Precipitation δ18O Research Articles

A modern snapshot of the isotopic composition of lacustrine biogenic carbonates – records of seasonal water temperature variability

Abstract. Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition (δ13C) of such carbonates depends on the δ13C of dissolved inorganic carbon (DIC). Their oxygen isotopic composition (δ18O) is controlled by the δ18O of the lake water and by water temperature during carbonate precipitation. Lake water δ18O, in turn, reflects the δ18O of atmospheric precipitation in the catchment area, water residence time and mixing, and evaporation. A paleoclimatic interpretation of carbonate isotope records requires a site-specific calibration based on an understanding of these local conditions. For this study, samples of different biogenic carbonate components and water were collected in the littoral zone of Lake Locknesjön, central Sweden (62.99∘ N, 14.85∘ E, 328 ma.s.l.) along a water depth gradient from 1 to 8 m. Carbonate samples of living organisms and subfossil remains in surface sediments were taken from the calcifying alga Chara hispida, from bivalve mollusks of the genus Pisidium, and from adult and juvenile instars of two ostracod species, Candona candida and Candona neglecta. Our results show that neither the isotopic composition of carbonates nor the δ18O of water vary significantly with water depth, indicating a well-mixed epilimnion. The mean δ13C of Chara hispida encrustations is 4 ‰ higher than the other carbonates. This is due to fractionation related to photosynthesis, which preferentially incorporates 12C into the organic matter and increases the δ13C of the encrustations. A small effect of photosynthetic 13C enrichment in DIC is seen in contemporaneously formed valves of juvenile ostracods. The largest differences in the mean carbonate δ18O between species are caused by vital offsets, i.e., the species-specific deviations from the δ18O of inorganic carbonate which would have been precipitated in isotopic equilibrium with the water. After subtraction of these offsets, the remaining differences in the mean carbonate δ18O between species can mainly be attributed to seasonal water temperature changes. The lowest δ18O values are observed in Chara hispida encrustations, which form during the summer months when photosynthesis is most intense. Adult ostracods, which calcify their valves during the cold season, display the highest δ18O values. The seasonal and interannual variability in lake water δ18O is small (∼ 0.5 ‰) due to the long water residence time in the lake. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate δ18O differences between species when vital offsets are corrected. Temperature reconstructions based on paleotemperature equations for equilibrium carbonate precipitation using the mean δ18O of each species and the mean δ18O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate δ18O variability of samples within a species, on the other hand, leads to a large scatter in the reconstructed temperatures based on individual samples. This implies that care must be taken to obtain a representative sample size for paleotemperature reconstructions.

Isotopic characteristics and vapor sources of atmospheric precipitation in the loess region of North Shaanxi, China

To explore the hydrogen and oxygen stable isotopes and vapor sources of atmospheric precipitation in the loess region of north Shaanxi, we collected 107 samples of precipitation during 2018 to 2020 in the Dingbian County, Shaanxi Province. After measuring δ18O and δ2H of precipitation, we analyzed the isotopic characteristics and vapor sources of precipitation in different seasons. There was obvious seasonal variability in δ18O and δ2H, in that they were more depleted in the wet season (June-September) but more enriched in the dry season (April-May, October-November). In addition, the deuterium excess values were high in the dry season but low in the wet season. The local meteoric water line was regressed as δ2H=7.35δ18O+4.19 (R2=0.96, P<0.01) with slope and intercept lower than that of the global meteoric water line, indicating an impact of evaporation on isotope fractionation. There was temperature effect in the precipitation isotopes for the whole year and for the dry season, while the precipitation isotopes in the wet season were affected by the combined effects of temperature and rainfall amount. The HYSPLIT model showed that the dry season water vapor mainly came from the Atlantic Ocean and the polar Arctic Ocean, while the wet season vapor mainly came from the Indian Ocean and Pacific subject to the Westerly.

Moisture Sources and Climatic Controls of Precipitation Stable Isotopes Over the Tibetan Plateau in Water‐Tagging Simulations

AbstractUnderstanding the climate controls of precipitation δ18O in the Tibetan Plateau (TP) is crucial for paleoclimate reconstructions from a wealth of regional archives. We use the outputs of iCAM5 model to quantify the different moisture contribution to local precipitation δ18O and to understand the climate controls of precipitation δ18O in the TP based on water‐tagging. The model shows some deficiencies in simulating the spatial and temporal characteristics of precipitation δ18O and the local climatic controls across the TP. Among all the tagged source regions, South Asia and the Indian Ocean contribute the most to the precipitation δ18O in the monsoon‐controlled domain, followed by the East Asia source region. The westerlies are identified as major moisture sources to the precipitation δ18O in the westerlies‐controlled domain. South Asia and the Indian Ocean also contribute substantially for the westerlies‐controlled domain. On interannual time scales, summer precipitation δ18O in the monsoon‐controlled domain is dominated by rainout processes occurring along the moisture transport pathway, indicating that precipitation δ18O variations here potentially record changes in the regional upstream convection. The δ18O signal can be altered by changes in the moisture source location, which implies that enhanced moisture delivery from remote source regions leads to more negative precipitation δ18O due to an increase in the rainout effect during transport. Our results have implications for the interpretation of past variations of archives with precipitation stable isotopes, such as ice cores, tree rings, lake sediments, and speleothems in the TP and surrounding regions.

Validation and Bias Correction of Monthly δ18O Precipitation Time Series from ECHAM5-Wiso Model in Central Europe

Simulated stable oxygen isotopic composition (δ18O) of precipitation from isotope-enabled GCMs (iGCMs) have gained significant visibility nowadays. This study evaluates bias correction techniques to reduce the systematic and dispersion biases of the modelled δ18O by the ECHAM5-wiso model compared to the Global Network of Isotopes in Precipitation (GNIP) observations over Central Europe. mean bias error (MBE) and Root Mean Square Error (RMSE) are substantially reduced by more than 70% and 10%, respectively, depending on the bias correction scheme, with better results for Generalized Additive Model (GAM) and linear scaling approach (SCL) methods. The bias-corrected δ18OECHAM5-wiso values successfully describe the long-term isotopic composition of precipitation and the isotopic amplitude with the best performances for the EQM method. The necessity of applying bias correction algorithms is verified by the excellent agreement between the corrected δ18OECHAM5-wiso with GNIP in high-altitude areas where ECHAM5-wiso fails to reproduce the observed isotopic variability. The results are expected to bring valuable insights into the utilization of iGCMs’ relationships in climate studies for understanding the present and past water cycle under the isotopic perspective.

Ultra-high resolution δ18O of land snail shell: A potential tool to reconstruct frequency and intensity of paleoprecipitation events

Extreme rainfall events cause tremendous threats to people’s lives and economic well-being. Given the short time span of instrumental records, reconstructing the full range of past rainfall variability using available proxies is impossible when trying to unravel the history and underlying mechanisms of extreme precipitation events. However, this work is usually limited by the temporal resolution of current paleoclimate records. Here we demonstrate that the ultra-high resolution measurements of δ18O of terrestrial snail shells of Cathaica fasciola (Draparnaud, 1801) can be used to reconstruct the rainfall variability on weather time scales. Broadly, the modern daily time resolution snail shell’s δ18O profile shows a negative trend from late March to September in 2020, which resembles the variations of monitored δ18O and amount of precipitation. Meanwhile, six abrupt negative deviations were observed in the δ18O profiles and these deviations matched well with the instrumental precipitation events for the same period. The high correlation of shell’s δ18O with rainfall amount during the six precipitation events indicates that the ultra-high resolution δ18O records of gastropod shells have the potential to qualitatively record local synoptic rainfall events. Our study highlights that the terrestrial gastropod shell could provide daily to weekly geochemical proxies and has the potential to be used as an ultra-high resolution archive for characterizing terrestrial paleoweather, such as extreme precipitation events.

Stable isotope variability of precipitation and cave drip-water at Jumandy cave, western Amazon River basin (Ecuador)

Monitoring studies are necessary to better understand the hydrological processes affecting the isotopic signature of cave waters, which are ultimately recorded in speleothems that are used as paleoclimate archives. This research examines changes in the isotopic composition (δ18O and δ2H) of precipitation as it infiltrates through the epikarst and into the Jumandy cave, located in the western Amazon Basin (Ecuador). Meteorological and hydrological parameters were monitored outside and inside the cave, and isotope analyses were carried out in waters from rainfall, an underground river, and drip-water at two sampling sites in the cave between April 2019 and February 2020. At monthly timescale, the rainfall weighted isotopic composition monitored at our stations was strongly correlated with the mean precipitation amount. However, when considered at weekly time-steps, the correlation is only moderate. This implies that the variation of the isotopic composition in the study area cannot be interpreted exclusively as an amount effect. Isotopic values and back-trajectory modeling show that the isotopic signature was affected by the moisture source effect associated with upstream rainout. The moisture flux is dominantly from an east to northeast direction and moisture mainly originates over the Atlantic Ocean, passing through the Amazon Basin. A significant fraction of moisture is associated with local sources within the Amazon Basin. This aspect is confirmed by d-excess values of rainfall and the Local Meteoric Water Lines (LMWLs) that indicate an influence of the high evapotranspiration rate of the Amazon region on the isotopic composition of local rainfall. The infiltrated water resides for about three weeks in the epikarst and then precipitates forming speleothems (residence time). However, this short residence time needs to be confirmed with a longer monitoring period. Despite the different magnitudes of the dripping rates, the isotopic values at the two monitored sites are similar. This suggests that the dripping discharge rate is affected by the karst structure, but the isotopic signature reflects the mixing of individual rainfall events above the cave. Therefore, ẟ18O in speleothems from these caves is mainly recording short-term precipitation changes linked to regional and large-scale atmospheric circulation.

An hourly-scale assessment of sub-cloud evaporation effect on precipitation isotopes in a rainshadow oasis of northwest China

In arid northwest China where the precipitation intensity is relatively low, the stable hydrogen and oxygen isotopes (δ2H and δ18O) in precipitation are usually impacted by the sub-cloud evaporation. To understand the sub-cloud evaporation effect under an arid climate, we used the hourly meteorological data at 14 stations in the Kaxgar-Yarkant River Oasis, a rainshadow oasis of northwest China, and estimated the monthly and hourly isotopic change in falling drops from the cloud base to the ground. The results showed that the hourly meteorological observations are an effective way to assess the spatiotemporal pattern of sub-cloud evaporation effect. Across the 14 stations, the annual mean changes in deuterium excess below cloud base ranged from −19.1‰ to −6.8‰. The impacts of sub-cloud evaporation during spring and autumn are larger than those during winter and weaker than those during summer. The exponential regression (R2 = 0.96), instead of linear regression (R2 = 0.85), is better at describing the relationship between the raindrop remaining fraction and the isotopic changes from the cloud base to the ground. The sensitivity analysis of isotopic changes to different relative humidity scenarios shows that the drying scenario may lead to slightly larger sensitivity than the wetting scenario. The impact of sub-cloud evaporation on stable isotope compositions in precipitation may be underestimated when low precipitation events (especially less than 1 mm/h) are not sampled.

Isotopic Characterization of Rainwater for the Development of a Local Meteoric Water Line in an Arid Climate: The Case of the Wadi Ziz Watershed (South-Eastern Morocco)

For any hydrological or hydrogeological system, the arrival of new rains is the input signal to the system. This isotopic signature of precipitation is of major interest in understanding the recharge processes of the aquifer system. On the scale of a given basin, staged stations at different altitudes and spread out in space allow this input signal to be well characterized and to draw the local meteoric water line. In south-eastern Morocco, specifically, in the Errachidia region, several chemical and isotopic studies of the waters of the various aquifers have been carried out. In the absence of a local meteoric water line, these studies were based on the use of the global meteoric water line (GMWL). Thus, the objective of this work is the isotopic characterization and the elaboration of the local meteoric water line of the rainwater of the Ziz watershed. This characterization of the input signal in the study area is based on 41 measurements of stable isotopes (δ18O and δ2H) relating to the precipitations collected during the period from December 2019 to November 2020 in four staged stations at different altitudes and spread over the space from upstream to downstream of the watershed. The linear relationship of δ2H as a function of δ18O describes the local meteoric water line (LMWL) by equation δ2H = 7.5 ± 0.3 δ18O + 4.6 ± 1.7; R2 = 0.93. This equation displays evaporation confirmed by the arrival of continental currents in an arid environment. The variation in precipitation δ18O as a function of the sampling altitudes for the rains highlighted the relationship δ18O = −0.0026 ∗ Z − 1.67, with R2 = 0.93, which means an altitudinal gradient of −0.26‰ per 100 m of altitude. In this regard, the development of the local meteoric water line and the determination of the altitudinal gradient for the first time in this arid to semi-arid region of the watershed will be of great use to researchers and water resource managers; for example, to help determine the groundwater recharge areas, determine the exchanges between surface water and groundwater, and analyze many other hydrological problems.

Asian-Australian monsoon evolution over the last millennium linked to ENSO in composite stalagmite δ18O records

The Asian-Australian monsoon (AAM) system distinctly influences global climate. However, the linkage between ENSO (El Niño–Southern Oscillation) and AAM is still poorly understood over the last millennium. Here, we combined stalagmite δ18O (δ18Os) records with exceptionally high-resolution and high-precision chronologies to demonstrate a tight remote coupling between AAM and tropical ocean hydrology on the multi-year, decadal, and centennial timescales. Our results reveal that (1) There is a significant negative correlation between the multi-year weighted average precipitation δ18O (δ18Oaw) from the AAM and Southern Oscillation Index (SOI), providing a modern analogous coupling for linking stalagmite δ18O (δ18Os) and ENSO over the past 100 years. (2) Over the last millennium, the integrated δ18Os record exhibit positive excursions during the Little Ice Age (LIA), marked by a tendency for more El Niño-like state conditions in the tropical Pacific, which is consistent in SST-gradient ENSO reconstruction but different to precipitation reconstruction. (3) Wavelet and power spectrum analysis for the integrated δ18Os record in the AAM region show that there is a significant multi-year periodicity (2.2, 2.8, 3.5, 4.6 yr), conceding with the ENSO cycle (2–7 yr), whereas, some multi-decadal and centennial cycles (10.1, 17, 28.6, 52.7, 143 yr) are significant too. We provide evidence from instrumental and paleocliamte datasets (GNIP and NOAA) and propose a possible “circulation effect” mechanism responsible for variations of δ18Os in the East Asian Monsoon (EAM) and Australian Monsoon (AM) region over the last millennium.

The Seasonally Altered Atmosphere Moisture Circulations With Rainfall and Rainfall Isotopes in Southwest China

To interpret the climatic signals of precipitation/speleothem δ18O, it is critical to identify the importance of the factors affecting the precipitation δ18O. This study presents new stable isotope data for precipitation δ18O and δD in the site of Shenqi cave, southwest China, from November-2015 to October-2016 (the “Super-El Niño” event), to investigate the regional-scale climate forcing on precipitation δ18O. The precipitation δ18O, δD and d-excess have an obvious seasonality, relatively low values in the wet season and high in the dry season. The further analysis of seasonally altered LMWL and moisture circulations suggested that changes in atmosphere moisture circulations would be the key factor underlying the precipitation/speleothem δ18O fluctuations in our study area at least on seasonal timescales. Combined with the seasonal-monthly variations of the IsoGSM δ18O, GPCP/CRU rainfall and NCEP/NCAR moisture fluxes, we detected that the super-El Niño of 2016 have changed the distributions of monthly rainfall in wet season through the Western Pacific Subtropical High, but not mainly the precipitation isotopic compositions and moisture circulations in our study area.

Arctic and sub-Arctic lake water δ2H and δ18O along a coastal-inland transect: Implications for interpreting water isotope proxy records

High-latitude lakes are sensitive to climate change and store information about large-scale circulation changes and catchment-integrated processes. Lakes are mainly recharged by meteoric water, meaning that some lake sediment proxies may indirectly archive the stable isotopic composition of hydrogen (δ2H) and oxygen (δ18O) of past precipitation. Yet, despite similar precipitation input, lakes within a region may exhibit a wide range of isotopic values due to the varying influence of inflow seasonality and evaporation. Moreover, the relative sensitivity of each lake to these controls may vary through time, something that is difficult to account for. Here, we evaluate the impact of variable inflow δ2H and evaporation on the lake water isotopic composition across northern Fennoscandia (Norway, Finland, and Sweden). We measured lake water δ2H and δ18O of 135 lakes spanning from the north Norwegian coast along a 460 km transect to the Bothnian Bay, sampled from 2018 to 2020. Our data show that both coastal and inland lakes are sensitive to distillation during moisture transport, and that lakes farther from the Atlantic Ocean are additionally impacted by evaporation. We estimated the isotopic composition of lake water inflow values for evapo-concentrated transect lakes (δ2HI) using a Bayesian method. Resampled transect lakes had more depleted δ2HI in 2020 than in 2019, indicating either that precipitation was 2H-depleted or that more winter precipitation contributed inflow to the lakes in 2020 compared to in 2019. We suggest that the more 2H-depleted values in 2020 were a response to a snow-rich winter, associated with extremely positive Arctic Oscillation (AO+) conditions and increased moisture supply from the North Atlantic. We find evidence that lake water isotopic variability in this region reflects a combination of seasonal precipitation changes associated with atmospheric circulation changes, and catchment-integrated evaporation. Careful consideration of the variable sensitivity to these processes is essential when making inferences about past climate based on lake water isotope proxies.

Evidence for high-elevation salar recharge and interbasin groundwater flow in the Western Cordillera of the Peruvian Andes

Abstract. Improving our understanding of hydrogeological processes on the western flank of the central Andes is critical to communities living in this arid region. Groundwater emerging as springs at low elevations provides water for drinking, agriculture, and baseflow. However, the high-elevation sources of recharge and groundwater flow paths that convey groundwater to lower elevations where the springs emerge remain poorly quantified in the volcanic mountain terrain of southern Peru. In this study, we identified recharge zones and groundwater flow paths supporting springs east of the city of Arequipa and the potential for recharge within the high-elevation closed-basin Lagunas Salinas salar. We used general chemistry and isotopic tracers (δ18O, δ2H, and 3H) in springs, surface waters (rivers and the salar), and precipitation (rain and snow) sampled from March 2019 through February 2020 to investigate these processes. We obtained monthly samples from six springs, bimonthly samples from four rivers, and various samples from high-elevation springs during the dry season. The monthly isotopic composition of spring water was invariable seasonally in this study and compared to published values from a decade prior, suggesting that the source of recharge and groundwater flow paths that support spring flow is relatively stable with time. The chemistry of springs in the low-elevations and mid-elevations (2500 to 2900 m a.s.l.) point towards a mix of recharge from the salar basin (4300 m a.s.l.) and mountain-block recharge (MBR) in or above a queñuales forest ecosystem at ∼4000 m a.s.l. on the adjacent Pichu Pichu volcano. Springs that clustered along the Río Andamayo, including those at 2900 m a.s.l., had higher chloride concentrations, indicating higher proportions of interbasin groundwater flow from the salar basin likely facilitated by a high degree of faulting along the Río Andamayo valley compared to springs further away from that fault network. A separate groundwater flow path was identified by higher sulfate concentrations (and lower Cl-/SO4-2 ratios) within the Pichu Pichu volcanic mountain range separating the city from the salar. We conclude that the salar basin is not a hydrologic dead end. Instead, it is a local topographic low where surface runoff during the wet season, groundwater from springs, and subsurface groundwater flow paths from the surrounding mountains converge in the basin, and some mixture of this water supports groundwater flow out of the salar basin via interbasin groundwater flow. In this arid location, high-elevation forests and the closed-basin salar are important sources of recharge supporting low-elevation springs. These features should be carefully managed to prevent impacts on the down-valley water quality and quantity.

Oxygen isotopes of land snail shells in high latitude regions

The present study investigates the environmental significance of the oxygen isotopic composition of several modern land snail species collected along two north-to-south transects in Alaska and Scandinavia at latitudes between 60 and 70 °N. We tested the hypothesis that land snail shell δ18O values primarily track precipitation δ18O. The results show that shell δ18O values from Scandinavia were ∼5.1‰ enriched in 18O with respect to snails from Alaska, equivalent to differences in precipitation δ18O values between the two regions. Within the Alaskan transect, shell δ18O values increased with observed increasing air temperature and precipitation δ18O, whereas shell δ18O values from Scandinavia did not correlate to instrumental climate data because of a reduced climatic gradient across the locations sampled. In addition, shell δ18O values differed significantly among sympatric species, with larger species consistently exhibiting higher δ18O values, which implies that species-level isotopic variations should be considered at the local and microhabitat scale. However, when snail shell δ18O values from this study are combined with previously published data from North America and Europe, we see evidence that shell δ18O values track precipitation δ18O across latitudes, even when different species are combined because climate gradients are greater than variations among taxa.

Indian summer monsoon variations during the Younger Dryas as revealed by a laminated stalagmite record from the Tibetan Plateau

High-resolution and precisely dated hydroclimate records in the southeastern Tibetan Plateau (TP) remain sparse beyond the Holocene, which hampers our ability to understand the hydroclimate variability in this important Indian summer monsoon (ISM) fringe area and its global teleconnection. Here we present 3-y resolution δ18O and δ13C records from a laminated stalagmite (RG-3) from Rige Cave in the southeastern TP, spanning the Younger Dryas (YD). The records allow us to precisely characterize the timing, structure, and particularly centennial-scale events within the YD, and probe the control factors of precipitation δ18O (δ18Op) in the ISM fringe area. On centennial–millennial timescales, the Rige δ18O record shows coherent pattern with the East Asian summer monsoon (EASM) counterpart, combined with modeled δ18Op results and spatial analysis, suggesting that δ18Op in this part of the TP is largely controlled by the large-scale atmosphere circulation (or the ISM strength), and the altitude increase in the TP may not potentially result in an opposite δ18Op pattern at least in the monsoonal TP regime. We also found significant δ13C-δ18O covariation on the centennial timescale, suggesting a coincided ISM rainfall and biomass change during the YD. In the Rige records, one weak centennial-scale ISM event (namely the intra-Allerød cold period, IACP) and three strong centennial-scale ISM events within the YD (namely A1′-A2′-A3′) were prominent and occurred between ∼12,470 and 12,310 ± 14, ∼12,210–12,090 ± 12 and ∼12,010–11,920 ± 12 y BP (before present, where present = 1950 CE), respectively. Spectral analyses of Rige records also revealed a significant ∼200-y periodicity, which is nearly in-phase with observed centennial-scale variations of the North Atlantic temperature and mid-latitude westerly-jet during that time. These observations support the hypothesis that the solar de Vries cycle (207-y) triggered the centennial-scale climate variations in high northern latitude, leading to the ISM variations via fast atmospheric processes.

A 5000-year lacustrine sediment oxygen isotope record of late Holocene climate change in Newfoundland, Canada

The spatiotemporal patterns and underlying causes of climate variations spanning the mid-to late-Holocene in the North Atlantic region are largely unsettled. Here, we present a decadally resolved carbonate oxygen isotope (δ18O) record spanning the last ∼5000 calibrated years before present (cal yr BP) from Norman's Pond, a small, hydrologically open lake located in west-central Newfoundland, Canada. Stable isotope data from regional lakes, rivers, and precipitation samples indicate the δ18O record primarily reflects changes in the integrated, annual δ18O of precipitation, which is largely controlled by local atmospheric temperature and to a lesser extent, changes in the seasonality of precipitation. The record exhibits a general trend of decreasing δ18O values over the last ∼5000 cal yr BP that is consistent with gradually declining Boreal summer insolation and long-term cooling. Large decadal to centennial timescale fluctuations in δ18O values occurred during the last millennium, with persistent low δ18O values during the early Medieval Climate Anomaly (MCA, 950 to 1250 CE) and more variable and low values across the Little Ice Age (LIA, 1450 to 1850 CE). Relatively low δ18O values during the early MCA at ∼950 cal yr BP (∼1000 CE) suggest colder temperatures in Newfoundland and/or an increase in cold-season or decrease in warm-season precipitation across the island. MCA δ18O values are lower than those of the LIA, revealing that climate conditions in Newfoundland were variable and differed from the broader North Atlantic region. For example, the shift to very low δ18O during the early MCA is coincident with a transition to colder sea surface temperatures (SSTs) in the Labrador Sea region, which was potentially triggered by melting of the Greenland Ice Sheet and alpine glaciers and ice caps in/around Baffin Bay, and subsequent delivery of cold waters further south via the Labrador Current. The early MCA interval of inferred cooling is coincident with Norse settlement at L'Anse aux Meadows in northern Newfoundland at ∼929 cal yr BP (∼1021 CE). Subsequent low and variable δ18O during the LIA suggests regional shifts in the seasonality of precipitation and/or cooling, consistent with other terrestrial climate records from the region. Proxy evidence from Norman's Pond therefore reinforces the idea of substantial spatiotemporal variability in climatic change in the broader North Atlantic region since at least the mid-Holocene, and in particular during the MCA, when cooler conditions prevailed in Newfoundland as a result of lower offshore sea surface temperatures that were likely driven by greater meltwater delivery via the Labrador current.

Stable Isotopes of Precipitation in the Eastern Tarim River Basin and Water Vapor Sources

The in-situ monitoring of stable hydrogen and oxygen isotopes (δ18O and δ2H) in precipitation is helpful to understand the processes of water phase change, mixing, and transportation. Due to the arid climate in the eastern Tarim River basin, water resources are the key factor affecting the regional sustainable development. However, the understanding of stable hydrogen and oxygen isotopes in precipitation is still limited for this region. Based on the isotope data of 103 precipitation samples collected at four stations in the eastern Tarim River basin from June 2019 to September 2020, the spatial and temporal characteristics of stable hydrogen and oxygen isotopes in precipitation were analyzed, and the connections between stable isotopes and moisture sources were discussed. The findings provide a reference for the application of environmental isotope tracers in arid areas. The results show that, ① the four sampling stations generally presented an increasing trend in precipitation stable isotopic values from north to south, and the isotopic values were higher in summer and lower in winter. The d-excess value in the study region was smaller than the national average, and the slope of the local meteoric water line exhibited a significant arid characteristic. ② There was a positive correlation between precipitation stable isotopes and air temperature in the study region, whereas a weak negative relationship was seen between d-excess and air temperature. The negative correlation occurred between δ18O and relative humidity, and the d-excess value exhibited a positive correlation with relative humidity. ③ The backward trajectory showed that the eastern Tarim River basin is mainly controlled by the westerlies paths. The proportion of air mass that comes from the middle and short distance is relatively large. The concentration weighted trajectory method showed that the farther from the precipitation sampling site, the more stable the d-excess value was. The closer to the sampling site, the more frequently the concentration changed, indicating that the d-excess value was greatly affected by the local water vapor recycling.

Concurrent Changepoints in Greenland Ice Core δ18O Records and the North Atlantic Oscillation over the Past Millennium

Structural changes, or changepoints, coinciding in multiple ice core records over the Greenland Ice Sheet (GrIS) may reflect a widespread response of the GrIS to atmospheric forcing. Thus, to better understand how atmospheric circulation may regulate sudden changes in δ18O of Greenland precipitation, we seek synchronous changepoints occurring in ice core-derived δ18O time series across the GrIS and in the North Atlantic Oscillation (NAO) over the past millennium. By utilizing a Bayesian changepoint detection method, four changepoint horizons were revealed: at the beginning of the 20th century, in the late-15th century, and around the turn of the 11th and 10th centuries. Although the changepoints in ice core δ18O records exhibited distinctive spatial arrangements in each horizon, all corresponded to changepoints in the NAO, indicative of a consistent atmospheric influence on GrIS surface changes over the past millennium.

Spatial distribution of meteorological factors controlling stable isotopes in precipitation in Northern Chile

A knowledge of the evolution of isotopic composition of air masses humidity and precipitation in the Western Cordilleras of the Central Andes is still incomplete. This study contributes to a better understanding of the factors that control the δ18O and δ2H contents in precipitation water in the north of Chile, above 2000 m a.s.l.. This paper deals with: (1) the relevant effects and processes that control the spatial (longitude, latitude and altitude) distribution of stable isotope contents of precipitation events in northern Chile, (2) the influence of local meteorological variables: temperature, precipitation and relative humidity on the δ18O and δ2H of precipitation, and (3) the estimation of these meteorological and isotopic variables at specific sites. To achieve these objectives, the relationships between geospatial and meteorological values are identified and analysed, followed by the estimation with empirical models. These estimation models (linear and non-linear) are obtained after examining, validating and calibrating techniques to find the best fit. This results in models for temperature, relative humidity and precipitation for each month of the year. In the same way, three isotopic models are derived from the spatial and meteorological variables (summer, winter and annual). Temperature has been shown to be controlled to a greater extent by altitude and latitude, while relative humidity is by latitude and precipitation in summer is by altitude and latitude. Monthly meteorological variables have been estimated throughout the study area. Precipitation δ18O and δ2H are controlled mainly by temperature and altitude and to a lesser extent by latitude, longitude and precipitation. In the same way, three isotopic models are derived from the spatial and meteorological variables: summer, winter and annual. This opens a new perspective of precipitation and its isotopic contents, but also allows the calculation of runoff and aquifer recharge and the path for linking future precipitation and aquifer recharge through their isotopic composition.

Monitoring of Sierra Nevada Caves Reveals the Potential for Stalagmites to Archive Seasonal Variability

In the southwestern United States, California (CA) is one of the most climatically sensitive regions given its low (≤250 mm/year) seasonal precipitation and its inherently variable hydroclimate, subject to large magnitude modulation. To reconstruct past climate change in CA, cave calcite deposits (stalagmites) have been utilized as an archive for environmentally sensitive proxies, such as stable isotope compositions (δ18O, δ13C) and trace element concentrations (e.g., Mg, Ba, Sr). Monitoring the cave and associated surface environments, the chemical evolution of cave drip-water, the calcite precipitated from the drip-water, and the response of these systems to seasonal variability in precipitation and temperature is imperative for interpreting stalagmite proxies. Here we present monitored drip-water and physical parameters at Lilburn Cave, Sequoia Kings Canyon National Park (Southern Sierra Nevada), CA, and measured trace element concentrations (Mg, Sr, Ba, Cu, Fe, Mn) and stable isotopic compositions (δ18O, δ2H) of drip-water and for calcite (δ18O) precipitated on glass substrates over a two-year period (November 2018 to February 2021) to better understand how chemical variability at this site is influenced by local and regional precipitation and temperature variability. Despite large variability in surface temperatures and precipitation amount and source region (North Pacific vs. subtropical Pacific), Lilburn Cave exhibits a constant cave environment year-round. At two of the three sites within the cave, drip-water δ18O and δ2H are influenced seasonally by evaporative enrichment. At a third collection site in the cave, the drip-water δ18O responds solely to precipitation δ18O variability. The Mg/Ca, Ba/Ca, and Sr/Ca ratios are seasonally responsive to prior calcite precipitation at all sites but minimally to water-rock interaction. Lastly, we examine the potential of trace metals (e.g., Mn2+ and Cu2+as a geochemical proxy of recharge and find that variability in their concentrations has high potential to denote the onset of the rainy season in the study region. The drip-water composition is recorded in the calcite, demonstrating that stalagmites from Lilburn Cave, and potentially more regionally, could record seasonal variability in weather even during periods of substantially reduced rainfall.

Quantifying the effect of moisture source and transport on the precipitation isotopic variations in northwest Ethiopian Highland

Understanding the controlling factors for variations of precipitation isotopes is essential for accurate interpretation of isotope-based paleoclimate proxy data as well as better apprehension of the hydrological cycle. These are particularly important for Ethiopia, an arid region and important location for studying human evolution. Past studies have established the importance of moisture source on isotopic variation in precipitation, but few studies have identified exact moisture sources and quantified their contributions. In this study, we use a Lagrangian moisture diagnostic technique and daily precipitation isotope data collected at Debre Markos on the northwest Ethiopian Highland to quantify the contributions of different moisture sources to precipitation. We also quantitatively establish the relative importance of local climate conditions, climate conditions of moisture uptake locations, and different moisture sources on the isotopic variation in precipitation at the study site. Our results show that recycled continental moisture from adjacent areas is the most dominant moisture source throughout the year (67%), and its contribution is higher in the wet season (79%) than in the dry season (56%). Secondary moisture sources include the southern Indian Ocean (16%) for the wet season and the northern Indian Ocean (14%) for the dry season. Based on correlation analyses, we find that the most important control over the precipitation isotopic variation is the convective intensity at both the study site and the moisture uptake locations. Strong connective activities are linked to low isotopic values, and the seasonal movement of the deep convection region could explain the seasonal precipitation δ18O patterns with low values in the wet season and high values in the dry season. Other climate factors that affect the isotopic composition of precipitation include local temperature, relative humidity, rainfall amount as well as total rainout along the trajectories and relative humidity at moisture uptake locations. Our study could provide new perspectives for interpreting δ18O-based paleoclimate proxies in this region, as well as crucial information for understanding hydrocliamte processes in a region where sustainable water resources are of particular importance for socioeconomic development.