Precipitation δ18O Research Articles

Temperature Trends in the Northwestern Tibetan Plateau Constrained by Ice Core Water Isotopes Over the Past 7,000 Years

AbstractThe reasons for the Holocene temperature conundrum, known as the inconsistency between the reconstructed cooling and the inferred warming simulations during the Holocene, remain unclear. Temperature reconstructions from the Tibetan Plateau (TP) provide important insights for understanding the Holocene temperature conundrum due to enhanced sensitivity to climate at high altitudes. Given the significant positive correlation between air temperature and δ18O in precipitation over the northern TP, the stable isotopic records of ice cores recovered from this area are widely used for paleotemperature reconstruction. Here we present a new high‐resolution δ18O record from the Chongce ice cores to bedrock, dated back to 7 ka BP by the accelerator mass spectrometry (AMS) 14C dating technique. Our reconstructed temperature record shows a long‐term warming trend until ~2 ka BP, followed by an abrupt change to a relatively cool period until the start of the industrial‐era warming. This record challenges the widely recognized Holocene reconstruction from the neighboring Guliya ice core. It is also different from many previous temperature reconstructions, most of which have summer biases and show a long‐term cooling trend over the past two millennia. In addition, our record shows that temperatures during the recent decades are almost the highest during the past 7 ka BP, highlighting the unusual warming forced by anthropogenic greenhouse gases.

Determination of soil-groundwater systems recharge mechanism in the middle Inner Mongolia Plateau by isotopic tracers

The steppe located in the middle Inner Mongolia Plateau (IMP) has scarce water resources and fragile ecosystems and has an arid-semiarid monsoon characterized by low and uneven seasonal precipitation and high evaporation. Until now, problems relating to the hydrologic processes in the area remain poorly understood, and thus strategies for the exploitation and utilization of the area’s local water resource remain limited. This study aimed to investigate the recharge and discharge processes of soil water and the source and circulation of groundwater through geochemical methods. The local meteoric water line (LMWI) of our study area in the middle IMP was first proposed and compared with the local meteoric water line of Baotou in Inner Mongolia and Ulan Bator. The δ18O and δD values of precipitation showed an increasing trend from June to August and decreasing trend from August to late October. The H and O isotopes of soil water varied with time, location, and soil profile depth. Besides the geographical and climatic factors, the Cl concentrations of soil water can be affected remarkably by land use type and land cover change. In our monitoring period, variations in the Cl concentrations and δ18O and δD values of shallow soil water indicated that the evaporation of soil water was in the sequence August > June > October. Furthermore, the detailed evaporation proportions of soil water were estimated. The isotope compositions of groundwater plotted on the LMWI and there could be a rapid recharge mechanism of the groundwater.

Isotope hydrology and hydrogeochemical modeling of Troodos Fractured Aquifer, Cyprus: The development of hydrogeological descriptions of observed water types

The origin of groundwater recharge and subsequent flow paths are often difficult to establish in fractured, multi-lithological, and highly compartmentalized aquifers such as the Troodos Fractured Aquifer (TFA). As the conjunctive use of stable isotopes and hydrogeochemical data provides additional information, we established a monitoring network for stable isotopes in precipitation in Cyprus. The local meteoric water line, altitude effect and seasonal variation of stable isotopes in precipitation are derived from monitoring data. Stable isotopes and hydrogeochemical data are combined to model water-rock interactions and groundwater evolution along a complete ophiolite sequence. As a result a generic hydrogeologic description for the observed water types is developed. Isotope hydrology was applied in conjunction with hydrogeochemical modelling in Kargiotis Watershed, a major north-south transect of the TFA. PHREEQC was used for hydrogeochemical modelling to establish generic descriptions for observed water types. Mean precipitation-weighted values from 16 monitoring stations were used to calculate the Local Meteoric Water Line (LMWL), which was found to be equal to δ2H = (6.58 ± 0.13)*δ18O + (12.64 ± 0.91). A general decrease of 1.22‰ for δ2H and 0.20‰ for δ18O in precipitation was calculated per 100 m altitude. A generic groundwater evolution path was established: 1. Na/MgClHCO3, 2. MgHCO3, 3. Ca/MgHCO3, 4. Ca/MgNaHCO3, 4a. MgNa/CaHCO3/Cl, 5. NaMg/CaHCO3/Cl, 6. NaHCO3, 7. Na/MgHCO3SO4, 8. NaSO4Cl/HCO3. Hydrogeologic descriptions, consisting of groundwater origin, flow path and possible active water-rock processes, have been realised for the observed water types. The first two water types occur in serpentine and ultramafic-gabbro springs. Type 3 waters represent early stages of recharge and/or short flow paths, in gabbro whereas types 4 and 5 are typical for further percolating waters in gabbro and diabase. Water types 6 and 7 occur both in diabase and in the basal group and represent the regional flow. Water type 8 is the end member of regional, upwelling groundwater in the basal group. The presented descriptions and methods have practical applications in groundwater exploration, characterization, and protection. The methodology can be applied in other complex aquifer systems.

Atmospheric constraints on δ18O and d-excess in precipitation at the middle latitude in the southwestern Atlantic region

ABSTRACT The mid-latitude coastal area at the western South-Atlantic out of the tropics is under the combined effect of different atmospheric circulation patterns at different temporal scales, which can be shown by the isotope composition of precipitation. This pattern effect is more changing and complex than that for the well-studied tropical areas, the isotope studies being an interesting proxy for identifying major processes. This study is focused on the isotope composition of precipitation at a mid-latitude zone, in the western South-Atlantic coastal area of Argentina. δ18O and d-excess were analysed in a data series of 14 years, obtained from the integration of three neighbouring rain collectors at 38°S. A seasonality is observed in both parameters, but with some differences in the extreme months. δ18O showed a seasonality according to the temperature effect, but with a displacement of high values to spring months. Significant linear links between δ18O and Southern Annular Mode (SAM) index and Southern Oscillation Index (SOI) were recognised for the summer and spring seasons, respectively.

Modern calibration of <i>Poa flabellata</i> (tussac grass) as a new paleoclimate proxy in the South Atlantic

Abstract. Terrestrial paleoclimate records are rare in the South Atlantic, limiting opportunities to provide a prehistoric context for current global changes. The tussock grass, Poa flabellata, grows abundantly along the coasts of the Falkland Islands and other subantarctic islands. It forms extensive peat records, providing a promising opportunity to reconstruct high-resolution regional climate records. The isotopic composition of leaf and root tissues deposited in these peats has the potential to record variation in precipitation, temperature, and relative humidity over time, but these relationships are unknown for P. flabellata. Here, we measured the isotopic composition of P. flabellata and precipitation and explore relationships with seasonal temperature and humidity variations across four study locations in the Falkland Islands. We reveal that inter-seasonal differences in carbon and oxygen stable isotopes of leaf α-cellulose of living P. flabellata correlated with monthly mean temperature and relative humidity. The carbon isotope composition of leaf α-cellulose (δ13Cleaf) records the balance of CO2 supply through stomata and the demand by photosynthesis. The positive correlation between δ13Cleaf and temperature and negative correlation between δ13Cleaf and relative humidity suggest that photosynthetic demand for CO2 relative to stomatal supply is enhanced when conditions are warm and dry. Further, the positive correlation between δ13Cleaf and δ18Oleaf (r=0.88; p<0.001; n=24) indicates that stomatal closure during warm dry periods explains seasonal variation in δ13Cleaf. We observed significant differences between winter and summer seasons for both δ18Oleaf and δ13Cleaf and among study locations for δ18Oleaf but not δ13Cleaf. δ18O values of monthly composite precipitation were similar between seasons and among study locations, yet characteristic of the latitudinal origin of storm tracks and seasonal winds. The weak correlation between δ18O in monthly composite precipitation and δ18Oleaf further suggests that relative humidity is the main driver of the δ18Oleaf. The oxygen isotopes in root α-cellulose did not reflect, or only partially reflected (at one study location), the δ18O in precipitation. Overall, this study supports the use of peat records formed by P. flabellata to fill a significant gap in our knowledge of the long-term trends in Southern Hemisphere climate dynamics.

An Asian Summer Monsoon-Related Relative Humidity Record from Tree-Ring δ18O in Gansu Province, North China

The monsoon fringe region in North China (NC) is also an ecologically fragile zone. Improving our comprehension of the paleoclimate variations and their driving mechanisms in this region has great significance for environmental protection and agricultural economic development. In order to provide more reliable data for future climate forecasting and reduce the effects of climatic disasters in NC, we established a 328-year stable oxygen isotope (δ18O) chronology based on four Pinus tabulaeformis Carr. from Mt. Hasi, Gansu Province, and found that the tree-ring δ18O inherited the signals of summer (July–August) monsoonal precipitation δ18O (δ18OP). Correlation function analysis indicated that the tree-ring δ18O series responded significantly to the observed local relative humidity from July to August (RHJA) with r = −0.65 (n = 55, p < 0.001). Based on the clear physiological mechanism, we reconstructed the RHJA variations from 1685 to 2012 using a transfer function. Our reconstruction was very stable and had strong spatial representativeness, it was significantly positively correlated with Asian summer monsoon (ASM) indices, indicating that our reconstruction reflected the variations of ASM to a large extent. The RHJA series successfully captured the weakening of the ASM since the 1930s. There was a close connection between the reconstructed sequence and the East Pacific sea surface temperature (SST). Further analyses revealed that El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) may play important roles in the summer monsoon precipitation in NC.

Characteristics of Stable Isotopes and Moisture Sources of Two Typical Precipitation Events in Lanzhou City

To better understand the isotope variations on a short time scale, this study focused on a long-term rainfall event with light precipitation (June 26-27) and a short-term one with heavy precipitation (July 28) in Lanzhou City in the summer of 2019. Combined with HYSPLIT model, samples collected during a continuous precipitation event every 10 min and 30 min were analyzed to explore the characteristics and mechanism of stable hydrogen and oxygen isotopes in precipitation. The results indicate that the effect of sub-cloud secondary evaporation makes the slope of the sequential meteoric water line (SMWL) smaller at the beginning of the rainfall event. Most of the continuous sampling points are distributed above the global meteoric water line (GMWL) and local meteoric water line (LMWL). Moreover, the deuterium excess is larger than the local average annual deuterium (8.13), indicating that the samples have experienced moisture recycling to a certain extent. During two consecutive days (June 26-27) of rainfall, the variations in oxygen isotope δ18O did not follow the effect of precipitation amount; the precipitation δ18O of the first day was "L" shaped, and it fluctuated the next day. On July 28, δ18O steadily decreased, and the range of δ18O exceeded 9‰. On June 26, the moisture transport path was short at the height of 500 m and on June 27 local evaporation was the main pathway. On July 28, with a relatively stable air mass, the moisture source of the entire precipitation event did not change significantly, neither did the isotope value. Therefore, for a single precipitation event on a short time scale, the difference in moisture sources is one of the reasons for isotope variations.

Controls on spatiotemporal variations of stable isotopes in precipitation across Bangladesh

Indian summer monsoon (ISM) driven moisture from the Indian Ocean is important for moisture transport to the Asian Water Towers and causing floods and droughts in Bangladesh. Stable isotopic compositions of precipitation (δ18O and δD) are crucial tracers of ISM moisture transport processes. Here we presented spatiotemporal variations of stable isotopes in precipitation at three stations over Bangladesh in 2017–2018 to evaluate the influence of ISM on intra-seasonal variations of stable isotopes in precipitation in the south of Himalaya, combined with local meteorological data, ERA5 reanalysis and HYSPLIT model. We found that precipitation δ18O displayed the highest values in May following gradual decrease till the lowest in October. The spatial decrease of δ18O and δD from southwest to northeast appeared in pre-monsoon and monsoon seasons together with increasing of d-excess eastwards. The weak temperature effect and amount effect were detected at Satkhira only at the daily scale in specific seasons, and the significantly negative correlations between relative humidity and δ18O were almost detected at three stations in both monsoon and non-monsoon seasons. We suggested that convections over Bay of Bengal (BOB) and tropical Indian Ocean and variable contributions of moisture sources are important factors controlling distinct variations of precipitation δ18O from west to east Bangladesh. Our results provided a better understanding of the ISM influence on precipitation stable isotopes and variations of the ISM moisture sources, which is crucial to address hydroclimatic interpretations relevant with ISM.

Stable Isotope Hydrology of Cave Groundwater and Its Relevance for Speleothem-Based Paleoenvironmental Reconstruction in Croatia

Speleothems deposited from cave drip waters retain, in their calcite lattice, isotopic records of past environmental changes. Among other proxies, δ18O is recognized as very useful for this purpose, but its accurate interpretation depends on understanding the relationship between precipitation and drip water δ18O, a relationship controlled by climatic settings. We analyzed water isotope data of 17 caves from different latitudes and altitudes in relatively small but diverse Croatian karst regions in order to distinguish the dominant influences. Drip water δ18O in colder caves generally shows a greater resemblance to the amount-weighted mean of precipitation δ18O compared to warmer sites, where evaporation plays an important role. However, during glacial periods, today’s ‘warm’ sites were cold, changing the cave characteristics and precipitation δ18O transmission patterns. Superimposed on these settings, each cave has site-specific features, such as morphology (descending or ascending passages), altitude and infiltration elevation, (micro) location (rain shadow or seaward orientation), aquifer architecture (responsible for the drip water homogenization) and cave atmosphere (governing equilibrium or kinetic fractionation). This necessitates an individual approach and thorough monitoring for best comprehension.

Geographic variations in the slope of the δ 2 H – δ 18 O meteoric water line over Europe: a record of increasing continentality

Abstract δ 2 H and δ 18 O values of precipitations follow an empirical linear relationship at the global scale that is called the Global Meteoric Water Line (GMWL) and characterized by a slope of 8. However, Local Meteoric Water Lines (LMWLs) may have different slopes S depending on their geographic situation. Monthly δ 2 H and δ 18 O of precipitation have been compiled from European International Atomic Energy Agency (IAEA) stations. Those data allowed the calculation of the slopes S of the δ 2 H– δ 18 O LMWL determined for each station. S increases with longitude ϕ from c. 5 (Portugal) to c. 9 (Russia) – they are positively correlated with relative humidity (RH), negatively with temperature and positively with the mean intra-annual amplitude of temperatures, which is a proxy of continentality. Slopes of 5–6, recorded in SW Europe, reflect mean RH (70–75%) and sea surface temperatures ( c. 25°C) of the Central Atlantic Ocean where the main flux of moisture is formed before being transported by the westerlies. In addition, falling water droplets within an air column with a high RH (>80%) and low temperature are expected to escape sub-cloud evaporation. Therefore, slopes with values close to 9 are considered to reflect isotopic equilibrium conditions during the condensation of water vapour in clouds.

Tree-ring δ18O from Southeast China reveals monsoon precipitation and ENSO variability

Considerable advances have been made in use of tree-ring δ18O for climate reconstructions in Southeast China, the East Asian summer monsoon region. However, the relationships among tree-ring cellulose δ18O (δ18Ocell), local hydroclimate, precipitation δ18O (δ18Opre) and El Niño–Southern Oscillation (ENSO) have not yet been fully resolved. The usual interpretation has been that local hydroclimate influences δ18Ocell through both the “amount effect” on δ18Opre and evaporative enrichment, but it cannot fully explain the high inter-site correlation of δ18Ocell chronologies or their response pattern to ENSO. In this study, we use a newly-developed δ18Ocell chronology of Pinus massoniana from a water-stressed site in Zhejiang province, in combination with another three δ18Ocell chronologies in Southeast China, to investigate their climatic implications from a regional perspective. The results show that besides local hydroclimate, δ18Opre is also significantly correlated with δ18Ocell, but with different effects. Spatially homogeneous δ18Opre variation causes high spatial correlations among δ18Ocell from different sites. Analyses show that ENSO variations are responsible for the large-scale common signals in δ18Ocell by modulating δ18Opre. Therefore, combining δ18Ocell chronologies from different locations can enhance ENSO signals, which provides us new opportunities to reconstruct paleo-ENSO activities. Furthermore, eliminating the part of ENSO signal that is not related to the growing season hydroclimate from δ18Ocell can increase the explained variance of observed values. These findings provide a guide for future optimized ENSO and local paleoclimate reconstructions using δ18Ocell.

Assessment of streamflow components and hydrologic transit times using stable isotopes of oxygen and hydrogen in waters of a subtropical watershed in eastern China

Streamflow components (e.g., young water vs old water) and hydrological transit times play an important role in water resource and water quality management. We collected a four-year record of stable isotopes of oxygen and hydrogen (δ18O and δ2H) in precipitation, groundwater and stream water for six catchments in the Yongan watershed of eastern China. The stable isotope records were used to identify spatio-temporal variations in the young water fraction (Fyw, defined as the proportion of the transit-time distribution younger than a threshold age) and mean transit time (MTT) based on sine-wave fitting and convolution integral methods, respectively. The Fyw ranged from 14 to 35% in the Yongan watershed. Cumulative transit time showed contrasting distributions, suggesting considerable heterogeneity and a complex interplay between catchment characteristics. Estimated MTTs ranged from 3.2 to 6.3 years and may be explained by catchment characteristics (e.g., elevation and topographic gradient). Observed spatial trends in MTTs likely result from contrasting contributions of different-aged subsurface water flows across the six catchments. The use of Fyw constraints in estimating MTTs reduced uncertainty in some catchments, suggesting the potential benefits of combining multiple approaches (e.g., Fyw) to optimize the results of traditional calibration methods. The relatively low Fyw and long MTTs highlight the importance of groundwater contributions to streamflow generation and imply a considerable lag time in river water quantity and quality responses to catchment-scale water resource management. Coupling multiple metrics (e.g., Fyw and MTT) and isotope models enhances our understanding of watershed-scale hydrologic processes and hydrograph separation.

A Climatological Interpretation of Precipitation δ18O across Siberia and Central Asia

Siberia and Central Asia are located at middle to high latitudes, encompassing a large landlocked area of the Eurasian continent and vast tracts of permafrost, which are sensitive to global climate change. Here, we investigated the data from 15 Global Network of Isotopes in Precipitation (GNIP) stations to clarify the relationship between precipitation δ18O (δ18OP) and the local temperature and precipitation amount on the monthly, seasonal, and annual timescales. Three main conclusions as following: (1) On the monthly time scale, the variation in δ18OP is mainly controlled by the “temperature effect”. (2) The weighted average value of precipitation δ18O (δ18Ow) exhibited “temperature effect” over 60° N–70° N. However, δ18Ow was dominated by multiple factors from 40° N to 60° N (e.g., the North Atlantic Oscillation (NAO) and water vapor source changes). (3) The variations of δ18OW can be attributed to the changes in pathway of the westerly dominated by the NAO at annual timescale. Therefore, it is possible to reconstruct the histories of past atmospheric circulations and water vapor sources in this region via δ18O in geologic archives, e.g., speleothem and ice core records.

Temperature-Dependent Oxygen Isotope Fractionation in Plant Cellulose Biosynthesis Revealed by a Global Dataset of Peat Mosses

The oxygen isotope composition (δ18O) of plant cellulose has been widely used to study ecohydrological processes of ecosystems as well as to reconstruct past climate conditions in terrestrial climate archives. These applications are grounded on a key assumption that the biochemical fractionation during cellulose synthesis is a constant around +27‰ and is not affected by environmental factors. Here we revisit the influence of temperature on biochemical fractionation factor during cellulose synthesis using a global compilation of Sphagnum cellulose δ18O data. Sphagnum (peat mosses) are known for inhabiting waterlogged peatlands and possessing unique physiological strategy in that their cellulose δ18O could closely reflect growing-season precipitation δ18O. Although within-site cellulose δ18O variability shows a median standard deviation of 0.7–0.8‰ resulting from different degree of evaporative enrichment of 18O in metabolic leaf water, this evaporative enrichment is a small quantity due to the external capillary “water buffer” in Sphagnum mosses. Using site-specific minimum cellulose δ18O data that most likely reflect the signal of unevaporated source water, we show that the apparent enrichment factor between cellulose and precipitation δ18O increases with decreasing air temperature. In particular, the apparent enrichment factor could reach as high as 32‰ when growth temperature is below 5 °C. This observational dataset extends the support for the temperature-dependent oxygen isotope fractionation in plant cellulose synthesis previously demonstrated in laboratory experiment, with implications for paleoclimate and plant physiology studies that employ cellulose δ18O measurements particularly in alpine regions.

Analysis of Hydrogen and Oxygen Stable Isotope Characteristics and Vapor Sources of Precipitation in the Guanzhong Plain

To improve the understanding of hydrogen and oxygen stable isotope characteristics and vapor sources in the Guanzhong Plain, we collected 98 precipitation samples and corresponding meteorological data between 2015 and 2018 in Yangling, Shanxi Province, which is located in the central area of the Guanzhong Plain. The composition characteristics of the local hydrogen and oxygen stable isotopes of precipitation (δ2H, δ18O, and δ17O) and their environmental controls were analyzed, and the local meteoric water line (LMWL) and the meteoric water line of the triple oxygen isotopes were established. Three indicators (δ18O, d-excess, and 17O-excess) were used to explore the possible vapor sources of local precipitation and to quantify the contributions of ocean-source and inland-source water vapor to the precipitation. The results showed that there were obvious seasonal changes in the hydrogen and oxygen stable isotopes of precipitation in the Yangling area:water isotopes were depleted in the wet season (May to October) and enriched in the dry season (November to April of the next year). Both the slope (7.7) and intercept (9.1) of the LMWL were lower than those of the global meteoric water line (GMWL), indicating that the annual precipitation in the research area experienced variable degrees of secondary evaporation under cloud cover. The slope of the meteoric water line of the triple oxygen isotopes is 0.528, which is between that of seawater equilibrium fractionation (0.529) and water vapor diffusion into dry air (0.518), consistent with the fact that the Guanzhong area is located on the migration path of marine air mass to inland arid regions. Comprehensive analysis of δ18O, d-excess, and 17O-excess confirmed that the precipitation in the study area is jointly contributed to by the warm and humid air mass from the southeast monsoon and the dry and cold air mass from the westerly wind. Of these, approximately 55%-79% of the precipitation water vapor comes from the ocean, mainly in June to August, and about 21%-45% of the water vapor comes from inland and local evaporation, mainly from October to April. The water vapor sources of precipitation in May and September are complex and may intermittently originate from ocean and inland water vapor.

East Asian Precipitation δ18O Relationship With Various Monsoon Indices

AbstractChinese speleothem oxygen isotope (δ18O) variations have been widely interpreted as recording the evolution of the Asian Monsoon. However, calibration of δ18O to monsoon intensity has not yet been carried out in a quantitative way. To understand the climatic significance of δ18O, we measured monthly precipitation δ18O data at Heshang Cave, China, where speleothem δ18O records were previously obtained. We examined the influence of local climate and large‐scale atmospheric circulation on the δ18O by correlating to local meteorological data and various monsoon indices. We find neither a significant amount effect nor temperature effect, and therefore suggest that local climate is not the primary driver of δ18O variability. On seasonal timescales, monthly δ18O is significantly correlated to all monsoon indices, especially the Indian Monsoon (IM) and Western North Pacific Monsoon (WNPM) indices, proving δ18O responds dominantly to the tropical monsoon circulation. On interannual timescales, there are significant negative correlations between annual δ18O and the WNPM and IM monsoon indices defined by zonal winds, reflecting the impact of the El Niño‐Southern Oscillation on δ18O. Similar correlations between δ18O and precipitation amount in these monsoon regions lend further support to this finding. As the driving force of seasonal and interannual monsoon changes can be compared to variations on orbital and suborbital scales, respectively, we argue that speleothem δ18O in southern China is influenced by both external forcing and internal variability. Solar radiation plays an indispensable role on the orbital and seasonal scale δ18O variations, whereas ocean–atmosphere interactions are dominant on suborbital timescales.

Geographical variations in the slope of the δ2H–δ18O meteoric water line over Europe: a record of increasing continentality

δ2H and δ18O values of precipitations follow an empirical linear relationship at the global scale that is called the Global Meteoric Water Line (GMWL) and characterized by a slope of 8. However, Local Meteoric Water Lines (LMWLs) may have different slopes S depending on their geographic situation. Monthly δ2H and δ18O of precipitation have been compiled from European International Atomic Energy Agency (IAEA) stations. Those data allowed the calculation of the slopes S of the δ2H–δ18O LMWL determined for each station. S increases with longitude ϕ from c. 5 (Portugal) to c. 9 (Russia) – they are positively correlated with relative humidity (RH), negatively with temperature and positively with the mean intra-annual amplitude of temperatures, which is a proxy of continentality. Slopes of 5–6, recorded in SW Europe, reflect mean RH (70–75%) and sea surface temperatures (c. 25°C) of the Central Atlantic Ocean where the main flux of moisture is formed before being transported by the westerlies. In addition, falling water droplets within an air column with a high RH (>80%) and low temperature are expected to escape sub-cloud evaporation. Therefore, slopes with values close to 9 are considered to reflect isotopic equilibrium conditions during the condensation of water vapour in clouds.

Individual event, seasonal and interannual variations in δ18O values of drip water in Maomaotou Big Cave, Guilin, South China, and their implications for palaeoclimatic reconstructions

Accurately interpreting the stable oxygen isotopic composition (δ18O) of cave drip water is critical to the retrieval of palaeoclimatic information based on the δ18O records preserved in speleothem calcite. Substantial cave monitoring data sites of interest can make essential contributions. In this study, six speleothem drip water sites and one pool water site were selected for a five‐year (2013–2017) monitoring of O and H stable isotope ratios in Maomaotou Big Cave, Guilin, South China. The results reveal that the drip water δ18O generally inherits the signals in the precipitation but may lag behind by times ranging from 1 to 6 months. Variations in drip water δ18O are determined by climatic variables operating over several time scales. At the individual storm‐event scale, drip water δ18O is a function of rainfall amount, moisture source δ18O and hydrological processes within the vadose zone. Moisture from nearby Beibu Bay in the northwest of the South China Sea often causes the rainfall and thus drip water to be enriched in 18O at Maomaotou Cave, for example. Seasonally or annually, the higher amounts of precipitation associated with the occurrence of El Niño events (e.g. in 2015/2016) result in more depleted drip water δ18O. On the interannual scale, variations of drip water δ18O are linked to changes in the intensity of the East Asian summer monsoon (EASM), the Southern Oscillation Index (SOI) and the West Pacific Subtropical High (WPSH). An enhanced EASM coupled with La Niña‐like conditions likely results in depleted precipitation δ18O, as does a stronger WPSH that leads to more rainfall. This study illustrates the importance of understanding these complex and varying external environmental conditions that contribute to δ18O changes in drip water, when interpreting the palaeoclimatic information contained in speleothems.

Isotopic ‘Altitude’ and ‘Continental’ Effects in Modern Precipitation across the Adriatic–Pannonian Region

It is generally observed that precipitation is gradually depleted in 18O and 2H isotopes as elevation increases (‘altitude’ effect) or when moving inland from seacoasts (‘continental’ effect); the regionally accurate estimation of these large-scale effects is important in isotope hydrological or paleoclimatological applications. Nevertheless, seasonal and spatial differences should be considered. Stable isotope composition of monthly precipitation fallen between January 2016 and December 2018 was studied for selected stations situated along an elevation transect and a continental transect in order to assess the isotopic ‘altitude’ and ‘continental’ effects in modern precipitation across the Adriatic–Pannonian region. Isotopic characteristics argue that the main driver of the apparent vertical depletion of precipitation in heavy stable isotopes is different in summer (raindrop evaporation) and winter (condensation), although, there is no significant difference in the resulting ‘altitude’ effect. Specifically, an ‘altitude’ effect of −1.2‰/km for δ18O and −7.9‰/km for δ2H can be used in modern precipitation across the Adriatic–Pannonian region. Isotopic characteristics of monthly precipitation showed seasonally different patterns and suggest different isotope hydrometeorological regimes along the continental transect. While no significant decrease was found in δ18O data moving inland from the Adriatic from May to August of the year, a clear decreasing trend was found in precipitation fallen during the colder season of the year (October to March) up to a break at ~400 km inland from the Adriatic coast. The estimated mean isotopic ‘continental’ effect for the colder season precipitation is −2.4‰/100 km in δ18O and −20‰/100 km in δ2H. A prevailing influence of the Mediterranean moisture in the colder season is detected up to this breakpoint, while the break in the δ18O data probably reflects the mixture of moisture sources with different isotopic characteristics. A sharp drop in the d-excess (>3‰) at the break in precipitation δ18O trend likely indicates a sudden switch from the Mediterranean moisture domain to additional (mainly Atlantic) influence, while a gradual change in the d-excess values might suggest a gradual increase of the non-Mediterranean moisture contribution along the transect.

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 (<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.