Seasonal Variation Of δ18O Research Articles

Spatiotemporal dynamics of stable isotopes of different water sources in western Himalayas: Insights into regional hydrological processes

The spatiotemporal variations in stable isotope ratios are primarily related to atmospheric moisture transport. However, the isotopic composition of stream water is significantly influenced by the diverse landscape of glaciated and snow-dominated basins. The present study investigates the spatiotemporal variation of the isotopic composition of stream water and potential water sources such as rain, snowmelt, glacier melt, lakes, and groundwater in the Upper Jhelum River Basin (UJRB). Our findings reveal that the significant seasonal variability of precipitation is mainly associated with the transition between westerlies (WDs) and the Indian Southwest Monsoon (ISWM), each conveying distinct moisture to the UJRB. The isotopic signature in rain is actively controlled by the combined effect of temperature (T) and relative humidity (RH) during WDs. However, the impact of “amount effect” with lower condensation temperature is dominant during the latter half of ISWM. Seasonal variations in δ18O and d-excess in stream water are distinct, influenced by different contributing sources and topographic controls. A linear relationship is observed between δ18O vs. altitude and glacier area ratio (GAR). Lower δ18O in stream water and higher d-excess values are observed in the southwest at higher elevations in glacier-fed tributaries. On the northern side, higher δ18O in stream water indicates contributions from non-glacier fed tributaries and enhanced groundwater contributions rather than merely following the lower δ18O of precipitation along the strengthened WDs gradient. The δ18O isotopic lapse rate (ILR) of rain (−0.19‰/100 m) is higher compared to ILR for the river (−0.05‰/100 m), reflecting the mixing of water sources in river water. However, both are lower than the global average (−0.28‰/100 m). This study helps in understanding the influence of meteorological, topographic factors, and other water sources on stream water, a prerequisite for projecting a potential future change in the complex Himalayan region.

Precipitation δ18O paced the seasonal δ18O variations of terrestrial snail body water and shells in the East Asian monsoon region

Terrestrial snails are sensitive to climate changes and their shell's oxygen isotope composition (δ18Oshell) is widely used in studies of paleoclimate reconstructions. However, the interpretation of δ18Oshell remains complex due to the combined effects from different factors such as precipitation δ18O (δ18Op), relative humidity (RH), and temperature. Furthermore, no systematic studies have addressed the transformation of oxygen isotope signals from rainfall to the aragonite shell via snail body water. Here we present the results of Cathaica fasciola body water δ18O (δ18OBW) and its content, which were sampled every two days throughout the growing season in 2021 in Xi'an. Meanwhile, the soil water δ18O (δ18OSW), δ18Op, rainfall amount, RH and temperature were also simultaneously monitored, and two high-resolution (at ∼0.3 mm interval) δ18Oshell series were obtained from two live snails collected in September and October 2021, respectively. These data reveal that the δ18OBW mimic the variations of δ18Op, and they are positively correlated (r2 = 0.84) for precipitation events and the correlation coefficient only increases slightly (r2 = 0.88) when RH is included in the multivariate analysis. During non-precipitation intervals, there is an obvious negative correlation between δ18OBW and RH. Moreover, the variations of two δ18Oshell series are broadly consistent with the theoretically calculated δ18Oshell from δ18OBW/δ18Op at seasonal scale. Collectively, this study demonstrates that δ18Op is the first-order control on the seasonal variations of δ18OBW and δ18Oshell, while RH play its role during non-precipitation periods. Importantly, this study lays good foundation for reconstructing terrestrial seasonal distribution of precipitation and extreme rainfall events using fossil shells in the geological past.

Extreme precipitation stable isotopic compositions reveal unexpected summer monsoon incursions in the Qilian Mountains

Isotope composition and moisture sources of precipitation are important for understanding water cycles and reconstructing paleoclimate. Based on 15-years' precipitation stable Isotope composition (δ18O and δ2H) from four stations of the Qilian Mountains, we found unique δ18O and δ2H features associated with the incursion of the summer monsoon over the Qilian Mountains, northwestern China. In 12 of the 15 years, similar seasonal variations of δ18O and δ2H confirmed a dominant source of moisture from Westerly circulation, and higher intercepts of the local meteoric water line (LMWL) indicated strong recycling of continental moisture. However, in August 2016 and 2018, extremely low slopes and intercepts of the LMWL, and more negative δ18O and δ2H revealed substantial contributions of the Asian summer monsoon to precipitation of the Qilian Mountains, with extremely heavy precipitation in August 2016. The column moisture flux, land-sea thermal contrast, correlations of precipitation δ18O with East Asian Summer Monsoon Index and Westerlies Index, HYSPLIT modeling results and precipitation δ18O along backward trajectories confirmed incursions of the summer monsoon in August 2016 and 2018. Our redefining of the boundary of the summer monsoon region confirmed the summer monsoon incursion zone can extend to the west of longitude 96°E and north of latitude 40°N in strong monsoon years, corresponding to boundaries of monsoon incursions in the mid-Holocene. Temperature correlated with precipitation δ18O at monthly and shorter time scales, but not for whole seasons or at yearly scale, revealing that summer monsoon incursions are therefore more likely than changing temperature to explain the multi-year cycles in the Qilian Mountains ice archives. Continent-scale shifts in atmospheric circulation strongly influence water resources in the Qilian mountains, and may change in frequency as climate warms. This study therefore has important implications for understanding water resources in the Qilian mountains in the past and into the future.

Sources and transformations of nitrogen in an agricultural watershed on the Jianghan Plain, China: an integration of δ15N–NH4+, δ15N–NO3-, δ18O–NO3- and a Bayesian isotope mixing model

Due to the complexity of nitrate sources and transformation processes under extensive human activities and climate change, there are a great number of uncertainties associated with ascertaining nitrogen sources and transformations in agricultural watersheds. In this study, multiple isotopes of ammonium (δ15N–NH4+), nitrate (δ15N–NO3-, δ18O–NO3-), water (δ18O–H2O) and hydrogeochemical data were integrated to identify nitrogen sources and elucidate the mechanisms of nitrogen transformation in the Quanshui River watershed, a typical agricultural watershed in central China. The characteristics of nitrate nitrogen and oxygen isotopes, lower δ15N–NO3- values (<9.73‰), and the results from the Bayesian isotope mixing model showed that the main sources of nitrogen in the watershed were mostly ammonium fertilizers (AF), nitrate fertilizer (NF) and soil organic nitrogen (SON). In the fertilization season, the enrichment of δ15N–NH4+ and the negative correlation between δ15N–NH4+ and δ15N–NO3- indicated that the volatilization and partial nitrification of nitrate in chemical fertilizers were the main factors affecting nitrogen isotope changes in the water. Seasonal changes in δ15N–NH4+ and δ15N–NO3- values can provide more direct isotopic evidence for the occurrence of nitrification, which may provide more useful insights into nitrogen transformation in future studies. Seasonal variations in δ18O–NO3- can effectively indicate the impact of rainfall events on the nitrogen cycle.

Chronological characteristics for snow accumulation on Styx Glacier in northern Victoria Land, Antarctica

AbstractUnder the potential to reconstruct the past climatic and atmospheric conditions from a deep ice core in the coastal Antarctic site (Styx Glacier), an 8.84 m long firn core (73°50.975′ S, 163°41.640′ E; 1623 m a.s.l.) was initially studied to propose a reliable age scale for the local estimation of snow accumulation rate. The seasonal variations of δ18O, methanesulfonic acid (MSA) and non-sea-salt sulfate (nssSO42–) were used for the firn core dating and revealed 25 annual peaks (from 1990 to 2014) with volcanic sulfate signal. The observed declining trend in annual accumulation rate with a mean value of 146 ± 60 kg m–2a–1is likely to be linked to the changes of sea-ice extent in the Ross Sea region. Moreover, the temporal variation of the annual mean δ18O, an annual flux of MSA and nssSO42–also likely to be under the influence of ice-covered and open water area. This study suggests a potential to recover past changes in an oceanic environment and will be useful for the interpretation of the long ice core drilled at the same site.

How does varying water supply affect oxygen isotope variations in needles and tree rings of Scots pine?

In many regions, drought is suspected to be a cause of Scots pine decline and mortality, but the underlying physiological mechanisms remain unclear. Because of their relationship to ecohydrological processes, δ18O values in tree rings are potentially useful for deciphering long-term physiological responses and tree adaptation to increasing drought. We therefore analyzed both needle- and stem-level isotope fractionations in mature trees exposed to varying water supply. In a first experiment, we investigated seasonal δ18O variations in soil and needle water of Scots pine in a dry inner Alpine valley in Switzerland, comparing drought-stressed trees with trees that were irrigated for more than 10years. In a second experiment, we analyzed twentieth-century δ18O variations in tree rings of the same forest, including a group of trees that had recently died. We observed less 18O enrichment in needle water of drought-stressed compared with irrigated trees. We applied different isotope fractionation models to explain these results, including the Péclet and the two-pool correction, which considers the ratio of unenriched xylem water in the needles to total needle water. Based on anatomical measurements, we found this ratio to be unchanged in drought-stressed needles, although they were shorter. The observed lower 18O enrichment in needles of stressed trees was therefore likely caused by increased effective path length for water movement within the leaf lamina. In the tree-ring study, we observed lower δ18O values in tree rings of dead trees compared with survivors during several decades prior to their death. These lower values in declining trees are consistent with the lower needle water 18O enrichment observed for drought-stressed compared with irrigated trees, suggesting that this needle-level signal is reflected in the tree rings, although changes in rooting depth could also play a role. Our study demonstrates that long-term effects of drought are reflected in the tree-ring δ18O values, which helps to provide a better understanding of past tree physiological changes of Scots pine.

Acquisition of Post-Depositional Effects on Stable Isotopes (δ18O and δD) of Snow and Firn at Dome A, East Antarctica

Water stable isotopes (δ18O and δD) in Antarctic snow pits and ice cores are extensively applied in paleoclimate reconstruction. However, their interpretation varies over some climate change processes that can alter isotope signals after deposition, especially at sites with a low snow accumulation rate (&lt;30 mm w.e. year−1). To investigate post-depositional effects during the archival processes of snow isotopes, we first analyzed δ18O and δD variations in summer precipitation, surface snow and snow pit samples collected at Dome A. Then, the effects of individual post-depositional processes were evaluated from the results of field experiments, spectral analysis and modeling simulations. It was found that the sublimation–condensation cycle and isotopic diffusion were likely the dominant processes that modified the δ18O at and under the snow–air interface, respectively. The sublimation–condensation cycle can cause no significant isotopic modification of δ18O from field experiments with ~3 cm snow. The diffusion process can significantly erase the original seasonal variation of δ18O driven by atmospheric temperature, leading to an apparent cycle of ~20 cm average wavelength present in the δ18O profile. Through the comparison with the artificial isotopic profile, the noise input from the diffusion process was the dominant component in the δ18O signal. Although some other processes (such as drifting, ventilation and metamorphism) were not fully considered, the quantitative understanding for the sublimation–condensation and diffusion processes will contribute to the paleoclimate construction using the ice core water isotope records at Dome A.

Overview of the Chemical and Isotopic Investigations of the Mareza Springs and the Zeta River in Montenegro

The Mareza karst aquifer is the most important drinking water resource for the water supply system of the City of Podgorica, the capital of Montenegro. This study presents the first assessment for the determination of the Mareza catchment area. Water chemistry and stable isotopic composition (δ18O and δ2H) of monthly precipitation samples (as inputs) are presented, in order to determine the Local Meteoric Water Line (LMWL) for the study area, and to analyze the behavior of the karst spring Mareza (as output) and the Zeta River water. The possible impact of the river on the Mareza springs was also investigated. Stable isotope compositions were used to analyze the origin of the four springs of the Mareza aquifer. Seasonal variations of δ18O and δ2H values and deuterium excess (d excess) changes in precipitation are explained by the mixing of air masses, such that a Mediterranean source prevails in the winter period, while in the summer period, the area is rather under the influence of air mass originating from the Atlantic Ocean. All spring water samples have lower δ values than the local precipitation and they plot above the LMWL, which may indicate recharge at a higher altitude in the distant mountainous area. The d excess values of all water samples (higher than 10‰) indicate the prevalence of the Mediterranean as a moisture source. Based on the analysis of the seasonal variations of δ18O and δ2H in precipitation and the Mareza spring, it has been estimated that the groundwater mean transit time (MTT) is 92–129 days, and that the young water fraction (Fyw) amounts to 40.9%–53.3%. These values are typical for the strong karstic springs of highly karstified terrains.

Isotopic evidence for the moisture origin and influencing factors at Urumqi Glacier No.1 in upstream Urumqi River Basin, eastern Tianshan Mountains

The stable isotope has been extensively applied as an effective tracer especially in precipitation. In glacierized area of arid northwest China, temperature is widely considered to be a major factor affecting isotopes in precipitation, while the influences of precipitation amount, relative humidity and other meteorological parameters are still not clear. Based on analyses on stable isotope values of water samples and NCEP/NCAR (National Centers of Environmental Prediction/National Center for Atmospheric Research, USA) re-analysis data, the moisture source and characteristics of isotopes in the precipitation, meltwater and river water isotopes at Urumqi Glacier No.1 of the upstream Urumqi River Basin, eastern Tianshan Mountains from spring to autumn during four years (from 2008 to 2011) was studied. Results indicated that meltwater are the main source of water for the upper Urumqi River. Seasonal variation of δ18O in precipitation demonstrated that δ18O was more enriched in summer and depleted in spring and autumn. Temperature was positively correlated with isotopes, while precipitation amount and relative humidity was negatively correlated with isotopes. The water vapor was affected by westerly air mass and regional water vapor cycle. Meanwhile, back trajectory clustering analyses showed that the moisture mainly from Europe and central Asia. The moisture was more likely to be locally sourced with the ratio was 46.8%~52.1%.

Climate information preserved in seasonal water isotope at NEEM: relationships with temperature, circulation and sea ice

Abstract. Analyzing seasonally resolved δ18O ice core data can aid the interpretation of the climate information in ice cores, also providing insights into factors governing the δ18O signal that cannot be deciphered by investigating the annual δ18O data only. However, the seasonal isotope signal has not yet been investigated in northern Greenland, e.g., at the NEEM (North Greenland Eemian Ice Drilling) ice core drill site. Here, we analyze seasonally resolved δ18O data from four shallow NEEM ice cores covering the last 150 years. Based on correlation analysis with observed temperature, we attribute about 70 and 30 % of annual accumulation to summer and winter, respectively. The NEEM summer δ18O signal correlates strongly with summer western Greenland coastal temperature and with the first principal component (PC1) of summer δ18O from multiple seasonally resolved ice cores from central/southern Greenland. However, there are no significant correlations between NEEM winter δ18O data and western Greenland coastal winter temperature or southern/central Greenland winter δ18O PC1. The stronger correlation with temperature during summer and the dominance of summer precipitation skew the annual δ18O signal in NEEM. The strong footprint of temperature in NEEM summer δ18O record also suggests that the summer δ18O record rather than the winter δ18O record is a better temperature proxy at the NEEM site. Despite the dominant signal of the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) in the central–southern ice core data, both NAO and AMO exert weak influences on NEEM seasonal δ18O variations. The NEEM seasonal δ18O is found to be highly correlated with Baffin Bay sea ice concentration (SIC) in the satellite observation period (1979–2004), suggesting a connection of the sea ice extent with δ18O at NEEM. NEEM winter δ18O significantly correlates with SIC even for the period prior to satellite observation (1901–1978). The NEEM winter δ18O may reflect sea ice variations of Baffin Bay rather than temperature itself. This study shows that seasonally resolved δ18O records, especially for sites with a seasonal precipitation bias such as NEEM, provide a better understanding of how changing air temperature and circulation patterns are associated with the variability in the δ18O records.

Identifying evaporation fractionation and streamflow components based on stable isotopes in the Kaidu River Basin with mountain–oasis system in north‐west China

AbstractBased on stable isotopes in stream water, groundwater, and meltwater in the Kaidu River Basin, NW China, we estimated the evaporation enrichment of stable oxygen isotopes in different types of water and separated the contribution of each streamflow component in river run‐off. Our results indicated that δ18O and δ2H in stream water did not vary with altitude regularly but with seasons, with low concentrations in spring and high concentrations in summer. However, the seasonal variations of δ18O and δ2H in groundwater were not as obvious. The mean evaporation enrichment was between 26% and 44% for δ18O. Of the various water types under investigation, we found glaciers were influenced the most, showing an evaporation enrichment of 44%, followed by oasis groundwater (37%), stream water (36%), and mountain groundwater (26%). Overall, meltwater and groundwater were the predominant streamflow components, with their contributions were governed by temperature, and varied both temporally and specially. In the oasis region, groundwater was the predominant contributor (64% in spring, 50% in summer, and 66% in autumn), whereas in the mountains, groundwater was the dominant in spring (53%) and autumn (51%), and meltwater contributed the most in summer (52%). Precipitation contributed less than 15% to the streamflow.

The potential of near-entrance stalagmites as high-resolution terrestrial paleoclimate proxies: Application of isotope and trace-element geochemistry to seasonally-resolved chronology

Sub-annually resolved environmental proxies can be valuable archives of climate change, but they are rare in terrestrial settings, and it can be difficult to verify their annual nature. We suggest that speleothems that grow in well-ventilated zones of caves may preserve such high-resolution records. Near-entrance cave environments are characterized by year-round, near-atmospheric CO2 concentrations and are significantly influenced by surface air temperature fluctuations, particularly in temperate latitudes. Previous monitoring studies of a well-ventilated, temperate-latitude cave (Westcave Preserve, central Texas) have documented seasonal variations in the oxygen isotope composition of calcite grown on glass substrates (with winter δ18O maxima and summer δ18O minima) as well as seasonal variations in drip water trace element compositions. Extending this work to a stalagmite (WC-3) from the same drip site, we find that stalagmite δ18O variations are similar in magnitude to the seasonal δ18O variations previously observed for calcite grown on glass substrates, that stalagmite [Mg] variations have a similar seasonal period with winter minima and summer maxima, and that geochemical variations follow stalagmite growth fabric as mm-scale couplets comprising thin, slow-growing, compact sparry calcite laminae (winters) and thicker, fast-growing, porous-elongate columnar calcite laminae (summers). We interpret a high-resolution (weekly to monthly) 52-year record of δ18O, Mg, Sr, and Ba in WC-3, and report new monthly measurements of drip water and associated calcite grown on glass substrates. We find drip water δ18O and [Mg]/[Ca] are essentially invariant and that seasonal variations in WC-3 calcite δ18O and Mg concentration agree well with predicted temperature-dependent fractionation between water and calcite. WC-3 calcite Sr and Ba also vary, but with higher and more variable frequencies compared to δ18O and [Mg]. The annual nature of δ18O and [Mg] cycles is supported by monitoring and 14C bomb-peak chronology. We suggest that stalagmite δ18O and [Mg] vary primarily in response to large seasonal temperature changes in this setting, allowing for unambiguous differentiation between summer and winter calcite growth. From such δ18O- and [Mg]- derived sub-annual chronologies, the timing of enrichments in other geochemical species that are less directly coupled to external cave temperature (e.g., calcite Sr and Ba) can be considered as proxies of other important processes such as water-rock interaction in the epikarst, precipitation events, or subsurface respiration rates. The potential for this kind of multi-proxy, seasonally-resolved dating may add near-entrance stalagmites to the list (ice cores, lake varves, tree rings) of high-resolution terrestrial proxies available for paleoclimate studies.

Seasonal variation of δ18O and δ2H in leaf water of Fagus sylvatica L. and related water compartments

The study aims to assess variability in leaf water isotopic enrichment occurring in the field under natural conditions. We focused on seasonal variation and difference between sun-exposed and shaded leaves. Isotopic composition (δ18O, δ2H) of leaf water was monitored in a beech tree (Fagus sylvatica L.) growing in the forest-meadow ecotone together with δ18O (2H) of water compartments which are in close relation to this signal, namely twig and soil water. The sampling was carried out in approximately two-week intervals during five consecutive vegetation seasons.The δ18O (2H) data showed a distinct seasonal pattern and a consistency in relative differences between the seasons and sample categories. Leaf water was the most isotopically enriched water compartment. The leaf water enrichment decreased toward the autumn reflecting the change in δ18O (2H) of source water and evaporative demands. The soil and twig water isotopic signal was depleted against current precipitation as it partly retained the isotopic signature from winter precipitation however the seasonal pattern of soil and twig water followed that of precipitation. No significant differences between sun-exposed and shaded samples were detected. Nevertheless, the observed strong seasonal pattern of isotope composition of leaf, twig and soil water should be taken into account when using leaf water enrichment for further calculations or modeling.

Sample preparation of ring-less tropical trees for δ&lt;sup&gt;18&lt;/sup&gt;O measurement in isotope dendrochronology

Radial variation of δ18O before and after cellulose extraction was assessed in Acacia auriculiformis, Eucalyptus camaldulensis, and Celtis timorensis growing in northeastern Thailand to examine the necessity of extracting α-cellulose to detect annual rings from trees without visible rings. Optimum sampling resolution to detect peaks in the radial variation of δ18O values was also examined. Cored samples were sectioned into 0.2mm thickness in the radial direction. Each circular section sliced from a wood core sample were divided along wood grain into two semicircular sections, both of which were located at the same radial and longitudinal positions, and were side-by-side tangentially. One half was used for bulk analysis and the other for extraction. Peak positions were assigned from the seasonal variation of δ18O. The δ18O values cyclically changed in both bulk wood and α-cellulose. The correlation coefficient between bulk wood and α-cellulose δ18O was high in every species, and the offset was almost constant across the radial position. The mean cycle length of one sample was longer than those of the other two samples, although annual increment based on dendrometer monitoring was smaller than those of the other two samples. That is, the seasonal variation in δ18O values recorded in the xylem was not completely detected because of low amplitude or insufficient radial resolution. Therefore, we concluded that α-cellulose extraction is unnecessary for annual ring detection. It is necessary to determine an appropriate sampling resolution based on growth rate for effective peak detection.

Impacts of Recent Warming and the 2015/2016 El Niño on Tropical Peruvian Ice Fields

AbstractData collected between 1974 and 2016 from snow pits and core samples from two Peruvian ice fields demonstrate the effect of the recent warming over the tropical Andes, augmented by El Niño, on the preservation of the climate record. As the 0°C isotherm is approaching the summit of the Quelccaya ice cap in the Andes of southern Peru (5,670 meters above sea level (masl)), the distinctive seasonal δ18O oscillations in the fresh snow deposited within each thermal year are attenuated at depth due to melting and percolation through the firn. This has become increasingly pronounced over 43 years. In the Andes of northern Peru, the ice field on the col of Nevado Huascarán (6050 masl) has retained its seasonal δ18O variations at depth due to its higher elevation. During the 2015/2016 El Niño, snow on Quelccaya and Huascarán was isotopically (δ18O) enriched and the net sum of accumulation over the previous year (NSA) was below the mean for non–El Niño years, particularly on Quelccaya (up to 64% below the mean) which was more pronounced than the NSA decrease during the comparable 1982/1983 El Niño. Interannual large‐scale oceanic and middle to upper‐level atmospheric temperatures influence δ18O in precipitation on both ice fields, although the influences are variably affected by strong El Niño–Southern Oscillation events, especially on Quelccaya. The rate of ice wastage along Quelccaya's margin was dramatically higher during 2015/2016 compared with that of the previous 15 years, suggesting that warming from future El Niños may accelerate mass loss on Peruvian glaciers.

Ambient Conditions and Feeding Strategy Influence δ18O of Milk Water in Cows (Bos taurus).

There are increasing concerns by consumers regarding agricultural product traceability and authenticity. Oxygen isotope composition (δ18O) has been used in this context based on the relationship between δ18O of animal products and annual precipitation. However, in dairy products this relationship is affected by the seasonality of δ18O in milk water which in turn depends on the feeding system used. We measured 608 milk samples from 28 farms with various feeding strategies in southern Germany throughout the year, investigating the influences of ambient conditions, drinking water source, and feeding strategies on seasonal variation of δ18O in milk water (δmilk). The mechanistic Munich-Kohn model reflecting these influences predicted the seasonal and farm-specific variation of δmilk well. The relationship between δ18O of precipitation and δmilk varied in different feeding strategies. The interplay of ambient conditions and feeding strategy on δmilk should thus be carefully considered when identifying the origin of milk.

Model explanation of the seasonal variation of \u03b418O in cow (Bos taurus) hair under temperate conditions

Oxygen isotopes (δ18O) in animal and human tissues are expected to be good recorders of geographical origin and migration histories. However, seasonal variation of δ18O may diminish the origin information in the tissues. Here the seasonality of δ18O in tail hair was investigated in a domestic suckler cow (Bos taurus) that underwent different ambient conditions, physiological states, keeping and feeding during five years. A detailed mechanistic model was built to explain this variation. The measured δ18O in hair significantly related (p < 0.05) to the δ18O in meteoric water in a regression analysis. Modelling suggested that this relation was only partly derived from the direct influence of feed moisture. Ambient conditions (temperature, moisture) also affected the animal itself (drinking water demand, transcutaneous vapor etc.). The clear temporal variation thus resulted from complex interactions with multiple influences. The twofold influence of ambient conditions via the feed and via the animal itself is advantageous for tracing the geographic origin because δ18O is then less influenced by variations in moisture uptake; however, it is unfavorable for indicating the production system, e.g. to distinguish between milk produced from fresh grass or from silage. The model is versatile but needs testing under a wider range of conditions.

Snowdrift effect on snow deposition: Insights from a comparison of a snow pit profile and meteorological observations in east Antarctica

A high-frequency and precise ultrasonic sounder was used to monitor precipitated/deposited and drift snow events over a 3-year period (17 January 2005 to 4 January 2008) at the Eagle automatic weather station site, inland Antarctica. Ion species and oxygen isotope ratios were also generated from a snow pit below the sensor. These accumulation and snowdrift events were used to examine the synchronism with seasonal variations of δ18O and ion species, providing an opportunity to assess the snowdrift effect in typical Antarctic inland conditions. There were up to 1-year differences for this 3-year-long snow pit between the traditional dating method and ultrasonic records. This difference implies that in areas with low accumulation or high wind, the snowdrift effect can induce abnormal disturbances on snow deposition. The snowdrift effect should be seriously taken into account for high-resolution dating of ice cores and estimation of surface mass balance, especially when the morphology of most Antarctic inland areas is similar to that of the Eagle site.

Record of Nile seasonality in Nubian neonates

ABSTRACTThe oxygen isotope compositions of bones (n = 11) and teeth (n = 20) from 12 Sudanese individuals buried on Sai Island (Nubia) were analysed to investigate the registration of the evolution of the Nile environment from 3700 to 500 years BP and the potential effects of ontogeny on the oxygen isotope ratios. The isotopic compositions were converted into the composition of drinking water, ultimately originating from the Nile. δ18O values decrease during ontogeny; this is mainly related to breastfeeding and physiology. Those of neonates present very large variations. Neonates have a very high bone turnover and are thus able to record seasonal δ18O variations of the Nile waters. These variations followed a pattern very similar to the present one. Nile δ18O values increased from 1.4 to 4.4 ‰ (Vienna Standard Mean Ocean Water) from the Classic Kerma (∼3500 BP) through the Christian period (∼1000 BP), traducing a progressive drying of Northeast Africa.

Numerical experiments on the impacts of surface evaporation and fractionation factors on stable isotopes in precipitation

The isotope enabled atmospheric water balance model is applied to examine the spatial and temporal variations of δ18O in precipitation, amount effect and meteoric water lines (MWL) under four scenarios with different fractionation nature and surface evaporation inputs. The experiments are conducted under the same weather forcing in the framework of the water balance and stable water isotope balance. Globally, the spatial patterns of mean δ18O and global MWLs simulated by four simulation tests are in reasonably good agreement with the Global Network of Isotopes in Precipitation observations. The results indicate that the assumptions of equilibrium fractionation for simulating spatial distribution in mean annual δ18O and the global MWL, and kinetic fractionation in simulating δ18O seasonality are acceptable. In Changsha, four simulation tests all reproduce the observed seasonal variations of δ18O in precipitation. Compared with equilibrium fractionation, the depleted degree of stable isotopes in precipitation is enhanced under kinetic fractionation, in company with a decrease of isotopic seasonality and inter-event variability. The alteration of stable isotopes in precipitation caused by the seasonal variation of stable isotopes in vapour evaporated from the surface is opposite between cold and warm seasons. Four simulations all produce the amount effect commonly observed in monsoon areas. Under kinetic fractionation, the slope of simulated amount effect is closer to the observed one than other scenarios. The MWL for warm and humid climate in monsoon areas are well simulated too. The slopes and intercepts of the simulated MWLs decrease under kinetic fractionation.