δ18O Trends Research Articles

The Paleoecological Environment during the Ediacaran–Cambrian Transition in Central Guizhou Province, China: Evidence from Zn Isotopes

During the Ediacaran–Cambrian transition, a series of stratal continuous and well-preserved siliceous rock and phosphorite assemblages developed in Qingzhen, Guizhou Province, China, facilitating research on the biological evolution, marine chemistry, and palaeoecological environment of this period. Therefore, we investigated the paleontology, trace and rare earth elements, total organic carbon, total sulfur content, and Zn isotopes of the phosphorus-bearing rock series in the Taozichong Formation of the Cambrian period in Qingzhen. Geochemical analysis reveals that the sedimentary rocks in this area were formed in the oxygen-rich seawater environment and were not affected by high-temperature hydrothermal activity. The upwelling ocean current provided abundant rare earth elements and other nutrient elements, as well as conditions for the prosperity of biota in Qingzhen. In addition, the δ66Zn value (−0.21‰–0.41‰ range and 0.17‰ mean) in the Qingzhen phosphorous rock series was much lower than that in seawater, indicating a strong level of biological productivity. The variation trend of δ13C, δ18O, and δ66Zn exhibited four stages and three obvious drift events. The results suggest that climate change during this period led to the intermittent flourishing and extinction of organisms, which triggered the negative drift of δ13C and δ18O in the ocean, resulting in a coordinated response of δ66Zn. The unique ecological environment of the Taozichong Formation in Qingzhen also provides favorable conditions for the population continuation of Ediacaran-type benthic soft-bodied metazoon dominated by discoid fossils, Shaanxilithes, worm fossils, and sponge fossils in the Cambrian strata, as well as participation in the global biological explosion events. The study area provides new insights for rebuilding the global Ediacaran–Cambrian ecosystem during the transition period.

Does zircon geochemistry record global sediment subduction?

Abstract Global compilations of zircon geochemistry have been used as evidence for changes in plate tectonic styles and surface environments. In particular, zircon δ18O has been used as a proxy for global sediment subduction and incorporation into igneous melts. However, research employing such compilations commonly ignores geologic and geographic context. We analyze a newly georeferenced zircon δ18O database from 1000 Ma to present. The compilation shows positive and negative isotopic excursions, both of which have been interpreted in the context of global phenomena. Sensitivity testing demonstrates that these excursions are the product of regional tectonic environments. Specifically, low Tonian–Cryogenian δ18O, previously interpreted as a climate signal of snowball Earth glacial meltwater, is isolated to ultra-high-pressure rocks of the Dabie Sulu orogen in central Asia, ophiolites in Egypt, and juvenile arc development in Madagascar, predating Cryogenian glaciation. Positive anomalies in the Ediacaran–Cambrian and Devonian, previously interpreted to record snowball Earth erosion and the rise of land plants, are the result of upper-plate, supracrustal sources of Gondwana. A Neogene anomaly is the result of volcanic rocks associated with Yellowstone and Iceland. Sampling bias in zircon geochemical compilations is analogous to that of Lagerstätten (sites of exquisite fossil preservation) and the “monograph effect” (large contributions from individual researchers) in paleontology, which both result in artifacts of over-representation. Long-term zircon δ18O trends broadly track the generation of collisional granites and continental arcs through time, recording the melting of sediments and sedimentary rocks in individual orogens, and not variations in sediment subduction globally.

An integrated study of constraining the initial 230Th of a stalagmite and its implications

High-precision U–Th age is the key to promoting speleothem as an important archive for paleoclimate study. Notably, obtaining reliable U–Th age primarily relies on the accurate initial 230Th correction. However, correcting the U–Th age of stalagmite with the average 232Th/238U value in the crust is sometimes not enough to eliminate the effect of the initial 230Th, using other dating methods (e.g., 14C) and referring to the characteristics of speleothem δ18O values are useful to guide its age correction and establish the chronology. This paper presents a comprehensive study of the chronology and isotopic record of a stalagmite XBL-5 from Xiaobailong Cave, Yunnan, China, using a combination of stalagmite U–Th dating, 14C dating, and δ18O wiggle match dating. The low uranium content of this stalagmite and the apparent inversion of U–Th dating prevent us from establishing the proxy profiles with reliable chronology. We determined the actual age range based on 14C ages assuming various dead carbon proportion (DCP) and uncorrected U–Th ages, along with the characteristic of stalagmite δ18O variations, and then established a credible chronology of this stalagmite through wiggle match of δ18O records. Taking the tuned ages as a reference, the initial 230Th/232Th values were reciprocally calculated. The initial 230Th/232Th atomic ratio of the studied samples was back-calculated to be 75.7 ± 11.4 × 10−6, an order of magnitude higher than the commonly used correction value. The reconstructed XBL-5 δ18O record spans the Younger Dryas (YD) period, indicating that the precipitation δ18O generally increased within the YD chronozone with significant centennial scale oscillations in regions affected by the Indian summer monsoon, in contrast to the speleothem records from East Asian summer monsoon domains which show no or even weakly decreased long-term trend in precipitation δ18O. This study highlights the importance of initial thorium correction in stalagmite chronology study and provides a paradigm for sequence reconstruction of similar samples.

The world-class carbonate-hosted Fankou Zn-Pb deposit in China. Part II. Alterations: C-O isotopic halos footprint the nature and transfer pathway of ore-fluids, and quantitative water-rock interaction process

The giant Fankou Zn-Pb deposit (more than 10 Mt Zn + Pb metals, grading at 15 %), situated in northern margin of the Quren Basin, is one of the largest Zn-Pb deposits in China. The stratiform orebodies are jointly bounded by both the strata (in the specific layers of Middle to Upper Devonian Donggangling and Tianziling carbonate sequences) and the fault system (a three-order thrusting fault system). Ore textures of carbonate dissolution, replacement-infilling and cementation are widespread in Fankou, analogous to a MVT (Mississippi Valley-type) deposit. Owing to the poorly developed fluid inclusions, the nature of the Fankou ore fluids remains indeterminate. To address this problem, we herein utilize micro-petrography, carbon-oxygen (C-O) isotopes and quantitative isotopic exchange modelling to trace the ore-fluids and water-rock interaction process. A total of ninety-three (93) carbonate samples, covering the majorities of stratigraphic horizons, planar locations, and various alteration intensities, were systematically collected. Micro-petrography reveals the primitively sedimentary bioclastic limestone suffer from heterogeneous dolomitization and carbonatization followed by massive Zn-Pb precipitation. These carbonate samples display a widely-ranged, δ18O- and δ13C-depleted isotope signature (δ18O = 12.57 to 23.82 ‰, δ13C = −7.79 to −0.46 ‰), relative to normal marine carbonates (δ18O = 25 ‰, δ13C = 0 ‰). Further modelling of isotopic exchange indicates a low-temperature (100 to 250 °C), δ18O-depleted (−3 to 7 ‰) and δ13C-depleted (−10 to −7‰) ore fluid equilibrated with the host rock by a relatively low water-rock ratio (W/R = 1 to 10). In terms of space, we identified a relatively decreasing trend of δ18O and δ13C isotopic compositions of carbonates, from southeast to northwest, and from bottom to up. Based upon the previous structural analysis, we interpret that the δ18O and δ13C isotopic distribution patterns are caused by a northwestwards transfer pathway of ore-fluids along the thrusting fault system, and the metalliferous, δ18O- and δ13C-depleted fluids are sourced from the interior of the Quren Basin. Collectively, we clarify the deep southeastern part of the Fankou Zn-Pb deposit and the interiors of the Quren Basin is the favorable exploration target in future.

High-resolution record of Paleoclimate during the late Quarternary, recovered from Con Moong cave - North Vietnam

To clarify the paleoclimatic events in Northern Vietnam during the Late Quaternary, 132 Con Moong - Thanh Hoa cave sediment samples were collected to analyze magnetic susceptibility (χ), δ18O, and δ13C. Based on the tendency in the value of χ Con Moong's sediment, 14 climate zones that change continuously from warm to cold were identified. These climate zones also correspond to the changing trend of δ18O and δ13C. The period of 14 magnetic zones is referenced from the extrapolation of four 14C data. These alternating cold-warm χ sequences correspond to the Composite Reference Section (CRS) developed in Southern Europe from a χ zonation of multiple caves. The paleoclimate characteristics, including the Heinrich H2 and H1 climate events, the Bølling/Allerød, and Younger Dryas warm-cold events, were elucidated for Northern Viet Nam from ~28.000 to ~10.000 yr BP.

Coupled biostratigraphy and chemostratigraphy at Lanna, Sweden: A key section for the Floian–lower Darriwilian interval (Lower–Middle Ordovician)

The Lanna area in the province of Närke, south-central Sweden, hosts a succession of cool-water carbonate strata that are largely devoid of tectonic and diagenetic alteration, and span the Floian through lower Darriwilian Global stages (Ottenbyan–Kundan Regional stages). In this study we assess the integrated biostratigraphic and chemostratigraphic framework for the Ordovician sequence at Lanna, utilizing trilobites and conodonts coupled with bulk-rock carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb). The local succession is thin but relatively complete; biostratigraphic details are similar to coeval successions in Baltoscandia. The new high-resolution δ13C and δ18O data are broadly consistent with other chemostratigraphic records at the regional scale, but notable differences in δ13C are obvious compared to global records. δ13C trends diverge markedly in the Dapingian and intra- and inter-regional differences become increasingly pronounced through younger strata. This may largely be explained by an overall low eustatic sea level, which led to more widespread oceanic restriction of epeiric seas. As a result, δ13C records became more influenced by local variables, muddling the global δ13C signal. This, in turn, suggests that sea level played a larger role than long-term changes within the global carbon cycle for regulating local–regional δ13C gradients in the Middle Ordovician. Regardless of the underlying cause(s), the documented variability poses significant challenges for using δ13C as a tool for detailed stratigraphic correlations. The limited expression of many named ‘carbon isotope excursions’ (CIEs) in the studied interval indicate that they are artefacts of local overprinting rather than reflective of changes to the global carbon cycle. Thus, these CIEs are of questionable correlative importance and validity. Although the scatter in numeric values is relatively large, long-term δ18O trends are overall consistent at the global scale, tentatively supporting the notion that climate cooled during the studied time interval.

Impacts of climate and tree morphology on tree-ring stable isotopes in central Mongolia.

Recent climate extremes in Mongolia have ignited a renewed interest in understanding past climate variability over centennial and longer time scales across north-central Asia. Tree-ring width records have been extensively studied in Mongolia as proxies for climate reconstruction, however, the climate and environmental signals of tree-ring stable isotopes from this region need to be further explored. Here, we evaluated a 182-year record of tree-ring δ13C and δ18O from Siberian Pine (Pinus sibirica Du Tour) from a xeric site in central Mongolia (Khorgo Lava) to elucidate the environmental factors modulating these parameters. First, we analyzed the climate sensitivity of tree-ring δ13C and δ18O at Khorgo Lava for comparison with ring-width records, which have been instrumental in reconstructing hydroclimate in central Mongolia over two millennia. We also compared stable isotope records of trees with partial cambial dieback ('strip-bark morphology'), a feature of long-lived conifers growing on resource-limited sites, and trees with a full cambium ('whole-bark morphology'), to assess the inferred leaf-level physiological behavior of these trees. We found that interannual variability in tree-ring δ13C and δ18O reflected summer hydroclimatic variability, and captured recent, extreme drought conditions, thereby complementing ring-width records. The tree-ring δ18O records also had a spring temperature signal and thus expanded the window of climate information recorded by these trees. Over longer time scales, strip-bark trees had an increasing trend in ring-widths, δ13C (and intrinsic water-use efficiency, iWUE) and δ18O, relative to whole-bark trees. Our results suggest that increases in iWUE at this site might be related to a combination of leaf-level physiological responses to increasing atmospheric CO2, recent drought, and stem morphological changes. Our study underscores the potential of stable isotopes for broadening our understanding of past climate in north-central Asia. However, further studies are needed to understand how stem morphological changes might impact stable isotopic trends.

Equilibrium Fractionation of Oxygen Isotopes in the U3O8-Atmospheric Oxygen System.

As a major component in the nuclear fuel cycle, octoxide uranium is subjected to intensive nuclear forensics research. Scientific efforts have been mainly dedicated to determine signatures, allowing for clear and distinct attribution. The oxygen isotopic composition of octoxide uranium, acquired during the fabrication process of the nuclear fuel, might serve as a signature. Hence, understanding the factors governing the final oxygen isotopic composition and the chemical systems in which U3O8 was produced may develop a new fingerprint concerning the history of the material and/or the process to which it was subjected. This research determines the fractionation of oxygen isotopes at different temperatures relevant to the nuclear fuel cycle in the system of U3O8 and atmospheric O2. We avoid the retrograde isotope effect at the cooling stage at the end of the fabrication process of U3O8. The system attains the isotope equilibrium at temperatures higher than 300 °C. The average δ18O values of U3O8 in equilibrium with atmospheric oxygen have been found to span over a wide range, from -9.90‰ at 300 °C up to 18.40‰ at 800 °C. The temperature dependency of the equilibrium fractionation (1000 ln αU3O8-atm.O2 ) exhibits two distinct regions, around -33‰ between 300 °C and -500 °C and -5‰ between 700 °C and -800 °C. The sharp change coincides with the transition from a pseudo-hexagonal structure to a hexagonal structure. A depletion trend in δ18O is associated with the orthorhombic structure and may result from the uranium mass effect, which might also play a role in the depletion of 5‰ versus atmospheric oxygen at high temperatures.

Orbital-scale climate changes detected in Lower and Middle Ordovician cyclic limestones using oxygen isotopes of conodont apatite

Recent studies using δ18O trends from conodont apatite suggest that the Ordovician greenhouse-to-icehouse climate transition was characterized by repeated Myr to multi-Myr-scale sea surface temperature (SST) oscillations throughout the Ordovician making it difficult to pinpoint the temporal onset of cooling and glaciation. This study utilizes high-resolution δ18Oapatite trends from Lower and Middle Ordovician meter-scale carbonate cycles to evaluate orbital-scale (~104–105 yr) climate changes superimposed on the Myr-scale oscillations. Detailed facies analysis and δ18Oapatite variations from 14 cycles deposited along the western Laurentian margin (Nevada and Utah) show trends indicating glacio- and/or thermo-eustatic sea-level changes along with associated wetter versus drier climate shifts controlled meter-scale cycle formation and δ18O signals. Using the range of magnitudes of intracycle isotopic shifts (0.45–1.4‰) and reasonable isotopic proportionalities, ~25–45 m glacio-eustatic changes with ~2–6 °C SST variations are estimated. However, observed cyclostratigraphic facies changes do not support the higher end of estimated eustatic changes suggesting that the apparent overestimation is due to local δ18O shifts related to variations in paleotropical surface ocean evaporation and/or dilution by freshwater runoff during orbital-scale wetter versus drier climatic changes. These new δ18Oapatite results indicate that orbital-scale climatic oscillations, including glaciation, were superimposed upon Myr-scale and ~107 yr-scale Ordovician SST variations as early as the late Early Ordovician and that the greenhouse-to-icehouse climatic transition was more complex and protracted than previously thought.

Early dolomitization and subsequent recrystallization of Middle Ordovician carbonates in the western flank of the Ordos Basin, Northern China: Implications for hydrocarbon exploration

The pervasive dolostone from the Middle Ordovician Zhuozishan Formation in the Ordos Basin (Northern China) are considered as potential hydrocarbon reservoirs. Three types of matrix dolomites (Md) are recognized: 1) very fine to fine crystalline, nonplanar-a to planar-e dolomite (Md1); 2) fine to medium crystalline, nonplanar-a to planar-e dolomite (Md2); and 3) medium crystalline, planar-s to nonplanar-a dolomite (Md3). The initial dolomitization of Md1 and Md2 dolomites is inferred to have taken place in the presence of seepage reflux of slightly evaporated seawater in the near-surface to shallow burial settings, and insignificant recrystallization occurred in the process of being buried again, according to petrography, trace elements, and oxygen-carbon-strontium isotopic geochemistry. Coarser crystals with cloudy cores and clear rims consist of the Md3 dolomites, and the texture shows irregular contacts between planar-s and nonplanar-a crystal. Geochemical analysis reveals that the Md3 dolomites have a negative shift in δ18O values with increasing Mn contents, and a negative δ18O trend with depth in the same well, while the δ13C does not tend to correlate with the oxygen isotopes. Additionally, the 87Sr/86Sr ratios of Md3 dolomites are relatively higher due to contribution of radiogenic 87Sr. Petrological attribute and geochemical evidence suggest that the massive Md3 dolomites can be due to significant recrystallization of earlier formed dolomites during burial period. The unevenly distributed pores in Md3 dolomites suggest the heterogeneous structure of the precursor dolomites, which may be the Md2 dolomites. The residual particles of Md2 dolomites indicate the precursor limestones were deposited in a relatively high-energy part of the shoal. Hence, the wide distributed Md3 dolomites show salient facies-controlled characteristics, which suggests strategies for hydrocarbon exploration of potential heterogeneous dolomite reservoir.

Nd isotopic evidence for enhanced mafic weathering leading to Ordovician cooling

Abstract It remains unclear whether waning of the volcanic degassing CO2 source or enhancement of the mafic (Ca, Mg-silicate) weathering CO2 sink, or both, caused global cooling leading to the Ordovician greenhouse–icehouse transition. We present a uniquely age-constrained and integrated Middle–Late Ordovician (470–450 Ma) continental weathering isotopic proxy data set (87Sr/86Sr and εNd(t)) from carbonate rocks of the Antelope Range of central Nevada, USA, paired with published paleotemperature proxy measurements (δ18O) of conodont apatite from the same locality. This suite of proxy records signals an increase in mafic weathering of the Taconic mountains (eastern United States) at ca. 463 Ma, which forced a period of global cooling. We adapt a 87Sr/86Sr and pCO2 mass balance approach to model CO2 drawdown during the Ordovician, and show that a combined decrease in volcanic degassing and increase in mafic weathering approximately halves pCO2 in agreement with δ18O trends and paleotemperature reconstructions.

Groundwater dynamic influenced by intense anthropogenic activities in a dried-up river oasis of Central Asia

AbstractIntense anthropogenic activities in arid areas have great impacts on groundwater process by causing river dried-up and phreatic decline. Groundwater recharge and discharge have become hot spot in the dried-up river oases of arid regions, but are not well known, challenging water and ecological security. This study applied a stable isotope and end-member mixing analysis method to quantify shallow groundwater sources and interpret groundwater processes using data from 186 water samples in the Wei-Ku Oasis of central Asia. Results showed that shallow groundwater (well depth < 20 m) was mainly supplied by surface water and lateral groundwater flow from upstream, accounting for 88 and 12%, respectively, implying surface water was the dominant source. Stable isotopes and TDS showed obviously spatiotemporal dynamic. Shallow groundwater TDS increased from northwest to southeast, while the spatial variation trend of groundwater δ18O was not obvious. Surface water and groundwater in non-flood season had higher values of stable isotopes and TDS than those in flood season. Anthropogenic activities greatly affect groundwater dynamics, where land-cover change and groundwater overexploitation are the main driving factors. The findings would be useful for further understanding groundwater sources and cycling, and help restore groundwater level and desert ecosystem in the arid region.

Significance of sedimentary provenance reconstruction based on borehole records of the North China Plain for the evolution of the Yellow River

The Yellow River (YR) flows from Northwest China to North China, crossing three topographic steps: the Tibetan Plateau, the Chinese Loess Plateau (CLP), and the North China Plain (NCP). Determining the river's geomorphological evolution is significant for understanding the formation of macro-geomorphic patterns in China. However, the formation age of YR channelization into the Bohai Sea is highly controversial and uncertain, with estimates ranging from the Late Miocene to the Late Pleistocene. This study conducted grain size analysis, whole-rock geochemistry, and detrital zircon U-Pb dating on ~5.2 Ma sediments from borehole G4 drilled in the Bohai Bay. The geochemical results from borehole G4 indicate that chemical weathering was weak during 5.2–1.6 Ma, but showed diverse characteristics since ~1.6 Ma. Moreover, concordant detrital zircons from borehole G4 (~1.6 Ma) yield a major age population of 350–550 Ma, which, however, is scarcely produced by detrital zircons from the nearby Yanshan and Taihang Mountains. Geochemical signals and detrital zircon U-Pb geochronology suggest that the YR reached the NCP after cutting through the Sanmen Gorge and entered the Bohai Bay since ~1.6 Ma. The massive presence of sediments with an age population of 650–1350 Ma in the top of borehole G4 demonstrates that the present topography of the middle and upper reaches of the YR initially formed at ~0.6 Ma. Subsequently, changes in provenance since ~1.6 Ma influenced the records of paleoclimatic signals indicated by elemental geochemistry in the Bohai Bay. Because of detrital inputs from the YR, the (La/Sm)N and (La/Yb)N ratios of borehole G4 abruptly increased after ~1.6 Ma. Other geochemical indicators exhibited no noticeable variation trend, which differs from the decreasing trend of deep-sea δ18O records, the Chemical Index of Alteration (CIA), and the 87Sr/86Sr ratio of the loess from the Chinese Loess Plateau after the YR entered the NCP at ~1.6 Ma. This paper proposes that the YR gradually triggered fluvial erosion up to its upper reaches after entering the NCP, which is primarily in response to climate change.

Tree-ring stable isotopes indicate mass wasting processes at Radicofani in the upper Orcia Valley (Tuscany, Italy)

Tree-ring carbon (C) and oxygen (O) stable isotope (SI) chronologies spanning the period 1983–2012 were analysed at three Pinus nigra Arn. sites located in the upper Orcia Valley (Tuscany, Italy) in a badlands landscape. The goal of the study was to determine the extent to which tree-ring stable isotopes (SI) can serve as a proxy for mass wasting processes. To this end, we applied an established dual-isotope model to detect physiological changes between trees growing at three sites, one along the upper border of a well-studied shallow landslide, one on the landslide, and one in a stable area, as control. We further analysed whether trees at the three sites showed different δ18O responses to climate and to precipitation δ18O. Tree-ring δ13C and δ18O variations and trends revealed impairments of the photosynthetic process at the landslide site. We found that trees growing on the landslide show signs of reduced photosynthetic capacity since the onset of the landslide in 1993, whereas since 2000, while producing compression wood during periods of landslide activity, the trees show trends of higher average maximum net photosynthesis. The correlation analysis performed between the SI chronologies and the climatic variables revealed that the climatic signals at the site located on the landslide are masked by growth stress induced by the mass wasting processes. The most distinct differences in climate responses between sites were found in tree-ring δ13C in response to mean temperature and to mean temperature range, and in tree-ring δ18O in response to precipitation δ18O. Our research confirms that it is possible to reconstruct mass-wasting processes on forested slopes and to date geomorphological events by considering the trees' physiological conditions as recorded by stable C and O isotopes in tree rings, and by comparing affected with unaffected sites.

Pingo drilling reveals sodium–chloride‐dominated massive ice in Grøndalen, Spitsbergen

AbstractDrilling of a 21.8‐m‐deep borehole on top of the 10.5‐m‐high Nori pingo that stands at 32 m asl in Grøndalen Valley (Spitsbergen) revealed a 16.1‐m‐thick massive ice enclosed by frozen sediments. The hydrochemical compositions of both the massive ice and the sediment extract show a prevalence of Na+ and Cl− ions throughout the core. The upper part of the massive ice (stage A) has low mineralization and shows an isotopically closed‐system trend in δ18O and δD isotopes decreasing down‐core. Stage B exhibits high mineralization and an isotopically semi‐open system. The crystallographic structure of Nori pingo’s massive ice provides evidence of several large groundwater intrusions that support the defined formation stages. Analysis of local aquifers leads to suggest that the pingo was hydraulically sourced through a local fault zone by low mineralized sodium–bicarbonate groundwater of a Paleogene strata aquifer. This groundwater was enriched by sodium and chloride ions while filtering through marine valley sediments with residual salinity. The comparison between the sodium–chloride‐dominated massive ice of the Nori pingo and the sodium–bicarbonate‐dominated ice of the adjacent Fili pingo that stands higher up the valley may serve as an indicator for groundwater source patterns of other Nordenskiöld Land pingos.

A data–model approach to interpreting speleothem oxygen isotope records from monsoon regions

Abstract. Reconstruction of past changes in monsoon climate from speleothem oxygen isotope (δ18O) records is complex because δ18O signals can be influenced by multiple factors including changes in precipitation, precipitation recycling over land, temperature at the moisture source, and changes in the moisture source region and transport pathway. Here, we analyse >150 speleothem records of the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database to produce composite regional trends in δ18O in monsoon regions; compositing minimises the influence of site-specific karst and cave processes that can influence individual site records. We compare speleothem δ18O observations with isotope-enabled climate model simulations to investigate the specific climatic factors causing these regional trends. We focus on differences in δ18O signals between the mid-Holocene, the peak of the Last Interglacial (Marine Isotope Stage 5e) and the Last Glacial Maximum as well as on δ18O evolution through the Holocene. Differences in speleothem δ18O between the mid-Holocene and the Last Interglacial in the East Asian and Indian monsoons are small, despite the larger summer insolation values during the Last Interglacial. Last Glacial Maximum δ18O values are significantly less negative than interglacial values. Comparison with simulated glacial–interglacial δ18O shows that changes are principally driven by global shifts in temperature and regional precipitation. Holocene speleothem δ18O records show distinct and coherent regional trends. Trends are similar to summer insolation in India, China and southwestern South America, but they are different in the Indonesian–Australian region. Redundancy analysis shows that 37 % of Holocene variability can be accounted for by latitude and longitude, supporting the differentiation of records into individual monsoon regions. Regression analysis of simulated precipitation δ18O and climate variables show significant relationships between global Holocene monsoon δ18O trends and changes in precipitation, atmospheric circulation and (to a lesser extent) source area temperature, whereas precipitation recycling is non-significant. However, there are differences in regional-scale mechanisms: there are clear relationships between changes in precipitation and δ18O for India, southwestern South America and the Indonesian–Australian regions but not for the East Asian monsoon. Changes in atmospheric circulation contribute to δ18O trends in the East Asian, Indian and Indonesian–Australian monsoons, and a weak source area temperature effect is observed over southern and central America and Asia. Precipitation recycling is influential in southwestern South America and southern Africa. Overall, our analyses show that it is possible to differentiate the impacts of specific climatic mechanisms influencing precipitation δ18O and use this analysis to interpret changes in speleothem δ18O.

The speleothem oxygen record as a proxy for thermal or moisture changes: a case study of multiproxy records from MIS 5–MIS 6 speleothems from the Demänová Cave system

Abstract. Speleothems are an important source of palaeoclimatic information about the terrestrial environment. The basic advantages of speleothems are their high preservation potential, the possibility of precise dating using the uranium-series (U-series) method, and many different proxies, such as stable isotopes, trace elements, and microfabrics, which can be interpreted in terms of palaeoclimatic conditions. Currently, central Europe is located in a transitional climate zone under the influence of both oceanic and continental climates. However, in the past, the region could have been under a stronger continental climate influence during cold glacial episodes or a stronger oceanic climate influence during wetter interglacial episodes. Long-term speleothem records can add new beneficial data about past climate changes in the region. The multiproxy record of the JS9 stalagmite, collected in the Demänová Cave system (Slovakia), represents a ca. 60 kyr period (143–83 ka). A multiproxy interpretation of the JS9 record shows that long-term δ18O trends can be interpreted as global/regional temperature changes, whereas short-term δ18O signals reflect changes in humidity. In contrast to the records from the Alps and the northern Tatra Mountains, the δ18O record of speleothem JS9 shows instantaneous decreasing episodes during Termination II. This indicates that the Carpathian Belt was an important climatic barrier at that time.

Insight into Canary Island pine physiology provided by stable isotope patterns of water and plant tissues along an altitudinal gradient.

The Canary Islands, an archipelago east of Morocco's Atlantic coast, present steep altitudinal gradients covering various climatic zones from hot deserts to subalpine Mediterranean, passing through fog-influenced cloud forests. Unlike the majority of the Canarian flora, Pinus canariensis C. Sm. ex DC. in Buch grow along most of these gradients, allowing the study of plant functioning in contrasting ecosystems. Here we assess the water sources (precipitation, fog) of P. canariensis and its physiological behavior in its different natural environments. We analyzed carbon and oxygen isotope ratios of water and organics from atmosphere, soil and different plant organs and tissues (including 10-year annual time series of tree-ring cellulose) of six sites from 480 to 1990m above sea level on the Canary Island La Palma. We found a decreasing δ18O trend in source water that was overridden by an increasing δ18O trend in needle water, leaf assimilates and tree-ring cellulose with increasing altitude, suggesting site-specific tree physiological responses to relative humidity. Fog-influenced and fog-free sites showed similar δ13C values, suggesting photosynthetic activity to be limited by stomatal closure and irradiance at certain periods. In addition, we observed an 18O-depletion (fog-free and timberline sites) and 13C-depletion (fog-influenced and fog-free sites) in latewood compared with earlywood caused by seasonal differences in: (i) water uptake (i.e., deeper ground water during summer drought, fog water frequency and interception) and (ii) meteorological conditions (stem radial growth and latewood δ18O correlated with winter precipitation). In addition, we found evidence for foliar water uptake and strong isotopic gradients along the pine needle axis in water and assimilates. These gradients are likely the reason for an unexpected underestimation of pine needle water δ18O when applying standard leaf water δ18O models. Our results indicate that soil water availability and air humidity conditions are the main drivers of the physiological behavior of pine along the Canary Island's altitudinal gradients.

Spatial distribution and interannual trends of δ18O, δ2H, and deuterium excess in precipitation across North-Eastern Italy

NE Italy presents high-quantity, high-quality and easily exploitable groundwater resources that are seriously threatened by anthropogenic pressures. This study analyzes the oxygen and hydrogen isotopic composition of 2250 precipitation samples collected in 36 sites across the north-easternmost region of Italy, Friuli Venezia Giulia, between 1984 and 2015. This is an unprecedented dataset for North Italy with a high density of sampling sites and a decadal temporal extension. A series of both routine and original chemometric approaches were applied to investigate the temporal and spatial variability of the isotopic composition through relationships with geographical and weather variables. New statistical approaches were presented to model the seasonal and spatial patterns of isotopic composition as well as to summarize the large amount of isotopic data. Significant gradients of δ18O and δ2H were detected in the area due to the peculiar orography and climate of the region; the amplitude of the monthly patterns also presented similar gradients. The deuterium excess did not present a clear seasonality, but higher values were found in autumn. The deuterium excess-to-δ18O ratio exhibited typical patterns throughout the region when grouping the sites for altitude and continentality; in winter, large differences of δ18O were detected among groups, but deuterium excess remained almost unchanged. The inter-site correlations were moderately high across all the territory even for deseasonalized data. The local meteoric water line (using all the single samples δ2H=7.8·δ18O+8.9) was also estimated at annual and seasonal basis, evidencing the presence of spatial gradients according to the orographic and weather characteristics of the region. Statistically significant increasing interannual trends (0.23–0.87‰/y) were detected in 11 sites for deuterium excess; the presence of these trends was linked to local processes. Two multiple linear regression models were applied to reconstruct the isotopic composition of precipitation at a regional scale. The stepwise approach returned the best results with root mean square errors in the 0.5-1‰, 3.5–8.2‰ and 0.9–1.5‰ intervals for δ18O, δ2H, and deuterium excess, respectively. The deuterium excess was not modelled in winter, where no relationships were found with geographic variables.

60-year trends of δ18O in global precipitation reveal large scale hydroclimatic variations

Abstract Sixty years of monthly data series of δ18O in precipitation revealed distinctive decadal long-term patterns and break points for the isotope composition of global precipitation. Our results showed that at the multi-decadal scale, δ18O in precipitation displayed changes due to large-scale hydroclimate processes, and particularly, from atmospheric oscillations. Break point and segmented regression trend analyses for δ18O and air temperature revealed influences of the North Atlantic Oscillation (NAO) on the long-term isotope variability for most continental stations in Northern Hemisphere. Long-term trends of δ18O and precipitation amount for Southern Hemisphere stations were mostly indicative of changes in regional hydroclimate processes driven by the larger scale Atlantic and Pacific atmospheric oscillations. For oceanic island stations, the δ18O variations could be described by sinusoidal trends with a ~ 20–40 years-time cycle and δ18O amplitude of ~1–2‰, suggesting these stations have a higher sensitivity to multi-decadal atmospheric circulation. Our findings reveal that coordinated long-term monitoring of stable isotopes in precipitation, coupled with basic meteorological parameters such as air temperature and precipitation amount, are essential to better understand the impact of larger scale hydroclimate variation on regional and local climate variability, and to help interpret long-term hydroclimatic changes of the past, present and future.