Precipitation δ18O Research Articles

Interannual modulation of the East and South Asian summer precipitation δ18O by the Indian and western North Pacific summer monsoon strength

Interannual modulation of the East and South Asian summer precipitation δ18O by the Indian and western North Pacific summer monsoon strength

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.

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

The scientific explanation of cave δ18O in the East Asian Summer Monsoon (EASM) region is a crucial issue restricting stalagmite-inferred paleoclimate research. The connection between El Niño Southern Oscillation (ENSO) and monsoonal precipitation/cave δ18O and the mechanism involved also remain contentious. Long-term cave monitoring is an effective solution to deal with these issues. Here, we present 11-year-long precipitation, dripwater, and modern calcite δ18O data from Yuhua Cave in Southeast China, located in the frontal zone affected by the EASM. The upstream convection and rainout processes mainly regulate seasonal and interannual variations in precipitation δ18O (δ18Op). The δ18Op series shows a strong response to EASM activities associated with the Intertropical Convergence Zone migrations and the activities of the West Pacific Subtropical High affected by ENSO activities. Due to the influence of the epikarst reservoir mixing effect, the seasonal variation of dripwater δ18O (δ18Od) is irregular, and its amplitude is much smaller than that of δ18Op. The δ18Od can reflect integrated δ18Op changes within the residence time of rainwater in the karst reservoir and can inherit the ENSO signal in δ18Op. The inferred recharge models for the Yuhua drip sites reveal differences in the response to ENSO in terms of both timing and amplitude, which can be attributed to variations in the residence time of rainwater and the mixing ratio of new and old water within the karst aquifer. Higher rainfall would lead to a more sensitive response of δ18Od to ENSO activities, with a shorter time lag and greater amplitude. The modern calcite δ18O (δ18Oc) series show significant seasonal variations controlled by the fractionation coefficient varied with cave temperature. On the inter-annul time scale, the change of δ18Oc is mainly controlled by δ18Od and can record the ENSO-related variations. Our findings imply that high-resolution stalagmite δ18O reconstructions from Yuhua Cave may be able to identify historical ENSO-related variability in the EASM on annual to decade time scales.

Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part I: Rainfall Response

Abstract Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation and precipitation δ18O (δ18Op) response have remained controversial, and their response mechanisms are unclear. In particular, it is not clear whether the regional precipitation represents the intensity of the millennial South American summer monsoon (SASM). Here, we study the TSA hydroclimate variability during the last deglaciation (20–11 ka ago) by combining transient simulations of an isotope-enabled Community Earth System Model (iCESM) and the speleothem records over the lowland TSA. Our model reasonably simulates the deglacial evolution of hydroclimate variables and water isotopes over the TSA, albeit underestimating the amplitude of variability. North Atlantic meltwater discharge is the leading factor driving the TSA’s millennial hydroclimate variability. The spatial pattern of both precipitation and δ18Op show a northwest–southeast dipole associated with the meridional migration of the intertropical convergence zone, instead of a continental-wide coherent change as inferred in many previous works on speleothem records. The dipole response is supported by multisource paleoclimate proxies. In response to increased meltwater forcing, the SASM weakened (characterized by a decreased low-level easterly wind) and consequently reduced rainfall in the western Amazon and increased rainfall in eastern Brazil. A similar dipole response is also generated by insolation, ice sheets, and greenhouse gases, suggesting an inherent stability of the spatial characteristics of the SASM regardless of the external forcing and time scales. Finally, we discuss the potential reasons for the model–proxy discrepancy and pose the necessity to build more paleoclimate proxy data in central-western Amazon. Significance Statement We want to reconcile the controversy on whether there is a coherent or heterogeneous response in millennial hydroclimate over tropical South America and to clearly understand the forcing mechanisms behind it. Our isotope-enabled transient simulations fill the gap in speleothem reconstructions to capture a complete picture of millennial precipitation/δ18Op and monsoon intensity change. We highlight a heterogeneous dipole response in precipitation and δ18Op on millennial and orbital time scales. Increased meltwater discharge shifts ITCZ southward and favors a wet condition in coastal Brazil. Meanwhile, the low-level easterly and the summer monsoon intensity reduced, causing a dry condition in the central-western Amazon. However, the millennial variability of hydroclimate response is underestimated in our model, together with the lack of direct paleoclimate proxies in the central-west Amazon, complicating the interpretation of changes in specific paleoclimate events and posing a challenge to constraining the spatial range of the dipole. Therefore, we emphasize the necessity to increase the source of proxies, enhance proxy interpretations, and improve climate model performance in the future.

Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part II: δ18Op Response

Abstract Understanding the hydroclimate representations of precipitation δ18O (δ18Op) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), characterized by a northwest–southeast (NW–SE) dipole in both rainfall and δ18Op, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of such δ18Op dipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the intertropical convergence zone (ITCZ) migrates southward, causing a moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to the δ18Op depletion in eastern Brazil (SE pole). In contrast, the moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated subcloud layers) contribute nearly equally to the δ18Op enrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship between δ18Op and rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennial δ18Op change in the central-western Amazon, while δ18Op does not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when using δ18Op as a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennial δ18Op dipole response and pose a challenge in reconciling the discrepancy. Significance Statement We want to comprehensively understand the hydroclimate footprints of δ18Op and the mechanisms of the millennial variability of δ18Op over tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennial δ18Op change in eastern Brazil mainly documents the moisture source location change associated with ITCZ migration and the change of the isotopic value of the incoming water vapor, instead of the local rainfall amount. In contrast, the central-western Amazon partially documents the moisture source location shift and local precipitation change. Our study cautions that one should not simply resort to the isotopic “amount effect” to reconstruct past precipitation in tropical regions without studying the mechanisms behind it.

The Intra‐Seasonal Oscillation of Precipitation δ18O Over the Asian Equatorial and Monsoon Regions

AbstractWater isotopes‐climate correlations are used to reconstruct paleoclimate from various natural archives. Intra‐seasonal oscillation is one of the major atmospheric patterns that modulate the ratio of precipitation isotope in low latitudes, yet their spatial and temporal distribution patterns are unclear. Here we presented a detailed analysis of how the intra‐seasonal oscillations (MJO, Madden‐Julian Oscillation, and BSISO, Boreal Summer Intra‐seasonal Oscillation) modulate precipitation δ18O and vapor δ18O over the Asian monsoon region and the equatorial region. This analysis found consistent intra‐seasonal variations between the ISO and rain δ18O. We interpret it as the ISO regulating the active and inactive convective systems on the regional scale, leading to the intra‐seasonal oscillations in precipitation δ18O, with amplitudes from 4‰ up to 15‰. The MJO (BSISO) leads to eastward (northeastward) propagation of δ18O intra‐seasonal oscillations in the Asian equatorial (monsoon) region, coinciding with the spatial patterns of OLR oscillations, reflecting the response of precipitation/vapor δ18O to the oscillation of large‐scale convective activity or accumulated depletion of regional precipitation. We proved that the amplitudes of intra‐seasonal variation of precipitation δ18O are comparable to seasonal variations, and both of them are the main patterns for precipitation δ18O in study regions. Our results are conducive to the accurate interpretation of δ18O in climate proxies such as precipitation, ice cores, tree cellulose, cave stalagmites, et al. on the intra‐seasonal scale in this region.

The spatio-temporal pattern of Asian summer monsoon during glacial Termination II recorded by Chinese stalagmite δ18O

The difficulty in constraining the large-scale Asian summer monsoon (ASM) variability in the Chinese monsoon region (CMR) during glacial Termination II lies in our limited knowledge of the western part, primarily due to sparse paleoclimate records. To get a better picture of the ASM during Termination II, we examined a precisely dated stalagmite δ18O record between 133.1 and 127.0 kyr B.P. from Wanxiang Cave located at the ASM's northern edge in western China. In combination with published δ18O data from this cave, we have identified the ‘Weak Monsoon Interval’ (WMI) in the Wanxiang δ18O record and confirmed that the Heinrich 11 cold event in the North Atlantic caused the weakened ASM over the CMR via reorganization of the large-scale ocean-atmospheric circulation. However, the post-WMI change in δ18O is gradual, in contrast with the abrupt shift shown in the other cave records from southern and northeastern China. The rapid northward migration of the westerly jet relative to the Qinghai-Tibet Plateau is probably responsible for this discrepancy. This northward-positioned westerly jet prevented the more 18O-depleted moisture from the Indian Ocean from reaching the study site. Simultaneously, it facilitated the earlier northward movement of the East Asian summer monsoon (EASM) rainband that carries positive precipitation δ18O to obscure the abrupt decrease in our δ18O record. After the onset of the last interglacial, no obvious Younger Dryas (YD)-like event was recorded in Wanxiang Cave. This result is consistent with most stalagmite δ18O records in the CMR and further suggests a minimal impact of the YD-like event on ASM variabilities. The relatively large amplitude of δ18O variations observed in Wanxiang Cave between the late penultimate glacial and the last interglacial corresponds to a dominant control of the Indian summer monsoon (ISM), whereas smaller δ18O amplitudes were recorded in cave sites mainly under the influence of both ISM and EASM. Therefore, we posit that the heterogeneity of the hydroclimate in the CMR during Termination II resulted from a combination of multiple processes, that is, the westerly jet, ISM and EASM, rather than a single one.

Atmospheric processes control the stable isotopic variability of precipitation in the middle–lower reaches of the Yangtze River Basin, East Asian monsoon region

Paleoclimatic records in the East Asian monsoon region (EAM) depend on precipitation isotopes as proxies for past hydroclimatic variability. However, the potential mechanisms controlling precipitation isotopic variability in this region remain poorly understood. This study collected daily precipitation isotopes in the middle-lower reaches of the Yangtze River Basin from 2017 to 2020 to explore how convective behaviors and moisture sources determining the variability of precipitation stable isotopes. Daily precipitation δ18O presented a weak inverse relationship with local precipitation amount and air temperature. Results of correlation analysis showed that the maximum correlation zones of precipitation δ18O between different cloud levels and outgoing long wave radiation were identified in the upstream regions such as the Indian Peninsula, BoB, and Indo-China Peninsula during the rainy season. Back-trajectory analysis showed that substantial water vapor from maritine sources in the Indian Ocean and the Bay of Bengal (BoB) largely contributed to the lower δ18O values during the rainy season (−7.5 ‰) because of intense rainout processes in the upstream sourced regions, demonstrated by significant regional correlation between precipitation δ18O and regional precipitation amount. The regional correlations were stronger in the upstream regions than the local influences, indicating that precipitation isotopes in this region are more influenced by regional convective processes in the upstream region during the rainy season. Using the modified Stewart model, the mean subcloud evaporation fraction estimated was greater in rainy season (24.1 % vs 36.1 % for HK and HZ, respectively) than that in dry season (15.1 % vs 19.9 % for HK and HZ, respectively). These findings highlight the importance of upstream processes in controlling precipitation isotopes in the EAM region and provide reliable evidence on the interpretation of monsoonal climate variability.

Re-evaluation of the effects of precipitation amount and temperature on precipitation δ18O at the monthly and interannual timescales

Abstract Proxy records of the oxygen isotope ratio of 18O/16O of past precipitation (δ18Op) have played an important role in revealing past hydroclimatic changes, on the basis of global observed relationships between monthly precipitation δ18Op and both precipitation amount and temperature only of a few years as reported by Dansgaard in 1964. It is therefore crucial to systematically re-evaluate the relationships using modern instrumental data. We analysed monthly and annual mean correlations from 108 global stations over the past about 60 years. Consistent with previously reported results, monthly δ18Op values in the high latitudes (≥60°) show a significant positive correlation with temperature (referred to as ‘temperature effect’) and a negative trend with precipitation amount in the low latitudes (≤20°) (‘amount effect’). However, these correlations do not hold true for yearly mean data for more than three-quarters of the stations evaluated. This indicates that the relationships between the different temporal resolutions could be more complicated than previously thought. For the related natural archives, such as ice cores, sediments, and carbonates, further careful evaluation is required to establish the robustness of their paleoclimatic implications.

Environmental drivers of precipitation stable isotopes and moisture sources in the Mongolian Plateau

Τhe absence of precipitation-isotope stations in the wide flatland of the Mongolian Plateau (MP) hinders the analysis of precipitation processes. For this reason, a precipitation-isotope station was established in Erenhot in 2015. Based on precipitation samples collected during all precipitation events between January 2015 and December 2019, we analyzed the temporal variations of stable isotopes (δ2H, δ18O) in precipitation and their influencing factors, including meteorological factors, sub-cloud evaporation, and moisture sources. Oxygen and hydrogen isotopes in precipitation in Erenhot are richer than those in Ulaanbaatar, which is situated close to the mountains and depleted compared to, e.g., Baotou. In Erenhot, oxygen and hydrogen isotopes in precipitation are controlled by the temperature effect, similarly to Ulaanbaatar, but also by the amount effect in summer. The low slope of the local meteoric water line indicates that raindrops have undergone sub-cloud evaporation. The annual mean sub-cloud evaporation fraction was 5.2% from 2015 to 2019, while sub-cloud evaporation fraction was highest in summer (7.5%). Sub-cloud evaporation is mainly correlated by relative humidity, precipitation amount, regional (pan) evaporation, and air temperature. Relative humidity has the greatest effect and masks the effect of air temperature. The study area is mainly controlled by the west wind and winter monsoon, which brought 75.6% of the total precipitation with −7.75‰ δ18O. During summer, the Pacific monsoon resulted in 13.5% precipitation with −10.34‰ δ18O in the study area, while the Indian monsoon had negligible impact. Except for moisture advection, the moisture recycling fraction accounted for 11.1% of local precipitation, with −10.32‰ δ18O. This study compensates for the lack of global network of isotopes in precipitation (GNIP) stations in the MP, reveals the MP precipitation-isotope signatures and their influencing factors, and provides basic precipitation-isotope information for researching the distribution of precipitation-isotope signatures in Asia and the water cycle in the MP.

Spatiotemporal variability of stable isotopes in precipitation and stream water in a high elevation tropical catchment in the Central Andes of Colombia

AbstractThe Colombian Andean Mountains include the headwaters of the main basins of the country. However, the isotope composition of water in these high mountain ecosystems has been poorly studied. In this study, we analysed the first set of stable isotope data collected along a wide elevation range (2600–4950 m a.s.l.) in the Central Andes of Colombia. The stable isotope composition of stream water and precipitation was determined for a period between 2017 and 2018 in the Upper Claro River basin. The driving factors influencing the spatial and temporal variability of δ2H, δ18O, and d‐excess were identified, and compared with daily air temperature and precipitation data from seven meteorological stations. The local regression line was described by δ2H = 8.2 δ18O + 12.3, R2 = 0.98. The δ2H and δ18O values showed more depletion in heavy isotopes, and the d‐excess values were more negative during the rainy season. An altitude effect of −0.11‰/100 m and −0.18‰/100 m was estimated for stream water and precipitation δ18O values, respectively, with the latter showing non‐linear behaviour. The dataset was compared with Colombian stations of the Global Network of Isotopes in Precipitation database, and a back‐trajectory analysis of air masses was conducted and compared with the d‐excess values. The δ18O weighted mean values changed with respect to the position in the Central Andes, indicating contrasting altitude effects depending on the moisture sources. The most positive d‐excess values were attributed to moisture recycling enhanced by local ecosystem conditions and the origin of precipitation from the Amazon basin, which change during the year and across the northern Andes. The results showed a high level of variation because of differences in elevation, seasonality, and atmospheric circulation patterns during the year. This study contributes to knowledge of spatial and temporal isotope composition data in the northern Andes, delineation of water supply basins, and to the definition of ecosystem boundaries in the high mountains of Colombia.

Identifying moisture transport pathways for north‐west India

The isotopic composition (δ18O and δ2H) of precipitation is widely used as a moisture source tracer. In north‐west India, the δ18O, δ2H and D‐excess values of precipitation correlate mainly with air temperature; however, the moisture sources of water vapour are unclear. Therefore, we collected daily precipitation isotope data (n = 425) from 13 rain gauge stations in northwest India in 2013. We established a regional meteoric water line (MWL) for northwest India and local MWLs for all 13 rain gauge stations separately. We observed an altitudinal gradient of about 0.11‰/100 m in slope and about 1.22‰/100 m in intercept from the western to the northeast part of the study area. The isotopic composition of precipitation shows spatial and temporal variability across the study area. We found higher Deuterium excess values during winter (December–February) and lower during the monsoon season (July–September), which may be associated with the different moisture sources, namely, the Bay of Bengal, Arabian Sea and Westerly disturbances. Our results suggest local moisture recycling may also occur during the study period. The present study can enhance the knowledge of the isotopic evolution of precipitation and moisture sources in northwest India.

Quaternary fluvial carbonate deposits of the Almonda River Valley, Central Portugal

This paper discusses the formation and preservation of a fluvial tufa system influenced by Atlantic climate based on stratigraphical, chronological (amino-acid racemization, AAR), sedimentological and stable-isotope analyses. On the southwestern Iberian Peninsula, the tufas and associated deposits of the Almonda River valley occur as isolated terraced bodies and reach 25 m thick. AAR dated most deposits to within the warm Marine Isotope Stage 5 (MIS-5). Two Holocene ages were reset within MIS-5 based on diverse criteria. Widely varied carbonate and minor allochthonous coarse detrital facies occur arranged in four simple vertical associations. The deposit geometry and facies association distribution correspond to a low- to moderate-sloped fluvial valley consisting of several short knickpoints and extensive flat areas between them. The latter are occupied by slow-flowing water facies (carbonate sand, lime mud, phytoclast and oncoid rudstones, and up-growing stem boundstones). Facies that formed in moderate- to high-slope substrates were stromatolite, moss and down-growing stem boundstones. The homogeneous Miocene bedrock lithology and gentle structural deformation propitiated this depositional architecture. Calcite δ13C and δ18O values suggest that the aquifer water provided the outflowing Almonda water with (1) 18O-enriched water, compared with present precipitation and groundwater δ18O values, and (2) 13C-depleted CO2 from bituminous rocks and vegetation cover in the catchment. The proximity to the Atlantic coast favoured the Mesozoic-rock aquifer recharge with 18O-enriched water precipitation, assuring water availability during the formation of the studied tufas. No evidence of frequent intense erosion phases might indicate stable precipitation regimes, which would have allowed the preservation of loose fine-grained and palustrine deposits.

Seasonal variations and controlling factors of speleothem multi-proxy in southeastern China: Implications for the reconstruction of precipitation seasonality

Precipitation in southeastern China exhibits strong seasonal variability, which significantly impacts local agricultural production and social development. However, the reconstruction of precipitation seasonality has been limited by the lack of precisely dated high-resolution paleoclimatic records. This study presents seasonal-scale multi-proxy records (trace elements: Mg/Ca, Sr/Ca Ba/Ca and stable isotopes: δ18O and δ13C) of a modern (1810–2009 AD) annually laminated stalagmite (EM1) from E’Mei Cave, Jiangxi Province in southeastern China. Comparative analysis of observation data with meteorological and simulation data shows that the seasonal variation of EM1 δ18O, derived from precipitation δ18O, is mainly controlled by the large-scale circulation and precipitation seasonality on an interannual timescale. The seasonal EM1 δ18O variation is controlled by the seasonal precipitation δ18O, however, the most negative values of EM1 δ18O are higher than those of simulated calcite δ18O and the average amplitude of the seasonal variation of EM1 δ18O (∼1.93‰) is much smaller than that of simulated calcite δ18O (∼9.72‰) because of evaporation and mixing of waters in the epikarst system. On the decadal timescales, variations of δ13C, Mg/Ca, Sr/Ca, and Ba/Ca were found to be strongly correlated, consistent with the variation of the local dry/flood index, indicating a common influencing factor of local hydroclimate change. However, their seasonal phase relationships vary between strong and weak summer monsoon conditions. We find a strong covariation between Mg/Ca and δ13C with Sr/Ca and Ba/Ca during the strong monsoon period (1951–1976), and they are in antiphase with seasonal δ18O variation. However, the seasonal variations of Sr/Ca and Ba/Ca transformed to dominantly antiphase with Mg/Ca and δ13C during the weak monsoon period (1977–1991 AD). Therefore, we suggest that Mg/Ca and δ13C are dominantly controlled by the local hydroclimate changes on seasonal timescales but Sr/Ca and Ba/Ca might have been affected by complex processes in the epikarst under different hydrothermal configuration conditions. The findings indicate the potential of the variation of the seasonal phase relationships between multi-proxy records in reconstructing precipitation seasonality changes under different hydrothermal backgrounds.

Simulation of Hydrologic Change of Linggo Co during 1979–2012 Using Hydrologic and Isotopic Mass Balance Model

The Tibetan Plateau (TP) and surrounding areas contain the largest number of glaciers outside the polar regions. The region affects downstream water supply and food security, thereby directly influencing one-third of the world’s population. The lakes in the central TP expanded rapidly in recent decades, which has attracted growing attention. Glacier meltwater was considered as a major component in the water balance of TP lakes, although few studies quantified its contribution. Stable isotope analysis is a powerful tool to trace hydrologic circulation, while its interpretation in paleoclimate records has been controversial. To bridge the gap between hydrologic and paleoclimatic studies, we performed a hydrologic and isotopic mass balance model to simulate the lake level change of Linggo Co in the central TP. The model was forced by the meteorological data, calibrated through observed lake level changes, and validated by oxygen isotope compositions (δ18O) of lake water. Our results indicated that glacier meltwater contributed 73.94% of the inflow water to Linggo Co before 1993 but decreased thereafter. Increasing glacier meltwater together with positive water balance (precipitation/evaporation) in the catchment contributed to the rapid expansion of Linggo Co after the mid-1990s. Lake water δ18O in Linggo Co was more sensitive to changes in the precipitation amount and precipitation δ18O than temperature. Our findings could shed light on the usage of δ18O proxy in future paleoclimate research on the TP.

Estimation of Precipitation Fraction in the Soil Water of the Hillslope Vineyard Using Stable Isotopes of Water

This paper presents research related to the estimation of the precipitation fraction in the soil water of a sloped vineyard at the SUPREHILL Critical Zone Observatory (CZO) in Zagreb, Croatia. Numerous investigations have shown that exploration of hillslope soils can be very challenging due to the existence of heterogeneity and different soil properties, as well as due to anthropogenically induced processes, which can affect precipitation infiltration and soil water flow. Within this research, physicochemical soil properties, soil water content (SWC), and isotopic composition of soil water and precipitation (δ2H and δ18O) have been examined. The isotopic signature of soil water was monitored in 24 points, at 4 depths, throughout the hillslope vineyard. Soil water isotopic composition from all monitoring points coincided with the Local Meteoric Water Line (LMWL), with almost no variability at 100 cm depth, which was consistent with the smallest variation of SWC at 80 cm depth and indicated that most of water mixing takes place in the shallower part of the hillslope. Results suggested the existence of heterogeneity, uneven erosion processes in the footslope of the observed vineyard, and different infiltration patterns. Fractions of precipitation varied significantly depending on the depth and position in the vineyard, from approximately 1% up to 98%, where more precipitation fraction has been determined in the surface and subsurface runoff. Additionally, statistical analysis and a more detailed evaluation of precipitation fractions at the 40 cm depth, where wick lysimeters are installed, have shown that Corg content is related to the silt fraction, while the first results indicate that the infiltration patterns were dependent on the common influence of all observed physicochemical properties.

Variability of precipitation-stable isotopes and moisture sources of two typical landforms in the eastern Loess Plateau, China

Study regionTwo typical landforms in the eastern Loess Plateau, China Study focusHydrogen and oxygen stable isotopes in precipitation are highly efficient tracers for quantitatively identifying the regional water cycle. Based on 237 precipitation event data points from seven precipitation stable isotope monitoring stations in the eastern Loess Plateau (ECLP), this study first presented the spatial-temporal variation of precipitation isotopes. It explored the mainly controlling factor and source of water vapor for regional precipitation. New hydrological insights for the regionThis research found a consistent temporal trend was observed in the ECLP except for the Jiexiu station, characterized by enrichment from December to May and depletion from June to October. Signals of the summer monsoon reaching the ECLP were recorded using precipitation δ18O (δ18Op), especially in July. Relatively smaller slope of the local meteoric water line (LMWL) of summer precipitation reflects that sub-cloud evaporation strongly influences regional precipitation process. δ18Op values did not exhibited significant relationships with air temperature and an apparent "precipitation amount effect" was observed in precipitation samples with precipitation amount less than 10 mm. An apparent "anti-altitude" effect appeared in precipitation samples with the elevation between 400 and 750 m of the ECLP. The sub-cloud evaporation had stronger influence on the precipitation process of the the Fenhe river basin valley (FRV) especially in the Jiexiu station. The water vapor of precipitation in the ECLP mainly originated from the Bohai Sea and the East China Sea in the near-surface and southeast path directions, especially in summer. These studies are essential in regional water resources allocation, especially under the influence of climate change.

Variations in water sources used by winter wheat across distinct rainfall years in the North China Plain

Quantifying water availability in different soil layers to crop root water uptake (RWU) in different rainfall years helps deliver strategies to combat water scarcity and improve water productivity. To investigate the responses of RWU to soil water availability across distinct rainfall years, a field experiment of winter wheat was conducted in 2017–2018 (wet year), 2018–2019 (dry year), and 2019–2020 (ordinary year) in the North China Plain. Stable isotopes (δ18O and δD) in precipitation, irrigation water, xylem water, and soil water, as well as soil water content (SWC), were investigated during the three wheat growth seasons and were used to determine the soil water evaporation line (SWEL: the linear relationship between δD and δ18O in soil water), the line-conditioned excess (lc-excess: the deviation between δD and the local meteoric water line in the dual-isotope space) and the proportion of RWU from different soil layers with the MixSIAR model. The results indicated that there were significant differences in δ18O and δD in soil water and xylem water across different rainfall years. The slopes of SWEL and lc-excess of the 0–100 cm soil layer in the dry year were significantly smaller than those in the ordinary and wet years, which suggests that more substantial soil evaporation occurred in the dry year. The MixSIAR results revealed that the three-year wheat RWU exhibited different patterns, which were related to soil water availability. Wheat mainly utilized soil water from the 0–20 cm and 20–60 cm layers during the greening to filling stage under the dry year, while it mainly utilized the 0–20 cm soil layer under the wet and ordinary years. However, at the harvest stage, wheat under all precipitation years could transfer its main RWU zone to the 20–60 cm soil layer. There were significantly different proportions of RWU from 0 to 60 cm under different rainfall years, shown as 75.3 %, 67.5 %, and 69.5 % in the wet year, dry year, and ordinary year, respectively. These findings indicated that the appropriate irrigation wetting depth should be set as<60 cm for all kinds of rainfall years.

The severe drought event of 2009–2010 over southwest China as recorded by a cave stalagmite from southeast Yunnan: Implication for stalagmite δ18O interpretation in the Asian monsoon region

It is still debated whether the stalagmite δ18O in the East Asian monsoon region indicates the intensity of the summer monsoon or the precipitation of the summer monsoon season on an annual scale. One key reason for this debate is the lack of accurate comparison between stalagmite δ18O and instrumental records. To better decipher the stalagmite δ18O in the Asian monsoon region on an annual scale, stalagmite PL1 from Pailong Cave, southeast of Yunnan, was selected. The calcite deposited from 2004 to 2019 within stalagmite PL1 was determined by laminar counting, 210Pb dating and seasonal variations in δ13C values. Seasonal scale δ18O, δ13C and Mg/Ca ratios were found to record the severe drought over Southwest China that occurred in 2009–2010. In 2009, when the severe drought was the most intense, the stalagmite δ18O, δ13C, and Mg/Ca ratios changed significantly, reflecting the hydrological response caused by climate change (decrease in precipitation). However, distinct from the δ13C and Mg/Ca ratios, the highest δ18O value was found in 2010, which reflects the influence of the evaporation in the soil and epikarst. On the annual scale, the stalagmites/precipitation δ18O in both Yunnan and eastern China are dominated by El Niño-Southern Oscillation (ENSO) activity. During the El Niño/La Niña phase, reduced/enhanced convective activities but strong/weak water vapor flux transportation occurred in the Bay of Bengal and the South China Sea, and weaker/stronger water vapor flux transportation in the Arabian Sea induced enriched/depleted precipitation δ18O and hence stalagmite δ18O in Yunnan and eastern China. On the interannual scale, the stalagmite δ18O of Yunnan is mainly affected by the strength of the South Asian summer monsoon and water vapor flux transportation from the Arabian Sea. The stronger (weaker) South Asian summer monsoon intensity will induce more (less) water vapor flux transported from the Arabian Sea and depleted (enriched) stalagmite δ18O in Yunnan. Distinct from stalagmite δ18O in Yunnan, the precipitation/stalagmite δ18O in eastern China is significantly correlated with ENSO indices on the interannual scale. Precipitation/stalagmite δ18O in eastern China is dominated by convective activities in the source area (Bay of Bengal-South China Sea).

Differences in watershed evaporation indicated by hydrogen and oxygen single and dual isotopes: Evidence from controlled simulation tests under different land uses

Partitioning watershed evapotranspiration into evaporation and transpiration is essential for studying the water–carbon cycle, and the key step in this partitioning is to calculate watershed evaporation based on isotopic indexes. The δ18O and δ2H single isotopes and d-excess or lc-excess dual isotopes derived from δ18O and δ2H are widely used for this purpose. Although δ18O, δ2H, and d-excess have different characteristics, few studies have assessed how these differences influence the usefulness of these isotopes as indicators of watershed evaporation. Here, we simulated five watersheds with different land uses and degrees of evaporation in a region of Southwest China with a subtropical monsoon climate. We then investigated differences in the ability of the isotopic indexes to indicate watershed evaporation over two hydrological years. We found that d-excess and lc-excess dual isotopes were better indicators of watershed evaporation than δ18O and δ2H single isotopes. This is because the δ18O and δ2H in precipitation are easily influenced by rainout during the rainy season, when groundwater is primarily recharged. They are more variable than d-excess and lc-excess, which leads to δ18O and δ2H in groundwater being more sensitive to changes in the isotopic signal input in precipitation. This probably interferes with, or even masks watershed evaporation signals carried by δ18O and δ2H. The annual mean isotopic compositions were more depleted in groundwaters (δ18O = − 8.7 ‰ to − 8.5 ‰, δ2H = − 61.9 ‰ to − 58.7 ‰) than in precipitation (δ18O = − 8.3 ‰, δ2H = − 54.3 ‰) from November 2015 to October 2016; these compositions fail to support the isotopic enrichment of groundwater compared with that of precipitation. Evaporation rates over two years derived from d-excess dual isotopes agreed well with those from the water balance in bare-rock and bare-soil lands, which had no data from plants available for verification. Compared with hydrogen and oxygen single isotopes, their dual isotopes reflected watershed evaporation more accurately, especially in Asian monsoon regions.