The geochemical and isotopic record of evaporite recycling in spas and salterns of the Basque Cantabrian basin, Spain

Evaporite outcrops are rare in the Basque Cantabrian basin due to a rainy climate, but saline springs with total dissolved solids ranging from 0.8 to 260 g/L are common and have long been used to supply spas and salterns. New and existing hydrochemistry of saline springs are used to provide additional insight on the origin and underground extent of their poorly known source evaporites. Saline water hydrochemistry is related to dissolution of halite and gypsum from two evaporitic successions (Triassic “Keuper” and Lower Cretaceous “Wealden”), as supported by rock samples from outcrops and oil exploration drill cuttings. The δ³⁴S value of gypsum in the Keuper evaporites and sulfate in the springs is δ³⁴S_SO4 = 14.06 ± 1.07‰ and δ¹⁸O_SO4 = 13.41 ± 1.44‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 124 and 288 ppm. The δ³⁴S value of gypsum in the Wealden evaporites and sulfate in the springs is δ³⁴S_SO4 = 19.66 ± 1.76‰, δ¹⁸O_SO4 = 14.93 ± 2.35‰, and the relationship between Cl/Br ratio of halite and water shows that waters have dissolved halite with Br content between 15 and 160 ppm. Wealden evaporites formed in a continental setting after the dissolution of Keuper salt. Gypsum δ³⁴S_SO4 and δ¹⁸O_SO4 modification from Keuper to Wealden evaporites was due mainly to bacterial SO₄ reduction in an anoxic, organic matter-rich environment. Saline springs with Wealden δ³⁴S_SO4 values are present in a 70 × 20 km wide area. Saline water temperatures, their δ²H_H2O and δ¹⁸O_H2O values, and the geological structure defines a hydrogeological model, where meteoric water recharges at heights up to 620 m above spring levels and circulates down to 720 m below them, thereby constraining the height range of evaporite dissolution. Groundwater flow towards saline springs is driven by gravity and buoyancy forces constrained by a thrust and fault network.     

A chlorine isotope study of DSDP/ODP serpentinized ultramafic rocks: Insights into the serpentinization process

Eight DSDP/ODP cores were analyzed for major ion concentrations and δ³⁷Cl values of water-soluble chloride (δ³⁷Cl_WSC) and structurally bound chloride (δ³⁷Cl_SBC) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition.The average total Cl content of all 86 samples is 0.26 ± 0.16 wt.% (0.19 ± 0.10 wt.% as water-soluble Cl (X_WSC) and 0.09 ± 0.09 wt.% as structurally bound Cl (X_SBC)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl⁻ site and the water-soluble Cl⁻ site varies from − 1.08‰ to + 1.16‰, averaging to + 0.21‰. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk δ³⁷Cl values (+ 0.05‰ to + 0.36‰); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk δ³⁷Cl values (− 1.26‰ and − 0.54‰). The cores with negative δ³⁷Cl values also have variable Cl⁻ / SO₄² ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ∼1‰ with depth for both the water-soluble and structurally bound Cl fractions.Non-zero bulk δ³⁷Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive δ³⁷Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of ³⁷Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low δ³⁷Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative δ³⁷Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.     

Chlorine isotopic composition in seafloor serpentinites and high-pressure metaperidotites. Insights into oceanic serpentinization and subduction processes

Bulk-rock chlorine content and isotopic composition (δ³⁷Cl) were determined in oceanic serpentinites, high-pressure metaperidotites and metasediments in order to gain constraints on the global chlorine cycle associated with hydrothermal alteration and subduction of oceanic lithosphere. The distribution of insoluble chlorine in oceanic serpentinites was also investigated by electron microprobe. The hydrothermally-altered ultramafic samples were dredged along the South West Indian Ridge and the Mid-Atlantic Ridge. The high-pressure metamorphic samples were collected in the Western Alps: metaperidotites in the Erro-Tobbio unit and metasediments in the Schistes Lustrés nappe.Oceanic serpentinites show relatively large variations of bulk-rock Cl contents and δ³⁷Cl values with mean values of 1105 ± 596 ppm and −0.7 ± 0.4‰, respectively (n = 8; 1σ). Serpentines formed after olivine (meshes) show lower Cl content than those formed after orthopyroxene (bastites). In bastites of two different samples, Cl is positively correlated with Al₂O₃ and negatively correlated with SiO₂. These relationships are interpreted as reflecting preferential Cl-incorporation into the bastite structure distorted by Al (substituted for Si) rather than different alteration conditions between olivine and orthopyroxene minerals. High-pressure metaperidotites display relatively homogeneous Cl contents and δ³⁷Cl values with mean values of 467 ± 88 ppm and −1.4 ± 0.1‰, respectively (n = 7; 1σ). A macroscopic high-pressure olivine-bearing vein, formed from partial devolatilization of serpentinites at ∼2.5 GPa and 500-600 °C, shows a Cl content and a δ³⁷Cl value of 603 ppm and −1.6‰, respectively. Metasediments (n = 2) show low whole-rock Cl contents (<15 ppm Cl) that did not allow Cl isotope analyses to be obtained.The range of negative δ³⁷Cl values observed in oceanic serpentinites is likely to result from water-rock interaction with fluids that have negative δ³⁷Cl values. The homogeneity of δ³⁷Cl values from the high-pressure olivine-bearing vein and the metaperidotite samples implies that progressive loss of Cl inherited from oceanic alteration throughout subduction did not significantly fractionate Cl isotopes. Chlorine recycled in subduction zones via metaperidotites should thus show a range of δ³⁷Cl values similar to the range found in oceanic serpentinized peridotites.     

Chlorine isotope evidence for the anthropogenic origin of tris-(4-chlorophenyl)methane

Compound-specific Cl-isotope analysis was performed on the persistent and bioaccumulating compound tris-(4-chlorophenyl)methane (4,4′,4″-TCPMe, referred to as TCPMe in this study) to elucidate whether its main source is natural or anthropogenic. Blubber from the Baltic grey seal (Halichoerus grypus) was extracted by continuous acetonitrile partitioning, and the TCPMe was isolated from the extract by preparative-capillary gas chromatography. Chlorine isotope analysis was subsequently performed by sealed-tube combustion in conjunction with thermal-ionization mass spectrometry (TIMS). The δ³⁷Cl of TCPMe was −3.5 ± 0.5‰, similar to the previously reported δ³⁷Cl of technical grade p,p′-DDT (referred to as DDT in this study). The data is not consistent with a putative marine natural source of TCPMe, as enzymatic (biotic) production is reported to give values of δ³⁷Cl < −10‰. The δ³⁷Cl–TCPMe data thus supports the hypothesis that TCPMe is produced as a byproduct during DDT synthesis and is released to the environment through the same pathways as DDT. It is also consistent with tris-(4-chlorophenyl)methanol as the primary biotransformation product of TCPMe.     

Stable silicon isotopes of groundwater, feldspars, and clay coatings in the Navajo Sandstone aquifer, Black Mesa, Arizona, USA

We present some of the first analyses of the stable isotopic composition of dissolved silicon (Si) in groundwater. The groundwater samples were from the Navajo Sandstone aquifer at Black Mesa, Arizona, USA, and the Si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed. Silicon isotope compositions were measured using high-resolution multi-collector inductively coupled mass spectrometry (HR-MC-ICP-MS) (Nu1700 & NuPlasma HR). The quartz dominated bulk rock and feldspar separates have similar δ³⁰Si of −0.09 ± 0.04‰ and −0.15 ± 0.04‰ (±95% SEM), respectively, and clay separates are isotopically lighter by up to 0.4‰ compared to the feldspars. From isotopic mass-balance considerations, co-existing aqueous fluids should have δ³⁰Si values heavier than the primary silicates. Positive δ³⁰Si values were only found in the shallow aquifer, where Si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals. However, δ³⁰Si decreases along the flow path from 0.56‰ to −1.42‰, representing the most negative dissolved Si isotope composition so far found for natural waters. We speculate that the enrichment in ²⁸Si is due to dissolution of partly secondary clay minerals and low-temperature silcretes in the Navajo Sandstone. The discovery of the large range and systematic shifts of δ³⁰Si values along a groundwater flow path illustrates the potential utility of stable Si isotopes for deciphering the Si cycling in sedimentary basins, tracing fluid flow, and evaluating global Si cycle.     

Sulfur and carbon isotope records from 1700 to 800 Ma carbonates of the Jixian section, northern China: Implications for secular isotope variations in Proterozoic seawater and relationships to global supercontinental events

This study is a comprehensive, stable isotope survey of the marine carbonate-dominated, upper Paleo- to lower Neoproterozoic stratigraphy of Jixian County, China. Carbonate-associated sulfate (CAS) was extracted and measured for δ³⁴S_CAS using the same samples analyzed for δ¹³C_carbonate. This integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of Proterozoic seawater. We specifically sampled marine carbonate intervals from the lower section of the Chuanlinggou Formation, Changcheng Group (ca. 1700 Ma) to the top of the Jingeryu Formation, Qingbaikou Group (ca. 800 Ma). δ¹³C_carbonate values are mostly negative in the upper Paleoproterozoic Changcheng Group, with an ascending trend from −3‰ to 0‰. We observed variation of approximately 0 ± 1‰ in the Mesoproterozoic Jixian Group, and positive values of +2 ± 2‰ characterize the lower Neoproterozoic Qingbaikou Group. Stratigraphic variations in δ³⁴S_CAS are more remarkable in their ranges and magnitudes, including conspicuously high values exceeding +30‰ in the three intervals at ca. 1700 Ma, 1300-1100 Ma, and 1000-900 Ma. In the Changcheng Group, δ³⁴S_CAS values are typically higher than +25‰, with only a few values of less than +15‰. In contrast, most of the data spanning from the Mesoproterozoic Tieling Formation of the Jixian Group to the lower Neoproterozoic Jingeryu Formation of the Qingbaikou Group are highly variable between +10‰ and +25‰, with some values exceeding +25‰.In the late Paleoproterozoic (1700-1600 Ma), a >10‰ decrease in δ³⁴S_CAS and ∼3‰ increase in δ¹³C_carbonate are coincident with, and likely related to, the breakup of Columbia, a supercontinent that predated Rodinia. Carbon and sulfur isotope data from the Mesoproterozoic, when global tectonic activity was comparatively weaker, fall mostly in the ranges of +15 ± 10‰ and 0 ± 1‰, respectively, but fluctuations of >20‰ for δ³⁴S_CAS and >3‰ for the δ¹³C_carbonate at ca. 1450-1400 Ma may reflect subduction and large-scale magmatic activity in island arcs marking the end of Columbia breakup. From the late Mesoproterozoic (ca. 1300-1100 Ma) to the early Neoproterozoic (ca. 800 Ma), the δ¹³C and δ³⁴S of seawater increased gradually with increasing variability. Most impressive areδ³⁴S_CAS values that exceed +30‰ in two intervals at ca. 1300-1100 Ma and ca. 1000-900 Ma, which may reflect the assembly and early breakup of Rodinia. Although gaps in the record remain, and studies of even higher resolution are warranted, our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of C and S. Furthermore, periods of the Proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity.     

Copper isotope fractionation in acid mine drainage

We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ⁶⁵Cu values (based on ⁶⁵Cu/⁶³Cu) of enargite (δ⁶⁵Cu = −0.01 ± 0.10‰; 2σ) and chalcopyrite (δ⁶⁵Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ < δ⁶⁵Cu < 1‰). These mineral samples show lower δ⁶⁵Cu values than stream waters (1.38‰ ? δ⁶⁵Cu ? 1.69‰). The average isotopic fractionation (Δ_aq-min = δ⁶⁵Cu_aq − δ⁶⁵Cu_min, where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14‰ and 1.60 ± 0.14‰ for chalcopyrite and enargite, respectively. To interpret this field survey, we leached chalcopyrite and enargite in batch experiments and found that, as in the field, the leachate is enriched in ⁶⁵Cu relative to chalcopyrite (1.37 ± 0.14‰) and enargite (0.98 ± 0.14‰) when microorganisms are absent. Leaching of minerals in the presence of Acidithiobacillus ferrooxidans results in smaller average fractionation in the opposite direction for chalcopyrite (Δ_aq-min^o=-0.57±0.14‰, where min^o refers to the starting mineral) and no apparent fractionation for enargite (Δ_aq-min^o=0.14±0.14‰). Abiotic fractionation is attributed to preferential oxidation of ⁶⁵Cu⁺ at the interface of the isotopically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of ⁶⁵Cu_aq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ⁶⁵Cu in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.     

The record of temperature, wind velocity and air humidity in the δD and δO of water inclusions in synthetic and Messinian halites

Deuterium and oxygen isotope fractionations between liquid and vapor water were experimentally-determined during evaporation of a NaCl solution (35 g L⁻¹) as a function of water temperature and wind velocity. In the case of a null wind velocity, slopes of δD-δ¹⁸O trajectories of residual waters hyperbolically decrease with increasing water temperatures in the range 23-47 °C. For wind velocities ranging from 0.8 to 2.2 m s⁻¹, slopes of the δD-δ¹⁸O trajectories linearly increase with increasing wind velocity at a given water temperature. These experimental results can be modeled by using Rayleigh distillation equations taking into account wind-related kinetics effects. Deuterium and oxygen isotope compositions of water inclusions trapped by the precipitated halite crystals were determined by micro-equilibration techniques.These isotopic compositions accurately reflect those of the surrounding residual waters during halite growth. Isotopic compositions of water inclusions in twenty natural halites from the Messinian Realmonte mine in Sicily suggest precipitation temperatures of that match the homogenization temperatures obtained by microthermometry (median = 34 ± 5 °C). The similarity between the measured and experimental slopes of the δD-δ¹⁸O evaporation trajectories suggests that the effect of wind was negligible during the genesis of these halite deposits. Hydrogen and oxygen isotope compositions of water inclusions from Realmonte halite also define a linear trend whose extrapolation until intersection with the Mediterranean Meteoric Water Line allows the characterization of the water source with δD and δ¹⁸O values of −70 ± 10‰ and −11.5 ± 1.5‰, respectively. These results reveal that the huge amounts of salts deposited in Sicily result from the evaporation of seawater mixed with a dominant fraction (?50%) of meteoric waters most likely deriving from alpine fluvial discharge.     

Abundance of mass 47 CO2 in urban air, car exhaust, and human breath

Atmospheric carbon dioxide is widely studied using records of CO₂ mixing ratio, δ¹³C and δ¹⁸O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally ¹³C¹⁸O¹⁶O) provides an additional constraint. In particular, the mass 47 anomaly (Δ₄₇) can distinguish between CO₂ produced by high temperature combustion processes vs. low temperature respiratory processes. Δ₄₇ is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured ¹³C and ¹⁸O values. In this study, we estimate the δ¹³C (vs. VPDB), δ¹⁸O (vs. VSMOW), δ47, and Δ₄₇ values of CO₂ from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO₂ from these sources. δ47 is defined as , where is the R⁴⁷ value for a hypothetical CO₂ whose δ¹³C_VPDB = 0, δ¹⁸O_VSMOW = 0, and Δ₄₇ = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO₂] from 383 to 404 μmol mol⁻¹, in δ¹³C and δ¹⁸O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ₄₇ from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ¹³C (similar to the δ¹³C of local gasoline), δ¹⁸O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ₄₇ of 0.41 ± 0.03‰. Both δ¹⁸O and Δ₄₇ values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO₂ and water generated by combustion of gasoline-air mixtures. Samples of CO₂ from human breath were found to have δ¹³C and δ¹⁸O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ₄₇ in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.     

Note on the isotopic geochemistry of fossil-lacustrine tufas in carbonate plateau—A study from Dungul region (SW Egypt)

Lithological, chemical, and stable isotope data are used to characterize lacustrine tufas dating back to pre-late Miocene and later unknown times, capping different surfaces of a Tertiary carbonate (Sinn el-Kedab) plateau in Dungul region in the currently hyperarid south-western Egypt. These deposits are composed mostly of calcium carbonate, some magnesium carbonate and clastic particles plus minor amounts of organic matter. They have a wide range of (Mg/Ca)_molar ratios, from 0.03 to 0.3. The bulk-tufa carbonate has characteristic isotope compositions: (δ¹³C_mean = −2.49 ± 0.99‰; δ¹⁸O_mean = −9.43 ± 1.40‰). The δ¹³C values are consistent with a small input from C4 vegetation or thinner soils in the recharge area of the tufa-depositing systems. The δ¹⁸O values are typical of fresh water carbonates. Covariation between δ¹³C and δ¹⁸O values probably is a reflection of climatic conditions such as aridity. The tufas studied are isotopically similar to the underlying diagenetic marine chalks, marls and limestones (δ¹³C_mean = −2.06 ± 0.84‰; δ¹⁸O_mean = −10.06 ± 1.39‰). The similarity has been attributed to common meteoric water signatures. This raises large uncertainties in using tufas (Mg/Ca)_molar, δ¹³C and δ¹⁸O records as proxies of paleoclimatic change and suggests that intrinsic compositional differences in material sources within the plateau may mask climatic changes in the records.     

Paleowaters in Silurian-Devonian carbonate aquifers: Geochemical evolution of groundwater in the Great Lakes region since the Late Pleistocene

Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater-saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from −10 to −2‰, ⁸⁷Sr/⁸⁶Sr ratios range from 0.7080 to 0.7090, and δ³⁴S-SO₄ values range from +10 to 30‰. A few waters have elevated δ¹³C_DIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ¹⁸O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14-50 ka BP, −15 to −13‰ δ¹⁸O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ¹⁸O values within the range of Holocene precipitation: −11 to −8‰ and −7 to −4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions (δ¹³C_DIC = −14.7 to−11.1‰ and ⁸⁷Sr/⁸⁶Sr = 0.7080-0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.     

Authigenic dolomite cementation in the Upper Cretaceous Phosphate Formation, Western Desert, Egypt

The Upper Cretaceous Phosphate Formation in the Western Desert of Egypt displays a characteristic facies association that includes marine phosphorites interbedded with black shales and glauconitic sandstones. The upper part of the formation is characterized by the presence of thin phosphatic beds, which are filled-extensively-with disordered and non stoichiometric (mean MgCO₃ = 41.4 ± 0.34 mol%) authigenic dolomite cement. SEM and the back scattered images of these coarse crystalline dolomite cements reveal that they display planar euhedral crystal boundaries, polymodal crystal size distribution and variable inclusion pattern. The relatively low and wide ranged δ¹⁸O (− 0.87 to − 4.15‰ VPDB) values of the dolomite cements coupled with their depleted Sr (mean = 187 ± 26 ppm) and high iron and manganese values (mean = 6851 ± 554 ppm and 11599 ± 229 ppm respectively) invoke that they were formed from mixed hypo-saline fluids within a mixing marine-meteoric zone probably during a low stand period at the vicinity of the Maastrichtian/Early Tertiary unconformity. Meanwhile, their negative δ¹³C (− 1.31 to − 3.56‰ VPDB) values argue for a possible involvement of isotopically light carbon, derived from degradation of organic matter, during their precipitation.     

Effect of precipitation regime on δD values of soil n-alkanes from elevation gradients - Implications for the study of paleo-elevation

We measured δD values of long chain n-alkanes isolated from 30 surface soil samples along two elevation transects on the Tibetan Plateau differing in precipitation regime and water source. The East Asian Monsoon precipitation dominates the wetter regime on the eastern slope (from 1230 to 4300 m) of Gongga Shan on the eastern Tibetan Plateau. Precipitation from the Polar Westerlies dominates the drier region on the slope from 1900 to 5000 m in the West Kunlun Shan on the northwestern Tibetan Plateau. The decrease in δD value with elevation in the wetter region greatly exceeded that in drier region by, −1.9 ± 0.1‰/100 m and −1.4 ± 1.0‰/100 m respectively. The apparent fractionation between leaf wax and precipitation ε_wax-p values in the wetter region (ca. −164‰) were more negative than those in drier region (ca. −125‰ above 3200 m).We also measured δD values in leaves of six common living trees (values from −287‰ to −193‰) from Gongga Shan, ranging from about 2900-4200 m. The abundance-weighted average values of the n-alkanes (δD_wax) show a strong reverse correlation with sample source elevation (R² 0.78 for soils from Gongga Shan; R² 0.85 for soils from West Kunlun Shan above 3200 m), suggesting that n-alkane δD_wax faithfully records the precipitation δD and that the isotopic altitude effect of precipitation controls δD_wax altitudinal gradients in the mountains. The data show a fairly strong monotonic dependency of n-alkane δD values on elevation for the eastern Plateau, but a complex relationship between n-alkane δD values and elevation for the northwestern Plateau. The δD_wax values at sites below 3200 m from the Kunlun Shan area exhibit an unexpected positive correlation with elevation. The study confirms the potential for using sediment δD_wax values to reconstruct paleo-elevation in wetter regions, but suggests caution in applying the approach to dry regions. Our results also show it is essential to consider the intricacy of the pattern of atmospheric circulation and water sources and their influence on the lapse rate of δD values with elevation.     

Tissue N content and N natural abundance in epilithic mosses for indicating atmospheric N deposition in the Guiyang area,SW China

Tissue N contents and δ¹⁵N signatures in 175 epilithic mosses were investigated from urban to rural sites in Guiyang (SW China) to determine atmospheric N deposition. Moss N contents (0.85–2.97%) showed a significant decrease from the urban area (mean = 2.24 ± 0.32%, 0–5 km) to the rural area (mean = 1.27 ± 0.13%, 20–25 km), indicating that the level of N deposition decreased away from the urban environment, while slightly higher N contents re-occurred at sites beyond 30 km, suggesting higher N deposition in more remote rural areas. Moss δ¹⁵N ranged from −12.50‰ to −1.39‰ and showed a clear bimodal distribution (−12‰ to −6‰ and −5‰ to −2‰), suggesting that there are two main sources for N deposition in the Guiyang area. More negative δ¹⁵N (mean = −8.87 ± 1.65‰) of urban mosses mainly indicated NH₃ released from excretory wastes and sewage, while the less negative δ¹⁵N (from −3.83 ± 0.82‰ to −2.48 ± 0.95‰) of rural mosses were mainly influenced by agricultural NH₃. With more negative values in the urban area than in the rural area, the pattern of moss δ¹⁵N variation in Guiyang was found to be opposite to cities where N deposition is dominated by NO_x–N. Therefore, NH_x–N is the dominant N form deposited in the Guiyang area, which is supported by higher NH_x–N than NO_x–N in local atmospheric deposition. From the data showing that moss is responding to NH_x–N/NO_x–N in deposition it can be further demonstrated that the variation of moss δ¹⁵N from the Guiyang urban to rural area was more likely controlled by the ratio of urban-NH_x/agriculture-NH_x than the ratio of NH_x–N/NO_x–N. The results of this study have extended knowledge of atmospheric N sources in city areas, showing that urban sewage discharge could be important in cities co-generic to Guiyang.     

Chlorine stable isotope studies of old groundwater,southwestern Great Artesian Basin,Australia

Stable Cl isotope ratios (³⁷Cl/³⁵Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl⁻ sources and transport mechanisms. δ³⁷Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl⁻ concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl⁻ in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between δ³⁷Cl and Cl⁻ concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl⁻ concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect. δ³⁷Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl⁻ for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl⁻ is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.     

Stable chlorine isotope evidence for non-marine chloride in Badenian evaporites, Carpathian mountain region

Routine trace-element geochemistry suggests that components in putative marine halite evaporites may be partly of nonmarine origin, but such interpretations are commonly ambiguous. Stable chlorine isotopes may provide a less-ambiguous marker of chloride origin where δ³⁷Cl departs from the range predicted for evaporite formation from seawater. Bedded halite with primary sedimentary textures preserves original δ³⁷Cl values. Measurable change in δ³⁷Cl can be generated by incongruent dissolution of halite, but only if less than half the original halite remains. Badenian (middle Miocene) halite from the Forecarpathian and from the East Slovakian and Transcarpathian basins has a δ³⁷Cl range of – 0.2 to 0.8‰. Two phenomena cannot be explained by simple evaporation of 0.0‰ seawater. At Wieliczka, the Shaft Salt has distinctive δ³⁷Cl values (– 0.2 to 0.0‰) relative to neighbouring salt beds (0.2 to  0.6‰), requiring a large, abrupt input of brine with negative δ³⁷Cl. Halite with high (0.6 – 0.8‰) δ³⁷Cl near the base of the East Slovakian and Transcarpathian evaporites requires a large input of chloride with positive δ³⁷Cl into the basins. Expulsion of basin brine with non-0‰δ³⁷Cl into the evaporite basins may account for the nonmarine chloride sources.     

Germanium isotopic variations in igneous rocks and marine sediments

A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC-ICPMS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of ⁷⁴Ge/⁷⁰Ge ratios are expressed as δ⁷⁴Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2‰ for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4‰ in δ⁷⁴Ge, representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of ⁷⁴Ge/⁷⁰Ge, ⁷³Ge/⁷⁰Ge and ⁷²Ge/⁷⁰Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of δ⁷⁴Ge in igneous rocks is only 0.25‰ without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a δ⁷⁴Ge of 1.3 ± 0.2‰ can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher δ⁷⁴Ge values between 2.0‰ and 3.0‰. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the δ⁷⁴Ge of the seawater to +3.0‰.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to seawater. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have δ⁷⁴Ge values ranging from 0.7‰ to 2.0‰. The variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification.     

Application of sulphur isotope ratios to examine weaning patterns and freshwater fish consumption in Roman Oxfordshire, UK

This study investigates the application of sulphur isotope ratios (δ³⁴S) in combination with carbon (δ¹³C) and nitrogen (δ¹⁵N) ratios to understand the influence of environmental sulphur on the isotopic composition of archaeological human and faunal remains from Roman era sites in Oxfordshire, UK. Humans (n = 83), terrestrial animals (n = 11), and freshwater fish (n = 5) were analysed for their isotope values from four locations in the Thames River Valley, and a broad range of δ³⁴S values were found. The δ³⁴S values from the terrestrial animals were highly variable (−13.6‰ to +0.5‰), but the δ³⁴S values of the fish were clustered and ³⁴S-depleted (−20.9‰ to −17.3‰). The results of the faunal remains suggest that riverine sulphur influenced the terrestrial sulphur isotopic signatures. Terrestrial animals were possibly raised on the floodplains of the River Thames, where highly ³⁴S-depleted sulphur influenced the soil. The humans show the largest range of δ³⁴S values (−18.8‰ to +9.6‰) from any archaeological context to date. No differences in δ³⁴S values were found between the males (−7.8 ± 6.0‰) and females (−5.3 ± 6.8‰), but the females had a linear correlation (R² = 0.71; p < 0.0001) between their δ¹⁵N and δ³⁴S compositions. These δ³⁴S results suggest a greater dietary variability for the inhabitants of Roman Oxfordshire than previously thought, with some individuals eating solely terrestrial protein resources and others showing a diet almost exclusively based on freshwater protein such as fish. Such large dietary variability was not visible by analysing only the carbon and nitrogen isotope ratios, and this research represents the largest and most detailed application of δ³⁴S analysis to examine dietary practices (including breastfeeding and weaning patterns) during the Romano-British Period.     

Calcium isotopes in fossil bones and teeth — Diagenetic versus biogenic origin

We present the first systematic study of Ca isotopes (δ^44/40Ca) in Late Triassic to Late Cretaceous dinosaur bones and teeth (enamel and dentin) from sympatric herbivorous and carnivorous dinosaurs. The samples derive from five different localities, and data from embedding sediments are also presented. Additional δ^44/40Ca in skeletal tissues from modern reptiles and birds (avian dinosaurs) were measured for comparison in order to examine whether the original Ca isotopic composition in dinosaur skeletal apatite was preserved or might have changed during the diagenesis and fossilization process.δ^44/40Ca of fossil skeletal tissues range from −1.62‰ (Tyrannosaurus rex enamel) to +1.08‰ (Brachiosaurus brancai bone), while values in modern archosaur bones and teeth range from −1.63‰ (caiman enamel) to −0.37‰ (ostrich bone). The average δ^44/40Ca of the three types of fossil skeletal tissue analyzed - bone, dentin and enamel - show some systematic differences: while δ^44/40Ca in bone exhibits the highest values, while δ^44/40Ca in enamel has the lowest values, and dentin δ^44/40Ca falls in between. Values of δ^44/40Ca in the remains of herbivorous dinosaurs (0.1-1.1‰) are generally higher than those of bones of modern mammalian herbivores (−2.6‰ to −0.8‰) and from modern herbivorous archosaurs, which exhibit intermediate δ^44/40Ca (−0.8‰ to −0.4‰). These systematic isotopic shifts may reflect physiological differences between dinosaurs, mammals and reptiles representing different taxonomic groups of vertebrates.Systematic offsets in skeletal apatite δ^44/40Ca between herbivorous and carnivorous dinosaurs are not obvious, indicating a lack of a clear-cut Trophic Level Effect (TLE) shift between herbivores and carnivores in dinosaurs. This observation can be explained if the carnivorous dinosaurs in this study fed mainly on soft tissues from their prey and did not ingest hard (calcified) tissue to much extent. The most striking indication that the primary δ^44/40Ca is actually preserved in most of the fossil teeth is a difference in δ^44/40Ca of about 0.35 ± 0.10‰ (1SD) between dentin and enamel, based upon 11 of 16 analyzed dentin-enamel pairs. This difference is close to that found in modern reptiles (0.28 ± 0.05‰), and strongly suggests that this tell-tale signature is a primary feature of the fossilized dinosaur material as well. Furthermore, simple mass balance calculations show that changes of the original δ^44/40Ca in bones and teeth by diagenetically-formed calcium-bearing minerals are either small or would require implausible high original δ^44/40Ca values in the skeletal apatite.     

An evaporated seawater origin for the ore-forming brines in unconformity-related uranium deposits (Athabasca Basin, Canada): Cl/Br and δCl analysis of fluid inclusions

Analyses of halogen concentration and stable chlorine isotope composition of fluid inclusions from hydrothermal quartz and carbonate veins spatially and temporally associated with giant unconformity-related uranium deposits from the Paleoproterozoic Athabasca Basin (Canada) were performed in order to determine the origin of chloride in the ore-forming brines. Microthermometric analyses show that samples contain variable amounts of a NaCl-rich brine (Cl concentration between 120,000 and 180,000 ppm) and a CaCl₂-rich brine (Cl concentration between 160,000 and 220,000 ppm). Molar Cl/Br ratios of fluid inclusion leachates range from ∼100 to ∼900, with most values between 150 and 350. Cl/Br ratios below 650 (seawater value) indicate that the high salinities were acquired by evaporation of seawater. Most δ³⁷Cl values are between −0.6‰ and 0‰ (seawater value) which is also compatible with a common evaporated seawater origin for both NaCl- and CaCl₂-rich brines.Slight discrepancies between the Cl concentration, Cl/Br, δ³⁷Cl data and seawater evaporation trends, indicate that the evaporated seawater underwent secondary minor modification of its composition by: (i) mixing with a minor amount of halite-dissolution brine or re-equilibration with halite during burial; (ii) dilution in a maximum of 30% of connate and/or formation waters during its migration towards the base of the Athabasca sandstones; (iii) leaching of chloride from biotites within basement rocks and (iv) water loss by hydration reactions in alteration haloes linked to uranium deposition.The chloride in uranium ore-forming brines of the Athabasca Basin has an unambiguous dominantly marine origin and has required large-scale seawater evaporation and evaporite deposition. Although the direct evidence for evaporative environments in the Athabasca Basin are lacking due to the erosion of ∼80% of the sedimentary pile, Cl/Br ratios and δ³⁷Cl values of brines have behaved conservatively at the basin scale and throughout basin history.