Isotopic Composition Of Pore Water Research Articles

Vadose zone water stable isotope profiles for assessing groundwater recharge: Sensitivity to seasonal soil sampling

Groundwater recharge is widely recognized as being the most important parameter for the sustainable management of water resources. In semiarid environments, groundwater recharge can be quantified using the piston displacement method (PDM). From a single soil sampling campaign, the PDM relies on linking the deeper vadose zone soil pore water stable isotope composition (δ2HH2O and δ18OH2O) to the local meteoric water line (LMWL). However, the isotopic composition of precipitation changes seasonally, influencing the water isotope composition of the vadose zone over time. Thus, it is important to test whether the PDM is sensitive to seasonal soil sampling and whether the assessed recharge rate is independent of the time of sampling. This study investigates the effect of seasonal soil sampling on the distribution of vadose zone stable isotope composition to determine whether the sampling time influences the estimate of recharge rate from PDM. Soil samples were obtained along vertical profiles through the vadose zone in a semiarid region during the spring, summer, and autumn seasons. Specifically, the δ2HH2O and δ18OH2O of the soil pore water were determined along vertical profiles, and the PDM was applied to quantify the annual recharge. The δ2HH2O and δ18OH2O values range from −7.3 to −3.5 ‰ and from −54.4 to +7.41 ‰, respectively, and plot along a continuum with a slope less than the LMWL. Samples from deeper in the vadose zone profile had distinct ranges in isotopic composition between the three soil sampling campaigns, with isotopic composition of spring sampling dominated by lower values and those from autumn with higher values. Despite these differences, the resulting annual recharge rates from the different sampling campaigns are comparable (1.5 to 2 % of annual precipitation). Even though the pore water isotopic composition changed over time, the shift between the deeper vadose zone isotopic compositions and the LMWL remained relatively constant, leading to a similar recharge estimate over time. Therefore, the PDM-based recharge assessment in the tested semiarid environment is independent of the sampling time, which indicates that sampling for assessing groundwater recharge can be undertaken during any season.

Calcium isotopic fractionation during aragonite and high-Mg calcite precipitation at methane seeps

The formation of authigenic carbonate in marine environments represents a process that buffers ocean chemistry and atmospheric carbon dioxide levels. The isotopic composition of calcium (δ44/40Ca) in authigenic carbonate can be used to investigate the calcium (Ca) cycle, seawater chemistry, and diagenesis of carbonate rocks, archiving key information on the evolution of the Earth's surface environments. Laboratory experiments have shown that Ca isotopic fractionation during carbonate precipitation – the basis of the δ44/40Ca proxy – is mainly determined by both mineral phase and precipitation rate. However, the precipitation rate of submarine authigenic carbonate is much lower than rates in laboratory experiments. This study investigated the Ca isotopic composition of pore water and carbonate nodules collected from two gravity piston cores and one push core from the Haima seeps in the northern South China Sea. Methane fluxes calculated for the three cores range from 12 to 134 µmol cm−2 yr−1. The core with intermediate methane flux showed evidence of aragonite precipitation, while high-Mg calcite was the dominant mineral phase in the other two cores as indicated by Mg/Ca and Sr/Ca ratios of pore water. Numerical modeling of pore water geochemistry revealed that: (1) precipitation rates of carbonate per unit reactive surface area range from 0.05 to 21 µmol m−2 h−1, which is one to three orders of magnitude lower than rates in precipitation experiments; (2) Ca isotopic fractionation for aragonite precipitation is –1.8 ± 0.3 ‰, close to the reported equilibrium isotopic fractionation in precipitation experiments; and (3) Ca isotopic fractionation for high-Mg calcite precipitation is similar for the two cores (−0.9 ± 0.3 ‰ and −0.8 ± 0.3 ‰), although the modeled precipitation rates differed by two orders of magnitude. These results indicate that mineralogy rather than precipitation rate is the primary control on Ca isotopic fractionation during carbonate precipitation at seeps, resulting in lower δ44/40Ca values of aragonite (0.91±0.06 ‰, N = 4) than of high-Mg calcite (1.41±0.04 ‰, N = 9) reported relative to NIST SRM915a. This study documents distinguishable Ca isotopic fractionation during the formation of methane-derived high-Mg calcite and aragonite, providing fundamental parameters for further exploration of the calcium isotopic composition of marine authigenic carbonates in the geological record.

Authigenesis of carbonates in the deposits of the gas-hydrate-bearing structure of the CHAOS (Sea of Okhotsk)

The paper presents the results of studies of authigenic carbonates sampled in the near-surface sediments of the gas-hydrate-bearing CHAOS structure located on the western slope of the Deryugin Basin in the Sea of Okhotsk. Carbonates were at different stages of formation, the most common morphological forms of which were concretions. Microscopic studies have shown that the concretions are formed by pelitomorphic carbonate cementing the terrigenous clay-clastic matrix and organic detritus. According to X-ray phase analysis, the dominant mineral composing nodules is cryptocrystalline high-Mg calcite (14.2-16.9 mol% Mg). Based on the geochemical modeling the intervals of the sediments with favorable conditions for the formation of basic carbonate minerals (aragonite, calcite, and dolomite) were identified. Almost over the entire area of the structure, the formation of carbonates occurs at more than 1m and up to 5m sub-bottom depths. In the central part of the structure (station LV31-27GC) the formation of carbonates is also possible in the upper part of the sedimentary section - from 0 to 1,5 m. On the basis of balance calculations of the δ13C isotopes content in nodules, the contribution of the main carbon sources involved in their formation was estimated. These sources turned out to be organic matter and microbial methane in a ratio of 67.5: 32.5. It was found that the conversion of carbon dioxide into bicarbonate ion occurs under conditions of early diagenesis due to the interaction of the fluid with aluminosilicates. Calculations of the isotopic composition of pore waters (the main source of oxygen in carbonates) and paleotemperatures indicate crystallization of carbonates at low (close to 0 ° C) temperatures. Based on 230Th/U-dating of carbonates, the age of the studied unloading source was determined as ~ 3.5 thousand years. It was found that the CHAOS structure is one of the “youngest” on the northeastern slope of the Sakhalin Island.

Effects of ultrafiltration on salt migration and isotopic composition of pore water in groundwater depression area in Hengshui, NCP

AbstractClay aquitards are semipermeable membranes that allow groundwater flow while retarding solute migration has been researched extensively but also subjected to much debate. At present, there is no evidence of whether the physical and chemical properties of clay soil and the isotopic composition of pore water affect the semipermeable membrane effect. In this study, we collected clay samples from drilling cores (30–90 m) in the Hengshui area located in the North China Plain (NCP), then extracted pore water using a high‐pressure squeezing device. Vertical hydrochemical and isotopic profile variation trends for the pore water were revealed using hydrochemical (Cl−, Na+, Ca2+, K+, Mg2+, and SO42−) and stable isotopic measurements of H, O and Cl. The results showed that the hydrochemical clay interlayer pore water of the saline aquifer is Cl/SO4‐Na/Mg type and the average total dissolved solids (TDS) are 10.17 g/L. However, the hydrochemical clay aquitard pore water is of the Cl/SO4‐Na/Ca type with an average TDS of 1.90 g/L. The hydrochemical clay interlayer pore water of aquifer II is of Cl‐Na/Ca type with an average TDS of 1.10 g/L. Our results showed that the water quality of the aquifer II is not affected by the upper part of the saline aquifer, thus the clay aquitard acts as a significant barrier to salt movement. A polarization layer concentrated in ions was formed between the upper part of the saline aquifer and the clay aquitard. The concentration polarization layer increases the salt‐inhibition effect. H, O and Cl isotopic composition results showed significant fractionation. The pore water of aquifer II lacked heavy isotopes (2H, 18O, 37Cl), but had significant heavy isotope enrichment in the concentrated polarized layer (the δ2H value was −76‰, the δ18O value was −8.4‰, and the δ37Cl value was 1.59‰). Hyperfiltration thus played a significant role in isotope fractionation.

Can Soil Hydraulic Parameters be Estimated from the Stable Isotope Composition of Pore Water from a Single Soil Profile?

Modeling water and solute transport in the vadose zone for groundwater resource management requires an accurate determination of soil hydraulic parameters. Estimating these parameters by inverse modeling using in situ observations is very common. However, little attention has been given to the potential of pore water isotope information to parameterize soil water transport models. By conducting a Morris and Sobol sensitivity analysis, we highlight the interest of combining water content and pore water isotope composition data in a multi-objective calibration approach to constrain soil hydraulic property parameterization. We then investigate the effect of the sampling frequency of the observed data used for model calibration on a synthetic case. When modeling is employed in order to estimate the annual groundwater recharge of a sandy aquifer, it is possible to calibrate the model without continuous monitoring data, using only water content and pore water isotopic composition profiles from a single sampling time. However, even if not continuous, multi-temporal data improve model calibration, especially pore water isotope data. The proposed calibration method was validated with field data. For groundwater recharge estimate studies, these results imply a significant reduction in the time and effort required, by avoiding long-term monitoring, since only one sampling campaign is needed to extract soil samples.

Pore water isotope fingerprints to understand the spatiotemporal groundwater recharge variability in ungauged watersheds

AbstractReliable groundwater recharge quantification at the regional scale (e.g., watershed or subwatershed) is fundamental to sustainable water resource management. Although modeling at the watershed scale is gaining wide support, the long‐term monitoring needed for model calibration is often not readily available, as many watersheds worldwide remain ungauged. In response to this situation, we propose a new approach to estimate groundwater recharge at the watershed scale. This approach is fast and accurate and takes into account the existing variability without requiring long‐term monitoring. Only a single field campaign to acquire soil water content and pore water isotopic composition depth profiles is needed. The principle is to extend a physically based, one‐dimensional unsaturated zone flow model from the local (i.e., profile) to the watershed scale, using an index method for distributed recharge based on a GIS. The methodology was validated in a gauged watershed, where previous studies have estimated recharge using a spatialized water balance model calibrated using long‐term discharge monitoring data. Scaling was investigated by comparing recharge values obtained using the local‐scale approach at 10 study sites within the watershed with coinciding values obtained at the watershed scale. Recharge values were similar in terms of both dynamics and quantity. Using the pore water isotopic fingerprint of ungauged watersheds is therefore confirmed to be a suitable approach for understanding spatiotemporal recharge variability.

The effect of extraction techniques on calcium concentrations and isotope ratios of marine pore water

ABSTRACTComparing two different techniques applied for the extraction of marine pore water samples from sediments, the well-established whole round (WR) method and the more recent Rhizon method, in terms of their effects on stable calcium isotope ratios in extracted pore waters, we recognize a systematic offset between the two sampling methods. Higher δ44/40Ca values are associated with lower Ca concentrations for the Rhizon sampling technique and lower δ44/40Ca values are associated with higher Ca concentrations for the corresponding WR-derived pore water samples. Models involving Rayleigh fractionation and mixing calculation suggest that the observed offset is most likely caused by a combined process of CaCO3 precipitation and ion exchange taking place during Rhizon sampling-induced CO2 degassing. Changing pressure, extraction time or extraction yield during WR pressing does not lead to a variation in δ44/40Ca, indicating that no Ca isotope fractionation takes place during the sampling of pore water. On the basis of analytical and modelling results, WR samples appear to provide δ44/40Ca values that are more representative of the ‘true’ pore water isotopic composition. While the difference between the sampling techniques is close to the present-day analytical precision of Ca isotope analysis, it may become more relevant with increasing analytical precision in the future.

Wine origin authentication linked to terroir – wine fingerprint

Global wine and alcohol trade faces a serious economic problem linked to counterfeiting of these commodities. Recently applied authentication methods and techniques pose more difficulties for counterfeiters but they are apparently not effective once we consider economical losses identified by EU legal authorities. The presented solution links isotopic characteristics of the soil, plant, technological intermediate product and the final food product (wine, grapes) on the basis of 87Sr/86Sr isotopes ratios. For the isotopic signature of wines, the average isotope composition of the substrate cannot be a reliable indicator. Only the isotopic composition of pore water can, as it leaches various mineral phases at different stages and passes into vine root system. Instead of complicated sampling of pore water, an original method of preparing and processing soil samples and consequently must & wine samples was developed. Based on both, soil and biological material analysis, we can unquestionably determine not only geographical but also regional and local authenticity of the wine. Determination of red wines isotopic signature is more straightforward process in comparison to white wines, because of technologically different processing of grapes. That is the reason why, in case of white vines, the 87Sr/86Sr ratio of bentonites (natural purifier and absorbent useful in the process of winemaking) must also be taken into consideration. Results of analyses of Slovak wines from geographically diverse regions as well as from sites in close-by distances have clearly established reliability of presented concept, in which the soil is linked to the plant and to the final food product (wine or table grapes).

Carbon diagenesis in different sedimentary environments of the subtropical Beibu Gulf, South China Sea

Abstract The depositional environment in the subtropical Beibu Gulf area is highly complex, and the sediments accumulate under specific combinations of hydrodynamics, physiography and sediment sources. In the present study, six sedimentary gravity cores were collected from three different depositional zones of the Beibu Gulf. We measured the geochemical and carbon isotopic compositions of pore water and solid sediments to characterize the benthic early diagenesis of carbon. Our results indicate that marine organic matter is rapidly remineralized in delta deposits and the Central Beibu Gulf. Terrestrial organic matter, however, seem to be more important at the bottom of the Central and Northern Coastal Beibu Gulf, which probably formed in an estuarine or fluvial environment before the Holocene or Pleistocene transgression. Organic matter degradation via microbial sulfate reduction is the most important source for isotopically light DIC (dissolved inorganic carbon) added to the pore waters at most sites. In the Central Beibu Gulf, however, the carbon isotope signatures of DIC indicate an additional source of light carbon and pore water profiles exhibit a characteristic pattern reflecting the occurrence of the anaerobic oxidation of methane. Moreover, we find that the pore water δ13Cadded is greater than the δ13C of total organic carbon net loss in solid sediments in the zones with relatively strong current environments. This result hints that the frequent physical reworking and reoxidation of bottom deposits are likely key factors controlling carbon isotopic fractionation between the sedimentary organic carbon and DIC. Overall, our results further reveal that carbon sources and transformations in the sediments sensitively respond to changes in depositional conditions. Pore water modeling give strong hints for non-steady state processes impacting diagenesis at several sites.

Gas hydrate estimates in muddy sediments from the oxygen isotope of water fraction

Gas hydrate in deep-sea sediments is a potential energy resource, and has been the focus of extensive drilling research. However, direct evaluation of the amount of the gas hydrate in marine sediments has been difficult because the gas hydrate in recovered sediment cores is at least partly dissociated due to the drop in pressure and increase in temperature during onboard recovery. In this study, we apply a new method based on oxygen isotopic composition of the H2O fraction of both hydrate and mud sub-samples (δ18OH and δ18OM, respectively) in order to evaluate the volume percentage of gas hydrate in core sections collected from the Japan Sea off Joetsu and Oki, which contain different fabrics of the hydrate within hemipelagic mud. We measured isotopic composition of CO2 equilibrated with H2O of the sub-samples of a small size (typically 0.3cm3) carefully separated from the core sediments and sealed in glass vials. The volume percentage of gas hydrate (H in %) was determined using porosity of the mud sub-samples and oxygen isotopic composition of the bulk pore water (δ18OPW) squeezed from a certain length of core sediment (typically 20cm) including dissociated hydrate. 28 out of the 29 examined core sections indicate the relation in the isotopic values of the three components, δ18OH>δ18OPW>δ18OM, as expected from isotopic fractionation that enriches 18O in the hydrate component. Evaluated H-values of the 28 sections ranged from 1.0% to 95.4% and, for most of the section, the H-value was clearly larger than the value estimated by the hydrate distribution on core images. Our new method can, in a simple manner, correct for the underestimation of hydrate amount caused as a result of dissociation during core handling. Our oxygen isotopic data of the hydrate and mud sub-samples fits poorly with the isotopic evolutional curve that assumes Rayleigh fractionation in a closed system. This implies that the pore water isotopic composition may have been homogenized by diffusion and advection of less 18O-depleted pore water from the surrounding sediments. Presence of micro-scale hydrate in the mud matrix was suspected for some sections from the Joetsu site, which present a small difference between δ18OH and δ18OM as well as high CH2/CO2 ratios in headspace gas. We suggest that this method, if carried out with careful and quick onboard sampling, is appropriate for the estimation of gas hydrate as an energy resource based on the amount of hydrate present in marine mud.

Br/Cl, I/Cl and chlorine isotopic compositions of pore water in shallow sediments: implications for the fluid sources in the Dongsha area, northern South China Sea

The Dongsha area is one of the most promising target areas for gas hydrate exploration in the South China Sea (SCS). The study of pore water geochemistry has played a key role in Chinese gas hydrate exploration. Br/Cl, I/Cl and δ37Cl in pore water were applied here in tracing gas hydrate occurrence, chemical evolution of pore fluids and water/rock interactions in low temperature sediment environments. The samples were collected from Sites HD255PC and HD309PC in the Dongsha area in 2004. At Site HD255PC, we found the elevated Br/Cl, I/Cl and decreased SO4/Cl at the depth of 4–5 m, suggestive of a laterally migrated fluid probably generated from the gas hydrate occurrence. The range of δ37Cl is–0.54‰ to +0.96‰, and positive δ37Cl at 4–5 m interval should be related with different diffusion rates between 35Cl and 37Cl. At Site HD309PC, a laterally migrated fluid was also found at the depth of 3–4 m, with the Br/Cl two times to that of the seawater and decreased I/Cl, indicating the fluid has no relationship with the gas hydrate. In this site, the chlorine isotopic composition varies from–0.7‰ to +1.9‰. Extra high Br/Cl might relate with the deep generated fluid. At higher temperature and pressure, the Br/Cl of the fluid is elevated during the hydrous silicate formation, while positive δ37Cl is also associated with the same mechanism.

Microbial metabolism inferred from chemical and isotopic compositions of pore water around bananas discovered on the deep-sea floor in the Tenryu Submarine Canyon

Microbial metabolism inferred from chemical and isotopic compositions of pore water around bananas discovered on the deep-sea floor in the Tenryu Submarine Canyon

Intercomparison of soil pore water extraction methods for stable isotope analysis

AbstractMeasurements of δ2H and δ18O composition of pore waters in saturated and unsaturated soil samples are routinely performed in hydrological studies. A variety of in‐situ and lab‐based pore water extraction methods for the analysis of the stable isotopes of water now exist. While some have been used for decades (e.g. cryogenic vacuum extraction) others are relatively new, such as direct vapour equilibration or the microwave extraction technique. Despite their broad range of application, a formal and comprehensive intercomparison of soil water extraction methods for stable isotope analysis is lacking and long overdue. Here we present an intercomparison among five commonly used lab‐based pore water extraction techniques (high pressure mechanical squeezing, centrifugation, direct vapour equilibration, microwave extraction, and cryogenic extraction). We applied these extraction methods to two physicochemically different soil types that were dried and rewetted with water of known isotopic composition at three different water contents. Our results showed that the extraction approach can have a significant effect on pore water isotopic composition as all methods exhibited significant deviations from the spiked reference water, depending secondarily on the soil type and soil water content. Most pronounced, cryogenic water extraction showed large deviations from the spiked reference water, whereas mechanical squeezing and centrifugation provided results closest to the spiked water for both soil types. We also compared results for each extraction method – where liquid water was obtained – on both an OA‐ICOS and IRMS. Differences between these two analytical instruments were negligible for these organic compound‐free waters. We suggest that users of soil water extraction approaches carefully choose an extraction technique that is suitable for the specific research question, adapted to the dominant soil type and water content of the study. Copyright © 2016 John Wiley & Sons, Ltd.

Hydrochemical reactions and origin of offshore relatively fresh pore water from core samples in Hong Kong

Summary The existence of relatively fresh pore water offshore has been well recognised over the globe but studies on the chemistry of the pore water from offshore geological formations are extremely limited. This study aims to characterize the hydrochemistry of the submarine groundwater body in Hong Kong. It looks into the major ion concentrations and the stable isotopic compositions of pore water extracted from core samples from an offshore 42.30-m vibrocore in the southwestern Hong Kong waters. A minimum Cl − level of about one-third of that in typical seawater was noted in the terrestrial sediments, suggesting the presence of offshore relatively fresh water. Unexpectedly high NH 4 + levels are attributed to organic matter decomposition in the terrestrial sediments. The leaching of shells due to exposure of marine sediments at sea-level low stands raises the Mg 2+ and Ca 2+ concentrations. Base Exchange Indices show weak cation exchange reactions in which Na + and K + are released while Mg 2+ and Ca 2+ are adsorbed. Isotopic compositions of pore water reveal that the low-salinity water is probably the relic water sequestered in fluvial systems during relative sea-level low stands. Cores properly stored in a freezer for a long time has been used to study the pore water chemistry. For the first time, this study introduces an approach to correct the measured data by considering the possible evaporation effect during the transportation and storage of the samples. Corrections for evaporation were applied to the major ion concentrations and the stable isotopic compositions of pore water measured. It is found that the corrections determined by the Cl − mass balance approach are more reliable. The corrected measurements give more reasonable observations and hence allow sensible conclusions on the hydrochemical reactions and the origin of pore water.

An evaluation of materials and methods for vapour measurement of the isotopic composition of pore water in deep, unsaturated zones

ABSTRACTThe development of in situ vapour sampling methods to measure δ2H and δ18O in pore water of deep, unsaturated soil profiles, including mine tailings and waste rock, is required to improve our ability to track water migration through these deposits. To develop appropriate field sampling methods, a laboratory study was first undertaken to evaluate potential materials and sampling methods to collect and analyse vapour samples from unsaturated mine waste. Field methods were developed based on these findings and tested at two mine sites using either on-site analyses with a portable isotope laser spectrometer or sample collection and storage prior to laboratory analyses. The field sites included a series of deep (>50 m) multiport profiles within a coal waste rock dump and open wells installed in a sand tailings dyke at an oil sands mine. Laboratory results show that memory effects in sample bags and tubing require 3–5 pore volumes of vapour flushing prior to sample collection and sample storage times are limited to 24 h. Field sampling highlighted a number of challenges including the need to correct for sample humidity and in situ temperature. Best results were obtained when a portable laser spectrometer was used to measure vapour samples in situ.

Pore-water squeezing from indurated shales

High-pressure mechanical squeezing was applied to sample pore waters from a sequence of highly indurated and overconsolidated sedimentary rocks in a drillcore from a deep borehole in NE Switzerland. The rocks are generally rich in clay minerals (28–71wt.%), with low water contents of 3.5–5.6wt.%, resulting in extremely low hydraulic conductivities of 10−14–10−13m/s. First pore-water samples could generally be taken at 200MPa, and further aliquots were obtained at 300, 400 and 500MPa. Chemical and isotopic compositions of squeezed waters evolve with increasing pressure. Decreasing concentrations of Cl−, Br−, Na+ and K+ are explained by ion filtration due to the collapse of the pore space during squeezing. Increasing concentrations of Ca2+ and Mg2+ are considered to be a consequence of pressure-dependent solubilities of carbonate minerals in combination with sorption/desorption reactions. The pressure dependence was studied by model calculations considering equilibrium with carbonate minerals and the exchanger population on clay surfaces, and the trends observed in the experiments could be confirmed. The compositions of the squeezed waters were compared with results of independent methods, such as aqueous extraction and in-situ sampling of ground and pore waters. On this basis, it is concluded that the chemical and isotopic composition of pore water squeezed at the lowest pressure of 200MPa closely represents that of the in-situ pore water. The feasibility of sampling pore waters with water contents down to 3.5wt.% and possibly less opens new perspectives for studies targeted at palaeo-hydrogeological investigations using pore-water compositions in aquitards as geochemical archives.

Origin and transport of pore fluids in the Nankai accretionary prism inferred from chemical and isotopic compositions of pore water at cold seep sites off Kumano

We used push corers during manned submersible dives to obtain sediment samples of up to 30 cm from the subseafloor at the Oomine Ridge. The concentrations of B in pore water extracted from the sediment samples from cold seep sites were higher than could be explained by organic matter decomposition, suggesting that the seepage fluid at the site was influenced by B derived from smectite-illite alteration, which occurs between 50°C and 160°C. Although the negative δ18OH2O and δDH2O values of the pore fluids cannot be explained by freshwater derived from clay mineral dehydration (CMD), we considered the contribution of pore fluids in the shallow sediments of the accretionary prism, which showed negative δ18OH2O and δDH2O values according to the results obtained during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316. We calculated the mixing ratios based on a four-end-member mixing model including freshwater derived from CMD, pore fluids in the shallow (SPF) accretionary prism sediment, seawater (SW), and freshwater derived from methane hydrate (MH) dissociation. However, the Oomine seep fluids were unable to be explained without four end members, suggesting that deep-sourced fluids in the accretionary prism influenced the seeping fluids from this area. This finding presents the first evidence of deep-sourced fluids at cold seep sites in the Oomine Ridge, indicating that a megasplay fault is a potential pathway for the deep-sourced fluids.

Determining the Diagenetic Conditions of Concretion Formation: Assessing Temperatures and Pore Waters Using Clumped Isotopes

Carbonate-δ^(18)O paleothermometry is used in many diagenetic studies to unravel the thermal history of basins. However, this approach generally requires an assumed pore-water δ^(18)O (δ^(18)O_(pw)) value, a parameter that is difficult to quantify in past regimes. In addition, many processes can change the original isotopic composition of pore water, which further complicates the assignment of an initial δ^(18)O_(pw) and can lead to erroneous temperature estimates. Here, we use clumped-isotope thermometry, a proxy based on the ^(13)C–^(18)O bond abundance in carbonate minerals, to evaluate the temperatures of concretion formation in the Miocene Monterey Formation and the Cretaceous Holz Shale, California. These temperatures are combined with established carbonate–water fractionation factors to calculate the associated δ^(18)O_(pw). Results demonstrate that diagenetic processes can modify the δ^(18)O of ancient pore water, confounding attempts to estimate diagenetic temperatures using standard approaches. Clumped-isotope-based temperature estimates for Monterey Formation concretions range from ∼ 17 to 35°C, up to ∼ 12°C higher than traditional δ^(18)O carbonate–water paleothermometry when δ^(18)O_(pw) values are assumed to equal Miocene seawater values. Calculated δ^(18)O_(pw) values range from +0.3 to +2.5‰ (VSMOW)—higher than coeval Miocene seawater, likely due to δ^(18)O_(pw) modification accompanying diagenesis of sedimentary siliceous phases. Clumped-isotope temperatures for the Holz Shale concretions range from ∼ 33 to 44°C, about 15 to 30°C lower than temperatures derived using the traditional method. Calculated δ^(18)O_(pw) values range from −5.0 to −2.9‰ and likely reflect the influx of meteoric fluids. We conclude that the use of clumped isotopes both improves the accuracy of temperature reconstructions and provides insight into the evolution of δ^(18)O_(pw) during diagenesis, addressing a longstanding conundrum in basin-evolution research.

Stable isotope evidence for the hydrogeological characteristics of clay-rich till in northern East Anglia

Abstract The intrinsic vulnerability of the Chalk aquifer in East Anglia in eastern England to surface-derived contamination is dependent on the nature and juxtaposition of overlying Quaternary deposits. The study presented here is a direct investigation of the physical characteristics and pore water isotope composition of the glacial deposits found overlying the Chalk at five drilled locations in north Norfolk. The pore water isotopic composition demonstrated a range of δ 18 O values from −8.13 to −6.47‰, similar to the range for Chalk groundwater in the area of −8.18 to −7.01‰. A lithology-dependent model is developed that recognizes the presence of isotopically depleted palaeowater of late Pleistocene origin contained within layers of clay-rich till, adjacent to pore water containing modern meteoric water within layers of sand-rich till. The presence of (weathered) vertical fractures and intercalated lenses of sand- and gravel-rich layers is inferred to explain the occurrence of isotopically enriched water within the general mass of clay-rich till. It is apparent that groundwater movement in the heterogeneous lodgement till is controlled by advection in the more permeable sand-rich layers, whereas in the mass of clay-rich till, diffusion is more dominant. At one confined site, the observed pore water isotopic profile is modelled as having developed by downward diffusion of modern meteoric water during the past 5–10 ka.

Sources of pore water in a Tanegashima mud volcano inferred from chemical and stable isotopic studies

distance from seafloor. This paper reports and discusses the pore waters collected at the summit (CV) site. The concentrations of Cl ‐ decrease from 540 at the seafloor to 375 mmol/kg at a depth of ~200 cm and remain constant at around 350 mmol/kg (64% of the concentration of seawater) below the depth. The concentrations of CH4 are two to three orders of magnitude higher than those at other sites and have a maximum value of 715 µmol/kg at around 120‐140 cm bsf. Core samples collected at depths deeper than 180 cm bsf show collapsing gas bubbles and empty voids when they were split open. It was also observed that liquid seeped out from the surface of the split core. Considering the physical condition is favorable for the formation of methane hydrate, the observations suggest the existence of methane hydrates. High concentration of C2H6, which had similar depth profile to that of CH4, was also observed. C2H6/CH4 ratio remained larger than 10 ‐3 and δ 13 CCH4 also remained around ‐45‰ below 180 cm bsf. The data suggest presence of thermogenic methane in the CV site. δ 18 O H2O and δD H2O profiles exhibited an opposite depth dependence, and only δD H2O showed a decreasing depth profile similar to the concentration profile of Cl ‐ . They were inversely correlated with the concentration of Cl ‐ . The data of these two isotope compositions suggest a dilute fluid originates mainly from clay mineral dehydration but meteoric water. A simple mixing model of fluids from three sources (ambient seawater, water dissociated from methane hydrates, and diagenetic water ascending from deeper depth) with isotopic fraction during methane hydrate dissociation was applied for the observation result below 280 cm bsf to constrain ranges of δ 18 O H2O and δD H2O of diagenetic water. Using the observed depth profile of Cl ‐ as a conservative component of ambient seawater, contribution of ambient seawater is estimated to be 64% whereas 36% from other two sources. Considering an isotopic fractionation during methane hydrate dissociation and using the estimated source fractions and observed isotopic composition of pore water, δ 18 O H2O and δD H2O of the diagenetic water were estimated to range from +15 to +22‰ and from ‐103 to ‐43‰, respectively, which are in good agreement with isotopic compositions of water formed from clay minerals during their dehydration but quite different from those of meteoric water, supporting negligible contribution of meteoric water in the Tanegashima mud volcano fluid.