δ18O Variations Research Articles

Modeling spatial distributions of hydrogen and oxygen stable isotopes in surface soils of the upper reaches of Minjiang River based on machine languages

To study the feasibility of simulating the spatial distribution of hydrogen and oxygen stable isotopes composition (δ2H and δ18O) in the surface soil based on the machine learning method and to investigate large-scale distribution of δ2H and δ18O in the upper reaches of Minjiang River, 183 soil samples were collected from the 0-10 cm soil layer. After variable selection, back propagation (BP) neural network, random forests (RF) and support vector machine (SVM) were used to model the δ2H and δ18O of the study area, with the accuracies being evaluated. The structural equation model (SEM) was used to reveal the mechanism between the auxiliary variables and the δ2H and δ18O of soil water. The results showed that the RF model had the highest prediction accuracy, and could explain 75.0% and 64.0% of the variations of δ2H and δ18O in the surface soil, respectively. In this model, soil water content was the most important auxiliary variable, contributing 48.9% and 37.4% to δ2H and δ18O. Vegetation factors had stronger influence on δ2H and δ18O in the surface soil than climate factors, and the influence of climate factors on δ2H and δ18O was media-ted by vegetation factors. Among all the auxiliary variables, hydrogen/oxygen isotope of precipitation had the lowest effect on δ2H and δ18O due to the fractionation. The δ2H and δ18O in the surface soil of the upper reaches of the Minjiang River changed significantly across different months during the growing season. The increases of δ2H and δ18O in the early growing season and the decreases in the late growing season were mainly affected by vegetation, while climate change led to a small fluctuation in the middle growing season.

Investigating the influence of temperature and seawater δ18O on Donax obesulus (Reeve, 1854) shell δ18O

The coastline of Peru lacks long-lived marine organisms useful for paleoclimatic reconstructions generating a need for novel archives. Short-lived (<5 years) bivalves are commonly found in geological and archaeological deposits and thus can provide “snapshots” of past climatic variability (i.e., seasonal range), similar to data obtained by individual foraminifera analysis, rather than continuous, cross-dated time series (e.g., trees and corals). Previous studies have found success using the short-lived intertidal clam Mesodesma donacium. However, M. donacium are vulnerable to die-offs from the warmer sea surface temperatures (SST) associated with El Niño events and are functionally extinct in northern Peru thus limiting the possibility of modern analog studies for that region. Here we investigate the short-lived (1–3 years) surf clam, Donax obesulus, commonly found in northern Peru, as a paleoclimate archive. Donax obesulus populations are able to survive the warmer SSTs present during El Niño years although they are vulnerable to colder SSTs associated with La Niñas. We assessed the environmental drivers underlying subannual δ18O variability in D. obesulus from live collected shells from fish markets and coastal beaches near the Nepeña Valley, Peru in 2012 (La Niña), 2014 (ENSO-neutral), and 2016 (El Niño). Forward modeling of pseudo-shell δ18O reveals that SST variations are a dominant driver with secondary contributions from seasonally-varying seawater δ18O (δ18Osw). By accounting for varying δ18Osw, we isolated the temperature dependent variable resulting in a paleotemperature equation for D. obesulus δ18O. We verified our results with the δ18O record of a D. obesulus shell collected in 2006. Our results suggest that the paleotemperature equation we developed is useful for reconstructing El Niño-Southern Oscillation (ENSO)-related climatic variations in this region and the pseudo-shell approach may be useful for understanding shell δ18O in other locations.

Spatiotemporal Variation of Hydrogen and Oxygen Stable Isotopes in the Yarlung Tsangpo River Basin, Southern Tibetan Plateau

Characterization of spatiotemporal variation of the stable isotopes δ18O and δD in surface water is essential to trace the water cycle, indicate moisture sources, and reconstruct paleoaltimetry. In this study, river water, rainwater, and groundwater samples were collected in the Yarlung Tsangpo River (YTR) Basin before (BM) and after the monsoon precipitation (AM) to investigate the δ18O and δD spatiotemporal variation of natural water. Most of the river waters are distributed along GMWL and the line of d-excess = 10‰, indicating that they are mainly originated from precipitation. Temporally, the δ18O and δD of river water are higher in BM series (SWL: δD = 10.26δ18O+43.01, R2 = 0.98) than AM series (SWL: δD = 9.10δ18O + 26.73, R2 = 0.82). Spatially, the isotopic compositions of tributaries increase gradually from west to east (BM: δ18O = 0.65Lon (°)-73.89, R2 = 0.79; AM: δ18O = 0.45Lon (°)-57.81, R2 = 0.70) and from high altitude to low (BM: δ18O = −0.0025Alt(m)-73.89, R2 = 0.66; AM: δ18O = −0.0018Alt(m)-10.57, R2 = 0.58), which conforms to the “continent effect” and “altitude effect” of precipitation. In the lower reaches of the mainstream, rainwater is the main source, so the variations of δ18O and δD are normally elevated with the flow direction. Anomalously, in the middle reaches, the δ18Omainstream and δDmainstream values firstly increase and then decrease. From the Saga to Lhaze section, the higher positive values of δ18Omainstream are mainly caused by groundwater afflux, which has high δ18O and low d-excess values. The δ18Omainstream decrease from the Lhaze to Qushui section is attributed to the combined action of the import of depleted 18O and D groundwater and tributaries. Therefore, because of the recharge of groundwater with markedly different δ18O and δD values, the mainstream no longer simply inherits the isotopic composition from precipitation. These results suggest that in the YTR Basin, if the δ18O value of surface water is used to trace moisture sources or reconstruct the paleoaltimetry, it is necessary to rule out the influence from groundwater.

A framework for triple oxygen isotopes in speleothem paleoclimatology

Speleothem oxygen isotope (δ18O) records provide key insight into the rate and timing of terrestrial paleoclimate changes during the late Quaternary. However, it can be difficult to deconvolve the δ18O signal into individual components, which include processes related to moisture source, moisture transport, temperature, precipitation amount, infiltration, and the cave environment. We developed a framework that uses triple oxygen isotope distributions in speleothems to refine interpretations of δ18O speleothem records. This framework identifies the influence of dominant processes on δ18O values through time by their characteristic (although not necessarily unique) trends in Δ′17O vs. δ′18O space, where Δ′17O = δ′17O – 0.528δ′18O and δ′xO = ln(δxO + 1). Following Guo and Zhou (2019a), we expect that ‘cave kinetic’ processes (e.g., fast degassing at the drip site, prior calcite precipitation) will drive positive trends between δ′18O and Δ′17O. In contrast, we can identify hydrologic processes from near-horizontal trends that reflect Rayleigh-type meteoric water processes and negative trends driven by changes in evaporation processes at the moisture source region or at the cave site, mineralization temperature, and seasonality in precipitation/infiltration amount. We applied this framework to four western USA speleothems from Cave of the Bells (Arizona), Leviathan Cave (Nevada), and Lehman Caves (Nevada). The Cave of the Bells and Leviathan data have near-horizontal to negative trends indicating δ18O variability was driven largely by changes in Rayleigh distillation of atmospheric moisture and moisture source conditions, supporting prior interpretations. We analyzed two Lehman Caves records because they were likely influenced by non-equilibrium processes and the data show weak to moderate negative trends. For sample LMC-12b, chosen for its extreme 7.5‰ δ18O range, the trend is statistically distinct from the near-horizontal Rayleigh-process trend and most consistent with changes in local evaporation intensity and infiltration seasonality as primary drivers. None of these records displays a positive covariation slope between δ′18O and Δ′17O, suggesting limited variability in cave kinetic processes through time or unknown limitations to the kinetic model of Guo and Zhou (2019a). Additionally, reconstructed formation waters for all sites fall near the Δ′17O vs. δ′18O Local Meteoric Water Line, a correlation we suggest as a novel test of the absolute magnitude of isotopic offset due to cave kinetic processes. More broadly, our framework adds context to the only other study of carbonate speleothem triple oxygen isotope composition (Sha et al., 2020). We find that positive to negative Δ′17O vs. δ′18O trends likely exist in speleothem data that may reasonably be expected from regional climate processes and that, combined with other proxy data, triple oxygen isotope data will be useful in constraining interpretations of δ18Ospeleothem records.

Hydrological behavior of natural springs/streams in mid hills of Chakrata Himalaya, India

&lt;p&gt;The present study was carried out to understand the hydrological behavior of natural springs/streams in Semalta watershed, Kalsi block, Dehradun. The surface catchment areas of springs/streams were delineated using Survey of India Toposheet (1:50,000 scale) whereas drainage, slope, and land use maps of the watershed were prepared using open source ILWIS GIS software. The drainage map of the watershed indicated the ephemeral, intermittent and perennial nature of streams and the slope map of the watershed indicated that the majority of the area falls under scrubland. The discharge trend at different gauging sites from October 2015 to March 2016 indicated that the flow rate of the stream gradually decreased from post-monsoon to pre-monsoon period. The geohydrological and geomorphological study indicated that most of the springs/streamflow were controlled by fracture/joints with slate/phyllite/limestone/quartzite as country rock. The pH of most of the stream water was mildly alkaline (7.85-8.82) whereas EC was non-saline (0.087-0.425) and temperature varied from 14-23°C. Results on major ions indicated that a higher percentage of NH4+ together with NO3- was possible because of anthropogenic influence. In addition, the higher percentage of SO4- and F- was due to lithological pyrite whereas the higher percentage of HCO3- was due to pyrite weathering of slate. Results on the isotopic variation of δ18O showed that samples were collected with a total height difference of 700 m with a variation of -0.21‰ per 100 m for stream water and -0.5‰ for rainwater. On higher altitudes mainly narrow subsurface pathways are found and therefore only small size of structures are required for recharging and vice versa for the recharge of springs located in lower altitudes. The present study discusses various sampling techniques to ascertain the altitude of recharge of springs which would form the basis for taking appropriate on field activities for spring/stream rejuvenation.&lt;/p&gt;&#x0D;

Ephemeral Magma Reservoirs During the Incremental Growth of the Neoproterozoic Jiuling Composite Batholith in South China

AbstractLarge‐volume magma bodies in the crust are crucial to magmatic differentiation and intracrustal mass redistribution. Their magmatic residence timescales during solidification in the upper crust reflect the intensity of magmatic activity and control the formation of economic deposits. Here we explore the use of zircon crystal size distributions (CSDs) and oxygen isotopic diffusion modeling to constrain the magmatic timescales of the four main intrusions of the Neoproterozoic Jiuling composite batholith in eastern Jiangnan Orogen, South China. We present laser‐ablation–inductively coupled plasma–mass spectrometry zircon U–Pb ages and co‐registered secondary ion mass spectrometry zircon O isotopes for the four main intrusions. The dating results show that the intrusions were emplaced at 826–810 Ma, with a few inherited zircon domains in the range of 3,800–3,835 Ma. The magmatic zircon grains/rims commonly have wide δ18O variations from 5.9‰ to 13.1‰, and magmatic centers have similar δ18O ranges (3.9‰–12.2‰) to those of inherited zircon domains (3.8‰–11.4‰). The application of zircon CSD measurements reveals short zircon crystallization timescales and thus possible magmatic residence times in the order of 103–104 yr, consistent with the maximum residence times (18–47 kyr, as an upper limit) constrained by zircon oxygen isotopic diffusion modeling. This study of such ephemeral reservoirs provides unique insights into the state of magmas and the processes that occur within magma reservoirs.

Variability of δ2H and δ18O in Soil Water and Its Linkage to Precipitation in an East Asian Monsoon Subtropical Forest Plantation

The linkage between δ2H and δ18O of soil water and precipitation provides a way of understanding precipitation infiltration, residence time, and soil water source. Soil water at 0–5, 15–20, and 40–45 cm depths and event-based precipitation were collected in a subtropical forest plantation. Correlations between the δ18O of soil water and precipitation on the same day were used to determine the critical threshold of precipitation infiltration. Residence time of precipitation in soil was determined with correlations between the δ18O of soil water and cumulative precipitation before sampling. Soil water source was determined by the intersection points of Soil Water Evaporation Lines (SEL) and local meteoric water lines. The results showed that precipitation &gt;5–6 mm could pass through canopy and litter, and infiltrate into soil. Residence times varied from a few days to several months, and increased with soil depth. The model-based approach for SEL estimation were more robust than the regression-based approach due to the inverse variability in the δ2H and δ18O of soil water source and soil evaporative fractionation. Soil water at a 0–5 cm depth originated mainly from precipitation in the current season, while those at 15–20 and 40–45 cm depths originated mainly from precipitation in the previous season.

Eastern Mediterranean climate change deduced from the Soreq Cave fluid inclusion stable isotopes and carbonate clumped isotopes record of the last 160 ka

A fundamental issue in the interpretation of speleothem calcite δ18Occ records is the correct partitioning of the effects of temperature and water δ18O variations. This study explores the paleo-environmental evolution of the Eastern Mediterranean Sea (EMS) region using Soreq Cave speleothems in the last 160 ka, a period covering glacial MIS6 to the present, through the independent determinations of temperatures via carbonate clumped isotopes (Δ47) and fluid inclusion δ18Ow, δDw and d-excess values of cave water entrapped in the speleothems. The observed general agreement between the trends in water δ18Ow composition directly measured from fluid inclusions and calculated from Δ47 temperatures and δ18Occ support the robustness of the data. Δ47 temperatures, derived using a revised thermometer calibration that includes recent modifications in 17O correction, vary from high values of 20–22 °C during interglacial periods to low values of 11–12 °C in cold glacial and stadial periods. Temperatures of 19–22 °C are observed during the time of deposition in the EMS of sapropel S5 in the last interglacial MIS5e period and sapropel S1 in the early Holocene, immediately following Terminations II and I, respectively. The Soreq Cave Δ47 temperatures generally agree with previously estimated EM sea surface temperatures, in support of the absence of a significant land-sea temperature gradient. Cave water δ18Ow values show large variations (from ∼-3 to −7.5‰), with high glacial and low interglacial values. These variations are largely explained by changes in Mediterranean Sea water composition due to glacial ice melting and freshwater input during sapropel periods, combined with the effect of rainfall amounts during interglacial periods. An analysis of relative timing shows that temperature rise precedes hydrological changes in the initial stages of Termination events. Fluid inclusion d-excess values show clear variations between values similar to the Global Meteoric Water Line during cold periods and values similar to the local Eastern Mediterranean Meteoric Water Line (EMMWL) in warm periods.

Continuous monitoring of surface water vapour isotopic compositions at Neumayer Station III, East Antarctica

Abstract. In this study, the first fully continuous monitoring of water vapour isotopic composition at Neumayer Station III, Antarctica, during the 2-year period from February 2017 to January 2019 is presented. Seasonal and synoptic-scale variations in both stable water isotopes H218O and HDO are reported, and their links to variations in key meteorological variables are analysed. In addition, the diurnal cycle of isotope variations during the summer months (December and January 2017/18 and 2018/19) has been examined. Changes in local temperature and specific humidity are the main drivers for the variability in δ18O and δD in vapour at Neumayer Station III, on both seasonal and shorter timescales. In contrast to the measured δ18O and δD variations, no seasonal cycle in the Deuterium excess signal (d) in vapour is detected. However, a rather high uncertainty in measured d values especially in austral winter limits the confidence of this finding. Overall, the d signal shows a stronger inverse correlation with specific humidity than with temperature, and this inverse correlation between d and specific humidity is stronger for the cloudy-sky conditions than for clear-sky conditions during summertime. Back-trajectory simulations performed with the FLEXPART model show that seasonal and synoptic variations in δ18O and δD in vapour coincide with changes in the main sources of water vapour transported to Neumayer Station III. In general, moisture transport pathways from the east lead to higher temperatures and more enriched δ18O values in vapour, while weather situations with southerly winds lead to lower temperatures and more depleted δ18O values. However, on several occasions, δ18O variations linked to wind direction changes were observed, which were not accompanied by a corresponding temperature change. Comparing isotopic compositions of water vapour at Neumayer Station III and snow samples taken in the vicinity of the station reveals almost identical slopes, both for the δ18O–δD relation and for the temperature–δ18O relation.

Oxygen Isotopic Compositions in a Plagioclase‐Olivine Inclusion from Ningqiang Similar to Those in Al‐rich Chondrules

AbstractWe report the petrology and oxygen isotopic composition, using a Cameca NanoSIMS 50L ion microprobe, of a plagioclase–olivine inclusion, C#1, found in the Ningqiang carbonaceous chondrite. In addition to major phases (plagioclase, spinel and olivine), C#1 is also surrounded by a pyroxene rim (64 vol % Ca‐rich and 36 vol % Ca‐poor pyroxenes). On a three‐isotope oxygen diagram, δ17O vs. δ18O, the compositions of individual minerals analyzed in C#1 fall along the carbonaceous chondrite anhydrous mineral line (CCAM), and oxygen isotopic compositions in C#1 show significant variability in δ18O and δ17O. The oxygen isotopic compositions of the pyroxene rim minerals are similar to those of the other host minerals, which suggests that the rim likely formed from the same melting process as the host. The rim is considered to have formed as a result of interaction between an 16O‐poor gas and a melt. Some spinel grains are typically 16O‐rich and likely of relict origin, which is similar to 16O‐rich Ca‐, Al‐rich inclusions, which are probably a precursor of C#1. The inclusion then likely melted in an 16O‐poor region where chondrules form, accompanied by oxygen isotope exchange with an 16O‐poor gas. Some anorthite, pyroxene and spinel might have undergone fluid‐assisted thermal metamorphism on the Ningqiang chondrite parent body. The oxygen isotope data and evolution of the C#1 plagioclase–olivine inclusion are similar with those of Al‐chondrules in chondrites.

Isotopic signatures of moisture recycling and evaporation processes along the Western Ghats orography

The water isotopic composition (δ18O and δ2H) of humid tropical mountain belts is influenced by the limited isotopic elevation effect due to the narrow temperature gradient, high relative humidity, and the moisture feedback processes. The Western Ghats mountain forest ecosystem, recognized as a biological hotspot, contributes significantly to characterizing the hydrological and weather patterns over its windward and leeward sides. The Indian Summer Monsoon southwesterly winds advancing towards the Indian subcontinent through the Arabian Sea between June to September are obstructed by this mountain belt and get orographically uplifted, producing heavy rainfall. The present study intends to improve understanding of the variation of precipitation isotopic composition along multiple stations located at different elevations of the heavily regulated Periyar river basin (PRB), southern Western Ghats, and a coastal station (TRV). The δ18O variation with elevation across PRB reveals an abrupt depletion at the highland station and a pseudo-elevation effect due to the supply of evaporated local moisture from reservoirs and lakes. The contribution of recycled precipitation estimated using back trajectories suggests a maximum of 8.5% of locally recycled rainfall over midland station towards the September month, the daily contribution reaching a maximum of 50.7%. Though this approach did not show any signatures of moisture recycling in the highland station, the surface water (reservoirs and lakes) lc-excess (line-conditioned excess), and the d-excess (deuterium excess) values of precipitation suggest the higher rates of evaporation from large water bodies and their successive role in generating local rainfall. Statistically significant amount effects visible only along coastal stations with lower rates of precipitation and higher temperature indicate that precipitation along the Ghats region (midland and highland) is significantly controlled by orographic uplift of air moisture and the contribution from locally recycled moisture. This is further supported by the significant correlation of δ18O with regional convective processes along the Arabian Sea till midland station and further decrease towards the highland. The present study provides important information on the moisture feedback mechanism in the Western Ghats and the factors controlling the isotopic signatures over the region.

Oxygen Isotope Compositions of Cellulose in Earlywood of Larix cajanderi Determined by Water Source Rather Than Leaf Water Enrichment in a Permafrost Ecosystem, Eastern Siberia

AbstractThe oxygen isotopic composition of tree‐ring cellulose (δ18Ocell) has been widely used to reconstruct historical environmental changes; however, the control factors on δ18Ocell have not been fully constrained—especially in high latitudes. To evaluate the influence of metabolic processes and related environmental factors on δ18Ocell, we analyzed the δ18O values of soil water (1998–2015), stem water (1997–2016), leaf water (3 days in each 2014 and 2015), and tree‐ring cellulose (1981–2016), on a dominant larch species (Larix cajanderi) in an eastern Siberian boreal forest. We determined that the δ18O variability of water sources is dependent on the precipitation, and the 18O enrichment in leaf water is determined by relative humidity. These findings suggest that both water source uptake and leaf water enrichment processes can affect the δ18O values of oxygen‐containing compounds in larch trees in the study site. However, the δ18Ocell (one of the end oxygen‐containing products) was found dependent on water sources on which was related to the amount of summer rainfall in the previous year. This finding significantly differs from that of studies in other areas, which infer the positive correlation between δ18Ocell and leaf water enrichment rather than precipitation or water sources. These differences are predominantly due to the specific conditions of high‐latitude areas, such as the large seasonal δ18O difference in precipitation, the existence of permafrost, and the low growth rate. Our findings contribute toward the development of tree‐ring paleoclimate reconstructions—especially in eastern Siberia.

Continuous monitoring of the isotopic composition of surface water vapor at Lhasa, southern Tibetan Plateau

The stable isotopes (δ18O and δD) of water vapor are used to characterize continuous variations in large-scale and boundary-layer atmospheric processes. We presented continuous measurements of δ18O in surface water vapor at Lhasa, southern Tibetan Plateau, from October 2018 to September 2019 to investigate how large-scale and local atmospheric processes influence variations in water vapor δ18O at different time scales. The water vapor δ18O measurements reveal different seasonal characteristics and diurnal patterns. At the seasonal scale, δ18O exhibits a W-shape with two maxima in May–June and October as well as two minima in July–August and February. The diurnal variations in the water vapor δ18O and meteorological data throughout the year present distinct occurrences of maxima and minima during different periods. We found that the significant seasonal variability is mainly associated with the transition between the Indian summer monsoon and the westerlies, which transport distinct moisture to the southern Tibetan Plateau. The local temperature, specific humidity and boundary layer height impact the diurnal variations in water vapor δ18O to some extent with remarkable seasonal differences.

Tree-ring oxygen isotope across monsoon Asia: Common signal and local influence

Tree-ring cellulose oxygen isotope (δ18O) in monsoon Asia (MA) is affected collectively by local hydroclimate and remote oceanic-atmospheric variations. Variations of tree-ring δ18O have been widely used for quantitative reconstructions of monsoon-season precipitation, relative humidity and drought severity. However, local hydroclimate and remote large-scale circulation's influences on tree-ring δ18O has not been fully resolved. Here, we collected 24 tree-ring δ18O chronologies covering the period of 1884–1999 across a wide range of MA and investigated the common signals of these chronologies using principal component analysis. The first principal component (PC1) of the 24 tree-ring δ18O chronologies explained 25.2% of the total variances and showed significant positive correlations with the observed Asian summer monsoon oxygen isotope index (r = 0.69) during the period of 1978–1999, suggesting that a common signal of precipitation δ18O does exist in MA and that it is highly correlated with El Niño–Southern Oscillation (ENSO) (r = 0.72) and the South Asian summer monsoon precipitation (r = −0.66). South Asian summer monsoon precipitation and ENSO are the main factors controlling this common signal of tree-ring δ18O across MA. PC1 of tree-ring δ18O chronologies in MA also depicted a negative (positive) correlation with the Atlantic Multidecadal Oscillation (Pacific Decadal Oscillation), indicating the large-scale circulation influences. After removing the large-scale circulation influences on tree-ring δ18O chronologies, however, the residual tree-ring δ18O chronologies exhibit negative correlations with the regional hydroclimate, highlighting the role local hydroclimate on the individual tree-ring δ18O variations.

Speleothem Records of the Hydroclimate Variability throughout the Last Glacial Cycle from Manita peć Cave (Velebit Mountain, Croatia)

We present stable carbon (δ13C) and oxygen (δ18O) isotope records from two partially coeval speleothems from Manita peć Cave, Croatia. The cave is located close to the Adriatic coast (3.7 km) at an elevation of 570 m a.s.l. The site experienced competing Mediterranean and continental climate influences throughout the last glacial cycle and was situated close to the ice limit during the glacial phases. U-Th dating constrains the growth history from Marine Isotope Stage (MIS) 5 to MIS 3 and the transition from MIS 2 to MIS 1. 14C dating was used to estimate the age of the youngest part of one stalagmite found to be rich in detrital thorium and thus undatable by U-Th. On a millennial scale, δ18O variations partly mimic the Dansgaard–Oeschger interstadials recorded in Greenland ice cores (Greenland Interstadials, GI) from GI 22 to GI 13. We interpret our δ18O record as a proxy for variations in precipitation amount and/or moisture sources, and the δ13C record is interpreted as a proxy for changes in soil bioproductivity. The latter indicates a generally reduced vegetation cover towards MIS 3–MIS 4, with shifts of ~8‰ and approaching values close to those of the host rock. However, even during the coldest phases, when a periglacial setting and enhanced aridity sustained long-residence-time groundwater, carbonic-acid dissolution remains the driving force of the karstification processes. Speleothem morphology follows changes in environmental conditions and complements regional results of submerged speleothems findings. Specifically, narrow sections of light porous spelaean calcite precipitated during the glacial/stadial sea-level lowstands, while the warmer and wetter conditions were marked with compact calcite and hiatuses in submerged speleothems due to sea-level highstands. Presumably, the transformation of this littoral site to a continental one with somewhat higher amounts of orographic precipitation was a site-specific effect that masked regional environmental changes.

Moisture source identification based on the seasonal isotope variation of precipitation in the Poyang Lake Wetland, China

Study regionThe wetland ecosystem of Poyang Lake, the largest freshwater lake in China. Study focusThe focus of this study is to understand the climatic influence on precipitation isotopic composition and the mechanism underlying the seasonal isotope variation of precipitation in the Poyang Lake Wetland. New hydrological insights for the regionThe Local Meteoric Water Line was established as δ2H = (8.83 ± 0.18) × δ18O + (12.77 ± 0.86) (R2 = 0.93, p < 0.01, n = 186). A total of seven moisture sub-sources contributing to precipitation were identified using the HYSPLIT model, among which distinct differences in isotopic signatures were revealed. The seasonal isotopic variation of precipitation in the study area was found not to respond to local meteorological factors. An increase in the relative occurrence frequency and relative contribution to precipitation of the continental source was found from summer to winter, which explains the seasonal variation in precipitation δ18O and d-excess. Combining the results of back trajectory analysis and isotopic statistics for the sub-source classification, the seasonal evolution of precipitation isotopes is considered to be caused by variations in the contribution from various moisture sources with distinct isotopic characteristics, which are further controlled by monsoon activities. This study highlights the importance of regional-scale moisture source dynamics and atmospheric circulation for the seasonal isotope variation of precipitation in the Poyang Lake Wetland.

Seasonal/Interannual Variation and Controlling Factors for Oxygen and Carbon Isotopes of Ostracod Shells Collected From a Time-Series Sediment Trap in Lake Qinghai

Because the shell substance of an ostracod is derived entirely from the water body where it lives, its chemical compositions are sensitive to aquatic environment and thus have been used to reconstruct past climatic and environmental changes. However, there is controversy about the controlling factors for the different compositions of ostracod shells from various water bodies. In this study, seasonal and interannual variations in daily flux and stable oxygen-carbon isotopic compositions (δ18O, δ13C) for two species of ostracod shells (Limnocythere inopinata and Eucypris mareotica) and their controlling factors are discussed, based on ostracod shell samples collected from a time-series sediment trap from July 2010 through September 2012 and from surface sediments in Lake Qinghai, which were correlated with the state-of-the-art sensing data of the lake water. The results show that the daily flux of L. inopinata shells is an order of magnitude higher than that of E. mareotica. The δ18O and δ13C of both L. inopinata and E. mareotica shells have distinctly interannual and seasonal variations, with species differences. Interannual differences of δ18O for the two species of ostracod shells directly reflect the systematic differences of the summer water temperature between 2010 and 2012. We propose that seasonal variations of both δ18O and δ13C for the two species are affected by the precipitation of authigenic carbonates in microenvironment induced by high water temperature in summers, highlighting their environmental implications in Lake Qinghai.

ISODRIP, a model to transfer the δ18O signal of precipitation to drip water — Implementation of the model for Eagle Cave (central Spain)

The isotope signature of cave waters provides an excellent opportunity to better understand the recharge in karst regions and the complexity of drainage systems in the vadose zone. We have developed a cave isotope hydrological model (ISODRIP) that requires entering basic hydrometeorological information and a precipitation δ18O record to simulate the discharge and δ18O signals of different drip sites. The model includes four different modules to simulate various flow route regimes: continuous and discontinuous drips under diffuse or preferential flows. We use precipitation and cave water δ18O records that were obtained in Eagle Cave (central Spain) during a 5-year period to test our model and to better understand the dynamics of karst aquifers. Eagle Cave waters do not record evaporation. The δ18O signals do not have seasonality, although they record intra-annual and inter-annual variability. Additionally, cave water δ18O signal falls within the range of the annual average weighted isotope composition of precipitation. Well-mixed cave waters, that characterize diffuse flows, record 1‰ δ18O variability, whereas partially-mixed waters, that flow along preferential drainage routes, have up to 3‰ δ18O variability. The results suggest that precipitation takes on average 15 months to reach the cave through the diffuse flow network, whereas under preferential flow the transit time is highly variable depending on the previous condition of the system. ISODRIP includes a soil layer above the vadose zone that controls large recharge events, together with direct recharge components that bypass the soil layer enabling at least some recharge all year round. Thus, the simulations reproduce the observed lack of seasonal bias in the cave water δ18O composition in relation to the average weighted isotope composition of precipitation. This research highlights the importance of understanding recharge dynamics and the configuration of particular drips sites to properly interpret speleothem δ18O records.

Skarn fluid sources as indicators of timing of Cordilleran arc emergence and paleogeography in the southwestern United States

Abstract Oxygen isotope ratios of garnet provide well-established means to investigate crustal fluid histories. Traditionally, δ18O values from skarn garnets have been used to track the hydrothermal evolution of an individual skarn body through time. We, however, use garnet from 14 skarns from the Jurassic (ca. 175 to ca. 148 Ma) Cordilleran margin arc (southwestern United States) to provide regional tectonic context to arc magmatism and hydrothermal activity. We document arc-wide garnet δ18O variability of ~19‰ (−8.9‰ to +10.3‰, n = 159), providing a record of contrasting meteoric fluid ingress between northern (Sierra Nevada) and southern (Mojave Desert) arc segments. Strongly negative garnet δ18O values (≤−3‰) are limited to the Mojave Desert arc segment and can only form in the presence of meteoric fluid, requiring shallow formation in subaerial crust. When combined with U-Pb garnet ages, the δ18O data provide a minimum radiometric age of local subaerial arc emergence and temporal constraint on the migration of the Jurassic paleoshoreline in the Mojave Desert section of the arc.

Hydrological characteristics of the Bay of Bengal water column using δ18O during the Indian summer monsoon

The Bay of Bengal, the eastern part of the northern Indian Ocean, experiences a reversal of monsoonal winds that control its surface and subsurface hydrological characteristics. Due to the limited number of studies, many aspects of the Bay of Bengal water column, particularly its deep hydrodynamic characteristics, are not thoroughly understood. For this purpose, oxygen isotopic compositions of water (δ18O), a potential tracer of oceanographic study, were measured at twelve discrete depths at eight locations in the central Bay of Bengal during the summer monsoon. The water columns were sampled to include surface (≤ 10 m), subsurface zone (25, depth of Deep Chlorophyll Maxima, 75, and 100 m), intermediate zone (150, 200, 300, and 500 m) and deep zone (1000, 1500, and 2000 m). Slopes of δ18O-salinity relationships at the surface and subsurface zones were similar. The δ18O and salinity showed an increasing southward trend at the surface, subsurface, and intermediate zones, which could be attributed to the influx of relatively saline and 18O enriched water from the Arabian Sea through Southwest Monsoon Current. The TS characteristics along with the negative slope of δ18O-salinity in the intermediate zone (− 0.51 ± 0.14) indicated the presence of Indonesian Throughflow water in the Bay of Bengal. Temporal variation in δ18O and slope of δ18O-salinity relationship in the intermediate zone was also observed, possibly indicating variable contribution of Indonesian Throughflow water mass through time. The δ18O of the deeper water showed the signature of existence of North Atlantic Deep Water in the Bay. Mesoscale eddies, a frequent phenomenon in the Bay of Bengal, also seem to modulate the δ18O-salinity relationship in the basin.