Using 81Kr and Noble Gases to Characterize and Date Groundwater and Brines in the Baltic Artesian Basin on the One-Million-Year Timescale

Analyses for $^{81}$Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water (\delta^{18}\text{O} \approx -10.4\unicode{x2030}) with a poorly evolved chemical and noble gas signature, glacial meltwater (\delta^{18}\text{O} \leq -18\unicode{x2030}) with elevated noble gas concentrations, and an old, high-salinity brine component (\delta^{18}\text{O} \geq -4.5\unicode{x2030}, \geq 90 \text{g Cl}^{-}/\text{L}) with strongly depleted atmospheric noble gas concentrations. The $^{81}$Kr measurements are interpreted within this mixing framework to estimate the age of the end-members. Deconvoluted $^{81}$Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA 3 instrument of 1.3 Ma. The radiogenic noble gas components $^{4}$He* and $^{40}$Ar* are less conclusive but also support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system.Comment: Accepted for publication in Geochimica et Cosmochimica Act

A comparison of isotope ratio mass spectrometry and cavity ring‐down spectroscopy techniques for isotope analysis of fluid inclusion water

Rationale Online oxygen (δ18O) and hydrogen (δ2H) isotope analysis of fluid inclusion water entrapped in minerals is widely applied in paleo‐fluid studies. In the state of the art of fluid inclusion isotope research, however, there is a scarcity of reported inter‐technique comparisons to account for possible analytical offsets. Along with improving analytical precisions and sample size limitations, interlaboratory comparisons can lead to a more robust application of fluid inclusion isotope records. Methods Mineral samples—including speleothem, travertine, and vein material—were analyzed on two newly setup systems for fluid inclusion isotope analysis to provide an inter‐platform comparison. One setup uses a crusher unit connected online to a continuous‐flow pyrolysis furnace and an isotope ratio mass spectrometry (IRMS) instrument. In the other setup, a crusher unit is lined up with a cavity ring‐down spectroscopy (CRDS) system, and water samples are analyzed on a continuous standard water background to achieve precisions on water injections better than 0.1‰ for δ18O values and 0.4‰ for δ2H values for amounts down to 0.2 μL. Results Fluid inclusion isotope analyses on the IRMS setup have an average 1σ reproducibility of 0.4‰ and 2.0‰ for δ18O and δ2H values, respectively. The CRDS setup has a better 1σ reproducibility (0.3‰ for δ18O values and 1.1‰ for δ2H values) and also a more rapid sample throughput (<30 min per sample). Fluid inclusion isotope analyses are reproducible at these uncertainties for water amounts down to 0.1 μL on both setups. Fluid inclusion isotope data show no systematic offsets between the setups. Conclusions The close match in fluid inclusion isotope results between the two setups demonstrates the high accuracy of the presented continuous‐flow techniques for fluid inclusion isotope analysis. Ideally, experiments such as the one presented in this study will lead to further interlaboratory comparison efforts and the selection of suitable reference materials for fluid inclusion isotopes studies.ISSN:1097-0231ISSN:0951-419

Intrusion of Saline Water into a Coastal Aquifer Containing Palaeogroundwater in the Viimsi Peninsula in Estonia

The Viimsi peninsula is located north-east of Tallinn, capital of Estonia. The Cambrian-Vendian (Cm-V) aquifer system is a sole source of drinking water in the area. Historically, the groundwater exploitation has led to freshening of groundwater in the peninsula, but in recent years an increase in chloride concentrations and enrichment in &delta;18O values has been detected, but in recent years hydrochemical parameters indicate an increasing influence of a saline water source. The exact origin of this saline water has remained unclear. The aim of the current study is to elucidate whether the increase in Cl&minus; concentrations is related to seawater intrusion or to the infiltration of saline water from the underlying crystalline basement. To identify the source of salinity, chemical composition of the groundwater and the isotope tracers (e.g., &delta;18O and radium isotopes) were studied in the Viimsi peninsula in the period from 1987 to 2018. Our results show that chemical composition of Cm-V groundwater in the peninsula is clearly controlled by three-component mixing between glacial palaeogroundwater, saline water from the underling crystalline basement and modern meteoric water. The concentrations of Ra are also significantly affected by the mixing, but the spatial variation of radium isotopes (226Ra and 228Ra) suggests the widespread occurrence of the U in the surrounding sedimentary sequence. Our hypothesis is that, in addition to U originating from the crystalline basement, some U could be associated with secondary U deposits in sedimentary rocks. The formation of these secondary U deposits could be related to glacial meltwater intrusion in the Pleistocene. Although the results suggest that the infiltration of saline groundwater from the underlying crystalline basement as the main source of salinity in the study area, the risk of seawater intrusion in the future cannot be ruled out. It needs to be highlighted that the present groundwater monitoring networks may not be precise enough to detect the potential seawater intrusion and subsequent changes in water quality of the Cm-V aquifer system in the Viimsi peninsula

Dating of glacial palaeogroundwater in the Ordovician-Cambrian aquifer system, northern Baltic Artesian Basin

The Ordovician-Cambrian aquifer system in the northern Baltic Artesian Basin contains glacial palaeogroundwater that originates from the Scandinavian Ice Sheet that covered the study area in the Pleistocene. Previously, no absolute dating of this palaeogroundwater has been attempted. In this multi-tracer study, we use H-3, C-14, He-4 and stable isotopes of water to constrain the age distribution of groundwater. We apply the geochemical modelling approach developed by van der Kemp et al. (2000) and Blaser et al. (2010) to calculate the theoretical composition of recharge waters in three hypothetical conditions: modern, glacial and interstadial for( 14)C model age calculations. In the second phase of the geochemical modelling, the calculated recharge water compositions are used to calculate the C-14 model ages using a series of inverse models developed with NETPATH. The calculated C-14 model ages show that the groundwater in the aquifer system originates from three different climatic periods: (1) the post-glacial period; (2) the Late Glacial Maximum (LGM) and (3) the pre-LGM period. A larger pre-LGM component seems to be present in the southern and north-eastern parts of the aquifer system where the radiogenic He-4 concentrations are higher (from similar to 3.0.10(-5) to 5.5.10(-4) cc.g(-1)) and the stable isotopic composition of water is heavier (delta O-18 from - 13.5 parts per thousand to -17.3 parts per thousand). Glacial palaeogroundwater from the north-western part of the aquifer system is younger and has C-14 model ages that coincide with the end of the LGM period. It is also characterized by lower radiogenic( 4)He concentrations (similar to 2.0.10(-5) cc.g(-1)) and lighter stable isotopic composition (delta O-18 from -17.7 to - 22.4 parts per thousand). Relations between radiogenic He-4 and C-14 model ages and between radiogenic He-4 and Cl(- )concentration show that groundwater in the aquifer system does not have a single well-defined age. Rather, the groundwater age distribution has been influenced by mixing between waters originating from end-members with strongly differing ages. Overall the results suggest, that in the shallower northern part of the aquifer system, significant changes in groundwater composition can be brought about by glacial meltwater intrusion during a single glaciation. However, multiple cycles of glacial advance and retreat are needed to transport glacial meltwater to the deeper parts of the aquifer system

Constraints for precise and accurate fluid inclusion stable isotope analysis using water-vapour saturated CRDS techniques

Hydrogen (δ 2 H) and oxygen (δ 18 O) isotopes of water extracted from speleothem fluid inclusions are important proxies used for paleoclimate reconstruction. In our study we use a cavity ring-down laser spectroscopy system for analysis and modified the approach of Affolter et al. (2014) for sample extraction. The method is based on crushing of small sub-gram speleothem samples in a heated and continuously water-vapour purged extraction line. The following points were identified: Injection of reference water shows a precision (1σ) of 0.4-0.5 ‰ for δ 18 O values and 1.1-1.9 ‰ for δ 2 H values for water amounts of 0.1-0.5 μl, which improves with increasing water amount to 0.1-0.3 ‰ and 0.2-0.7 ‰, respectively, above 1 μl. The accuracy of measurements of water injections and water-filled glass capillaries crushed in the system is better than 0.08 ‰ for δ 18 O and 0.3 ‰ for δ 2 H values. The reproducibility (1σ) based on replicate analysis of speleothem fluid inclusion samples with water amounts >0.2 μl is 0.5 ‰ for δ 18 O and 1.2 ‰ for δ 2 H values, respectively. Isotopic differences between the water vapour background of the extraction system and the fluid inclusions have no significant impact on the measured fluid inclusion isotope values if they are within 10 ‰ for δ 18 O and 50 ‰ for δ 2 H values of the background. Tests of potential adsorption effects with inclusion free spar calcite confirm that the isotope values are unaffected by adsorption for water contents of about 1 μl (fluid inclusion) water per g of carbonate or above. Fluid inclusion analysis on three different modern to late Holocene speleothems from caves in northwest Germany resulted in δ 18 O and δ 2 H values that follow the relationship as defined by the meteoric water line and that correspond to the local drip water. Yet, due to potential isotope exchange reactions for oxygen atoms, hydrogen isotope measurements are preferentially to be used for temperature reconstructions. We demonstrate this in a case study with a Romanian stalagmite, for which we reconstruct the 20th century warming with an amplitude of approximately 1 °C, with a precision for each data point of better than ±0.5 °C