δ18O Variations Research Articles

Atmosphere-Snow Exchange Explains Surface Snow Isotope Variability.

The climate signal imprinted in the snow isotopic composition allows to infer past climate variability from ice core stable water isotope records. The concurrent evolution of vapor and surface snow isotopic composition between precipitation events indicates that post-depositional atmosphere-snow humidity exchange influences the snow and hence the ice core isotope signal. To date, however, this is not accounted for in paeleoclimate reconstructions from isotope records. Here we show that vapor-snow exchange explains 36% of the summertime day-to-day δ18O variability of the surface snow between precipitation events, and 53% of the δD variability. Through observations from the Greenland Ice Sheet and accompanying modeling we demonstrate that vapor-snow exchange introduces a warm bias on the summertime snow isotope value relevant for ice core records. In case of long-term variability in atmosphere-snow exchange the relevance for the ice core signal is also variable and thus paleoclimate reconstructions from isotope records should be revisited.

A method for predicting hydrogen and oxygen isotope distributions across a region's river network using reach-scale environmental attributes

Abstract. Stable isotope ratios (isotope values) of surface water reflect hydrological pathways, mixing processes, and atmospheric exchange within catchments. Development of maps of surface water isotope values (isoscapes) is limited by methods to interpolate point measures across river networks. Catchment attributes that alter surface water isotope values affect downstream river reaches via flow, but some attributes such as artificial dams are no more likely to affect nearby unconnected catchments than distant ones. Hence, simple distance-based geospatial and statistical interpolation methods used to develop isoscapes for precipitation and terrestrial systems are less appropriate for river networks. We used a water-balance-based method to map long-term average δ2H and δ18O for New Zealand rivers, incorporating corrections using catchment environmental predictors. Inputs to the model are national rainfall precipitation isoscapes, a digital elevation layer, a national river water isotope monitoring dataset (3 years of monthly sampling at 58 sites), and river environmental databases covering around 600 000 reaches and over 400 000 km of rivers. Much of the spatial variability in δ2H and δ18O of New Zealand river water was explained using the initial combination of precipitation isoscapes and a simple water balance model. δ2H and δ18O isoscapes produced by subsequently applying residuals from the water balance model as a correction factor across the river network using regression kriging showed improved fits to the validation data compared to the correction using ordinary kriging. Predictors of high importance in the regression included upstream lake and wetland area, which was not strongly spatially autocorrelated nationally. Hence, additional hydrological process information such as evaporation effects can be incorporated into river isoscapes using regression kriging of residuals. The resulting isoscapes have potential applications in ecological, hydrological, and provenance studies that consider differences between surface water isotope values and those of other components of the hydrological cycle (e.g. subsurface runoff or local precipitation).

Biomarker and Pollen Evidence for Late Pleistocene Pluvials in the Mojave Desert

AbstractThe climate of the southwestern North America has experienced profound changes between wet and dry phases over the past 200 Kyr. To better constrain the timing, magnitude, and paleoenvironmental impacts of these changes in hydroclimate, we conducted a multiproxy biomarker study from samples collected from a new 77 m sediment core (SLAPP‐SRLS17) drilled in Searles Lake, California. Here, we use biomarkers and pollen to reconstruct vegetation, lake conditions, and climate. We find that δD values of long chain n‐alkanes are dominated by glacial to interglacial changes that match nearby Devils Hole calcite δ18O variability, suggesting both archives predominantly reflect precipitation isotopes. However, precipitation isotopes do not simply covary with evidence for wet‐dry changes in vegetation and lake conditions, indicating a partial disconnect between large scale atmospheric circulation tracked by precipitation isotopes and landscape moisture availability. Increased crenarchaeol production and decreased evidence for methane cycling reveal a 10 Kyr interval of a fresh, productive, and well‐mixed lake during Termination II, corroborating evidence for a paleolake highstand from shorelines and spillover deposits in downstream Panamint Basin and Death Valley during the end of the penultimate (Tahoe) glacial (140–130 ka). At the same time brGDGTs yield the lowest temperature estimates (mean months above freezing = 9°C ± 3°C) of the 200 Kyr record. These limnological conditions are not replicated elsewhere in the 200 Kyr record, suggesting that the Heinrich stadial 11 highstand was wetter than the last glacial maximum and Heinrich 1 (18–15 ka).

Recharge variability in Australia's southeast alpine region derived from cave monitoring and modern stalagmite δ18O records

Oxygen isotopic (δ18O) variations in stalagmite records have the potential to provide new insights about past climates beyond the instrumental record. This paper presents the first high-resolution oxygen isotope time series of three coeval stalagmite records from the alpine region of south-eastern Australia covering the period 1922–2006 CE. We use extended surface and cave monitoring datasets, petrographic investigation, modelled recharge time series and farmed calcite precipitates to assess the controls on speleothem δ18O and investigate the coherence between three records from Harrie Wood Cave. The drip water response to recent interannual rainfall variability shows that cave drip water Cl−, δ18O and drip rate display a clear response to an increase in rainfall recharge. It is demonstrated that stalagmites from the same drip sites also record variability in interannual recharge, where an increase in δ18O values is observed with lower recharge, while a decrease in δ18O values correspond to higher recharge amounts. The three stalagmite δ18O records are in broad agreement, showing common responses to relatively higher recharge between 1945 and 1995 CE and the low recharge periods between 1937 and 1945 CE (World War II drought) and late 1996 to 2006 CE (beginning of the Millennium Drought). However, differences in the magnitude of the relative response of each stalagmite δ18O record varies. Based on evidence from our cave monitoring study and farmed calcites, we conclude that the differences between the three stalagmite records is attributed to variability in the contribution of preferential flows during recharge events and the store reservoir volume supplying the drip site. When the δ18O decreases in response to enhanced recharge, the speleothem δ13C also decreases, and this is interpreted to reflect a soil respiration response to changes in soil moisture availability due to recharge. Hence, stalagmite δ18O from the Australian alpine region can be applied to reconstruct periods of relatively higher and lower rainfall recharge and thus extend our knowledge of the timing and relative magnitude of droughts as well as past periods of higher recharge in this region.

Precipitation stable isotope composition, moisture sources, and controlling factors in Xi'an, Northwest China

Seasonal and interannual variations in precipitation stable isotopes are crucial for clarifying complex moisture sources at the inland margins of the summer monsoon such as Northwest China. Stable hydrogen and oxygen isotope data (δD, δ18O) of totally 317 event-based precipitation samples from 2016 to 2020 in Xi'an show a consistent annual cycle in which, maximum δD and δ18O values occur in the pre-monsoon season (April–June), and minimum in the late monsoon to post-monsoon (November–March) seasons. Values of d-excess are lowest in the monsoon season. The local meteoric water line is δD = 7.5δ18O + 9.7. Precipitation δ18O in monthly time scale correlates weakly with relative humidity in the monsoon season, and with precipitation amount in the pre-monsoon season. Maps of potential water sources and wind fields, considered with monsoon intensity indices and the isotope data, are consistent with moisture sources as follows: during the monsoon, moisture advected by air flow mainly from the Bay of Bengal or recycled from land surfaces along this path; during the pre-monsoon, moisture advected in westerly circulation passing north of the Tibetan Plateau; and during the post-monsoon, recycled monsoon moisture advected from the south or southwest of Xi'an, and influenced by the intensity of the preceding South Asian Summer Monsoon (SASM). Moisture recycled from land surfaces is important in all seasons. The SASM index controlled the variation of mean seasonal δ18O, but not precipitation amount in the monsoon season. The westerly index (WI) controlled the precipitation amount in the pre-monsoon season and mean seasonal δ18O in the post-monsoon season. Seasonal change in moisture sources is the main reason of the seasonal variation of mean precipitation δ18O.

Spatial variability and post-depositional diffusion of stable isotopes in high accumulation regions of East Antarctica

Stable isotopes of Antarcticice cores are used extensively in reconstructing the past climate. Variations in snow accumulation patterns and post-depositional processes such as changes in isotope ratios due to diffusion may complicate these records. We analyzed the spatio-temporal variations of snow accumulation, stable isotopic composition, and factors controlling their distribution along two transects in the Dronning Maud Land (DML) and Princess Elizabeth Land (PEL) regions of East Antarctica. The δ18O and δD variations are dominantly influenced by snow accumulation in the DML region and temperature in the PEL region. To evaluate the extent of diffusion in isotope records, the isotope record of an ice core (IND-33) drilled after 5 years close to the DML transect was compared with the snow isotope records. A quantitative comparison of the snow core δ18O records with that of the ice core revealed signal attenuation ranging between 55 and 70%. Using a firn diffusion model, we estimated a diffusion length of 6 cm in IND-33 over 5 years. Our study revealed that while isotope diffusion exists even in high accumulation sites of coastal Antarctica, it does not significantly impact the dating and paleoclimatic interpretation of isotope records, unlike in low accumulation areas.

Model-based orbital-scale precipitation δ18O variations and distinct mechanisms in Asian monsoon and arid regions

The past Asian precipitation δ18O (δ18Op) records from stalagmites and other deposits have shown significant orbital-scale variations, but their climatic implications and regional differences are still not fully understood. This study, as the first attempt of a 300-kyr transient stable isotope-enabled simulation, investigated the characteristics and mechanisms of the orbital-scale δ18Op variations in three representative regions of Asia: arid Central Asia (CA), monsoonal South Asia (SA) and monsoonal East Asia (EA). The modelling results showed that the variations in the CA, SA and EA annual δ18Op exhibited significant but asynchronous 23-kyr precession cycles. Further analyses revealed that although the precession-induced insolation variation was the ultimate cause of the δ18Op variation in all three regions, the dominant mechanisms and the involved physical processes were distinct among them. For the CA region, the rainy-season (November-March) temperature effect and water vapour transport by the westerly circulation were identified as the key precession-scale processes linking the October-February boreal mid-latitude insolation to the rainy-season or annual δ18Op. In the SA region, the rainy-season (June-September) precipitation amount effect and upstream depletion of the monsoonal water vapour δ18O served as the main mechanisms linking the rainy-season or annual δ18Op to the April-July insolation variation at the precession scale. For the EA region, however, the precession-scale annual δ18Op was mainly controlled by the late-monsoon (August-September) and pre-monsoon (April-May) water vapour transport patterns, which were driven by the July-August insolation and the global ice volume, respectively. These results suggest that the climatic implications of the orbital-scale Asia δ18Op variations are sensitive to their geographic locations as determined by the combined effects of insolation and regional circulation patterns associated with the respective rainy seasons. This study provides new insights into understanding the regional differences and formation mechanisms of the Asian orbital-scale δ18Op variations.

Fine-scaled climate variation in equatorial Africa revealed by modern and fossil primate teeth

Variability in resource availability is hypothesized to be a significant driver of primate adaptation and evolution, but most paleoclimate proxies cannot recover environmental seasonality on the scale of an individual lifespan. Oxygen isotopecompositions (δ18O values) sampled at high spatial resolution in the dentitions of modern African primates (n = 2,352 near weekly measurements from 26 teeth) track concurrent seasonal precipitation, regional climatic patterns, discrete meteorological events, and niche partitioning. We leverage these data to contextualize the first δ18O values of two 17 Ma Afropithecus turkanensis individuals from Kalodirr, Kenya, from which we infer variably bimodal wet seasons, supported by rainfall reconstructions in a global Earth system model. Afropithecus' δ18O fluctuations are intermediate in magnitude between those measured at high resolution in baboons (Papio spp.) living across a gradient of aridity and modern forest-dwelling chimpanzees (Pan troglodytes verus). This large-bodied Miocene ape consumed seasonally variable food and water sources enriched in 18O compared to contemporaneous terrestrial fauna (n = 66 fossil specimens). Reliance on fallback foods during documented dry seasons potentially contributed to novel dental features long considered adaptations to hard-object feeding. Developmentally informed microsampling recovers greater ecological complexity than conventional isotope sampling; the two Miocene apes (n = 248 near weekly measurements) evince as great a range of seasonal δ18O variation as more time-averaged bulk measurements from 101 eastern African Plio-Pleistocene hominins and 42 papionins spanning 4 million y. These results reveal unprecedented environmental histories in primate teeth and suggest a framework for evaluating climate change and primate paleoecology throughout the Cenozoic.

Defining a Precipitation Stable Isotope Framework in the Wider Carpathian Region

The eastern part of Europe is very poorly represented in the Global Network for Isotopes in Precipitation (GNIP) database, mainly because the monitoring of the stable isotopes in precipitation started only recently compared with other regions. In this respect, the main objective of this article is to fill the gap in the GNIP database over the eastern part of Europe and show the temporal variability and potential drivers of an extended network of δ18O values in precipitation collected from 27 locations in Romania and the Republic of Moldova. We also present the first high-resolution map of the spatio-temporal distribution of δ18O values in precipitation in Romania and the Republic of Moldova, according to an observational dataset. According to our results, the stations from western and northern Romania tend to have LMWLS with higher values than those from southwestern Romania. The monthly variation of the δ18O and δ2H showed a clearly interannual variation, with distinct seasonal differences, following the seasonal temperatures. The analysis of the spatial distribution of stable isotopes in precipitation water was made on the basis of both observational data and modeled data. This allowed us to study the origin of the air moisture and the interaction with regional and local patterns and to analyze the link between the spatial δ18O variations and the large-scale circulation patterns on a seasonal scale.

Orbital-scale hydroclimate variations in the southern Tibetan Plateau over the past 414,000 years

Speleothem oxygen isotope (δ18O) constitutes one of key proxies to reconstruct the Indian and East Asian summer monsoon (ISM and EASM) hydroclimate history on a wide range of timescales. Here, we use three new well-dated speleothem δ18O records from Tianmen cave, in the south-central Tibetan Plateau (TP), together with previous records from the same cave, to reconstruct a composite Tianmen record spanning over the past 414,000 years. The Tianmen δ18O record follows Northern Hemisphere summer insolation (NHSI), broadly coherent with speleothem δ18O records from both ISM and EASM regimes. In comparison with previous model simulations and modern hydroclimate analyses, the Tianmen δ18O was interpreted primarily as a proxy indicating the ISM intensity. The Tianmen δ18O record comprises both high and low NHSI time periods, which reveals an exceptional large amplitude (∼14‰) of δ18O variations at the precession band, possibly resulted from a combination effect of precipitation amount, moisture source, transport pathway, high elevation and rainout. Besides, a broad consistency on orbital-scale variability among Tianmen δ18O, δ13C and trace element records is found, suggesting that the ISM intensity regulates the effective rainfall at the cave site as well. Notably, Tianmen speleothem growth occurred merely during warm-wet interglacial periods, even during low NHSI times near the precession maximum, and in contrast, hiatuses persistently occurred throughout cold-dry glacial periods. This pattern is similar to that found in travertine formations in the region, both suggesting that the interglacial temperature might be a prerequisite for speleothem/travertine formations in the region, while the strong ISM precipitation in turn provides a sufficient condition. Our new Tianmen record, combined with a large set of speleothem records from both Asian Westerlies and Asian monsoon domains, gives further evidence to reinforce a previous hypothesis: a consistent temporal pattern in the δ18O of precipitation on orbital and millennial scales over the vast Asian monsoon and Westerlies regimes. As such, in order to reconcile the precipitation δ18O variations between speleothem and TP ice cores, we argue that the Guliya ice-core chronology may have a basal age of ∼70 ka BP, which is supported by recent dating results from the Guliya ice cap.

Rapid Warming Over East Antarctica Since the 1940s Caused by Increasing Influence of El Niño Southern Oscillation and Southern Annular Mode

El Niño Southern Oscillation (ENSO), Interdecadal Pacific Oscillations (IPO), and their phase relation with the Southern Annular Mode (SAM) largely control Antarctic climate variability. The relative roles of these climate modes remain elusive, particularly in the backdrop of global warming. In this study, we present a seasonally resolved new ice core (IND33) record of oxygen isotope (δ18O) for the past two centuries (1809–2013 CE) from coastal Dronning Maud Land (DML) to investigate the role of these climate modes in the Antarctic temperature variability and trend. Our investigation based on this record combined with available records from the DML region reveals that ∼32% variability in δ18O records is related to late spring to summer (Nov–Dec–Jan) temperature rather than the mean annual temperature. This indicates that reconstructed annual temperature based on Antarctic ice core δ18O records could be biased toward the temperature of the months/seasons of higher precipitation with low-moderate wind speed, which are suitable for better preservation of the ice core signal. We have reconstructed the DML temperature record of the past two centuries (1809–2019 CE) at an annual resolution based on the δ18O ice core record (1809–1993 CE) combined with the recent ERA5 surface air temperature record (1994–2019 CE). The reconstructed temperature anomaly record reveals a significant cooling trend in the 19th century during 1809–1907 CE with a rate of −0.164 ± 0.045°C decade−1 followed by a warming trend from the mid-20th to early 21st centuries (1942–2019 CE) with a rate of +0.452 ± 0.056°C decade−1. This long-term warming trend since the 1940s coincides with the increase in ENSO events and its strong antiphase relation with SAM, suggesting an increasing influence of SAM–ENSO coupling in modulating the DML temperature in recent decades.

Kinetic isotope effects in H2O2 self-decomposition: Implications for triple oxygen isotope systematics of secondary minerals in the solar system

Hydrogen peroxide (H2O2) is a ubiquitous molecule in nature that shapes the redox state of planetary surfaces. Given that H2O2 is a major oxidant, isotope effects associated with H2O2 chemistry play a key role in determining triple oxygen isotopic compositions (δ17O and δ18O) of secondary aerosols and minerals, which are powerful proxies for understanding terrestrial/Martian atmospheric chemistry, and chemical evolution in the solar nebular. However, isotope effects in H2O2 self-decomposition processes, which actively occur in nature due to the thermal instability of H2O2, remains poorly understood. Here, we report a hitherto overlooked large and mass-dependent isotope fractionation in aqueous H2O2 self-decomposition processes quantified in a series of kinetic experiments. δ18O in remaining H2O2 is several tens of per mil (‰) with respective to initial H2O2. By synthesizing triple oxygen isotope measurements of natural H2O2, ozone, various oxyanions formed in the atmosphere (sulfate, nitrate, perchlorate, and carbonate), and oxygen-bearing secondary minerals in meteorites, we find that a decoupled Δ17O–δ18O pattern in natural H2O2 is attributed in part to the high degree of mass-dependent δ18O variation in H2O2 decomposition, and further argue that this unique signature may play a crucial role in triple oxygen isotope systematics in a board spectrum of secondary minerals in our solar system including aerosols, sediments, and meteorites. Our results shed fresh insights into recent debates on the role of H2O2 in the formation of these secondary minerals in the modern Earth, geological past, and other planets. The isotope effect experimentally quantified in this study are needed for future improvements of planetary atmosphere and solar nebular evolution models. We highlight the importance of further experimental and theoretical efforts to quantify isotope effects in H2O2 chemistry that are representative of natural systems.

New Data on Paleogeography of Quaternary Coast Sediments of the Rivers of the European North of Russia Using Isotope-Geochemical Methods

(1) Background: The Quaternary river bank deposits of the European North of Russia contain several mollusk fossils that can give important past-climate information which is relevant to understand the evolution of climate in the Holocene period and predict future scenarios on climate change. (2) The purpose of the research: reconstruction of the main physical and chemical parameters of the habitat of mollusks based on the determination of the species of mollusks and the study of the isotope-geochemical composition of shells and their host deposits. (3) Object of study: Quaternary deposits of coastal outcrops of the rivers of the European North of Russia: Pechora Sula, Peza, bivalve shells, and their fragments. (4) Results: The revealed macrofauna complexes represented by bivalves (Arctica islandica, Hiatella arctica, Mytilus edulis, Tridonta borealis, Mya truncata, Neptunea despecta) were determined and the variations of δ13C and δ18O (‰) isotopes in the studied fossil bivalve shells were calculated. The data on the concentrations of chemical elements in fossil shells were determined and summarized. The content of elements in mollusk shells varied within the following limits: Fe 101.7−403.3 µg/g (Vastyansky Kon’, river Pechora), Mg 0.38–2.61 µg/g (Vastyansky Kon’, river Pechora), Ba 18.27–60.44 µg/g (village Oksino-River Sula), and Sr 1128.3–2275.34 µg/g (Vastyansky Kon’-village Oksino, river Pechora). The ratio between the elements varies within Ba/Sr 0.011–0.041 (village Oksino- River Sula) and Mg/Fe 0.01–0.15 (Vastyansky Kon’- village Oksino, river Pechora).

Intra-annual tree-ring δ18O and δ13C reveal a trade-off between isotopic source and humidity in moist environments.

Tree-ring intra-annual stable isotopes (δ13C and δ18O) are powerful tools for revealing plant ecophysiological responses to climatic extremes. We analyzed interannual and fine-scale intra-annual variability of tree-ring δ13C and δ18O in Chinese red pine (Pinus massoniana) from southeastern China to explore environmental drivers and potential trade-offs between the main physiological controls. We show that wet season relative humidity (May-October RH) drove interannual variability of δ18O and intra-annual variability of tree-ring δ18O. It also drove intra-annual variability of tree-ring δ13C, whereas interannual variability was mainly controlled by February-May temperature and September-October RH. Furthermore, intra-annual tree-ring δ18O variability was larger during wet years compared with dry years, whereas δ13C variability was lower during wet years compared with dry years. As a result of these differences in intra-annual variability amplitude, process-based models (we used the Roden model for δ18O and the Farquhar model for δ13C) captured the intra-annual δ18O pattern better in wet years compared with dry years, whereas intra-annual δ13C pattern was better simulated in dry years compared with wet years. This result suggests a potential asymmetric bias in process-based models in capturing the interplay of the different mechanistic processes (i.e., isotopic source and leaf-level enrichment) operating in dry versus wet years. We therefore propose an intra-annual conceptual model considering a dynamic trade-off between the isotopic source and leaf-level enrichment in different tree-ring parts to understand how climate and ecophysiological processes drive intra-annual tree-ring stable isotopic variability under humid climate conditions.

Variation and reason analysis of groundwater hydrochemical characteristics in Beiluhe Basin, Qinghai–Tibet Plateau during a freezing–thawing period

Abstract Groundwater in the Qinghai–Tibet Plateau (QTP) is mainly distributed in the active layer. Rising temperatures and human activities have influenced the hydrogeochemical characteristics of the QTP in recent years. This study investigates the hydrochemistry and stable isotope (δ18O and δD) variations of the groundwater in the Beiluhe Basin, which is located in the QTP of China, during a freezing–thawing period of the active layer. Results show the chemical types of the groundwater are mainly HCO3 • Cl–Na • Ca • Mg, which are converted to Cl • HCO3–Na • Ca • Mg during the freezing period of the active layer. At different stages of the freezing–thawing period of the active layer, evaporation, concentration, and rock weathering control the chemical composition of the groundwater in this region. The main ion ratio coefficients of groundwater and the saturation indices of related minerals in groundwater indicate that halite, dolomite, and calcite control the relevant chemical components of water in the study area. The stable isotopic results show that δ18O and δD fluctuations in the lake water and the groundwater are mainly affected by groundwater runoff, temperature, and evaporation. This study provides the scientific basis for groundwater evolution and utilization in high-altitude areas.

Stalagmite evidence for Early Holocene multidecadal hydroclimate variability in Ethiopia

AbstractA multiproxy oxygen and carbon isotope (δ13C and δ18O), growth rate, and trace element stalagmite paleoenvironmental record is presented for the Early Holocene from Ethiopia. The annually laminated stalagmite grew from 10.6 to 10.4 ka and from 9.7 to 9.0 ka with a short hiatus at ~9.25 ka. Statistically significant and coherent spectral frequencies in δ13C and δ18O are observed at 15–25 and 19–23 years, respectively. The observed ~1‰ amplitude variability in stalagmite δ18O is likely forced by nonequilibrium deposition, due to kinetic effects during the progressive degassing of CO2 from the water film during stalagmite formation. These frequencies are similar to the periodicity reported for other Holocene stalagmite records from Ethiopia, suggesting that multidecadal variability in stalagmite δ18O is typical. Several processes can lead to this multidecadal variability and operate in different directions. A hydroclimate forcing is likely the primary control on the extent of the partial evaporation of soil and shallow epikarst water and associated isotopic fractionation. The resulting oxygen isotope composition of percolation water is subsequently modulated by karst hydrology. Further isotopic fractionation is possible in-cave during nonequilibrium stalagmite deposition. Combined with possible recharge biases in drip-water δ18O, these processes can generate multidecadal δ18O variability.

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.

Temporal variation of water-use efficiency and water sources of Phragmites australis in the coastal wetland: Based on stable isotopic composition (δ13C and δ18O)

Water-use characteristics are important aspects of plant adaptation to environmental factors. While water-use efficiency (WUE) and water sources used by plants in arid habitats are commonly studied, water-use strategies of plants in coastal wetlands are poorly studied. Although ample water is available in coastal wetlands, water conditions vary among seasons, suggesting that the plant in coastal wetlands may need to adapt to seasonal changes in water availability.To uncover temporal variation in plant water-use strategies in the coastal wetland, we investigated WUE and water sources used by Phragmites australis in the tidal marsh of the Yellow River Delta employing stable isotopic composition indicators. Temporal variation of δ13C and δ18O in leaf organic matter were measured in two years (2018 and 2019) along with plant traits and environmental factors, to determine the factors affecting δ13C and δ18O in leaf organic matter. Water sources (rainfall, soil water in different depths, tidal water, and groundwater) were sampled for δ18O composition in the early growth season (2019 May, June, and July) to determine water sources using the Bayesian mixing model MixSIAR.We found that P. australis had high WUE in spring and autumn, but WUE declined during summer. Changes in WUE were explained by variation in plant height, leaf nutrient concentrations (C: N, total N, total S), leaf traits (leaf length and leaf number), soil electronic conductivity (EC), soil pH, soil nutrients (NO3-N at 0–30 cm soil depth, total S at 0–30 cm soil depth, available P at the soil depths of 0–10 cm and 40–50 cm, and total C at the 30–40 cm soil depth). Spring drought may enhance WUE through increasing soil salinity (i.e., EC) and lowering pH. Water uptake in the early growth season was composed mainly of rainfall, soil water, and groundwater. However, tidal water (i.e., ocean water) was not a main water source. To sum up, leaf WUE varied with seasons in the coastal wetland, which may be related to plant traits and soil factors, particularly salinity and pH. The plant in the tidal marsh may not take up ocean water, even though it is really abundant. Precipitation is still an important water source promoting growth of these plants in the coastal wetland, therefore, to conserve them, supplementing fresh water in the drought season is suggested.

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

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