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Fractionation of Mg isotopes by clay formation and calcite precipitation in groundwater with long residence times in a sandstone aquifer, Ordos Basin, China

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Abstract

Compared with the numerous studies on river and soil waters, studies on Mg isotopes of groundwater are limited. In this study, a sandstone aquifer in the Ordos Basin, China with contrast contents of Mg in shallow and deep groundwater is selected to examine the behavior of Mg isotopes during groundwater circulation. The δ²⁶Mg values of shallow groundwater are within the range of widely reported results of groundwater, while those of deep groundwater are found to be as light as −3.30‰ to −2.13‰. Assuming that shallow groundwater is an endmember, ⁸⁷Sr/⁸⁶Sr ratios show that calcite dissolution has contribution to low δ²⁶Mg of deep groundwater, but mixing alone cannot explain the coupled low δ²⁶Mg and low Mg contents. The removal of Mg in deep groundwater is found to be mainly caused by incorporating into neoformed clay minerals, which further lowers δ²⁶Mg. For the deep groundwater samples denoted as G1 and G3, the relationship between δ²⁶Mg and 1/Mg has been quantitatively explained by the superposition of calcite dissolution and clay formation with a fractionation factor (αclay–water) of 1.0003. For samples denoted as G2, in addition to calcite dissolution and clay formation, high proportion of Mg in the residual solution are further removed via precipitation of low-Mg calcite, which leads to increased δ²⁶Mg. There are increasingly stronger degrees of clay formation in G3, G1, and G2 due to the increasingly longer travel distances and travel times of groundwater from recharge to discharge areas. This study enhances understanding on the factors controlling Mg isotopes of groundwater, as well as the geochemical processes of subsurface water-rock interactions in sandstone aquifers.

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... The low precipitation temperatures (≤50 • C) of columnar calcite, which is suggested by all-liquid fluid inclusion composition, combined with its low salinity (0.5 wt% NaCl eq.) and low δ 13 C VPDB (− 8.5‰ to − 3.9‰) suggest precipitation by incursion of meteoric waters, which has presumably occurred during late uplift and erosion phae of the Zagros Orogeny (ca. 12 Ma; Mansurbeg et al., 2016). Wombacher et al., 2009;Li et al., 2010, Zhang et al., 2018. The δ 26 Mg DSM3 values reported in the literature for hydrothermal dolomites vary widely and are considerably lower than the values expected from these three sources (δ 26 Mg = − 3.50‰ to − 0.45‰; Fig. 12; Fig. 13) (Young and Galy, 2004;Teng et al., 2010;Azmy et al., 2013;Lavoie et al., 2014;Geske et al., 2015a). ...
... The source of Mg 2+ -rich hydrothermal brines (CaCl 2 -MgCl 2 -NaCl) was suggested by Mansurbeg et al. (2016) to be overpressured mudstones (Fig. 3). Clay minerals (typically Mg smectite) in mudstones have δ 26 Mg DSM3 values − 0.49‰ to − 0.39‰ (Zhang et al., 2018). The Zagros orogeny during the Upper Cretaceous (Koyi, 1998) might have breached regional seals and formed syndepositional faults and fractures thus mobilization of Mg-rich, deep connate marine waters (Horbury, 2013). ...
... The influx of Mg-rich detrital grains derived from the erosion of uplifted ophiolites and deposited as important component of the Tanjero Formation in the basin (Al-Naqib and Al-Juboury, 2014;Jones et al., 2020) is potential source of Mg for the formation of hydrothermal as was suggested by Lavoie and Morin (2004) for the Lower Silurian Sayabec Formation in Québec. However, this potential Mg source is not very likely because: (i) the higher Sr isotopic ratios (0.707721-0.707912) in dolomite than in mafic igneous rocks, which suggests input of Sr from felsic (K-rich) rocks, and (ii) the lower δ 26 Mg DSM3 values of Bekhme dolomites (− 0.81‰ to − 0.76‰) than those of ophiolites (− 0.28‰ to +0.05‰; Su et al., 2015;Zhang et al., 2018). Upper Cretaceous granitoid intrusions have been suggested to be a potential heat source for hydrothermal fluids in NE Turkey (Kırmacı et al., 2018). ...
Article
This paper integrates Mg-, C-, O-, Sr-isotopes, fluid-inclusions microthermometry, and radiometric dating of dolostones and vug-filling saddle dolomite and calcite of the Upper Cretaceous Bekhme Formation, the Kurdistan Region of Iraq, provide new insights into the origin, timing and geochemical evolution of hydrothermal dolomitizing fluids. Similar isotopic compositions and fluid-inclusion microthermometry of the dolostones and saddle dolomite suggest formation by similar fluids. The U-Pb ages of dolostones and saddle dolomite suggest that dolomitization (74.8 Ma) was accomplished in 300 Ka at near Earth's surface by flow of hydrothermal fluids followed by dolostone dissolution and precipitation of vug-filling saddle dolomite (73.8 Ma). Mg isotopic compositions of the dolomites (²⁶MgDSM3 = -0.81‰ to -0.76‰) are interpreted to indicate derivation of Mg from marine connate waters entrapped in overpressured basinal mudstones. Relatively high Sr-isotope ratios of the dolomites precludes the role of ophiolites as an important source of Mg in the basin.
... The low precipitation temperatures (≤50 • C) of columnar calcite, which is suggested by all-liquid fluid inclusion composition, combined with its low salinity (0.5 wt% NaCl eq.) and low δ 13 C VPDB (− 8.5‰ to − 3.9‰) suggest precipitation by incursion of meteoric waters, which has presumably occurred during late uplift and erosion phae of the Zagros Orogeny (ca. 12 Ma; Mansurbeg et al., 2016). Wombacher et al., 2009;Li et al., 2010, Zhang et al., 2018. The δ 26 Mg DSM3 values reported in the literature for hydrothermal dolomites vary widely and are considerably lower than the values expected from these three sources (δ 26 Mg = − 3.50‰ to − 0.45‰; Fig. 12; Fig. 13) (Young and Galy, 2004;Teng et al., 2010;Azmy et al., 2013;Lavoie et al., 2014;Geske et al., 2015a). ...
... The source of Mg 2+ -rich hydrothermal brines (CaCl 2 -MgCl 2 -NaCl) was suggested by Mansurbeg et al. (2016) to be overpressured mudstones (Fig. 3). Clay minerals (typically Mg smectite) in mudstones have δ 26 Mg DSM3 values − 0.49‰ to − 0.39‰ (Zhang et al., 2018). The Zagros orogeny during the Upper Cretaceous (Koyi, 1998) might have breached regional seals and formed syndepositional faults and fractures thus mobilization of Mg-rich, deep connate marine waters (Horbury, 2013). ...
... The influx of Mg-rich detrital grains derived from the erosion of uplifted ophiolites and deposited as important component of the Tanjero Formation in the basin (Al-Naqib and Al-Juboury, 2014;Jones et al., 2020) is potential source of Mg for the formation of hydrothermal as was suggested by Lavoie and Morin (2004) for the Lower Silurian Sayabec Formation in Québec. However, this potential Mg source is not very likely because: (i) the higher Sr isotopic ratios (0.707721-0.707912) in dolomite than in mafic igneous rocks, which suggests input of Sr from felsic (K-rich) rocks, and (ii) the lower δ 26 Mg DSM3 values of Bekhme dolomites (− 0.81‰ to − 0.76‰) than those of ophiolites (− 0.28‰ to +0.05‰; Su et al., 2015;Zhang et al., 2018). Upper Cretaceous granitoid intrusions have been suggested to be a potential heat source for hydrothermal fluids in NE Turkey (Kırmacı et al., 2018). ...
... It has been well documented that light Mg is preferentially incorporated into carbonate during its precipitation by experimental and field studies (e.g. Galy et al., 2002;Mavromatis et al., 2013;Trostle et al., 2014;Zhang et al., 2018). Thus, the formation of secondary carbonate in river basin has the potential to drift the Mg isotope composition of river water heavier. ...
... Mg/Ca ratio in solution usually affects the Mg content in precipitated carbonate (Folk and Land, 1975). These ongoing processes have been reported in soil water and groundwater at basin scale (Capo et al., 2000;Whipkey et al., 2002;Trostle et al., 2014;Zhang et al., 2018), and regulate the river water geochemistry. ...
... Even for the Mg isotope compositions of the carbonate unsaturated water samples, they cannot be fully explained by conservative mixing (Fig. 5a). Although the in-place river waters are unsaturated with respect to carbonate, carbonate kinetic precipitation reactions are widely documented in groundwater/soil pore water systems in the basin (Capo et al., 2000;Whipkey et al., 2002;Trostle et al., 2014;Zhang et al., 2018). Fractionation resulted solution (as a Mg reservoir) will be produced and serves as a new end-member, which recharges the river water and regulates river Mg isotope composition. ...
Article
Magnesium isotope behavior in dissolution and precipitation reactions during chemical weathering have been well documented. However, mechanisms of mineral dissolution and precipitation impact on the riverine Mg isotope composition under different climatic and geology background are not well constrained, which limits Mg isotope application in weathering research. Mg isotopic compositions for solute and suspended sediments in Jinsha River Basin, located in Tibetan Plateau, China, were examined to address this issue. The δ²⁶Mg values for dissolved loads range from −1.67‰ to −0.5‰, and the suspended loads show systematically heavier Mg isotope compositions (−1.15‰ to −0.06‰). Conservative mixing between different rock weathering end-members fails to fully explain Mg isotopic composition variation of Jinsha River waters based on mass balance and mixing model with the river geochemistry data. Mg in rivers draining dominantly carbonate and evaporite is isotopically heavier compared with the value of catchment bedrocks, and water pH and δ²⁶Mg values are negatively correlated in carbonate (calcite and dolomite) oversaturated river waters, which suggests the precipitation of secondary carbonate as an important mechanism driving the δ²⁶Mg of dissolved loads heavier. For carbonate unsaturated waters, Mg concentrations and δ²⁶Mg values are intermediate between those of silicate dominated basins and carbonate oversaturated waters. The results suggest two possible different mechanisms controlling river solute δ²⁶Mg values: fractionation during carbonate precipitation incorporating of Mg; and conservative mixing between a solute end-member formed from carbonate precipitation and end-members from rock weathering. This study provides new evidence and insights in carbonate precipitation processes regulating river Mg isotope signature and its potential influence on Mg cycling.
... The available geochemical investigations of groundwater have been just carried out at a regional scale in the Ordos Plateau [15,17,18], while detailed investigations on the groundwater geochemistry are rather scattered so far, and analyses conducted are limited to the major ions chemistry of groundwater in the Lake Basins, NE Ordos Plateau, such as Dake Lake, Mukai Lake and Hutongchahan (HTCH) lake ( Figure 1b) [14,16,[19][20][21][22]. In addition, the HTCH Lake Basin is chosen as the key study area (Figure 1c) because it is located in the lowest reaches of the Wushenzhao Basin and might serve as the fingerprint of nested flow systems. ...
... The groundwater flow is driven by the wavy topography in the Ordos Plateau, which develops hierarchically nested groundwater flow systems. The local flow system generally develops between groundwater divides and neighboring depressions with a shallow circulation depth of around 200 m; intermediate flow systems occurs between the major divides and the relatively larger lakes with a relatively deep circulation depth; and the regional flow system has a much larger lateral extent and a higher circulation depth (up to 500 m) when compared with the local and intermediate flow systems [14,15,18,20,22]. ...
... mg/L; mean: 36.1 mg/L) than type 2 (range: 3.1-31.1 mg/L; mean: 18.5 mg/L) and type 3 (range: 13.6-180.1 mg/L; mean: 2.4 mg/L) (Table S3 and Figure 5). A similar contamination phenomenon caused by human activities is also observed in the regions close to the study area in the Ordos Plateau [20,32]. ...
Article
Full-text available
The lowest reaches of a large-scale basin could be the discharge areas of local, intermediate and regional groundwater flow systems with significantly distinct travel distances and travel times. This study aims to delineate the groundwater chemical characteristics and the mechanism controlling the chemical evolution in the lowest reaches of the Wushenzhao Cretaceous basin, NW China. A total of 38 groundwater samples were collected and were chemically classified into five distinct water types by means of a Piper Plot. According to the hydrogeological setting and groundwater age, the spatial distribution of these water types is found to be associated with hierarchically nested groundwater flow systems (local and regional system): Types 1, 2, 3 and 4 belong to the local groundwater flow system, while type 5 belongs to the regional flow system. Graphical plots, stable isotopes and geochemical modeling techniques were used to interpret the observed compositions. The results show the dominance of carbonate and gypsum dissolution in type 1 waters; ion exchange in types 2, 3 and 4; and evaporite dissolution in type 5. In addition, human activities in the form of extensive irrigation also affect the chemical compositions of type 1 water. These findings are important for the sustainable management of groundwater resources in the study area.
... Similarly, surface weathering causes 26 Mg enrichment in the weathering residual, producing Mg-isotope signatures that appear long lasting and robust during subsequent deep burial, potential fluid alteration and partial melting (Shen et al., 2009;Teng et al., 2016;Wang et al., 2017). Concomitant formation of secondary Mg carbonate appears in some cases to balance 26 Mg uptake in secondary silicate phases (Beinlich et al., 2014), while precipitation of 24 Mg enriched carbonate during open system alteration will alter the composition of the weathering runoff, thus contributing to the variability of measured Mg isotope ratios in natural continental waters (Tipper et al., 2006b;Zhang et al., 2018). ...
... Both hydrothermal alteration and low-temperature weathering result in fractionation of Mg isotopes into 26 Mg enriched secondary silicate and 24 Mg enriched carbonate phases (Fig. 4). The distinct Mg-isotope signatures of Mg-silicate and Mg-carbonate minerals have previously been used to distinguish changing weathering regimes (Immenhauser et al., 2010;Kasemann et al. 2014;Tipper et al., 2006b), to infer the evolution of groundwater (Zhang et al. 2018) and to identify the formation of Mg clay rather than Mg-carbonate during subsurface interactions of CO2-bearing fluids with basalt at the CarbFix site . The latter study highlights the importance of understanding the fate of Mg during CO2 sequestration in mafic and ultramafic rocks, as the formation of Mg-clay (as opposed to Mg-carbonate) does not lead to the desired storage of CO2 in carbonate minerals. ...
Article
The Mg-isotope systematics of peridotite weathering and low-temperature carbonation have not yet been thoroughly investigated, despite their potential to provide insights into reaction pathways and mechanisms of lithosphere-hydrosphere transfer of Mg and sequestration of CO2 in carbonate minerals. Here, we present new observations of the evolution of Mg isotope ratios during subtropical ultramafic rock weathering and associated magnesite formation, including the lowest δ²⁶Mg of magnesite reported so far. At the investigated field sites in eastern Australia, the proximity of the ultramafic Mg source rocks and associated magnesite deposits provides boundary conditions that constrain Mg isotope fractionation during low-temperature alteration. Saprolite samples from Attunga, New South Wales, show that weathering of serpentinite is accompanied by Mg loss and formation of secondary Mg-bearing clay minerals. Furthermore, Mg isotope ratios increase systematically with weathering intensity, indicating that incorporation of ²⁶Mg into clay mineral structures controls Mg isotope fractionation during ultramafic rock weathering. The Mg-bearing clay formed by decomposition of serpentine minerals has a δ²⁶Mg value of ∼0.35‰, which is up to ∼0.6‰ heavier than the ultramafic precursor. In contrast, nodular magnesite hosted in ultramafic rock shows δ²⁶Mg values between −3.26‰ and −2.55‰ that are significantly lower than those of magnesite and dolomite formed by hydrothermal alteration of peridotite at higher temperature (δ²⁶Mg = −0.69‰ and −0.62‰). The strong enrichment of ²⁴Mg in nodular magnesite does not reconcile with simple fractionation during direct precipitation from ultramafic host rock buffered meteoric fluids and instead suggests multiple formation steps involving dissolution and re-precipitation of pre-existing carbonate accompanied by fractionation between species of dissolved Mg. Our data highlight the potential of Mg isotope studies for distinguishing the formation pathways of low temperature magnesite and for tracing Mg in low temperature alteration processes based on the distinct signatures of secondary silicate and carbonate minerals.
... Note that these flowing wells belong to topography-controlled flowing wells in homogeneous unconfined aquifers (Freeze and Cherry 1979), not geologically controlled flowing wells in confined aquifers. In the Dosit River Watershed, the mixed groundwater samples collected at the outlets of uncased flowing wells with depths of several hundred meters have hydrochemistry and isotopes of H, O, Mg and Sr representative of deep groundwater in discharge areas- Zhang et al. (2018a); Wang et al. (2015a); see electronic supplementary material (ESM) for a detailed summary. Based on the mixed groundwater samples in both recharge and discharge areas, the major geochemical processes of water-rock interactions during the flowpaths from recharge to discharge areas have been successfully identified (Zhang et al. 2018a), which indicates that existing long-screen wells have the potential to be used in basin-scale hydrogeochemical studies. ...
... In the Dosit River Watershed, the mixed groundwater samples collected at the outlets of uncased flowing wells with depths of several hundred meters have hydrochemistry and isotopes of H, O, Mg and Sr representative of deep groundwater in discharge areas- Zhang et al. (2018a); Wang et al. (2015a); see electronic supplementary material (ESM) for a detailed summary. Based on the mixed groundwater samples in both recharge and discharge areas, the major geochemical processes of water-rock interactions during the flowpaths from recharge to discharge areas have been successfully identified (Zhang et al. 2018a), which indicates that existing long-screen wells have the potential to be used in basin-scale hydrogeochemical studies. In a recent study, Zhang et al. (2018b) identified the sources of groundwater inflow in topography-controlled flowing wells; however, the physical mechanism of the mixing of hydrochemical components in such a flowing well, which leads to a mixed groundwater sample at the outlet representative of the deep part of the well, is not clear. ...
Article
Full-text available
Groundwater sampled at the outlets of uncased flowing wells in a thick unconfined aquifer, which has undergone mixing, has been found to have hydrochemistry similar to deep groundwater in discharge areas. To identify the hydrodynamic causes, transient models of groundwater flow and age in a three-dimensional homogeneous unit basin with flowing wells are constructed to obtain flow rates in wells and groundwater mean age around wells. Inflow of groundwater to the well in the deep part leads to mixing of groundwater from different sources, and the finally mixed groundwater is found to have the same age as groundwater in the aquifer at a specific depth, termed the equivalent position (EP). EP is always found in the lower half of the flowing well, indicating that a mixed sample at the outlet could represent deep groundwater. Outflow from the well to the unconfined aquifer in the shallow part results in aging of groundwater around the well. For fully penetrating flowing wells in confined aquifers, EP is found in the upper half of the aquifer. The different relative depths of EP to the screen interval in the two types of flowing wells are mainly due to the profiles of horizontal velocity in the inflow segment, which is basically uniform in a confined aquifer but increases from zero to a maximum value in unconfined aquifers. Thus, groundwater at the outlets of topography-controlled flowing wells is a window of the deep part of a basin, and existing long-screen wells could have the potential for groundwater sampling.
... At present, the information on solute isotopic fractionation associated with adsorption-desorption or/and ion exchange on clays or/and organic matter is limited (Tipper et al., 2016;Zhang et al., 2018). One of the most important reasons is a lack of accurate samples in field and information on potential endmembers (especially the exchangeable component). ...
Article
High-temperature aquifer thermal energy storage (HT-ATES) is a cost-effective and suitable technology to store large amounts of energy. HT-ATES has been demonstrated to be an efficient and stable tool to buffer seasonal imbalances and significantly reduce greenhouse gas emissions. Fractured reservoirs are widespread in sedimentary basins worldwide. However, naturally fractured reservoirs have received little attention as potential formations for HT-ATES. The main concern regarding thermal energy storage in naturally fractured formations is the high fracture permeability, which may result in fast fluid flow and an increase in thermal losses. Therefore, quantification of the effects of fracture flow is essential to HT-ATES site verification. An HT-ATES system in a generic fractured reservoir is simulated with a 3D stochastically generated discrete fracture network (DFN) model combined with a fluid flow and heat transport model. The pressure and temperature evolutions at the well are analyzed, and the total extracted energy and thermal recovery efficiency are evaluated. The results confirm that the presence of the DFNs enhances thermal loss and thereby results in a lower thermal recovery efficiency compared with those of an unfractured reservoir. Such a decrease can reach 20%. However, simulations show that naturally fractured reservoirs might be potential candidates for HT-ATES due to a larger injectivity. A further discussion is conducted to characterize the relative importance of the hydrogeological properties of the fractures and fracture network structure on the thermal behavior of the HT-ATES system. This study provides preliminary insights into the impacts of complex natural DFN on the thermal performance of HT-ATES in fractured reservoirs.
... Without considering the degradation and attenuation of pollutants in the aquifer, the pumping wells are recharged from the aquifers in the distance under long-term pumping conditions. At this time, the pollutants in the distance will still be affected by pumping and migrate with the groundwater until they reach the pumping well [62,63]. In other words, not all pollutants in the pumping wells come from sources close to the pumping wells, but some also come from the aquifers outside the range of the influence radius. ...
Article
Full-text available
To facilitate understanding and calculation, hydrogeologists have introduced the influence radius. This parameter is now widely used, not only in the theoretical calculation and reasoning of well flow mechanics, but also in guiding production practice, and it has become an essential parameter in hydrogeology. However, the reasonableness of this parameter has always been disputed. This paper discusses the nature of the influence radius and the problems of its practical application based on mathematical reasoning and analogy starting from the Dupuit formula and Thiem formula. It is found that the influence radius is essentially the distance in the time–distance problem in physics; therefore, it is a function of time and velocity and is influenced by hydrogeological conditions and pumping conditions. Additionally, the influence radius is a variable and is essentially different from the hydrogeological parameters reflecting the natural properties of aquifers such as the porosity, specific yield, and hydraulic conductivity. Furthermore, the parameterized influence radius violates the continuity principle of fluids. In reality, there are no infinite horizontal aquifers, and most aquifers are replenished from external sources, which is very different from theory. The stable or seemingly stable groundwater level observed in practice is simply a coincidence that occurs under the influence of various practical factors, which cannot be considered to explain the rationality of applying this parameter in production calculations. Therefore, the influence radius cannot be used to evaluate the sustainable water supply capacity of aquifers, nor can it be used to guide the design of groundwater pollution remediation projects, the division of water source protection areas, and the scheme of riverbank filtration wells. Various ecological and environmental problems caused by groundwater exploitation are related to misleading information from the influence radius theory. Generally, the influence radius does not have scientific or practical significance, but it can easily be misleading, particularly for non-professionals. The influence radius should not be used in the sustainable development and protection of groundwater resources, let alone in theoretical models. From the perspective of regional overall planning, the calculation and evaluation of sustainable development and the utilization of groundwater resources should be investigated in a systematic manner.
... 3 Mg has three stable isotopes, 24 Mg, 25 Mg, and 26 Mg with relative abundances of 78.99%, 10.00%, and 11.01%, respectively. 4 Due to their properties of high abundance and large mass difference, Mg isotopes become a powerful indicator to trace different geological processes such as magmatic processes, 5-7 planet formation, [8][9][10][11] carbonate precipitation, [12][13][14] chemical weathering, [15][16][17][18][19][20] and biogeochemical cycling. [21][22][23][24][25][26][27][28] The equilibrium fractionation of Mg isotopes during the igneous process at high temperatures is considered limited, which results in a narrow range of d 26 Mg in igneous rocks. ...
Article
We present a method to obtain high quality Mg isotopic data for rocks with diverse Mg concentrations by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The separation efficiency of Mg is greatly improved by optimizing acid molarity, column size and introducing HF to the eluting acid. The chromatographic separation uses a single column without additional procedures and efficiently achieves the purification of Mg from matrix elements including Na, K, Ca, Al, Fe, Ti, and K. The recovery of the purification method is 100.5% ± 1.4%. The effect of concentrations mismatch, acid mismatch as well as matrix effect were systematically tested by using a Nu Plasma 3 MC-ICP-MS with wet plasma mode. The average δ26Mg of three synthetic standard solutions, made by doping mono-elemental Mg standard solution Alfa-Mg, GSB-Mg, and IRMM-009 with matrix elements, agrees with the recommended values within error. The robustness and reproducibility of the established method was further examined by replicated measurements of reference materials including rock standards, seawater, and standard solution. Biological reference materials GSB-14 and GSB-19 have for the first time been characterized for Mg isotopic composition. The long-term precision based on repeated measurement of standards is better than ± 0.06‰ for δ26Mg. Such precision and accuracy of Mg isotope measurement can be more widely used to obtain accurate Mg isotopic compositions for different samples, and to trace geochemical processes in future studies.
... Generally, groundwater residence time (or water-rock interaction time) increases with an increase in the groundwater flow path (Edmunds and Smedley, 2000). Thus, the active elements (e.g., B, Cs, and Mg) will be partly removed by newly-formed secondary minerals (e.g., clays, metal oxy-hydroxides, and carbonate) from groundwater though adsorption and/or co-precipitation along the groundwater flow path (Edmunds et al., 2003;Zhang et al., 2018). As a result, the ratio of the active element (As, B, and Cs) to inert (or non-conservative) element Na or Cl in groundwater gradually decreases with an increase in groundwater flow path, and further significant decreases in these ratios in tributary and main channel in lower reaches of the basin (Fig. S5). ...
Article
High levels of dissolved arsenic (As) have been reported in many rivers running though the Tibetan Plateau (TP), the “Water Tower of Asia”. However, the source, spatiotemporal variations, and geochemical behavior of dissolved As in these rivers remain poorly understood. In this study, hot spring, river water, and suspended particulate material samples collected from the Yarlung Tsangpo River (YTR) (upper reaches of the Brahmaputra River) system in 2017 and 2018 were analyzed. Spatial results shown that the upper reaches of YTR (Zone I) have comparatively high levels of dissolved As ([As]dissolved: mean 31.7 μg/L; 4.7–81.6 μg/L; n = 16), while the tributaries of the lower reaches (Zone II) have relatively low levels (mean 0.54 μg/L; 0.11–1.3 μg/L; n = 7). Seasonal results shown that the high [As]dissolved (6.1–22.4 μg/L) were found in September to June and low [As]dissolved (1.4–3.7 μg/L) were observed in July to August. Geothermal water is suspected as the main source of the elevated As levels in YTR due to the extremely high [As]dissolved in hot springs (1.13–9.76 mg/L) and abundance of geothermal systems throughout TP. However, the seasonal results suggested that weathering of As-containing rocks and minerals is also a key factor affecting the [As]dissolved in the river water in July to August (wet-season). Natural attenuation of As in main channel is dominated by dilution process due to the lower As concentrations in tributaries, but mostly occurred by both dilution and adsorption (or co-precipitation) processes in tributaries. This work highlights that the weathering process may have an important contribution to the dissolved As in the river waters in wet-season, and the geochemical behavior of As is largely transported conservatively in the main channel and relative non-conservatively in the tributaries in YTR system.
... Riverine is generally regarded as a reservoir of the light Mg isotopes, corresponding to the 26 Mg-rich regolith (Brenot et al., 2008;Tipper et al., 2006Tipper et al., , 2012Tipper et al., , 2008Wimpenny et al., 2011). Light Mg isotopes are dominantly transported to the riverine systems through groundwater flow (Tipper et al., 2006;Zhang et al., 2018). Generally, groundwater flows through deeper part of soil profiles, and apart from receiving 24 Mg-rich soil solutions from infiltration (Tipper et al., 2010). ...
Article
Continental weathering is a fundamental process in releasing magnesium (Mg) from crystalline rocks to the hydrosphere and biosphere. Mg isotopes can be substantially mobilized, re-distributed, and fractionated during weathering, and therefore can be used as a powerful tool to trace the biogeochemical cycle of Mg. Causes of significant Mg isotopic fractionation and behaviors during silicate weathering are still not well understood, hindering further application of the Mg isotopes to probe different geological processes. In this study, we demonstrate that dissolution and formation of phyllosilicates are the main control of Mg isotopic fractionation during sub-tropical weathering of granite. Furthermore, different formation and dissolution mechanisms for the same mineral phase could also cause variations in magnitude and directionality of fractionation. In incipient weathering, supergene phyllosilicates form mainly through topotactic transformation. Vermiculitization of parental chlorite tends to release ²⁴Mg and causes significant ²⁶Mg enrichment in the saprock. In an advanced stage of weathering, Mg isotopic compositions of supergene phyllosilicates are more influenced by the interaction with the soil solutions. Minerals formed mainly through a dissolution-precipitation mechanism with Mg in neoformed phyllosilicates dominantly sourced from the contemporary soil solutions. ²⁶Mg would be firstly incorporated into neoformed phyllosilicates, such as vermiculite, interstratified biotite/vermiculite and chlorite/vermiculite. Therefore, soil solutions became more enriched in ²⁴Mg with depth in the pedolith, from which relatively ²⁴Mg-rich phyllosilicates would form. However, in the saprolite, precipitation of illite may have preferentially scavenged ²⁴Mg, enriching the soil solutions with ²⁶Mg. Varying relative abundances of different phyllosilicate minerals along the profile could cause large variations in the Mg isotopic compositions of regolith. Our study shows that Mg isotopic composition of the slightly weathered materials could be significantly heavy. Hence, entrainment of ²⁶Mg-rich but slightly weathered materials could be an alternative to explain the high δ26Mg as recorded in some sedimentary rocks, especially of aeolian source. Whereas low δ26Mg widely archived in groundwater and river water could be alternatively explained by interaction with the saprock and ²⁶Mg scavenging during phyllosilicate transformation, instead of severe depletion of ²⁶Mg in soil solutions due to intense weathering and vast formation of secondary minerals, as previously suggested. Comprehensive characterization of the weathering processes and the resultant products is essential to interpret the observed Mg isotopic fractionation and trace the biogeochemical cycle of Mg.
... The reaction between the groundwater and rock mineralogy is controlled by limited-rate kinetics and the increase in groundwater travel time implies an increase in the rock-water interaction (Kiro et al., 2013;Santoni et al., 2016;Sun et al., 2016;Zhang et al., 2018). Thus, it is expected that older groundwater exhibits greater enhancement of rock-water interactions than younger groundwater. ...
Article
Aquifers possess distinct mineralogy, diagenetic history, and geochemical constraints, which control the pore water’s hydrochemistry. Whenever aquifers exchange groundwater, the incoming groundwater establishes a new chemical driving force, imposing new reactions related to rock-water interactions. This study developed a simple approach to assess the mass balance of the rock-water interaction of the groundwater exchanged between interconnected aquifers. To illustrate the suitability of this approach, it was applied to the Bauru Aquifer System located in Sao Paulo State in southeastern Brazil. The hydrochemical characterization identified three major stratified hydrochemical facies with typical chemical compositions as well as the authigenic mineralogy assemblages. The first and shallowest facies was composed of a variable groundwater composition along with highly diluted water from a recent recharge. The second facies was composed of unconfined bicarbonate calcium-rich water with a mineralogy mainly composed of K-feldspar, Mg-montmorillonite, and calcite. The third facies, which was confined and the deepest, was composed of bicarbonate sodium-rich water and mineralogically dominated by Na-, Mg-, and Ca-montmorillonite, calcite, and analcime. Although the replacement of Na by Ca suggests an extensive cation exchange in the deep-confined portion, sodium-rich silicate dissolution and calcium-rich clay precipitation were identified as the operating mechanisms promoting Na enrichment and Ca consumption in the water of the closed system. This finding is supported by the authigenic mineral characterization, δ13C of the dissolved inorganic compounds, and numerical analysis. The approach developed for this study, which used numerical simulations to calculate the mass balance, was capable of consistently reproducing the water composition and mineralogy changes.
... It is interesting that groundwater collected at the flowing wells has a hydrochemical facies of Na-HCO3, does not contain NO3 -, and is depleted in δ 2 H and δ 18 O, all of which are quite different from groundwater in recharge areas (Wang et al., 2015a). Moreover, Mg in groundwater collected from flowing wells has been greatly removed by the process of clay formation, leading to much lower δ 26 Mg than samples in recharge areas (Zhang et al., 2018a). These 670 hydrochemical and isotopic evidences show that groundwater collected at the outlets of flowing well could represent deep groundwater and is seldom mixed with shallow groundwater (Zhang et al., 2019). ...
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The spewing of groundwater in flowing wells is a phenomenon of interest to the public, but little attention has been paid to the role of flowing wells on the science of groundwater. This study reviews that answering to problems related to flowing wells since the early 19th century led to the birth of many fundamental concepts and principles of groundwater hydrology. The concepts stemmed from flowing wells in confined aquifers include permeability and compressibility, while the principles include Darcy's law, role of aquitards on flowing well conditions and the piston flow pattern, steady-state well hydraulics in confined aquifers, and transient well hydraulics towards constant-head wells in confined or leaky aquifers, all of which are applicable even if flowing well conditions have disappeared. Due to the widespread occurrence of aquitards, there is a long-lasting misconception that flowing wells must be geologically-controlled. The occurrence of flowing wells in topographic lows of unconfined aquifers was anticipated in 1940 and later verified in the 1960s, accompanying with the birth of the theory of topographically-driven groundwater flow, which has been considered as a paradigm shift in groundwater hydrology. Based on studies following this new paradigm, several preconditions of flowing wells given in the 19th century have been found to be not necessary at all. This historical perspective of the causes of flowing well conditions and the role of flowing wells on the science of groundwater could lead to a deeper understanding of the evolution of groundwater hydrology.
... The magnesium isotopic value for the groundwater source (−1.2‰ ± 0.2‰, 1σ, n = 27) is calculated as the mean value measured in silicate-and carbonatedominated shallow groundwater reservoirs 4,[72][73][74][75][76] . The mean isotopic value of groundwater in shallow silicate reservoirs (−1.28‰ ± 0.17‰, 1σ, n = 12) is within the 1σ error of the mean value in shallow carbonate reservoirs (−1.18‰ ± 0.17‰, 1σ, n = 15). ...
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The oceanic magnesium cycle is largely controlled by continental weathering and marine authigenic mineral formation, which are intimately linked to long-term climate. Uncertainties in the magnesium cycle propagate into other chemical budgets, and into interpretations of paleo-oceanographic reconstructions of seawater δ26Mg and Mg/Ca ratios. Here, we produce a detailed global map of the flux of dissolved magnesium from the ocean into deeper marine sediments (greater than ∼1 meter below seafloor), and quantify the global flux and associated isotopic fractionation. We find that this flux accounts for 15-20% of the output of magnesium from the ocean, with a flux-weighted fractionation factor of ∼0.9994 acting to increase the magnesium isotopic ratio in the ocean. Our analysis provides the best constraints to date on the sources and sinks that define the oceanic magnesium cycle, including new constraints on the output flux of magnesium and isotopic fractionation during low-temperature ridge flank hydrothermal circulation.
... Magnesium is a major element in evaporites and brines, and the Mg isotope composition of brine in the terrestrial basin is controlled by the mass balance of source (river input) and sink (carbonate precipitation) under different hydrological conditions (Shalev et al., 2018a). River waters show large Mg isotope variability (e.g., Tipper et al., 2006;Zhang et al., 2018), and precipitation of carbonate is associated with significant Mg isotope fractionation (e.g., Chen et al., 2020;Immenhauser et al., 2010;Li et al., 2015;Mavromatis et al., 2013;Wang et al., 2013). Thus the δ 26 Mg value of brine and precipitated halite has the potential to record the history of basin hydrology. ...
Article
Saline lakes are sensitive to climatic changes, however, it is challenging to reconstruct paleoclimate based on terrestrial evaporite records using conventional elemental and isotopic proxies. Magnesium is a major element in saline lakes, and the Mg isotope composition of brine is responsive to climate-driven processes such as carbonate precipitation and freshwater input. However, little has been explored on the application of Mg isotopes to studies of saline lakes. In this study, the Middle Pleistocene halite deposits of the Xiaoliangshan (XLS) evaporite section in the Qaidam Basin, Northwest China, was selected as a case to evaluate the response of Mg isotopes in the saline lake to environmental events. The Mg isotope data of the halite are complemented by geochemical analyses major elements and SrCl isotopes of the halite component, and CO isotopes of the associated carbonates. The ⁸⁷Sr/⁸⁶Sr ratios of the halite remained homogeneous (0.7111 to 0.7112) throughout the section, suggesting that the material source did not change significantly during the precipitation of the halite beds. By contrast, element ratio (Mg/Na and K/Na) and stable isotope ratios of C, O, Cl, and Mg show remarkable fluctuations along with the sediment profile. Based on the C-O-Cl isotope data, we identified events of freshwater recharging and desalination in an overall dry climatic background that were recorded in the halite record. Notably, halite δ²⁶Mg values vary by up to 2‰ in the section (from −1.63‰ to 0.46‰), and the low δ²⁶Mg signature of halite is interpreted to reflect the input of light Mg isotopes into the saline lake in a freshwater recharging event, while the high δ²⁶Mg values were produced by the precipitation of carbonate minerals under arid climatic conditions. Collectively, we suggest that Mg isotopes in terrestrial halite could be a sensitive tracer of basin hydrology. This study for the first time demonstrates the potential of Mg isotopes in terrestrial evaporites to unravel paleoclimatic events.
... At present, the information on solute isotopic fractionation associated with adsorption-desorption or/and ion exchange on clays or/and organic matter is limited (Tipper et al., 2016;Zhang et al., 2018). One of the most important reasons is a lack of accurate samples in field and information on potential endmembers (especially the exchangeable component). ...
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In this study we present the first Mg isotope data that record the fate of Mg during mineralisation of atmospheric CO2 in ultramafic mine tailings. At the Woodsreef Asbestos Mine, New South Wales, Australia, weathering of ultramafic mine waste sequesters significant amounts of CO2 in hydromagnesite [Mg5(CO3)4(OH)2·4H2O]. Mineralisation of CO2 in above-ground, sub-aerially stored tailings is driven by the infiltration of rainwater dissolving Mg from bedrock minerals present in the tailings. Hydromagnesite, forming on the surface of the tailings, has lower δ²⁶Mg (δ²⁶MgHmgs = -1.48 ± 0.02 ‰) than the serpentinised harzburgite bedrock (δ²⁶MgSerpentinite = -0.10 ± 0.06 ‰), the bulk tailings (δ²⁶MgBulk tailings = -0.29 ± 0.03 ‰) and weathered tailings containing authigenic clay minerals (δ²⁶MgWeathered tailings = +0.28 ± 0.06 ‰). Dripwater (δ²⁶MgDripwater = -1.79 ± 0.02 ‰) and co-existing hydromagnesite (δ²⁶MgHmgs = -2.01 ± 0.09 ‰), forming in a tunnel within the tailings, and nodular bedrock magnesite [MgCO3] (δ²⁶MgMgs = -3.26 ± 0.10 ‰) have lower δ²⁶Mg than surficial fluid (δ²⁶Mg = -0.36 ‰) and hydromagnesite. Complete dissolution of source minerals, or formation of Mg-poor products during weathering, is expected to transfer Mg into solution without significant alteration of the Mg isotopic composition. Aqueous geochemical data and modelling of saturation indices, along with Rayleigh distillation and mixing calculations, indicate that the ²⁶Mg-depletion in the drip water, relative to surficial water, is the result of brucite dissolution and/or precipitation of secondary Mg-bearing silicates and cannot be assigned to bedrock magnesite dissolution. Our results show that the main mineral sources of Mg in the tailings (silicate, oxide/hydroxide and carbonate minerals) are isotopically distinct and that the Mg isotopic composition of fluids and thus of the precipitating hydromagnesite reflects both isotopic composition of source minerals and precipitation of Mg-rich secondary phases. The consistent enrichment and depletion of ²⁶Mg in secondary silicates and carbonates, respectively, underpins the use of the presented hydromagnesite and fluid Mg isotopic compositions as a tracer of Mg sources and pathways during CO2 mineralisation in ultramafic rocks.
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Understanding and quantifying the biogeochemical cycle of carbon during the alteration process was modeled according to different parameters. The assessment of the alteration balance of rocks and the determination of control factors is a major tool for understanding the Earth's climate. Chemical erosion of silicate and carbonate minerals in the long term plays an important role in the (Berner et al., 1983). Magnesium is a chemical soluble in water, it is mainly due to the alteration of rocks. The phenomenon of alteration is a preponderant factor in the regularization of the terrestrial climate by consuming atmospheric CO₂. Moreover, this chemical element (Mg) represents a nutrient essential to life (plant growth). The objective of my thesis work is to develop a tool isotope to trace the biogeochemical cycle of Mg. My job will be to quantify and qualify the biogeochemical cycle at the small catchment scale. On the other hand, we will be interested in basins on a larger scale. The Himalayan and Taiwanese basins which are characterized by of alteration strongly influenced by the reaction kinetics.
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Lithium (Li) isotopes have shown large potential in tracing weathering in various water bodies, but there is limited study on Li isotopes in subsurface conditions where CO2 has been largely consumed. In this study, we use a thick sandstone aquifer in the Ordos Basin, NW China, as a natural setting to investigate the behaviors of Li isotopes in hydrogeochemical conditions with different concentrations of dissolved CO2. For young groundwater in the recharge area (group R) where CO2 is abundant (mean PCO2 = 10-2.5 atm), clay formation accompanying with weathering leads to the enrichment of ⁷Li in groundwater. The four deep samples in the recharge area have uniform Li/Na ratios (with a mean of 2.52 μmol/mmol) and δ⁷Li (with a mean of 25.0‰), corresponding to a mean Li removal rate of 81.2% compared with the sandstone leachate. For groundwater in the shallow part of the discharge area (group D1), Li was firstly removed by clay formation during weathering in the recharge area and was later removed by physisorption when CO2 becomes much lower (mean PCO2 = 10-3.1 atm). Different degrees of weathering lead to a wide range of δ⁷Li varying from 19.7‰ in the deepest well to 33.0‰ in the shallowest well. The proportion of Li removal caused by physisorption is found to increase with groundwater age. After the stage of Li removal by adsorption, Li was released in the deeper part of the discharge area (group D2), and the positive correlation of δ⁷Li versus Li/Na is explained by a ternary mixing model. The endmember of water brought by cation exchange is inferred to have a heavier δ⁷Li than sandstone leachate, demonstrating that cation exchange could cause an enrichment of ⁷Li in water. This study enhances our understanding of the controlling factors of Li isotopes in deep groundwater with low dissolved CO2, which have implications for the application of Li isotopes in subsurface water.
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The gushing of water from flowing wells attracted public attention and scientific curiosity as early as the 17th century, but little attention has been paid to the influence of flowing wells on the evolution of groundwater science. This study asserts that questions posed by flowing wells since the early 19th century led to the birth of many fundamental concepts and principles of physical hydrogeology. Due to the widespread occurrence of flowing wells in basins with regional aquitards, there is a long-lasting misconception that flowing wells could only occur in regionally confined aquifers. However, the recognition of possible occurrence of flowing wells in unconfined aquifers was anticipated at the turn of the 20th century based on observed increases in hydraulic head with depth in topographic lows of basins without apparent aquitards. This was later verified in the 1960s by field and modeling studies that gave birth to quantitative analysis of topographically driven groundwater flow systems, which was a paradigm shift in hydrogeology. Following this paradigm, several preconditions for flowing wells established in the 19th century were found unnecessary. Intermingled in the evolution of flow system concepts are inconsistencies and confusion concerning the use of the term “artesian”, so we propose avoidance of this term. This historical perspective of the causes of flowing well conditions and the influence of flowing wells on groundwater science could lead to a deeper understanding of the evolution of groundwater science and guide future studies on hydraulics of flowing wells.
Thesis
La compréhension et la quantification du cycle biogéochimique du Carbone lors du processus d’altération a été modélisé en fonction de différents paramètres. L’évaluation du bilan d’altération des roches et la détermination des facteurs de contrôle sont un outil majeur pour la compréhension du climat terrestre. L’érosion chimique des minéraux silicatés et carbonatés, sur le long terme, joue un rôle important dans la régularisation du climat (Berner et al. 1983). Le Magnésium est un élément chimique soluble dans l’eau, il provient principalement de l’altération des roches. Le phénomène d’altération est un facteur prépondérant dans la régularisation du climat terrestre en consommant du CO₂ atmosphérique. Par ailleurs, cet élément chimique (Mg) représente un nutriment essentiel à la vie (croissance des plantes). L’objectif de mon travail de thèse est de développer un outil isotopique pour retracer le cycle biogéochimique du Mg. Mon travail consistera à quantifier et qualifier le cycle biogéochimique à l’échelle de petit bassin versant. D’autre part, nous allons nous intéresser à des bassins à plus grande échelle. Les bassins Himalayen et Taiwanais qui sont caractérisés par des processus d’altération fortement influencés par la cinétique réactionnelle.
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Causes and water sources of flowing artesian wells attracted the interest of many hydrogeologists throughout history, however, a quantitative model that satisfactorily considers the roles of topography, groundwater recharge/discharge and aquitards on hydraulics of flowing wells is still lacking. In this study, a three-layer river-valley basin with a recharge boundary is used to obtain the basinal flow field, spatial distribution and transient discharge rates of flowing wells. Even if there is an aquitard separating the unconfined and the confined aquifer, groundwater discharge to the river still plays a critical role in occurrence of flowing wells. An aquitard with lower permeability would enlarge the zone with flowing wells, implying that flowing wells are more likely to occur in basins with continuous aquitards. By comparing flowpaths and inter-aquifer leakages before and after numerically installing a flowing well in a three-layer basin, we find decreased upward leakage in the discharge limb plays a much larger role than increased downward leakage in the recharge limb in the stable discharge rate of a flowing well. This recognition is different from previous studies where the stable discharge rate is supported only by downward leakage from the overlying aquifer. By considering well hydraulics in a basinal background flow field, this study improves understanding of the mechanisms of flowing artesian wells and the interaction of surface water and deep groundwater, indicating that flowing wells are valuable for sampling deep groundwater representative of deep and old groundwater in the discharge area of a basin.
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The occurrence of flowing wells in basins has been found to be closely related to the discharge area with an upward hydraulic gradient. Unfortunately, previous studies on upward gradient induced wellbore flow with equaling total inflow (Qin) in the deep and total outflow (Qout) in the shallow could not explain the occurrence of flowing wells. By representing wells using the MNW2 Package imbedded in MODFLOW 2005, we obtain the exchange of groundwater between the aquifer and the well in the discharge area of 3D unit basins and identify three scenarios: Qin=Qout, Qin>Qout>0 and Qin>Qout=0. The relationship of Qin>Qout well explains why flowing wells only develop in a limited part of the discharge area. Sensitivity analysis shows that well location, water table undulation and basin length‐depth ratio do not change the profile of the ratio of cumulative flow rate in the flowing well to total inflow (Qv/Qin) versus the relative elevation in the inflow segment, zin*, but could significantly change the length of the inflow segment; well depth could change both the length of the inflow segment and the profile of Qv/Qin versus zin*. Based on numerical results in homogeneous and isotropic basins with different dimensions, the ratio of inflow in the lower half part of a flowing well to the total inflow is found to be at least 68% and could be close to 100%, indicating that water at the outlets of flowing wells with long open sections is mainly from the deep part of the well.
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The Ordos Basin is one of the most intensively studied groundwater basins in China. The Ordos Plateau, located in the north part of the Ordos Basin, is ideal to study the pattern of regional groundwater circulation induced by water-table undulations due to the wavy topography and the relatively simple aquifer systems with macroscopically homogeneous sandstone. In catchments located near the first-order divide, the water table is found to be a subdued replica of the topography, and the nonclosed water-table contours in topographic highs of a catchment are indicative of regional groundwater outflow to other catchments. In topographic lows, groundwater-fed lakes/rivers, topography-driven flowing wells, water-loving and/or salt-tolerant vegetation, and soap holes are all indicative of discharge areas. In discharge areas, although groundwater inflow from recharge areas is relatively stable, seasonal variations in groundwater recharge and evapotranspiration lead to significant seasonal fluctuations in the water table, which can be used to estimate groundwater inflow and evapotranspiration rates based on water balance at different stages of water-table change. In the lowest reaches of a complex basin, superposition of local flow systems on regional flow systems has been identified based on groundwater samples collected from wells with different depths and geophysical measurements of apparent resistivity, both of which can be used for characterizing groundwater flow systems. This study enhances understanding of the pattern of regional groundwater circulation in the Ordos Plateau, and also tests the effectiveness of methods for groundwater flow-system characterization.
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Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ26Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ26Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ26Mg values (− 0.46 to − 0.26‰ and − 0.41 to − 0.17‰, respectively) significantly lighter than the mantle (− 0.25 ± 0.07‰, 2SD). No correlation of δ26Mg with either SiO2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ26Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high 87Sr/86Sr ratios (0.70814 to 0.71105) and negative correlation between δ26Mg and δ18O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120 km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources.
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Although it has been reported that flowing artesian wells could be topographically-controlled, there is no quantitative research on artesian flow conditions in unconfined aquifers. In this study, the water table, which has a lower amplitude than the land surface, is damped from the topography and used as the boundary condition to obtain the analytical solution of hydraulic head of a unit basin with a single flow system. The term artesian head is defined to characterize the condition of flowing artesian wells. The zone with positive artesian head is called artesian zone while with negative artesian head is non-artesian zone. The maximum artesian head and the size of artesian zones are found to increase with the damping factor and the anisotropy ratio, and decrease with the ratio of basin width to depth and the depth-decay exponent of hydraulic conductivity. Moreover, the artesian head increases with depth nearby the valley and decreases with depth near by the divide, and the variation rates are influenced by the decay exponent and the anisotropy ratio. Finally, the distribution of flowing artesian wells and the artesian head measurements in different depths of a borehole in a small catchment in the Ordos Plateau, Northwestern China is used to illustrate the theoretical findings. The change in artesian head with depth was used to estimate the anisotropy ratio and the decay exponent. This study opens up a new door to analyze basin-scale groundwater flow. This article is protected by copyright. All rights reserved.
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The major ions and Sr concentrations and the Sr isotopic compositions of rainwater from four weather stations in the Ordos desert, Northwest China, were measured in this study. In the studied rainwater, Ca2+ was the most abundant cation with a volume-weighted mean (VWM) value of 387 µmol/L, and SO42− was the dominant anion, with a VWM value of 229 µmol/L. The Sr concentrations varied from 0.1 to 72 µmol/L, and the strontium isotopic ratios (87Sr/86Sr) range from 0.7098 to 0.7110, with an average of 0.7106, which are higher than that of seawater. The 87Sr/86Sr ratios showed the potential to trace sources of rainwater solutes when combined with other chemical composition data. The covariation between Mg/Ca vs. Ca/Na and Sr isotopic ratios vs. Ca/Sr in the rainwater suggested the mixture of at least three sources: soil dust derived from the local area and/or desert and loess areas in northwest China (87Sr/86Sr values of 0.7114), seawater (87Sr/86Sr value of 0.70917), and anthropogenic inputs (87Sr/86Sr value of 0.7110 and high Ca/Sr, due to coal combustion or automobile exhausts). The human activity inputs are likely the major sources of atmospheric contaminants in the Ordos rainwater. From back-trajectory analysis, principal component analysis and a comparison between the major ion compositions of other selected sites in China, we conclude that the Ordos rainwater ion composition is more significantly influenced by desert and soil dust and anthropogenic sources (primarily industrial and traffic emissions from the surrounding cities) than by marine sources.
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The Dalucao deposit in western Sichuan Province, southwest China, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE Belt. Moreover, this is the only deposit identified in the southern part of the belt. The deposit contains the No. 1, 2, and 3 orebodies. The No. 1 and 3 orebodies are hosted in two breccia pipes within syenite–carbonatite rocks that intrude a Proterozoic quartz-diorite pluton. Both breccia pipes have elliptical horizontal cross-sections at the surface, being 200–400 m long, 180–200 m wide, and extending to > 450 m depth. The No. 1 and No. 3 orebodies have total thicknesses of 55–175 m and 14–58 m, respectively. REE mineralization is associated with four brecciation events that are recorded in both pipes. The ore grades in the No. 1 and 3 orebodies are similar, with the rocks containing 1.0–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a mineral assemblage comprising fluorite + barite + celestite + bastnäsite (i.e., Type I), whereas the No. 3 orebody is characterized by an assemblage comprising fluorite + celestite + pyrite + muscovite + bastnäsite + strontianite (i.e., Type II). Significant amounts of weathered high-grade REE ore (up to 60 wt% of the rock mass) is mainly present in the No. 1 orebody. This is the main ore-type targeted for exploration within the Dalucao deposit, but is rarely present in other deposits in the Mianning–Dechang REE Belt.
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Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: δ26Mg values (− 0.6‰ to − 0.1‰) of stream and soil pore waters are about ~ 0.5‰ to 1‰ lighter than the shale bedrock δ(26Mg values of + 0.4‰), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg2 + concentrations but lower δ26Mg values (− 1.1‰) in groundwater, ~ 1.5‰ lighter than the bedrock. δ26Mg values (+ 0.2‰ to + 0.4‰) of soil samples on the planar hillslope are either similar or up to ~ 0.2‰ lighter than the bedrock. Hence a heavy Mg isotope reservoir – complementary to the lighter Mg isotopes in soil pore water and stream water – is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in δ26Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution–precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments (δ26Mg values of + 0.3‰ to + 0.5‰) accumulated on the valley floor are ~ 0.2‰ heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir.
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Groundwater flow systems and stagnant zones in drainage basins are critical to a series of geologic processes. Unfortunately, the difficulty of mapping flow system boundaries and no field example of detected stagnant zones restrict the application of the concept of nested flow systems. By assuming the variation in bulk resistivity of an aquifer with uniform porosity is mainly caused by groundwater salinity, the magnetotelluric technique is used to obtain the apparent resistivity of a profile across a groundwater-fed river in the Ordos Plateau, China. Based on the variations in apparent resistivity of the Cretaceous sandstone aquifer, the basin-bottom hydraulic trap below the river has been detected for the first time, and its size is found to be large enough for possible deposition of large ore bodies. The boundary between local and regional flows has also been identified, which would be useful for groundwater exploration and calibration of large-scale groundwater models.
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Magnesium isotopic compositions of a set of clay-rich saprolites developed on the Neogene tholeiitic basalt from Hainan Island in southern China have been measured in order to document the behavior of Mg isotopes during continental weathering. Compared with unaltered basalts (δ26Mg=−0.36‰), the overlying saprolites are strongly depleted in Mg (i.e., τTh,Mg=−99.1% to −92.9%), and display highly variable δ26Mg, ranging from −0.49‰ to +0.40‰. Magnesium concentration and δ26Mg value of the saprolites display a general increasing trend upwards in the lower part of the profile, but a decreasing trend towards the surface in the upper part. The variations of Mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes: (1) adsorption of Mg to kaolin minerals (kaolinite and halloysite), with preferential uptake of heavy Mg isotopes onto kaolin minerals; and (2) desorption of Mg through cation exchange of Mg with the relatively lower hydration energy cations in the upper profile. Evidence for adsorption is supported by the positive correlation between δ26Mg and the modal abundance of kaolin minerals in saprolite of the lower profile, while negative correlations between δ26Mg and concentrations of lower hydration energy cations (e.g., Sr and Cs) in the upper profile support the desorption process. Our results highlight that adsorption and desorption of Mg on clay minerals play an important role in behavior of Mg isotopes during extreme weathering, which may help to explain the large variation in Mg isotopic composition of river waters.
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Seasonal changes in river chemistry offer the potential to assess how weathering processes respond to changing meteorological parameters and ultimately how chemical weathering might respond to climatic parameters. Systematic seasonal variations in magnesium isotope ratios (the 26Mg/24Mg ratio expressed as δ26Mg in per mil units) are reported in stream waters from a mono-lithological granitic, weathering-limited, first order catchment from the Swiss Alps (Damma glacier). Rain, ground, and pore-waters, in addition to plants, rocks, mineral separates and soil are also reported. The concentration response of the river waters is attenuated compared to the large changes in discharge. However, the systematic trends in the isotope data imply that either the source of the Mg changes in a systematic manner, or that the process by which Mg is released into solution changes as a function of discharge.
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The stagnant zones in nested flow systems have been assumed to be critical to accumulation of transported matter, such as metallic ions and hydrocarbons in drainage basins. However, little quantitative research has been devoted to prove this assumption. In this paper, the transport of age mass is used as an example to demonstrate that transported matter could accumulate around stagnation points. The spatial distribution of model age is analyzed in a series of drainage basins of different depths. We found that groundwater age has a local or regional maximum value around each stagnation point, which proves the accumulation of age mass. In basins where local, intermediate and regional flow systems are all well developed, the regional maximum groundwater age occurs at the regional stagnation point below the basin valley. This can be attributed to the long travel distances of regional flow systems as well as stagnancy of the water. However, when local flow systems dominate, the maximum groundwater age in the basin can be located around the local stagnation points due to stagnancy, which are far away from the basin valley. A case study is presented to illustrate groundwater flow and age in the Ordos Plateau, northwestern China. The accumulation of age mass around stagnation points is confirmed by tracer age determined by 14C dating in two boreholes and simulated age near local stagnation points under different dispersivities. The results will help shed light on the relationship between groundwater flow and distributions of groundwater age, hydrochemistry, mineral resources, and hydrocarbons in drainage basins.
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We present here a field geochemical study of controls on carbonate weathering within rapidly circulating, shallow groundwater-surface water systems in the glaciated mid-continent region. Groundwaters and surface waters in three watersheds spanning the Upper to Lower Peninsulas of Michigan consist of Ca2+-Mg2+-HCO3 - solutions derived from the open-system dissolution of calcite and dolomite in soils developed on mixed mineralogy glacial drift. The thermodynamic stabilities of calcite and dolomite both decrease with decreasing temperature, with dolomite more strongly affected. Thus, the low mean annual temperature of these temperate weathering environments maximizes the absolute solubility of dolomite as well as its solubility relative to calcite. Many groundwaters in the study area approach equilibrium with respect to the more soluble dolomite and are moderately supersaturated with respect to calcite. Groundwaters in each watershed have distinct and relatively narrow ranges of carbon dioxide partial pressure (PCO2) values, which increase significantly from north to south (log PCO2 of -3.0 to -2.2 atm), suggesting that there are landscape-level differences in carbon transformation rates in soil weathering zones. Increases in weathering-zone PCO2 values produce HCO3 - concentrations that vary by a factor of five, but the Mg2+/Ca2+ and Mg2+/HCO3 - ratios of all groundwaters are similar, suggesting relatively constant weathering input ratios of calcite and dolomite. Although surface waters commonly are between 2 and 10 times supersaturated with respect to calcite, the Mg2/HCO3 - ratios of surface waters are very close to initial groundwater values, suggesting that back precipitation of calcite is not a significant process in these systems. The enhanced solubility of dolomite at low temperatures coupled with the landscape-level differences in carbon cycling suggest that temperate-zone weathering reactions in glaciated terrains are significant contributors to continent-scale fluxes of both Mg2+ and HCO3-.
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The high precision with which Mg isotope ratios can be measured using MC-ICPMS opens new opportunities for using Mg as a tracer in both terrestrial and extraterrestrial materials. A key advance is the ability to resolve kinetic from equilibrium mass-dependent fractionation processes. From these new data it appears that Mg in waters is related to mantle and crustal reservoirs of Mg by kinetic fractionation while Mg in carbonates is related in turn to the waters by equilibrium processes. Resolution of different fractionation laws is only possible for measurements of Mg in solution at present; laser ablation combined with MC-ICPMS (LAMC-ICPMS) is not yet sufficiently precise to measure different fractionation laws. Variability in Mg isotope ratios among ch ondritic meteorites and their constituents is dominated by mixing between a radiogenic CAI-like reservoir and a reservoir resembling ordinary chondrites. The mixing is evident in δ25Mg and δ26Mg, Al/Mg, and Δ17O values but will require substantiation by collection of more MC-IPMS Mg isotope data together with oxygen isotope and elemental compositions of bulk objects. Laser ablation combined with LA-MC-ICPMS provides a new dimension to the analysis of Mg isotopes in calcium aluminum-rich inclusions from primitive meteorites. Dispersion in 26Mg*-27 Al/24Mg evolution lines can be correlated with mass-dependent variations in δ 25Mg that distinguish open-system from closed-system processes. The ultimate product of such studies will be a better understanding of the chronological significance of variations in 26Mg* in these objects.
Book
Understand the Environmental Processes That Control Groundwater Quality The integration of environmental isotopes with geochemical studies is now recognized as a routine approach to solving problems of natural and contaminated groundwater quality. Advanced sampling and analytical methods are readily accessible and affordable, providing abundant geochemical and isotope data for high spatial resolution and high frequency time series. Groundwater Geochemistry and Isotopes provides the theoretical understanding and interpretive methods and contains a useful chapter presenting the basics of sampling and analysis. This text teaches the thermodynamic basis and principal reactions involving the major ions, gases and isotopes during groundwater recharge, weathering and redox evolution. Subsequent chapters apply these principles in hands-on training for dating young groundwaters with tritium and helium and ancient systems with radiocarbon, radiohalides and noble gases, and for tracing reactions of the major contaminants of concern in groundwaters. • Covers the basics of solutes, gases and isotopes in water, and concentration-activity relationships and reactions • Describes tracing the water cycle, weathering, and the geochemical evolution of water quality • Explores dating groundwater as young as a few years to over hundreds of millions of years • Uses case studies to demonstrate the application of geochemistry and isotopes for contaminated groundwaters. Accessible to consultants and practitioners as well as undergraduates, Groundwater Geochemistry and Isotopes presents the basics of environmental isotopes and geochemistry, and provides you with a full understanding of their use in natural and contaminated groundwater.
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As an indispensable component of groundwater circulation, groundwater evapotranspiration rate (ETG) estimation using water table fluctuations is a hot research topic in the past decades. However, most existing methods for estimating ETG using either diurnal or seasonal water table fluctuations are based on the assumptions that groundwater recharge equals 0 in the whole period and ETG equals 0 in a specific duration of time. The aim of the current study is to develop a method to estimate ETG when both assumptions do not apply. In the discharge area of a semi-arid catchment in the Ordos Plateau, NW China, it is found out that the water table fluctuations are influenced greatly by recharge, ETG and the vertical inflow rate (qin) in the seasonal scale but are controlled by atmospheric pressure instead of evapotranspiration in the diurnal scale, which implies that the existing methods are not applicable. Therefore, we propose a method to estimate ETG and qin using the seasonal water table fluctuations based on the assumption that ETG has a linear relationship to the reference evapotranspiration (ET0), which is readily available based on meteorological data. It is found out that qin is around 1.12 mm/d, and the ratio of ETG to ET0 is around 0.4. ETG in April through July with the highest ET0 of the year is found to be around 3 mm/d, which is much larger than qin, thus leads to significant decline in water table. ETG in November with the lowest ET0 of the unfrozen period is found to be 1.0 mm/d, which is slightly smaller than qin, thus results in a weak increase in water table. The vertical component of regional groundwater inflow is found to exceed precipitation recharge, indicating that regional groundwater inflow plays the major role on maintaining the shallow water table in the discharge area. Groundwater evapotranspiration is also found to exceed precipitation recharge in most time of the unfrozen period, indicating a specified-flux upper boundary with water loss in the discharge area of an arid catchment. The proposed method could be applied to similar study areas and is critical to understand the budget of groundwater.
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This work focuses on the behavior of the stable Mg and Si isotope compositions of the largest Arctic river, the Yenisey River and 28 of its major and minor tributaries during the spring flood period. Samples were collected along a 1500 km latitudinal profile covering a wide range of permafrost, lithology, and vegetation. Despite significant contrasts in the main physico-geographical, climate, and lithological parameters of the watersheds, the isotope composition of both dissolved Mg and Si was found to be only weakly influenced by the degree of the permafrost coverage, type of vegetation (forest vs. tundra), and lithology (granites, basalts, carbonates or terrigenous rocks). This observation is generally consistent with the lack of chemical uptake of Mg and Si by soil mineral formation and vegetation during the early spring. The radiogenic Sr isotope composition of the Yenisey and its tributaries varied within a narrow range (0.708 ⩽ ⁸⁷Sr/⁸⁶Sr ⩽ 0.711) reflecting the dominance of Phanerozoic rock weathering and/or atmospheric deposition on these compositions. The Mg and Si isotopic compositions of riverine samples reflect two main processes with distinct isotopic signatures. First, isotopically heavier Mg (δ²⁶Mg = −1.0 ± 0.2‰) and isotopically lighter Si (δ³⁰Si = 1.0 ± 0.25‰) are added to the waters by river suspended matter dissolution and leaching from vegetation biomass/topsoil litter. Second, isotopically lighter Mg (δ²⁶Mg = −1.5 to −1.75‰) and isotopically heavier Si (δ³⁰Si = 1.75–2.0‰) are delivered to the Yenisey's tributaries from deep underground water feeding the rivers via taliks. This lighter Mg and heavier Si isotopic composition is interpreted to originate from Precambrian dolomite dissolution and aluminosilicate dissolution coupled with authigenic mineral precipitation, respectively, in deep underground water reservoirs. Taking account of the isotopic composition evolution over the course of the year established earlier on mono-lithological watersheds of the Yenisey basin, the average annual isotopic signatures of the Yenisey river arriving to the Arctic Ocean are estimated to be δ²⁶Mg = −1.58 ± 0.30‰ and δ³⁰Si = +1.60 ± 0.25‰. As the Yenisey is the largest river feeding the Arctic Ocean and as it samples a large variety of environments and lithologies, these values may be reasonable estimates for the average Mg and Si isotopic composition of the dissolved riverine flux to the Arctic Ocean.
Book
This text presents a series of thematic chapters together with chapters on representative groundwater systems in Europe which illustrate the main processes and evolution of water quality. Brings together the research of a consortium of leading European scientists who have conducted detailed studies of water quality in Europe. Includes a synthesis of findings, highlighting the thematic and regional results, with recommendations regarding aquifer evaluation, indicators, monitoring, and drinking water standards. Creates a key reference work on natural water quality of aquifers, at a time when the Groundwater Directive (GD) will shortly be brought in to supplement The European Water Framework Directive (WFD) to ensure good status of groundwater.
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A comprehensive world-wide database has been established of all research, development, demonstration and commercial activities in thermochemical biomass conversion. This paper explains the objectives and gives some preliminary results, a complete listing of all activities identified to date, and a sample printout of one entry.
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Combined with groundwater chemistry data and using Sr content and 87 Sr/86 Sr ratios of hydrous media and groundwater, this paper studies the mechanism of water-rock interaction in the Cretaceous aquifer of Ordos Desert Plateau. The result indicates that K-feldspar, plagioclase and carbonate minerals in hydrous media have different 87 Sr/ 86 Sr ratios and the 87 Sr/86 Sr ratios of groundwater are between 0. 709871 and 0. 711792. Sr in groundwater is primarily derived from carbonate minerals, sulfate minerals and plagioclase of silicate minerals. The water-rock interactions in the groundwater of Huan River Group in the Cretaceous aquifer are mainly carbonate mineral dissolution, sulfate mineral dissolution and plagioclase dissolution,and the interactions of Luo River Group are sulfate mineral dissolution and K-feldspar dissolution. The main water-rock interactions include the carbonate mineral dissolution,sulfate mineral dissolution and silicate mineral dissolution in the Cretaceous aquifer of Ordos Desert Plateau.
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Earliest marine diagenetic sabkha type dolomites are widespread in Earth’s geological record. Potentially, these carbonates may act as archives of past seawater magnesium isotope (δ26Mg) ratios. At present, however, the fractionation of magnesium isotopes (Δ26Mg) between seawater - here evaporated marine porewater - and sabkha dolomite is not constrained. In order to explore Δ26Mgdol-Mg(porewater), we make use of actualistic sabkha type dolomite precipitation in the Gulf region (Trucial coast, United Arab Emirates). This paper documents and discusses the first detailed sabkha δ26Mg data set of Mg-bearing solids including stoichiometric dolomites (degree of ordering > 0.9; mean δ26Mgdol = -0.79‰ ± 0.41 2σ, n = 17) and related marine pore waters. The presence of dolomite crystals with a broad range of ordering in shallow cores is documented using X-ray diffraction and scanning electron microscopy, but individual crystals are too small (< 10 microns) to be mechanically separated from their host sediment. Hence, a method was developed to chemically separate the most stoichiometric dolomite crystals from coexisting less stoichiometric dolomites and other Mg-bearing minerals and fluid phases by using disodium ethylenediaminetetracetic acid. Sabkha shallow ground water collected in trenches displays δ26Mg values of about -0.59‰, i.e. is only moderately enriched in 26 Mg relative to the present-day seawater signature of -0.83‰. Conversely, the δ26Mg of evaporated porewater is enriched by + 0.43‰ relative to that of δ26Mgseawater. Here we use the term “apparent” fractionation for the variable Δ26Mgdol-Mg(porewater) (+ 0.1 and -0.7‰) obtained that cannot be compared with experimentally deduced fractionation factors from controlled laboratory settings. Moreover, evaporated sabkha porewater differs, in terms of its isotope signature, from seawater. With regard to the current level of knowledge, the hypothesis that earliest diagenetic sabkha dolomites represent direct archives of secular changes of seawater δ26Mg values with time is not supported by our dataset. Our present understanding is that the magnesium isotope signature of sabkha dolomites is related to complex kinetics of precursor formation, dissolution/precipitation reactions including microbiological effects and involves variable Mg sources and sinks in a temporally and spatially variable microenvironment.
Article
Geochemical variations in marine biogenic carbonates that are preserved in the geological record serve as proxies of past environmental change. However, interpreting most proxies is complicated by biologically-mediated vital effects, highlighting the need to develop new tools for reconstructing paleoenvironmental change. Recently, magnesium (Mg) isotope variability in carbonates has been explored extensively to determine its utility as a paleoenvironmental proxy. We review the results of these works, which have yielded valuable information on the factors affecting Mg isotope fractionation between carbonates and solution (Δ26Mgcarb-sol) in biogenic and abiogenic carbonate minerals. Strong evidence exists for a mineralogical control on Δ26Mgcarb-sol, with the negative offset from 0‰ following the sequence aragonite < dolomite < magnesite < calcite. Abiogenic carbonates precipitated from solutions with relatively high Mg/Ca ratios (>∼3 mol/mol) and saturation states (Ω >∼3) that are similar to seawater suggest that Δ26Mgcarb-sol has a temperature dependence of ∼0.01‰ °C-1 and is insensitive to precipitation rate. In contrast, a significant precipitation rate dependence is observed in calcites precipitated from solutions with relatively low Mg/Ca ratios (<∼3 mol/mol) and saturation states (Ω <∼3). This difference likely reflects varying mineral growth mechanisms and we discuss the degree to which Δ26Mgcarb-sol may be affected by factors such as fluid inclusions, amorphous calcium carbonate precursors, ion attachment/detachment kinetics, surface entrapment and Mg speciation. High-Mg calcite organisms, which likely precipitate from relatively unmodified seawater, also exhibit a temperature dependence of ∼0.01‰ °C-1, albeit sometimes with a systematic offset toward smaller fractionations. In contrast, strong vital effects in low-Mg calcite organisms, which exclude Mg from their calcifying fluids, lead to Δ26Mgcarb-sol values that exhibit no clear temperature dependence and are offset from abiogenic experiments. The majority of biogenic aragonites have Δ26Mgcarb-sol values that are slightly more positive than those in abiogenic experiments, but bivalves and one sclerosponge species can exhibit significantly larger fractionations. Although vital effects and analytical uncertainties will limit Δ26Mgcarb-sol paleotemperature reconstructions to anomalies of at least ±10 °C, Mg isotope variability in biogenic carbonates may be a useful proxy for the Mg isotope composition of seawater, which reflects continental weathering, dolomitization and hydrothermal activity.
Article
Magnesium (Mg) isotopes can be a proxy for directly constraining the sources of riverine Mg, but the dominant controls on riverine Mg isotope ratios are still uncertain. Here, we report Mg isotope ratios for river waters, experimental leachates and digestions, bulk rocks, and fertilizers in the Han River (HR), South Korea. The HR is composed of two lithologically distinct tributaries: the North Han River (NHR) that flows over only silicate rocks, and the South Han River (SHR) that flows over silicate and carbonate rocks. The lithological differences between the NHR and SHR are reflected in major ion, 87Sr/86Sr, and δ26Mg geochemistry. In particular, the NHR has lower major ion concentrations but higher 87Sr/86Sr ratios and δ26Mg values than the SHR. Simple mass balances and mixing equations indicate that if the riverine δ26Mg values in the HR system are mainly controlled by conservative mixing between silicate and carbonate weathering, the average carbonate end-member δ26Mg value should be unlikely lower than what are measured in this study. Although multiple process-related fractionations occur in the HR system, the enrichment of 24Mg in river waters relative to silicate rocks they drain could be mostly controlled by either fractionation or mixing between isotopically distinct reservoirs, such as minerals or fractions (labile and structural Mg), during dissolution, while the little depletion of 24Mg in the SHR waters relative to carbonate rocks they drain could be likely due to the input of groundwater with lower δ26Mg value rather than fractionation. However, it is difficult to identify the contribution of anthropogenic inputs to riverine δ26Mg because their effects are little. This study suggests that the potential of Mg isotopes for constraining Mg sources in a lithologically varied river basin can be enhanced with a better understanding of process-related fractionation.
Article
We present elemental and isotopic data detailing how the Mg isotope system behaves in natural and experimentally synthesized clay minerals. We show that the bulk Mg isotopic composition (δ26Mg) of a set of natural illite, montmorillonite and kaolinite spans a 2‰ range, and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable Mg. After acid leaching, these natural clays become relatively enriched in isotopically heavy Mg by between 0.2 and 1.6‰. Results of exchange experiments indicate that the Mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light Mg compared to the residual clay. The isotopic composition of this exchangeable Mg (-1.49 to -2.03‰) is characteristic of the isotopic composition of Mg found in many natural waters. Further experiments with an isotopically characterized MgCl2 solution shows that the clay minerals adsorb this exchangeable Mg with little or no isotopic fractionation, although we cannot discount the possibility that the uptake of exchangeable Mg does so with a slight preference for 24Mg. To characterize the behaviour of Mg isotopes during clay mineral formation we synthesized brucite (Mg(OH)2), which we consider to be a good analogue for the incorporation of Mg into the octahedral sheet of Mg-rich clay minerals or into the brucitic layer of clays such as chlorite. In our experiment the brucite mineral becomes enriched in the heavy isotopes of Mg while the corresponding solution is always relatively enriched in isotopically light Mg. The system reaches a steady state after 10 days with a final fractionation factor (αsolid-solution) of 1.0005 at near-neutral pH. This result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy Mg during their formation. However our results also show that exchangeable Mg is an important component within bulk clay minerals and can have an important influence over the bulk clay δ26Mg value. Modeling shows that in certain soils or sediments where the percentage of exchangeable Mg is >40% and the isotopic composition of the exchangeable Mg is around -2‰, the generation of bulk δ26Mg values of <-0.5‰ is likely. On a broader scale, Mg-rich minerals such as smectite and illite are likely to impart a stronger control over the Mg budget in clay rich sediments, and their high structural Mg component is likely to result in bulk sediment δ26Mg values that are closer in composition to the UCC. Despite this, results of modeling, together with experimental observation suggests that the uptake of exchangeable Mg into these clay rich sediments could cause a decrease in the bulk δ26Mg value by up to ∼0.3-0.4‰. This should be accounted for when assessing the δ26Mg value of sediments on a crustal scale.
Article
Calcite growth experiments have been performed in the presence of aqueous Mg at 25 °C and 1 bar pCO2 to quantify magnesium partition coefficient DMg=(Mg/Ca)solid(Mg/Ca)fluid and Mg isotope fractionation between calcite and reactive fluid (Δ26Mgcalcite–fluid) as a function of calcite precipitation rate rp (mol m−2 s−1). Mg partition coefficient, DMg, increases with calcite growth rate according to:LogDMg=0.2517(±0.0150)×Logrp+0.0944(±0.0182);R2=0.93,(10-8.3⩽rp⩽10-6.6molm-2s-1) Δ26Mgcalcite–fluid was found to depend heavily on calcite growth rate with preferential incorporation of 24Mg in calcite and the extent of isotope fractionation decreasing with increasing calcite growth rate in accord with:Δ26Mgcalcite-fluid=0.7918(±0.0452)×Logrp+3.2366(±0.3360);R2=0.97(10-8.3⩽rp⩽10-6.6molm-2s-1) The negative Δ26Mgcalcite–fluid values found in this study, with calcite overgrowths enriched in light Mg, are consistent with (i) recent experimental data on Mg isotope fractionation during low-Mg calcite homogeneous precipitation (Immenhauser et al., 2010) and magnesite growth (Pearce et al., 2012) and (ii) with theoretical values calculated for Mg-calcite by density-functional electronic structure models (Rustad et al., 2010). The deviation of the isotopic composition of precipitated Mg-calcite from the equilibrium mass fractionation line in a three isotope diagram is a linear function of calcite growth rate. The equilibrium Δ26Mgcalcite–fluid value at 25 °C derived from this linear extrapolation, Δ26Mgcalcite–fluid = −3.5 ± 0.2‰ (2σ), is in good agreement with the theoretical value calculated by Rustad et al. (2010) for Mg-calcite (Δ26Mgcalcite–fluid = −3.6‰; BP86 functional). A striking feature of the results of this study is the decrease of the extent of Mg isotope fractionation (from −3.16‰ to −1.88‰) with the increase of calcite precipitation rate (from 10−8.3 to 10−6.6 mol m−2 s−1) which is opposite to the variation of Ca, Ba and Sr isotope fractionation with calcite precipitation rate. This behavior likely stems from the strong free energy of hydration of the Mg2+ ion compared to Ca2+, Ba2+ and Sr2+ which leads, during fast calcite growth, to the entrapment in calcite overgrowths of hydrated Mg ions whose isotopic composition is close to that of aqueous Mg2+. The strong dependence of Mg isotope fractionation on calcite growth rate suggests that, using the three isotopes method, Mg isotopic signatures of calcite in association with those of other divalent metals (Zn2+, Cu2+) have the potential to reveal mineral precipitation rates and thus environmental conditions of the oceans over geological time.
Article
Magnesium (Mg) stable isotopes are increasingly used as a weathering proxy in soils and rivers, but the impact of the mineralogy of secondary phases on isotope fractionation remains obscure. A better understanding of the behaviour of Mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone. Here we investigate isotopic variations in δ26Mg in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from Guadeloupe formed under contrasting climatic conditions. Soils formed in drier conditions (low precipitation) contain smectite, whereas soils formed under wet conditions (high rainfall) are characterized by halloysite and Fe-oxides or kaolinite.All clay fractions have Mg isotopic compositions (δ26Mg −0.41‰ to −0.10‰) similar to or heavier than their parent andesite (δ26Mg −0.47‰) supporting the preferential incorporation of heavy Mg isotopes in secondary Mg-bearing clay minerals with the first direct measurements on clay fractions. Soils with lighter Mg isotope compositions have greater quantities of exchangeable Mg. The data support a contribution from sea spray to the exchangeable Mg pool correlated to the soil weathering degree. This study highlights for the first time that the soil δ26Mg not only depend on δ26Mg of the parent rock, and on any fractionation that might occur, but also on the Mg retention on the exchange complex, which could in turn be controlled by external inputs such as sea spray.
Article
a b s t r a c t During chemical weathering, magnesium (Mg) is released by the dissolution of both carbonate and silicate sources. The degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs, or cycling by a series of processes associated with weathering is poorly constrained. In the river waters of the Mackenzie Basin (Canada), the Mg content is high and Mg isotope ratios (26 Mg/ 24 Mg expressed as d 26 Mg) show in excess of one per mil variability. Part of this variability is attributed to the 3% range in the carbonate and silicate rocks drained. Despite this inherent lithological control on river water d 26 Mg values, there is also evidence for a fractionation control. A linear positive covariation between lithium (7 Li/ 6 Li, expressed as d 7 Li) and Mg isotope ratios in the river waters of the Mackenzie Basin is reported. This covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with Mg isotope ratios. This continental-scale covariation can be resolved by either process-related fractionation or mixing. Evidence for fractionation associated with clays is provided firstly by comparing Mg and Li isotopes in both the waters and sediments carried in suspension. Secondly a linear covariation between the sediment concentrations of large ion lithophile elements caesium and rubidium (a proxy for clay content of the sediment) and d 26 Mg values of the water suggests that processes linked to clay, such as
Article
This study investigates the potential of Mg isotopes as tracers of biogeochemical processes in a small-forested catchment located on sandstones extremely poor in Mg-bearing minerals. The average d 26 Mg is À0.63 ± 0.12& and 0 ± 0.14& for local rainwater and bedrock, respectively. From the C horizon to the upper eluvial (E) horizon, soil d 26 Mg (from 0.0 ± 0.14& to 0.25 ± 0.14&) is close to the bedrock value, while more than 70% of Mg is lost, suggesting a small isotopic shift during illite dissolution. The surface soil horizon (A h) d 26 Mg is close to plant d 26 Mg, and especially to the grass d 26 Mg value (À0.49&). The bulk d 26 Mg of trees and grass (À0.32& and À0.41&, respectively) are higher than the average d 26 Mg values of the soil exchangeable fraction (À0.92& to À0.42&), and of rainwater (À0.65&). Within plants, roots are enriched in heavy isotopes, whereas light isotopes are preferentially translocated and stored in the above ground parts. In Norway spruce, the older nee-dles, forming the annual litterfall, are isotopically lighter and strongly depleted in Mg compared to more recent needles. Soil solution d 26 Mg shifts seasonally, from low values, lower than rainwater and close to litterfall during a high rainfall period in spring, to higher values, close to soil d 26 Mg in dryer periods of winter or summer. At the watershed scale, streamwater d 26 Mg varies between À0.85 ± 0.14& and À0.08 ± 0.14& and d 26 Mg values decrease linearly with discharge. The high streamwater d 26 Mg at low flow, close to bedrock d 26 Mg, most likely reflects dissolution processes in the deep saprolite in relation to the very long water residence time. Conversely, we suggest that low stream level d 26 Mg values are at least partly related to the contribution of surface flows from wet areas. Using a simple mass and isotopic balance approach, we compute that mineral dissolution rates in the soil (0.35 kg Mg ha À1 year À1) presently compensate for Mg losses from the soil.
Article
In order to evaluate the importance of hydrologic processes in controlling chemical weathering rates, a reactive transport analysis is used to interpret chemical weathering rate data for a range of systems. An analysis of weathering rates for granitic material shows that weathering rates depend most strongly on fluid residence times and fluid flow rates, and depend very weakly on material age. Over moderate fluid residence times from 5. days to 10. yr, characteristic of soils and some aquifers, transport-controlled weathering explains the orders of magnitude variation in weathering rates to a better extent than material age. For fluid residence times greater than 10. yr, characteristic of some aquifers, saprolites, and most marine sediments, a purely thermodynamic-control on chemical weathering rates sustains chemical weathering-this control may be due to clay precipitation, which can drive weathering of primary minerals, or microbial processes which alter the fluid chemistry via the oxidation of organic matter. In addition, this analysis suggests that the apparent time dependence of chemical weathering rates commonly used to model the evolution of Earth's landforms may be attributable to transport-controlled weathering and the evolution of hydrologic properties over time. If hydrologic processes are the primary control on chemical weathering rates, the nature of the temperature dependence of chemical weathering rates is also altered.
Article
The magnesium (Mg) isotope composition of rivers is sensitive to changes in the balance of primary mineral dissolution and secondary mineral formation. As these processes are regulated by climate then changes to the Earth's climate system, such as the onset of glaciation, could potentially alter the isotopic behaviour of Mg in rivers. To investigate how Mg isotopes behave during glacial weathering, we have determined the Mg concentration and Mg isotope ratio for the dissolved, suspended and bedload phases of glacial and non-glacial rivers in west Greenland. This region is essentially monolithological and there is little biological activity; hence isotope variations largely result from differences in weathering processes. Dissolved Mg in rivers is always enriched in the light Mg isotopes relative to the bedload (δ26Mg=−0.4‰), and the glacial rivers have lighter Mg isotope compositions (δ26Mg −1 to −1.3‰) than the non-glacial rivers (δ26Mg ~ −0.6‰). Enrichment of light Mg in the dissolved load is consistent with the preferential uptake of heavy Mg from solution during the formation of secondary silicate minerals. However, evidence from saturation state modelling suggests that little secondary mineral formation is likely to have occurred in the glacial rivers because the concentration of dissolved solids is very low. Moreover, the glacial derived suspended sediment has identical Mg isotope ratios to the bedload. These observations suggest that the formation of secondary weathering minerals is not controlling the behaviour of Mg isotopes in these rivers. Rather, variations in the δ26Mg value of the dissolved load are attributed to incongruent weathering of the solid phase: preferential dissolution of carbonate minerals, which have low δ26Mg (−1.1 to −5.2‰), imparts a more negative δ26Mg signal to the glacial river waters. Although our study indicates that the Mg isotopic composition of the dissolved load is sensitive to changes in weathering congruence, it is unlikely that glaciation will result in major changes in the δ26Mg composition of continental runoff.
Article
In rivers draining the Himalaya-Tibetan-Plateau region, the 26Mg/24Mg ratio has a range of 2‰ and the 44Ca/42Ca ratio has a range of 0.6‰. The average values of tributaries from each of the main lithotectonic units (Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Series (HHCS) and Lesser Himalayan Series (LHS)) are within 2 standard deviation analytical uncertainty (0.14‰). The consistency of average riverine values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average values by more than 2‰. Tributaries draining the dolostones of the LHS differ in their values compared to tributaries from the TSS and HHCS. The chemistry of these river waters is strongly influenced by dolostone (solute Mg/Ca close to unity) and both (−1.31‰) and (0.64‰) values are within analytical uncertainty of the LHS dolostone. These are the most elevated values in rivers and rock reported so far demonstrating that both riverine and bedrock values may show greater variability than previously thought.
Article
Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the ²³⁴U/²³⁸U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11–15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).
Article
High-precision Mg isotopic analysis was performed on a suite of well-characterized I-type granitoids and associated hornblende and biotite minerals from the Dabie Orogen in central China, to address the behavior of Mg isotopes during granite differentiation. Although these granitoids formed through different degrees of partial melting and fractional crystallization, with large variations in elemental and mineral compositions, their δ26Mg values vary from −0.26 to −0.14 and are indistinguishable within our analytical precision (±0.07‰; 2SD). Coexisting hornblendes and biotites in these granitoids display similar Mg isotopic composition, with δ26Mg ranging from −0.31 to −0.14 in hornblendes and −0.23 to −0.12 in biotites. The inter-mineral fractionation factors (Δ26MgHbl-Bt=δ26MgHbl−δ26MgBt) vary from −0.10 to −0.02, with an average=−0.06±0.08 (2SD). The limited inter-mineral fractionation agrees with the theoretic prediction that Mg cations in both hornblende and biotite are octahedrally coordinated with oxygen, which restricts the magnitude of equilibrium isotope fractionation. Overall, data from both bulk granitoids and associated mineral separates suggest that Mg isotope fractionation during I-type granite differentiation is limited.Collectively, granitoids studied here have Mg isotopic composition similar to that of terrestrial basalts and peridotites (δ26Mg=−0.21±0.07 vs. −0.25±0.07; 2SD), confirming that magmatic processes do not significantly fractionate Mg isotopes. The continental crust in the Dabie Orogen, as sampled by these I-type granitoids, has a mantle-like Mg isotopic composition. Given that significant Mg isotope fractionation occurs during chemical weathering processes, Mg isotopes may potentially be used for tracing granite genesis, in particular, if sedimentary materials are involved in granite sources.
Article
Magnesium-isotope time series MC-ICP-MS analyses from a Pleistocene speleothem – collected in a limestone cave in NW Africa (Morocco) – are reported and discussed in a process-oriented context. In addition, high-resolution C, O and Sr-isotope data, and Mg and Sr element abundances were compiled from the same stalagmite. Sub-samples were collected along the stalagmite growth axis and a second data set was drilled perpendicularly within one growth interval (Hendy test). The analytical results show clearly co-variant, systematic and cyclical fluctuations for all proxies collected along the growth axis and – with respect to the analytical error – invariant data within one growth increment. Magnesium-isotope ratios (δ26Mg) fluctuate between −4.39‰±0.02 2σ and −4.17‰±0.05 2σ and are within the range of published results of speleothem calcite from limestone caves. The difference of 0.22‰ is significantly beyond the error of the external reproducibility of ±0.03‰ 2σ for δ26 Mg. Considering the analytical error, neither a purely kinetic nor an equilibrium fractionation process explains the observed isotope pattern. For the time being, it is suggested that two external factors drive the speleothem Mg-isotope cyclicity: (1) climate-driven (arid versus humid) variances in the precipitation rate of a carbonate phase from meteoric water within the karstic system prior to entering the cave system; and (2) changing rates of silicate (aeolian material) versus carbonate weathering. Both of these processes fractionate the Mg-isotopic composition of runoff/seepage water. There is evidence that the magnesium-isotope system, applied to speleothem archives, bears significant information concerning continental climatic variability in arid zones and deserves further research.
Article
The effect of pressure on the solubility of CaCO3(calcite), CaF2, and SrSO4 in water was investigated for temperatures between 0 and 35 °C for pressures up to 1000 atm. It was found that for SrSO4 at 0 °C, and at 22 °C above 500 atm, and CaCO3 the high pressure solubility products were appreciably lower than those expected from calculations based on the anhydrous salts. It is postulated that this effect may be due to the formation of a bulk or surface hydrate. Consequently, care must be taken in assigning the solid phase when predicting the pressure dependence of the solubility of a sparingly soluble salt.
Book
This book is designed to fill the gap between elementary nonscience books on global environment and change and specialized volumes. The scope is on the global environment and specific examples encompass the globe and have strong reference support. Topics covered in the chapters include the following: global environment in terms of atmospheric, water, and oceanic circulations; air chemistry, the greenhouse effect and the ozone hole; global chemical environment; chemical weathering and water chemistry; lakes in terms of physical, biological and chemical processes and cycles; marine littoral environments; and the open ocean.
Article
This study presents lithium, magnesium and silicon isotope ratios from pore waters and soils from a well-characterised Histic Andosol in south-west Iceland. The soil δ7Li composition ranges between values slightly lighter than basalt, to those that are much heavier (−1.1‰ to +26.8‰), and are possibly influenced by sea salt. In contrast, precipitation-corrected dissolved (pore water) δ7Li values (1.8–10.0‰) appear to reflect preferential adsorption of 6Li onto secondary minerals, where allophane supersaturation results in high δ7Li values. Conversely low δ7Li together with high [Li] are probably due to destabilisation of allophane at low pH, and thus desorption of Li. When compared to Icelandic river values, it would appear that soil pore waters reflect an intermediate isotope composition between basalts and river waters. Precipitation corrected pore water Mg isotope ratios (δ26Mg) range between −0.46‰ and −0.12‰, and correlate with the amount of heavy Mg adsorbed onto the soil exchange complex. Silicon isotopes in the soils are isotopically lighter (δ30Si=−0.91‰ to −0.53‰) than basalt (−0.29‰), whereas pore waters are heavier (+0.13‰ to +1.03‰). Soil δ30Si values show a clear evolution between unweathered basalt and a hypothetical isotopically light endmember representing secondary minerals. Dissolved Si isotopes also respond to chemical weathering processes, and show that isotopically heavy δ30Si corresponds to high cation fluxes and high secondary mineral formation. However, comparison of all these proposed isotopic weathering tracers suggests that they respond differently to the same chemical weathering conditions. This indicates a differing behaviour during secondary mineral neoformation or adsorption depending on whether the incorporated element is a major or trace constituent. In turn, this behaviour can potentially yield important information on secondary mineral behaviour and destabilisation, and thus on the chemical weathering processes.
Article
Magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the Moselle basin, northeastern France. This small basin is remarkable for its well-constrained and varied lithology on a small distance scale, and this is reflected in river water Sr isotope compositions. Upstream, where the Moselle River drains silicate rocks of the Vosges mountains, waters are characterized by relatively high 87Sr/86Sr ratios (0.7128–0.7174). In contrast, downstream of the city of Epinal where the Moselle River flows through carbonates and evaporites of the Lorraine plateau, 87Sr/86Sr ratios are lower, down to 0.70824.
Article
To constrain how Mg isotopes behave during chemical interactions and physical transport in carbonate-rich settings, we measured δ26Mg values of surface water, groundwater, and dolomite samples from the Madison Aquifer, South Dakota. Groundwater in the Madison Aquifer chemically evolves by dedolomitization during transport along a 236 km flow path. Surface streams recharging the aquifer have δ26Mg values of − 1.08 and − 1.18‰. Following recharge, groundwater δ26Mg values vary between − 1.10 and − 1.63‰ up to a distance of 20 km. Between 20 and 189 km, δ26Mg values remain nearly constant at − 1.40‰, and a final sample at 236 km shows an increase to − 1.09‰. Dolomite exhibits a wide range of δ26Mg values between − 2.21 and − 1.27‰. Reactive-transport modeling and isotope mixing calculations employing previously published major ion mass-balances, 87Sr/86Sr ratios, and δ44Ca values were used to determine whether dedolomitization reactions, namely dolomite dissolution, calcite precipitation, and Mg-for-Na ion-exchange, fractionate Mg isotopes. We tentatively attribute the final δ26Mg value to preferential uptake of 24Mg during Mg-for-Na ion-exchange. Otherwise, we find little evidence of isotopic fractionation and observe instead that δ26Mg conservatively traces lithologic and hydrologic sources. Either isotope exchange between dolomite and water, with a fractionation factor of 0‰, or mixing between different water sources establishes the δ26Mg value of − 1.40‰ at 20 km. This value remains unchanged for the next 169 km of water transport because dolomite adds Mg with an average δ26Mg value near − 1.40‰, and no other processes cause fractionation. Calcite precipitation is unimportant either because calcite is not a significant sink for Mg or because Mg uptake during calcite precipitation under conditions of chemical equilibrium does not fractionate Mg isotopes. This study suggests Mg isotopes undergo conservative transport in carbonate-rich settings where waters are in chemical equilibrium with respect to major sources and sinks of Mg.
Article
Experimental precipitations of calcite and other carbonate minerals were performed under various conditions of pH, tem-perature and solution Mg/Ca to determine the Mg partition coefficient and Mg isotope fractionation. Fifteen experiments were performed at pH ranging from 7.41 ± 0.07 to 8.51 ± 0.39, temperature ranging from 16.2 ± 0.7 to 26.5 ± 0.3 °C and Mg/Ca solution ranging from 0.11 to 0.52 mol/mol. The apparent Mg partition coefficient between calcite and solution (D Mg) spans a large range of values from 0.018 ± 0.014 to 0.15 ± 0.11 and carbonate Mg isotope fractionation (D 26 Mg) ranges from À2.53 ± 0.25& to À1.33 ± 0.14& and does not correlate with either pH or temperature. The range in D Mg and D 26 Mg suggests non-equilibrium partitioning controlled by the processes of calcite growth, i.e. mixing between calcite grown at equilibrium and fluid inclusions, and entrapment of a surface Mg-rich calcite layer in isotopic equilibrium with the solution. The equilibrium Mg isotope fractionation between inorganic calcite and solution is estimated to be À2.13 ± 0.24&. Additional Mg elemental and isotopic fractionations are observed to occur during biogenic formation of calcite due to variable removal of Mg by the organisms (high-Mg calcite corals, foraminifera) of seawater Mg from their calcification medium.
Article
The Ordos Basin is a large-scale sedimentary basin in northwestern China. The hydrostratigraphic units from bottom to top are pre-Cambrian metamorphic rocks, Lower Paleozoic carbonate rocks, Upper Paleozoic to Mesozoic clastic rocks and Cenozoic deposits. The total thickness is up to 6000 m. Three groundwater systems are present in the Ordos Basin, based on the geological settings, i.e. the karst groundwater system, the Cretaceous clastic groundwater system and the Quaternary groundwater system. This paper describes systematically the groundwater flow patterns of each system and overall assessment of groundwater resources.