Calcium isotope fractionation in a silicate dominated Cenozoic aquifer system

doi:10.1016/j.jhydrol.2018.02.039

Journal of Hydrology

Volume 559, April 2018, Pages 523-533

https://doi.org/10.1016/j.jhydrol.2018.02.039 Get rights and content

Highlights

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Ca, C_DIC and Sr isotope patterns were investigated in a silicate groundwater system.
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Along the groundwater flowpath, ⁸⁷Sr/⁸⁶Sr values in groundwater gradually decrease.
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Chemical weathering of silicate generates depleted groundwater δ^44/40Ca ratios.
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Cation exchange between CaX₂ and NaX on clay minerals enriches ⁴⁴Ca in groundwater.
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Microbial-mediated carbonate precipitation elevates groundwater δ^44/40Ca ratios.

Abstract

To understand the characteristics of Ca isotope composition and fractionation in silicate-dominated Quaternary aquifer system, hydrochemical and isotope studies (⁸⁷Sr/⁸⁶Sr, ¹³C_DIC and ^44/40Ca) were conducted on groundwater, sediment and rock samples from the Datong basin, China. Along the groundwater flow path from the basin margin to the center, groundwater hydrochemical type evolves from Ca-HCO₃ to Na-HCO₃/Na-Cl type, which results from aluminosilicate hydrolysis, vertical mixing, cation exchange between CaX₂ and NaX, and calcite/dolomite precipitation. These processes cause the decrease in groundwater Ca concentration and the associated modest fractionation of groundwater Ca isotopes along the flowpath. The groundwater δ^44/40Ca value varies from −0.11 to 0.49‰. The elevated δ^44/40Ca ratios in shallow groundwater are attributed to vertical mixing involving addition of irrigation water, which had the average δ^44/40Ca ratio of 0.595‰. Chemical weathering of silicate minerals and carbonate generates depleted δ^44/40Ca signatures in groundwater from Heng Mountain (east area) and Huanghua Uplift (west area), respectively. Along the groundwater flow path from Heng Mountain to central area of east area, cation exchange between CaX₂ and NaX on clay mineral results in the enrichment of heavier Ca isotope in groundwater. All groundwater samples are oversaturated with respect to calcite and dolomite. The groundwater environment rich in organic matter promotes the precipitation of carbonate minerals via the biodegradation of organic carbon, thereby further promoting the elevation of groundwater δ^44/40Ca ratios.

Introduction

Recent advances in geochemical studies of stable isotopes such as Sr and Ca in groundwater have substantially improved our understanding of water–rock interactions and solute transport processes (i.g. (Cartwright et al., 2010, Gosselin et al., 2004, Zhu and Macdougall, 1998)). Calcium is one of the more abundant metal elements in the Earth’s crust, and understanding the effects of various physical and chemical processes on its isotope fractionation could provide new perspectives on the evolution of many natural systems. Ca isotope variation and fractionation have been inferred to be due to secondary precipitation of Ca-bearing phases (Hindshaw et al., 2013), source mixing processes (Jacobson et al., 2015), ion-exchange (Ockert et al., 2013), biological processes (Böhm et al., 2006) and plant uptake (Bagard et al., 2013). Generally, inorganic calcite grown at moderate solution oversaturation preferentially excludes ⁴⁴Ca, and the degree of ⁴⁰Ca enrichment is dependent on precipitation rate and solution temperature (Tang et al., 2008).

Since the elemental role played by Ca in the lithosphere, biosphere and hydrosphere, the understanding the mechanism of Ca isotope fractionation helps to develop the Ca isotope as a potential tracer. In the oceanic setting, Ca sedimentation is considered to be a critical recorder of ocean evolution, and therefore, Ca isotope is helpful to reconstruct the fluctuation of ocean Ca concentration through geologic time (Griffith et al., 2008). The variations of Ca isotope of marine carbonate also provide information on the weathering and/or depositional mode within the ocean (Fantle, 2010). Additionally, the Ca isotopes have been proposed as a proxy for the global carbon cycle and paleo-sea surface temperatures (Griffith et al., 2008, Nägler et al., 2000).

Compared to studies on Ca isotope fractionation in carbonate-dominated settings, little work has been conducted on Ca cycling and isotope fractionation in silicate systems (Jacobson et al., 2015, Moore et al., 2013). Silicate rocks and minerals have a somewhat larger range of δ^44/40Ca than carbonate rocks (Fantle and Tipper, 2014). Therefore, it is not surprising that water in silicate systems would exhibit different Ca isotope signatures from those in carbonate systems. Up to now, the Ca isotope signatures of groundwater in silicate aquifers have not been systematically evaluated. Therefore, theoretical and case studies on silicate-dominated Cenozoic aquifer systems under different hydrogeological conditions may provide new insights about Ca isotope signature and fractionation.

At the Datong basin in northern China which is evolved from a fossil lake, hydrochemical evolution of its Quaternary aquifer system is dominantly controlled by the incongruent dissolution of aluminosilicates, which have been well documented in our previous studies using hydrochemical and stable isotope tracers (Li et al., 2016, Li et al., 2012, Xie et al., 2014, Wang et al., 2009, Xie et al., 2013). Therefore, we selected Datong basin as our case study area for assessing the effects of several hydrogeochemical processes on Ca isotope fractionation in silicate aquifers.

Section snippets

Study area

The Datong basin, located in northern China, is a NW-SE oriented Cenozoic rifted basin, which experienced multiple periods of sedimentary deposition and volcanism since the late Pleistocene. The four major types of rocks and sediments at Datong are (Li et al., 2009, Xie et al., 2011) (Fig. 1): (1) Archean metamorphic rocks (granites-gneiss greenstone terrain) in the east margin (Heng Mountain), (2) Cambrian to Ordovician limestone and dolomite with clastic rocks in the southwest margin

Sampling and chemical analysis

In August 2013, a total of 26 water samples (24 groundwater and 2 upstream reservoir water samples) and 5 rock samples were collected from the Datong basin (Fig. 1). All the sampling wells can be mainly divided into two types, including multiple screening wells (MSW) and confined screening wells (CSW). The groundwater from MSW is the mixture of water from all aquifers screening the well depth, while the groundwater from CSW is mainly from the lower confined aquifers. 16 of 24 groundwater

Groundwater chemistry

Hydrochemical compositions and isotope signatures of groundwater from Datong basin are shown in Table 1. Groundwater pH ranged from 7.26 to 8.93, indicating a near-neutral to weakly alkaline condition in groundwater system. The groundwater TDS had a variable range of 365–10150 mg/L with a median value of 1143 mg/L. Groundwater Na (9.18–1968 mg/L) and Ca (2.86–453.6 mg/L) were the dominant cations in discharge and recharge areas, respectively, while groundwater HCO₃ and Cl were the major anions

Hydrochemical evolution

The basin-scale Sr isotope signature of groundwater depends on the type of minerals and Sr concentrations in the aquifer matrix. In the west area, relatively narrower range (0.70872–0.71151) of groundwater ⁸⁷Sr/⁸⁶Sr ratio indicates the contribution from Ordovician limestone weathering (0.70979) of Hongshou Mountain (Fig. 1). Comparably, the wide range of groundwater ⁸⁷Sr/⁸⁶Sr ratio in the east area indicates the dominant effect of hydrolysis of metamorphic rocks on groundwater chemistry (

Conclusions

The characteristics of hydrochemical and stable isotopes (⁸⁷Sr/⁸⁶Sr, ¹³C_DIC and ^44/40Ca) compositions were investigated in groundwater, sediment and rock samples collected from the silicate and OC-rich groundwater system of the Datong basin, northern China. The hydrochemical type of groundwater evolves from Ca-HCO₃ to Na-HCO₃ and Na-Cl from the recharge to discharge area. Groundwater samples have ⁸⁷Sr/⁸⁶Sr, δ¹³C_DIC and δ^44/40Ca ranges of 0.70872–0.72155, −18.5–−5.6, and −0.11–0.49,

Acknowledgements

We would like to thank Thomas L. Owens of University of California, Berkeley and Lanping Feng of State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences for their assistance in Ca isotope analysis in laboratory. The research work was financially supported by National Natural Science Foundation of China (No. 41521001 and No. 41502230), the program of the China Postdoctoral Science Foundation (DE-AC02-05CH11231). The Berkeley laboratory facilities are

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Research papersCalcium isotope fractionation in a silicate dominated Cenozoic aquifer system

Highlights

Abstract

Introduction

Section snippets

Study area

Sampling and chemical analysis

Groundwater chemistry

Hydrochemical evolution

Conclusions

Acknowledgements

Earth Sci. Rev.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Sci. Rev.

USA Appl. Geochem.

Quat. Int.

China. Appl. Geochem.

J. Geochem. Explor.

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

China Sci. Total Environ.

J. Geochem. Explor.

Chem. Geol.

Geochim. Cosmochim. Acta

Research papers
Calcium isotope fractionation in a silicate dominated Cenozoic aquifer system