18O Content Research Articles

Geological Trap Controlling the Residence Time of Groundwater in Assessment of Exploitation Zone for Its Sustainable Resources Case Study: The Slope of Karang Mount, Banten Province, Indonesia

The southern slope of Mount Karang is covered by complex volcanic deposits with complicated texture and structure. The study on zone or location of water resources which would be exploited required a comprehensive hydrogeological approach. Through detailed geological mapping, spring sampling, and well drilling were carried out. Representative spring water samples were taken to be analyzed in the laboratory, and to obtain the data of physical groundwater, chemical groundwater, stable isotopes 18O (oxygen-18), and deuterium contents, as well as the age of the groundwater. In general, the groundwater facies of the studied area showed Ca, Na, KHCO3 with several sites indicating changes to CaHCO3 during the dry and rainy seasons. The synthesis results of the stable isotope 18O (oxygen-18) and deuterium contents, verified by the physical and chemical groundwater controlled by geological setting in the groundwater subbasins, show the anomaly of residence time as trapped by the normal fault in the middle of the studied area. The existing normal fault might control this anomaly of residence time of groundwater surrounding site JH1, JH9, and JH20. However, the distribution of three different water source zones occurred. All group of groundwater indicated a complex flowing with geological setting controlling the physical, chemical content, and the age of the groundwater. At last, sites JH4, JH5, and JH9 show that the zones are proper to be developed as sustainability groundwater resources. Keywords: groundwater facies, stable isotope, groundwater flow, sustainable water

Research on the characteristics of 2H and 18Ostable isotopic composition of precipitation in Hanoi

The variability in the content of environmental isotopes 2H and 18O in precipitation over time is applied to study the characteristics of rainwater and the time-varying process in the atmospheric water cycle and the variable processes in the hydrological cycle. 26 rainwater samples were collected according to rainy days from August 3 to October 29, 2020 in Ha Noi. The stable isotope composition in rainwater ranged from -108.04‰ to -40‰ and -15.87‰ to -6.53‰ for 2H and 18O, respectively. Besides, the values of 2H and 18O were located around the local meteorological waterline. The stable isotope values also clearly reflect the correlation with the daily rainfall as well as clearly reflect the influence of season and ambient temperature on the change of stable isotope value with rainwater.

Oxygen Isotope Exchange between Carbon Dioxide and Iron Oxides on Mars' Surface.

An investigation of the fundamental processes leading to the incorporation of 18O isotopes in carbon dioxide and in iron oxides is critical to understanding the atmospheric evolution and geochemistry of Mars. Whereas signatures of 18O have been observed by the Phoenix Lander and the sample analysis at Mars for carbon dioxide, the underlying isotopic exchange pathways with minerals of the crust of Mars are still elusive. Here, we reveal that reactions of gaseous 18O-carbon dioxide over goethite (FeO(OH)) and hematite (Fe2O3) lead to an 18O transfer from the atmosphere that enriches the 18O content of the iron oxides in the absence of water and light. This proof-of-concept study shows that isotopic enrichment processes on Mars not only are limited to the atmosphere but also proceed via chemical interaction with dry iron oxides. These processes are decisive to comprehending the 18O cycle between the atmosphere and the surface on the planetary scale.

Isotopic signature of groundwater and climate change within a semi-arid environment

The negative effects of climate change on water resources from arid and semi-arid environments are of great interest to academics and policymakers. In this regard, this paper aims to assess the climate change impact on water resources within semi-arid environments, with a focus on the Essaouira Basin. To assess these effects we based on the isotopic signature of groundwater. This approach shows that (a) the recharge of groundwater is ensured by the precipitation of Atlantic origin without significant evaporation. (b) low recharge rate following the decrease in the precipitation rate (16% for the period 1978–2015) experienced by the study area in recent years, (c) The presence of old or fossil water in the study area suggests a low rate of renewal and the overexploitation of the groundwater in this basin, (d) The temporal evolution of the 18O contents generally shows an enrichment character of 1–1.5‰, which can probably be due to the evaporation phenomenon following the increase in the air temperature caused by climate change. However, it can be concluded that the isotopic signature of the groundwater within the Essaouira basin has not been spared from the effects of climate change.

Physico-Chemical and Isotopic (<sup>18</sup>O, <sup>2</sup>H and <sup>3</sup>H) Characterization of the Upper Dallol Maouri Watershed (Niger)

Dallol Maouri is one of the fossil valleys on the left bank of the Niger River. Its watershed drains water from valleys in the Tahoua and Dosso regions of Niger. The watercourse is not continuous and has areas of surface water accumulation (ponds) and areas of spreading and infiltration. The surface water potential in this area is limited, with most of the ponds having a temporary to the semi-permanent regime. Groundwater resources are, therefore, the main sources of withdrawal for human food, agriculture, and livestock. Climate change through the variation in rainfall, the often-anarchic multiplication of catchment works (wells and boreholes) and agricultural activities can have impacts on the qualitative and quantitative evolution of surface and groundwater. The objective of this study is to characterize the surface and underground water of the upper dallol Maouri through the physicochemical parameters of the water and the use of the isotopes of the water molecule (18O, 2H and 3H). A total of 53 water samples were analyzed at the laboratory of the Regional Directorate of Hydraulics and Sanitation of Tillabéri (Niger) for chemical parameters and 73 samples were analyzed at the Radio Analysis and Environment Laboratory (LRAE) of the National School of Engineers of Sfax (ENIS) of Tunis for isotopy. The water of the ponds is characterized by three types of facies: calcium bicarbonate, calcium and magnesium bicarbonate, and calcium nitrate chloride. The groundwater is 75% of the chloride-nitrate-calcium facies and the CT3/CT2 groundwater is calcium and magnesium bicarbonate. The results of the stable isotopes made it possible to highlight the presence of four distinct water groups: ponds characterized by highly evaporated water, the sheets of Continental Terminal 1 and 2 (CT1 and CT2) with an isotopic cachet of old water, the water table consisting of a two-layer system in the southwestern part of the area. The water is not very mineralized (40 μS to 600 μS) and is slightly acidic (pH = 6.4). The analysis of the radioactive isotope (3H) content of the water from the water table has made it possible to identify the areas of recharge of the water table. These results also highlight: the presence of an upward drainage zone, and the contribution of lowland ponds to the recharge of alluvial aquifers ranging from 24% to 84%. No structure in this zone reaches the Continental Intercalaire and the Continental Hamadien, the depth of which is estimated at an average of 600 m. The relationship between the 18O and 3H contents of groundwater water shows two types of water with tritium contents higher than 2 TU for alluvial aquifers and tritium contents lower than 2 TU for groundwater (CT3). The analysis of the radioactive isotope (3H) contents of the groundwater has made it possible to identify recent recharges at the level of the alluvial aquifers and in the outcrop zone of CT3. There is no hydraulic continuity between the plateau ponds and groundwater.

Stable isotopes changes in groundwater: a case study in Mudal and Clapar springs, West Progo

Hydroisotope studies were carried out on Mudal and Clapar springs located in the central part of the West Progo Dome. The research was conducted by taking samples of groundwater in each spring for three periods, representing the rainy (2016), dry (2017), and rainy (2018) seasons. Data on stable isotope content of 18O and D were analyzed to see the hydroisotope characteristics of groundwater and their relationship to climate change. The results show that the stable isotope content of groundwater in both springs was relatively stable, with insignificant changes over time and season. Mudal springs tend to show light isotopes, indicating deep aquifer or high elevation recharge, less affected by the season. Clapar spring shows heavy isotopes, which may be sourced from a shallow aquifer with mixing/evaporation processes and are more influenced by the season. Meanwhile, the range value of δD in the two springs shows slightly - totally changes, indicating that the D content also changes due to seasons, although it is small. The δD enrichment shows the medium-large change in both spring springs, but uncertainty in Mudal. However, the D-excess value shows that the dry and rainy season conditions, which may be related to temperature or precipitation, are not much different.

Use of Isotope Technique in Groundwater Investigation around Erbil City- Northern Iraq

The fundamental characteristics of environmental isotopes in water resources management is dependable on the nature of geographical region of the catchment boundaries. The method of applying different isotopes has a main character in quantity and quality estimations of water resources that sometimes cannot be achieved by old techniques. Nowadays the detailed inquiry of surface water or ground water difficulties can practically have been explained against the support of performing the isotopes. The present study assembled some investigations and applications on the environmental isotopes that predict the water management systems in different catchment areas and implementing the isotope techniques in Kurdistan region of Iraq. This should help us to solve many problems of surface and ground water such as the recognition a source of ground water, ground water dating, problems of ground water contamination, trace the direction and velocity of flow, the interaction of surface water with ground water, physical transportation link between different aquifers, some characteristics of an aquifer and renewability of water. The objectives of this study are to implicate and investigate on the most intensive science of environmental isotope in different catchment areas of Kurdistan region in northern Iraq. The data were taken by (Dana,2016) for different springs and wells in the region starting from Haji-Omaran to Barzan. There are no meteorological or hydrographic measuring stations. The ground water compositions of 10 wells and 12 springs were illustrates that the seasonal does not conclusive a part while the assumptions would be modified for the precipitation data. The results show that, groundwater saves the 18O and deuterium (D) content out of which they were formed. The isotope contents of most groundwater lie somewhere near the local respective meteoric water line (MWL). Whereas, the proportionality pf high degree in deuterium surplus (d) is about 10 which is representative for the Mediterranean Sea region. also, the high concentration of isotopes was taken place at Haji-Omaran and Jundian w that about 8.93 and 8.37 degrees respectively. This is leads to offer the possibility with a relatively small financial expense to use isotopic hydrological methods to explore the underground water balance in northern Iraq.

A New Theory of Change in the Insolation of the Earth over Millions of Years against Marine Isotope Stages

Results from a new astronomical theory of paleoclimate for the last 1, 5, and 20 Myr are presented. Evolutions of obliquity e and insolation at 65°N $$Q_{s}^{{65\,{\text{N}}}}$$ are considered for the summer half year and insolation I at equivalent latitudes over the last 1 Myr; the results are compared with the evolution of these parameters obtained from the former theory. The oscillations in the obliquity e range from 14.7° to 32.1° according to the new theory and from 22.08° to 24.45° according to the former theory; i.e., the amplitude of oscillations have increased 7–8 times. The insolation oscillations have increased to the same extent. The summer insolation $$Q_{s}^{{65\,{\text{N}}}}$$ was used to introduce six (from very cold to very warm) climate gradations. Oscillations in the parameters of orbital and rotational motions of the Earth and various insolation components, as well as their periods and amplitudes, are considered for the last 5 Myr. Orbit eccentricity e varies with periods of 94.6 kyr, 413 kyr, and 2.31 Myr, while the periods of perihelion rotation relative to the equator are nonuniform and vary from 13.8 to 41.8 kyr. These periods can also be found in changes in the obliquity and insolation. The distribution of insolation over the Earth’s latitude for the summer and winter caloric half-years and for the year in three different (modern, coldest, and warmest) epochs is considered. The maximum changes in summer and annual insolation occur at high latitudes. The statistics of coldest and warmest periods are considered for the last 20 Myr. On average, there can be six such periods per 1 Myr; their number can reach 10 in unquiet time intervals and can decrease to 2 in quiet time intervals. The results of measurements of the oxygen isotope 18O content in marine sediments are analyzed. The content of 18O is shown to be inconsistent with the change in the Earth’s insolation according to both the former theory and the new one; in addition, the variation in 18O is inconsistent with known paleoclimate changes. An analysis of short-period variations in radioactive isotopes, as well as 18O in stalagmites and glacial cores, indicates that variations in 18O in marine sediments can be caused by a change in the 18O content in the Earth’s atmosphere, rather than by climate fluctuations.

The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds.

The transport of carbon into Earth's mantle is a critical pathway in Earth's carbon cycle, affecting both the climate and the redox conditions of the surface and mantle. The largest unconstrained variables in this cycle are the depths to which carbon in sediments and altered oceanic crust can be subducted and the relative contributions of these reservoirs to the sequestration of carbon in the deep mantle1. Mineral inclusions in sublithospheric, or 'superdeep', diamonds (derived fromdepths greater than250 kilometres) can be used to constrain these variables. Here we present oxygen isotope measurements of mineral inclusions within diamonds from Kankan, Guineathat are derived from depths extending from the lithosphere to the lower mantle (greater than 660 kilometres). These data, combined with the carbon and nitrogen isotope contents of the diamonds, indicate that carbonated igneous oceanic crust, not sediment, is the primary carbon-bearing reservoir in slabs subducted to deep-lithospheric and transition-zone depths (less than 660 kilometres). Within this depth regime, sublithospheric inclusions are distinctly enriched in 18O relative to eclogitic lithospheric inclusions derived from crustal protoliths. The increased 18O content of these sublithospheric inclusions results from their crystallization from melts of carbonate-rich subducted oceanic crust. In contrast, lower-mantle mineral inclusions and their host diamonds (deeper than 660 kilometres) have a narrow range of isotopic values that are typical of mantle that has experienced little or no crustal interaction. Because carbon is hosted in metals, rather than in diamond, in the reduced, volatile-poor lower mantle2, carbon must be mobilized and concentrated to form lower-mantle diamonds. Our data support a model in which the hydration of the uppermost lower mantle by subducted oceanic lithosphere destabilizes carbon-bearing metals to form diamond, without disturbing the ambient-mantle stable-isotope signatures. This transition from carbonate slab melting in the transition zone to slab dehydration in the lower mantle supports a lower-mantle barrier for carbon subduction.

The effects of moisture sources and local parameters on the 18O and 2H contents of precipitation in the west of Iran and the east of Iraq

Iran and Iraq face a water shortage crisis especially in the recent decade. Hence, the regions with high potential water resources are highly important to these countries. Precipitation sampling for 18O and 2H analyses was conducted in 10 stations in the west of Iran and 2 stations in the east of Iraq during this study. Studying precipitation and its moisture sources using the backward trajectories of the HYSPLIT model, developing seasonal meteoric water lines (MWLs), and simulating the spatial variations of stable isotopes in precipitation using stepwise regression yielded valuable results regarding the hydrometeorology of the study region. The precipitation moisture sources in the study region were determined using the backward trajectories of the HYSPLIT model. These backward trajectories demonstrated the domination of various moisture sources including the Mediterranean Sea, the Black Sea, the Red Sea, and also the Persian Gulf in different seasons as reflected in the seasonal MWLs. In addition to moisture sources, the effects of meteorological parameters (temperature and precipitation amount) on the stable isotope content of precipitation also showed a notable correlation (with R2 = 0.58 and 0.46, respectively). Surface water resources were also plotted on the developed seasonal MWLs, while some samples deviated from seasonal MWLs due to the evaporation effect.

Identification Of Sinter Silica Evolutionary Manifestation As A Bicarbonate Travertine Based On Isotope 18O And 2H Review Of Local Meteoric Water Line Curves As Determinant Of Outflow Fluid Reservoir In Cisolok-Cisukarame Geothermal System

Cisolok geothermal system located in Cisolok district, Sukabumi regency has undergone an evolution. One of the indication is the transformation of its surface manifestation from Silica sinter to Bicarbonate travertine. The existence of this travertine indicates the presence of bicarbonate spring and reservoir systems temperatures below 150°C, which are the hallmark of two-phase liquid dominated geothermal systems. Analysis of stable isotopes (18O and 2H) is used to identify the reservoir water’s source affecting the evolution of Cisolok’s surface manifestations. The stable isotopes are then compared with the global and local meteoric water line. The contents of isotope 18O and 2H, after being compared with the global and local meteoric water line, gives a visual data that’s adjacent and in line with the local meteoric line. This indicates that the hot water from Cisukarame manifestation originates from local meteoric water, which is the result of evaporation and precipitation from the local water. The evolution of the geothermal system in this area is indicated by the increase of isotope 18O content in water and a decrease of the same isotope among the rocks. Indication of the temperature reduction in this area, which is estimated to have happened over a period of 10,000-50,000 years, involves a diagenesis reaction and an amorphous silica hydrolysis that caused the increase of 18O and dD levels in the water. With two-phase liquid dominated geothermal systems, there’ll be many outflows that bring diverse manifestations. The results of the data analysis show that the evolution of the Cisolok geothermal system occurs due to the contact between reservoir fluid and limestone lithology at a depth of 350-600 meters which causes the Cl water content in the system to transform into HCO3 water content and alter the surface manifestations of silica sinter containing amorphous silica into a travertine containing bicarbonate.

Controls of extreme isotopic enrichment in modern microbialites and associated abiogenic carbonates

Microbialites and abiogenic carbonates of the closed-basin hypersaline lake Laguna Negra (Catamarca Province, Argentina) are highly enriched in both 13C and 18O. These carbonates precipitate in the zone of recharge where lake water mixes with groundwater. We examined the processes controlling the isotopic evolution of input waters in order to better interpret the isotopic compositions of the carbonates. Large enrichments of 13C in dissolved inorganic carbon and 18O in water occur as input groundwater chemically evolves. These enrichments can be explained through the abiotic processes of water-equilibration, evaporation, degassing, and carbonate precipitation. The 13C and 18O contents of lake carbonates are consistent with equilibrium precipitation from lake water indicating that the currently ongoing processes can explain the isotopic patterns observed in carbonates. Isotopic compositions of these microbialites are largely unrelated to the biological processes controlling microbialite formation – a pattern that may be generalizable to other settings. However, the isotopic compositions of these microbialites record information about their depositional environment.

Reprocessing of Irradiated [18O]H2O under the Conditions of a PET Center

The possibility of reprocessing of irradiated [18O]H2O after isolation of [18F]fluoride ion from it (regenerate) under the conditions of the cyclotron-radiochemical laboratory of an operating PET center was examined. The radionuclide and chemical composition of [18O]H2O and the degree of its enrichment in 18O before and after distillation at atmospheric pressure were studied comprehensively. The suggested method allows efficient removal of long-lived γ-emitting radionuclides (56Co, 57Co, 58Co, 65Zn, 54Mn, 51Cr) from the [18O]H2O regenerate. The distillation reduces the content of residual solvents (acetaldehyde, acetone, isopropanol, ethanol, acetonitrile), whereas the content of 3H and 18O in [18O]H2O changes within the measurement uncertainty. The distillation residues and decontamination solutions after neutralization can be jointly cemented to incorporate the γ-emitters into a compact matrix with the aim of its subsequent delivery to specialized organizations performing long-term storage and disposal of solid radioactive waste.

Anthropogenic disturbances of natural ecohydrological processes in the Matlabas mountain mire, South Africa

Matlabas is a mountain mire in Marakele National Park, located within the headwaters of the Limpopo River in South Africa. This mire consists of a complex of valley-bottom and seepage wetlands with small elevated peat domes. The occurrence of one decaying peat dome, which has burnt, and desiccated wetland areas with terrestrial vegetation has raised concerns. The aim of this study was to understand the mire features and water flows in order to identify the potential drivers causing wetland degradation. Wells and piezometers were installed to monitor the hydraulic head and collect water samples for analysis of ion composition, 18O and 2H stable isotope content, and δ13C and 14C isotope content for radiocarbon dating. Moreover, peat temperature profiles were measured and peat deposits were also dated using radiocarbon. Results indicate that the Matlabas mire developed in the lowest central-east side of the valley by paludification at the onset of the Holocene. During the Mid-Holocene, peat development was extended laterally by autogenic and allogenic processes. Three types of water flows driving peat development were identified – sheet flow, phreatic groundwater flow and deep groundwater flow – two of which are surface or near surface flows. The recent occurrence of decaying peat domes and desiccated wetland areas is possibly related to loss of exfiltrating deep groundwater flows that have resulted from drainage by the head-cut channels in the mire and interception of near surface water flow by an access road, respectively. Interventions should be undertaken to prevent further degradation of the mire.
 Significance:
 
 This study is the first, as far as we are aware, on the ecohydrology of an inland mountainous mire in southern Africa.
 The results highlight the importance of the current wetland management (including rehabilitation) initiatives in South Africa.
 The integrative ecohydrological methods can be applied in other headwater wetlands in southern Africa.

Micro-milling super-fine powdered activated carbon decreases adsorption capacity by introducing oxygen/hydrogen-containing functional groups on carbon surface from water

Superfine powdered activated carbon (SPAC) of micron to submicron particle size is produced by micro-milling of conventionally sized powdered activated carbon. SPAC has attracted attention because of its high adsorption capacity; however, milling to the submicron particle size range lowers its adsorption capacity. Here, we found that this decrease of adsorption capacity was due to the introduction of oxygen/hydrogen-containing functional groups into the graphene structure of the carbon from water during the milling, causing it to become less hydrophobic. This finding was supported by three analyses of SPAC particles before and after milling: 1) elemental analysis revealed increased oxygen and hydrogen content, 2) Boehm titration analysis revealed increased amounts of acidic functional groups, including carboxylic and phenolic hydroxyl groups, and 3) Fourier-transform infrared spectroscopy showed increased peaks at 1200, 1580, and 3400 cm−1, confirming the presence of those groups. Dissolved oxygen concentration did not strongly affect the increase of oxygen content in SPAC, and no evidence was found for hydroxyl radical production during micro-milling, suggesting that a mechanochemical reaction underlies the increase in oxygen/hydrogen-containing functional groups. An increase in 18O content in the SPAC particles after milling in water-18O indicated that the oxygen in the functional groups originated from the surrounding water.

Origin and Age of the surface water and groundwater of the Ouargla basin-Algeria

The chemical and isotopic data of groundwater and surface water of the Ouargla basin have been examined the relation and the spatial models that could be useful to determine the vertical and genetic relation between the mass water, recharge zones, directions of groundwater flow and geochemical processes. The surface distributions of water types and some chemical and isotopic data are discussed in this paper, as are the apparent relationships between constituents. The hydrogeological and geochemical implications and characteristics of the observed data are presented as well. The saturation indices for the selected minerals discussed below were calculated using the PHREEQC geochemical package (Parkhurst, 1995) [1].The objective of this work is to apply isotopic and hydro-chemical techniques to determine the chemical features of the water and the state of saturation, the origin, the ages and the mechanisms of supply of surface water and recharge of deep aquifers in the region of Ouargla. The basin of Ouargla is located in the North-East of the Algerian Sahara, presents an area of 95 000 ha; characterized by a very arid climate. Hydrochemically, water quality is very degraded, the chemical features are almost homogeneous. The 18O and 2H contents of groundwater are highly enriched and show a highly evaporated character on the majority of the surface water and are thus distinguished from the waters of two deep aquifers. On the surface a mixture between the different tanks has been demonstrated by the isotopes (18O and 2H). The 14C and 3H activities of the surface water at the basin scale show quite variable activities confirm the existence of several recharging episodes over time and validate the mixture evidenced by stable isotopes. For the terminal complex 14C activities are very low and show a decrease in the direction of flow.

Seasonal Subsurface Water Contributions to Baseflow in the Mountainous Uhlířská Catchment (Czech Republic)

Abstract Nine years of seasonal δ18O values in precipitation, soilwater and groundwater were evaluated in the Uhlířská catchment between 2008 and 2016 and recharge winter/summer ratios were calculated using δ18O values. The longterm average 18O content in groundwater is lower than the mean weighted 18O content in precipitation. This is explained by more than 50% of winter- and snowmelt- induced groundwater recharge that occurs in all years except of 2010 and 2013. The recharge of the peat organic soil water is balanced between summer and winter, whereas the mineral hillslope soil is dominantly recharged by summer precipitation. The 67% portion of baseflow, dominantly generated in the winter season, is composed of groundwater and peat organic soil water, according to the hydrochemical distribution of runoff components. Isotopic mass balance of individual winters shows that precipitation in warmer winters is entirely transformed into outflow until the end of the winter season, generating no significant water storage for potential drought periods

Isotopic composition of daily precipitation along the southern foothills of the Himalayas: impact of marine and continental sources of atmospheric moisture

Abstract. The flow of the Himalayan rivers, a key source of fresh water for more than a billion people primarily depends upon the strength, behaviour and duration of the Indian summer monsoon (ISM) and the western disturbances (WD), two contrasting circulation regimes of the regional atmosphere. An analysis of the 2H and 18O isotope composition of daily precipitation collected along the southern foothills of the Himalayas, combined with extensive backward trajectory modelling, was used to gain deeper insight into the mechanisms controlling the isotopic composition of precipitation and the origin of atmospheric moisture and precipitation during ISM and WD periods. Daily precipitation samples were collected during the period from September 2008 to December 2011 at six stations, extending from Srinagar in the west (Kashmir state) to Dibrugarh in the east (Assam state). In total, 548 daily precipitation samples were collected and analysed for their stable isotope composition. It is suggested that the gradual reduction in the 2H and 18O content of precipitation in the study region, progressing from δ18O values close to zero down to ca. −10 ‰ in the course of ISM evolution, stems from regional, large-scale recycling of moisture-driven monsoonal circulation. Superimposed on this general trend are short-term fluctuations of the isotopic composition of rainfall, which might have stem from local effects such as enhanced convective activity and the associated higher degree of rainout of moist air masses (local amount effect), the partial evaporation of raindrops, or the impact of isotopically heavy moisture generated in evapotranspiration processes taking place in the vicinity of rainfall sampling sites. Seasonal footprint maps constructed for three stations representing the western, central and eastern portions of the Himalayan region indicate that the influence of monsoonal circulation reaches the western edges of the Himalayan region. While the characteristic imprint of monsoonal air masses (increase of monthly rainfall amount) can be completely absent in the western Himalayas, the onset of the ISM period in this region is still clearly visible in the isotopic composition of daily precipitation. A characteristic feature of daily precipitation collected during the WD period is the gradual increase of 2H and 18O content, reaching positive δ2H and δ18O values towards the end of the period. This trend can be explained by the growing importance of moisture of continental origin as a source of daily precipitation. High deuterium-excess (d-excess) values of daily rainfall recorded at the monitoring stations (38 cases in total, range from 20.6 to 44.0 ‰) are attributed to moisture of continental origin released into the atmosphere during the evaporation of surface water bodies and/or soil water evaporation.

Effects of elevated CO2 on shell 13C and 18O content and growth rates in the clam Scapharca broughtonii

The effects of elevated CO2 levels on growth and stable carbon and oxygen isotope compositions (δ13C and δ18O) of aragonitic shells were experimentally evaluated by rearing the bloody clam, Scapharca broughtonii, at two different temperatures and at six different pCO2 levels (17 °C: 269, 382, 550, 757, 939, and 1114 µatm; 25 °C: 332, 463, 653, 872, 1137, and 1337 μatm). Ambient CO2 levels did not much affect shell growth rates. Mass spectrometric analysis of the outer shell layer indicated a negative correlation between seawater pH and shell δ18O (equivalently, a positive correlation between seawater pCO2 and shell δ18O) at both temperatures (regression slopes: −0.70‰ ± 0.14‰/pH unit at 17 °C; −0.36‰ ± 0.17‰/pH unit at 25 °C). A positive correlation was observed between seawater pH and shell δ13C at both temperatures, but the regression slopes were less steep than those between seawater pH and the δ13C of seawater DIC. As CO2 levels rose (and pH dropped to about 7.6), shell δ18O and δ13C increased relative to ambient dissolved inorganic carbon (DIC). Both approached apparent isotopic equilibrium in acidified waters. Shell δ18O was below that of the ambient mix of bicarbonate and carbonate ions, and usually below apparent isotopic equilibrium with water. Kinetic discrimination against heavy isotopes during CO2 hydration and hydroxylation reactions may contribute to this isotopic “enlightenment”, and elevated ambient CO2 may “wash out” this effect. Our findings thus generally support models of calcification physiology and shell isotopic content, and have implications for isotopic paleontology.

Stability of Residual Oxides in Oxide‐Derived Copper Catalysts for Electrochemical CO2 Reduction Investigated with 18O Labeling

AbstractOxide‐derived (OD) Cu catalysts have high selectivity towards the formation of multi‐carbon products (C2/C3) for aqueous electrochemical CO2 reduction (CO2R). It has been proposed that a large fraction of the initial oxide can be surprisingly resistant to reduction, and these residual oxides play a crucial catalytic role. The stability of residual oxides was investigated by synthesizing 18O‐enriched OD Cu catalysts and testing them for CO2R. These catalysts maintain a high selectivity towards C2/C3 products (ca. 60 %) for up to 5 h in 0.1 m KHCO3 at −1.0 V vs. RHE. However, secondary‐ion mass spectrometry measurements show that only a small fraction (<1 %) of the original 18O content remains, showing that residual oxides are not present in significant amounts during CO2R. Furthermore, we show that OD Cu can reoxidize rapidly, which could compromise the accuracy of ex situ methods for determining the true oxygen content.