Variation of O18 content of waters from natural sources
Abstract
A number of marine water and fresh water samples were examined for the relative ratio, and the variation of this ratio was determined to a precision of ± 0.1%. In the case of surface marine waters, for a range of salinity of 29.40%., the O16 content varies over a range of approximately 6%. The low ratios were obtained from surface marine waters contaminated with meltwater from the ice fields, while the marine waters of high salinity were richest in O18. The observed relation between O18 content and salinity of the oceanic waters can be explained by a process of multiple stage distillation which produces a continuous loss of fresh water to the ice regions from the surface waters of the warm oceans. The lower salinities of cold ocean currents, such as the Alaskan and Californian currents, are due primarily to mixing with meltwater from cold regions. The effect of glaciation upon the isotopic method of measuring paleotemperatures is discussed. The results for deep oceanic samples and for non-typical water samples are also discussed.
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Distinguishing between non-magmatic and magmatic hydrothermal systems coexisting in a magmatic geothermal belt is of great significance. Shidian, located in the Yunnan-Sichuan-Tibet geothermal belt (YST), is representative of those hydrothermal areas unexplored or explored to a low degree. The hydrochemical types of the Shidian geothermal waters reflect the lithology of the reservoirs there. Lack of acid sulfate springs and much lower chloride concentrations in the neutral-alkaline springs are the major differences of the Shidian hydrothermal area from the Rehai hydrothermal area. Typically, co-existence of Na-Cl type, Na-HCO3 type, and H-SO42− type hot springs in a geothermal system can be indicative of magmatic heat source, which is not the case in Shidian. The water isotopic compositions of the Shidian geothermal waters are controlled by water-rock interactions and their mixing with shallow groundwaters. In contrast, the enrichment of oxygen-18 and deuterium in the Rehai geothermal waters is the result of magmatic fluid input and adiabatic cooling of parent geothermal fluid (PGF) during its ascent. Moreover, the reservoir temperatures in Shidian calculated using chemical geothermometers range from 70 to 118 °C, much lower than those in Rehai. To sum up, a combination of the hydrochemical, water isotopic, and reservoir temperature data shows little possibility of the existence of a magmatic heat source in Shidian. The ways to identify the heat source of a hydrothermal system used in this study are helpful for further exploration and exploitation of not only the Shidian system but also the other unexplored systems in YST.
Significant role of permafrost in regional hydrology of the Upper Indus Basin, India
2024, Science of the Total EnvironmentUpper Indus Basin (UIB), being climatologically sensitive and socio-economically important, has emerged as a hotspot for eco-hydrological studies. Permafrost, one of the essential components of the regional hydrological cycle with a critical role in microclimate, is also an important water resource in the UIB. Despite being an important component of the cryospheric system, permafrost is least studied in the UIB. In present study, we used stable oxygen and hydrogen isotopic composition in supra-permafrost water (SPFW) and aufeis along with precipitation, snowpack, glacier and other groundwaters to assess their variability and estimate their contribution to regional hydrology. The sources are evolving isotopically, depending on physiographic and hydrometeorological factors, with each source attaining different (if not distinct) isotopic signatures. The isotopic signatures (with different ranges) of sources help in estimating the contribution from these sources. A significant altitude gradient of δ18O is observed in stream water, SPFW and other groundwaters. Isotopic composition in SPFW is differentially modulated by fractionation, resulting in isotopic variability from the source waters. The results suggest snowmelt and/or glacier melt as the source of SPFW. To stream flow, SPFW is the dominant contributor (43 ± 18 %) at higher elevations (> 4300 m a.m.s.l.) in July, followed by snowmelt (26 ± 10 %). In September, SPFW contribution decreases (14 ± 8 %), but the contribution from other groundwaters becomes dominant (39 ± 11 %) to stream flow. The results indicate the significant role of seasonal thawing and freezing of active layer on the contribution from SPFW. This study highlights the significant role of permafrost in the hydrological system of the basin. The study also emphasizes the need to understand the dynamics of permafrost, taliks of various types (e.g., supra-permafrost subaerial talik) and active layer under changing climate to define the subsequent implications to regional hydrology, eco-hydrological systems and micro-climate of permafrost regions.
Unravelling the sources contributing to the urban water supply: An isotope perspective from Ljubljana, Slovenia
2024, Journal of HydrologyIn cities experiencing rapid urbanization, we must continually update our understanding of the partitioning of drinking water sources concerning its supply if it is to be managed sustainably. This need is especially crucial given the pressure on water resources arising from evolving land use patterns and climate change. For this reason, a city-wide study of stable water isotopes (δ2H and δ18O) in precipitation, surface water and groundwater across Ljubljana, Slovenia, was undertaken. The goal was to characterise the temporal dynamics of urban water cycling and trace the various sources contributing to the city’s drinking water supply. Monthly water sampling, combined with hydrogeochemical and in-situ data, permitted the identification of local precipitation and surface water contributions to its two groundwater supply aquifers. In addition, a re-examination of the mean residence times (MRT) of surface waters revealed an MRT of 3–4 years, which is much longer than previously reported. Also, changes in the contributions of surface water and precipitation to groundwater were observed compared to previous studies. These findings improve our understanding of local water partitioning and provide valuable insights for water managers addressing future urban water resource management.
Assessment of hydraulic connectivity among shallow and deep aquifers and surface water of Central Gujarat: An isotopic approach
2024, Applied Radiation and IsotopesThe primary objective of this study is to identify the surface water - groundwater interaction and interaction among groundwater at different depths using stable isotopes (δ18O and δ2H). The Fence diagram of the study area describes the subsurface geology where the main aquifer system includes both confined and unconfined in the VRB area and is complex multi-layered with different sources of recharge. The flow pattern of groundwater is generally from the highland of north-east to low-lying south-west of VRB and is indicated by the piezometric surface map of pre-monsoon, and post monsoon, 2016. The δ2H (‰ VSMOW) and δ18O (‰ VSMOW) values do not show any symmetric distribution among them. The values for shallow groundwater ranges between −23.0‰ and 1.9‰, and −3.2‰–2.3‰, respectively with a mean value for δ18O and δ2H are −1.26‰ and −11.194‰ respectively. The depth wise interpretation revealed that the locations Virpur and Nana Pura show connectivity. Therefore, deep and shallow aquifers are contaminated due to this connectivity. The plots of d-excess vs. electrical conductivity, d-excess vs δ18O and electrical conductivity vs. δ18O indicate that most of the groundwaters are primarily influenced by evaporation process, high mineral dissolution and enrichment of δ18O. The plot of δ18O vs. nitrate indicate the anthropogenic influences like agricultural activities in this region. The SI values of calcite and dolomite indicates oversaturated condition along with dissolution of these minerals in the aquifer matrix.
Moisture sources and climatic effects controlling precipitation stable isotope composition in a western Mediterranean island (Pianosa, Italy)
2023, Atmospheric ResearchThe Mediterranean basin is indicated as a hot spot of climate change, which is an area whose climate is especially responsive to variations. The insular environment is one of the most threatened by the current climate change, especially in terms of drought events, with serious consequences for water scarcity and water stress. This issue is even enhanced in small islands, whose ecosystems are among more sensitive to climatic changes and water availability. The stable isotope composition of hydrogen (δ2H) and oxygen (δ18O) in precipitation is globally recognized as a powerful natural tracer in the water cycle and represents the starting point to investigate hydrological processes. The understanding of the prevailing factors that drive the isotopic variability of precipitation in the Mediterranean is therefore essential to unravel the hydrological processes and to ensure proper and sustainable management of potentially vulnerable resources to climate change. Here, we discuss the results of multi-year isotopic monitoring in the period 2014–2021 of monthly precipitation collected on Pianosa Island (Italy), a small island located in the northern Tyrrhenian (western Mediterranean). The lower slope and intercept of the Local Meteoric Water Line of the island compared to the Global Meteoric Water Line indicated warmer and drier climatic conditions, suggesting the existence of sub-cloud evaporation processes of raindrops during precipitation, especially in summer. The mean δ18O of precipitation was lower with respect to other sites placed at higher elevation in this Mediterranean region, due to the lack of summer precipitation which were generally enriched in heavy isotopes. Temperature and amount effects may explain part of the δ18O variability observed at the monthly and seasonal scale. An HYSPLIT-based moisture uptake analysis indicated the area between the western Mediterranean basin, Italy, and the Adriatic Sea as the region that supplied most of the humidity associated with monthly precipitation samples on Pianosa Island. Less moisture was picked from the northwestern areas of Europe, the North Atlantic Ocean, the proximal Atlantic Ocean, the Iberian Peninsula and North Africa. Consistently with the rainout effect, the higher the moisture fraction picked from the more proximal regions, the more positive the δ18O of precipitation occurring on Pianosa Island; conversely, the higher the percentage of moisture sourced from more distal regions, the more negative the δ18O. A multiple linear model was proposed to predict the δ18O of monthly precipitation from temperature, precipitation amount and moisture origin data, which explained 45% of the δ18O variability. The deuterium excess variability on the island was partly controlled by the local climatic variables, whose effect potentially modifies the original d-excess signature imprinted at the moisture source. No relationship was found between the precipitation deuterium excess and moisture sources, suggesting that more attention should be paid when using the deuterium excess as a tracer of moisture origin, especially in the Mediterranean.
Hydrogeological controls on microbial activity and habitability in the Precambrian continental crust
2024, Geobiology
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Present address: Division of Geological Sciences, California Institute of Technology, Pasadena, California.