Oxygen Isotope Analyses Research Articles

The Guttenberg δ13C excursion (GICE) and paleoclimate change recorded in the early Late Ordovician carbonate succession of the Ordos Basin, Northern Chinas

The well-known Guttenberg isotopic carbon excursion (GICE) is a global palaeoceanographic event in the early Katian, Late Ordovician. Here, based on analysis of the carbon oxygen isotopes and K-bentonite characteristics, we documented the evidence of the GICE from the Upper Ordovician carbonate succession in the Ordos Basin, Northern China. The results revealed a positive δ13C excursion occurred in the Jinsushan Formation. Based on SHRIMP U-Pb dating of the K-bentonites and biostratigraphic constraints, the carbon isotope excursion of X1 (in Xilingggou section) and Z1 (in Zhaolaoyu section) identified in the area can be regarded as local expressions of the GICE. It is the first time that the GICE was identified in the Ordos Basin. Compared with the GICE profiles from the Dolly Ridge, North America and the Mehikoorma Drill Core from Europe, the X1 profile exhibits a similar feature with a positive δ13C excursion of 2.11‰. While the Z1 profile shows a two-peaks curve with a positive shift up to 5.61‰, representing one of the largest shifts in δ13C for the GICE so far. Considering that the X1 and Z1 profiles started immediately above the K-bentonites, and the GICE in North America and Europe were also in close association with marked bentonites, we argued that the carbon cycle perturbation depicted by the GICE and its equivalents were genetically related to increased volcanic eruptions in the early Late Ordovician, which may have led to a climate cooling and enhanced organic burial, thus resulted in the positive δ13C excursion.

Oxygen Isotopic Studies of a Species of Pitar (Hyphantosoma) from Quilon Formation, Kerala, Southwest India: Inferences on Seasonality during the Miocene (late Burdigalian)

Abstract The carbonaceous shale of the Miocene (late Burdigalian) Quilon Formation (Kerala Basin, southwest India) were collected and processed to retrieve the microfaunal component. In the present study, the oxygen isotope analysis of the growth bands of the recovered bivalves Pitar (Hyphantosoma) simonnei and Ostrea sp. (both retrieved from the Quilon Formation) is emphasized to help reconstruct the palaeomonsoonal seasonality. The oxygen-isotope record for the specimens of P.(Hyphantosoma) simonnei and Ostrea sp. exhibits regular, high amplitude δ18O cycles caused by the yearly seasonal temperature and monsoonal variations. Based on the number of δ18O cycles,P. (Hyphantosoma) simonnei and Ostrea sp. have been considered to be approximately 2- and 4-year-old, respectively, at the time of deposition. The calculated palaeo- seawater δ18O values between -0.6 to -1.1‰ [for P. (Hyphantosoma) simonnei] allow us to distinguish between the wet and dry seasons. In addition, the two-component mixing model is suggestive of seasonal freshwater discharge of ~70-80% during wet season and ~20-40% during the dry season into the depositional system. This reconstructed palaeo-seasonality clearly indicates presence of monsoon system in the south west of India during the Miocene (late Burdigalian).

Generation of REE-rich syenite-(carbonatite) complex through lithosphere-asthenosphere interaction: An in-situ Sr–Nd–O isotopic study of the Mesozoic Weishan pluton, Northern China

Carbonatite-alkaline complexes are commonly derived from the mantle. However, whether the source is lithospheric or asthenospheric mantle remains debated. In order to better understand this issue, this paper conducts a Sr-Nd-O isotopic study on the Early Cretaceous, Weishan REE-rich syenite-(carbonatite) complex, Northern China. Two types of apatite are identified from the Weishan complex, namely magmatic and xenocrystic apatite. In-situ Sr-Nd isotopic analyses on magmatic apatite yielded (87Sr/86Sr)i from 0.70690 to 0.70719, and εNd(t) from −7.6 to −8.8. The pristine calcite from carbonatite has similar (87Sr/86Sr)i (0.70727–0.70739) with magmatic apatite. In-situ oxygen isotopic analyses on zircon yielded δ18OVSMOW(‰) from 5.9 to 6.7 ‰. Previous studies reveal that coeval, mantle lithosphere-derived rocks in the region have more enriched Sr (87Sr/86Sri greater than 0.709) and Nd [εNd(t) 〈 −1 2] isotopes than the Weishan complex; whereas the asthenosphere-derived rocks in the region are characterized by depleted Sr-Nd isotopes (87Sr/86Sri < 0.706; εNd(t) greater than 0) and low δ18OVSMOW values (mostly < 5.4 ‰). It is well known that Northern China experienced destruction in the Early Mesozoic, which was accompanied with asthenosphere upwelling along trans-lithosphere faults. Considering that the Weishan complex locates adjacent to the trans-lithosphere Tanlu fault, generation of the complex may be related to asthenosphere-lithosphere interaction. Notably, the Weishan complex differs from coeval REE-barren carbonatites in that its source involved asthenosphere-derived components; hence, involvement of asthenosphere-derived components may be important for the formation of such a REE-rich complex. In general, this study highlights the importance of lithosphere-asthenosphere interaction in supplying necessary heat and materials for the generation of the Weishan complex.

Accurate and Efficient SIMS Oxygen Isotope Analysis of Composition-Variable Minerals: Online Matrix Effect Calibration for Dolomite.

High-quality oxygen isotope analysis of composition-variable minerals (e.g., ubiquitous carbonates) using secondary ion mass spectrometry (SIMS) is extremely challenging. The classical off-line procedure, which requires additional electron probe microanalyzer (EPMA) chemical compositions for calibrating instrumental mass fractionation (IMF), is inherently inaccurate and analytically inefficient. In this study, the first accurate and paired SIMS analysis of δ18O and Fe# [molar Fe/(Mg + Fe)] in dolomite is reported. Based on five newly developed dolomite O-isotopic standards with an Fe# range of 0.01-0.35 obtained by SIMS, a novel accurate and rapid online matrix effect calibration method for dolomite O-isotope analysis was developed using concurrent SIMS 18O-16O-56Fe16O-24Mg16O measurements without additional chemical electron probe microanalysis. A logistic equation was proposed as the best-fit curve to represent the δ18O matrix effect based on the 56Fe16O/24Mg16O ratios. For CTD-4 carbonatitic dolomite with variable Fe# but homogeneous oxygen isotopes, the off-line method exhibited highly variable apparent δ18O values in the range of 5.74-10.11‰. The online method yielded a homogeneous δ18O value of 7.94 ± 0.34‰ (2SD, n = 40), which is comparable with that of bulk analysis (7.94 ± 0.20‰; 2SD). Comprehensive analyses validated the online method as the best strategy for performing accurate δ18O analysis of samples with highly heterogeneous compositions. Based on its accuracy, simplicity, and economic feasibility, this method has potential applications in the analysis of composition-complex dolomites, detrital dolomites, and other precious terrestrial and extraterrestrial materials.

Mass Bias Corrections for Hydrogen and Oxygen Isotope Analysis of Tourmaline by Secondary Ion Mass Spectrometry

ABSTRACT Hydrogen (δ2H) and oxygen (δ18O) stable isotopes are used to trace fluid sources, metals, and contaminants in the environment and Earth's subsurface. Tourmaline-supergroup minerals provide an opportunity to quantify both δ2H and δ18O from the same grain using in situ analytical techniques (e.g., Secondary Ion Mass Spectrometry – SIMS). These minerals occur in a wide variety of geological environments and have a wide range of chemical compositions. However, large differences in chemical composition are problematic during SIMS analysis, as instrumental mass fractionation (IMF) often varies with the chemical composition of the mineral. Therefore, calibration models derived by analyzing tourmalines of different chemical composition must be developed for accurate analysis by SIMS. Hydrogen and oxygen isotope analysis was done on six reference tourmaline samples using a CAMECA 7f SIMS instrument operating at extreme energy filtering. Spot-to-spot repeatability for tourmalines was in the range 4–5‰ and 0.6–1.0‰ for δ2H and δ18O, respectively. There is a strong correlation between IMF and several elements (B, Si, Ca, Fe, and Fe#). Iron content is the most robust predictor of IMF, and we report two calibration curves for the correction of δ2H and δ18O measured by SIMS using reference tourmaline crystals with different Fe contents, ranging from 0.00 to 14.00 wt.% Fe. This is the first calibration curve used to correct for the fractionation of hydrogen isotope ratios in tourmaline as measured by SIMS. Tourmaline-supergroup minerals require a suite of at least three, with a range of Fe content, to ensure accurate and precise H and O analysis by SIMS. Crystallographic orientation effects were not observed for these tourmalines.

New insights in Neanderthal palaeoecology using stable oxygen isotopes preserved in small mammals as palaeoclimatic tracers in Teixoneres Cave (Moià, northeastern Iberia)

The northeastern region of Iberia constitutes a natural pass-area for arriving populations into the peninsula and becomes a key area to understand Neanderthal resilience to changing environmental conditions experienced during Marine Isotope Stage 3 (MIS 3; 60–30 ka). Short-term but repeated occupations by Neanderthal groups occurred in Teixoneres Cave (Moià, Barcelona) in alternation with large and small carnivores during MIS3. Abundant small-mammal remains accumulated in units III and II of this fossiliferous deposit, providing local climatic and environmental information. This work focuses on the taphonomic history of small-mammal faunas, which a is clue to validate previous palaeoecological interpretations. As was observed with leporids and bird remains, raptors are considered the major source of small-mammal remains. The most likely accumulator is an opportunistic predator, the eagle owl, with very rare inputs by mammalian carnivores. In parallel, high-resolution palaeoclimatic data are provided through oxygen isotope analyses (δ18O) of rodent teeth from four subunits (IIIb to IIa), which are compared with independent methods of palaeotemperature estimations. According to air temperatures estimated from δ18O rodent teeth, cooler conditions than present day (− 1.6/ − 0.5 °C) are recorded along the sequence, but homogenous (< 1 °C). Complementary methods also explain higher rainfall than present day (+ 44/ + 682 mm). Only slight changes between units III and II show climatic instability, which could be related to palimpsests of stadial-interstadial events. Climatic stable conditions are reported from coeval isotopic and palaeodiet analyses from northeastern Iberia in agreement with the palynological records that underline how the Mediterranean area could have sustained rich ecosystems that assured the Neanderthal subsistence during the abrupt climatic pulsations of the Late Glacial.

The effects of highly reduced magmatism revealed through aubrites

AbstractEnstatite‐rich meteorites, including the aubrites, formed under conditions of very low oxygen fugacity (ƒO2: iron‐wüstite buffer −2 to −6) and thus offer the ability to study reduced magmatism present on multiple bodies in our solar system. Elemental partitioning among metals, sulfides, and silicates is poorly constrained at low ƒO2; however, studies of enstatite‐rich meteorites may yield empirical evidence of the effects of low ƒO2 on elemental behavior. This work presents comprehensive petrologic and oxygen isotopic studies of 14 aubrites, including four meteorites that have not been previously investigated in detail. The aubrites exhibit a variety of textures and mineralogy, and their elemental zoning patterns point to slow cooling histories for all 14 samples. Oxygen isotope analyses suggest that the aubrite parent bodies may be more heterogeneous than originally reported or may have experienced incomplete magmatic differentiation. Contrary to the other classified aubrites and based on textural and mineralogical observations, we suggest that the Northwest Africa 8396 meteorite shows an affinity for an enstatite chondrite parentage. By measuring major elemental compositions of silicates, sulfides, and metals, we calculate new metal–silicate, sulfide–silicate, and sulfide–metal partition coefficients for aubrites that are applicable to igneous systems at low ƒO2. The geochemical behavior of elements in aubrites, as determined using partition coefficients, is similar to the geochemical behavior of elements determined experimentally for magmatic systems on Mercury. Enstatite‐rich meteorites, including aubrites, represent valuable natural petrologic analogues to Mercury and their study could further our understanding of reduced magmatism in our solar system.

Asteroids accretion, differentiation, and break-up in the Vesta source region: Evidence from cosmochemistry of mesosiderites

The cosmochemistry of meteorites provides unique clues on asteroids accretion, differentiation, collisional break-up and reassembly - processes of critical importance for understanding planet formation in the early solar system. Mesosiderites are a complex group of achondrites whose nearly 50:50 metal-silicate composition is interpreted in the literature as resulting from the mixing of core and crustal materials derived from differentiated asteroids. Because of their complex nature and contrasting geochemical, isotopic, and spectroscopic data, the formation mechanism of mesosiderites is still poorly understood and is open to a large variety of planetary differentiation scenarios and collisional histories. In this study, based on new petrographic and geochemical data of 16 mesosiderites, we investigate in detail the proposal that mesosiderites are related to the howardite-eucrite-diogenite (HED) meteorite group, whose parent body is widely considered to be asteroid 4 Vesta (∼500 km diameter), the target of the recent NASA’s Dawn mission. We present the first high precision oxygen isotope analyses on the matrix of a set of mesosiderite samples, coupled with new chemical and petrographic analyses of mesosiderites Um Hadid, Estherville, and Mount Padbury. Concordant Δ17O values between mesosiderites (–0.241 ± 0.015 (2σ)) and howardite-eucrite-diogenites (−0.241 ± 0.017‰ (2σ)) indicate that they derived from the same oxygen isotope reservoir, but petrological evidence, in particular the distinctly lower Fe/Mn ratios and the larger lithological diversity in mesosiderites, indicates that they formed within different parent bodies. This suggests that mesosiderites and howardite-eucrite-diogenites originated in distinct parent bodies that accreted in the 4 Vesta source region but experienced different geologic evolution in terms of crustal differentiation and impact history, which were more complex and catastrophic in the mesosiderite parent body.

Garnet secondary ion mass spectrometry oxygen isotopes reveal crucial roles of pulsed magmatic fluid and its mixing with meteoric water in lode gold genesis

SignificanceThere is a common consensus that lode gold deposits mostly precipitated from metamorphic fluids via fluid boiling and/or fluid-rock interaction, but whether magmatic hydrothermal fluids and the mixing of such fluids with an external component have played a vital role in the formation of lode gold deposits remains elusive. We use garnet secondary ion mass spectrometry oxygen isotope analysis to demonstrate that the world-class Dongping lode gold deposit has been formed by multiple pulses of magmatic hydrothermal fluids and their mixing with large volumes of meteoric water. This study opens an opportunity to tightly constrain the origin of lode gold deposits worldwide and other hydrothermal systems that may have generated giant ore deposits in the Earth's crust.

Middle to Late Jurassic stable isotopes and element ratios of fossils from western India: Developing a reference temperature curve for northeastern Gondwana

The stable isotope (δ13C, δ18O) and element (Mg/Ca, Sr/Ca) composition of 374 well-preserved belemnites, bivalves, and brachiopods of the Kachchh and Jaisalmer basins in western India were used to reconstruct climatic changes in the Middle and Late Jurassic time interval. Absolute water temperatures reconstructed from δ18Oshell values depend on the used δ18Osea value and equation, but indicate a long-term temperature decrease of around 8–9 °C in the study area from the Bajocian to the Tithonian. Measured Mg/Ca ratios also point to decreasing temperatures, but additionally reflect species-specific factors and a lower Mg/Ca ratio of Jurassic seawater compared to today. The recorded cooling in western India agrees with previous results from neighbouring Madagascar and the Indian Himalayas and can be connected to a drift of Eastern Gondwana into higher latitudes. This palaeogeographic shift is accompanied by changes in sedimentation indicating an increasingly humid climate and in the composition of ecosystems reflecting increasingly cooler water temperatures. High-resolution stable oxygen isotope analyses of three oyster shells point to seasonal temperature changes of 4–5 °C in the Kimmeridgian. The recorded δ13C values reflect sea-level changes with a positive excursion in the Oxfordian connected to a major transgressive event. Sr/Ca ratios are observed to follow changes in global ocean chemistry.

Genesis of wavy carbonate flowstone deposits in Bossea Cave (North Italy) and their hydroclimatic significance

Speleothems show an array of shapes. Flowstones are commonly tabular sheet-like deposits, which cover cave floors and walls. They can procure detailed information about past hydrogeological conditions through their morphology, geochemical composition and stratigraphic properties, which in turn are related to the climate conditions at the surface. This is possible if the climate and environmental factors controlling the formation of these deposits are well understood. In Bossea Cave (north-east Italy), we have investigated highly symmetrical wavy flowstones that have never been described in detail before. These deposits show a cyclical repetition of sloped and sub-vertical layers, with knickpoints migrating downslope. This paper investigates the origin of these peculiar carbonate deposits, using: i) terrestrial laser scanning surveys; ii) petrographic observations; iii) oxygen and carbon stable isotope analyses; iv) hydrochemical monitoring data of feeding waters. According to these analyses, we have generated a genetic model that demonstrates: 1) laminar flow of CaCO3–rich water prevails during the deposition of the flowstone; in accordance, the final flowstone architecture does not appear to be influenced by random irregularities of the underlying bedrock substrate. 2) The peculiar morphology is inherited by ripples occurring in the flowing water films during carbonate deposition, which trigger the precipitation of regularly spaced CaCO3 deposits along the bedrock slopes. Because of these ripple-induced initial deposits, calcite then deposits as sloped and sub-vertical layers, giving rise to a wavy-like morphology. 3) Wavy calcite layers are only deposited after heavy rainfall events occurred within two to four consecutive rainy days (10 to 20 mm/hour during rainfall peaks; average rates between 3.5 and 7.0 mm/hour); in contrast, calcite deposition cannot occur during low rainfall events (less than 5 mm/hour during rainfall peaks, average rates around 0.8 mm/hour). We therefore propose that the studied wavy flowstones are hydroclimatic indicators, testifying the occurrence of flashy type recharge related to heavy rainfall events. Similar deposits could record these same hydro-climatic conditions in other karst areas.In recent decades, storms have caused more frequent flooding in highly populated Mediterranean areas, especially in Italy. Wavy flowstones may offer a new archive to gain a better understanding of the long-term dynamics of intense precipitation events and, thus, help to improve future climate scenarios.

Carbon and oxygen isotope analysis of CO2 trapped in silicate minerals

Knowledge of the carbon and oxygen isotope composition of CO2-rich inclusions in crust- or mantle-derived minerals is crucial for a more quantitative understanding of cycling of volatiles between Earth's deep and surficial reservoirs. In this study, we present a crushing method for analysis of combined carbon and oxygen stable isotope compositions of small (≤ nmole) amounts of CO2 in fluid and melt inclusions. During crushing, the released CO2 was cryo-trapped, and subsequently introduced into a Gas Bench II coupled to a Deltaplus Isotope Ratio Mass Spectrometry (IRMS). Reference standard CO2 introduced in the crusher chamber was analysed to a precision better than ~0.3 and ~ 0.4‰ (1σ) for carbon and oxygen, respectively. Analysis of previously analysed samples validates that the crushing technique can be applied to CO2-rich fluid inclusions in quartz samples as small as 0.2 to 5 mg. CO2 –rich quartz samples were analysed from the UHT-Bakhuis Granulite Belt (Suriname) and Val Nalps (Switzerland) as well as forsterite-rich olivine-hosted melt inclusions from Vulsini volcanic district (Italy). The carbon isotope compositions obtained from the quartz samples allow discrimination between different sources within the crust and mantle. In contrast, the oxygen isotopic signatures of the fluid inclusions containing water appear affected by isotopic exchange between CO2 and H2O in the inclusions, where for the free-water fluid inclusions can reflect the pristine δ18O value. Crushing of olivine grains with CO2 melt-inclusions displayed instantaneous adsorption of the released CO2, which leads to low CO2 yields, and considerable oxygen and carbon isotope fractionation, hampering reliable isotopic determinations. We conclude that the δ13C and δ18O values from CO2-rich inclusions can be used as an indicator of the CO2 source and to evaluate mantle CO2 outgassing and release from storages within Earth's crust. The method presented can be readily applied to determine carbon and oxygen isotopic compositions of CO2 (4.5−10 mol) in small amounts of fluid-inclusion bearing quartz.

Contribution of Spring Snowmelt Water to Soil Water in Northeast China and Its Dynamic Changes

Snowmelt water in spring is an important source of soil water, which is critical to supporting crop growth. Quantifying the contribution of snowmelt water to soil water and its dynamic changes is essential for evaluating soil moisture and allocating agricultural water resources. In this paper, through controlled outdoor experiments, different snow depths and soil depth gradients were set; and snow, precipitation, and soil samples were collected regularly. To analyze the contribution of snowmelt water to soil water and its dynamic changes, the MAT-253 stable isotope ratio mass spectrometer was adopted for hydrogen and oxygen isotope analyses. The results showed that the snowmelt water for snow depths of 10 cm, 30 cm, and 50 cm all contributed to the 0–30 cm soil layer. The contribution increased with soil depth, contributing 8.13%, 8.55%, and 11.24%, respectively. The contribution of the snow cover at the same depth to the soil moisture at different depths also varied, i.e., the contribution increased with increasing soil depth. The snowmelt water retention time at depths of 10 cm, 30 cm, and 50 cm was inconsistent, i.e., it was the longest at 0–10 cm (average of 69 days), followed by 20–30 cm (average of 59 days), and the shortest at 10–20 cm (average of 54 days). The greater the snow depth, the shorter the retention time of the snowmelt water in the different soil layers. For surface soil, the contribution of the snowmelt water at greater depths was significantly different; while for deep soil, the contribution was more sensitive to the snow depth. Regardless of snow depth, soil contributions at different depths were significantly different. Precipitation also affected the contribution of the snowmelt water to the soil water, exhibiting different effects at different depths.

Looking upstream with clumped and triple oxygen isotopes of estuarine oyster shells in the early Eocene of California, USA

Abstract The δ18O of carbonate minerals that formed at Earth's surface is widely used to investigate paleoclimates and paleo-elevations. However, a multitude of hydrologic processes can affect δ18O values, including mixing, evaporation, distillation of parent waters, and carbonate growth temperatures. We combined traditional carbon and oxygen isotope analyses with clumped (Δ47) and triple oxygen isotopes (Δ′17O) analyses in oyster shells (Acutostrea idriaensis) of the Goler Formation in southern California (USA) to obtain insights into surface temperatures and δ18O values of meteoric waters during the early Eocene hothouse climate. The Δ47-derived temperatures ranged from 9 °C to 20 °C. We found a correlation between the δ18O of growth water (δ18Ogw) (calculated using Δ47 temperatures and δ18O of carbonate) and the δ13C values of shells. The Δ′17O values of shell growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from Δ′17O of carbonate (−0.087‰ to −0.078‰ VSMOW-SLAP) were lower than typical meteoric waters. These isotopic compositions are consistent with oyster habitation in an estuary. We present a new triple oxygen isotope mixing model to estimate the δ18O value of freshwater supplying the estuary (δ18Ofw). The reconstructed δ18Ofw of −11.3‰ to −14.7‰ (VSMOW) is significantly lower than the δ18Ogw of −4.4‰ to −9.9‰ that would have been calculated using “only” Δ47 and δ18O values of carbonate. This δ18Ofw estimate supports paleogeographic reconstructions of a Paleogene river fed by high-elevation catchments of the paleo–southern Sierra Nevada. Our study highlights the potential for paired Δ47 and Δ′17O analyses to improve reconstructions of meteoric water δ18O, with implications for understanding ancient climates and elevations.

Petrology, mineralogy and geochemistry of Ordovician rocks in the southwest of Tarim Basin, implications for genetic mechanism and evolution model of the hydrothermal reformed-paleokarst carbonate reservoir

Paleokarst carbonate reservoir plays an important role in the exploration of oil and gas from shallow to ultra-deep strata. Multistage karstification is superimposed to form the present ultra-deep karst-hydrothermal reformed carbonate reservoirs in the Yubei area of the Tarim Basin. Based on petrology identification, stable carbon and oxygen isotope analysis, inductively coupled plasma mass spectrometry (ICP-MS), and Laser Ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we investigated paleokarst periods, chemical property of hydrothermal fluids, and the evolution model and formation mechanism of paleokarst-hydrothermal reformed Ordovician carbonate reservoirs. The results show that the Ordovician strata experienced five periods of karstification from the middle Caledonian period to the late Himalayan period. The Caledonian quasi-contemporaneous karst and the Hercynian epigenetic karstification provided the most fundamental pore – fracture framework for carbonate reservoirs, and the later multistage deep burial karstification re-transformed the established reservoir space by hydrothermal dissolution under thermochemical sulfate reduction (TSR) reaction. It is proposed that the thermal convection between the hydrothermal fluids from the upper mantle and carbonate rocks formed two kinds of hydrothermal fluids in Ordovician strata. The first kind of hydrothermal fluid is performed with a SiO2-enriched fluid under the acidic environment based on TSR, forming siliceous cements and hydrothermal dissolution pores in the Lower Ordovician strata. The second kind of hydrothermal fluids appeared as an alkaline CO32—enriched fluid with the reversible reaction between HCO3− and CaCO3, forming hydrothermal calcite and dolomite near the migration channel in Upper Ordovician carbonate rocks. The periodic changes of in situ trace element concentrations of multistage quartz fillings indicated that paleokarst reservoirs are controlled by thermal convection under episodic tectonic activity. As a summary, we established the migration pattern of hydrothermal fluids in the thin section scale and the evolution model of the hydrothermal reformed paleokarst reservoirs in basin scale.

Hydrogeochemistry Characteristics of Groundwater in the Nandong Karst Water System, China

The hydrogeochemical characteristics of groundwater are an important element in the study of the spatial and temporal variation of groundwater resources, which is crucial to water resources utilization, ecological environmental protection, and human development. Water samples were collected at eight observation of Nandong Karst Water System (NKWS) sites in each month of 2019, and the main ions and isotopes of the water samples were examined. The hydrogeochemistry characteristics of groundwater and its differences with surface water were explored by using the methods of multivariate statistics, Gibbs model. Results showed that the water chemistry types of groundwater were mainly HCO3–Ca and HCO3–Ca·Mg. The analysis of hydrogen and oxygen isotope showed that the initial recharge source of surface water and groundwater were atmospheric precipitation, and the measured hydrogen and oxygen isotopes of surface water were heavier due to the strong evaporation effect. The natural and anthropogenic processes contributed to the chemical composition of surface water and groundwater in the study area. However, the main factor affecting the quality of surface water and groundwater was the input of anthropogenic contaminants. In terms of natural factors, the main chemical ions of surface water and groundwater were mainly controlled by water-rock action originating from weathering and hydrolysis of rocks and soils. Ca2+, Mg2+, and HCO3− mainly originated from natural dissolution of carbonate rocks. K+, Na+, SO42−, and Cl− were partly from atmospheric precipitation. For human activities, Na+ and Cl− were partly from domestic water for local residents. SO42− in surface water mainly came from mining. NO3− in groundwater mainly came from chemical fertilizers, and NO3− in surface water were mainly from human waste and domestic sewage.

Hydrogen and Oxygen Isotopic Characteristics of Different Water and Indicative Significance in Baiyangdian Lake

In order to better understand the water cycle processes in Baiyangdian Lake, samples for precipitation, river water, lake water, and groundwater were collected in 2020 via the analysis of oxygen and hydrogen isotopes and the total dissolved solids. A combined approach including correlation analysis, end-member modeling, and evaporation modeling was used to identify hydrogen and oxygen isotopic characteristics of different water types and their indicative significance to evaporation, hydrodynamics, and lake-groundwater interactions. The results showed that the compositions of hydrogen and oxygen isotopes in surface water and groundwater differed from each other. The average values of δD and δ18O were the highest in samples from lake water in June, secondary for lake water in October, and the lightest in groundwater. The slope of the lake water line was lower than that of the local meteoric water line, which could be attributed to a greater evaporative enrichment in lake water. The water loss ratio was estimated to be 18.8%-42.3% in June and 2.7%-30.3% in October by applying an evaporation model using deuterium excess. Lake water isotopes exhibited highly spatial heterogeneity, which indicated that Baiyangdian Lake was a poor-mixed lake controlled by the complex hydrodynamic conditions. The values of δD and δ18O were lighter around the estuary area and higher in the district far from the estuary. Although the main water isotopes in areas such as channels had a short residence time, the spatial differences in lake water isotopes were not remarkable, which suggested that the well-mixed conditions of the lake water contributed to reducing the heterogeneity of the lake water isotopic compositions on a spatial scale. The contribution ratios of lake leakage to groundwater were 0-91.7% and varied spatially. It had a negative relationship with the buried depth of groundwater and the distance to the lakeshore. No obvious relationship was found between lake leakage magnitude and groundwater level gradient. This study could provide a theoretical basis and technical support for the sustainable development of water resources and ecological environment protection in Baiyangdian Lake.

Hypersalinity accompanies tectonic restriction in the eastern Mediterranean prior to the Messinian Salinity Crisis

This study describes the hydroclimate evolution of the eastern Mediterranean Basin during the early Messinian (7.2 to 6.5 Ma) time-interval based on analysis of a succession at Agios Myron (Crete, Greece), prior to the onset of the Messinian Salinity Crisis (5.96–5.33 Ma). Specifically, we report sea surface temperature and salinity reconstructions based on a combined analysis of biomarkers and oxygen isotopes of planktonic foraminifera. Data reveal that a negative water budget and strong hydrologic and climate variability characterized the eastern Mediterranean Basin at this time, and we identify three distinct phases. In Phase 1 (7.2–6.9 Ma), a shift to more positive oxygen isotope values in planktonic foraminifera at ~7.2 Ma is attributed to progressive gateway restriction of Mediterranean–Atlantic corridors and subsequent cooling until 6.9 Ma. In Phase 2 (6.9–6.7 Ma), distinct warm and hypersaline events (at 6.9–6.82 and 6.72 Ma) resulted in stressed marine microfauna during periods of strong evaporation. An important step-change in the Mediterranean restriction at 6.72 Ma may have resulted from shallowing of the Mediterranean gateways and reduced Mediterranean marine outflow. During Phase 3 (6.7–6.5 Ma) this gateway shallowing reduced the oceanic input into the Mediterranean Basin causing significant hydrological changes, reflected in a wide range of temperature and salinity fluctuations accompanied by enhanced water-column stratification. The data presented here counterbalance the general lack of quantitative temperature and especially salinity estimates available for the Mediterranean Messinian, time interval where we still highly rely on modelling for such evaluations. This study highlights the severity of preconditioning stages leading to the Messinian Salinity Crisis in the Mediterranean and sets values for extreme salinity conditions that could still host marine life. • Three restriction phases affected the eastern Mediterranean between 7.2 and 6.5 Ma. • Colder water and normal salinity characterized the Mediterranean during 7.2–6.9 Ma. • Periods of strong evaporation observed for the 6.9–6.7 Ma interval. • Warm-hypersaline events (6.9–6.82 and 6.72 Ma) caused extreme environmental stress. • Restriction at 6.7–6.5 Ma was induced by tectonics and modulated by climate.

Restoration of formation processes of dolomite reservoirs based on laser U-Pb dating: A case study of Ordovician Majiagou Formation, Ordos Basin, NW China

To address the issue of non-unique interpretation of dolomite reservoir diagenetic and porosity evolution in the previous qualitative or semi-quantitative studies, we investigate two dolomite reservoir types, i.e. weathering-crust karstic reservoirs and mound-beach reservoirs, in the Ordovician Majiagou Formation, Ordos Basin by using in-situ laser ablation U-Pb dating as well as carbon and oxygen isotopic composition analysis. The results show that: (1) According to the dating of 8 reservoir samples, the Majiagou Formation experienced 5 diagenetic stages (Stage 1: deposition of matrix dolomite or penecontemporaneous dolomitization, in 444.0–494.0 Ma; Stage 2: dogtooth- or blade-shaped dolomite cementation, in 440.0–467.0 Ma; Stage 3: dolomitic silt filling, in 316.5–381.0 Ma; Stage 4: crystalline dolomite filling, in 354.0 Ma; Stage 5: crystalline calcite filling, in 292.7–319.0 Ma). (2) Supra-salt weathering-crust karstic dolomite reservoirs went through several diagenetic processes including penecontemporaneous dolomitization, compaction, weathering-crust karstification, packing, and rupturing in succession. Gypsum mold pores formed in the phase of hypergenic karstification and were filled with such minerals as dolomitic silts and calcites, and thus the porosity decreased from 10%–40% to 3%–8%. (3) Sub-salt mound-beach dolomite reservoirs went through the diagenetic processes including penecontemporaneous dolomitization, compaction, subsea cementation, penecontemporaneous corrosion, infiltration backflow dolomitization, packing, and rupturing. The porosity of reservoirs was originally 10%–30%, decreased to 0–6% due to seawater cementation, rose back to 5%–15% owing to penecontemporaneous corrosion, and finally declined to 2%–6% as a result of crystalline dolomites and calcites packing. The above methodology for the restoration of dolomitization and porosity evolution may be helpful for the restoration of porosity evolution in other basins or series of strata.

Early Pleistocene (Marine Isotope Stage 40–36) paleoceanography in the northwestern Pacific: Evidence from faunal and oxygen isotope analyses of planktonic foraminifera

The Mid-Pleistocene Transition (MPT) is characterized by a shift from a 41-kyr to a 100-kyr climate cycle of approximately 1.25–0.7 Ma. However, proxy records before the MPT are sparse in the northwestern Pacific region. To investigate the marine environment around the oceanographic convergence zone of the northwestern Pacific prior to the MPT, we conducted biostratigraphic and oxygen isotope analyses of planktonic foraminifera derived from the upper part of the Kiwada Formation of the Kazusa Group, which is distributed in the Boso Peninsula, central Honshu, Japan. The interval investigated was deposited from Marine Isotope Stages (MIS) 41 to 36 (ca. 1.31–1.20 Ma). The last occurrence of Globigerinoides obliquus is emplaced 30 m above the tephra layer Kd1 during MIS 37. The first occurrence of Neogloboquadrina inglei is placed somewhere below the tephra layer Kd8 and is older than 1.310 Ma. Time-series environmental proxy changes in the planktonic foraminiferal assemblages and oxygen isotopes suggest that the marine environment off the Boso Peninsula was influenced by the Kuroshio Current and the transitional water close to the Kuroshio Front, while the latitudinal shifts of the Kuroshio Front were associated with the glacial-interglacial periods prior to the MPT. Our results suggest that obliquity variability played a significant role in the mid-latitude Pacific Ocean before the MPT, and the hydrographic change in the North Pacific Subtropical Gyre seems to be connected to high-latitude climatic changes. Moreover, the temporary southward migration of the Kuroshio and Subarctic fronts occurred during the late MIS 38, which may be associated with the stronger East Asian winter monsoon. Thus, mid-latitude Pacific oceanographic changes that occurred before the MPT may be linked not only to global remote connections but also to local atmospheric circulation. • We conducted the biostratigraphic and oxygen isotope analyses of the planktonic foraminifera in the Kiwada Formation, central Japan. • The last occurrence of Globigerinoides obliquus was correlated with the late MIS 37. • Palaeoceanographic changes in the mid-latitudes seems to be connected to high-latitude climatic changes.