Oxygen Isotope Composition Of Water Research Articles

Authigenic quartz reveals the triple oxygen isotope composition and diagenetic temperature of saline brines

Triple oxygen isotope compositions of authigenic minerals are an emerging tool for the reconstruction of fluid temperatures and the fluid isotopic composition. In this study, we analyzed euhedral authigenic quartz crystals from a halite deposit of the Upper Permian Zechstein evaporitic basin for their triple oxygen isotope composition using laser fluorination isotope ratio mass spectrometry. In the triple oxygen isotope space, additional information, such as the triple oxygen isotope composition of the ambient water, provides insights into equilibrium conditions during quartz formation that cannot be identified from δ18O alone. The triple oxygen isotope composition of authigenic quartz shows, that single oxygen isotope (δ18O) thermometry is not applicable for samples from evaporitic environments, where the fluids' δ18O deviates from dynamic oxygen isotope equilibrium. By combining the Craig and Gordon evaporation model with H2O – SiO2 triple oxygen isotope equilibrium fractionation, we reconstruct the formation temperature of authigenic quartz and the triple oxygen isotope composition of the marine-derived saline brine pore fluid in equilibrium with the quartz. We obtain temperatures of 90 to 130 °C that is interpreted as crystallization temperatures of microcrystalline quartz in pore fluid of halite during burial diagenesis. The triple oxygen isotope composition of authigenic quartz from evaporitic environments is suited as a geochemical tracer for fluid oxygen isotope compositions and ancient fluid or diagenetic temperatures.

Late Holocene climate dynamics in the Azores archipelago

The location of the Azores Archipelago makes this group of islands an excellent setting for investigating past long-term temperature and precipitation changes in the central North Atlantic region. Here, we present a chironomid-based quantitative temperature reconstruction and a record of oxygen isotope composition of chironomid head capsules for the last ca. 1200 years, based on the Lake Prata core (São Miguel Island, Azores). Chironomids from Lake Prata show that the temperature trend is aligned with the prevailing past global temperature trends, indicating warming during the Medieval Climate Anomaly (MCA; ca. 950–1300 CE) and subsequent cooling during the Little Ice Age (LIA; ca. 1300–1800 CE). Furthermore, changes in lake water oxygen isotope composition from chironomid head capsules were recorded with more positive δ18O values (18.2 ± 0.2‰) during the MCA and more negative values (varying between 14.8‰ and 17.6‰) during the LIA, indicating a positive shift in the precipitation-evaporation balance and/or heavier precipitation in winter relative to summer. Our study revealed that, in subtropical regions with a temperate oceanic climate and low thermal variation, there is a limited correlation between the isotopic composition of precipitation and temperature. Additionally, the temporal sequence of oxygen isotope values obtained from chironomid head capsules closely corresponds to variations in precipitation seasonality inferred from distinct climatic proxies. This consistency underscores the robustness of quantitative reconstructions and indicates that δ18Ochironomid records are promising for future investigations, especially in the context of lakes located on oceanic islands characterised by limited thermal variability.

Rapid decrease of the Labrador Sea’s influence on black spruce ecosystems with distance inland

In eastern Canada, Black spruce (Picea mariana Mill. B.S.P.) grows in a wide variety of climates, from maritime-oceanic conditions near the Labrador Sea, to more continental climates, inland. Along this gradient, timing and provenance of heat and moisture that support growth are uncertain, weakening our capacity to predict the response of boreal ecosystems to climate variability. Here, we measured the stable oxygen isotopic composition of black spruce tree-ring cellulose at three sites in eastern Canada and provide evidence of a rapid decrease of Labrador Sea’s influence on adjacent ecosystems. Our results report a landwards decrease in the oxygen isotope composition of both tree-ring cellulose (δ18OTRC) and precipitation water (δ18Op). We also reveal a rapid landwards decoupling between δ18OTRC variability (1950-2013), maximum temperature and Sea Surface Temperature variations over the Northwest Atlantic. Thus, despite their apparent ecological homogeneity, eastern Canada’s black spruce ecosystems rely on heterogeneous sources of heat and moisture.

Hydrogen and oxygen isotope compositions of bottom waters collected around off southwestern Tokara Islands

Hydrogen and oxygen isotope compositions of bottom waters collected around off southwestern Tokara Islands

Isotopic evolutionary track of water due to interaction with rocks and its use for tracing water cycle through the lithosphere

Hydrogen and oxygen isotopic compositions of natural spa waters from hot and/or mineral-rich springs (including flowing/pumping wells) potentially provide important clues about water cycle in Earth's interior, because some of the waters are deep-seated and non-meteoric. However, the isotopic compositions of these waters are typically altered by contamination with meteoric water and are thus poorly known. To reconstruct their original (i.e., before these waters mix with meteoric water) isotope compositions, here we develop an isotopic evolution model of the lithosphere-enclosed waters and derive the “ocean-origin lithospheric water curve” (OLWC; δ2H = 60 / (δ18O – 11)) as a typical evolutionary track starting from ocean water. Then we apply them to spa waters from central Japan, a globally unique geotectonic region where double subduction of slabs occurs. The OLWC explains well observed isotope data of subseafloor pore waters, submarine mud volcano pore waters, coastal oil-field brines, and subduction zone volcanic steam. Mixing analysis indicates that lithospheric water accounts for 19–95 % of each spa water we investigated. The nonlinear relationships of the reconstructed water δ18O and Cl or Li concentrations justify our hypothesis that the lithospheric waters are undergoing evolution from ocean water to magmatic water by rock-water isotopic exchange and that they are not simply mixtures of the two. A comparison of the reconstruction results with model predictions under slab subduction settings suggests that most spa waters in the southern to central part of the study region contain progressively evolving lithospheric waters released from the subducting Philippine Sea slab, and that a few at western edge of the region contain waters released from the Pacific slab. The others in the northern and eastern part of the region contain waters at the early-to-mid evolutionary stage, which are discharged from Tertiary marine sedimentary rocks (including tuffs). All these waters were enclosed in the lithosphere for more than 1–5 Ma. Our approach provides useful tools for tracing sources of non-meteoric water and highlights water cycle through lithosphere for more than one million years.

ENSO sentinels in the Americas' humid tropics: We need combined hydrometric and isotopic monitoring for improved El Niño and La Niña impact prediction

AbstractThis Scientific Briefing presents results from a nearly 10‐year hydrometric and isotope monitoring network across north‐central Costa Rica, a region known as a headwater‐dependent system. This monitoring system has recorded different El Niño and La Niña events and the direct/indirect effects of several hurricane and tropical storm passages. Our results show that El Niño‐Southern Oscillation (ENSO) exerts a significant but predictable impact on rainfall amount anomalies, groundwater level and spring discharge, as evidenced by second‐order water isotope parameters (e.g., line conditioned‐excess or line‐conditioned (LC)‐excess). Sea surface temperature anomaly (El Niño Region 3) is correlated with a reduction in mean annual and cold front rainfall across the headwaters of north‐central Costa Rica. During El Niño conditions, rainfall is substantially reduced (up to 69.2%) during the critical cold fronts period, limiting groundwater recharge and promoting an early onset of minimum baseflow conditions (up to 5 months). In contrast, La Niña is associated with increased rainfall and groundwater recharge (up to 94.7% during active cold front periods). During La Niña, the long‐term mean spring discharge (39 Ls−1) is exceeded 63–80% of the time, whereas, during El Niño, the exceedance time ranges between 26% and 44%. The regional hydroclimatic variability is also imprinted on the hydrogen and oxygen isotopic compositions of meteoric waters. Drier conditions favoured lower LC‐excess in rainfall (−17.3‰) and spring water (−6.5‰), whereas wetter conditions resulted in greater values (rainfall = +17.5‰; spring water = +10.7‰). The lower and higher LC‐excess values in rainfall corresponded to the very strong 2014–2016 El Niño and 2018 La Niña, respectively. During the recent triple‐dip 2021–23 La Niña, LC‐excess exhibited a significant and consistently increasing trend. These findings highlight the importance of combining hydrometric, synoptic and isotopic monitoring as ENSO sentinels to advance our current understanding of ENSO impacts on hydrological systems across the humid Tropics. Such information is critical to constraining the 21st century projections of future water stress across this fragile region.

Determination of water balance maintenance in Orcinus orca and Tursiops truncatus using oxygen isotopes.

The secondary adaptation of Cetacea to a fully marine lifestyle raises the question of their ability to maintain their water balance in a hyperosmotic environment. Cetacea have access to four potential sources of water: surrounding salt oceanic water, dietary free water, metabolic water and inhaled water vapour to a lesser degree. Here, we measured the 18O/16O oxygen isotope ratio of blood plasma from 13 specimens belonging to two species of Cetacea raised under human care (four killer whales Orcinus orca, nine common bottlenose dolphins Tursiops truncatus) to investigate and quantify the contribution of preformed water (dietary free water, surrounding salt oceanic water) and metabolic water to Cetacea body water using a box-modelling approach. The oxygen isotope composition of Cetacea blood plasma indicates that dietary free water and metabolic water contribute to more than 90% of the total water input in weight for cetaceans, with the remaining 10% consisting of inhaled water vapour and surrounding water accidentally ingested or absorbed through the skin. Moreover, the contribution of metabolic water appears to be more important in organisms with a more lipid-rich diet. Beyond these physiological and conservation biology implications, this study opens up questions that need to be addressed, such as the applicability of the oxygen isotope composition of cetacean body fluids and skeletal elements as an environmental proxy of the oxygen isotope composition of present and past marine waters.

A speleothem record of seasonality and moisture transport around the 8.2 ka event in Central Europe (Vacska Cave, Hungary)

AbstractA stalagmite was collected in northern Hungary from the Vacska Cave, where monitoring and ventilation-based site selection had been conducted. The stalagmite covers the 10–8 ka (relative to AD 1950) period, including the so-called 8.2 ka event, and showed preceding signs of climate change that were evaluated by petrographic observations, 14C activities, Sr concentrations, and stable isotope compositions of calcite and inclusion-hosted water. Comparisons of speleothem records show that isotope peaks at ca. 8.5 ka are related to a regional climate anomaly, rather than to a continental-scale event. In accordance with regional proxy records, the 8.2 ka event was associated with a series of temperature and precipitation amount changes, starting with cooling and a reduction in the winter-to-summer precipitation ratio, and then becoming a humid and warm phase at 8.15 ka. X-ray diffraction-based crystallinity parameter (FWHM) values provided evidence for diagenetic alteration of the stable oxygen isotope compositions of inclusion waters. Nevertheless, the stable hydrogen isotope compositions of inclusion waters and the oxygen isotope values of the host calcite revealed elevated d-excess values, and therefore increased Mediterranean moisture contribution during the 8.2 ka event, which indirectly indicate the southward displacement of moisture transport from the Atlantic Ocean.

The measurement of ambient air moisture stable isotope composition for the accurate estimation of evaporative losses

Accurate estimation of evaporative losses from a water body, using the Craig-Gordon model and the stable hydrogen and oxygen isotope composition of water, requires knowledge of the stable isotope composition of ambient air moisture. This is rarely measured in the field, and it is usually estimated assuming that recent rainfall remains in isotopic equilibrium with atmospheric moisture. However, the ambient air moisture stable isotope composition may vary significantly at different heights above the water body.In this study, we set up outdoor pan evaporation experiments and simultaneously measured the stable isotope composition of ambient moisture in the atmosphere at three different heights. Using these measurements, we calculated evaporative losses, compared them with the observed losses in the pan, and assessed the uncertainty introduced by differences in ambient moisture measurements.Three main steps in the experimental method:•Daily water sampling from the evaporation pan for analysis of stable hydrogen and oxygen isotope compositions.•Recording the stable isotope composition of ambient air moisture at three different heights using the Picarro L2130-i system over a period of experiments.•Calculating evaporative losses from the pan using the Craig-Gordon model and ambient air stable isotope composition measured at three different levels and comparing to the observed losses.

Examination of the parameters controlling the triple oxygen isotope composition of grass leaf water and phytoliths at a Mediterranean site: a model–data approach

Abstract. Triple oxygen isotopes (17O-excess) of water are useful to trace evaporation at the soil–plant–atmosphere interface. The 17O-excess of plant silica, i.e., phytoliths, inherited from leaf water, was previously calibrated in growth chambers as a proxy of atmospheric relative humidity (RH). Here, using a model–data approach, we examine the parameters that control the triple oxygen isotope composition of bulk grass leaf water and phytoliths in natura, at the O3HP experimental platform located in the French Mediterranean area. A grass plot was equipped to measure for 1 year, all environmental and plant physiological parameters relevant for modeling the isotope composition of the grass leaf water. In particular, the triple oxygen and hydrogen isotope composition of atmospheric water vapor above the grass was measured continuously using a cavity ring-down spectrometer, and the grass leaf temperature was monitored at plot scale using an infrared (IR) radiometer. Grass leaves were collected in different seasons of the year and over a 24 h period in June. Grass leaf water was extracted by cryogenic vacuum distillation and analyzed by isotope ratio mass spectrometry (IRMS). Phytoliths were analyzed by IR–laser fluorination–IRMS after chemical extraction. We showed that the traditional Craig–Gordon steady-state model modified for grass leaves reliably predicts the triple oxygen isotope composition of leaf water during daytime but is sensitive to uncertainties on the leaf-to-air temperature difference. Deviations from isotope steady state at night are well represented in the triple oxygen isotope system and predictable by a non-steady-state model. The 17O-excess of phytoliths confirms the applicability of the 17O-excessphyto vs. RH equation established in previous growth chamber experiments. Further, it recorded average daytime RH over the growth period rather than daily RH, related to low transpiration and silicification during the night. This model–data approach highlights the utility of the triple oxygen isotope system to improve the understanding of water exchange at the soil–plant–atmosphere interface. The in natura experiment underlines the applicability of 17O-excess of phytoliths as a RH proxy.

Caves demonstrate decrease in rainfall recharge of southwest Australian groundwater is unprecedented for the last 800 years

Billions of people worldwide rely on groundwater. As rainfall in many regions in the future is projected to decrease, it is critical to understand the impacts of climate change on groundwater recharge. The groundwater recharge response to a sustained decrease in rainfall across southwest Australia that began in the late 1960s was examined in seven modern speleothems and drip waters from four caves. These show a pronounced increase or uptick in regional drip water and speleothem oxygen isotopic composition (δ18O) that is not driven by a change in rainfall δ18O values, but is in response to the shallow karst aquifers becoming disconnected from rainfall recharge due to regional drying. Our findings imply that rainfall recharge to groundwater may no longer be reliably occurring in this region, which is highly dependent on groundwater resources. Examination of the longer speleothem record shows that this situation is unprecedented over the last 800 years.

Vacuum extraction of high-salinity water for the determination of oxygen and hydrogen isotopic compositions using cavity ring-down spectroscopy

Tracking the hydrogen and oxygen isotopic compositions of water samples can aid in understanding the water cycle. Compared to that of fresh water, analyzing the hydrogen and oxygen isotopic composition of high-salinity water is still a challenging task owing to the influence of dissolved salts. In this study, desalination was achieved by vacuum extraction, and the oxygen and hydrogen isotope ratios (δ18O and δ2H) were measured using cavity ring-down spectroscopy (CRDS). Different solutions of the individual salts (NaCl, KCl, MgCl2, and CaCl2) were prepared with deionized water (DI) having a known stable isotopic composition. After vacuum extraction, the maximum differences in isotopic composition was no more than 0.09‰ for δ18O and 0.30‰ for δ2H at 4 mol/L NaCl and KCl and 1 mol/L MgCl2 and CaCl2. The isotopic compositions of two high-salinity samples (IAPSO standard seawater and brine water) were determined by the proposed procedure as well as thermal conversion/elemental analyzer-isotope ratio mass spectrometry for comparison. The δ18O and δ2H results obtained using the two methods were consistent. The influence of isotope-salts was completely overcome by the proposed vacuum extraction desalination procedure. Thus, the method enables the rapid extraction of a large number of samples, regardless of the type and origin of the salt solutions, provided the concentrations of MgCl2 and CaCl2 are below 1.0 mol/L.

Water sources used by artificial Salix psammophila in stands of different ages based on stable isotope analysis in northeastern Mu Us Sandy Land

Understanding the water use pattern of artificially restored vegetation is important to provide reasonable water management strategies. However, the characteristics, controls and differences of water sources of planted Salix psammophila at different developmental stages remain poorly understood. In this study, the hydrogen and oxygen isotope compositions of xylem water, soil water, and rainwater were measured and coupled with the MixSIAR model to investigate the variations and controls of water use patterns of different aged S. psammophila in a semi-arid region in northeastern Mu Us Sandy Land. Results showed that >60% of water used by 6-yr S. psammophila derived from 0 to 120 cm soil layer throughout the growing season due to the relatively shallow roots (0–100 cm). In contrast, water sources used by 12- and 18-yr S. psammophila differed markedly between the dry and rainy seasons, which were attributed to the developed dimorphic root systems and seasonal changes in soil water profile. In the dry season, 12- and 18-yr S. psammophila extracted 71.93% and 68.91% of water from 120 to 300 cm and 40–200 cm soil layer, respectively; in the rainy season, however, they both shifted main water sources to 0–120 cm soil layer (65.09% and 56.14%). The severe soil desiccation and dead roots observed in the 18-yr stand decreased the ability of old S. psammophila to extract water from the deep soil layer (200–300 cm) in the dry season. Thus, changes in the vertical distribution of roots and soil water contents controlled the seasonal water use pattern of different aged S. psammophila. Our study demonstrates that the age-dependent water use pattern of S. psammophila should be considered to implement the corresponding management measures in future afforestation plans to avoid overuse of water and maintain sustainable vegetation restoration in the water-limited desert regions.

Simulation of Hydrologic Change of Linggo Co during 1979–2012 Using Hydrologic and Isotopic Mass Balance Model

The Tibetan Plateau (TP) and surrounding areas contain the largest number of glaciers outside the polar regions. The region affects downstream water supply and food security, thereby directly influencing one-third of the world’s population. The lakes in the central TP expanded rapidly in recent decades, which has attracted growing attention. Glacier meltwater was considered as a major component in the water balance of TP lakes, although few studies quantified its contribution. Stable isotope analysis is a powerful tool to trace hydrologic circulation, while its interpretation in paleoclimate records has been controversial. To bridge the gap between hydrologic and paleoclimatic studies, we performed a hydrologic and isotopic mass balance model to simulate the lake level change of Linggo Co in the central TP. The model was forced by the meteorological data, calibrated through observed lake level changes, and validated by oxygen isotope compositions (δ18O) of lake water. Our results indicated that glacier meltwater contributed 73.94% of the inflow water to Linggo Co before 1993 but decreased thereafter. Increasing glacier meltwater together with positive water balance (precipitation/evaporation) in the catchment contributed to the rapid expansion of Linggo Co after the mid-1990s. Lake water δ18O in Linggo Co was more sensitive to changes in the precipitation amount and precipitation δ18O than temperature. Our findings could shed light on the usage of δ18O proxy in future paleoclimate research on the TP.

Clumped Isotope Thermometry in Plant‐Derived Carbonates

AbstractThe genus Celtis includes widespread trees that produce drupes with aragonite endocarps, or “hackberries.” These carbonate endocarps are preserved in the fossil record, often in cave deposits or packrat middens, and thus are targets for paleoclimate reconstructions. Stable oxygen isotopes in Celtis endocarps have been used as proxies for oxygen isotopic composition of past stream water and for paleothermometry. Here, we explore the suitability of hackberry carbonates for paleoclimate reconstructions based on carbonate clumped‐isotope thermometry. We sampled modern hackberries grown at sites across North America (n = 37) for stable and clumped isotope analyses. Measured clumped‐isotope temperatures are found to be within the range of measured local modern growing season surface temperatures and typically in dual clumped‐isotope equilibrium. As such, we propose that hackberry clumped‐isotope measurements can be used to reconstruct past Earth‐surface air temperatures.

Micrometallogeny and hydrothermal fluid evolution of the Iju porphyry Cu deposit, NW Kerman, Iran: Evidence from fluid inclusions, Laser Raman spectroscopy, and S[sbnd]O isotope systematics

The Iju Cu porphyry is located in the NW part of the Kerman Magmatic Copper Belt (KMCB). It is related to a ~ 9 Ma granodiorite porphyry intrusion, with three main stages of hydrothermal activity. The homogenization temperatures for the fluid inclusions are in the ranges of 200–494 °C, and their salinities vary from 4.0 to 42.8 wt% NaCl equiv., which are typical magmatic-hydrothermal fluids. The δ34S values of sulfides range from −0.4 to +3.2 ‰ (V-CDT), and the δ34S values of anhydrite samples range from +11.6 to +16.8 ‰. The δ34S values of sulfides show a narrow range, implying a homogeneous sulfur source. The oxygen isotopic composition of hydrothermal water in equilibrium with quartz samples ranges from +3.4 to +6.0 ‰ (V-SMOW) consistent with the hydrothermal fluids having a magmatic signature, but diluted with meteoric waters in the main mineralizing stage. The most important factors responsible for metal precipitation in the Iju porphyry deposit are fluid boiling, oxygen fugacity decrease and cooling followed by dilution with meteoric water. The primary fluids of the Iju Cu deposit are characterized by relatively high temperature and moderate salinity, and are CO2-rich, indicating a typical post-collisional porphyry system.

Transformation mechanism of methylphosphonate to methane by Burkholderia sp: Insight from multi-labeled water isotope probing and transcriptomic

Methylphosphonate (MPn), has been identified as a likely source of methane in aerobic ocean and may be responsible for the “ocean methane paradox”, that is oversaturation of dissolved methane in oxic sea waters. However, the mechanism underlying the cleavage of C–P bonds during microbial degradation is not well understood. Using multi-labeled water isotope probing (MLWIP) and transcriptome analysis, we investigated the phosphate oxygen isotope systematics and mechanisms of microbial-mediated degradation of MPn in this study. In the aerobic culture containing MPn as the only phosphorus source, there was a significant release of inorganic phosphate (149.4 μmol/L) and free methane (268.3 mg/L). The oxygen isotopic composition of inorganic phosphorus (δ18OP) of accumulated released phosphate was 4.50‰, 23.96‰, and 40.88‰, respectively, in the corresponding 18O-labeled waters of −10.3‰, 9.9‰, and 30.6‰, and the slope obtained in plots of δ18OP versus the oxygen isotopic composition of water (δ18OW) was 0.89. Consequently, 89% of the oxygen atoms (Os) in phosphate (PO4) were exchanged with 18O-labeled waters in the medium, while the rest were exchanged with intracellular metabolic water. It has been confirmed that the C–P bond cleavage of MPn occurs in the cell with both ambient and metabolic water participation. Moreover, phn gene clusters play significant roles to cleave the C–P bond of MPn for Burkholderia sp. HQL1813, in which phnJ, phnM and phnI genes are significantly up-regulated during MPn decomposition to methane. In conclusion, the aerobic biotransformation of MPn to free methane by Burkholderia sp. HQL1813 has been elucidated, providing new insights into the mechanism that bio-cleaves C–P bonds to produce methane aerobically in aqueous environments for representative phosphonates.

Changes in the oxygen isotope composition of the Bering Sea contribution to the Arctic Ocean are an independent measure of increasing freshwater fluxes through the Bering Strait.

A large volume of freshwater is incorporated in the relatively fresh (salinity ~32–33) Pacific Ocean waters that are transported north through the Bering Strait relative to deep Atlantic salinity in the Arctic Ocean (salinity ~34.8). These freshened waters help maintain the halocline that separates cold Arctic surface waters from warmer Arctic Ocean waters at depth. The stable oxygen isotope composition of the Bering Sea contribution to the upper Arctic Ocean halocline was established as early as the late 1980’s as having a δ18OV-SMOW value of approximately -1.1‰. More recent data indicates a shift to an isotopic composition that is more depleted in 18O (mean δ18O value ~-1.5‰). This shift is supported by a data synthesis of >1400 water samples (salinity from 32.5 to 33.5) from the northern Bering and Chukchi seas, from the years 1987–2020, which show significant year-to-year, seasonal and regional variability. This change in the oxygen isotope composition of water in the upper halocline is consistent with observations of added freshwater in the Canada Basin, and mooring-based estimates of increased freshwater inflows through Bering Strait. Here, we use this isotopic time-series as an independent means of estimating freshwater flux changes through the Bering Strait. We employed a simple end-member mixing model that requires that the volume of freshwater (including runoff and other meteoric water, but not sea ice melt) flowing through Bering Strait has increased by ~40% over the past two decades to account for a change in the isotopic composition of the 33.1 salinity water from a δ18O value of approximately -1.1‰ to a mean of -1.5‰. This freshwater flux change is comparable with independent published measurements made from mooring arrays in the Bering Strait (freshwater fluxes rising from 2000–2500 km3 in 2001 to 3000–3500 km3 in 2011).

Stalagmite evidence for Early Holocene multidecadal hydroclimate variability in Ethiopia

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

The Isotope Composition, Nature, and Main Mechanisms of Formation of Different Types and Subtypes of Salt Lakes in Transbaikalia

Abstract —More than a hundred lakes, groundwater, rivers, and precipitation of their catchments have been studied in Transbaikalia. The chemical composition of natural waters and the mineral composition of rocks and lacustrine bottom sediments have been analyzed. The isotope composition of waters, the isotope ratios in dissolved and deposited carbonates and in bacterial mats, and the content of heavy oxygen in the aluminosilicate fraction of bottom sediments and rocks in catchments have been determined. It is shown that all secondary minerals inherit the oxygen isotope composition of water, but their oxygen is isotopically much heavier than the water oxygen, because the water–rock interaction leads to the fractionation of oxygen and the transition of its heavy isotope into secondary carbonates and aluminosilicates. Bacterial mats using oxygen of carbon dioxide ions for their vital activity are also enriched in its heavy isotope. We have established that the water of soda lakes, being less saline than other lake waters, is more enriched in heavy oxygen isotope. Mineral analysis of lacustrine bottom sediments and material from sedimentation traps revealed chemogenic carbonates and secondary aluminosilicates. The performed thermodynamic calculations confirm their possible formation in lakes. It is shown that the lake water is in equilibrium not only with carbonates, clays, and hydromica but also with zeolites, and the most alkaline and mineralized water is in equilibrium with albite. The bottom sediments of soda lakes have a greater portion of clays and carbonates than those of fresh, chloride, and sulfate waters. The enrichment of salt lake waters with heavy isotopes is due not only to evaporation but also to the water–rock interaction. The contribution of aluminosilicate hydrolysis to oxygen fractionation is confirmed by the direct dependence of the “oxygen shift” value on pH, whose value is determined by the degree of the above interaction. We have established that the diversity of the chemical and isotope compositions of lake waters is due to their multifactorial formation related to the different degrees of lake water evaporation, the different mineralization of organic matter, and the different duration of the interaction of lake waters and groundwater with rocks.