Elucidating modern geochemical cycles at local, regional, and global scales using calcium isotopes

Journal Pre-proof Elucidating modern geochemical cycles at local, regional, and global scales using calcium isotopes

Elizabeth M. Griffith, Anne-Désirée Schmitt, M. Grace Andrews, Matthew S. Fantle PII:

S0009-2541(19)30574-1

DOI:

https://doi.org/10.1016/j.chemgeo.2019.119445

Reference:

CHEMGE 119445

To appear in:

Chemical Geology

Received date:

31 July 2019

Revised date:

16 December 2019

Accepted date:

17 December 2019

Please cite this article as: E.M. Griffith, A.-D. Schmitt, M.G. Andrews, et al., Elucidating modern geochemical cycles at local, regional, and global scales using calcium isotopes, Chemical Geology (2019), https://doi.org/10.1016/j.chemgeo.2019.119445

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© 2019 Published by Elsevier.

Journal Pre-proof Elucidating modern geochemical cycles at local, regional, and global scales using calcium isotopes

Elizabeth M. Griffith1, Anne-Désirée Schmitt2, M. Grace Andrews3, Matthew S. Fantle4

1 - School of Earth Sciences, The Ohio State University [email protected] 2 - Université de Strasbourg, CNRS, ENGEES, LHyGeS UMR 7517, 1, rue Blessig 67000

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Strasbourg, France, [email protected]

3 - Ocean and Earth Sciences, University of Southampton [email protected]

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4 - Department of Geosciences, The Pennsylvania State University [email protected]

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Abstract

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In Earth’s surface environment, calcium (Ca) is an important mobile metal that is actively and passively transported in solution and within organic and mineral phases, being cycled and recycled during various biogeochemical processes. With the development of modern mass

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spectrometric techniques small variations in the stable and radiogenic isotopic compositions of Ca can be measured, revealing insight in these complex biogeochemical cycles and tracing and

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quantifying components across a range of spatial and temporal scales similar to other more

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routine isotope systems. More than three decades of work reveal systematic variations in the partitioning of Ca isotopes due to both abiotic and biological processes. An overview of processes that fractionate Ca isotopes at local, regional, and global scales is outlined here. We present detailed examples of instances in which Ca isotopes have provided unique insight into the functioning of Earth surface processes and the cycling of Ca at multiple scales. Future studies should target questions for which Ca isotopic analysis provide unique insight and, when combined with other isotope and trace element multi-proxy studies, better constrain the system of interest. At the same time, we challenge the scientific community to explore new frontiers including polar regions and other extreme environments.

Key Words (6 max.): calcium, isotopes, soil, weathering, catchment, hydrosphere

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Highlights: 

Ca isotopes can be applied to gain novel information on (bio)geochemical cycles across temporal and spatial scales.

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Ca isotopes should be applied with other geochemical proxies to uniquely constrain system of interest.

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Ca isotopes in Earth-surface are buffered by primary mineral dissolution or dominated by secondary processes. Secondary processes fractionating Ca are biological cycling, secondary mineral

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

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precipitation and adsorption/desorption.

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1. Introduction

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Calcium atoms in a sample of rock, water, vegetation or animal are composed of some mixture

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of the isotopes calcium-40, calcium-42, calcium-43, calcium-44, calcium-46, and calcium-48 (i.e., 40Ca, 42Ca, 43Ca, 44Ca, 46Ca, and 48Ca), all of which are treated as stable. Because isotopes

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have identical atomic numbers and therefore electron arrangements, they behave chemically the same. However, the relative differences in masses of the isotopes (due to different numbers of

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neutrons) result in partitioning of the isotopes during chemical reactions or phase changes that “fractionate” the isotopes, either due to kinetics or equilibrium thermodynamics or some

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combination. Mass-dependent variations in the Ca isotopic composition of Earth’s surface environment are therefore influenced by a variety of abiotic and biological processes. These mass-dependent variations allow Ca isotopes to trace and quantify components in complex biogeochemical processes across a range of spatial and temporal scales, similar to more traditional stable isotope systems such as carbon and oxygen.

Until recently, strontium (Sr), an alkaline-earth element with similar ionic radius and charge to Ca, and radiogenic Sr isotope ratios (87Sr/86Sr), in combination with Ca concentrations, have been used in conjunction with each other to understand the behavior of Ca in surface environments and biogeochemical cycles. Studies of forested ecosystems, for instance, have shown that Sr is not a perfect chemical analog to Ca, stimulating the development of Ca isotopes

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Journal Pre-proof as a tracer in such settings (Baes and Bloom, 1988; Poszwa et al., 2000; Bullen and Bailey, 2005; Drouet et al., 2005; Berger et al., 2006). Indeed, Ca isotope analyses have confirmed that Sr and Ca are decoupled in many of these terrestrial ecosystems (Schmitt et al., 2003; Schmitt and Stille, 2005; Cenki-Tok et al., 2009; Holmden and Bélanger, 2010; Schmitt et al., 2017). Furthermore, because Ca fractionates mass dependently during biological processes, it can be used to trace processes occurring in the biosphere that 87Sr/86Sr cannot. The most abundant isotope of Ca (40Ca) has a radiogenic component as a consequence of the

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radioactive decay of 40K (- emission). The long half-life of 40K (1.25 billion years; Steiger and

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Jager, 1977) results in minimal contribution of radiogenic 40Ca for many modern Earth systems (considering our current level of analytical precision; e.g., Mills et al., 2018). Regardless, the

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radiogenic component of 40Ca, which can be isolated by internal normalization the same way 87

Sr/86Sr data are, can be used to trace Ca sources in forested ecosystems, as well as silicate

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weathering fluxes from old, potassic minerals and/or rocks (i.e., Precambrian or older) (DePaolo,

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2004; Fantle and Tipper, 2014). In cases where carbonate 87Sr/86Sr complicates the use of radiogenic Sr to trace silicate weathering, radiogenic 40Ca can be used to trace silicate weathering fluxes in natural systems (e.g., Davenport et al., 2014). Similar approaches have been

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used in forested ecosystems to trace Ca sources (Farkaš et al., 2011).

Several comprehensive reviews have been published on Ca isotopes in Earth-surface processes,

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the critical zone, and global Ca cycles that detail previous work over the past thirty years, and the interested reader is referred to these for more information and references (e.g., Nielsen et al., 2012; Schmitt et al., 2012; Bullen, 2014; Fantle and Tipper, 2014; Wiederhold, 2015; Schmitt, 2016; Tipper et al., 2016). Here we present an overview of processes that fractionate Ca isotopically at the Earth’s surface that are important to consider when applying Ca isotopes at local, regional, and global scales. We then give examples and emphasize where Ca isotope analyses provide unique insight for identifying and quantifying mineral sources of Ca during weathering, biological sources of Ca during Ca cycling in forested ecosystems, and Ca sources and carbonate production in marginal marine environments, in particular when combined with other isotope systems.

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Journal Pre-proof 2. Calcium Isotopic Fractionation in Earth-Surface Processes

Calcium isotopic compositions in the Earth-surface can either be buffered by primary mineral dissolution (i.e., chemical weathering) or dominated by secondary processes such as biological cycling, secondary mineral precipitation (e.g., Schmitt, 2016 and references therein), or adsorption/desorption on phyllosilicate/phyllomanganate minerals (Brazier et al., 2019). Processes imparting Ca isotopic fractionation during biological cycling and water-rock interactions such as chemical weathering, secondary mineral precipitation, and

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adsorption/desorption are briefly outlined below.

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2.1.Water-rock interactions

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2.1.1. Abiotic reactions

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Chemical weathering - Although congruent low-temperature dissolution of primary or secondary minerals does not impart mass-dependent fractionation of Ca isotopes (Fig. 1; Hindshaw et al.,

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2011; Ryu et al., 2011; Cobert et al., 2011a), the relative weatherability of isotopically distinct components and seasonal precipitation and dissolution of secondary (e.g., carbonate) minerals (Hindshaw et al., 2011) can impart variations in Ca isotopes during chemical weathering.

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Igneous differentiation and crystallization have been shown to fractionate stable Ca isotopes

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(Amini et al., 2009; Huang et al., 2010) and the subsequent weathering of distinct components could potentially result in dissolved Ca with a Ca isotopic composition different from bedrock. Formation of secondary clay minerals during incongruent weathering is also thought to fractionate Ca isotopes with the lighter Ca isotopes enriched in the solid phase (secondary clay minerals) and heavier Ca isotopes enriched in the dissolved phase. Secondary Mineral Precipitation – Precipitation (and dissolution) of secondary minerals fractionate Ca isotopes between solutions from which they precipitate and solid phases. The magnitude of the isotopic fractionation depends on the precipitation rate, dissolved phase, and precipitated mineral such as CaCO3 (aragonite, calcite, vaterite, ikaite), gypsum, and Ca-oxalates (Gussone et al., 2003, 2005, 2011; Lemarchand et al., 2004; Reynard et al., 2011; Tang et al.,

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Journal Pre-proof 2012; Harouaka et al., 2014; Yan et al., 2016). The relationship between CaCO3 precipitation and the measured Ca isotopic fractionation is dependent on the experimental set-up which impact the physicochemical conditions during precipitation (see Gussone and Dietzel, 2016 and references therein). Heavier isotopes occur preferentially in the dissolved phase compared to the solid one (to varying degrees), although no isotopic fractionation was seen during marine calcite precipitation from pore waters under chemical equilibrium conditions (Fantle and DePaolo, 2007). In contrast, during travertine precipitation (of calcite) in nature, significant Ca isotopic fractionations are observed (Yan et al., 2016). The larger Ca isotopic fractionations between Ca

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in CaCO3 and solution are correlated with higher precipitation rates in agreement with some

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experimental work (Tang et al., 2008) suggesting kinetic fractionation dominates this and likely

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other natural systems (Yan et al., 2016).

Ion-Exchange – Adsorption of Ca on clays and phyllomanganates in the terrestrial environment

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(Brazier et al., 2019) are similar to that studied in the marine environment where the lighter Ca

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isotopes adsorb preferentially on the solid phase (Teichert et al., 2009; Ockert et al., 2013). A recent study demonstrated that during the adsorption of Ca on soil model phyllosilicates (KGa kaolinite, Swy montmorillonite, Tuftane muscovite), the light isotope (40Ca) is preferentially

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adsorbed (compared to 44Ca; for only Swy montmorillonite and Tuftane muscovite) and the amplitude of this isotopic fractionation is a function of the size of the mineral (0.1 to 1 μm versus

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50 to 200 μm), the surface charge, the layer charge and the specific surface area of the

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phyllosilicate mineral considered, as well as the presence of cation-free interfoliar space (Brazier et al., 2019). Isotopic fractionation associated with desorption has been more challenging to study. It is suggested that partial desorption could fractionate Ca isotopically (Ockert et al., 2013; Brazier et al., 2019), the magnitude depending on the isotopic fractionation factor associated with adsorption, the specific mineral, and physicochemical conditions (Brazier et al., 2019).

2.1.2. Importance of microbial processes

Production of carbon dioxide by microbial processes can induce mineralization of CaCO3 in the presence of Ca2+ within microenvironments on small spatial scales, and cell surfaces can act as nucleation sites for mineralization (Hammes and Verstraete, 2002). Therefore, microbes can

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Journal Pre-proof enhance the potential for secondary carbonate or sulfate mineral precipitation and can impact Ca isotopic fractionation as described above for abiotic secondary mineral precipitation. Krause et al. (2012) studied Ca isotopic fractionation by inducing dolomite crystal formation using sulfate reducing bacteria in laboratory cultures. Calcium uptake into or adsorption on exopolymeric substances (EPS) resulted in a significant Ca isotopic fractionation (preferring the lighter isotopes) as part of a hypothesized two-step Ca isotopic fractionation during microbially induced dolomite formation (Krause et al., 2012). Harouaka et al. (2016) inferred a biological isotope effect associated with gypsum precipitation in laboratory culture experiments, and observed

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significant fractionation in natural gypsum deposits impacted by microbial activity. In a similar

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vein, Harouaka et al. (2017) demonstrated a systematic effect of organics on the isotopic fractionation factor associated with gypsum formation, and presented ab inito models that

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supported the hypothesis that dissolved organics can influence mineral isotopic composition due to interactions with crystal surfaces. Microbially mediated carbonate precipitation has also been

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studied in the laboratory induced by “simultaneous ammonification and extracellular carbonic

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anhydrase activity” which result in Ca isotopic fractionation that follow observations made for

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inorganic precipitation (Krause et al., 2018).

2.2. Biological cycling

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In the critical zone, the balance between the supply of Ca from input fluxes (litterfall,

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atmospheric deposition and soil mineral weathering) and the uptake by vegetation dictates the Ca isotopic composition of the Ca soil pool reservoir. When the vegetative uptake is greater than recycling (i.e., in Ca-limited systems), the exchangeable pool is driven to higher 44/40Ca values (Fantle and Tipper, 2014). In a non-limiting system, the Ca isotopic composition is controlled by primary mineral dissolution (i.e., chemical weathering) and potentially secondary (e.g., carbonate) mineral precipitation. The biological processes fractionating Ca isotopes are briefly outlined below.

2.2.1. Vegetation effects

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Journal Pre-proof Two processes primarily fractionate Ca isotopes in vegetation: (1) root uptake, and (2) translocation within the tree (Fig. 2). Ca isotopic fractionation occurs at the nutritive solution/root interface when Ca forms exchangeable bonds with root surface during Ca adsorption on cation-exchange binding sites on the external surface of the cell wall with lighter isotopes being removed from nutritive solution into roots (Schmitt et al., 2013). Translocation of Ca within the tree in roots (fine vs. large), woody parts and leaves result in progressively heavier isotopes from roots to woody parts to leaves. The isotopic fractionation seems to be controlled by the Ca speciation within the tree, e.g., free-Ca, Ca linked to pectins or to oxalates (Schmitt et al.,

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2018). The intensity of Ca isotopic fractionation within the plants also seems to be dependent on

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the Ca content and pH (Cobert et al., 2011b). The beginning Ca isotope composition of the soil solution that is taken up by the roots (i.e., nutritive solution) also exerts a primary control on the

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Ca isotopic signature within plants (Schmitt et al., 2013).

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The isotopic fractionations associated with biological processes in forested ecosystems results in

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a pool of Ca with low isotope values. Ca can be recycled from this pool (e.g., litterfall, Fig. 2) back into the vegetation. Within soils, the degree of Ca isotopic fractionation (i.e., enrichment of 44

Ca) is a “function of the calcium uptake rate into trees, the magnitude of the fractionation

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factor and the supply rate of 40Ca-enriched input fluxes from litterfall, atmospheric deposition and soil mineral weathering” (Holmden and Bélanger, 2010). The balance between the supply of

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Ca from input fluxes and the uptake by vegetation dictates the Ca isotopic composition of the Ca

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soil pool reservoir. In a non-limiting Ca soil reservoir, the vegetation uptake will have little effect on the Ca isotopic composition of the Ca soil pool reservoir (Wiegand et al., 2005; Hindshaw et al., 2011). In other ecosystems where Ca is limited, Ca isotopic fractionation during vegetation uptake results in 40Ca enrichment in roots and 44Ca enrichments in soil solutions compared to the average soils or rocks (Cenki-Tok et al., 2009; Holmden and Bélanger, 2010). Soil colloids (enriched in 40Ca) in permafrost environments and organic matter degradation appear to control the Ca isotopic compositions of soil solutions in this environment (Bagard et al., 2013).

2.2.2. Microbial effects

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Journal Pre-proof Experimentally work with Scots pines and an apatite substrate in the presence or absence of the rhizosphere bacterial strain Bulkholderia glathei suggest a clear bacterial influence on Ca isotopic signatures (Cobert et al., 2011a). In the absence of bacteria, the analyzed soil percolates were enriched in the heavy 44Ca isotope through time whereas no fractionation was recorded in the presence of bacteria. These preliminary results suggested that bacteria could influence the Ca isotopic signatures of soil-vegetative system by dissolving Ca from apatite more efficiently resulting in a system where Ca is non-limiting. Without the bacteria, Ca appears to be limiting

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and a measurable Ca isotopic fractionation due to vegetative uptake is seen in the soil percolates.

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Similarly, Gangloff et al. (2014) suggested a clear isotopic signature of different bacterial strains in the field due to their impact on bio-weathering and dissolution of apatite (Ca source to soils).

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In soil solutions originating from the spruce parcel of the Strengbach Critical Zone Observatory (CZO) catchment (Vosges, France), spring samples reflect anoxic conditions after heavy

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rainfalls, which caused increased bacterial activity and weathering of apatite resulting in non-Ca

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limiting environment reducing any vegetative effect on the Ca isotopic composition of the soil solutions (Fig. 3). Oxic conditions resulted in Ca isotopic signatures that were more similar to litterfall. As can be seen from studying the data in Fig. 3, only by combining both 87Sr/86Sr and

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44/40Ca measurements on the soil solution can we accurately describe the role of bacterial

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activity in the soils. Strontium isotopes only reveal part of the story (i.e., the source of Ca). Calcium isotopes are needed to provide evidence for the role of bacteria in weathering apatite

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efficiently during anoxic conditions. If these preliminary studies are confirmed, then Ca isotopes could become an important tracer of bacterial activity in soils.

2.3.Other considerations

Isotopic disequilibrium has been observed between porewaters and Ca labile soil sites, suggesting that cation-exchange processes are non-instantaneous in soils (Bullen et al., 2004). If cation-exchange processes or the influence of secondary mineral phases during Ca movement in soil below the root level can be neglected (over specific time scales), then 44/40Ca can serve as a conservative hydrologic tracer of Ca transport in natural waters (soil-, ground-, and surface waters) (Schmitt, 2016). Calcium isotope variations with depth and/or age in a soil can also

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Journal Pre-proof reflect changes in the source of Ca and Ca uptake by vegetation (Wiegand et al., 2005). In this context, Ca isotopes can be used as a marker of vegetal turnover (Schmitt, 2016). Repeat dissolution and reprecipitation events described by Rayleigh fractionation (and not just single events) can also impact the Ca isotopic signatures in the Earth-surface and should be considered (Ewing et al., 2008).

Lastly, the atmospheric Ca reservoir includes a potential source of Ca isotopic fractionation or variation. The impact of atmospheric Ca on soils varies largely between locations, with many

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observing little to no impact (Wiegand et al., 2005; Perakis et al., 2006; Page et al., 2008; Hindshaw et al., 2011; Bagard et al., 2013). Atmospheric deposition includes wet and dry input.

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Wet deposition is dominated by terrestrial sources derived from carbonate minerals (Schmitt and

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Stille, 2005) and other continental rocks or basalts (Holmden and Bélanger, 2010; Farkaš et al., 2011; Bagard et al., 2013; Wiegand and Schwendenmann, 2013). However, wet depositions on

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forested ecosystems can be modified before reaching the soil as throughfall (interacts with tree

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canopy) or stemflow (interacts with tree trunk). Throughfall samples have been shown to be slightly enriched in 40Ca compared to wet precipitation due to mixing of dry and wet atmospheric

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deposits with recycled Ca from leaf and needle exudations (Schmitt and Stille, 2005; Fig. 2). The Ca isotopic composition of atmospheric dry deposition (dust) is strongly linked to weathering as measured by the acid soluble fraction (Ewing et al., 2008; Fantle et al., 2012). Interestingly,

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Fantle et al. (2012) found the water-soluble fraction of dust enriched in the heavier isotopes of

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Ca (possibly due to Ca adsorption on clays; Ockert et al., 2013).

3. Calcium Isotope Variations in Local, Regional, and Global Systems

The cycling of Ca on the continents and in the oceans are coupled through time and space, thus an understanding of both is critical to constrain either one (Tipper et al., 2016). The oceanic mass balance of Ca is key to our understanding of geological processes moderating atmospheric CO2 and climate because Ca is the key cation in the silicate chemical weathering reaction and calcium carbonate deposition in the oceans –the largest sink of carbon on long geological timescales (Berner et al., 1983; Ridgwell et al., 2003). Calcium is primarily supplied to the oceans through processes that occur on the continents and is eventually returned to the continents through

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Journal Pre-proof tectonism. In this overview, we first explore known Ca isotope variations in local and regional systems on the continents, then briefly outline variations in the global hydrosphere.

3.1. Local and regional systems

Continental Ca reservoirs influencing Ca isotope variations in local and regional systems on the continents include: (1) vegetation having the widest range of variation (~4.1‰) with values ranging from -2.20 and 1.76‰ (relative to SRM915a), (2) soil solutions, bulk and leachable soil

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fractions with a range of variation of 3.51‰ (from -0.74 to 2.77‰ relative to SRM915a), and (3) continental rocks including sedimentary rocks having a 2.5‰ variability, from -0.77 to 1.73‰

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(relative to SRM915a), and igneous rocks with low variability (~1‰), from 0.31 to 1.34‰

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(relative to SRM915a; see Schmitt, 2016 and references therein). Atmospheric deposition (rainwater, snow, dust, throughfall) is an additional source of Ca with a global variability of

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1.09‰, ranging from 0.27 to 1.36‰ (relative to SRM915a) with two outliers with very low

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44/40Ca values (see Schmitt, 2016 and references therein). It is important to remember that these ranges and observations summarized below are based on currently published studies which are

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limited in some systems.

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3.1.1. Small catchments / rivers

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Small catchments and “small rivers” that have been studied vary by 1.48‰, ranging from 0.27 to 1.70‰ (relative to SRM915a), similar to “world rivers” sampled along their course (1.38‰, ranging from 0.17 to 1.55‰ relative to SRM915a) (Schmitt, 2016; Lyons et al., 2017). Variations at a single small catchment can be impacted by seasonal variations, changes in depth of water sources, dry/humid periods, low/high water-flow including snowmelt, vegetation uptake and recycling, secondary calcite or calcrete precipitation in soils, inflow of groundwater, or mixing of water sources (e.g., Jacobson and Holmden, 2008; Cenki-Tok et al., 2009; Tipper et al., 2010; Hindshaw et al., 2011, 2013b; Wiegand and Schwendenmann, 2013; Fantle and Tipper, 2014; Lyons et al., 2017; Fig. 4). Stable Ca isotopes have been used to track catchmentscale weathering in many environments including glaciated landscapes (Tipper et al. 2006, 2008; Hindshaw et al. 2011, 2013a, 2014; Moore et al. 2013; Jacobson et al. 2015; Lyons et al., 2017),

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Journal Pre-proof deserts (Ewing et al., 2008), and groundwaters ( Tipper et al., 2006; Jacobson and Holmden, 2008; Holmden et al., 2012; Jacobson et al., 2015; Li et al., 2018; Shao et al., 2018). However, more extreme environments remain relatively unexplored (e.g., Lyons et al., 2017).

3.1.2. Lakes

Lake waters have the largest range of aqueous Ca isotope values in small-scale terrestrial systems (3.05‰, spanning from 0.72 to 3.77‰ (relative to SRM915a)), although few systems

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have been investigated (Nielsen and DePaolo, 2013; Lyons et al., 2017; Shao et al., 2018). The high-alkalinity Mono Lake in California has the most 44Ca enriched natural water because

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precipitation of carbonates from the lake waters removes 40Ca preferentially, resulting in

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enrichment of 44Ca in the lake waters (Nielsen and DePaolo, 2013). Furthermore, the variability of Ca isotopes in the lake waters suggest that the lake is out of steady state with respect to Ca

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isotopes (Nielsen and DePaolo, 2013). In the McMurdo Dry Valleys, Antarctica, no isotopic

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fractionation was inferred during the cryo-concentration process in ponds and lakes (Lyons et al.,

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2017), but carbonate precipitation played a role in modifying some of the waters.

3.1.3. Shallow water, coastal settings

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The Ca isotopic composition of waters in coastal waters are sensitive to mixing of freshwaters

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and seawaters due to their different Ca isotopic signatures as documented in coastal waters of Florida Bay (Holmden et al., 2012) and a lagoon in southeastern Australia (Shao et al., 2018). In addition, Ca isotopic composition of waters from the carbonate-producing lagoon system in southeastern Australia are influenced by Ca removal as CaCO3 precipitated primarily as aragonite, acting as a net sink for dissolved inorganic carbon (Shao et al., 2018). Carbonate precipitation preferentially removes the light isotopes of Ca. The more hydrologically restricted waters show the largest Ca isotopic effect.

3.2 The global hydrosphere

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Journal Pre-proof The three major reservoirs of Ca in the hydrosphere are the ocean, rivers and groundwater (Fig. 5). The atmosphere constitutes a small reservoir of Ca described previously (Section 2.3. and 3.1).

3.2.1. Large rivers (at mouth) When considering only waters samples from large “world” rivers at the river mouth, the range in 44/40Ca reduces to 0.58‰ compared to the larger spread of ~1.5‰ measured in small catchments

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and “small rivers”. At the river mouth, these large rivers have 44/40Ca values spanning between

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0.67 and 1.25‰ (relative to SRM915a; Schmitt, 2016 and references therein) and averages 0.86 ±0.06‰ (2SE relative to SRM915a; see also Tipper et al., 2016). This reduction in variance is

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undoubtedly related to integration of the Ca isotopic signature across larger continental scale areas. Therefore, this homogenization during flow downstream results in no lithology-dependent

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44/40Ca signatures in waters sampled at the mouth of these large rivers (Schmitt et al., 2003;

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Tipper et al., 2010).

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The average 44/40Ca value could suggest dominance of silicate weathering over carbonate weathering in contrast with previous work (Meybeck, 1987; Berner and Berner, 1996; Gaillardet

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et al., 1999; Tipper et al., 2010). However, a more recent study by Fantle and Tipper (2014) suggests that secondary processes such as Ca uptake by plants, secondary mineral precipitation

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(e.g., pedogenic carbonates, travertines, or calcrete), or adsorption on clays influence the global riverine Ca isotopic composition and are necessary to explain the global riverine Ca isotopic composition which is isotopically heavier than carbonate rocks assumed to be the dominant source of Ca to rivers. They suggest that it is not just simple mixing of sources (e.g., carbonates of various ages and silicate rocks). Furthermore, Fantle and Tipper (2014) suggest that the present-day riverine value reflects a terrestrial Ca cycle that is currently out of steady state, i.e. global net growth of the biosphere could explain the riverine value or that there is an uncharacterized mechanism that fractionates Ca isotopes in the terrestrial weathering environment.

3.2.2. Groundwater

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Interestingly, the average measured ground and spring water value is 0.86 ±0.10‰ (2mean relative to SRM915a, n = 65; Tipper et al., 2016 and references therein) identical to average world rivers at their mouth. This value is not significantly different from that calculated by Holmden et al. (2012) using mass balance constraints on the Ca isotopic budget in the oceans and assuming the ocean Ca cycle is in steady state. However, the measured groundwater data has a much higher variability of 1.91‰ (ranging from 0.17 to 2.08‰ relative to SRM915a) than world rivers at their mouth and slightly higher variability than small rivers (Schmitt, 2016 and

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references therein). Presumably the controls on the isotopic composition of groundwater are

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similar to that of large rivers – with a primary control being weathering of the primary rocks and secondary influence from Ca uptake by plants, secondary (e.g., carbonate) mineral precipitation,

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or adsorption on clays.

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3.2.3. The global ocean

The major sources of Ca to the ocean are rivers, groundwater, and hydrothermal fluids.

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Additional smaller, but poorly quantified fluxes include cation exchange with clay minerals and diagenesis of marine sediments (Berner and Berner, 1996; Tipper et al., 2016). The major sink of

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Ca from the ocean is biogenic carbonate in deep-sea and shallow-water environments, which preferentially removes the lighter isotopes of Ca (Skulan et al., 1997; De La Rocha and DePaolo,

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2000; DePaolo, 2004; Gussone and Heuser, 2016). Burial of pore water constitutes a minor sink of Ca (Drever, 1997).

The high concentrations of Ca in seawater and relatively small input (or output) flux yield a long residence time for Ca in seawater (~1 Myr) relative to the mixing time of the oceans (~1 kyr) together with small differences in Ca concentrations and isotopic compositions within and between ocean basins suggest seawater has a homogenous isotopic composition that reflects the balance between the sources and sinks of Ca to the ocean and their isotopic compositions (at steady state: 𝛿 44/40 𝐶𝑎𝑠𝑤 = 𝛿 44/40 𝐶𝑎𝑖𝑛𝑝𝑢𝑡 − Δ44/40 𝐶𝑎𝑠𝑒𝑑 ). Modern seawater is isotopically heavy, and has an average 44/40Ca value of 1.89 ± 0.02‰ (2mean relative to SRM 915a, n = 62; Holmden et al., 2012; Fantle and Tipper, 2014; Tipper et al., 2016). With the exception of some

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Journal Pre-proof lake waters and springs (e.g., Nielsen and DePaolo, 2013), seawater is the isotopically heaviest reservoir in the global Ca cycle. Past work has been aimed at understanding if the modern Ca cycle is at steady state, and how this has changed in the past (Skulan et al., 1997; De La Rocha and DePaolo, 2000; Griffith et al., 2008; Fantle and Tipper, 2014; Tipper et al., 2016; Blättler and Higgins, 2017; Gussone et al., this volume). A clear assessment is limited to some extent by incomplete characterization of the Ca cycle, though efforts are being, and continue to be, made in this area. The similarity in the Ca isotopic composition of the marine inputs and outputs suggests that the modern ocean is in steady state, and recent work suggests this is also true over geologic

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time scales (Blättler and Higgins, 2017). However, it is unclear if, to what extent, and over what

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kind of time scales seawater 44/40Ca can evolve over time (e.g., De La Rocha and DePaolo, 2000; Fantle and DePaolo, 2005; Farkaš et al., 2007a; 2007b; Fantle, 2010; Fantle and Tipper,

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Case Studies Using Calcium Isotopes as a Unique Constraint

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4.

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2014). Such an understanding is critical to the development of the Ca isotope proxy.

Calcium isotopic analyses can provide unique insight into Earth-surface processes and cycling of this element. To date, radiogenic Sr isotopes (87Sr/86Sr) are the most common tracer of Ca

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sources during weathering because Sr substitutes for Ca in all materials and 87Sr/86Sr ratios are a

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relatively straightforward isotope system. 87Sr/86Sr ratios vary between primary rocks and minerals due to radiogenic ingrowth but any mass-dependent fractionation is eliminated during

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isotope measurement. This is in contrast to δ44/40Ca values which can be affected by both sources and fractionation processes including biological processes, as outlined previously, and therefore may be more challenging to interpret. Nevertheless, when radiogenic Sr and stable Ca isotopes are combined along with other isotope and trace element multi-proxy studies, Ca isotopes can often better constrain the system of interest and provide a more comprehensive picture of geochemical cycling of Ca. By combining two (or more) isotope systems, a more complete answer is possible to the question at hand, better than what one or the other isotope system could have provided independently. We highlight several studies here that use stable Ca isotopic analyses to provide unique constraints on processes occurring in Earth’s surface environment and encourage thoughtful application of both stable and radiogenic Ca isotopes to answer previously inaccessible questions about the Earth in the present and past.

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4.1. Identifying and quantifying mineral sources of Ca during weathering

Calcium isotopic analyses can be critical to tracing the source of dissolved Ca in systems where Ca is thought to originate, in part, from the dissolution of Ca-bearing secondary (e.g., carbonate or sulfate) minerals. In these cases, 87Sr/86Sr ratios can be equivocal to sources because secondary carbonate minerals take on the 87Sr/86Sr ratio of the solution from which they form. There is often no distinction between the 87Sr/86Sr ratio of the secondary and primary minerals or

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bulk lithology. Conversely, because δ44/40Ca isotopes often fractionate during secondary mineral

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precipitation (e.g., formation of carbonates), they also have a δ44/40Ca signature distinguishable from silicates during subsequent dissolution. This can be used to track weathering of secondary

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carbonate minerals through space or time.

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4.1.1. Icelandic rivers

The source of Ca during weathering on Iceland was investigated to elucidate the fraction of

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riverine bicarbonate corresponding to silicate, rather than carbonate, weathering and thereby contribute to long-term atmospheric carbon dioxide regulation (Jacobson et al., 2015; Andrews and Jacobson, 2017). Analysis of radiogenic Sr (along with Ca/Sr) was used to identify sources

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of Ca in the Icelandic rivers, revealing three-component mixing between basalt, hydrothermal

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calcite, and atmospheric deposition (Fig. 6a; Hindshaw et al., 2013a; Andrews and Jacobson, 2017). Significantly more calcite-derived Sr was seen in glacial rivers, as compared to nonglacial rivers. However, hydrothermal calcite has an identical 87Sr/86Sr ratio as the host basalt bedrock and these two end-members are only distinguishable in Ca/Sr space. Proportions of Sr from each end-member source cannot be robustly quantified because Andrews and Jacobson (2017) found that the hydrothermal calcite Ca/Sr ratios vary by an order of magnitude between samples and do not define a discrete end-member composition.

The same suite of samples was also analyzed for Ca isotopes and plotted with Sr/Ca ratios (Fig. 6b; Hindshaw et al., 2013a; Jacobsen et al., 2015). Basalt and hydrothermal calcite have different δ44/40Ca values such that they are distinguishable in both Ca isotope and molar ratio space.

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Journal Pre-proof Furthermore, as the inverse of Ca/Sr ratios, Sr/Ca ratios insure end-members have well-defined compositions, which facilitate linear quantification of mixing proportions. Ultimately, both δ44/40Ca values and 87Sr/86Sr ratios arrive at the same qualitative observation that glacial rivers derive more Ca (and Sr) from hydrothermal calcite than non-glacial rivers, thereby largely validating Sr as a proxy for Ca. However, Jacobsen et al. (2015) show that dissolved riverine Ca isotopes can be explained by a simple two component mixing between basalt and hydrothermal calcite, where calcite contributes 0-65% of riverine Ca in non-glacial rivers, and 25-90% of Ca in glacial rivers. Unlike the Sr system, atmospheric deposition is not an end-member, likely due to a

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reservoir effect where the high riverine Ca concentrations derived from basalt and calcite weathering dwarf atmospheric inputs. This highlights an important point, that although Sr is

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frequently interpreted as a reliable proxy for Ca, the two systems can differ, and only δ44/40Ca

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values unequivocally trace Ca.

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The earlier study by Hindshaw et al. (2013a) attributed similar trends in δ44/40Ca values of

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Icelandic rivers to result from mixing between basalt, hydrothermal waters, and meltwaters (in part atmospheric deposition), but required processes imparting isotopic fractionation (and not

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just simple mixing) to explain the entire dataset due to differences in the way endmembers were defined. Adsorption or ion-exchange onto secondary clay minerals was suggested to modify the riverine Ca isotopic compositions (Hindshaw et al., 2013a). When quantifying sources of Ca (or

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is of critical importance.

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any other element) to a system, defining endmember geochemistry as representative as possible

4.1.2. Madison Aquifer, South Dakota, USA

In a study of the Madison aquifer, South Dakota, USA, the geochemical evolution of groundwater was studied along a ~240 km flow path (Jacobson and Wasserburg, 2005). The aquifer is primarily dolomite, contains minor amounts of highly soluble anhydrite, and is recharged by surface water draining the igneous Black Hills range. Along the groundwater flow path, 87Sr/86Sr ratios transitioned from values similar to surface waters to values similar to dolomite and anhydrite, which are effectively isotopically indistinguishable (Fig. 7a). Analysis of Ca isotopes revealed that the anhydrite had lower δ44/40Ca values as compared to dolomite,

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Journal Pre-proof consistent with fractionation during precipitation of the anhydrite. Measurements of groundwater δ44/40Ca values evolved to this lower value, thereby directly implicating anhydrite dissolution as a primary control on groundwater geochemistry (Fig. 7b) (Jacobson and Holmden, 2008). Strontium isotopes were not able to identify this source of Ca due to the overlapping 87Sr/86Sr signatures of the distinct minerals.

This study highlights a more general phenomena for continental waters. Broadly speaking, the evolution of continental waters to lower δ44/40Ca values (as seen here in the groundwaters with

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Ca sourced from anhydrite) is suggestive of a mixing control on δ44/40Ca values (with Ca sources

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of low δ44/40Ca), rather than a mass dependent isotopic fractionation process (e.g., Holmden et al., 2012), because very few reservoirs (e.g., the zeolite stilbite; Andrews and Jacobson, 2017)

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are thought to preferentially incorporate heavy Ca isotopes.

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4.2. Identifying and quantifying biological sources of Ca during Ca cycling in forested

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ecosystems

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4.2.1. Temporal evolution in a forested temperate ecosystem in Strengbach (Vosges, France)

The combination of stable Ca and radiogenic Sr isotopes applied to soil solutions under a beech

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cover at the Strengbach CZO forested catchment (NE France) by Schmitt et al. (2017) shows

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significant differences in the same forested temperate ecosystem between two time periods: 2004/2006 and 2011/2013. The Sr isotopic composition of the soil solutions from 2004/2006 are similar to those of corresponding roots (highlighted with blue rectangle in Fig. 8), whereas the more recent soil solutions (2011/2013) are more radiogenic than the older soil solutions (2004/2006) (green arrow in Fig. 8b) but some of the roots are less radiogenic (orange arrow in Fig. 8b). The Ca isotopic composition of the soil solutions have higher 44/40Ca values than the roots and leaves, i.e., the soil solutions represent the complement of uptake by roots. Altogether this points to a shift at the CZO site in Sr (and Ca) source towards decreasing contribution of lower radiogenic Sr fluxes alimenting the soil solution, i.e., a decrease in the apatite weathering flux, the atmospheric deposits or the vegetation cycling flux, between 2004/2006 and 2011/2013. Schmitt et al. (2017) suggested that the most recent surface soil solutions are less affected by

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Journal Pre-proof vegetation uptake, indicating a decline in vegetation growth. Such reduced importance of nutrient uptake and biomass production by the trees could be a result of the aging of the Strengbach trees and weakening by repeated storm events and drought episodes (Schmitt et al., 2017).

4.2.2. Soil, boreal, forested permafrost-dominated ecosystem in Central Siberia (Russia)

Stable Ca and radiogenic Sr isotopic compositions were measured in different compartments

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(stream water, soil solutions, rocks, soils and vegetation) of a small permafrost-dominated

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catchment in the Central Siberia Plateau to understand cycling of Ca in this system which is important for understanding ongoing changes in this system which is very sensitive to climate

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warming (Bagard et al., 2013). The Sr isotopic compositions vary between a geogenic (basalt) and an atmospheric (snow) end-member (Fig. 9). Large Ca isotopic fractionations are mainly

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recorded in biological samples (between roots and shoots from a larch tree), and to a lesser

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degree in soil solutions from the warmer, south-facing slope of the catchment, which is subject to melting of the upper part of the permafrost in summer (Fig. 9). No significant Ca isotopic

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fractionation is seen in the cold soil active layer of the north-facing slope. In this permafrostdominated environment, biomass degradation (more significant in the south-facing slope) rather than plant uptake significantly influences the Ca isotopic compositions of soil solutions via the

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release of light Ca. Another pool enriched in the light Ca isotopes, such as colloids, was also

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suggested to influence the Ca isotopic compositions of soil solutions from the south-facing slope of the catchment (Bagard et al., 2013). This enrichment of the light Ca isotopes in the soil solution is not seen in the streams or large rivers sampled over the course of a year, which appear to be primarily controlled by 44/40Ca heterogeneity in the lithology of Ca sources (Bagard et al., 2013).

4.2.3. Soil tropical climosequence Kohala Mountain, Hawaii, USA

In soils, biolifting can be an important process by which plant roots move mineral-derived nutrients from depth and concentrate them in surface soil horizons. Tracing biolifting is complicated by mechanisms such as atmospheric deposition which can also deliver rock-derived

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Journal Pre-proof nutrients to surface soils. Bullen and Chadwick (2016) used stable Ca and stable Ba (138/134Ba ) isotopes, among other tracers, to investigate the extent to which phosphorus (P) was biolifted along a soil climosequence at Kohala Mountain, Hawaii, USA. Radiogenic Sr isotopes of exchangeable soil within the profiles were largely homogenous and resembled local bedrock and are therefore equivocal to biolifting. Plants preferentially took up light isotopes of both Ca and Ba, relative to source rock, and left a fingerprint of isotopically light, biocycled Ca and Ba in exchangeable surface soils. However, these isotope systems also revealed that Ca and Ba were subject to different pedogenic processes down profile. Below the ~20cm depth, stable Ba isotope

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values sharply increased, a result of light isotope removal by vegetation, and then remained

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constant with depth (Fig. 10b). This demonstrated that biolifted Ba was tightly retained at the surface. Conversely, below ~20cm stable Ca isotopes gradually transitioned to heavier isotopic

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values, indicating that light, biocycled Ca leached from surface soils (Fig. 10a). Additionally, a comparison of multiple soil profiles demonstrated that in some cases, 44/40Ca values were offset

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to relatively heavier values both in surface soils and with depth due to the formation of

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pedogenic carbonate, which preferentially sequestered light Ca isotopes throughout the profile (Fig. 10a). Taken together, this study revealed that Ba isotopes are a robust tracer of biolifting

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while stable Ca isotopes trace different but coincident pedogenic processes such as cation leaching and secondary carbonate mineral formation.

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4.3. Quantifying Ca sources and carbonate production in marginal marine environments

4.3.1 Florida Bay, USA

Analyses of Ca isotopes in modern shallow-water settings and marginal marine environments provide insight into Ca-cycling and processes such as carbonate production in these environments. They serve as our best analogue for ancient epeiric seas whose sedimentary record has provided most of what is known about Earth History, especially prior to the Cenozoic (Pratt and Holmden, 2008). In Florida Bay, Holmden et al. (2012) demonstrated the impact of submarine groundwater discharge on Ca-cycling in this estuarine lagoon. Waters are typically <3 m deep and overly Pleistocene limestone bedrock which also are exposed to the east and south composing the Florida Keys that keep surface Atlantic waters out of Florida Bay (Holmden et

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Journal Pre-proof al., 2012). Sediment and waters from the Florida Bay record a 0.7‰ gradient in δ44/40Ca, decreasing towards the Florida Everglades, caused by local-scale Ca-inputs from submarine groundwater discharge with a high Ca concentration and low δ44/40Ca value compared to seawater (Fig. 11; Holmden et al., 2012). Mixing calculations show that submarine groundwater discharge and surface waters (Everglades runoff) contribute between 8% and 60% of the Ca to the waters in the Bay, with salinities between 30 and 14, respectively (Fig. 11; Holmden et al., 2012).

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Radiogenic Sr (87Sr/86Sr) was not analyzed on these same samples (C. Holmden, personal

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communication). The small difference in 87Sr/86Sr of Floridian groundwater (minimum value of ~0.70825; Janzen and Krupa, 2011) and seawater (0.70918; Hodell et al., 1989) would result in a

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relatively small range of 87Sr/86Sr values that are largely insensitive to small amounts of fresh or brackish waters in the Bay. In contrast, there are significant variations in δ44/40Ca in both

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sediment and waters which can be explained by simple mixing between Everglades groundwater,

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surface water and seawater (Fig. 11; Holmden et al., 2012). It is therefore possible, in a restricted or shallow marine environment for Ca cycling to impart large changes in δ44/40Ca due to mixing between seawater, surface or groundwater (e.g., Florida Bay) or due to carbonate production if

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Ca is limiting (Shao et al., 2018) that are largely undetectable in 87Sr/86Sr of the sediment if the source of Sr (e.g., from carbonate rocks) is similar to coeval seawater. Measuring a

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contemporaneous seawater 87Sr/86Sr value in a rock does not always indicate that it was

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deposited in an unrestricted marine setting, and that Ca isotopes (or other isotope systems) measured in these sediments are recording seawater as well (e.g., Swart, 2008). “Similar degrees of circulation restriction between epeiric seas and oceans in the geological past may have also led to overprinting of sedimentary carbonate δ44/40Ca values in nearshore regions of epeiric seas due to local-scale cycling of seawater through coastal carbonate aquifers” (Holmden et al., 2012).

4.3.2. Coorong lagoon system, Australia

Shao et al. (2018) used both Ca and Sr isotopes to constrain the local carbonate cycle of the Coorong lagoon system, Australia. Radiogenic Sr isotopes revealed that, of three potential

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Journal Pre-proof primary water sources (seawater, surface freshwater, and groundwater) northern lagoon water predominately originated from seawater (Fig. 12a). Southern lagoon waters were a mixture of seawater and surface freshwater, suggesting a more hydrologically restricted lagoon in the south. Indeed δ44/40Ca values of northern lagoon water were similar to seawater, thereby confirming its origin, but southern lagoon water had high δ44/40Ca values (greater than seawater), compared to end-member water sources suggesting an additional process was critical to the Ca cycle in these waters, i.e., not just simple mixing of seawater and surface freshwater (Fig. 12b). The high δ44/40Ca values are indicative of carbonate precipitation which preferentially remove 40Ca into the

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precipitated solid, leaving the lagoon enriched in the heavier isotopes. The latter observation is

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consistent with the more hydrologically restricted nature of the southern lagoon, which yields

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evaporative (hypersaline) conditions and oversaturation with respect to carbonate phases.

Ultimately, Shao et al. (2018) calculated that ~15% of dissolved Ca is sequestered as carbonate

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in the southern lagoon suggesting it acts as a net sink for dissolved inorganic carbon. Studies like

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these are important to better constrain local calcium carbonate cycles in coastal ecosystems

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(Macreadie et al., 2017).

Concluding Statements

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Much of the calcium isotope research over the past thirty years has focused on low-temperature

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processes at the Earth’s surface, where various (bio)geochemical processes cycle Ca between geochemical reservoirs. The examples we have presented clearly demonstrate that Ca isotopes are useful probes of Ca cycling at the local, regional, and global scales. Calcium isotopes fractionate substantially in a range of processes, from ion exchange to mineral precipitation to root uptake and translocation within a tree. Calcium isotopes have been used successfully to identify and quantify Ca sources and to trace mineral dissolution and biological processes. Calcium isotopes add a necessary constraint in complicated, and often underconstrained, natural systems, especially when the more traditional tools (Sr isotopes and elemental ratios) do not function as required. Radiogenic 40Ca, though less developed for elucidating Ca cycling at a range of spatial scales, holds promise for future work.

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Journal Pre-proof Current observations on the behavior of Ca isotopes at local, regional, and global scales at Earth’s surface are summarized here, however many of these observations are based on only one or two studies. These studies reveal that the magnitude of Ca isotopic fractionation during weathering and secondary mineral formation depends on mineralogy as well as different physicochemical variables (affecting precipitation rate, dissolution, ion-exchange) that require additional experimental, theoretical and field work to fully constrain. Future studies that focus on Ca isotope behavior across gradients in lithology, climate, ecosystems, and human impact are needed to refine predictive quantitative models using Ca isotopes to elucidate changes in the past

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and future due to changing climate, hydrology, and human impact (e.g., land use change). Where

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are the Ca reservoirs in soils? How are these Ca reservoirs mobilized by plants? How do they respond to changing temperature, dry/wet periods? Can an increase in our understanding

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improve ecosystem resilience and sustainability, and restore ecosystem function (e.g., White et al., 2015)? The impact of microbial activity (and microbially mediated mineral precipitation)

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also needs to be thoroughly investigated as initial work presents evidence for large biological

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isotope effects. How does bacterial activity impact Ca isotope fractionation during mineral formation? What are the function of bacteria and hyphae? Furthermore, what are the fundamental

speciation?

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mechanisms behind Ca isotopic fractionation – what is the role of coordination, hydration,

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We also encourage the exploration of new frontiers including polar regions and other extreme

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environments (e.g., hypersaline, acidic, alkaline, geothermal, anoxic, euxinic). Polar systems with limited biological activity and cold temperatures represent end-members for understanding Ca isotope behavior on a variety of spatial and temporal scales, akin to work done in deep-sea sediments to understand Ca isotope behavior on geologic timescales which is unattainable in laboratory environments (Fantle and DePaolo, 2007). Furthermore, Ca isotopes can be used to characterize different weathering regimes in polar regions associated with ice sheets vs. valley glaciers. The differences, especially when compared to nonglacial systems, could impact global elemental cycles and climate on longer timescales (via sulfate oxidation, silicate vs carbonate weathering; Tranter et al., 2002; Torres et al., 2017). These and other extreme environments also serve as potential analogues for habitats of early life on Earth and extraterrestrial environments. Future studies and collaborations are needed across disciplines in the Earth sciences that apply

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Journal Pre-proof Ca isotopes in new and interesting ways to gain unique and novel information tracing and constraining Ca (bio)geochemical cycles at local, regional, and global scales (e.g., Pogge von

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Strandmann et al., 2019).

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Journal Pre-proof References Amini, M., Eisenhauer, A., Böhm, F., Holmden, C., Kreissig, K., Hauff, F., Jochum, K.P., 2009. Calcium isotopes (δ44/40Ca) in MPI-DING reference glasses, USGS rock powders and various rocks: evidence for Ca isotope fractionation in terrestrial silicates. Geostand. Geoanal. Res. 33, 231–247. Andrews, M.G., Jacobson, A.D., 2017. The radiogenic and stable Sr isotope geochemistry of basalt weathering in Iceland: Role of hydrothermal calcite and implications for long-term climate regulation. Geochem. Cosmochim. Acta 215, 247-262.

of

Baes, A.U., Bloom, P.R., 1988. Exchange of alkaline earth cations in soil organic matter. Soil

ro

Sci. 146, 6-14.

Bagard, M.-L., Schmitt, A.-D., Chabaux, F., Pokrovsky, O.S., Viers, J., Stille, P., Labolle, F.,

-p

Prokushkin, A.S., 2013. Biogeochemistry of stable Ca and radiogenic Sr isotopes in a larchcovered permafrost-dominated watershed of Central Siberia. Geochem. Cosmochim. Acta

re

114, 169-187.

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Berger, T.W., Swoboda, S., Prohaska, T., Glatzel, G., 2006. The role of calcium uptake from deep soils for spruce (Picea abies) and beech (Fagus sylvatica). Forest Ecol. Manag. 229,

na

234-246.

Berner, E.K., Berner, R.A., 1996. Global environment: water, air and geochemical cycles. Prentice Hall, Englewood Cliffs, NJ.

ur

Berner, R.A., Lasaga, A.C., Garrels, R.M., 1983. The carbonate-silicate geochemical cycle and

Jo

its effect on atmospheric carbon-dioxide over the past 100 million years. American Journal of Science 283, 641-683.

Blättler, C.L., Higgins, J.A., 2017. Testing Urey's carbonate-silicate cycle using the calcium isotopic composition of sedimentary carbonates. Earth and Planetary Science Letters 479, 241-251. Brazier, J.-M., Schmitt, A.-D., Gangloff, S., Chabaux, F., Tertre, E., 2019. Calcium isotopic fractionation during adsorption and desorption onto common soil phyllosilicates. Geochim. Cosmochim. Acta 250, 324-347. Bullen, T.D., 2014. Metal stable isotopes in weathering and hydrology. In Treatise on Geochemistry, 2nd edition, Elsevier, New York, 7.10, 329–359.

24

Journal Pre-proof Bullen, T.D., Bailey, S.W., 2005. Identifying calcium sources at an acid deposition impacted spruce forest: a strontium isotope, alkaline earth element multi-tracer approach. Biogeochem. 74, 63-99. Bullen, T., Chadwick, O., 2016. Ca, Sr and Ba stable isotopes reveal the fate of soil nutrients along a tropical climosequence in Hawaii. Chem. Geol. 422, 25-45. Bullen, T.D., Fitzpatrick, J.A., White, A.F., Schulz, M.S., Vivit, D.V., 2004. Calcium stable isotope evidence for three soil calcium pools at a granitoid chronosequence. In: Wanty, R.B., Seal II, R.R. (Eds.), Water–Rock Interaction: Proceedings of the 11th International

of

Symposium on Water–Rock Interaction, Saratoga Springs, New York, vol. 1. Taylor &

ro

Francis, London, pp. 813–817.

Cenki-Tok, B., Chabaux, F., Lemarchand, D., Schmitt, A.-D., Pierret, M.-C., Viville, D., Stille,

-p

P., 2009. The impact of water–rock interaction and vegetation on calcium isotope fractionation in soil- and stream waters of a small, forested catchment (the Strengbach case).

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Geochim. Cosmochim. Acta 73, 2215–2228.

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Cobert, F., Schmitt, A.-D., Calvaruso, Ch., Turpault, M.-P, Lemarchand, D., Collignon, Ch., Chabaux, F., Stille, P., 2011a. Biotic and abiotic experimental identification of bacterial influence on Ca isotopic signatures. Rapid Com. in Mass Spectrom. 25, 2760-2768.

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Cobert, F., Schmitt, A.-D., Bourgeade, P., Labolle, F., Badot, P.-M., Chabaux, F., Stille, P., 2011b. Experimental identification of Ca isotopic fractionations in higher plants. Geochim.

ur

Cosmochim Acta 75, 5467-5482.

Jo

Davenport, J., Caro, G., France-Lanord, C., 2014. Tracing silicate weathering in the Himalaya using the 40K-40Ca system: A reconnaissance study. Procedia Earth and Planetary Science 10, 238-242.

De La Rocha, C.L., DePaolo, D.J., 2000. Isotopic evidence for variations in the marine calcium cycle over the Cenozoic. Science 289, 1176–1178. DePaolo, D.J., 2004. Calcium isotopic variations produced by biological, kinetic, radiogenic and nucleosynthetic processes. Rev. Mineral. Geochem. 55, 255–288. Drever, J.I., 1997. The geochemistry of natural waters, 3rd Edn. Prentice-Hall Inc., Upper Saddle River, NJ. 436 pages. Drouet, T., Herbauts, J., Gruber, W., Demaiffe, D., 2005. Strontium isotope composition as a tracer of calcium sources in two forest ecosystems in Belgium. Geoderma 126, 203–223.

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Journal Pre-proof Ewing, S.A., Yang, W., DePaolo, D.J., Michalski, G., Kendall, C., Stewart, B.W., Thiemens, M., Amundson, R., 2008. Non-biological fractionation of stable Ca isotopes in soils of the Atacama Desert, Chile. Geochim. Cosmochim. Acta 72, 1096-1110. Fantle, M.S., 2010. Evaluating the Ca isotope proxy. Am. J. Sci. 310, 194-230. Fantle, M.S., DePaolo, D.J., 2005. Variations in the marine Ca cycle over the past 20 million years. Earth and Planetary Science Letters 237, 102-117. Fantle, M.S., DePaolo, D.J., 2007. Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: the Ca2+ (aq)-calcite equilibrium fractionation factor and calcite recrystallization

of

rates in Pleistocene sediments. Geochim. Cosmochim. Acta 71, 2524–2546.

ro

Fantle, M.S., Tipper, E.T., 2014. Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy. Earth-Sci. Rev. 129, 148-177.

-p

Fantle, M.S., Tollerud, H., Eisenhauer, A., Holmden, C., 2012. The Ca isotopic composition of dust-producing regions: measurements of surface sediments in the Black Rock Desert,

re

Nevada. Geochim. Cosmochim. Acta 87, 178-193.

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Farkaš, J., Böhm, F., Wallmann, K., Blenkinsop, J., Eisenhauer, A., van Geldern, R., Munnecke, A., Voigt, S., Veizer, J., 2007a. Calcium isotope record of Phanerozoic oceans: Implications

Acta 71, 5117-5134.

na

for chemical evolution of seawater and its causative mechanisms. Geochim. Cosmochim. Farkaš, J., Buhl, D., Blenkinsop, J., Veizer, J., 2007b. Evolution of the oceanic calcium cycle

ur

during the late Mesozoic: Evidence from δ44/40Ca of marine skeletal carbonates. Earth and

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Planetary Science Letters 253, 96-111. Farkaš, J., Déjeant, A., Novák, M., Jacobsen, S.B., 2011. Calcium isotope constraints on the uptake and sources of Ca2+ in a base-poor forest: a new concept of combining stable (δ44/42Ca) and radiogenic (εCa) signals. Geochim. Cosmochim. Acta 75, 7031–7046. Gaillardet, J., Dupré, B., Louvat, P., Allègre, C.J., 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chem. Geol. 159, 3-30. Gangloff, S., Stille, P., Schmitt, A.-D., Chabaux, F., 2014. Impact of bacterial activity on Sr and Ca isotopic compositions (87S/86Sr and 44/40Ca) in soil solutions (the Strengbach CZO). Procedia Earth Planet. Sci. 10, 109-113. Griffith, E.M., Paytan, A., Caldeira, K., Bullen, T.D., Thomas, E., 2008. A dynamic marine calcium cycle during the past 28 million years. Science 322, 1671-1674.

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Journal Pre-proof Gussone, N., Dietzel, M., 2016. Calcium isotope fractionation during mineral precipitation from aqueous solution. In: Calcium Stable Isotope Geochemistry (eds. N. Gussone, A.-D. Schmitt, A. Heuser, F. Wombacher, M. Dietzel, E. Tipper and M. Schiller). Springer, pp. 75-110. Gussone, N., Heuser, A., 2016. Biominerals and biomaterial. In: Calcium Stable Isotope Geochemistry (eds. N. Gussone, A.-D. Schmitt, A. Heuser, F. Wombacher, M. Dietzel, E. Tipper and M. Schiller). Springer, pp. 111-144. Gussone, N., Eisenhauer, A., Heuser, A., Dietzel, A., Bock, B., Böhm, F., Spero, H.J., Lea, D.W., Bijma, J., Nägler, Th.F., 2003. Model for kinetic effects on calcium isotope

of

fractionation (44Ca) in inorganic aragonite and cultured planktonic foraminifera. Geochim. Cosmochim. Acta 67(7), 1375–1382.

ro

Gussone, N., Böhm, F., Eisenhauer, A. Dietzel, M. Heuser, A., Teichert, B.M.A., Reitner, J.,

Geochim. Cosmochim. Acta 69, 4485–4494.

-p

Wörheide, G., Dullo, W.-C., 2005. Calcium isotope fractionation in calcite and aragonite.

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vaterite. Chem. Geol. 285, 194-202.

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Gussone, N., Nehrke, G., Teichert, B.M.A., 2011. Calcium isotope fractionation in ikaite and

Gussone, N., Ahm, A.-S.C., Lau, K.V., Bradbury, H.J., submitted. Calcium isotopes in deep

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time: potential and limitations. Chem. Geol. this volume. Hammes, F., Verstraete, W., 2002. Key roles of pH and calcium metabolism in microbial

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carbonate precipitation. Environ. Sci.Biotechnol. 1, 3–7. Harouaka, K., Eisenhauer, A., Fantle, M.S., 2014. Experimental investigation of Ca isotopic

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fractionation during abiotic gypsum precipitation. Geochim. Cosmochim. Acta 129, 157-176. Harouaka, K., Mansor, M., Macalady, J.L., Fantle, M.S., 2016. Calcium isotopic fractionation in microbially mediated gypsum precipitates. Geochim. Cosmochim. Acta 184, 114-131. Harouaka, K., Kubicki, J.D., Fantle, M.S., 2017. Effect of amino acids on the precipitation kinetics and Ca isotopic composition of gypsum. Geochim. Cosmochim. Acta 218, 343-364. Hindshaw, R.S., Reynolds, B.C., Wiederhold, J.G., Kretzschmar, R., Bourdon, B., 2011. Calcium isotopes in a proglacial weathering environment: Damma glacier, Switzerland. Geochim. Cosmochim. Acta 75, 106-118. Hindshaw, R.S., Bourdon, B., Pogge von Strandmann, P.A.E., Vigier, N., Burton, K.W., 2013a. The stable calcium isotopic composition of rivers draining basaltic catchments in Iceland. Earth Planet. Sci. Lett. 374,173-184.

27

Journal Pre-proof Hindshaw, R.S., Reynolds, B.C., Wiederhold, J.G., Kiczka, M., Kretzschmar, R., Bourdon, B., 2013b. Calcium isotope fractionation in alpine plants. Biogeochem. 112, 373-388. Hindshaw, R.S., Rickli, J., Leuthold, J., Wadham, J., Bourdon, B., 2014. Identifying weathering sources and processes in an outlet glacier of the Greenland Ice Sheet using Ca and Sr isotope ratios. Geochim. Cosmochim. Acta 145, 50-71. Hodell, D.A., Mueller, P.A., McKenzie, J.A., Mead, G.A., 1989. Strontium isotope stratigraphy and geochemistry of the late Neogene ocean. Earth Planet. Sci. Lett. 92, 165–178. Holmden, C., Bélanger, N., 2010. Ca isotope cycling in a forested ecosystem. Geochim.

of

Cosmochim. Acta 74, 995–1015.

ro

Holmden, C., Papanastassiou, D.A., Blanchon, P., Evans, S., 2012. 44/40Ca variability in shallow water carbonates and the impact of submarine groundwater discharge on Ca-cycling in

-p

marine environments. Geochim. Cosmochim. Acta 83, 179-194. Huang, S., Farkaš, J., Jacobsen, S.B., 2010. Calcium isotopic fractionation between

re

clinopyroxene and orthopyroxene from mantle peridotites. Earth Planet. Sci. Lett. 292, 337-

lP

344.

Jacobson, A.D., Holmden, C., 2008. 44Ca evolution in a carbonate aquifer and its bearing on the

na

equilibrium isotope fractionation factor for calcite. Earth Planet. Sci. Lett. 270, 349-353. Jacobson, A.D., Wasserburg, G.J., 2005. Anhydrite and the Sr isotope evolution of groundwater

ur

in a carbonate aquifer. Chem. Geol. 214, 331-350. Jacobson, A.D., Andrews, M.G., Lehn, G.O., Holmden, C., 2015. Silicate versus carbonate

Jo

weathering in Iceland: New insights from Ca isotopes. Earth Planet. Sci. Lett. 416, 132-142. Janzen, J.H., Krupa, S., 2011. Water quality characterization of southern Miami-Dade County and nearby FPL Turkey Point power plant, Miami-Dade County, Florida. Technical Publication WS-31. South Florida Water Management District. Krause, S., Liebetrau, V., Gorb, S., Sánchez-Román, M., McKenzie, J.A., Treude, T., 2012. Microbial nucleation of Mg-rich dolomite in exopolymeric substances under anoxic modern seawater salinity: new insight into an old enigma. Geology 40, 587-590. Krause, S., Liebetrau, V., Löscher, C.R., Böhm, F., Gorb, S., Eisenhauer, A., Treude, T., 2018. Marine ammonification and carbonic anhydrase activity induce rapid calcium carbonate precipitation. Geochim. Cosmochim. Acta 243, 116-132.

28

Journal Pre-proof Lemarchand, D., Wasserburg, G.T., Papanastassiou, D.A., 2004. Rate-controlled calcium isotope fractionation in synthetic calcite. Geochim. Cosmochim. Acta 68(22), 4665–4678. Li, J.X., DePaolo, D.J., Wang, Y.X., Xie, X.J., 2018. Calcium isotope fractionation in a silicate dominated Cenozoic aquifer system. Journal of Hydrology 559, 523-533. Lyons, W.B., Bullen, T.D., Welch, K.A., 2017. Ca isotopic geochemistry of an Antarctic aquatic system. Geophys. Res. Lett. 44, 882-891. Macreadie, P.I., Serrano, O., Maher, D.T., Duarte, C.M., Beardall, J., 2017. Addressing calcium carbonate cycling in blue carbon accounting. Limnol. Oceanogr. Lett. 2, 195-201.

of

Meybeck, M., 1987. Global chemical weathering of surficial rocks estimated from river

ro

dissolved loads. Am. J. Sci. 287, 401-428.

Mills, R.D., Simon, J.I., DePaolo, D.J., 2018. Calcium and neodymium radiogenic isotopes of

-p

igneous rocks: tracing crustal contributions in felsic magmas related to super-eruptions and continental rifting. Earth Planet. Sci. Lett 495, 242-250.

re

Moore, J., Jacobson, A.D., Holmden, C., Craw, D., 2013. Tracking the relationship between

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mountain uplift, silicate weathering, and long-term CO2 consumption with Ca isotopes: Southern Alps, New Zealand. Chem. Geol. 341, 110-112.

na

Nielsen, L.C., DePaolo, D.J., 2013. Ca isotope fractionation in a high-alkalinity lake system: Mono Lake, California. Geochim. Cosmochim. Acta 118, 276–294. Nielsen, L.C., Druhan, J.L., Yang, W., Brown, S.T., DePaolo, D.J., 2012. Calcium isotopes as

ur

tracers of biogeochemical processes. In Handbook of Environmental Isotope Geochemistry,

Jo

Baskaran, M., Ed., Springer, Heidelberg, Chapter 7, 105–124. Ockert, C., Gussone, N., Kaufhold, S., Teichert, B.M.A., 2013. Isotope fractionation during Ca exchange on clay minerals in a marine environment. Geochim. Cosmochim. Acta 112, 374388. Page, B.D., Bullen, T.D., Mitchell, M.J., 2008. Influences of calcium availability and tree species on Ca isotope fractionation in soil and vegetation. Biogeochemistry 88, 1–13. Perakis, S.S., Macguire, D.A., Bullen, T.D., Cromack, K., Waring, R.H., Boyle, J.R., 2006. Coupled nitrogen and calcium cycles in forests of the Oregon coast range. Ecosystems 9, 6374. Pogge Von Strandmann, P., Burton, K., Snaebjornsdottir, S., Sigfusson, B., Aradottir, E., Gunnarsson, I., Alfredsson, H., Mesfin, K., Oelkers, E., Gislason, S., 2019. Rapid CO2

29

Journal Pre-proof mineralisation into calcite at the CarbFix storage site quantified using calcium isotopes. Nature Communications 10, 1983, doi:10.1038/s41467-019-10003-8. Poszwa, A., Dambrine, E., Pollier, B., Atteia, O., 2000. A comparison between Ca and Sr cycling in forest ecosystems. Plant Soil 225, 299–310. Pratt, B.R., Holmden, C., 2008. Introduction. Geological Association of Canada Special Paper 48, 1-5. Reynard, L.M., Day, C.C., Henderson, G.M., 2011. Large fractionation of calcium isotopes during cave-analogue calcium carbonate growth. Geochim. Cosmochim. Acta 75, 3726–

of

3740.

Neoproterozoic ice ages. Science 302, 859-862.

ro

Ridgwell, A.J., Kennedy, M.J., Caldeira, K., 2003. Carbonate deposition, climate stability, and

-p

Ryu, J.S., Jacobson, A.D., Holmden, C., Lundstrom, C., Zhang, Z., 2011. The major ion, δ44/40Ca, δ44/42, and δ26Mg geochemistry of granite weathering at pH = 1 and T = 25°C:

re

power-law processes and the relative reactivity of minerals. Geochim. Cosmochim. Acta 75,

lP

6004–6026.

Schmitt, A.-D., 2016. Earth-surface Ca isotopic fractionations. In: Calcium Stable Isotope

na

Geochemistry (eds. N. Gussone, A.-D. Schmitt, A. Heuser, F. Wombacher, M. Dietzel, E. Tipper and M. Schiller). Springer, pp. 145–172. Schmitt, A.-D., Stille, P., 2005. The source of calcium in wet atmospheric deposits: Ca–Sr

ur

isotope evidence. Geochem. Cosmochim. Acta 69, 3463–3468.

Jo

Schmitt, A.-D., Chabaux, F., Stille, P., 2003. The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance. Earth Planet. Sci. Lett. 6731, 1–16. Schmitt, A-D., Vigier, N., Lemarchand, D., Millot, R., Chabaux, F., Stille, P., 2012. Processes controlling the stable isotope compositions of Li, B, Mg and Ca in plants, soils and waters: A review. C.R. Geosci. 344, 704-722. Schmitt, A.-D., Cobert, F., Bourgeade, P., Ertlin, D., Labolle, F., Gangloff, S., Badot, P.M., Chabaux, F., Stille, P., 2013. Calcium isotope fractionation during plant growth under a limiting nutrient supply. Geochim. Cosmochim. Acta 110, 70-83. Schmitt, A.-D., Gangloff, S., Labolle, F., Chabaux, F., Stille, P., 2017. Ca biogeochemical cycle at the beech tree-soil solution interface from the Strengbach CZO (NE France): a clue from stable Ca and radiogenic Sr isotopes. Geochim. Cosmochim. Acta 213, 91-109.

30

Journal Pre-proof Schmitt, A.-D., Borrelli, N., Ertlen, D., Gangloff, S., Chabaux, F., Osterrieth, M., 2018. Stable calcium isotope speciation and calcium oxalate production within beech tree (Fagus sylvatica L.) organs. Biogeochem. 137 (1), 197-217. Shao, Y.X., Farkaš, J., Holmden, C., Mosley, L., Kell-Duivestein, .I, Izzo, C., Reis-Santos, P., Tyler, J., Torber, P., Fryda, J., Taylor, H., Haynes, D., Tibby, J., Gillanders, B.M., 2018. Calcium and strontium isotope systematics in the lagoon-estuarine environments of South Australia: Implications for water source mixing, carbonate fluxes and fish migration. Geochim. Cosmochim. Acta 239, 90-108.

of

Skulan, J., DePaolo, D.J., Owens, T.L., 1997, Biological control of calcium isotopic abundances

ro

in the global calcium cycle. Geochim. Cosmochim. Acta 61(12), 2505–2510. Steiger, R.H., Jager,E., 1977. Subcommission on geochronology: convention on the use of decay

-p

constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362. Swart, P.K., 2008. Global synchronous changes in the carbon isotopic composition of carbonate

re

sediments unrelated to changes in the global carbon cycle. Proc. Natl. Acad. Sci. 105, 13741–

lP

13745.

Tang, J., Deitzel, M., Böhm, F., Köhler, S.J., Eisenhauer, A., 2008. Sr2+/Ca2+ and 44Ca/40Ca

72, 3733-3745.

na

fractionation during inorganic calcite formation: II. Ca isotopes. Geochim. Cosmochim. Acta Tang, J., Niedermayr, A., Köhler, S.J., Böhm, F., Kısakürek, B., Eisenhauer, A., Dietzel, M.,

ur

2012. Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: III. Impact of

Jo

salinity/ionic strength. Geochim. Cosmochim. Acta 77(C), 432–443. Teichert, B.M.A., Gussone, N., Torres, M.E., 2009. Controls on calcium isotope fractionation in sedimentary pore waters. Earth Planet Sci. Lett. 279, 373-382. Tipper, E.T., Galy, A., Bickle, M.J., 2006. Riverine evidence for a fractionated reservoir of Ca and Mg on the continents: Implications for the oceanic Ca cycle. Earth Planet. Sci. Lett. 247, 267-279. Tipper, E.T., Galy, A., Bickle, M.J., 2008. Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: lithological or fractionation control? Geochim. Cosmochim. Acta 72, 1057-1075.

31

Journal Pre-proof Tipper, E.T., Gaillardet, J., Galy, A., Louvat, P., 2010. Calcium isotope ratios in the world’s largest rivers: a constraint on the maximum imbalance of oceanic calcium fluxes. Global Biogeochem. Cy. 24, GB3019, 13p. doi:10.1029/2009GB003574. Tipper, E.T., Schmitt, A.-D., Gussone, N., 2016. Global Ca cycles: coupling of continental and oceanic processes. In: Calcium Stable Isotope Geochemistry (eds. N. Gussone, A.-D. Schmitt, A. Heuser, F. Wombacher, M. Dietzel, E. Tipper and M. Schiller). Springer, pp. 173-222. Torres, M.A., Moosdorf, N., Hartmann, J., Adkins, J.A., West, A.J., 2017. Glacial weathering,

of

sulfide oxidation, and global carbon cycle feedbacks. Proc. Nat. Acad. Sci.,

ro

doi/10.1073/pnas.1702953114.

Tranter, M., Huybrechts, P., Munhoven, G., Sharp, M.J., Brown, G.H., Jones, I.W., Hodson,

-p

A.J., Hodgkins, R., Wadham, J.L., 2002. Direct effect of ice sheets on terrestrial bicarbonate, sulphate and base cation fluxes during the last glacial cycle: minimal impact on atmospheric

re

CO2 concentrations. Chem. Geol. 190, 33-44.

lP

White, T., Brantley, S., Banwart, S., Chorover, J., Dietrich, W., Derry, L., Lohse, K., Anderson, S., Aufdendkampe, A., Bales, R., Kumar, P., Richter, D., McDowell, B., 2015. The role of

Processes 19, 15-78.

na

Critical Zone Observatories in Critical Zone science. Developments in Earth Surface

Wiederhold, J.G., 2015. Metal stable isotope signatures as tracers in environmental

ur

geochemistry. Environ. Sci. Technol. 49, 2606-2624.

Jo

Wiegand, B.A., Schwendenmann, L., 2013. Determination of Sr and Ca sources in small tropical catchments (La Selva, Costa Rica)—A comparison of Sr and Ca isotopes. J. Hydrol. 488, 110–117.

Wiegand, B.A., Chadwick, O.A., Vitousek, P.M., Wooden, J.L., 2005. Ca cycling and isotopic fluxes in forested ecosystems in Hawaii. Geophys Res Lett 32, L11404. Yan, H., Schmitt, A.-D., Liu, Z., Gangloff, S., Sun, H., Chen, J., Chabaux, F., 2016. Calcium isotopic fractionation during travertine deposition under different hydrodynamic conditions; example of Baushuitai (Yunnan, SW China). Chem. Geol. 426, 60-70.

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Journal Pre-proof FIGURE CAPTIONS

Fig. 1: Overview of processes related to water-rock interactions that impart Ca mass dependent stable isotopic fractionations designated as “i”. 1 during secondary (e.g., carbonate) mineral precipitation from solution. 2 during adsorption onto phyllosilicate or phyllomanganate minerals. Hypothesized (designated with a “?”) Ca isotope fractionations include: 3 during incongruent weathering and formation of secondary clay minerals; 4 during (partial) desorption from phyllosilicate or phyllomanganate minerals; and 5 due to processes related to bacteria and

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hyphae. All processes that fractionate Ca isotopes preferentially incorporate the light isotopes of

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Ca into or onto the solid phase (40Ca enriched).

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Fig. 2: Overview of processes related to biological cycling of Ca that impart Ca isotopic

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fractionations designated as “i” including  uptake into vegetation,  translocation within the tree (roots to trunk to leaves), and hypothesized (designated with “?”) due to bacteria and

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hyphae. Uptake into vegetation preferentially incorporates the light isotopes of Ca, while translocation increases the heavier isotopes of Ca from roots to trunk to leaves. Recycling by

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vegetation of litterfall enriches the soil Ca pool in light isotopes. Not all of the processes

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influencing the Ca soil pool reservoir are pictured.

Fig. 3: Variation in soil solutions under spruce at different depths (-5, -10, -30 and -60 cm) at

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different seasons (modified from Gangloff et al., 2014). Strontium isotopes reveal the Ca source at depth, including more radiogenic minerals at the deepest depth, e.g., plagioclase. Calcium isotopes are also impacted by vegetation and provide evidence for the role of bacteria in weathering apatite efficiently during anoxic conditions in upper 10 cm. Deeper samples (30+ cm) were measured, but no clear end-member was identified limiting interpretation of the measured values (Gangloff et al., 2014).

Fig. 4: Overview of processes producing variation in Ca isotopes in local and regional settings. Mass dependent fractionations designated as “i” including uptake into vegetation (and translocation within trees), and carbonate production. If Ca is limited during carbonate production, then waters are enriched in heavy Ca isotopes. Mixing of Ca sources include 33

Journal Pre-proof decomposition of vegetation, soil solutions, weathering of continental rocks (including sedimentary rocks) and igneous rocks, atmospheric deposition, groundwater, and seawater.

Fig. 5. Schematic of the major reservoirs of Ca in the modern global hydrosphere and processes that impact their Ca isotopic composition. Mass dependent fractionation is designated as “” associated with neritic and pelagic carbonate production, the largest sink of Ca from oceans. The long residence time of Ca in the ocean (~1 Myr) relative to the mixing time of the ocean (~1 kyr) together with small differences in concentrations and isotopic compositions within and between

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ocean basins suggest a homogenous Ca isotopic composition of seawater that reflects the balance

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between the sources and sinks of Ca to the ocean and their isotopic compositions. Minor fluxes

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are indicated and may constitute relatively greater importance under past conditions.

Fig. 6: Sources of Sr (A) and Ca (B) in Icelandic glacial and non-glacial rivers determined by Sr/86Sr (A) and δ44/40Ca (B) values and molar cation ratios. (A) Note the order of magnitude

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difference in Ca/Sr scale in inset. Gray shading corresponds to the three- (A) or two- (B) component source mixing regime. All samples are within the mixing regime, within analytical uncertainty. Thin-outlined circles indicate data taken from Hindshaw et al. (2013a), δ44/40Ca

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values  0.14‰. Thick-outlined circles and end-member source data taken from (A) Andrews

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and Jacobson (2017) or (B) Jacobson et al. (2015); δ44/40Ca values  0.04‰. (A) modified from

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Andrews and Jacobson (2017) and (B) modified from Jacobson et al. (2015). Fig. 7: The evolution of groundwater 87Sr/86Sr (A) and δ44/40Ca (B) isotope values with distance in the Madison Aquifer. (B) Shading around dolomite and anhydrite values indicated δ44/40Ca  0.08‰. (A) data from Jacobson and Wasserberg (2005) and (B) modified from Jacobson and Holmden (2008). Fig. 8: 44/40Ca variation as a function of 87Sr/86Sr for the soil solutions, atmospheric inputs, roots, leaves and wooden organs as well as for the ambient geogenic samples in a temperate CZO forested catchment (modified from Schmitt et al., 2017). Samples for soil solutions (stars), roots (diamonds), and leaves (triangles) are given in (a) for the 2004/2006 period and in (b) for 2011/2013. Blue rectangle highlights small range of 87Sr/86Sr values in soil solutions, roots, and

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Journal Pre-proof leaves from 2004/2006 with a large range of 44/40Ca values where soil solutions are enriched in 44

Ca as a result of 40Ca uptake by roots. The green arrow in (b) shows a shift in many soil

solutions to higher 87Sr/86Sr values in 2011/2013 compared to 2004/2006. The orange arrow in (b) shows an opposite shift in some roots towards decreasing 87Sr/86Sr values in 2011/2013 compared to 2004/2006 suggesting variations in Sr (and Ca) source. The trend in 44/40Ca values is due to Ca isotopic fractionation during root uptake and translocation within the tree. Geogenic sources are plotted with tan thin cross (apatite) and tan star (plagioclase). Atmospheric sources (precipitation and throughfall) are shown with thick crosses. Circles are measured xylem and

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phloem. Squares are measured wood and bark. For full legend see Fig. 5 in Schmitt et al. (2017). Fig. 9: 44/40Ca variation as a function of 87Sr/86Sr for the soil solutions (circles), atmospheric

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inputs (squares), roots, leaves and wooden organs (triangles) as well as for the ambient geogenic (basalt indicated with solid diamond and soil represented by empty diamond) samples in a

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permafrost-dominated forested catchment from Central Siberia. The symbols correspond to

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individual samples measured for each category (represented as shaded envelopes). Ca isotopic fractionation mechanisms are represented with green arrow, Sr source variations are represented

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with grey arrows (modified from Bagard et al., 2013).

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Fig. 10: Exchangeable soil 44/40Ca (A) and 138/134Ba (B) values in two soil profiles from Kohala Mountain, Hawaii, USA. Gray shading indicates the isotopic composition of bedrock and

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the dashed line in (A) indicates the isotopic composition of pedogenic carbonate. Vegetation 44/40CaSRM915A values range from -2.01 to -0.6‰ and 138/134BaSRM3104a values range from -0.75 to -0.20‰ (not shown). Exchangeable soil stable Ca isotopes are controlled by the gradual leaching of biocycled, isotopically light 44/40Ca values from surface to depth (Site I) unless offset to higher 44/40Ca values through the uptake of light Ca isotopes by pedogenic carbonate formation (Site B). Exchangeable soil stable Ba isotopes are controlled by the close retention of isotopically light biolifted Ba in surface soils. Modified from Bullen and Chadwick (2016).

Fig. 11: Florida Bay (filled symbols) and Florida Keys (ocean side, open symbol) waters and sediment δ44/40Ca variations with salinity. Percentage of Ca attributed to submarine groundwater

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Journal Pre-proof discharge and surface waters (Everglades run-off) in the Florida Bay waters indicated next to samples with salinities less than 31 using a mixing model from Holmden et al. (2012).

Fig. 12: Controls on water geochemistry of the Coorong lagoon system, Australia. (A) Input water sources determined by 87Sr/86Sr ratios. Note difference in 87Sr/86Sr ratio scale of inset. (B) Secondary mineral (carbonate) precipitation in waters of the southern Coorong lagoon (yellow symbols) determined by δ44/40Ca values. Dashed lines represent mixing trends between endmember input sources. Modified from Shao et al. (2018). Northern Coorong lagoon waters are

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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