Negative Isotope Values Research Articles

Biochemically induced diagenesis of Jurassic micrite: evidence from phase analysis, carbon, oxygen, and strontium isotopes (Franconian Alb, Germany)

The marine Upper Jurassic rocks of the Franconian Alb consist largely of micritic carbonate of partly dolomitized reef mounds and bedded basinal limestone. All carbonates were lithified in the shallow (centimeters, meters) subsurface and have a wide range of ∂13C (≤ + 3‰ to − 10‰VPDB) but always negative ∂18O (− 1 to − 6‰VPDB). Dolomite and reef limestone show the highest ∂18O and ∂13C values. The most negative ∂13C (≥ − 10‰) occurs mainly as cement in dolomite of a basinal, partly dolomitic, biostrome interval. Basinal limestone shows intermediate ∂13C values. Because freshwater diagenesis and elevated temperatures cannot explain the observed isotope values, pH is here considered a major factor influencing the isotope signal of micritic limestone. The bulk sediment isotope signal was reset to lower values, from an original lime mud with ∂13C ≥ 3‰ and a ∂18O of ≥ + 1‰, as a result of biochemically induced diagenesis. Carbonate, probably mostly aragonite but occasionally including dolomite, was dissolved in a zone where low pH developed as a result of organic matter degradation. Dissolved carbonate was translocated by diffusion and re-precipitated as cement (ca. 50vol%) in a zone with elevated pH where all in situ lime mud ∂18O was reset. Imported cement carbonate precipitated in equilibrium with the pore fluid with negative isotope values, whereas ∂13C of the in situ lime mud remained unmodified. The negative shift of the bulk ∂13C and ∂18O is variable and depends on pH and the contribution of 12C from anaerobic sulfate reduction in the zone of cement precipitation. This produced an ubiquitous covariance of ∂18O and ∂13C. Incorporation of seawater-derived Mg2+ during recrystallization of carbonate can account for the local dolomitization. Elevated 87Sr/86Sr ratios are explained as a result of interaction of clay minerals with the stationary pore fluids. This study shows that the isotopic signal produced by biochemically induced shallow submarine subsurface carbonate diagenesis can be indistinguishable from freshwater diagenesis, that ∂18O and ∂13C of the bulk rock are always reset, and that carbonates can show, in the presence of clay minerals, elevated 87Sr/86Sr ratios even when the pore fluids were never exchanged.Graphical

The silicon isotopic compositions of silica sinters in Xizang, China: Implications for paleo-geothermal activities since 0.5 Ma B.P.

The diagenetic sequence from opal-A to opal-CT has been observed in geothermal systems worldwide. Xizang, a unique geothermal region in the world, has experienced several large-scale geothermal activities since 0.5 million years ago. The Targejia and Gulu geothermal systems were investigated in this study. The time for transformation from opal-A to opal-CT varies, indicating that the rates of silica phase transformation depend on specific conditions; however, their silicon isotopic compositions are not significantly different, ranging from −1.0‰ to −0.3‰ in Targejia and from −1.1‰ to −0.1‰ in Gulu. Additionally, silica sinters undergo varying degrees of diagenesis, as supported by different time periods and varying degrees of ordering. The δ30Si values of different phases, opal-A and opal-CT, also did not significantly change, ranging from −1.1‰ to −0.3‰ for opal-A and from −1.0‰ to −0.1‰ for opal-CT. Due to the lower dissolution rates of sediments, high aluminum content, and reduced reactivity of silica sinters, the δ30Si values of silica sinters are less susceptible to secondary modifications, and the initial information after the silica sinters formation is well preserved. Hence, the isotopic signatures of silica sinters reflect various depositional environments and can be used to reconstruct paleo-geothermal activity. The narrow range and slightly negative isotopic values of silica sinters suggest that large-scale high-temperature geothermal activity has occurred in different regions since 0.5 Ma. The diagenesis of silica sinters may have been influenced by postlarge-scale high-temperature geothermal activity. Such large-scale, high-temperature and long-term natural eruptions in geothermal fields are typically associated with magmatic activity, possibly caused by crustal remelting processes.

Spring tropical cyclones modulate near-surface isotopic compositions of atmospheric water vapour in Kathmandu, Nepal

Abstract. While westerlies are recognized as a significant moisture transport in Nepal during the pre-monsoon season, precipitation is also attributed to moisture from cyclones originating in the Bay of Bengal (BoB) or the Arabian Sea (AS). Tropical cyclones exhibit negative isotopic values in both precipitation and atmospheric water vapour; however, the factors influencing isotopic fractionation during tropical cyclones remain poorly understood. We present the results of continuous measurements of the isotopic composition of atmospheric water vapour (δ18Ov, δDv, and d-excessv) in Kathmandu from 7 May to 7 June 2021 during two pre-monsoon cyclones: cyclone Tauktae, formed over the Arabian Sea, and cyclone Yaas, formed over the Bay of Bengal. Our study reveals that tropical cyclones originating from the BoB and the AS during the pre-monsoon season modulate isotopic signals of near-surface atmospheric water vapour in Nepal. Comparing conditions before and after, we observed a significant depletion of δ18Ov and δDv during both cyclones, attributed to changes in moisture sources (local vs. marine). Convective activity plays a pivotal role in the variability of δ18Ov and δDv during both cyclones, confirmed by the spatial variations of outgoing longwave radiation (OLR) and regional precipitation during both cyclones. We also found a significant negative correlation between δ18Ov and/or δDv and rainfall amount along the trajectories during cyclone Tauktae, probably resulting from integrated upstream processes linked to the earlier Rayleigh distillation of water vapour via rainfall rather than local rainfall. The decrease in δ18Ov and/or δDv during cyclone Yaas is associated with the intensified convection and moisture convergence at the measurement site, while the lower cloud top temperatures (CTTs) and lower cloud top pressure (CTP) during intense convection contribute to higher d-excessv values during the final stage of cyclone Yaas. This characteristic is missing during cyclone Tauktae. Our results shed light on key processes governing the isotopic composition of atmospheric water vapour in Kathmandu with implications for the monsoon moisture transport and paleoclimate reconstructions of tropical cyclone activity.

Isotopic variability in tropical cyclone precipitation is controlled by Rayleigh distillation and cloud microphysics

Tropical cyclones produce rainfall with extremely negative isotope values (δ18O and δ2H), but the controls on isotopic fractionation during tropical cyclones are poorly understood. Here we studied the isotopic composition of rainfall at sites across central Texas during Hurricane Harvey (2017) to better understand these processes. Rainfall δ18O trend towards more negative values as a result of Rayleigh distillation of precipitation-generating airmasses as they travel towards the center of the storm. Superimposed on these gradual changes are abrupt isotopic shifts with exceptionally low deuterium excess values. These appear to be controlled by microphysical processes associated with the passage of spiral rainbands over the sampling locations. Isotope-enabled climate modeling suggests that it may be possible to identify the signature of tropical cyclones from annually resolved isotopic proxy records, but will depend on the size of the storm and the proximity of the site to the core of the storm system.

Spatiotemporal variability in stable isotopes of the Ganga River and factors affecting their distributions

Although the Ganga is an important fluvial system of India, the isotopic investigations of its water are limited and not reported for the whole length of the river. This limits the understanding of the hydrological processes of the river whose flow characteristics have been changed considerably over the years due to changes in the climate and land use/land cover patterns of the region. This study intends to fill this gap of data and knowledge. Hence, a robust isotope datasets were generated for a period of 2–3 years from 11 locations covering the entire length of the river (2250 km). These data were further analysed to study the controls on spatiotemporal patterns of river water isotopes and understand dominance of different hydrological processes affecting flow characteristics of the river in different reaches. The stable isotopes of oxygen and hydrogen in river water exhibited large spatial and temporal variation throughout the study periods. The most negative isotopic values (mean δ18O: −15‰ to −9.7‰) between 0 and 318 km in the mountainous region during pre-monsoon period attributes to the dominant glaciers melt contribution while the altitude effect in rainfall is mainly responsible during monsoon season. However, less negative isotopic values (mean δ18O: −9.7‰ to −4.3‰) between 318 and 1000 km correspond to the mixing of water from major tributaries. Other hydrological factors responsible for the increased isotopic values include evaporative enrichment and contribution of isotopically less negative groundwater. The observed relatively low isotopic values (δ18O: −4.3‰ to −6.9‰) in the downstream of 1000 km of the river are due to joining of tributaries originating from the Nepal Himalayas. Results substantiate that distinct isotopic values found in different reaches of the river are because of the variations in basin characteristics, hydro-meteorological processes, and water mixing. These findings would contribute in developing a better knowledge on hydrological behaviour of the Ganga River and help in taking appropriate measures for maintaining its sustainable flows.

Characterization of groundwater dynamics and contamination in an unconfined aquifer using isotope techniques to evaluate domestic supply in an urban area

In the urban area of Rio Cuarto groundwater is used for domestic supply. The objective of the current research was to investigate the water dynamics and nitrate contamination in an unconfined aquifer system of Rio Cuarto River basin. Stable isotopes of water (δ18O and δ2H) and nitrate (δ15N–NO3 and δ18O–NO3) were used and combined with conventional chemical techniques and mixing modelling approaches to determine the recharge areas and identify the main origin of nitrate pollution. More enriched water isotope values were recorded in Rio Cuarto city than in the piedmont and mountains indicating local recharge for well batteries 2 and 3. The well battery 1 and the infiltration gallery showed more negative isotopic values, demonstrating a strong influence by the recharge from the piedmont sector (impoverished groundwater). The δ15N–NO3 values and the Bayesian modeling showed that the dominant nitrate contamination source in the urban area is the on-site sanitation systems whereas in the peri-urban area nitrate contamination originates mostly from animal wastes. Both sources supply the aquifer with anthropogenic organic matter (>50%). The highest δ15N–NO3 values were correlated with low dissolved oxygen values, indicating the occurrence of denitrification processes in some places. High NO3 ions in the rural sector were attributed to the application of fertilizers and consequent nitrification processes. The samples from Rio Cuarto river located upstream of the Rio Cuarto City and close to the infiltration gallery showed the highest input from fertilizers (~40%), due to surrounding agricultural fields. The municipal well batteries showed good water quality (freshwater of low to extremely low nitrate concentration) which is linked to the high hydraulic conductivity of the aquifer that increases its capacity to dilute the contaminants. These results will be useful for local water administrators to improve water management.

Orbitally driven evolution of Asian monsoon and stable water isotope ratios during the Holocene: Isotope-enabled climate model simulations and proxy data comparisons

The variation of stable water isotopes (δ18O, δD) in climate archives is an important proxy to understand the evolution of South Asian monsoon (SA) precipitation over the Holocene. In this study, using an isotope-enabled climate-model, we examine the responses of water isotopes in precipitation over the SA region to orbital changes in the early to late Holocene (8 ka to 0 ka). Precipitation is enhanced during 8 ka to 4 ka in response to increased summertime insolation, and correspondingly, larger magnitudes of negative water isotope ratios in precipitation (δ18Oprecip) are simulated. The model-simulated wettest period and the corresponding period of maximum depletion of δ18Oprecip in the SA region is the 8 ka period. We find that strengthened circulation, increased convection, and precipitation led to the δ18Oprecip depletion in the SA region. In the tropical Indian monsoon region, where convective precipitation is dominant, the δ18Oprecip values are inversely correlated with the local convection and the amount of precipitation. The δ18Oprecip values in the East Asian Summer Monsoon (EASM) region are not well-correlated with local precipitation, likely due to the enhanced convection and depletion of vapor in upstream areas and mixed precipitation types in the region. The modeled δ18Oprecip values are evaluated against δ18O inferred from proxy archives. Proxy data and model-simulated isotope ratios in precipitation agree on the larger magnitude of negative isotopic values during the 4 ka, 6 ka, and 8 ka periods relative to the present, although model-simulated enrichment in the 2 ka period is inconsistent with some proxy-records.

Geochemical characterisation of the alkaline and hyperalkaline groundwater in the Othrys Ophiolite Massif, central Greece

The complex geology of Greece includes two important parallel running ophiolitic belts. The Othrys Massif in central Greece belongs to the westernmost of them. In the current study, 33 water samples from cold hyperalkaline and hypothermal (T < 40°C) alkaline springs and 30 gas samples (either dissolved or free) were collected at 17 different sites in and around this wide ophiolite outcrop, aiming to determine the origin of fluids and evidence gas-water-rock interaction processes taking place in the area. Water samples were analysed for their chemical (major ions and trace elements) and isotope (δ18O-H2O, δ2H-H2O) composition. They can be subdivided into alkaline (pH 11 and Ca-OH composition). Trace elements generally showed very low concentrations and mostly inversely correlated with pH. Gases were analysed for their chemical (He, Ne, Ar, H2, O2, N2, CH4, C2H6, CO2 and H2S) and isotope (δ13C-CH4, δ2H-CH4, δ13C-CO2) composition. Samples from alkaline waters were mainly dominated by CH4 (from 128,000 to 915,000 μmol/mol), while hyperalkaline waters showed a N2-rich composition (from 727,000 to 977,000 μmol/mol). Methane had a wide range of isotope compositions (δ13C-CH4 from -74.5 to -14.5 ‰ and δ2H-CH4 from -343 to -62 ‰). Alkaline waters present the most negative isotope values for CH4, evidencing a biogenic (both thermogenic and microbial) origin. Many of the hyperalkaline waters had CH4 isotope values compatible with an abiogenic origin through serpentinization processes but occasionaly very negative values were recorded, indicating sometimes a clear biogenic contribution. Finally, few samples both from alkaline and hyperalkaline waters showed some evidence of secondary oxidation processes.

Regional palaeoenvironmental influence on organic matter sequestration and characteristics of carbon isotope segment C5 in a hemipelagic sequence, Organyà Basin, northeast Spain

AbstractThe Cretaceous marine sedimentary record is punctuated by episodes of unusually high rates of organic carbon burial termed oceanic anoxic events. The El Pujal Section archives the response to perturbations of the global carbon cycle associated with Oceanic Anoxic Event 1a in a hemipelagic setting of the Organyà Basin. Here, the aim is to understand the lithological and geochemical responses to changing palaeoenviromental conditions associated with carbon isotope segment C5 of Oceanic Anoxic Event 1a. A multi‐proxy approach is applied to characterize how regional factors influenced the accumulation of organic‐rich sediments during this period of severe oceanic oxygen deficiency. Source specific biomarkers demonstrate that the bulk organic matter derives from in‐situ production, with lesser inputs from allochthonous sources. High‐resolution stable carbon isotope data (δ13Corg) were used to establish the refined details of segment C5. Pulses of fluvial fluxes associated with climate fluctuations supplied variable quantities of terrestrial organic matter with more negative isotopic values, which left their intrinsic signature superimposed on the global δ13Corg signal of segment C5 as minor negative spikes. Primary production sustained by fluvial inputs of biolimiting elements provided abundant labile organic matter conducive to oxygen‐deprived conditions as attested by concomitant peaks in total organic carbon, relatively lower benthic faunal counts, lower bioturbation index, higher concentration of pyrite and enrichments in redox sensitive trace elements. However, organic matter preservation was not chiefly controlled by redox conditions as physical encapsulation by clay minerals probably played a significant role. In addition, coincident inputs of terrestrial organic matter with more inert properties and prone to be bound to clay minerals supplemented the accumulation of organic matter, especially during intervals of enhanced terrigenous fluxes. The combined results underscore how despite sharing common global forcing factors, regional palaeoenvironmental conditions are more important in determining the sedimentary expression of Oceanic Anoxic Event 1a.

Barium isotope systematics of subduction zones

Subduction zones are the focal points of mass transfer between the surface and deep Earth. Despite their significance, there remains substantial debate regarding the specific mechanisms of material transport from the slab to the overlying magmatic arc. Broadly, models accounting for slab material transport focus on the relative sequence of events promoting arc volcanism and, in particular, whether mobilization of the down-going slab leads or lags mixing with the mantle wedge. To address these uncertainties, we outline the utility of barium (Ba) isotope mass balance in subduction zones as a means to test different slab material transport models. Barium is a highly fluid-mobile element that is significantly enriched in arc magmas and is thus thought to be a sensitive tracer of slab material transport in arcs.We also present qualitative Ba isotopic mass balances for two well-characterized subduction zones—the Aleutian and Ryukyu magmatic arcs—by analyzing the Ba isotope systematics of their respective subduction inputs and outputs. Despite the narrow (and similar) Ba-isotope range of slab inputs to both systems, we find that erupted magmas exhibit systematic variations indicative of a small negative isotope fractionation during Ba mobilization (≈20–40 ppm AMU−1). We suggest that AOC (altered oceanic crust) is not the principal source of these negative isotope values using other geochemical parameters (e.g., Rb/Ba, Pb isotopes), and infer that the Ba isotope composition of AOC—though contributing a minor amount of Ba in these systems—is isotopically heavier than the overlying sediment package and the depleted mantle. Altogether, these findings are significant as they indicate that the magnitude of isotope fractionation associated with Ba mobilization is small relative to the likely isotopic contrast between subduction inputs in other subduction zones, such as beneath areas of strong ocean upwelling (e.g., South Sandwich, Kamchatka). Thus, we propose that the Ba isotope composition of erupted arc magmas holds great promise for constraining the importance of different slab components, which could help address uncertainties regarding the mechanism of slab material transport in subduction zones.

Spatial and temporal variability of stable isotopes (δ18O and δ2H) in surface waters of arid, mountainous Central Asia

AbstractCharacterization of spatial and temporal variability of stable isotopes (δ18O and δ2H) of surface waters is essential to interpret hydrological processes and establish modern isotope–elevation gradients across mountainous terrains. Here, we present stable isotope data for river waters across Kyrgyzstan. River water isotopes exhibit substantial spatial heterogeneity among different watersheds in Kyrgyzstan. Higher river water isotope values were found mainly in the Issyk‐Kul Lake watershed, whereas waters in the Son‐Kul Lake watershed display lower values. Results show a close δ18O–δ2H relation between river water and the local meteoric water line, implying that river water experiences little evaporative enrichment. River water from the high‐elevation regions (e.g., Naryn and Son‐Kul Lake watershed) had the most negative isotope values, implying that river water is dominated by snowmelt. Higher deuterium excess (average d = 13.9‰) in river water probably represents the isotopic signature of combined contributions from direct precipitation and glacier melt in stream discharge across Kyrgyzstan. A significant relationship between river water δ18O and elevation was observed with a vertical lapse rate of 0.13‰/100 m. These findings provide crucial information about hydrological processes across Kyrgyzstan and contribute to a better understanding of the paleoclimate/elevation reconstruction of this region.

Geochemical features of the Menteşe Formation dolostones (Rhaetian) in the Karacahisar-Kasımlar area (Isparta-Turkey)

The purpose of this study is to determine the geochemical characteristics of the Mentese dolostones (Rhaetian) in the west Taurus Mountains, Turkey. The isotope characteristics of the Mentese dolostones show a variation in the δ18O values of the dolomites between + 1.35 and − 2.72‰ (VPDB) and a variation in the δ13C values of the dolomites between + 1.6 and + 3.33‰ (VPDB). Most of the Mentese dolostone samples have positive oxygen isotope values; others have weak negative isotope values, which indicate early diagenetic development. In addition, oxygen, carbon, and strontium isotopes and trace element analyses suggest that these dolostones were probably resulted from insignificant recrystallizaion process than chemically similar fluids. Furthermore, Mn/Sr values of the Mentese dolostones indicate that there is no significant diagenetic alteration. The total REE contents of the Mentese Formation dolostones display a strong positive correlation with Si, Al, Fe, K, Ti, Ba, Rb and negative correlation with Mg, hence REE content is associated with in the first degree titanium-bearing minerals (probably sphene, rutile, anatase), second degree phosphate (apatite), and barium and rubidium-bearing minerals (probably barite). Ce anomalies versus Nd concentrations of the Mentese dolostone samples indicate that these dolostones are formed in an anoxic environment. In addition, the K/Rb ratios of the Mentese dolostones are not exposed to any metamorphism and indicate some diagenetic alteration. Calculated temperature values using δ18O values of the Mentese dolostones yield precipitation temperatures of 17 to 42 °C, and suggest burial depths of 67 to 900 m. Sources of magnesium for dolomitization of the Mentese Formation carbonates were most likely fluids from seawater and/or diagenetic brines (slightly modified seawater). The Mentese dolostones formed as early diagenetic (Rhaetian to Cenomanian) in shallow burial areas at lower temperatures than seawater and/or diagenetic brines (slightly modified seawater).

Distribution of stable isotopes in water from an alpine river in China

AbstractThe Hailuogou River, on the south-eastern edge of the Tibetan Plateau, receives a substantial portion of its flow from meltwater. The stable isotopic composition and water temperature were observed for waters collected from the main stream and selected tributaries. The results indicate that the river water is generally more depleted in 18O and 2H than groundwater, but more enriched than meltwater. The river water in the upper reaches is characterised by more negative isotopic values, the isotopic fingerprint of meltwaters, and lower temperatures. Both δ18O and δ2H increase significantly during passage from the upper basin downward, at least partly due to enrichment from tributary-inputs. There are also an altitude δ18O-effect of −0.1‰ per 100-m rise, and an altitude δ2H-effect of −0.8‰ per 100-m rise, along the main stream. The positive correlation between isotopic composition and water temperature further highlights the role of meltwater in regulating the river's isotope hydrology. The fraction of meltwater inputs over the total river flow ranges from 67% at Section 11 to 89% at Section 7, in the proglacial headwater region. The tributary-input enrichments and ice-snow melting are the main mechanisms controlling the isotopic composition of river runoff, but depending strongly on altitude.

Petrologic and geochemical attributes of calcite cementation, dolomitization and dolomite recrystallization: an example from the Mississippian Pekisko Formation, west-central Alberta

Abstract Carbonate rocks of the Pekisko Formation make up an important reservoir in west-central Alberta, especially in fields along the Pekisko subcrop edge. They represent a transgressive-regressive carbonate platform sequence comprised of upward shallowing facies, which subsequently underwent extreme erosion leading to the development of karst topography. As a result, diagenetic alteration, mainly through dolomitization and karstification, has affected reservoir characterization for most of the carbonate facies. Several generations of calcite cementation and dolomite are the result of complex diagenetic changes. Calcite cements include isopachous fibrous, equant drusy mosaic, pendant/meniscus, blocky spar, syntaxial, fibrous, and bladed. These cements formed during early and late diagenetic events; pre- syn- and post exposure in shallow and deeper burial realms. There are five types of dolomite identified in the Pekisko Formation, based on petrographic and geochemical analyses: 1) pervasive, fine to coarse crystalline, subhedral to anhedral replacive dolomite;2) void-filling, coarse crystalline, euhedral dolomite cement;3) selective, fine to coarse crystalline, euhedral to anhedral dolomite; 4) dissolution seam-associated, fine crystalline, euhedral dolomite; and 5) saddle dolomite. Dolomite types 1), 3) and 4) are interpreted to have formed early in the diagenetic history and subsequently recrystallized, whereas void-filling, coarse crystalline, euhedral dolomite and saddle dolomite formed later in deeper burial setting. Petrographic evidence for recrystallization of dolomite types, excluding void-filling and saddle dolomite, includes: etching, displayed mainly on euhedral crystals; dissolved cores on many crystals of varying shapes; non-planar crystal boundaries, exclusively in pervasive dolomites; and coarsening crystal size, evident in both pervasive and selective dolomite types. Geochemical evidence, such as pronounced negative shift in oxygen isotopes (by up to 8‰ VPDB) and enrichment of radiogenic Sr isotopes further support this interpretation. There is a definite negative trend whereby wells closest to the subcrop edge have the most negative isotopic values and those farthest away show the least depletion. This trend in δ18O isotope values points to recrystallization of the earlier formed dolomites.

Isotopic effects of different preservation methods on scales of olive ridley sea turtles (Lepidochelys olivacea) from the Mexican Central Pacific.

Stable isotope analysis can be used to obtain information on olive ridley sea turtles in the Mexican Central Pacific (MCP). Tissue samples such as scale, muscle, and blood are usually not analyzed immediately and are preserved in different substances such as ethanol, formalin, sodium chloride, the ideal being liquid nitrogen. Due to logistical reasons, the latter preservation method cannot always be used. The objective of the present study was to evaluate the efficiency of dimethyl sulfoxide (DMSO) as a preservation agent for later stable isotope analysis. Forty-eight paired scale samples of olive ridley were collected in MCP waters and preserved in DMSO and liquid nitrogen. We determined their δ13 C and δ15 N values using a Carlo Erba 1108 elemental analyzer coupled to a ThermoFinnigan Delta Plus XP isotope ratio mass spectrometer (precision ±0.2‰ for each stable isotope). The mean isotopic ratios of each preservation method were compared. Significant differences were observed between the isotopic ratios of samples in the two preservatives. Samples preserved in DMSO resulted in more negative isotopic values than those in liquid nitrogen, and a correction factor was calculated. The correction factors were 2.1‰ for δ13 C values and 0.8‰ for δ15 N values. DMSO is not equivalent to liquid nitrogen for the preservation of olive ridley scales to be used for isotopic analysis. If DMSO has to be used for logistical reasons a replicate sample should also be collected to be able to estimate a correction factor. Copyright © 2016 John Wiley & Sons, Ltd.

δ18O and δD variations in Holocene massive ice in the Sabettayakha river mouth, northern Yamal Peninsula

The conditions of formation of massive ice near the South Tambey gas-condensate field in northern Yamal Peninsula are studied. It is shown that massive ice bodies up to 4.5 m thick occur in the Holocene deposits of the high laida and the first terrace. Therefore, they cannot be the remains of glaciers; they are ground ice formations. All three types of massive ice have quite various isotopic compositions: the values of δD range from–107 to–199.7, and δ18O from–15.7 to–26.48‰. Such a significant differentiation in isotopic composition is a result of cryogenic fractionation in a freezing water-saturated sediment. The most negative isotope values are even lower in this Holocene massive ice than in the Late Pleistocene ice-wedge ice of Yamal Peninsula.

Lithalsas in the Sentsa River Valley, Eastern Sayan Mountains, Southern Russia

AbstractPerennial frost mounds identified as lithalsas occur in the Sentsa River valley of the Eastern Sayan Mountains, Russia. We report the first detailed study of permafrost in this region, based on analysis of the cryostructure and distribution of stable isotopes of oxygen and hydrogen, and ion concentrations from the ice‐rich cores of two lithalsas 3–7 m high. Their main cryostructures are reticulate and lenticular, with visible ice contents exceeding 50–60%. The vertical and lateral distribution of δ18О and δD values show a step‐by‐step mechanism of lithalsa growth: in the first stage a large lithalsa formed, and in the second stage a small lithalsa formed. The more negative isotope values of ice in the large lithalsa ice probably resulted from gradual release of isotopically depleted water from the central to peripheral part of the massif during ice segregation in water‐saturated fine‐grained lake sediments. Minor variations of isotope values suggest intensive recharge of lake‐fen and meteoric water supply during freezing and lithalsa growth. Copyright © 2015 John Wiley &amp; Sons, Ltd.

The O and H isotope characteristics of water from major rivers in China

Isotopic compositions of the water sources and evapotranspiration in large rivers of China are not well explored. In this investigation, δ18O and δD values of water from major large rivers of China are integrated to understand the spatial and temporal variations in the isotopes of water and d-excess. River water samples were collected from the channels and tributaries of the Changjiang, Yellow, Songhua, and Liao Rivers in China. Water from upstream was generally enriched with light isotopes relative to that from downstream areas. Water from the Songhua River and upper reach of the Changjiang River has relatively negative isotopic values, impacted by latitude and altitude effects, respectively. The spatial pattern showed that δ18O-H2O values in rivers mainly ranged from −9 to −7 ‰ in most of China based on roughly mapped contours, and rivers enriched in light isotopes were primarily located in the Tibetan Plateau and northeast China. The slopes of linear regression from the Yellow and Liao Rivers suggested that the loss of water by direct evaporation shifted the isotopes of water, especially in some small tributaries. Additionally, there was a narrow range of low δ18O-H2O values from the main channel of rivers during the low-flow season relative to the high-flow season due to influence of the monsoon climate. No distinct spatial variation of d-excess values was found in the rivers of China. However, water samples in some rivers with low d-excess values had high δ18O values, indicating that evaporation in the tributaries has led to significant water loss, especially in tributaries of the Yellow and Liao Rivers.

Tracer-based analysis of spatial and temporal variations of water sources in a glacierized catchment

Abstract. Snow-dominated and glacierized catchments are important sources of fresh water for biological communities and for populations living in mountain valleys. Gaining a better understanding of the runoff origin and of the hydrological interactions between meltwater, streamflow and groundwater is critical for natural risk assessment and mitigation as well as for effective water resource management in mountain regions. This study is based on the use of stable isotopes of water and electrical conductivity as tracers to identify the water sources for runoff and groundwater and their seasonal variability in a glacierized catchment in the Italian Alps. Samples were collected from rainfall, snow, snowmelt, ice melt, spring and stream water (from the main stream at different locations and from selected tributaries) in 2011, 2012 and 2013. The tracer-based mixing analysis revealed that, overall, snowmelt and glacier melt were the most important end-members for stream runoff during late spring, summer and early fall. The temporal variability of the tracer concentration suggested that stream water was dominated by snowmelt at the beginning of the melting season (May–June), by a mixture of snowmelt and glacier melt during mid-summer (July–early August), and by glacier melt during the end of the summer (end of August–September). The same seasonal pattern observed in streamflow was also evident for groundwater, with the highest electrical conductivity and least negative isotopic values found during cold or relatively less warm periods, when the melt of snowpack and ice was limited. Particularly, the application of a two-component mixing model to data from different springs showed that the snowmelt contribution to groundwater recharge varied between 21% (±3%) and 93% (±1%) over the season, and the overall contribution during the three study years ranged between 58% (±24%) and 72% (±19%). These results provided new insights into the isotopic characterization of the study catchment presenting further understanding of the spatio-temporal variability of the main water sources contributing to runoff.

The geochemistry of biogenic carbonates as a paleoenvironmental tool for the Lower Pleistocene Barranco León sequence (BL-5D, Baza Basin, Spain)

The Barranco (creek) León section is located in the NE marginal area of the lacustrine Plio-Pleistocene Baza Basin (Southern Spain). The Barranco León BL-5D mammal and archeological site has delivered abundant lithic tools and a hominin tooth; the age ascribed to this site is comprised between the Olduvai and Jaramillo subchrons, close to 1.4 Ma, based on magnetostratigraphy, biostratigraphy and ERS dating. The studied BL-5 section consists from base to top, of dark sandy mudstones and lutites with interbedded sandy marls. The lower part of the dark sandy mudstones contains a brackish fauna of foraminifera, ostracods, and mollusks. Sandy carbonate mudstones and sandy lutites with freshwater-oligosaline ostracods and mollusks form the upper part of the archeological excavation sequence. Stable isotope (δ13C, δ18O) and trace element analyses on ostracod and mollusk shells allow depicting the paleoenvironmental conditions recorded in the lacustrine sequence. Moreover, indications on provenance for waters have been obtained from 87Sr/86Sr ratios from shells and opercula from mollusks. The isotopic composition of the biogenic carbonates shows a strong correlation between δ13C and δ18O, which is noticeable for some gastropod shells. This correlation is a characteristic feature of closed lake settings. In our case, the more negative isotopic values reflect the input of meteoric waters and the more positive ones the skeletal carbonates formed in more isotopically concentrated waters. Some of the more negative isotopic values correspond to biogenic carbonates from the BL-5D level. They correspond to isotopically diluted waters, although the trace element data and the invertebrate fauna indicate oligosaline to mesosaline waters. Moreover, the presence in BL-5D of shells from Melanopsis tuberculata, a thermophilous gastropod, together with their isotopic signatures suggest the input of oligosaline waters with a thermal influence. The available geochemical data indicate a first stage of varying influence of several water types under changing water level in a closed lake recorded in the base of the studied sequence (brackish fauna level). The overlying beds record a through flowing open lacustrine environment mainly fed by meteoric surface and ground waters with significant inputs of stream waters.