Depleted δ18O Values Research Articles

Dolomitization history and fluid evolution of end-ediacaran multi-phase dolomites from the near-surface to deep burial depths in the tarim craton, northwestern China

The deeply buried (>8 km) end-Ediacaran Qigebrak Formation dolostones in the Tarim Basin, China, have significant petroleum potential, but have experienced multi-stage dolomitization that has resulted in the occurrence of diverse dolomite phases. The features and origins of the different dolomite types were merely investigated, which hinders an in-depth understanding of the complex dolomitization processes. This paper presents an integrated petrological, geochemical (C–O–Sr isotope and in situ trace element), fluid inclusion microthermometric, and dolomite U–Pb dating study of outcrop and core samples, to investigate the dolomitization mechanisms, and the nature and evolution of the dolomitizing fluids. The results indicate the occurrence of early dolomitization triggered by seepage–reflux of mesosaline seawater, which retained the primary marine geochemical properties of most of the matrix dolomite. Subsequently, during early diagenesis, fibrous dolomite cement was precipitated from marine porewaters, which have similar geochemical characteristics to the matrix dolomites. Following this, during intermediate-depth burial diagenesis in the Ordovician, fine-to medium-crystalline dolomite cements formed at temperatures of 80–117 °C in association with dissolution–reprecipitation of the matrix and early fibrous cements or diagenetic interactions with clay minerals of the lower Cambrian Yurtus Formation shales. During deep burial diagenesis and dolomitization in the Carboniferous, coarse-crystalline dolomite cements formed at temperatures of 104–138 °C from saline brines enriched in middle rare earth elements. Saddle dolomites, formed by hydrothermal dolomitization (162–184 °C) that was closely related to Permian–Triassic volcanic activity, have positive Eu anomalies and depleted δ18O values, that resulted from the mixing of basinal brines with 87Sr-enriched hydrothermal fluids. This study provides a detailed description of multiple dolomitization events in deep time that affected deeply buried dolostones.

Evaluating middle to late Holocene climate variability from δ18O of aquatic invertebrate remains in southwestern Greenland

Holocene paleotemperature records from ice-free margins of Greenland suggest spatiotemporal heterogeneity in the timing of key climate transitions following retreat of the Greenland Ice Sheet. However, the paucity of high-resolution climate records from this region limits our understanding of the timing, spatial variation, and drivers of Holocene climate variability. Here, we present a middle to late Holocene climate reconstruction based on δ18O of aquatic invertebrate (chironomid and cladoceran) remains and sediment geochemistry from lake sediments to elucidate millennial to submillennial-scale drivers of climate in southwestern Greenland. We establish that our site, Arrowhead Lake, is a through-flowing basin with an integrated signal of annual meteoric water with no evidence of significant evaporative enrichment. Core sediment geochemistry captures the transition from minerogenic-dominated to organic-rich sediments ∼8 cal ka BP. Chitin-based δ18O measurements of both chironomid head capsules and ephippia exhibit similar temporal variability through the record, broadly decreasing from 8.2 cal ka BP to present, concurrent with decreasing summer insolation and regional Neoglacial cooling. A simplified assumption that temperature is the sole driver of changes in isotopes of precipitation yields an estimated average temperature change from the middle to late Holocene at Arrowhead Lake of 2.1–3.0 °C, comparable to existing estimates of middle to late Holocene temperature decline in western Greenland. Depleted δ18O values after 3.3 cal ka BP correspond to the onset of Neoglacial cooling and independent evidence for a slowdown of the West Greenland Current, and suggests that both temperature and sea-ice variability were important controls on isotopic variability at this site. Highly depleted isotopic values at 8.2 cal ka BP followed by high isotopic variability in the early part of the middle Holocene suggests that our record may capture regional 8.2 ka cooling. This new isotope record provides insight into climate influences during the middle and late Holocene along the western Greenland margin that are important for understanding how ongoing melt of the Greenland Ice Sheet and amplified warming in the Arctic may impact the climate system.

Seismic sequence stratigraphy of Late Albian-Early Turonian successions in SW Iran: Implication for reservoir characterizations and potential for improved reservoir quality prediction

The Late Albian-Early Turonian succession on the Arabian Platform stands out as one of the world's richest hydrocarbon provinces. This study delves into the Seismic Sequence Stratigraphy (SSS) of these successions to unravel reservoir characterization and characterized potential hydrocarbon plays. Utilizing data from five exploration wells, including well logs, selected core data, biostratigraphical information, and chemostratigraphy analysis, alongside a comprehensive 3D seismic volume, enabled the identification of four 3rd-order sequences and multiple 4th-order sequences within the studied successions. Sequence-1 (DS-I), dating back to the Late Albian age, features mudstone and bioclast wackestone in an open marine environment. DS-I is a key sequence, notably witnessing the disappearance of siliciclastic input from west to east in the study area. DS-II (Early Cenomanian) and DS-III (Mid-Cenomanian) were deposited on carbonate platforms with grain-supported facies in shoal and lagoonal depositional environments.A distinct regional unconformity at the Turonian base, evident on seismic sections, aligns with depleted δ18O values on the Cenomanian-Turonian Boundary. The maximum flooding surfaces of DS-I and DS-II are characterized by continuous reflectors with negative amplitude. Furthermore, the lateral grading of reservoir facies and their intercalation with non-reservoir facies within 4th-order sequences suggest that reservoir facies heterogeneity is strongly influenced by depositional processes at this scale. Potential reservoirs, particularly in the Highstand System Tract (HST) of each 3rd sequence, are highlighted, with DC-III's HST standing out, hosting patch reef facies resulting from local tectonic activities and carbonate build-up movements.

Isotope Discrimination of Source Waters, Flowpaths, and Travel Times at an Acid-Generating, Lead–Zinc–Silver Mine, Silver Valley, Idaho, USA

Precipitation infiltrates into the lead–zinc–silver Bunker Hill Mine, oxidizes pyrite, and produces acidic waters that discharge from the mine portal. The metasedimentary geology and alteration from 100+ yr of mining provide a heterogeneous environment for source water infiltration and flow within the mine. A university–industry partnership was developed to trace the mine water sources, flowpaths, and travel times to identify potential areas for infiltration reduction. Snowpack, creek, and mine water samples were collected over a 1-year period for the analysis of δ2H, δ18O, and 3H, along with the in situ measurement of temperature, specific conductance, pH, dissolved oxygen, and flow. The isotope tracers were used to identify the source waters, unmix mine water as it moved deeper in the mine, and examine flowpaths in and near the acid-generating pyritic zone. The results indicate creek water infiltrating relatively quickly through the anthropogenically-modified pathways and causing the largest amount of acidic water in the upper levels of the mine. Slower, natural pathways associated with faults, fractures, and bedding planes produce mostly neutral waters with the source waters typically originating at higher elevations. Travel times ranged from <1 to 22 years with shorter pathways to the upper levels of the mine and increasing contributions deeper in the mine from pathways containing older, higher-elevation snowmelt. These slower and older inflows were identified by depleted δ18O values, smaller 3H concentrations, the dampening of the variability of the isotope signals, and pH increases. Reduction of infiltration zones near the upper workings of the mine likely will decrease the acidic waters in the upper levels of the mine, but the higher elevation infiltration zones will continue to contribute snowmelt-derived waters at all mine levels.

The Obir caves adjacent to the Periadriatic Fault in southern Austria: Uplifted hypogene caves formed by carbonic acid speleogenesis

Hypogene karst caves, formed independently of surface processes, have garnered increasing attention in recent decades, revealing a wider distribution than previously assumed. While most studies have relied on morphological observations, this research combines morphological and geochemical approaches to present new insights from the Obir caves in the Northern Karawank Mountains, Austria. These caverns, discovered during mining activities, lack natural entrances and developed along a prominent master horizon about 500 m above the current spring level. Notably, large shafts within the caves originate in narrow fissures and terminate blindly at the top, also suggesting a phreatic regime during their formation, with some areas being modified by vadose waters later on. A comprehensive study of drill cores indicates a stable isotope halo in the first few centimeters of the wall rock, characterized by significantly depleted δ13C and moderately depleted δ18O values compared to the pristine limestone, accompanied by bleaching of the host rock. This geochemical evidence of wall rock alteration is only present where the original phreatic solutional morphology is preserved and absent where vadose waters resulted in rugged cave wall surfaces.Considering the lack of evidence for the involvement of sulfuric acid or thermal waters, the formation of the Obir caves is attributed to carbonic acid speleogenesis. This interpretation is supported by the observed strong C isotope shift in the wall rock and the presence of CO2-bearing springs both north and south of the Periadriatic Fault. In contrast to previous studies of carbonic acid speleogenesis suggesting large-scale upwelling of aggressive waters, we propose an alternative mechanism: the injection of carbon dioxide gas into limestone situated in the phreatic zone. These injections likely occurred along subvertical tectonic fissures, triggered by the tectonic transpression of the Northern Karawank Mountains during the middle Miocene. Subsequent tectonic uplift and the incision of valleys led to a drop in the groundwater table, widespread collapse of larger cave rooms and localized vadose overprint.This study highlights the strength of combining mega- to the microscale cave morphological observations with systematic geochemical analyses of wall rock alteration in identifying caves formed by carbonic acid speleogenesis.

Agulhas leakage extension and its influences on South Atlantic surface water hydrography during the Pleistocene

The inter-ocean transfer of warm and salty water through the Agulhas leakage is regarded as a significant factor influencing the South Atlantic's surface and intermediate water hydrography. To evaluate the extension of Agulhas leakage for the past 1.2 Ma, planktonic foraminiferal assemblage from Ocean Drilling Program Hole 1088B, within the vicinity of AL, and Hole 1090B, located south of the significant Agulhas retroflection region, were generated. The Hole 1088B shows higher abundance of tropical and sub-tropical assemblage. The combination of Globigerinoides ruber, Trilobatus sacculifer, Globigerinoides conglobatus and Globorotalia menardii has previously been defined as the Agulhas leakage fauna [ALF]. The pulse-like amplification of the ALF record at major glacial-interglacial transitions indicates these were periods of strengthened Agulhas leakage with subsequent shedding of Agulhas rings and filaments as far as ∼41°S. From ∼450 ka, the frontal zone exhibits strong ALF variations with evident increase at glacial-interglacial transitions of Marine Isotopic Stages (MIS) 12, 10, 8 and 6, suggesting vigorous eddy mixing and a shift in the leakage zone towards the subantarctic waters. The migration of the Subtropical Convergence and Neogloboquadrina pachyderma (s) coiling ratios during these glacial-interglacial transitions supports the southward expansion of Agulhas leakage. The ALF record is supported by the δ18O record of Globigerina bulloides from hole 1088B, which identify a major anomaly at MIS 10, with depleted δ18O values towards the glacial termination. Simultaneously, the results from Hole 1090B show restricted presence of tropical-subtropical assemblage with their appearance reported only at glacial-interglacial transitions of the MIS 16, 12, 10 and 8. The curtailed ALF index and dominance of the subpolar species N. pachyderma (s) indicate the termination of Agulhas leakage before reaching the Subantarctic zone. The ODP sites chosen for the study reveal the intensification of Agulhas leakage and southward shift of the major leakage zone into the subantarctic zone during the Pleistocene glacial-interglacial transitions.

Early dolomitization and subsequent hydrothermal modification of the middle Permian Qixia Formation carbonate in the northwest Sichuan Basin

High-quality dolomite reservoir are widely developed in Permian Qixia Formation in northwest Sichuan Basin. Understanding the genesis of dolomite is significant to predicting high-quality reservoir distribution. Based on the petrographical and geochemical characteristics, three types of dolomites and one type of dolomite cement were identified in the Qixia Formation in northwest Sichuan Basin: micritic to fine-crystalline dolomites (D1), medium-crystalline dolomite (D2), medium-to coarse-crystalline dolomite (D3) and saddle dolomite cement.Type D1 dolomites are characterized by micritic to fine-sized crystals, lamellar structure, generally dull to no luminescence, and relatively high strontium (Sr) concentrations. These characteristics suggest that type D1 dolomites were initially formed during the penecontemporaneous phase. The wide range of depleted δ18O values, slightly higher 87Sr/86Sr ratios and slightly positive europium (Eu) anomalies indicate that type-D1 dolomites were subsequently subjected to hydrothermal modification. Type D2 dolomite is characterized by unimodal middle-sized crystals and crystal texture dominated by nonplanar crystals. It usually occurs as thin or thick layers in the study area. The δ18O and δ13C values of type D2 dolomite generally fall within the estimated Middle Permian marine dolomite and show a significantly positive offset compared to those of coeval seawater. The 87Sr/86Sr ratios of type D2 dolomite are slightly higher than the coeval seawater. These features combined with the flat rare earth element (REE) profile and relatively high Sr concentrations (both are similar to the host limestone) indicate that type D2 dolomites first precipitated in the slightly evaporated Middle Permian seawater during the near-surface to shallow burial phase. Type D3 dolomites are mainly characterized by interlocking mosaic crystals with crystal sizes varying from 300 μm to 900 μm. It mainly occurs as patches in type D2 dolomites or as a thick layer sandwiched by type D2 dolomites. These features combined with its high 87Sr/86Sr ratios, δ18O depletion and positive Eu anomalies (which are very similar to those of the saddle dolomites) demonstrate that it is formed by hydrothermal recrystallization upon type D2 dolomites. Saddle dolomite generally occurs as the filling cement of pores, vugs, and fractures and is usually combined with matrix dolomite to form zebra dolomite and hydrothermal dolomite breccia. It has a curve crystal face, demonstrates wavy extinction under cross-polarization and red luminescence under a cathodoluminescence microscope. These features, together with the highest Th, most negative δ18O value and highest 87Sr/86Sr ratio compared to those of all types of dolomites in the study area as well as the positive Eu anomalies suggest that it should have been formed by hydrothermal fluid precipitation.Type D1 and type D3 dolomites are volumetrically minor and the pores are poorly developed. Saddle dolomite cemented and filled the pores and vugs of matrix dolomite, resulting in reservoir damage. Type D2 dolomites constitute the main body of high-quality reservoir in Qixia Formation. It precipitated in the slightly evaporated penesaline Middle Permian seawater during the near-surface to shallow burial phase. Potential high-quality reservoirs may occur in the shoal around the limited inter-shoal sea.

Conifer water-use patterns across temporal and topographic gradients in the southern Sierra Nevada.

Climate change is increasing the severity and duration of drought events experienced by forest ecosystems. Because water is essential for tree physiological processes, the ability of trees to survive prolonged droughts will largely depend on whether they have access to reliable water sources. While many woody plant species exhibit the ability to shift water sources between different depths of soil and rock water in response to changes in climate and water availability, it is unclear if Sierra Nevada conifers exhibit this plasticity. Here we analysed the δ18O and δ13C values of annual tree rings to determine the water-use patterns of large Sierra Nevada conifers during the 2012-16 California drought and 4 years before this drought event (2004-07). We analysed four species (Pinus jeffreyi Grev. & Balf. (Jeffrey pine), Pinus lambertiana Dougl. (sugar pine), Abies concolor (Gord. & Glend.) Lindl. Ex Hilderbr (white fir) and Calocedrus decurrens (Torr.) Florin (incense-cedar)) across a range of topographic positions to investigate differences in water-use patterns by species and position on the landscape. We found no significant differences in δ18O and δ13C values for the pre-drought and drought periods. This stability in δ18O values suggests that trees did not shift their water-use patterns in response to the 2012-16 drought. We did find species-specific differences in water-use patterns, with incense-cedar exhibiting more depleted δ18O values than all other species. We also found trends that suggest the water source used by a tree may depend on topographic and growing environment attributes such as topographic wetness and the surrounding basal area. Overall, our results suggest that the water source used by trees varies by the species and topographic position, but that Sierra Nevada conifers do not switch their water-use patterns in response to the drought. This lack of plasticity could make Sierra Nevada conifers particularly vulnerable to drought mortality as their historically reliable water sources begin to dry out with climate change.

Processes Associated with Multiphase Dolomitization and Other Related Diagenetic Events in the Jurassic Samana Suk Formation, Himalayan Foreland Basin, NW Pakistan

Foreland fold and thrust belts always represent a complex diagenetic history of carbonate succession, particularly multiphase dolomitization, due to the multi-sourcing nature of fluids affecting syn-to post-depositional successions. The present work documents a comprehensive study on the diagenetic changes, particularly dolomitization patterns in the Jurassic carbonates (Samana Suk Fm) in the Lesser Himalayan fold and thrust belt, NW Pakistan. To better understand the processes involved, integrated field/petrographic, geochemical, isotopic, and micro-thermometric studies were carried out. Field observations indicate that dolostones appear as light grey to brown stratabound and patchy units within the formation. Petrographic analysis reveals that fabric destructive matrix dolomite (RD-I) and fabric preserving replacive dolomite (RD-II) phases are present as distinct units. In addition, saddle dolomite cement (SD) and fracture-filling calcite (CC) are also observed in association with replacive dolomite cement. Geochemical analysis (EPMA) showed that Fe, Mn, and Ba concentrations in matrix dolomite are relatively less than those of replacive/saddle dolomite and fracture-filling calcite, suggesting a hydrothermal source of replacive/saddle dolomite in reducing conditions. Furthermore, stable isotope studies of RD-I showed non-depleted δ18O values, which represent coeval seawater signatures of Jurassic carbonates. RD shows depleted δ18O values and non-depleted δ13C, respectively, indicating burial or elevated temperature of dolomitization. Fracture-filled calcite represents lighter δ18O values and δ13C, indicative of relatively high temperatures. 87Sr/86Sr ratios of all diagenetic phases range from 0.707718 to 0.710747, showing more radiogenic values indicates interaction fluids with more radiogenic sources. Fluid inclusion micro-thermometry data of saddle dolomite shows TH ranging from 102.8 to 186 °C, and salinity ranging from 11.7 to 19.4 eq. wt.% NaCl, suggesting hot saline brines are responsible for the dolomitization. Fracture-filling calcite shows TH ranging from 68.0 to 98.4 °C and salinity ranging from 6.9 to 13.1 eq. wt.%. NaCl suggests moderately hot and saline solutions are responsible for their formation. In conclusion, the above-mentioned studies indicate two distinct processes of dolomitization are involved in the formation of matrix and replacive/saddle-type dolomites. RD-I has formed in the evaporative setting, whereas RD has formed due to the interaction of hydrothermal fluids during burial. Fracture-filling calcite is produced from hot subsurface solutions during uplift related to the Himalayan orogeny.

Multiphase Diagenetic Processes and Their Impact on Reservoir Character of the Late Triassic (Rhaetian) Kingriali Formation, Upper Indus Basin, Pakistan

Multiple episodes of dolomitization of the shallow marine carbonates of the Late Kingriali Formation resulted in regional scale mappable dolostone geobodies in the Kohat and Potwar sub-basins. With the exception of few unaltered patches of the host limestone, more than 90% of the carbonates of the studied formation are diagenetically altered by replacive dolomites with associated dolomite cementation. Petrographical and geochemical data interpretation reveals that during the initial stage of dolomitization, the precursor limestone was significantly modified by the fabric-retentive replacive dolomite (RD-I) and produced bulk dolostones with non-planar-a to planar-s crystals. Neomorphic recrystallization (RD-II) was observed as an overgrowth of the already formed RD-I dolomite crystals during progressive dolomitization. The seawater at shallow depths is enriched with Fe-ions due to its interaction with Fe-rich beds within the studied formation. The modified seawater actively participated in the formation of ferroan replacive dolomites (RD-III). Stable isotopic composition of the unaltered Echinoderm plates, calcite cement (CC-I), and RD-I demonstrates signatures of δ18O and δ13C within the limit of late Triassic marine seawater or modified seawater. Depletion in the stable oxygen isotopic composition (from −0.99‰ to −3.75‰ V-PDB) demonstrates that RD-II and RD-III were formed in a sequence with progressively higher temperature fluids than normal seawater. Precipitation of dolomite cements as cavity filling rhombs (DC-I) and crystal overgrowth (DC-II) with highly depleted δ18O values (−5.44‰ to −7.45‰ V-PDB) illustrates dolomite cementation at higher temperatures and greater depths. The highly depleted values of δ18O (up to −9.16‰ V-PDB) and (up to 0.42‰ V-PDB) for δ13C of saddle dolomite (SD-I) indicate the precipitation of SD-I as a cavity filling dolomite at considerable depth. Calcite cementation and calcitization actively participated in the early, middle, and late diagenetic modifications as interpreted from their petrographic and stable isotopic studies. Porosity enhancement is clearly demonstrated by dissolution, stylolization, fracturing, and replacement dolomitization. Dolomite and calcite cementation had a negative impact on the reservoir character and occluded the dolostone porosity to a greater extent.

Fluid Inclusion, Isotopic, and Elemental Geochemistry Studies of Cave-Filling Calcite in the Lower–Middle Ordovician Yingshan Formation of Tahe Oilfield, NW China: Implication for Karstification in Non-exposed Limestone

Improving the recognition of paleo-fluid circulation history is of great significance to reconstruct pore evolution during carbonate diagenesis. Integrated petrography, fluid inclusion, isotopic and elemental geochemistry (laser ablation inductively coupled plasma mass spectrometry) studies, calcites generated in paleocaves, and fractures were investigated. This study aims to reveal the paleo-fluid origin and karstification event within the nonexposed limestone of the Lower–Middle Ordovician Yingshan Formation in the Tarim Basin. The only generation of blocky calcite growing along the karst paleocave and fracture walls [cave-filling calcite (CFC)] crosscuts burial stylolites. The secondary fluid inclusions obtained from CFC are characterized by the coexistence of liquid-only and liquid-dominated aqueous inclusions with low salinities values (0–2.4 wt%), suggesting that the CFC has experienced a low-temperature environment (<50°C). The depleted δ18O values (−15.32‰ to 12.45‰), seawater-like yttrium and rare earth element patterns, and low ΣREE (<0.65 ppm) have recorded the major episode of meteoric water leaching the Yingshan limestone. This view was further confirmed by the calculated δ18Owater values of parent fluids (−14.3‰ to 2.2‰). The relatively higher 87Sr/86Sr ratios (0.70942–0.70994) are interpreted as the result of meteoric water interacting with the overlying Silurian detrital rock when percolated downward. The geochemical evidence recorded by CFC indicates that the karstification event for Yingshan nonexposed limestone possibly took place in the Early Hercynian period during the late Devonian. Therefore, meteoric water percolating downward along the fractures penetrating insoluble strata and/or migrating laterally along the permeable strata is deemed responsible for the karstification event, although the overlying thick insoluble strata in the coverage area.

Massive dolomitization driven by MgSO4-rich seawater and its effects on thermochemical sulfate reduction, Upper Permian Changxing Formation, northeastern Sichuan, China

Massive dolostones replacing build-ups and containing H2S of thermochemical sulfate reduction (TSR) origin occur in the Upper Permian reef and bank deposits, northeastern Sichuan, China, despite the absence of gypsum or anhydrite deposits, which are usually present in TSR cases. Fluid chemistry from fluid incluions and δ13C, δ18O and 87Sr/86Sr values from different diagenetic phases were measured to determine the dolomitization regime, and to assess the relationships between the occurrence of dolomitization and H2S accumulation. Dolomitization was initiated by seawater with slightly increased salinities (penesaline) at shallow depths prior to chemical compaction. The micro- to fine-crystalline cloudy dolomite formed with relatively high Na and Sr contents, and with δ13C, δ18O and 87Sr/86Sr values (2.2‰∼4.8‰, −3.9‰∼-5.0‰ and 0.70724∼0.70746, respectively) inherited from seawater. The evaporation of Permian seawater in back-reef and inter-reef lagoons during sea-level fall and the subsequent seepage reflux into reef-beach bodies led to greater Mg2+ and SO42− concentrations in higher-salinity pore waters. Further massive dolomitization was promoted by compactional flow of hotter residual seawater at shallow to intermediate depths and resulted in the formation of fine- to medium-crystalline clean dolomite with lower Na and Sr contents, more depleted δ18O values (-5.1‰∼-6.0 ‰), andδ13C (2.5‰∼4.8 ‰) and 87Sr/86Sr values (0.70726∼0.70741) similar to those of the coeval seawater. The relatively closed hydrodynamic system during burial facilitated SO42− preservation. The whole dolomitization process enriched porewater SO42−, which have been almost exhausted by subsequent TSR, accounting for the high present-day concentrations of H2S. The output of this study shows that similar scenarios involving dolomitization driven by condensed MgSO4 seawater and H2S accumulations can occur in evaporite-free settings across a broad range and deserves special attention during deep oil/gas exploitation.

Late Pleistocene–Holocene climatic implications of high‐resolution stable isotope profiles of a speleothem from south‐central Anatolia, Turkey

ABSTRACTHigh‐resolution 230Th ages and stable isotope (δ18O and δ13C) records from a stalagmite that grew between 39 and 2 ka in Incesu Cave located in south‐central Anatolia allow us to evaluate paleoclimate conditions for growth periods during Marine Isotope Stage 3 (MIS3) and the Holocene. High δ18O values and Heinrich events H3 and H4 are observed during the MIS3 interval. After a dry period in the Younger Dryas, low values between ca. 10 and 5.3 ka suggest a transition to wet mid‐ to early Holocene conditions. In the early Holocene, there are drier periods at 9.4 and 10.3 ka, coincident with cooling events recorded in the North Atlantic sediments and, after 5.3 ka a relatively dry late Holocene is seen. The IN‐01 isotope record is in phase with general trends of speleothem records in Turkey and the Eastern Mediterranean but differs in detail at the millennial scale. The more depleted δ18O values of IN‐01 compared to those of Eastern Mediterranean speleothems during the Holocene indicate that central Anatolian winter rainfall was isotopically influenced by the same air mass trajectories derived from the North Atlantic and Mediterranean with an isotopic rain shadow effect.

Continuous monitoring of surface water vapour isotopic compositions at Neumayer Station III, East Antarctica

Abstract. In this study, the first fully continuous monitoring of water vapour isotopic composition at Neumayer Station III, Antarctica, during the 2-year period from February 2017 to January 2019 is presented. Seasonal and synoptic-scale variations in both stable water isotopes H218O and HDO are reported, and their links to variations in key meteorological variables are analysed. In addition, the diurnal cycle of isotope variations during the summer months (December and January 2017/18 and 2018/19) has been examined. Changes in local temperature and specific humidity are the main drivers for the variability in δ18O and δD in vapour at Neumayer Station III, on both seasonal and shorter timescales. In contrast to the measured δ18O and δD variations, no seasonal cycle in the Deuterium excess signal (d) in vapour is detected. However, a rather high uncertainty in measured d values especially in austral winter limits the confidence of this finding. Overall, the d signal shows a stronger inverse correlation with specific humidity than with temperature, and this inverse correlation between d and specific humidity is stronger for the cloudy-sky conditions than for clear-sky conditions during summertime. Back-trajectory simulations performed with the FLEXPART model show that seasonal and synoptic variations in δ18O and δD in vapour coincide with changes in the main sources of water vapour transported to Neumayer Station III. In general, moisture transport pathways from the east lead to higher temperatures and more enriched δ18O values in vapour, while weather situations with southerly winds lead to lower temperatures and more depleted δ18O values. However, on several occasions, δ18O variations linked to wind direction changes were observed, which were not accompanied by a corresponding temperature change. Comparing isotopic compositions of water vapour at Neumayer Station III and snow samples taken in the vicinity of the station reveals almost identical slopes, both for the δ18O–δD relation and for the temperature–δ18O relation.

Comparison Of Isotope And Hydrochemical Characteristics Of Springs In Sembalun – Rinjani Area, East Lombok, West Nusa Tenggara, Indonesia Before And After The Earthquake Events In 2018

The current 2019 isotope and hydrochemical study of hot and cold springs in Sembalun - Rinjani area is a re-assessment of previous similar study in 2012. The aim of this study is to assess the isotope and hydrochemical characteristics of springs due to the earthquake events in 2018. After the earthquake events in 2018, the stable isotopes δ18O and δ2H composition of Sebau hot spring and most of cold springs is shifted into more depleted values which may indicate water-rock interaction or interaction with cold waters which has more depleted δ18O and δ2H values. Also, Sebau hot spring is still plotted at mixing line of meteoric and andesitic water, but still dominant meteoric water. The hydrochemical data of all cold springs and Orok river show the enrichment of Na, probably from silicates weathering or the cation exhchange. While hydrochemical composition of Sebau hot spring is significantly decreased, except SO4, probably due to dilution with cold waters before the thermal water reach the surface. The Piper diagram showed that cold springs and Orok river are Ca-Mg-HCO3 type before and after the earthquake events. While Sebau hot spring is shifted from Ca-Cl type into mixed Ca-Mg-Cl type after the earthquake events. The temperature of Sebau hot spring slightly decreased from 35.5 °C to 34.8 °C after the earthquake events, while Na/K geothermometer calculation also indicate decreasing of sub-surface temperature, i.e. from 146–165 °C to 130–150 °C.

Temporal variations and evaporation control effect of the stable isotope composition of precipitation in the subtropical monsoon climate region, Southwest China

Stable hydrogen and oxygen isotopes in precipitation, which are sensitive to climate change and the water cycle, are widely used as natural tracers to investigate hydrological processes and paleoclimate reconstruction. High resolution event-based precipitation sampling was conducted in Guilin, a subtropical monsoon climate region in Southwest China, from 2010 to 2016 to identify the temporal variability of stable isotopes of precipitation and its dominant meteorological factors. The results showed that the stable isotopes of precipitation exhibited significant seasonal variability, with lower values in the wet season and higher values in the dry season, reflecting the changes in the character of precipitation in response to seasonal variability of the moisture sources and atmospheric dynamics. δ18O values were significantly negatively correlated with temperature and potential evapotranspiration (ET0) on the daily and monthly timescales; however, there was no significant correlation between monthly δ18O and the amount of rainfall. Multivariate regression analysis showed that when temperature, precipitation amount and ET0 were considered, 39% of the variability of the monthly oxygen isotope composition of precipitation was explained. Monthly mean temperature had a higher covariation with precipitation δ18O, indicating that the variations of precipitation isotope were more dependent on temperature, but not substantially dependent on the amount effect of rainfall. Howerver, the main influence of meteorological factors on the isotopic variations of precipitation varied seasonally. Temperature strongly influenced the isotopic composition of precipitation in summer, while the amount effect was prominent due to the enhanced convective activity characterized by more depleted δ18O values. Although evapotranspiration is a minor contributor of precipitation isotope variability, the impact of evapotranspiration on the enrichment of precipitation isotopes in autumn is a substantial control and should be considered, which makes a major contribution to compensate for the depletion in heavy isotopes of rainfall. The non-linear relationship between d-excess and ET0 was attributed to the variable contribution of large-scale convective moisture and local evaporation, as well as water vapor transfer processes. The response of precipitation stable isotopes to convective events was characterized by distinct stage patterns, associated with the water vapor sources and physical processes during rainout. The significant depletion of precipitation δ18O values during convective events was not due to the ‘amount effect’, but due to large-scale convection activity. These findings provide further insight into intraevent-scale isotopic variations associated with organized convection and their key influencing factors, which ultimately improves our interpretation of the paleoclimate records in subtropical monsoon regions.

Possible Sources of Salinity in the Upper Dibdibba Aquifer, Basrah, Iraq

Salinity increase in groundwater was investigated in the area between Al-Zubair and Safwan, and close to the Khor Al-Zubair Channel of southern Iraq. Thirty-nine groundwater samples from the shallow aquifer and one sample from the Khor Al-Zubair Channel were analyzed. The mean total dissolved solids are 7556 mg/L. The δ2H and δ18O plot in two groups are below the global meteoric water line. Group A indicates the evaporation effect of irrigation return-flow, while group B is characterized by depleted δ18O values due to recharge under colder climate. Deuterium excess values plot within the region of modern precipitation and dilution of groundwater by precipitating water. The groundwater residence time is between 1000 and 2000 years and combining 14C -age with SO42− shows a contrasting effect on groundwater on both sides of Khedr Almai Fault and the Zubair anticline, which indicates the role of these geological structures on the hydrochemical evolution in the western part. Jabal Sanam shows no clear effect in this regard. The ratio Cl−/Br− and sulfate in groundwater showed that the measured salinity in groundwater is the result of a mixing process between groundwater, seawater intruding from Khor Al-Zubair Channel, and water from septic tanks in addition to dry and wet sea spray, and irrigation return-flow.

Large volcanic eruptions reduce landfalling tropical cyclone activity: Evidence from tree rings

Tropical cyclones (TCs) are one of the most devastating storm systems with high socioeconomic impacts around the world. The drivers of long-term changes in TC frequency and intensity, including the recent global climatic changes, are, however, poorly understood due to short instrumental measurements and a lack of accurate proxy records. Here we present the long-term impacts of large volcanic eruptions on TC activity in northeast Asia. For this purpose, we performed a reconstruction of the frequency and intensity of landfalling TCs based on tree-ring oxygen isotope ratios over the past 350 years. Our results revealed markedly depleted δ18O values (P < 0.01) for TC years and confirm tree-ring δ18O as a strong proxy for the detection of past TCs. The agreement between the δ18O chronology and the corresponding TC record (1950–2005) was 96.4% and it was relatively high also for the preceding periods covered by less reliable TC records, specifically 76.1% (1904–1949) and 66.4% (1652–1903). In addition to the prominent TC frequency signal, we found a strong negative correlation (R = −0.65; P < 0.001) between the δ18O chronology and TC intensity expressed by the amount of TC precipitation. Our reconstruction revealed that the recent high frequency of landfalling TCs is distinct on a long-term scale. We provide the first long-term evidence of reduced TC activity for two years following large volcanic eruptions. Our results indicate that volcanic ash is a relevant driver of TC activity over northeast Asia via its role on radiative climate forcing. We suggest that large volcanic eruptions modulate large-scale atmospheric and oceanic circulation determining TC genesis and thus TC activity in the western North Pacific.

Carbon and oxygen stable isotopes of freshwater fish otoliths from the São Francisco River, northeastern Brazil.

Fish otoliths store geochemical and isotopic signatures that can be used as proxies for environmental conditions in ecological investigations. We investigated the δ13C and δ18O of otoliths of four freshwater fish species (Astronotus ocellatus, Serrasalmus brandtii, Plagioscion squamosissimus, and Cichla ocellaris) from the Moxotó Reservoir in the São Francisco River, Brazil. The enriched δ13C signatures that distinguish A. ocellatus from other species could be caused by dissolved inorganic carbon (δ13CDIC) in reservoir waters. Macrophytes growing along the reservoir margins would favor photosynthetic uptake of 12C, leading to enriched δ13CDIC incorporated into the otoliths of those fish. Otolith δ18O signatures appear to reflect water column preferences, in which species preferentially inhabiting surface waters (such as S. Brandtii) show more enriched values (due to high surface evaporation ratios), while bottom water species show depleted δ18O values. Our results represent the first investigation of the isotopic compositions of fish otoliths in freshwater environments in the São Francisco River, and shed light on the interpretation of isotopic information stored in otoliths and how they can be used to infer the ecological strategies of freshwater fish.

Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan

The Jurassic carbonates of the Samana Suk Formation are extensively exposed in the foreland areas of Himalayas and form major reservoir of the upper Indus basin. These carbonates are composed of oolitic, pelitic fossiliferous and micritic limestone units which have been extensively modified by diagenetic alterations, particularly dolomitization. Field observations show two distinct types of dolostone geobodies (i) bedding parallel stratiform, and (ii) patchy dolostone units respectively. Bedding parallel stratiform dolostones are present in the basal part of the formation, while patchy dolostones are present at the middle and upper parts of the Samana Suk Formation. The dolomitization intensity of both geobodies increases from NW to SE in the study area. Petrographic studies reveal six phases of dolomites and three phases of calcites based on texture, crystal size and morphology. These phases are: matrix replacive dolomites (MD-I to MD-III); cementing dolomites include, replacive cementing dolomite (RD), saddle cementing dolomite (SD) and late stage cementing dolomites (CD); and calcite phases include CC-I and CC-II. XRD analyses reveal that stratiform matrix dolomites (MD-I, MD-II) are stoichiometric (51.08–51.86 mol percent of CaCO3) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non-stoichiometric (33.39–55.08 mol% of CaCO3) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO3) in origin. Stable isotope studies reveal non-depleted δ18O and δ13C values of matrix dolomites (MD-I, MD-II) represents coeval sea-water signatures of Jurassic carbonates, hence may have been formed by evaporative process. Dolomite cements (RD, SD) shows depleted δ18O values which represent elevated temperature, related to hydrothermal fluid source for their formation. The fracture filling calcite (CC-II) exhibits less depleted values indicative of meteoric fluids affected by shallow to moderate burial. The dedolomites shows depleted δ13C values suggests their formation from the meteoric water. Field, petrographic and geochemical studies suggest that diagenetic evolution of the Samana Suk Formation is the multistage process. In the first phase, marine diagenetic processes including marine cementation, stratiform dolomitization may have formed due to surface processes of marine water in peritidal to intertidal settings, while the second phase of diagenesis is due to burial associated processes which includes hydrothermal dolomitization occurred due to movement of magnesium rich fluids along weak planes such as fractures, faults, bedding planes and stylolites. Last stage includes formation of hydrothermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes.