(234)U/(238)U activity ratio disequilibrium technique for studying uranium mobility in the Opalinus Clay at Mont Terri, Switzerland

Mobility of naturally occurring U-238 and U-234 radionuclides was studied in a low permeability, reducing claystone formation (Opalinus Clay) near its contact with an overlying oxidising aquifer (Dogger Limestones) at Mont Terri, Switzerland. Our data point to a limited redistribution of U in some of the studied samples. Observed centimetre-scale U mobility is explained by slow diffusive transport of U-234 in the pore waters of the Opalinus Clay driven by spatially variable in situ supply (by alpha-recoil) of U-234 from the rock matrix. Metre-scale mobility is interpreted as a result of infiltration of meteoric water into the overlying aquifer which developed gradients of U concentration across the two rock formations. This triggered a slow in-diffusion of U with (U-234/U-238) > 1 into the Opalinus Clay as attested by a clear-cut pattern of decreasing bulk rock (U-234/U-238) inwards the Opalinus Clay, away from the Dogger Limestones.

Geochemical synthesis for the Effingen Member in boreholes at Oftringen, Gösgen and Küttigen

The Effingen Member is a low-permeability rock unit of Oxfordian age (ca. 160 Ma) that occurs across northern Switzerland. It comprises sandy calcareous marls and (argillaceous) limestones. This report describes the hydrogeochemistry, mineralogy and supporting physical properties of the Effingen Member in three boreholes in the Jura-Südfuss area: Oftringen, Gösgen and Küttigen, where it is 220–240 m thick. The top of the Effingen Member is at 420, 66 and 32 m depths at the three sites. Core materials are available from Oftringen and Gösgen, whereas information from Küttigen is limited to cuttings, in-situ hydrogeological testing and geophysical logging. Hydrogeological boundaries of the Effingen Member vary between locations. Ground-water flows were identified during drilling at the top (Geissberg Member), but not at the base, of the Effingen Member at Oftringen, at the base (Hauptrogenstein Formation) of the Effingen Member at Gösgen, and in a limestone layer (Gerstenhübel unit) within the Effingen Member at Küttigen. The marls and limestones of the Effingen Member have carbonate contents of 46–91 wt.-% and clay-mineral contents of 5–37 wt.-%. Pyrite contents are up to 1.6 wt.-%, but no sulphate minerals were detected by routine analyses. Clay minerals are predominantly mixed-layer illite-smectite, illite and kaolinite, with sporadic traces of chlorite and smectite. Veins filled with calcite ± celestite occur through the Effingen Member at Oftringen but not at Gösgen or Küttigen. They formed at 50–70 ºC from externally derived fluids, probably of Miocene age. Water contents are 0.7–4.2 wt.-%, corresponding to a water-loss porosity range of 1.9–10.8 vol.-%. Specific surface areas, measured by the BET method, are 2–30 m2/g, correlating with clay-mineral contents. Water activity has been measured and yielded surprisingly low values down to 0.8. These cannot be explained by pore-water salinity alone and include other effects, such as changes in the fabric due to stress release or partial saturation. Observed variations in measurements are not fully understood. Cation exchange capacity (CEC) and exchangeable cation populations have been studied by the Ni-en method. CEC, derived from the consumption of the index cation Ni, is 9–99 meq/kgrock at a solid:liquid ratio of 1, correlating with the clay-mineral content. Cation concentrations in Ni-en extract solutions are in the order Na+≥Ca2+>Mg2+>K+>Sr2+. However, the analytical results from the Ni-en extractions have additional contributions from cations originating from pore water and from mineral dissolution reactions that occurred during extraction, and it was not possible to reliably quantify these contributions. Therefore, in-situ cation populations and selectivity coefficients could not be derived. A suite of methods have been used for characterising the chemical compositions of pore waters in the Effingen Member. Advective displacement was used on one sample from each Oftringen and Gösgen and is the only method that produces results that approach complete hydrochemical compositions. Aqueous extraction was used on core samples from these two boreholes and gives data only for Cl- and, in some cases, Br-. Out-diffusion was used on core samples from Oftringen and similarly gives data for Cl- and Br- only. For both aqueous extraction and out-diffusion, reaction of the experimental water with rock affected concentrations of cations, SO42 and alkalinity in experimental solutions. Another method, centrifugation, failed to extract pore water. Stable isotope ratios (δ18O and δ2H) of pore waters in core samples from Oftringen were analysed by the diffusive exchange method and helium contents of pore water in Oftringen samples were extracted for mass spectrometric analysis by quantitative outgassing of preserved core samples. Several lines of evidence indicate that drillcore samples might not have been fully saturated when opened and subsampled in the laboratory. These include comparisons of water-loss porosities with physical porosities, water-activity measurements, and high contents of dissolved gas as inferred from ground-water samples. There is no clear proof of partial saturation and it is unclear whether this might represent in-situ conditions or is due to exsolution of gas due to the pressure release since drilling. Partial saturation would have no impact on the recalculation of pore-water compositions from aqueous extraction experiments using water-loss porosity data. The largest uncertainty in the pore-water Cl- concentrations recalculated from aqueous extraction and out-diffusion experiments is the magnitude of the anion-accessible fraction of water-loss porosity. General experience of clay-mineral rich formations suggests that the anion-accessible porosity fraction is very often about 0.5 and generally in a range of 0.3 to 0.6 and tends to be inversely correlated with clay-mineral contents. Comparisons of the Cl- concentration in pore water obtained by advective displacement with that recalculated from aqueous extraction of an adjacent core sample suggests a fraction of 0.27 for an Oftringen sample, whereas the same procedure for a Gösgen sample suggests a value of 0.64. The former value for anion-accessible porosity fraction is presumed to be unrepresentative given the local mineralogical heterogeneity at that depth. Through-diffusion experiments with HTO and 36Cl- suggest that the anion-accessible porosity fraction in the Effingen Member at Oftringen and Gösgen is around 0.5. This value is proposed as a typical average for rocks of the Effingen Member, bearing in mind that it varies on a local scale in response to the heterogeneity of lithology and pore-space architecture. The substantial uncertainties associated with the approaches to estimating anion-accessible porosity propagate into the calculated values of in-situ pore-water Cl- concentrations. On the basis of aqueous extraction experiments, and using an anion-accessible porosity fraction of 0.5, Cl- concentrations in the Effingen Member at Oftringen reach a maximum of about 14 g/L in the centre. Cl- decreases upwards and downwards from that, forming a curved depth profile. Cl- contents in the Effingen Member at Gösgen increase with depth from about 3.5 g/L to about 14 g/L at the base of the cored profile (which corresponds to the centre of the formation). Out-diffusion experiments were carried out on four samples from Oftringen, distributed through the Effingen Member. Recalculated Cl- concentrations are similar to those from aqueous extraction for 3 out of the 4 samples, and somewhat lower for one sample. Concentrations of other components, i.e. Na+, K+, Ca2+, Mg2+, Sr2+, SO42- and HCO3- cannot be obtained from the aqueous extraction and out-diffusion experimental data because of mineral dissolution and cation exchange reactions during the experiments. Pore-water pH also is not constrained by those extraction experiments. The only experimental approach to obtain complete pore-water compositions for samples from Oftringen and Gösgen is advective displacement of pore water. The sample from Oftringen used for this experiment is from 445 m depth in the upper part of the Effingen Member and gave eluate with 16.5 g/L Cl- whereas aqueous extraction from a nearby sample indicated about 9 g/L Cl-. The sample from Gösgen used for advective displacement is from 123 m depth in the centre of the Effingen Member sequence and gave eluate with about 9 g/L Cl- whereas aqueous extraction gave 11.5 g/L Cl-. In both cases the pore waters have Na-(Ca)-Cl compositions and SO42- concentrations of about 1.1 g/L. The Gösgen sample has a Br/Cl ratio similar to that of sea water, whereas this ratio is lower for the Oftringen sample. Taking account of uncertainties in the applied experimental approaches, it is reasonable to place an upper limit of ca. 20 g/L on Cl- concentration for pore water in the Effingen Member in this area. There are major discrepancies between pore-water SO42- concentrations inferred from aqueous extraction or out-diffusion experiments and those obtained from advective displacement in both the Oftringen and Gösgen cases. A general conclusion is that all or at least part of the discrepancies are attributable to perturbation of the sulphur system and enhancement of SO42- by sulphate mineral dissolution and possibly minor pyrite oxidation during aqueous extraction and out-diffusion. Therefore, data for SO42- calculated from those pore-water sampling methods are considered not to be representative of in-situ conditions. A reference pore-water composition was defined for the Effingen Member in the Jura Südfuss area. It represents the probable upper limits of Cl- contents and corresponding anion and cation concentrations that are reasonably constrained by experimental data. Except for Cl- and possibly Na+ concentrations, this composition is poorly constrained especially with respect to SO42- and Ca2+ concentrations, and pH and alkalinity. Stable isotope compositions, δ18O and δ2H, of pore waters in the Effingen Member at Oftringen plot to the right of the meteoric water line, suggesting that 18O has been enriched by water-rock exchange, which indicates that the pore waters have a long residence time. A long residence time of pore water is supported by the level of dissolved 4He that has accumulated in pore water of the Effingen Member at Oftringen. This is comparable with, or slightly higher than, the amounts of 4He in the Opalinus Clay at Benken. Ground waters were sampled from flowing zones intersected by boreholes at the three locations. The general interpretation is that pore waters and ground-water solutes may have similar origins in Mesozoic and Cenozoic brackish-marine formations waters, but ground-water solutes have been diluted rather more than pore waters by ingress of Tertiary and Quaternary meteoric waters. The available hydrochemical data for pore waters from the Effingen Member at these three locations in the Jura-Südfuss area suggest that the geochemical system evolved slowly over geological periods of time, in which diffusion was an important mechanism of solute transport. The irregularity of Cl- and δ18O profiles and spatial variability of advective ground-water flows in the Malm-Dogger system suggests that palaeohydrogeological and hydrochemical responses to changing tectonic and surface environmental conditions were complex.

Stable-Isotopic Composition of Precipitation Over the Northern Slope of the Central Himalaya

Stable-water-isotope data (deltaD and delta(18)O) from three groups of samples (fresh-snow and snow-pit samples collected on Qomolangma (Mount Everest) and Xixabangma during field seasons 1997,1998 and 2001, and precipitation samples collected at Tingri station during summer 2000) are presented and used to survey the isotopic composition of precipitation over the northern slope of the central Himalaya. Multi-year snow-pit samples on Qomolangma have a local meteoric water-line (slope = 8) close to the global value. Deuterium excess (d = deltaD - 8delta(18)O) values at Tingri are much lower than those in fresh snow from Qomolangma, probably due to differences in moisture source and air-mass trajectories as well as local weather conditions. There is no obvious seasonal trend for d values in the Qomolangma region. A negative relationship exists between delta(18)O and d values in both fresh snow on Qomolangma and precipitation at Tingri. Fresh-snow samples collected from different altitudes on Xixabangma allow us to investigate the altitude effect on delta(18)O values in snow. Of four storm events, only one has an obvious altitude effect on delta(18)O variation and a very low gradient of -0.1% per 100 in elevation.

Seasonal Deuterium Excess in a Tien Shan Ice Core: Influence of Moisture Transport and Recycling in Central Asia

Stable water isotope (delta(18)O, deltaD) data from a high elevation (5100 masl) ice core recovered from the Tien Shan Mountains, Kyrgyzstan, display a seasonal cycle in deuterium excess (d = deltaD - 8* delta(18)O) related to changes in the regional hydrologic cycle during 1994 - 2000. While there is a strong correlation (r(2) = 0.98) between delta(18)O and dD in the ice core samples, the regression slope (6.9) and mean d value (23.0) are significantly different than the global meteoric water line values. The resulting time-series ice core d profile contains distinct winter maxima and summer minima, with a yearly d amplitude of similar to 15 - 20parts per thousand. Local-scale processes that may affect d values preserved in the ice core are not consistent with the observed seasonal variability. Data from Central Asian monitoring sites in the Global Network of Isotopes in Precipitation (GNIP) have similar seasonal d changes. We suggest that regional-scale hydrological conditions, including seasonal changes in moisture source, transport, and recycling in the Caspian/ Aral Sea region, are responsible for the observed spatial and temporal d variability.

Stable-Isotope Time Series and Precipitation Origin from Firn-Core and Snow Samples, Altai Glaciers, Siberia

In the summers of 2001 and 2002, glacio-climatological research was performed at 4110-4120 m a.s.l. on the Belukha snow/firn plateau, Siberian Altai. Hundreds of samples from snow pits and a 21 m snow/firn core were collected to establish the annual/seasonal/monthly depth-accumulation scale, based on stable-isotope records, stratigraphic analyses and meteorological and synoptic data. The fluctuations of water stable-isotope records show well-preserved seasonal variations. The delta(18)O and delta D relationships in precipitation, snow pits and the snow/firn core have the same slope to the covariance as that of the global meteoric water line. The origins of precipitation nourishing the Belukha plateau were determined based on clustering analysis of delta(18)O and d-excess records and examination of synoptic atmospheric patterns. Calibration and validation of the developed clusters occurred at event and monthly timescales with about 15% uncertainty. Two distinct moisture sources were shown: oceanic sources with d-excess < 12 parts per thousand, and the Aral-Caspian closed drainage basin sources with d-excess > 12 parts per thousand. Two-thirds of the annual accumulation was from oceanic precipitation, of which more than half had isotopic ratios corresponding to moisture evaporated over the Atlantic Ocean. Precipitation from the Arctic/Pacific Ocean had the lowest deuterium excess, contributing one-tenth to annual accumulation.

Assessing the Environmental Hazard of Using Seawater for Ore Processing at the Lasail Mine Site in the Sultanate of Oman

The Lasail mining area (Sultanate of Oman) was contaminated by acid mine drainage during the exploitation and processing of local and imported copper ore and the subsequent deposition of sulphide-bearing waste material into an unsealed tailings dump. In this arid environment, the use of seawater in the initial stages of ore processing caused saline contamination of the fresh groundwater downstream of the tailings dump. After detection of the contamination in the 1980s, different source-controlled remediation activities were conducted including a seepage water collection system and, in 2005, surface sealing of the tailings dump using an HDPE-liner to prevent further infiltration of meteoric water. We have been assessing the benefits of the remediation actions undertaken so far. We present chemical and isotopic (δ18O, δ 2H, 3H) groundwater data from a long-term survey (8–16 years) of the Wadi Suq aquifer along a 28 km profile from the tailings dump to the Gulf of Oman. Over this period, most metal concentrations in the Wadi Suq groundwater decreased below detection limits. In addition, in the first boreholes downstream of the tailings pond, the salinity contamination has decreased by 30 % since 2005. This decrease appears to be related to the surface coverage of the tailings pond, which reduces flushing of the tailings by the sporadic, but commonly heavy, precipitation events. Despite generally low metal concentrations and the decreased salinity, groundwater quality still does not meet the WHO drinking water guidelines in more than 90 % of the Wadi Suq aquifer area. The observations show that under arid conditions, use of seawater for ore processing or any other industrial activity has the potential to contaminate aquifers for decades.

Assemblages and isotopic signals of modern ostracodes and host waters from Patagonia, Argentine

Ostracode species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracodes, together with delta18O and delta13C_DIC values of host water samples, provide a first data set that characterizes a wide range of modern aquatic environments in the Laguna Cari-Laufquen (41°S, 68 - 69°W) and the Lago Cardiel area (48 - 49°S, 70 - 71°W) in Patagonia, Argentina. This data set will ultimately be used to interpret and calibrate data acquired from lake sediment cores with the goal of reconstructing past climate. Species assemblages and isotope values can be assigned to three groups; (1) springs, seeps and streams, (2) permanent ponds and lakes, and (3) ephemeral ponds and lakes. Springs, seeps and streams are characterized by Darwinula sp., Heterocypris incongruens, Eucypris fontana, Amphicypris nobilis and Ilyocypris ramirezi. Ostracode and water isotope values range between -13 and -5 per mil for oxygen, and between -15 and -3 per mil for carbon. They are the most negative of the entire sample set, reflecting ground water input with little or no evaporative enrichment. Limnocythere patagonica, Eucypris labyrinthica, Limnocythere sp. and Eucypris aff. fontana are typical species of permanent ponds and lakes. Isotope values indicate high degree of evaporation of lake waters relative to feeder springs and streams and range between -7 and +5 per mil for oxygen, and -5 and +4 per mil for carbon. Limnocythere rionegroensis is the dominant species in ephemeral ponds and lakes. These systems display the most enriched isotope values in both ostracodes and host waters, extending from -5 to +7 per mil for oxygen, and from -5 to +6 per mil for carbon. Living ostracodes show a positive offset from equilibrium values of up to 2 per mil for oxygen. Carbon-isotope values are up to 6? more negative than equilibrium values in highly productive pools. Comparison of ostracode and host water isotope signals permits assessment of the life span of the aquatic environments. Valves from dead ostracodes collected from ephemeral ponds and lakes show a wide scatter with each sample providing a snapshot of the seasonal history of the host water. The presence of the stream species Ilyocypris ramirezi and a wide range of ostracode isotope values suggest that ephemeral ponds and lakes are fed by streams during spring run-off and seasonally dry. A temporary character is also indicated by Heterocypris incongruens, a drought-resistant species that occupies most springs and seeps. In addition, Limnocythere rionegroensis has adjusted its reproduction strategies to its environment. Whereas only females were collected in fresh host waters, males were found in ephemeral ponds and lakes with higher solute content. Sexual reproduction seems to be the more successful reproduction strategy in high and variable salinities and seasonal droughts. The temporary character of the aquatic environments shows that the availability of meteoric water controls the life span of host waters and underlines the sensitivity of the area to changes in precipitation.

(Table 1) Stabile isotopes at DSDP Leg 55 Holes

Deep sea drilling on four seamounts in the Emperor Seamount chain revealed that Paleogene shallow-water carbonate sediments of the "bryozoan-algal" facies crown the basalt edifices. According to the biofacies model of Schlanger and Konishi (1966, 1975), this bryozoan- algal assemblage suggests that the seamounts formed in cooler, more northerly waters than those presently occupied by the island of Hawaii; i.e., the paleolatitudes of formation were greater than 20 °N. Moving southward toward the youngest member of the seamount chain, a facies gradient indicative of warmer waters was observed. This gradient is interpreted as a reflection of a northward shift in isotherms during the time span in which the seamounts were progressively formed (Savin et al., 1975). On all seamounts, sedimentation at the drilling sites occurred in a high-energy environment with water depths of approximately 20 meters. Early-stage carbonate diagenesis began in the phreatic zone in the presence of meteoric water, but proceeded after subsidence of the seamounts into intermediate sea waters, where the bulk, stable isotopic composition was determined. The subsidence into intermediate waters was rapid, and permitted establishment of an isotopic equilibrium which, like the facies gradient, reflects the northward shift in isotherms during the Paleogene. Calcite and zeolite cements comprise the later-stage diagenesis, and originated from solutions arising from the hydrolysis of the underlying basalt. In conclusion, the results of this study of the shallow-water carbonate sediments are not inconsistent with a paleolatitude of formation for Suiko Seamount (Site 433) of 26.9 ±3.5 °N, as determined by paleomagnetic measurements (Kono, 1980).

Hydrogen and oxygen stable isotope analyses of precipitation sampled at Erlangen (southern Germany) between 2010 and 2013

Shallow groundwater aquifers are often influenced by anthropogenic contaminants or increased nutrient levels. In contrast, deeper aquifers hold potentially pristine paleo-waters that are not influenced by modern recharge. They thus represent important water resources, but their recharge history is often unknown. In this study groundwater from two aquifers in southern Germany were analyzed for their hydrogen and oxygen stable isotope compositions. One sampling campaign targeted the upper aquifer that is actively recharged by modern precipitation, whereas the second campaign sampled the confined, deep Benkersandstein aquifer. The groundwater samples from both aquifers were compared to the local meteoric water line to investigate sources and conditions of groundwater recharge. In addition, the deep groundwater was dated by tritium and radiocarbon analyses. Stable and radiogenic isotope data indicate that the deep-aquifer groundwater was not part of the hydrological water cycle in the recent human history. The results show that the groundwater is older than ~20,000 years and most likely originates from isotopically depleted melt waters of the Pleistocene ice age. Today, the use of this aquifer is strictly regulated to preserve the pristine water. Clear identification of such non-renewable paleo-waters by means of isotope geochemistry will help local water authorities to enact and justify measures for conservation of these valuable resources for future generations in the context of a sustainable water management.

Chemistry of interstitial waters of ODP Leg 112 samples

Two distinct hydrogeochemical regimes currently dominate the Peruvian continental margin. One, in shallower water (150-450 m) shelf to upper-slope regions, is characterized by interstitial waters with strong positive chloride gradients with depth. The maximum measured value of 1043 mM chloride at Site 680 at ITS corresponds to a degree of seawater evaporation of ~2 times. Major ion chemistry and strontioum isotopic composition of the interstitial waters suggest that a subsurface brine that has a marine origin and is of pre-early Miocene "age," profoundly influences the chemistry and diagenesis of this shelf environment. Site 684 at ~9°S must be closest to the source of this brine, which becomes diluted with seawater and/or interstitial water as it flows southward toward Site 686 at ~13?S (and probably beyond) at a rate of approximately 3 to 4 cm/yr, since early Miocene time. The other regime, in deep water (3000-5000 m) middle to lower-slope regions, is characterized by interstitial waters with steep negative and nonsteady-state chloride gradients with depth. The minimum measured value of 454 mM chloride, at Site 683 at ITS, corresponds to ~20% dilution of seawater chloride The most probably sources of these low-chloride fluids are gas hydrate dissociation and mineral (particularly clay) dehydration reactions. Fluid advection is consistent with (1) the extent of dilution shown in the chloride profiles, (2) the striking nonsteady-state depth profiles of chlorides at Sites 683 and 688 and of 87Sr/86Sr ratios at Site 685, and (3) the temperatures resulting from an average geothermal gradient of 50°C/km and required for clay mineral dehydration reactions. Strontium isotope data reveal two separate fluid regimes in this slope region: a more northerly one at Sites 683 and 685 that is influenced by fluids with a radiogenic continental strontium signature, and a southerly one at Sites 682 and 688 that is influenced by fluids with a nonradiogenic oceanic signatures. Stratigraphically controlled fluid migration seems to prevail in this margin. Because of its special tectonic setting, Site 679 at ITS is geochemically distinct. The interstitial waters are characterized by seawater chloride concentrations to -200 mbsf and deeper by a significantly lower chloride concentration of about two-thirds of the value in seawater, suggesting mixing with a meteoric water source. Regardless of the hydrogeochemical regime, the chemistry and isotopic compositions of the interstitial waters at all sites are markedly modified by diagenesis, particularly by calcite and dolomite crystallization.

Bromine and iodine concentrations in sediments and pore fluids of ODP Leg 112

At the Peruvian convergent margin, two distinct pore fluid regimes are recognized from differences in their Cl- concentrations. The slope pore fluids are characterized by low Cl- concentrations, but elevated Br- and I- concentrations due to biogenic production. The shelf pore fluids exhibit elevated Cl- and Br- concentrations due to diffusive mixing with an evaporitic brine. In the slope pore fluids, the Br-, I-, and NH4+ concentrations are elevated following bacterial decomposition of organic matter, but the I- concentrations are in excess of those expected based on mass balance calculations using NH4+ and Br- concentrations. The slope sediment organic matter, which is enriched in iodine from oxidationreduction processes at the oxygenated sediment-water interface, is responsible for this enrichment. The increases in dissolved I- and the I- enrichments relative to NH4+ and Br- correlate well with sedimentation rates because of differential trapping following regeneration. The pore-fluid I-/Br- ratios suggest that membrane ion fiitration is not a major cause of the decreases in Cl- concentrations. Other possible sources for low Cl- water, including meteoric water, clathrate dissociation, and/or mineral dehydration reactions, imply that the diluting component of the slope low-Cl- fluids has flowed at least 1 km through the sediment. The low bottom-water oxygenation in the shelf is responsible for the low (if any) enrichment of iodine in the shelf sediments. Fluctuations in bottom-water oxygen concentrations in the past, however, may be responsible for the observed variations in the sediment I/Br ratios. Comparison of Na+/Cl- and Br-/Cl- molar ratios in the pore fluids shows that the shelf high-Cl- fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution. This dense brine, located below the sediment sections drilled, appears to have flowed a distance >500 km through the sediment.

Chemical and isotopic composition of seawater and waters from thermal spring sites of the Yankicha Island and Kraternaya Bay

A study of chemical and isotopic composition of coastal thermal springs and waters of the Kraternaya Bay, Yankicha Island, revealed that the total mineralization and concentrations of d18O and d2H decrease from the thermal spring site I to V. These waters are of marine origin with various proportions of local meteoric water. Thermal waters of the site VI have considerably altered chemical and isotopic composition due to high temperatures of surrounding rocks. Base temperatures calculated for this area were 130-200°C. Coastal thermal springs affect isotopic composition of water throughout the bay.

Geochemistry of pore fluids from ODP Leg 134 sites

Depending on the temperature and the extent of diagenetic alteration of fluid chemistry, fluid flow at convergent margins may transfer important quantities of heat and mass between the crust and seawater, thereby influencing global mass, isotopic and heat budgets. In the North Aoba Basin, an intra-arc basin located at the New Hebrides Island Arc, alteration of volcanic ash to clay minerals and zeolites forms a CaCl2 brine, perhaps in less than 1 to 3 m.y. The brine results from an exchange of Ca for Na, K, and Mg, and an increase in Cl concentrations to a maximum of 1241 mM. The Cl increase is partly due to the transfer of H2O from the pore fluid into authigenic minerals, but water mass balances, d18O-Cl correlations, and Br/Cl ratios suggest that there is a source of Cl in the sediments. Concentration profiles indicate that Li is transferred from the fluid to solid phase at depths <300 meters below seafloor (mbsf), but at greater depths it is transferred from the solid to fluid phase, at temperatures possibly as low as 25°C. In the accretionary wedge extensive fluid flow appears to be confined to highly faulted regions. Although Cl concentrations less than seawater value are common at convergent margins, the New Hebrides margin contains little low-Cl fluid. Br/Cl ratios suggest the low-Cl fluid is from dilution, and d18O values indicate the water may be derived from mineral dehydration and mixing with meteoric water. The New Hebrides margin exhibits few surface manifestations of venting (e.g., sulfide-oxidizing benthic biological communities, carbonate crusts, mud volcanoes) and thus fluid fluxes may be smaller than at many other margins.

Oxygen and hydrogen isotopes measured in the Northeast Atlantic

Only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in NE Atlantic water masses, and data are especially sparse for intermediate and deep-water masses. The current study greatly expands this record with 527 d18O values from 47 stations located throughout the mid- to low-latitude NE Atlantic. In addition, dD was analyzed in the 192 samples collected along the GEOTRACES North Atlantic Transect GA03 (GA03_e=KN199-4) and the 115 Iberia-Forams cruise samples from the western and southern Iberian margin. An intercomparison study between the two stable isotope measurement techniques (cavity ring-down laser spectroscopy and magnetic-sector isotope ratio mass spectrometry) used to analyze GA03_e samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios. The surface (0-100 m) and central (100-500 m) water isotope data show the typical, evaporation related trend of increasing values equatorward with the exception for the zonal transect off Cape Blanc, NW Africa. Off Cape Blanc, surface water isotope signatures are modified by the upwelling of fresher Antarctic Intermediate Water (AAIW) that generally has isotopic values of 0.0 to 0.5 per mil for d18O and 0 to 2 per mil for dD. Along the Iberian margin the Mediterranean Outflow Water (MOW) is clearly distinguished by its high d18O (0.5-1.1 per mil) and dD (3-6 per mil) values that can be traced into the open Atlantic. Isotopic values in the NE Atlantic Deep Water (NEADW) are relatively low (d18O: -0.1 to 0.5 per mil; dD: -1 to 4 per mil) and show a broader range than observed previously in the northern and southern convection areas. The NEADW is best observed at GA03_e Stations 5 and 7 in the central NE Atlantic basin. Antarctic Bottom Water isotope values are relatively high indicating modification of the original Antarctic source water along the flow path. The reconstructed d18O-salinity relationship for the complete data set has a slope of 0.51, i.e., slightly steeper than the 0.46 described previously by Pierre et al. (1994, J. Mar. Syst. 5 (2), 159-170.) for the tropical to subtropical Northeast Atlantic. This slope decreases to 0.46 for the subtropical North Atlantic Central Water (NACW) and the MOW and to 0.32 for the surface waters of the upper 50 m. The dD-salinity mixing lines have estimated slopes of 3.01 for the complete data, 1.26 for the MOW, 3.47 for the NACW, and 2.63 for the surface waters. The slopes of the d18O-dD relationship are significantly lower than the one for the Global Meteoric Water Line with 5.6 for the complete data set, 2.30 for the MOW, 4.79 for the NACW, and 3.99 for the surface waters. The lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the subtropics.

Oxygen and hydrogen isotope ratios of pore waters from ODP sites in the Mediterranean Sea

Pore fluids from two ODP sites at Eastern Mediterranean mud volcanoes have been analyzed for their Cl concentration and their delta18O and deltaD isotopic composition. The Cl data span a wide range of concentrations, from extremely depleted with respect to seawater (as low as 60 mM) at the crest of Milano dome (site 970) to strongly enriched (up to 5.4 M) at Napoli dome (site 971). Chloride enrichment is known to be due to dissolving Messinian evaporites, whereas the source of the low-Cl fluid is deduced from stable isotope data presented here. The isotopic composition of the endmember fluid is found to be +10? for delta18O and -32? for deltaD for low- as well as for high-Cl waters. From this signature it can be concluded that neither gas hydrates nor meteoric water play a significant role in the freshening of the pore water. Several other processes altering the delta18O/deltaD composition of pore waters are discussed and considered to be of only negligible influence. The process characterizing the isotopic composition of the fluid is found to be clay mineral dehydration (mainly smectite-illite transformation), corresponding to a depth range of 3.5-7 km and an elevated temperature of about 120-165°C. A quantitative estimate shows that this reaction is capable of producing the observed extreme Cl depletion.