Average δ18O Values Research Articles

Oxygen three-isotope ratios of silicate particles returned from asteroid Itokawa by the Hayabusa spacecraft: A strong link with equilibrated LL chondrites

Oxygen three-isotope ratios of four mineral phases (olivine, low-Ca and high-Ca pyroxene, and plagioclase) in seven silicate particles from asteroid Itokawa were measured to investigate oxygen isotope systematics of asteroidal materials. In order to obtain highly precise and accurate oxygen isotope ratios (∼±0.3‰ in δ18O), we used a new indium mounting method that minimizes potential instrumental mass fractionation due to surface topography. The oxygen isotope data of the seven Itokawa particles are distributed above the terrestrial fractionation line with indistinguishable ΔO17(=δO17−0.52×δO18) values of +1.34±0.36‰ (2SD; n=22), which is similar to those of Itokawa particles analyzed by Yurimoto et al. (2011) and Nakamura et al. (2012). However, δ18O values of the seven Itokawa particles are tightly clustered (+3.9‰ to +5.9‰ VSMOW) compared to previous analyses of the Itokawa particles (+1.4‰ to +8.8‰).The average Δ17O values and 2SD of the two equilibrated ordinary chondrites (EOCs), which were analyzed for comparison, are +1.31±0.49‰ for St. Séverin (LL6) and +0.83±0.64‰ for Guareña (H6). The Δ17O values of the seven Itokawa particles are similar to those of St. Séverin. The average bulk δ18O value of the seven Itokawa particles is estimated as +4.8‰, which is within the δ18O range of bulk LL5 and LL6 chondrites. It is suggested that Itokawa particles are similar to those of equilibrated LL chondrites, which strengthens a link between asteroid Itokawa and equilibrated LL chondrites that fell to Earth.The seven Itokawa particles and two EOCs have δ18O values of high-Ca pyroxene that are lower than those of olivine, indicating that oxygen isotopes between the two mineral phases are not in equilibrium. Apparent oxygen isotope equilibrium temperatures estimated from a pair of low-Ca pyroxene and olivine are excessively higher than those from a pair of plagioclase and high-Ca pyroxene (∼800 °C) and peak metamorphic temperatures. Oxygen isotopes in olivine and low-Ca pyroxene do not represent equilibrium. On the other hand, oxygen isotope ratios in high-Ca pyroxene and plagioclase were mass-dependently fractionated accompanied by recrystallization (and possibly by diffusion) during thermal metamorphism. Values of δ18O in high-Ca pyroxene were lowered by exchange, which may have resulted in the reverse δ18O fractionation between olivine and high-Ca pyroxene.

STABLE ISOTOPE EVIDENCE FOR IMPACT-RELATED PSEUDOTACHYLITE FORMATION AT VREDEFORT BY LOCAL MELTING OF DRY ROCKS

We have determined the major element composition, δ18O and δD values, and water content of impact-related granophyre, and pseudotachylite, from various Vredefort Dome localities, aiming to constrain the mechanism of melt formation and the relationship between pseudotachylite and granophyre. The granitoid gneisses and the pseudotachylites they host have almost identical average δD and δ18O values (−67 and 8.6‰, and −67 and 8.4‰, respectively). The water contents of the pseudotachylites are extremely low, consistent with the isolation of the pseudotachylites from free water during and since their formation. There is a bimodal distribution of water content in pseudotachylites, with one group averaging 0.28 ± 0.03 weight % (n = 9) and the other 0.59 ± 0.06 weight % (n = 9). The Vredefort granophyre, which has been interpreted as the pooled product of impact melting, has average δD and δ18O values of −69‰ and 7.6‰, respectively (n = 2) and also has a very low water content (0.23 weight %). Differences in major element and O-isotope composition between the granophyre and the pseudotachylites are not consistent with a simple relationship, but can be explained by a higher component of greenstone in the granophyre. A strong correlation between host and pseudotachylite δ18O values is consistent with a system where the melt composition is controlled by the immediate surroundings. The pseudotachylites with higher water content have slightly higher δ18O values (9.1 compared to 8.1‰). This is opposite to the relationship predicted if water content is related to the proportion of biotite entering the melt. It is possible, instead, that this relates to the incorporation of higher proportions of material altered at low temperature in the high-water group.

The origin and evolution of low-δ18O magma recorded by multi-growth zircons in granite

We for the first time reported low-δ18O granites within craton in Bengbu uplift, southeast margin of North China Craton. Integrated ion microprobe study of δ18O and U–Pb age on single zircon grains and zircon populations, gives direct evidence for the origin and later magmatic evolution of the low-δ18O granites. Three types of zircon domains are recognized: inherited magmatic core (Type I), inherited metamorphic core/mantle (Type II) and overgrown magmatic rim (Type III). Type I zircons were formed in Neoproterozoic, and have the average δ18O values of 5.3±0.6‰ around mantle value. Triassic Type II zircons have extremely low and highly heterogeneous δ18O values ranging from −9.4‰ to 8.6‰, consistent with the values and variations exhibited by metamorphic zircons from Dabie–Sulu orogen. This feature argues for a Triassic metamorphic origin for the 18O-depletion signature of zircons from ultrahigh pressure metamorphic rocks in Dabie–Sulu orogen. Jurassic Type III zircons have isotopically homogeneous δ18O values of 3.3±0.5‰. Collectively, zircon populations recorded the δ18O variations from source to low-δ18O granite. The primary protolith of Jurassic granites in Bengbu uplift is a Neoproterozoic granitic intrusion in South China Block. Later high-temperature meteoric hydrothermal alteration has lowered the whole-rock δ18O values in various degrees. The low-δ18O protolith was then buried to the middle-lower crust by Triassic continental subduction and Jurassic anatexis produced the low-δ18O granitic magma. Moreover, δ18O values of Type III zircons are isotopically lighter than previously reported values of magmatic garnets in Jurassic granites, indicating later magmatic evolution of low-δ18O magma toward high δ18O values probably as a result of infiltration of high-δ18O fluid from wall-rocks.

Oxygen isotope fractionation between aragonite and seawater: Developing a novel kinetic oxygen isotope fractionation model

Oxygen isotope fractionation factors between aragonite and seawater are studied at T=25–55°C and pH=7.4–8.1 in a set of ‘free-drift’ precipitation experiments with various CO2-degassing rates (0–75cc/min). The measured fractionation factors correlate weakly with degassing rate, but strongly with temperature in the following form (R2=0.998):1000lnαaragonite–seawater=22.5(±0.5)(103/T)-46.1(±1.6)where α is the fractionation factor and T is the temperature in Kelvin. Along with results from previous studies, these experiments help calibrate two extreme cases of a new kinetic model – extremely fast (e.g., during spontaneous precipitation) and slow aragonite precipitation processes (e.g., during slow ‘free-drift’ precipitation experiments) – at pH >7.5, T=0–55°C and salinity=∼5–39.6‰. The model assumes little isotopic fractionation between DIC species and aragonite during their attachment and detachment on aragonite surfaces at low temperatures and the δ18O value of aragonite equals the average δ18O value of all contributing DIC species. During the fast precipitation of aragonite, the contribution of each DIC species to the δ18O value of aragonite is proportional to its concentration, whereas in a slow precipitation process when the system reaches a steady state, the contribution is determined by both its concentration and a modifying factor. Although the physical meaning of this modifying factor depends on the method of derivation and other assumptions made, three different derivations based on precipitation/dissolution kinetics, isotope exchange reactions, and isotope disequilibrium among DIC species all lead to the same formulation. At pH >7.5, the natural logarithm of the modifying factor (kHCO3-) for adjusting the contribution of the bicarbonate ion relative to the carbonate ion is calibrated, using experimental data covering a range of temperature, salinity, pH, and precipitation rate, as follows:lnkHCO3-=(2.22±0.08)×(pH-pH0)+(846±78)·S-(40.3±2.1)×103RT-(0.30±0.03)·S-(4.7±1.2)where S is the salinity, R is the gas constant, pH0 is the pH difference between free scale and the scale used to calculate the concentration of DIC species. Quantitative evaluation of these two extreme cases using our model suggests that oxygen isotope fractionation between aragonite and water varies significantly with a pH change between 7.5 and 10 (e.g., ∼4‰ at 25°C), but little with a salinity change between 5‰ and 35‰ (<0.1‰ at 25°C) during extremely fast precipitation, whereas during slow precipitation little variation occurs with pH varying between 7.5 and 10 at a given salinity (<0.3‰ in δ18O at T=0–55°C). This model connects aragonite precipitation kinetics with oxygen isotope fractionation between aragonite and water, and provides a framework to reconcile the discrepancy among experimental calibrations of aragonite–water fractionation in different labs and empirical calibrations using biogenic aragonites by variations in precipitation conditions (including pH, salinity and precipitation rate).

Environmental Factors Controlling the 13C and 18O Variations of Recent Fluvial Tufas: A 12-Year Record from the Monasterio de Piedra Natural Park (Ne Iberian Peninsula)

Abstract A 12-year study (2000–2012) of the stable isotopes of recent fluvial tufas has been performed in the Monasterio de Piedra Natural Park (River Piedra, NE Iberian Peninsula), an area of Mediterranean climate with intense tufa deposition since the Pleistocene. The biannual monitoring of the tufa calcite δ13C and δ18O signatures and of the water δ18O value has demonstrated a clear seasonal pattern for the tufa-calcite oxygen isotope composition, with less negative values for cool periods, which is consistent with regional temperature oscillations. The tufa calcite δ13C values only occasionally exhibit a pattern, which is inconclusive and opposite to the δ18O clear pattern. The water δ18O signature has a distinct seasonal pattern, also opposite to the tufa-calcite δ18O clear pattern. The temperature calculations from the tufa-calcite δ18O values and average water δ18O values agree with the seasonal pattern of measured water temperatures, with a mean difference of 2.3 °C between the calculated and measured temperatures. The anomalous values recorded for both the water and the tufa-calcite oxygen isotope composition during a two-year period are characterized by smoothing of the seasonal variations of the tufa calcite and by reversal of the water pattern. There is also an increase in the differences between the measured and estimated temperatures. The detected anomaly is roughly synchronous with a change in the isotopic signature of the regional precipitation δ18O values. This contribution demonstrates that tufas in this depositional and climatic context are good indicators of seasonal temperature oscillations, and that tufas can also record other interannual environmental changes such as variations in the isotopic composition of precipitation. These results can be extrapolated to ancient tufa systems developed in similar contexts, which would expand the potential of tufas as high-resolution records of paleoenvironmental conditions.

Unmixing of stable isotope signals using single specimen δ18O analyses

AbstractThe resolution at which foraminiferal stable isotopes are applied in paleo‐environmental studies is ever increasing, resulting in continuous sampling of sediment cores. The resolution of such continuously sampled records depends on the rate of sedimentation of foraminiferal shells in its relation to the intensity of bioturbation. Bioturbation essentially mixes sediment layers of different age, altering the primary climate signal, thereby impacting the accuracy of both the timing and magnitude of reconstructed climate changes. A new approach to assess and correct the impact of bioturbation is investigated here, based on the δ18O of individual specimens of planktonic foraminifera Globorotalia inflata from a series of boxcore samples in the Eastern North Atlantic. Average δ18O values decrease southward from 1.62 to 1.07‰ with the exception of site T86‐11 (1.35‰). The δ18O distribution of each station can be fitted with a uni‐ to polymodal distribution. A nonunimodal distribution strongly suggests admixing of bioturbated individuals. Quantification of these distributions allows deconvolving the original and bioturbated signals and subsequently provides a correction for bioturbation.

The application of an oxygen isotope aridity index to terrestrial paleoenvironmental reconstructions in Pleistocene North America

Geochemical tools, including the analysis of stable isotopes from fossil mammals, are often used to infer regional climatic and environmental differences. We have further developed an oxygen isotope aridity index and used oxygen (δ18O) isotope values and carbon (δ13C) isotope values to assess regional climatic differences between the southeastern and southwestern United States during the Pleistocene. Using data collected from previously published studies, we assigned taxa to evaporation-sensitivity categories by quantifying the frequency and magnitude of aridity index values (i.e., an average taxon δ18O value minus a site specific proboscidean δ18O value). Antilocapridae, Camelidae, Equidae, and Cervidae were identified as evaporation-sensitive families, meaning that a majority of their water comes from the food they eat, thus indicating that they are more likely to capture changing climatic conditions. Bovidae, Tayassuidae, and Tapiridae were identified as less sensitive families, possibly because of increased or more variable drinking behavior. While it is difficult to tease out individual influences on δ18O values in tooth enamel, the use of an aridity index will provide a more in-depth look at relative aridity in the fossil record. Greater aridity index values in the Southwest suggest a drier climate than in the Southeast during the Pleistocene, and δ13C values suggest that diet does not determine evaporation sensitivity. The combination of more-positive δ13C values and the lack of forest indicator taxa in the Southwest suggest that landscapes were more open than in the Southeast. Inferred higher aridity in the Southwest may indicate that aridity or seasonal aridity/precipitation, not temperature orpCO2, was a greater driver of C4abundance during the Pleistocene. Collectively, these data suggest that regional climatic and environmental interpretations can be improved by using an aridity index and a more detailed understanding of mammalian paleobiology.

Quaternary coral reefs of the Red Sea coast, Egypt: diagenetic sequence, isotopes and trace metals contamination

This study focuses on the diagenetic sequence under marine and meteoric conditions as well as isotopes and trace metals contamination in Quseir and Gebel Zeit areas along the Egyptian Red Sea coast through a series of modern and fossil corals, Porites lutea and Favites pentagona. The diagenetic sequence begins with deposition of thin fringes of syntaxial aragonite and micritic high-magnesian calcite in the modern corals to completely altered Porites and partially altered Favites to low-magnesium calcite in the oldest Pleistocene unit. Average δ18O and δ13C values of Pleistocene corals in the two studied areas were lower than those of modern corals. Values of modern corals and lower fossil unit indicated coralline limestone, while those of middle and upper fossil units indicated fresh water influences. Average values of trace metals in modern corals were higher than those of Pleistocene counterpart except for Mn. Modern coral samples recorded enrichment in the average values of Pb, Zn, and Mn at Quseir area and enrichment in Co, Cu, and Ni at Gebel Zeit area. This may be attributed mostly to different tourist activities, landfill due to increase urbanization and nearby of Quseir area from the old phosphate harbor at El Hamrawin area, as well as oil exploration and production activities in the Gulf of Suez area. Also, results indicated that most samples of Porites have high concentration of trace metals than in Favites, especially in Cu, Zn, Mn, and Pb. This may due to high amounts of intergranular porosity and high total surface area of Porites in contrast to Favites.

Quaternary interglacial environmental stability in San Salvador Island (Bahamas): A land snail isotopic approach

The 13C/12C and 18O/16O ratios of modern and fossil shells of two ecologically-distinct land snail taxa (Cerionidae and Annularidae) from San Salvador Island were analyzed to estimate the interglacial Pleistocene and Holocene paleoenvironmental conditions in the Bahamas. The average δ13C value of Cerion was −5.0±1.8‰ for modern (~0–0.1ka) specimens, −4.3±0.7‰ for late Holocene (~2–4.6ka) individuals, −3.4±1.6‰ for middle Holocene (~5–6.3ka) shells, −4.0±1.5‰ for ~85 or ~110ka specimens, and −3.6±0.4‰ for individuals that grew during OIS 5e (~125ka). Annularidae specimens displayed average δ13C values of −6.3±2.5‰, −6.1±1.0‰, −2.4±1.0‰, −4.4±1.1‰, and −5.0±2.9‰, respectively. These results suggest that snails consumed C3 and C4 plants arbitrarily through time, however, higher proportion of C4 plants may have been consumed in the past compared to the present. Cerion was ~1‰ higher in average δ13C value than Annularidae specimens for some time-intervals, indicating greater C4 plant consumption and/or higher ingestion of carbonate-rich sediments. The average δ18O value of Cerion was −0.5±0.8‰ for modern individuals, −0.5±0.5‰ for late Holocene specimens, −0.8±0.2‰ for middle Holocene shells, −0.4±0.6‰ for ~85 or ~110ka specimens, and −0.3±0.6‰ for OIS 5e individuals. Annularidae individuals exhibited average δ18O values of −0.7±0.8‰, −0.5±0.9‰, −0.9±0.3‰, −0.3±0.6‰, and −0.9±0.9‰, respectively. These results point to similar atmospheric conditions over time-intervals when shell was deposited. The shell δ18O values did not significantly differ between species despite the apparent differences in lifestyle. Calculations from a snail evaporative steady-state flux balance model and proxy data suggest that modern individuals deposited shell at air temperatures of ~24–25°C, ambient water δ18O values between −4.5‰ and −3.5‰ (SMOW), and relative humidity of ~87–89%. Late to middle Holocene and interglacial late Pleistocene shells appear to have precipitated under a similar environmental setting. Overall, atmospheric conditions during these Quaternary interglacial time-intervals at coastal sites of San Salvador were largely comparable to the present. This apparent environmental similarity is consistent with the relatively comparable snail richness and diversity observed at those same locales and time-intervals.

The Impact of Depositional Events and Burial Rate On Carbonate-Silica Diagenesis In A Middle Jurassic Stromatactis Carbonate Mud Mound, Sainte-Baume Massif, Se France

Abstract Understanding the processes and conditions of chertification in carbonates is a challenging problem to assess the marine silica cycle. This contribution highlights parameters of internal silica recycling processes from their biogenic source to quartz cementation by describing a unique case of carbonate–silica diagenesis in a Mesozoic mud mound. Stromatactis carbonate mud mounds exposed in the Sainte-Baume Massif (Basse Provence, France) developed in an outer-shelf environment during the late Aalenian concavum ammonite Zone (Middle Jurassic). They form part of a cherty succession punctuated by hardgrounds and stratigraphically condensed intervals, interpreted to record deepening episodes. These mud mounds, rich in siliceous sponge spicules and stromatactis, are similar to their widespread Paleozoic counterparts but are particular in being extensively silicified. Carbonate–silica paragenesis forms a polymud fabric including four microcrystalline carbonates (M1 to M4; all low-Mg calcite, LMC), five spar cements (C1 to C5; C1 with high-Mg calcite, HMC, precursor mineralogy), and four silica phases (S1 to S4; S1 to S3 replacive chalcedony after opal-CT, S4 euhedral quartz cement). M1, assumedly related to the degradative calcification of siliceous sponges, forms a labyrinthine network into which M2 to M4 were infiltrated. Spar cements C1 to C3 include a successive decrease of δ18 O at fairly invariable δ13 C, interpreted to represent marine to shallow burial conditions, suggesting burial and successive transformation of marine bottom waters. Temperature estimation, based on the average δ18O value of C4 (δ18O −7.9‰), indicates a temperature of around 50°C at a maximum burial depth of about 1000 meters. Replacive chalcedony (S1 to S3) occurs between calcite cementation C3 and C4 in combination with traces of corrosion on C3. Mg-calcite was preferably replaced, particularly the precursor of cement C1 that surrounds peloids of M2, and preserved some ghost structures. Corrosion and substrate selectivity suggest that acidification, Mg-hydroxyl complexes, and surface area are the triggers for flocculation of a silica gel. Because dissolution of opaline sponge spicules started almost contemporaneously with C1, the dissolved silica was retained in a connate fluid as phases C1 to C3 were precipitated, and stagnant conditions prevailed for around 1 Ma. Bulk δ18O/δD values (S1–S3) below the marine chert line, together with the range of S1–S3 δ18O (δ18O SMOW = 28.3‰ ± 1.0) suggest silica flocculation in a shallow burial environment at temperatures of 25–30°C. The establishment of acidic conditions might have been favored by sulfide-oxidizing micro-organisms. Numerical simulation of early diagenetic silica flux in such marine sediments is consistent with the interpretation that small grain-size changes and fluid barriers (that create diffusion-controlled conditions), such as those induced by stratigraphic condensation, are key parameters to retain pore-water dissolved silica over geologic time. Inversely, Paleozoic stromatactis carbonate mud mounds typically form part of large-scale coarsening- and shallowing-upward successions that correspond to rapid burial. These conditions support convergent pore-water flux and drainage of pore-water dissolved silica in the shallow burial realm. Hence, spicule-rich mounds should tend to silicify if they form part of a condensed section.

Palaeoprecipitation record using O-isotope studies of the Himalayan Foreland Basin sediments, NW India

The Himalayan foreland basin sediments were deposited during a crucial time period in the Himalayan orogeny. This period, in the Tertiary, has become major focus of research because of the large scale uplift of the Himalayan range which has greatly modified the climate of the Asian continent. The present South Asian monsoonal climate is believed to be a consequence of Himalayan uplift during this period. Also the Himalayan orogeny comprises a major regional tectonic event of the Earth. Hence, the study of these Himalayan foreland basin sediments are not only significant in revealing the palaeoclimatic history of the Asian continent but also paves a way for understanding of the effects of Himalayan orogeny on global climate. Here we report a palaeoprecipitation history of the Ramnagar sub-basin since Miocene times and its regional correlation to other sub-basins of the Himalayan mountain belt. An attempt has been made to use oxygen isotope ratios of pedogenic carbonates in Siwalik palaeosols as proxy palaeo-recorders to deduce the palaeoclimatic history of the Jammu region in the Ramnagar sub-basin for the first time.A total of 141 pedogenic carbonates have been analyzed from three Siwalik sections covering a lateral stretch of approximately 40-50Km along strike. The δ18O values vary from −12.31‰ to −5.89‰ and, when plotted against age, indicate that δ18O values, on average, become ~2‰ enriched from the oldest to the youngest analyzed sediments except for largely depleted δ18O values around 10Ma, 5Ma and 1.8Ma. This ~2‰ increase in calculated δ18O values could have been possible by an ~3°C increase in mean annual air temperature; conversely the largely negative δ18O values indicate periods of intense rainfall or stronger monsoonal conditions. An increase of 2.6‰ in δ18O values has been found from the 10Ma rainfall event to that at 1.8Ma which indicates a reduction of ~173mm in rainfall intensity and amount. Hence, precipitation reconstructions indicate a progressive increase in aridity from ~12Ma to Recent excluding a short term increases in rainfall or monsoon intensity at around 10Ma, 5Ma and 1.8Ma.This pattern of palaeoprecipitation change is similar to the changes documented in other sub-basins of the Himalayan mountain range. Further, monsoon intensification at around 10Ma, 5Ma and 1.8Ma is also observable in various other proxy records, thereby suggesting the role of significant regional tectonism in the evolution and intensification of the present South Asian monsoon climate. Our results indicate that from the Mid-Miocene to Recent, except for the periods of largely depleted δ18O values, the average δ18O value increased by ~2‰. The general increase in average δ18O values from Lower to Upper Siwalik sediments can be explained by changes in both continentality and latitude of the moisture source. It seems likely that this long term increase in δ18O values was due to the uplift of the Himalayas and the Tibetan Plateau in the north which acted as a barrier to the passage of airmasses from Central Asia in the north, thereupon with time restricting the source of airmasses to the south from the Indian Ocean.

Evaporative enrichment of oxygen-18 and deuterium in lake waters on the Tibetan Plateau

Stable isotopes (δ18O and δD) are useful tracers for investigating hydrologic and climatic variability on a variety of temporal and spatial scales. Since the early isotopic studies on mountainous glaciers in the late 1960s, a great deal of information has been generated on the isotopic composition of rainfall, snow, ice, surface waters, and lake carbonate sediments across the Tibetan Plateau. However, measurements of δ18O and δD values of lake water are scarce. Here we present a new dataset of δ18O and δD values of lake waters collected from 27 lakes across the plateau during a reconnaissance survey in summer 2009. δ18O and δD values of lake water range from −19.9 to 6.6‰ and from −153 to −16‰, respectively. The average values of δ18O and δD are −6.4 and −72‰, considerably greater than those of precipitation observed in this region. The derived Tibetan lake water line, δD = 5.2δ18O − 38.9, is significantly different from the global meteoric water line. Most of the lakes, including some freshwater lakes, contain water with negative values of d-excess (d). There is a negative correlation between d and total dissolved solids (TDS). Each of these findings indicates that evaporation-induced isotopic enrichment prevails in Tibetan lakes. Moreover, we develop an isotope modeling scheme to calculate E/P ratios for Tibetan lakes, using a combination of existing isotopic fractionation equations and the Rayleigh distillation model. We use the intersection of the local evaporation line and GMWL as a first approximation of δ18O and δD values of lake water inputs to infer an E/P ratio for each lake. Our modeling calculations reveal that although variable from lake to lake, the water budget across the plateau is positive, with an average E/P of 0.52. This is in good agreement with other observational and model data that show varying degrees of increases in lake size from satellite imagery and significant decreases in lake salinity in many lakes on the plateau over the last several decades. Together with the new isotopic dataset, the proposed modeling framework can be used to examine and quantify past changes in a lake’s hydrologic balance from the isotopic record of downcore carbonate sediments in the region.

Syn-extensional plutonism and peak metamorphism in the Albion-Raft River-Grouse Creek metamorphic core complex

The Cassia plutonic complex (CPC) is a group of variably deformed, Oligocene granitic plutons exposed in the lower plate of the Albion–Raft River–Grouse Creek (ARG) metamorphic core complex of Idaho and Utah. The plutons range from granodiorite to garnet-bearing, leucogranite, and during intrusion, sillimanite-grade peak metamorphism and ductile attenuation occurred in the country rocks and normal-sense, amphibolite-grade deformation took place along the Middle Mountain shear zone. U-Pb zircon geochronology from three variably deformed plutons exposed in the lower plate of the ARG metamorphic core complex revealed that each zircon is comprised of inherited cores (dominantly late Archean) and Oligocene igneous overgrowths. Within each pluton, a spread of concordant ages from the Oligocene zircon overgrowths is interpreted as zircon recycling within a long-lived magmatic system. The plutons of the CPC have very low negative whole rock εNd values of −26 to −35, and initial Sr values of 0.714 to 0.718, consistent with an ancient, crustal source. Oxygen isotope ratios of the Oligocene zircon overgrowths from the CPC have an average δ18O value of 5.40 ± 0.63 permil (2SD, n = 65) with a slight trend towards higher δ18O values through time. The δ18O values of the inherited cores of the zircons are more variable at 5.93 ± 1.51 permil (2SD, n = 29). Therefore, we interpret the plutons of the CPC as derived, at least in part, from melting Archean crust based on the isotope geochemistry. In situ partial melting of the exposed Archean basement that was intruded by the Oligocene plutons of the CPC is excluded as the source for the CPC based on field relationships, age and geochemistry. Correlations between Ti and Hf concentrations in zircons from the CPC suggest that the magmatic system may have become hotter (higher Ti concentration in zircon) and less evolved (lower Hf in zircon concentration) through time. Therefore, the CPC represents prolonged or episodic magmatism system (32-25 Ma), and the intrusions were each accompanied by sillimanite-grade deformation and extension. The Oligocene magmatism and peak metamorphism preserved in the ARG metamorphic core complex are likely related to regional trends in mantle-derived magmatism that led to protracted heating, melting and mobilization of the deeper crust.

An oxygen isotope study of two contrasting orogenic vein gold systems in the Meguma Terrane, Nova Scotia, Canada, with implications for fluid sources and genetic models

Sampling of quartz vein material from two gold deposits of similar geological setting but different ages (The Ovens, 408 Ma; Dufferin, 380 Ma) in the Meguma Terrane of Nova Scotia has been done to compare and contrast their δ18Oquartz signatures. Despite different ages of formation, quartz from all vein types in each of the deposits (i.e. saddle-reef, bedding-concordant leg reefs, en echelon, discordant) shows limited intra-deposit variation with similar average δ18O values of +15.2 ± 0.9‰ (n = 16) for The Ovens and +15.7 ± 0.6‰ (n = 12) for Dufferin. Using an average δ18O value of +15.4‰ for the two deposits, the corresponding δ18OH2O values, calculated for 400°C and 350°C based on constraints from mineral assemblages and fluid inclusion studies, indicate averages of 11.4 ± 0.2‰ and +10.2 ± 0.2‰, respectively. The isotopic data indicate that: (1) the vein-forming fluids have a metamorphic signature, (2) all vein types in the two deposits represent formation from a single, isotopically homogeneous fluid, and (3) a fluid of similar isotopic signature was generated by two contrasting tectono-thermal events in the Meguma Terrane that were separated by 30 Ma. Integration of these results with previously published data for 14 Meguma gold deposits indicate a general stratigaphic dependence in δ18OH2O values for deposits when arranged in their relative stratigraphic position such that δ18OH2O values increase upwards in the stratigraphy. This apparent trend cannot be explained by models involving either fluid mixing or cooling of the vein-forming fluids, but instead is modelled using fluid/rock interaction taking into account a change in the modal mineralogy of the metasedimentary rocks upwards in the stratigraphy.

The Origin of the Millimeter-Scale Lamination in the Neoproterozoic Lower Beck Spring Dolomite: Implications for Widespread, Fine-Scale, Layer-Parallel Diagenesis in Precambrian Carbonates

Abstract The Neoproterozoic lower Beck Spring Dolomite exhibits a prominent, millimeter-scale, light–dark, planar lamination commonly interpreted as a primary depositional fabric. Microscopic examination reveals that the dark laminae are composed of micrite and contain a micrometer-scale, wavy fabric, whereas the light laminae are composed of relatively coarse-crystalline pseudospar that engulfs, surrounds, and crosscuts the dark laminae. Nearly all of the light laminae surround, or occur in proximity to, elongate (long axis fabric-parallel), spar-filled cavity structures. The average δ18O values for dark and light laminae are −1.8‰ (std. dev. 1.6) and −3.0‰ (std. dev. 1.3) VPDB, respectively. Cavity-filling cements are even more depleted in 18O, with an average isotopic value of −6.2‰ (std. dev. 1.1). These characteristics suggest that the coarse-crystalline, light laminae are the result of the preferential, aggrading neomorphism of a thinly laminated, micritic host rock and that the (now cemented) cavity structures acted as conduits for the fluid responsible for recrystallization. The oxygen isotope distributions are consistent with progressive recrystallization by an 18O-depleted fluid, likely of meteoric origin. Therefore, we interpret the prominent millimeter-scale, light–dark layering in the lower Beck Spring Dolomite to result from widespread, laminae-parallel, diagenetic processes—perhaps controlled by fine-scale layer-parallel differences in initial porosity and permeability—and not from primary sedimentary processes. Extensive, layer-parallel diagenesis on such a fine scale is rare in Phanerozoic shallow marine deposits, due to disruption by bioturbation. However, before the advent of metazoan burrowing, fine-scale, layer-parallel fluid flow would have been more common. Thus, the diagenetic fabric of the lower Beck Spring Dolomite may represent a mode of carbonate diagenesis restricted to the Precambrian. Similar diagenesis may also occur in Phanerozoic environments devoid of vertical bioturbation.

Lessons learned from oxygen isotopes in modern precipitation applied to interpretation of speleothem records of paleoclimate from eastern Asia

Variability in oxygen isotope ratios collected from speleothems in Chinese caves is often interpreted as a proxy for variability of precipitation, summer precipitation, seasonality of precipitation, and/or the proportion of 18O to 16O of annual total rainfall that is related to a strengthening or weakening of the East Asian monsoon and, in some cases, to the Indian monsoon. We use modern reanalysis and station data to test whether precipitation and temperature variability over China can be related to changes in climate in these distant locales. We find that annual and rainy season precipitation totals in each of central China, south China, and east India have correlation length scales of ∼500km, shorter than the distance between many speleothem records that share similar long-term time variations in δ18O values. Thus the short distances of correlation do not support, though by themselves cannot refute, the idea that apparently synchronous variations in δ18O values at widely spaced (>500km) caves in China are due to variations in annual precipitation amounts. We also evaluate connections between climate variables and δ18O values using available instrumental measurements of δ18O values in precipitation. These data, from stations in the Global Network of Isotopes in Precipitation (GNIP), show that monthly δ18O values generally do not correlate well with either local precipitation amount or local temperature, and the degree to which monthly δ18O values do correlate with them varies from station to station. For the few locations that do show significant correlations between δ18O values and precipitation amount, we estimate the differences in precipitation amount that would be required to account for peak-to-peak differences in δ18O values in the speleothems from Hulu and Dongge caves, assuming that δ18O scales with the monthly amount of precipitation or with seasonal differences in precipitation. Insofar as the present-day relationship between δ18O values and monthly precipitation amounts can be applied to past conditions, differences of at least 50% in mean annual precipitation would be required to explain the δ18O variations on orbital time scales, which are implausibly large and inconsistent with published GCM results. Similarly, plausible amplitudes of seasonal cycles in amounts or in seasonal variations in δ18O values can account for less than half of the 4–5‰ difference between glacial and interglacial δ18O values from speleothems in China. If seasonal cycles in precipitation account for the amplitudes of δ18O values on paleoclimate timescales, they might do so by extending or contracting the durations of seasons (a frequency modulation of the annual cycle), but not by simply varying the amplitudes of the monthly rainfall amounts or monthly average δ18O values (amplitude modulation). Allowing that several processes can affect seasonal variability in isotopic content, we explore the possibility that one or more of the following processes contribute to variations in δ18O values in Chinese cave speleothems: different source regions of the precipitation, which bring different values of δ18O in vapor; different pathways between the moisture source and the paleorecord site along which exchange of 18O between vapor, surface water, and condensate might differ; a different mix of processes involving condensation and evaporation within the atmosphere; or different types of precipitation. Each may account for part of the range of δ18O values revealed by speleothems, and each might contribute to seasonal differences between past and present that do not scale with monthly or even seasonal precipitation amounts.

A 70-yr record of oxygen-18 variability in an ice core from the Tanggula Mountains, central Tibetan Plateau

Abstract. A 33 m ice core was retrieved from the Tanggula Mts, central Tibetan Plateau at 5743 m a.s.l. in August 2005. Annual average δ18O values were determined for the upper 17 m depth (14.6 m w.eq.), representing the time series since the mid-1930s. Data are compared to previous results of an ice core from Mt. Geladaindong, 100 km to the northwest, for the period 1935–2003. During the time 1935–1960, δ18O values differed by 2–3‰ between the two ice cores, with generally lower ratios preserved in the Tanggula 2005 core. Differences in interannual variability and overall average ratios between the two study locations highlight the spatially variable climate controls on ice core isotope ratios within the boundary of monsoon- and westerly-impacted regions of the central Tibetan Plateau. Average annual net accumulation was 261 mm w.eq. for the period 1935–2004. The overall average δ18O value was −13.2‰ and exhibited a statistically significant increase from the 1935–1969 average (−13.7‰) to the 1970–2004 average (−12.6‰). Despite the observed increase in isotope ratios, isotopic temperature dependence was not evident, based on comparison with long-term data from meteorological stations to the north and southwest of the study location. Lack of correlation between average δ18O values and temperature is likely due to monsoon influence, which results in relatively greater isotopic depletion of moisture during the warm season. Evidence of monsoon impacts on precipitation in the central Tibetan Plateau has been previously documented, and statistically significant negative correlation (r=−0.37, p&lt;0.01) between the annual average ice core δ18O values and North India monsoon rainfall was observed for the period 1935–2004. Although the δ18O data agree well with the monsoon rainfall amount, no significant correlation was observed between the core accumulation and the monsoon rainfall amount. Previous model and observational results suggest monsoon impact on δ18O in precipitation may extend beyond the immediate extent of heavy monsoon rainfall, reaching the central Tibetan Plateau. These results provide evidence that the δ18O variability at this study location may be sensitive to southern monsoon intensity.

Spatio-temporal variation of stable isotopes in precipitation in the Heihe River Basin, Northwestern China

An intensive investigation of the spatial and temporal variations of δD and δ 18O in precipitation was conducted during 2002–2004 in six sites in the Heihe River Basin, Northwestern China. The δD and δ 18O values for 301 precipitation samples ranged from +59 to −254 and +6.5 to −33.4‰, respectively. The relationship between δD and δ 18O defines a well-constrained line given by $$ \delta D = 7.82\delta {}^{18}{\text{O}} + 7.63 $$ , which is nearly identical to the meteoric water line in the Northern China. This wide range indicates that stable isotopes in precipitation were primarily controlled by different condensation mechanisms as a function of air temperature and varying sources of moisture. The results of backward trajectory of each precipitation day at Xishui show that the moisture of the precipitation in cold season (October–March) mainly originated from the west while the moisture source was more complicated in warm season (April–September). The simulation of seasonal δ 18O variation shows that the stable isotope composition of precipitation tended to a clear sine-wave seasonal variation. The spatial variation of δ 18O shows that the weighted average δ 18O values decreases with the increasing altitude of sampling sites. The great difference of air temperature which led to the differences of condensation mechanisms and local recycled continental moisture may have influence upon the isotopic composition of rain events in different sites.

Stable isotopes of modern herbivore tooth enamel in the Tibetan Plateau: Implications for paleoelevation reconstructions

Fossil tooth enamel from herbivores is considered one of the best proxies for paleoclimate and paleoelevation reconstructions, due to its low susceptibility to diagenetic alteration. A synthesis of oxygen and carbon isotope analyses of modern tooth enamel from herbivores such as Tibetan yaks, asses and antelopes is assessed. The average δ13C(PDB) value of herbivore tooth enamel in the Lhasa and south Qiangtang terrains is −11.3% ± 1.1%, whereas in the north Qiangtang and Hoh Xil terrains value is −10.2% ± 1.4% consistent with the current C3-dominant ecotype in the Tibetan Plateau. In addition, the average δ18O(PDB) values from tooth enamel show a northward systematic increase from −11.8% ± 3.4% in the Gyirong Basin, to −11.1% ± 1.1% in the Lhasa and southern Qiangtang terrains, and then to −9.0% ± 1.1% in the northern Qiangtang and the Hoh Xil areas, similar to those of the local river water and precipitation variation trends. It is suggested that δ18O(PDB) values of tooth enamel should not be used directly to evaluate the paleo-elevation of the Tibetan Plateau, because oxygen isotopic composition from tooth enamel is a combination of drinking water (precipitation- and river-sourced) and diet (plant-sourced). Moreover, an empirical relationship between oxygen isotope of tooth enamel and modern river water, and as well as elevation is recommended, which can be used in future studies of Tibetan Plateau paleoelevation.

Mixing and transport of water in a karst catchment: a case study from precipitation via seepage to the spring

Abstract. One of the best-known and largest karst areas in Germany, the Blautopf Catchment, offers unique access to waters of the unsaturated zone through a large cave system. It was investigated with stable isotopes (18O/16O and D/H ratios expressed in permille = ‰) in precipitation, seepage- and groundwater as tracers for water flow, mixing, and storage. The precipitation showed a distinct seasonality with δ18O values between −2.9 and −24.6‰ during summer and winter, respectively. However, the isotope signals in seepage water in the caves as well as the discharge were almost completely buffered and ranged around an average δ18O value of −10‰. This value was also close to the long-term average value of local precipitation, −9.3‰. The homogeneous isotopic composition of the Blautopf Spring was unexpected, as its highly variable discharge (0.3 to 32 m3 s−1) is typical for a fast responsive karst system. These isotopic similarities could be explained by nearly complete mixing of the water already in the vadose zone. The data set therefore presents a case study to narrow down zones of mixing in karst catchments. It also confirms the minor role of the fast conduit system in the water balance of the Blautopf Catchment.