, we have estimated glacial–interglacial changes in sea surface temperature (SST) using planktonic Mg/Ca records from core RC11-120 in the Subantarctic Indian Ocean (43°S, 80°E) and core E11-2 in the Subantarctic Pacific Ocean (56°S, 115°W). Our results suggest that glacial SST was about 4°C cooler in the Subantarctic Indian Ocean and 2.5°C cooler in the Subantarctic Pacific. Comparison of SST and planktonic δ¹⁸O records indicates that changes in SST lead changes in δ¹⁸O by on average 1–3 kyr. The glacial–interglacial temperature change indicated by the Subantarctic Mg/Ca records suggests that temperature accounts for 40–60% of the foraminiferal δ¹⁸O change. We have used the Mg/Ca-based SST estimates and δ¹⁸O determinations to generate site-specific seawater δ¹⁸O records, which suggest that seawater δ¹⁸O was on average 1‰ more positive during glacial episodes compared with interglacial episodes.     

Use of N stable isotope and microbial analyses to define wastewater influence in Mobile Bay, AL

We assessed short-term ecological and potential human health effects of wastewater treatment plant (WTP) effluent by measuring δ¹⁵N‰ and microbial concentrations in oysters and suspended particulate matter (SPM). We also tested male-specific bacteriophage (MSB) as an alternative to fecal coliforms, to assess potential influence of wastewater contamination on shellfish. WTP effluent did not affect oyster growth or survival, but SPM and oysters acquired wastewater-specific δ¹⁵N‰. δ¹⁵N values were depleted near the WTP, typical of low-level processed wastewater. Fecal coliform and MSB concentrations were higher in samples taken closest to the WTP, and MSB values were significantly correlated with δ¹⁵N‰ in oyster tissues. Overall, oysters demonstrated relatively rapid integration and accumulation of wastewater-specific δ¹⁵N‰ and indicator microorganisms compared to water samples. These data suggest oysters were superior sentinels compared to water, and MSB was a more reliable indicator of wastewater influence on shellfish than fecal coliforms.     

Oxygen isotope evidence for the origin of enriched mantle beneath the mid-Atlantic ridge

Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ¹⁸O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ¹⁸O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ¹⁸O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea.     

Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO₂–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (100°C) aquifer of mixed marine–meteoric water. The measured ³He/⁴He ratios and δ¹³C of both the free gases (4.5–7.3 R_a and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 R_a and −7.1 to −0.9‰), together with their covariation with the He/CO₂ ratio, constrain a ³He/⁴He ratio of 7.3±0.1 R_a and a δ¹³C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic ⁴He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in ³He and has a lower (closer to MORB) δ¹³C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 R_a, δ¹³C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, ³He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB ³He/⁴He ratio but higher than MORB CO₂/³He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.     

Origin and age of thermal waters in Cieplice Spa, Sudeten, Poland, inferred from isotope, chemical and noble gas data

Isotope and hydrochemical data of the thermal water system in Cieplice laskie Zdrój (Spa) indicate the existence of two subsystems that greatly differ in volume and which meet at the fault zones of a granitic horst, where they discharge at an altitude of about 340m. One of the subsystems is very small (about 4 × 10³ m³) as indicated by the tritium age of the order of 10 years and a low outflow rate. Its recharge area found from the δ¹⁸O and δD values, is about 200m above the springs, most probably on the slopes of the foothills of the Karkonosze Mountains south-southwest of the spa. The large subsystem contains water which is free of tritium and whose ¹⁴C content is from 1 to 8 pmc with δ¹³C = −8.0 to −9.2‰. The isotopic composition of this water reflects either the climatic effect (low-altitude recharge during a cooler pre-Holocene climate) or the altitude effect (recharge in the early Holocene period at about 1000m at the heights of the Karkonosze assuming that the ¹⁴C concentration is strongly reduced by exchange with calcite in veins). For the former hypothesis, the recharge area of this water is probably either at the foot of the southeastern slopes of the Kaczawa Mountains or/and at the foot of the Rudawy Janowickie Mountains, to the east of Cieplice. The noble gas temperatures are more consistent with the pre-Holocene recharge. Similarly, the ⁴He excess and ⁴⁰Ar/³⁶Ar ratio support the hypothesis of a pre-Holecene age. The constant ³He/⁴He ratio of 26 × 10⁻⁸ for highly different helium contents indicates crustal origin of helium. For the pre-Holocene age of water its volume is calculated at >- 10⁹m³ (stagnant water in micropores and mobile water in fractures) and the hydraulic conductivity of the host granite massif is estimated at about 7 × 10⁻⁸ ms⁻¹. Two outflows from this subsystem have different and variable fractions of a modern water component (bomb age), most probably originating from the bank infiltration of a nearby stream.     

A high resolution palaeoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter

Variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of Lake Bosumtwi, Ghana reflect climatically induced changes to the lake and the catchment flora. Cores spanning the last 27.5 kyr of sedimentation in the lake show large oscillations inδ¹³C_org PDB andδ¹⁵N_org Air. The late Pleistocene record is particularly detailed, showing changes of ca. 20‰ in carbon and ca. 10‰ in nitrogen isotopic compositions. These variations are of complex origin. Although different in magnitude, major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the Last Glacial Maximum. The Allerød-Younger Dryas period also seems to have been marked by generally dry conditions in this part of tropical West Africa. Nitrogen isotopic evidence suggests that during the period 9.2–3.2 kyr the lake had an extremely stable water column, probably due to the absence of a windy or cool, dry season, or both. Regular circulation of the water column recommenced during the late Holocene and has persisted until the present day.     

“Andesitic water”: a phantom of the isotopic evolution of water-silicate systems. Comment on “Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin” by W.F. Giggenbach

On account of the low porosity of the lithosphere, intracrustal fluids behave very differently from surface fluids, in that they are changing their geochemical and isotopic labels according to the geological environment. Given a heat source, meteoric waters can be supplied plentifully and their rates of throughput in geothermal systems are sufficiently high to exhaust the compositional signals of a given rock buffer. In contrast, fluids exsolved from magma, and subducted fluids, would be supplied at less than about one tenth of the meteoric rate over the life time of a system. Based on up-to-date flow models, the isotopic evolution of meteoric water interacting with crustal rock follows a curved to L-shaped track in the δD versus δ¹⁸O plot. Instantaneous (present-day) tie-lines between recharge and discharge are secants of such tracks and can have a range of slopes. At the start of an interaction, waters have δD and δ¹⁸O values approaching equilibrium with the original rock composition (water “W1”). Using known hydrogen isotope fractionation factors, W1 values generally plot in the region of “andesite” or “andesitic” waters of various authors. Since the W1 waters have δD values that are on average more positive, and also less variable than those of the meteoric recharges, most tracks and tie-lines have positive slopes, and the plotting of a large number of tie-lines will produce a focus on the field of W1 waters, regardless of the original water source.