Paleosol carbonate nodules may preserve environmental information despite later burial alteration, yet disentangling original signals from diagenetic overprints remains a central challenge. Here we apply paired clumped and triple oxygen isotope analyses (Delta(47)-Delta'O-17) to microsampled Eocene paleosol carbonates from the Gonjo Basin, southeastern Tibet. Intra-nodule T Delta(47) values of 9-58 degrees C define a spectrum of microscale thermal heterogeneity, spanning lower-temperature to more strongly burial-modified domains. In contrast, carbonate Delta'O-17 does not vary systematically with T Delta(47) (R-2 < 0.6), whereas reconstructed diagenetic-water compositions (delta O-18(w) and Delta'O-17(w)) covary with T Delta(47), suggesting progressive fluid-rock exchange during burial. Together with petrographic and geochemical observations, these data are most consistent with fluid-limited, rock-buffered recrystallization at low-water-rock ratios, with modeled solutions for most micritic domains falling at W/R < 0.05. Reconstructed Delta'O-17(w) values of diagenetic fluids range from -77 to -27 per meg, consistent with interaction with isotopically evolved meteoric waters and plausibly reflecting prior evaporative modification, although alternative fluid histories cannot be fully excluded. Rather than fully erasing environmental information, burial recrystallization in these carbonates appears to preserve a quantifiable record of fluid-rock interaction and hydroclimatic conditions. Our results show that paired Delta(47)-Delta'O-17 approach can help distinguish lower-temperature domains from more strongly burial-modified domains and trace diagenetic fluid evolution in ancient terrestrial carbonates.

Carbonate-bearing fluids derived from subducting slabs are important agents in crust-mantle interactions that can modify the composition and oxidation state of the overlying mantle wedge. Here we use magnesite-bearing metasomatic veins as well as fluid inclusions in the mantle wedge harzburgite to investigate multiple stages of fluid-mantle interactions which initialize the mantle oxidation and chemical transformation of carbon-bearing species. We find that compositional growth zones in metasomatic minerals record three stages of changes in fO2, fluctuating from FMQ-1 to FMQ-5 and back to FMQ-1.5. Such fO2 fluctuation corresponds to the presence of carbon-bearing species with varying valence states, including magnesite, amorphous carbon, graphite, nanosized diamond, as well as methane and organic compounds. We suggest that multiple stages of fluid-mantle interactions, regulated by oxidation states of the evolving carbonaceous fluid, provide a mechanism for the abiotic formation of organic carbon at the slab-mantle interface.

The stable chromium isotope system has been widely used as a redox proxy to reconstruct the oxygenation history of ocean atmosphere systems. However, the Cr isotope mass balance in modern oceans (i.e., inputs and outputs) remains poorly constrained. To investigate the influence of seawater-peridotite reaction on the global marine Cr isotope mass balance, we report high-precision Cr isotope data (delta 53Cr) on a series of fresh and altered abyssal peridotites from the Gakkel Ridge and the Southwest Indian Ridge (SWIR). The least altered peridotites give a delta 53Cr value of -0.08 +/- 0.06 parts per thousand (2SD, n = 4) for the oceanic mantle which is consistent with the established delta 53Cr of the Bulk Silicate Earth. Compared to fresh peridotites, a subset of altered peridotites exhibit a loss of isotopically light Cr with relatively positive delta 53Cr values (up to 0.04 parts per thousand). These altered peridotites are characterized by significant Cr loss and likely have been subject to serpentinization. By contrast, seafloor weathering has limited influence on the Cr concentrations and isotopic compositions of the altered peridotites. Monte Carlo (MC) simulations of marine alteration suggest a net Cr flux into seawater from altered abyssal peridotites of similar to 3.5 x 108 mol/yr, which is on the same order of magnitude as the riverine input flux of 108-109 mol/yr. Furthermore, the MC results suggest that the peridotite-sourced net Cr flux has a negative delta 53Cr signature (-0.33 +/- 0.21 parts per thousand, 2SD). Thus, seawater-peridotite interactions must be considered when evaluating the modern oceanic Cr isotope mass balance. The stable chromium isotope system is an emerging paleoredox proxy for tracing Earth's atmospheric oxygenation over geological timescales. However, accurately inferring past Earth's atmospheric oxygenation levels using chromium isotopes requires a more precise understanding of the elemental and isotopic cycling of chromium, particularly within the oceans. By studying abyssal peridotites which are rocks found beneath the oceanic crust, we find that specific types of alterations in these rocks, particularly the process of serpentinization, have an impact on the chromium isotopes. These findings provide insights into the mechanisms involved in transporting chromium from the deep Earth mantle to the oceans. We discover that these altered rocks release a flux of isotopically light chromium into the ocean, which is comparable to the amount coming from rivers. Our study sheds new light on the complex interactions between seawater and abyssal peridotites, which potentially exert a profound impact on the global marine Cr isotope mass balance. The first data set of stable Cr isotopic compositions for abyssal peridotites Seafloor weathering has insignificant impact on the Cr isotopic compositions of abyssal peridotites Seafloor alteration exerts a key control on the Cr isotopic composition of seawater

Water in nominally anhydrous minerals may exist as structural hydroxyl and molecular H2O, and their inter-transformation can occur during subduction zone metamorphism. This issue is examined here by a systematic study on water in garnet and omphacite for ultrahigh-pressure metamorphic eclogites from the Dabie orogen. IR spectra of garnet at different preheating temperatures imply that the broad absorption band at 3400-3450 cm-1 mainly represents molecular H2O. Molecular H2O in garnet can be intrinsic origin, but is mainly originated from external fluid during exhumation. The positive correlation between molecular H2O and structural hydroxyl in garnet suggests their inter-transformation. Molecular H2O that was formerly included in garnet or derived from decomposition of hydrous minerals was transformed gradually to structural hydroxyl in garnet during subduc-tion. OH- modes at higher wavenumber in garnet are more stable and more enriched in D, and D-poor molecular H2O is preferentially lost during dehydration. Hydroxyl in both anhydrous and hydrous minerals would be transformed to molecular H2O during exhumation and redistributed into garnet or retrograde minerals. The addition of molecular H2O into garnet during exhumation causes increase of structural hydroxyl but decrease of delta D value, and affects accurate estimate of water content in peak garnet. Different from the large variation of garnet water contents, omphacite tends to be water-saturated at high-pressure conditions and its water content is mainly controlled by Ca-Eskola. The subducting mafic crust can transport a considerable amount of water to subarc depths to result in the formation of relatively water-rich mantle regions.

Triple oxygen isotope ratios (Delta'O-17) offer new opportunities to improve reconstructions of past climate by quantifying evaporation, relative humidity, and diagenesis in geologic archives. However, the utility of Delta'O-17 in paleoclimate applications is hampered by a limited understanding of how precipitation Delta'O-17 values vary across time and space. To improve applications of Delta'O-17, we present delta O-18, d-excess, and Delta'O-17 data from 26 precipitation sites in the western and central United States and three streams from the Willamette River Basin in western Oregon. In this data set, we find that precipitation Delta'O-17 tracks evaporation but appears insensitive to many controls that govern variation in delta O-18, including Rayleigh distillation, elevation, latitude, longitude, and local precipitation amount. Seasonality has a large effect on Delta'O-17 variation in the data set and we observe higher seasonally amount-weighted average precipitation Delta'O-17 values in the winter (40 +/- 15 per meg [+/- standard deviation]) than in the summer (18 +/- 18 per meg). This seasonal precipitation Delta'O-17 variability likely arises from a combination of sub-cloud evaporation, atmospheric mixing, moisture recycling, sublimation, and/or relative humidity, but the data set is not well suited to quantitatively assess isotopic variability associated with each of these processes. The seasonal Delta'O-17 pattern, which is absent in d-excess and opposite in sign from delta O-18, appears in other data sets globally; it showcases the influence of seasonality on Delta'O-17 values of precipitation and highlights the need for further systematic studies to understand variation in Delta'O-17 values of precipitation.

We report measurements of resolved 12CH2D2 and 13CH3D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12CH2D2 from 13CH3D provides unprecedented insights into the origin and evolution of CH4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ12CH2D2 and Δ13CH3D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH4/H2 D/H exchange, underscoring the importance of reliable thermometry based on the CH4 molecules themselves. Where Δ12CH2D2 and Δ13CH3D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis vs. biological processes. Deficits in 12CH2D2 compared with equilibrium values in CH4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13CH3D that accompany the low 12CH2D2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ12CH2D2 values are a key tracer of microbial recycling.

Molecular nitrogen (N2) comprises three-quarters of Earth's atmosphere and significant portions of other planetary atmospheres. We report a 19 per mil (‰) excess of 15N15N in air relative to a random distribution of nitrogen isotopes, an enrichment that is 10 times larger than what isotopic equilibration in the atmosphere allows. Biological experiments show that the main sources and sinks of N2 yield much smaller proportions of 15N15N in N2. Electrical discharge experiments, however, establish 15N15N excesses of up to +23‰. We argue that 15N15N accumulates in the atmosphere because of gas-phase chemistry in the thermosphere (>100 km altitude) on time scales comparable to those of biological cycling. The atmospheric 15N15N excess therefore reflects a planetary-scale balance of biogeochemical and atmospheric nitrogen chemistry, one that may also exist on other planets.

Reliable forecasts of extra-tropical cyclones such as Superstorm Sandy require accurate understanding of their thermodynamic evolution. Within such systems, the evaporation, transport, and precipitation of moisture alters stable isotope ratios of cyclonic waters and creates spatio-temporal isotopic patterns indicative of synoptic-scale processes. Here, high-frequency records of precipitation isotope ratios from four sites (West Lebanon, NH; Baltimore, MD; State College, PA; and Colcord, WV) are used to investigate the development of Sandy as the storm made landfall and moved inland. These high-frequency records are also combined with a Lagrangian backward transport model to create a general relationship between precipitation deuterium-excess and moisture source conditions. Based on this general relationship, the evolution of precipitation efficiency within Superstorm Sandy is mapped through time using a set of distributed isotope collections. These maps identify a region of high-precipitation efficiency near storm’s core where intense rainfall rates likely exceeded the resupply of moisture as well as outlying rain-bands of lower precipitation efficiency possibly influenced by entrainment of a mid-western cold front.

High-precision triple oxygen isotope analysis of waters is an emerging tool in hydrological and paleoclimate research. The existing research on 17O-excess in waters includes surveys of meteoric waters and region-specific studies of high-latitude snow and tropical storms. However, a better understanding of the variation in 17O-excess of waters across large geographic regions is needed to expand the utility of triple oxygen isotope measurements. Here we present 17O-excess data from tap waters across the continental U.S., which we used as a proxy for precipitation. The 17O-excess values of tap waters ranged from −6 to +43 per meg and averaged 17±11 per meg which is lower than the average 17O-excess reported for global meteoric waters, but overlaps with reported 17O-excess values of rainfall from the tropics. We observed relatively high 17O-excess values (>25 per meg) of tap waters in the northwestern U.S. and some of the lowest 17O-excess values (<5 per meg) in the states bordering the Gulf of Mexico. The latitudinal variation of 17O-excess among tap waters likely reflects the different controls on 17O-excess in precipitation. For example, re-evaporation of precipitation and convective processes influence the isotopic composition of tap waters from the southern portions of the U.S., resulting in relatively low 17O-excess values. In contrast, these effects are reduced in tap waters from the northern portions of the U.S. where snow and cold-season rainfall are primarily responsible for the majority of annual precipitation. Exceptions to the latitudinal trend are prevalent in the central portions of the U.S., where mixing and convection are likely responsible for 17O-excess values that are lower than would be expected at their latitudes. The results of this study provide both a first look at the variation of 17O-excess in meteoric waters on a continental scale and a predictive map for 17O-excess of meteoric waters in the U.S.

The 17O anomaly (Δ17O) of natural waters has been shown to be sensitive to evaporation in a way analogous to deuterium excess, with evaporated bodies of water (e.g., leaf waters, lake waters, animal body waters) tending to have lower Δ17O than primary meteoric waters. In animal body water, Δ17O relates to the intake of evaporated waters, evaporative effluxes of water, and the Δ17O value of atmospheric O2, which itself carries signatures of global carbon cycling and photochemical reactions in the stratosphere. Carbonates have the potential to record the triple oxygen isotope compositions of parent waters, allowing reconstruction of past water compositions, but such investigations have awaited development of methods for high-precision measurement of Δ17O of carbonate. We describe optimized methods based on a sequential acid digestion/reduction/fluorination approach that yield Δ17O data with the high precision (∼0.010‰, 1σ) needed to resolve subtle environmental signals. We report the first high-precision Δ17O dataset for terrestrial carbonates, focusing on vertebrate biogenic carbonates and soil carbonates, but also including marine invertebrates and high-temperature carbonates. We determine apparent three-isotope fractionation factors between the O2 analyte derived from carbonate and the parent waters of the carbonate. These in combination with appropriate temperature estimates (from clumped isotope thermometry, or known or estimated body temperatures) are used to calculate the δ18O and Δ17O of parent waters. The clearest pattern to emerge is the strong 17O-depletion in avian, dinosaurian, and mammalian body water (from analyses of eggshell and tooth enamel) relative to meteoric waters, following expected influences of evaporated water (e.g., leaf water) and atmospheric O2 on vertebrate body water. Parent waters of the soil carbonates studied here have Δ17O values that are similar to or slightly lower than global precipitation. Our results suggest that Δ17O will have useful application to paleoenvironmental studies of continental environments where the effects of evaporation are important, and where vertebrate body water may record an isotopic signal of evaporated water sources and atmospheric oxygen.

Stable isotopes of water have long been used to improve understanding of the hydrological cycle, catchment hydrology, and polar climate. Recently, there has been increasing interest in measurement and use of the less-abundant (17)O isotope in addition to (2)H and (18)O. Off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and precise measurements δ(18)O, δ(17)O, and (17)O-excess in liquid water. OA-ICOS involves no sample conversion and has a small footprint, allowing measurements to be made by researchers collecting the samples. Repeated (514) high-throughput measurements of the international isotopic reference water standard Greenland Ice Sheet Precipitation (GISP) demonstrate the precision and accuracy of OA-ICOS: δ(18)OVSMOW-SLAP = -24.74 ± 0.07‰ (1σ) and δ(17)OVSMOW-SLAP = -13.12 ± 0.05‰ (1σ). For comparison, the International Atomic Energy Agency (IAEA) value for δ(18)OVSMOW-SLAP is -24.76 ± 0.09‰ (1σ) and an average of previously reported values for δ(17)OVSMOW-SLAP is -13.12 ± 0.06‰ (1σ). Multiple (26) high-precision measurements of GISP provide a (17)O-excessVSMOW-SLAP of 23 ± 10 per meg (1σ); an average of previously reported values for (17)O-excessVSMOW-SLAP is 22 ± 11 per meg (1σ). For all these OA-ICOS measurements, precision can be further enhanced by additional averaging. OA-ICOS measurements were compared with two independent isotope ratio mass spectrometry (IRMS) laboratories and shown to have comparable accuracy and precision as the current fluorination-IRMS techniques in δ(18)O, δ(17)O, and (17)O-excess. The ability to measure accurately δ(18)O, δ(17)O, and (17)O-excess in liquid water inexpensively and without sample conversion is expected to increase vastly the application of δ(17)O and (17)O-excess measurements for scientific understanding of the water cycle, atmospheric convection, and climate modeling among others.

Variations in triple oxygen isotopes have been used in studies of atmospheric photochemistry, global productivity and increasingly in studies of hydroclimate. Understanding the distribution of triple oxygen isotopes in plant waters is critical to studying the fluxes of oxygen isotopes between the atmosphere and hydrosphere, in which plants play an important role. In this paper we report triple oxygen isotope data for stem and leaf waters from Mpala, Kenya and explore how Δ17O, the deviation from an expected relationship between O17/O16 and O18/O16 ratios, in plant waters vary with respect to relative humidity and deuterium excess (d-excess). We observe significant variation in Δ17O among waters in leaves and stems from a single plant (up to 0.16‰ range in Δ17O in leaf water in a plant over the course of a signal day), which correlates to changes in relative humidity. A steady state model for evaporation in leaf water reproduces the majority of variation in Δ17O and d-excess we observed in leaf waters, except for samples that were collected in the morning, when relative humidity is high and the degree of fractionation in the system is minimal. The data and the steady state model indicate that the slope, λtransp, that links δ17O and δ18O values of stem and leaf waters and characterizes the fractionation during transpiration, is strongly influenced by the isotopic composition of ambient vapor when relative humidity is high. We observe a strong, positive relationship between d-excess and Δ17O, with a slope 2.2±0.2permeg‰−1, which is consistent with the observed relationship in tropical rainfall and in water in an evaporating open pan. The strong linear relationship between d-excess and Δ17O should be typical for any process involving evaporation or any other fractionation that is governed by kinetic effects.

Metamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction-zone PT conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh-pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite-bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz +/- K-feldspar +/- plagioclase +/- K-bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute-rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon UPb dating for the vein gave two groups of concordant ages at 217 +/- 2 and 210 +/- 2Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent 18O and D values, suggesting that the vein-forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water-soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.25.7 times) and NbTa (1.82.0 times) than those in the eclogite, indicating that these normally water-insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein-forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction-zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein-forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust.

The composition and evolution of metamorphic fluid/melt formed during continental collision are major concerns in chemical geodynamics of continental subduction zones.Ultrahigh-pressure(UHP)metamorphic rocks record the interactions between fluid/melt and rocks during the continental subduction and exhumation,presenting an excellent natural laboratory to study the composition and evolution of fluid/melt phases in deep subduction zones.In doing so,multiphase solid(MS)inclusions enclosed in minerals of UHP metamorphic rocks provide us the key information on the behavior of fuild/melt during UHP metamorphism of continental crustal rocks.Many studies have been devoted to the mineral and chemical composition of MS inclusions as well as their formation time and mechanism,with the aim to decipher the physicochemical properties of metamorphic melts.The occurrence of MS inclusions in UHP metamorphic rocks provides not only the important information on the composition and evolution of UHP metamorphic fluid/melt,but also insight into the metasomatism of fluid/melt at slab-arc depths.This paper presents an overview on the progress in the study of MS inclusions in UHP metamorphic rocks,with the emphasis on such outstanding issues as the formation mechanism,morphology and texture,mineral chemistry,trace element and fluid/melt origin.It is expected that microscale studies of MS inclusions within the framework of modern petrology will improve our understanding of continental subduction-zone metamorphism and its bearing on the chermical geodynamics of various subduction zones.