# Synthesis of In-House Produced Calibrated Silver Phosphate with a Large Range of Oxygen Isotope Compositions

Christophe Lécuyer, François Fourel, Magali Seris, Romain Amiot, Jean Goedert, Laurent Simon

► To cite this version:

Christophe Lécuyer, François Fourel, Magali Seris, Romain Amiot, Jean Goedert, et al. Synthesis of In-House Produced Calibrated Silver Phosphate with a Large Range of Oxygen Isotope Compositions. Geostandards and Geoanalytical Research, 2019, 43 (4), pp.681-688. 10.1111/ggr.12285. hal-02991841

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1 Synthesis of In-House Made Calibrated Silver Phosphate in a Large Range of Oxygen

2 Isotope Compositions

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6 Christophe **Lécuyer**¹,*, François **Fourel**², Magali **Seris**¹, Romain **Amiot**¹, Jean **Goedert**¹,³ and

7 Laurent **Simon**²

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11 ¹LGL-TPE, UMR 5276, CNRS, Université Claude Bernard Lyon 1, 69622 Villeurbanne,

12 France

13 ²LEHNA, UMR 5023, CNRS, Université Claude Bernard Lyon 1, ENTPE, 69622

14 Villeurbanne, France

15 ³PACEA, UMR 5199, CNRS, Université de Bordeaux, Bâtiment B18, Allée Geoffroy Saint-

16 Hilaire, CS 50023, FR-33615 Pessac, France

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18 *corresponding author. E-mail : christophe.lecuyer@univ-lyon1.fr

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**Abstract** – The large range of stable oxygen isotope values of phosphate-bearing minerals and dissolved phosphate of inorganic or organic origin requires the availability of in-house made calibrated silver phosphate of which isotopic ratios must closely bracket those of studied samples. We propose a simple protocol to synthesize Ag₃PO₄ in a wide range of oxygen isotope compositions based on the equilibrium isotopic fractionation factor and the kinetics and temperature of isotopic exchange in the phosphate-water system. Ag₃PO₄ crystals are obtained from KH₂PO₄ that is dissolved in water of known oxygen isotope composition. Isotopic exchange between dissolved phosphate and water takes place at a desired and constant temperature into pyrex tubes that are placed in a high precision oven for run-times defined by the user. Samples are withdrawn at desired times, quenched in cold water, and precipitated as Ag₃PO₄. We provide a calculation sheet that computes the δ¹⁸O of precipitated Ag₃PO₄ as a function of time, temperature and δ¹⁸O of both reactants KH₂PO₄ and H₂O at t = 0. Predicted oxygen isotope compositions of synthesized silver phosphate range from -7 to +31‰ VSMOW for a temperature range comprised between 110°C and 130°C and a range of water δ¹⁸O from -20 to +15‰ VSMOW.

Keywords: oxygen isotopes, isotopic fractionation, kinetics, silver phosphate, internal reference
41
42 Résumé – La large gamme de δ¹⁸O des minéraux phosphatés et de phosphates dissous d'origine
43 inorganique ou organique nécessite l'existence d'Ag₃PO₄ calibré dont les rapports isotopiques
44 doivent encadrer les échantillons étudiés. Nous proposons un protocole simple pour synthétiser
45 Ag₃PO₄ dans une large gamme de compositions isotopiques. Ce protocole repose sur les
46 fractionnements isotopiques à l'équilibre et les températures et cinétiques d'échange isotopique
47 dans le système phosphate-eau. Les cristaux d'Ag₃PO₄ sont obtenus à partir de KH₂PO₄ dissous
48 dans une eau de δ¹⁸O connu. Les échanges isotopiques entre le phosphate dissous et l'eau ont
49 lieu à une température constante dans des tubes en pyrex placés dans une étuve thermo-régulée
50 pour des durées de réactions définies par l'opérateur. Les échantillons sont retirés en temps
51 voulu, trempés dans de l'eau froide, et précipités sous la forme de cristaux de phosphate
d'argent. Nous fournissons une « feuille Excel » qui calcule le δ¹⁸O d'Ag₃PO₄ précipité en
52 fonction du temps, de la température et des δ¹⁸O de KH₂PO₄ et H₂O au temps t = 0. Les δ¹⁸O
53 prédits pour Ag₃PO₄ varient de -7 à +31‰ VSMOW pour une gamme de températures
54 comprises entre 110°C et 130°C et une gamme de δ¹⁸O de l'eau de -20 to +15‰ VSMOW.
55

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57 Mots-clés: isotopes de l'oxygène, fractionnement isotopique, cinétique, phosphate d'argent,
58 référence interne

59
60 1. Introduction

61
62 During the past decades, the rise of automated devices coupled to mass spectrometers
operating in continuous flow or in dual-inlet mode resulted in an explosive growth of the
number of data published in the scientific fields exploiting the stable isotope ratios of organic and inorganic compounds. Consequently, there is an increasing use and need for developing in-house made material calibrated against certified reference materials. Moreover, a large range of isotopic compositions is highly desirable to bracket the expected compositions of the studied sample collection as well as to be able to perform a two-point calibration, which is required for the acquisition of high-quality data.

In Earth and Archaeological Sciences, to only mention the most concerned research fields, the ¹⁸O/¹⁶O ratio of the phosphate radical (PO₄³⁻) is now widely used to reconstruct the paleoclimates of the Earth (Fricke and O'Neil, 1996; Kolodny and Raab, 1988; Amiot et al., 2004; Joachimski et al., 2012; Goedert et al., 2017), the thermophysiology and ecology of extinct vertebrates (Barrick and Showers, 1994; Fricke and Rogers, 2000; Amiot et al., 2006; Tütken and Vennemann, 2009; Bernard et al., 2010; Rey et al., 2018), the source and recycling of dissolved phosphate in natural waters (Markel et al., 1994; McLaughlin et al., 2006; Pistocchi et al., 2017) as well as the diet and living environment of past human populations (White et al., 1998; Evans and Chenery, 2006; Touzeau et al., 2013; Lightfoot and O'Connell, 2016; Pellegrini et al., 2016). The most common way to determine the ¹⁸O/¹⁶O ratio of the phosphate radical is to isolate it as silver phosphate (Ag₃PO₄) crystals through a wet chemistry procedure (Crowson et al., 1991). Then they are pyrolyzed in the presence of graphite at high temperature to produce either CO₂ or CO analysed either off-line or on-line with an isotopic ratio mass spectrometer (O'Neil et al., 1994; Lécuyer et al., 1998; 2007; Fourel et al., 2011). Fluorination technique has also been proven to be a precise and accurate technique that allowed the first determination of the δ¹⁸O*SMOW* of the Miocene Florida phosphorite SRM 120c (Lécuyer et al., 1993). Whatever the considered research field, the δ¹⁸O of calcium phosphate minerals (e.g.
biogenic, magmatic or hydrothermal apatite) and dissolved phosphate ($H_2PO_4^-$, $HPO_4^{2-}$, $PO_4^{3-}$) range worldwide from about a few per mil up to values close to 30‰. Indeed, high-temperature apatites of magmatic or hydrothermal origin have $\delta^{18}\text{O}$ ranging from 6 to 11‰. Sun et al., 2016). Concerning soils and their connected aquatic environments (rivers, ponds, lakes), dissolved phosphate also displays a large range of oxygen isotope compositions comprised between 8‰ and 25‰ (Markel et al., 1994; Gruau et al., 2005; Angert et al., 2012; Davies et al., 2014; Tamburini et al., 2014; Pistocchi et al., 2017; Granger et al., 2017; Bauke et al., 2018). In the case of dissolved phosphate being isotopically equilibrated with ambient water through biological recycling (Longinelli et al., 1976; Liang and Blake, 2009; Chang and Blake, 2015; von Sperber et al., 2017), it is expected to have $\delta^{18}\text{O}$ that range from 6‰ (high-latitude freshwater environments) to ≈ 20‰ for seawater and to ≈ 22‰ for low-latitude freshwater environments in agreement with the available isotopic fractionation equations (Kolodny et al., 1983; Lécuyer et al., 2013). In the case of biogenic apatites, their $\delta^{18}\text{O}$ values range from a few ‰ for vertebrates drinking highly $^{18}\text{O}$-depleted waters ($\delta^{18}\text{O}$ as low as -20‰) relative to SMOW (high-altitude or high-latitude environments; e.g. Rey et al., 2018) up to about 30‰ for vertebrates living in arid environments (Lécuyer et al., 1999a).

So far, researchers were using the Florida phosphorite SRM 120c, for which a consensual $\delta^{18}\text{O}_{\text{SMOW}}$ value of 21.7±0.2‰ (VSMOW) was only admitted these last years. Indeed, Chenery et al. (2010) proposed a comparable value of 21.7±0.7‰ after a 6-month period of repeated measurements of SRM 120c calibrated against NBS 127 barium sulfate, which is consistent with the average value of published data for SRM 120c (21.5±0.5‰) analyzed in different laboratories (Chenery et al., 2010). Thereafter, Halas et al. (2011) confirmed the absence of any sizable isotopic fractionation effect during the conversion of
109 Ag₃PO₄ into CO, and proposed a mean δ¹⁸O value of 21.8±0.2‰ for SRM 120c based on inter-
110 laboratory calibrations. It is also worthy to note that Vennemann et al. (2002) proposed a
111 significantly higher value of 22.5‰ for SRM 120c. But later on, in a communication given at
112 ‘IsoPhos’ meeting in Ascona, Switzerland, Vennemann (2012) concluded that the most accurate
113 value for SRM 120c is likely close to 21.7‰. This value of 21.7±0.2‰ for SRM 120c is now
114 widely accepted as exemplified by the recent study published by Huang et al. (2018). It has also
115 to be pointed out that an isotopic ratio of 21.7±0.16‰ for SRM 120c was determined for the
116 first time by using quantitative fluorination 25 years ago (Lécuyer et al., 1993). Nevertheless,
117 it must be underlined that the Florida phosphorite SRM 120c is a compositional, not an isotopic
118 international reference, hence its oxygen isotope ratio has never been officially certified. In
119 order to comply with the IUPAC recommendations (Brandt et al., 2014), a two-point calibration
120 can also be performed with NBS 127, which is a barium sulfate calibrated reference material
121 for oxygen isotopes (δ¹⁸O = 9.3‰ VSMOW), as we proceeded in some recent papers (e.g. Rey
122 et al., 2018; Goedert et al., 2018). However, we underline that NBS 127 is a different matrix,
123 i.e. a barium sulfate instead of a silver phosphate, a chemical difference that needs to be taken
124 into account during calibration procedures even though the recently developed “purge-and-
125 trap” technology was able to overcome this pitfall as shown by Fourel et al. (2011). Moreover,
126 the stock of NBS 127 international reference hosted by the IAEA is now exhausted, thus
127 seriously reducing the possibility of calibrating the oxygen isotope composition of solid
128 matrices. Therefore, the need for alternative calibrated material is becoming critical.

129 Here, we propose a simple and cheap protocol to synthesize silver phosphate in a wide
130 range (≈ 35‰) of oxygen isotope compositions based on thermodynamic properties of the
131 phosphate-water system, more specifically the equilibrium isotopic fractionation factor and the
132 kinetics and temperature of isotopic exchange according to the data published by Lécuyer et al.
133 (1999b). Beyond the theoretical considerations, an “Excel calculation sheet” is provided as a
supplementary material. This calculation sheet allows the δ¹⁸O of precipitated Ag₃PO₄ to be
predicted by tuning various parameters such as the temperature of oxygen isotope exchange,
the duration of the reaction and the isotopic compositions of reactants.

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## 2. Theoretical background

### 2.1. First-order kinetics of isotopic exchange between dissolved phosphate and water

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The most common pH-dependent speciation of phosphorus in aqueous solution of low ionic strength are (1) the dihydrogen phosphate ion (H₂PO₄⁻), (2) the hydrogen phosphate ion (HPO₄²⁻), and (3) the phosphate ion (PO₄³⁻) according to the three following chemical equilibria at 25°C (Zeebe and Wolf-Gladrow, 2001; Figure 1):

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$$146 \quad - \mathrm{H}_3\mathrm{PO}_4 \rightleftharpoons \mathrm{H}_2\mathrm{PO}_4^- + \mathrm{H}^+ \quad (\mathrm{p}K_{\mathrm{A}1} = 1.9) \quad (1)$$

$$147 \quad - \mathrm{H}_2\mathrm{PO}_4^- \rightleftharpoons \mathrm{HPO}_4^{2-} + \mathrm{H}^+ \quad (\mathrm{p}K_{\mathrm{A}2} = 6.1) \quad (2)$$

$$148 \quad - \mathrm{HPO}_4^{2-} \rightleftharpoons \mathrm{PO}_4^{3-} + \mathrm{H}^+ \quad (\mathrm{p}K_{\mathrm{A}3} = 8.9) \quad (3)$$

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For seawater (pH ≈ 7.6 to 8.4), the dominant species is HPO₄²⁻ while in the case of  
natural freshwater it is either H₂PO₄⁻ or HPO₄²⁻. Lécuyer et al. (1999b) performed kinetics and  
isotopic experiments at a pH close to 5 for which H₂PO₄⁻ was almost the only species present  
in solution (Figure 1). During first-order kinetics of reaction, the rate of isotopic exchange can  
be quantified by considering *f* as follows,

155
$$f = 1 - \exp(-kt) = \frac{\delta^{18}\text{O}(\text{PO}_4)_i - \delta^{18}\text{O}(\text{PO}_4)_t}{\delta^{18}\text{O}(\text{PO}_4)_i - \delta^{18}\text{O}(\text{PO}_4)_e} \quad (4)$$

which is the mole fraction of exchanged isotopes between the phosphate ions and water molecules, while k is the rate constant (s⁻¹) of the isotopic reaction:

$$k = A \cdot \exp\left(-\frac{E_a}{RT}\right) \quad (5)$$

with A being the Arrhenius pre-exponential factor, E_a the activation energy (kJ.mol⁻¹) of the isotopic reaction, R the universal gas constant, T the absolute temperature (K), t the time (s), δ¹⁸O(PO₄)ᵢ the oxygen isotope composition of the reactant at t = 0, δ¹⁸O(PO₄)t the oxygen isotope composition of H₂PO₄⁻ at any time t during the course of the isotopic reaction, and δ¹⁸O(PO₄)e the oxygen isotope composition of H₂PO₄⁻ at isotopic equilibrium with H₂O. Note that when t = 0; f = 0 and δ¹⁸O(PO₄)t = δ¹⁸O(PO₄)ᵢ while when t → ∞; f → 1 and δ¹⁸O(PO₄)t = δ¹⁸O(PO₄)e.

## 2.2. Experimental determination of thermodynamic variables (k, E_a and α)

### 2.2.1. The rate constant k

Lécuyer et al. (1999b) performed a best fit of their data with the Arrhenius law and found a strong temperature dependence of the rate constant k:

$$\log (k) = 12.35 \pm 0.63 - 6.98 \pm 0.24(10^3 T^{-1}) \quad (6)$$
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A convenient way to report graphically the rate of isotopic exchange against time is to rewrite equation (4) as follows:

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$$ \ln(1-f) = -kt \qquad (7) $$

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The absolute value of the slope of the straight line for a given temperature T is the rate constant k of the reaction. Equation (7) also allows the half-time ($t_{1/2}$) of the isotopic reaction to be determined by solving it for f = 0.5 as follows:

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$$ t = \frac{0.693}{k} \qquad (8) $$

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### 2.2.1. The activation energy $E_a$ of the reaction

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Due to the tetrahedral architecture and covalent bonds (P–O) of the phosphate ion, the activation energy $E_a$ necessary to promote the oxygen isotope exchange between phosphate and water molecules is very high with a value of 133.6±4.6 kJmol⁻¹ at pH = 5 (Lécuyer et al., 1999b). The activation energy $E_a$ is calculated from the slope of the straight line defined by equation (6) combined to equation (5).

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### 2.2.2. The fractionation coefficient $\alpha$ between $H_2PO_4^-$ and $H_2O$

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At thermodynamic equilibrium, and according to the Mass Action Law, we can write
the following isotopic exchange equation:

$$H_2P^{16}O_4^- + H_2^{18}O \leftrightarrow H_2P^{18}O_4^- + H_2^{16}O \quad (9)$$

Rearranging the terms in equation (9), the equilibrium constant or isotopic fractionation factor called α, which is determined for a given temperature T, is expressed as follows:

$$\alpha_{(H_2PO_4^- - H_2O)} = \frac{\left[ \frac{18O}{16O} \right]_{H_2PO_4^-}}{\left[ \frac{18O}{16O} \right]_{H_2O}} \quad (10)$$

It is worthy to note that this temperature-dependent fractionation factor α must be independent from the isotopic compositions of both reactants according to Northrop and Clayton (1966). Lécuyer et al. (1999b) determined experimentally α(H₂PO₄⁻ – H₂O) in the temperature range 75 to 135°C:

$$10^3 \ln \alpha_{(H_2PO_4^- - H_2O)} = 18.35 \pm 0.37 (10^3 T^{-1}) - 32.29 \pm 1.01 \quad (11)$$

### 3. Application to the synthesis of silver phosphate of known oxygen isotope ratio

#### 3.1. Experimental protocol of silver phosphate precipitation

We present a protocol of silver phosphate precipitation from a highly soluble salt in water (solubility = 25 g.100 mL⁻¹ at 25°C; Lide, 2005) such as the potassium dihydrogen
222 phosphate KH₂PO₄ (Lécuyer et al., 1999b). An aliquot of 50 mg of pure high-grade (> 99.5 wt%) Sigma-Aldrich™ synthetic KH₂PO₄ is dissolved in 35 mL of deionized water of known oxygen isotope composition. The resulting aqueous solutions have a concentration of 1 g.L⁻¹ (10.6 mmol.l⁻¹) of phosphate ions with a pH of 5 at ambient temperature. It is important to note that the amount of oxygen in water is much higher than in the pool of dissolved phosphate (oxygen molar ratio between H₂O and KH₂PO₄ ≈ 5000), consequently the change in the oxygen isotope composition before and after equilibrium is not detectable with respect to the analytical uncertainties (1σ = 0.05‰ for the δ¹⁸O of H₂O). The solutions are transferred into Ace Glass™ pyrex tubes sealed with a threaded teflon plug. Each set of tubes for a given temperature is placed in a high precision oven for run-times defined by the user. Samples are withdrawn at desired times and quenched in cold water to room temperature within a few minutes. Each sample of dissolved phosphate is quantitatively precipitated as silver phosphate (chemical yields are close to 100%) according to the protocol determined by Firsching (1961), which means that all the dissolved phosphate species (≈ H₂PO₄⁻), are converted into Ag₃PO₄. For a chemical yield of 100%, the expected amount of silver phosphate is close to 150 mg. Running an experiment batch with ten Ace Glass™ pyrex tubes at the same time ensures the production of about 1.5 g of silver phosphate crystals of predicted oxygen isotope composition.

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## 3.2. Oxygen isotope analysis of silver phosphate

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Oxygen isotope compositions are measured using a high-temperature pyrolysis technique involving a VarioPYROcube™ elemental analyzer (EA) interfaced in continuous flow (CF) mode to an Isoprime™ isotopic ratio mass spectrometer (IRMS) (EA-Py-CF-IRMS
technique (Lécuyer et al., 2007; Fourel et al., 2011) at the University Claude Bernard Lyon 1.
For each sample, 5 aliquots of 300 µg of Ag₃PO₄ are mixed with 300 µg of pure carbon black
powder and loaded in silver foil capsules. Pyrolysis is performed at a temperature of 1450°C.
Measurements are calibrated against the SRM 120c (natural Miocene phosphorite from Florida)
and the NBS 127 (barium sulfate, BaSO₄: δ¹⁸O = 9.3‰ VSMOW). The δ¹⁸O of SRM 120c is
fixed at 21.7‰ (VSMOW) according to Lécuyer et al. (1993) who determined this value by
fluorinating silver phosphate crystals (n = 25, 1σ = 0.16) at 600°C for 12 hours and calibrated
the oxygen isotope ratios with the calibrated reference material NBS 28 quartz (9.5±0.2‰), and
measurement standards that are the “Snowbird” quartz (16.1±0.1‰) and the tholeiitic basaltic
glass standard CIRCE 93 (5.6±0.1‰).

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### 3.3. Predicted oxygen isotope composition of synthesized silver phosphate

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According to equation (4), for any temperature T, k and δ¹⁸O(PO₄)ₐ are calculated, t
and δ¹⁸O(PO₄)ᵢ are measured, which means that δ¹⁸O(PO₄)ₜ can be predicted as follows:

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$$ \delta^{18}\text{O}(\text{PO}_4)_t = \delta^{18}\text{O}(\text{PO}_4)_i - \{\delta^{18}\text{O}(\text{PO}_4)_i - \delta^{18}\text{O}(\text{PO}_4)_e\}\{1 - \exp(-kt)\} \quad (12) $$

262

We emphasize that the recommended working range of temperature is from 110°C to
130°C. Indeed, below a temperature of 110°C, kinetics of reaction is so slow that significant
isotopic exchange takes months, or even years when temperature is below 100°C. Then, above
a temperature of 130°C, the Ace Glass™ pyrex tubes sealed with a threaded teflon plug could
start to leak, leading to a shift of the initial oxygen isotope ratio of water. Finally, we provide
an “Excel calculation sheet” (Supplementary File) that automatically computes the oxygen

268
269 isotope composition of the precipitated silver phosphate depending on time t and temperature
270 T of isotopic exchange as well as the oxygen isotope compositions of reactants KH₂PO₄ and
271 H₂O at t = 0. Predicted oxygen isotope compositions of synthesized silver phosphate range from
272 -7 to +31‰ VSMOW for a temperature range comprised between 110°C and 130°C and a range
273 of water δ¹⁸O from -20 to +15‰ VSMOW (Figure 2). For example, we have provided a
274 calculation table (Supplementary File) showing a possible tuning of the parameters described
275 above to obtain silver phosphate crystals with a δ¹⁸O value (‰ VSMOW) that mimics that
276 certified for the NBS 127 barium sulfate international reference.

277 In order to further demonstrate the efficiency of this method, we analysed the oxygen
278 isotope compositions of three silver phosphates with expected δ¹⁸O values ranging from about
279 +10 to +22 ‰ VSMOW (Table 1), which cover the most common range of documented δ¹⁸O
280 values for natural dissolved phosphate and apatites. We reported the measured δ¹⁸O values as
281 a function of the expected ones and we obtained a strong linear correlation with a slope close
282 to 1 (a = 1.032±0.029) and an intercept close to 0 (b = -0.794±0.466) (Figure 3). Within this
283 isotopic variation range of 12‰, the calculated standard error is 0.24‰, which is comparable
284 to the error associated with the admitted δ¹⁸O value of SRM 120c. Therefore, we show that this
285 method is suitable to produce robust in-house made calibrated Ag₃PO₄ for the oxygen isotope
286 analysis of phosphatic compounds.

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3.4. Potential application to the sulfate-water system

289
Furthermore, a similar protocol may be applied to the sulfate-water system to produce barium sulfate of known oxygen isotope composition. Indeed, the required thermodynamic properties such as the rate constant k, the activation E_a and the T-dependent isotopic fractionation factor α have been determined by Chiba and Sakai (1985). The oxygen isotope measurement of sulfates, whatever their natural or anthropogenic origin, could be very useful to trace the mechanisms of their formation. For instance, sulfates may form in the atmosphere from the oxidation of sulfur dioxide by hydroxyl radicals, hydrogen peroxide or ozone. Sulfates may also result from a high-temperature oxidation to produce sulfur trioxide during combustion processes before being ultimately hydrated to form sulfuric acid. Various oxygen isotope fractionations could be associated with those chemical reactions, requiring the production of calibrated barium sulfate with different δ¹⁸O_vSMOW values.

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## 4. Conclusions

303

The protocol we developed in this study is based on the equilibrium isotopic fractionation factor α, the kinetics k and temperature T of isotopic exchange in the dissolved phosphate-water system. It offers the possibility to produce in a reasonable amount of time (a few days or a couple of weeks) about 1.5 g (the equivalent of 1,000 measurements) of silver phosphate of known oxygen isotope composition. Moreover, the tuning of some parameters (time and temperature of isotopic exchange and the compositions of reactants) allows the synthesis of silver phosphate within a large range (≈35‰) of oxygen isotope ratios that match the documented natural variability for both phosphate-bearing minerals and dissolved phosphate of organic or inorganic origin.

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313
314 Acknowledgements – this study has been founded by CNRS and IUF (CL).

315
316

317 **References**

318

319 **Amiot R., Lécuyer C., Buffetaut E., Escarguel G., Fluteau F. and Martineau F. (2006)**

320 Oxygen isotopes from biogenic apatites suggest widespread endothermy in Cretaceous

321 dinosaurs. **Earth and Planetary Science Letters**, **246**, 41–54.

322

323 **Amiot R., Lécuyer C., Buffetaut E., Fluteau F., Legendre S. and Martineau F. (2004)**

324 Latitudinal temperature gradient during the Cretaceous Upper Campanian–Middle

325 Maastrichtian: $\delta^{18}$O record of continental vertebrates. **Earth and Planetary Science Letters,**

326 **226**, 255–272.

327

328 **Angert A., Weiner T., Mazeh S. and Sternberg M. (2012)**

329 Soil phosphate stable oxygen isotopes across rainfall and bedrock gradients. **Environmental**

330 **Science and technology**, **46**, 2156–2162.

331

332 **Barrick R.E. and Showers W.J. (1994)**

333 Thermophysiology of *Tyrannosaurus rex*: evidence from oxygen isotopes. **Science**, **265**, 222–

334 224.

335

336 **Bauke S.L., Sperber (von) C., Tamburini F., Gocke M.I., Honermeier B., Schweitzer K.,**

337 **Baumecker M., Don A., Sandhage-Hofmann A. and Amelung W. (2018)**

338 Subsoil phosphorus is affected by fertilization regime in long-term agricultural experimental

339 trials. **European Journal of Soil Science**, **69**, 103–112.

340
341 Bernard A., Lécuyer C., Vincent P., Amiot R., Bardet N., Buffetaut E., Cuny G., Fourel

342 F., Martineau F., Mazin J.-M., and Prieur A. (2010)

343 Regulation of body temperature by some Mesozoic marine reptiles. **Science**, **328**, 1379–1382.

345 Brandt W., Coplen T.B., Vogl J., Rosner M. and Prohaska T. (2014)

346 Assessment of international reference materials for isotope-ratio analysis (IUPAC Technical

347 Report). **Pure and Applied Chemistry**, **86**, 425–467.

349 Chang S.J. and Blake R.E. (2015)

350 Precise calibration of equilibrium oxygen isotope fractionations between dissolved phosphate

351 and water from 3 to 37 °C. **Geochimica et Cosmochimica Acta**, **150**, 314–329.

353 Chenery C., Müldner G., Evans J., Eckardt H. and Lewis M. (2010)

354 Strontium and stable isotope evidence for diet and mobility in Roman Gloucester, UK. **Journal**

355 of Archaeological Science, **37**, 150–163.

357 Chiba H. and Sakai H. (1985)

358 Oxygen isotope exchange rate between dissolved sulfate and water at hydrothermal

359 temperatures. **Geochimica et Cosmochimica Acta**, **49**, 993–1000.

361 Crowson R.A., Showers W.J., Wright E.K. and Hoering T.C. (1991)

362 A method for preparation of phosphate samples for oxygen isotope analysis. **Analytical**

363 Chemistry, **63**, 2397-2400.

365 Davies C.L., Surridge B.W. and Gooddy D.C. (2014)
366 Phosphate oxygen isotopes within aquatic ecosystems: Global data synthesis and future research priorities. **Science of the Total Environment**, 496, 563–575.

368
369 **Evans J.A., Chenery C.A. and Fitzpatrick A.P. (2006)**

370 Bronze age childhood migration of individuals near Stonehenge, revealed by strontium and oxygen isotope tooth enamel analysis. **Archaeometry**, 48, 309–321.

372
373 **Firsching F.H. (1961)**

374 Precipitation of silver phosphate from homogeneous solution. **Analytical Chemistry**, 33, 873-874.

376
377 **Fourel F., Martineau F., Lécuyer C., Kupka H.J., Lange L., Ojeimi C. and Seed M. (2011)**

378 $^{18}$O/$^{16}$O ratios measurements of inorganic and organic materials by EA–Pyrolysis–IRMS continuous flow techniques. **Rapid Communications in Mass Spectrometry**, 25, 2691–2696.

380
381 **Fricke H.C. and Rogers R.R. (2000)**

382 Multiple taxon–multiple locality approach to providing oxygen isotope evidence for warm-blooded theropod dinosaurs. **Geology**, 28, 799–802.

384
385 **Fricke H.C. and O'Neil J.R. (1996)**

386 Inter- and intra-tooth variation in the oxygen isotope composition of mammalian tooth enamel phosphate: implications for palaeoclimatological and palaeobiological research.
387
388 **Palaeogeography, Palaeoclimatology, Palaeoecology**, 126, 91–99.

389
390 Goedert J., Amiot R., Cuny G., Fourel F., Arnaud Godet F., Hernandez J.-A., Pedreira-Segade U. and Lécuyer C. (2017)

392 Miocene (Burdigalian) seawater and air temperatures estimated from the geochemistry of vertebrate remains from the Aquitaine Basin, France. **Palaeogeography, Palaeoclimatology, Palaeoecology, 481**, 14–28.

395 Goedert J., Lécuyer C., Amiot R., Wang X., Cui L., Cuny G., Douay G., Fourel F., Simon L., Steyer S. and Zhu M. (2018)

398 Euryhaline ecology of early tetrapods revealed by stable isotopes. **Nature, 558**, 68–72.

399 Granger S.J., Harris P., Peukert S., Guo R., Tamburini F., Blackwell M.S., Howden N.J.K. and McGrath S. (2017)

402 Phosphate stable oxygen isotope variability within a temperate agricultural soil. **Geoderma, 285**, 64–75.

404 Gruau G., Legeas M., Riou C., Gallacier E., Martineau F. and Hénin O. (2005)

406 The oxygen isotope composition of dissolved anthropogenic phosphates: a new tool for eutrophication research? **Water Research, 39**, 232–238.

408 Halas S., Skrzypek G., Meier-Augenstein W., Pelc A. and Kemp H.F. (2011)

410 Inter-laboratory calibration of new silver orthophosphate comparison materials for the stable oxygen isotope analysis of phosphates. **Rapid Communications in Mass Spectrometry, 25**, 579–584.

413 Huang C., Joachimski M.M. and Gong Y. (2018)
415 Did climate changes trigger the Late Devonian Kellwasser Crisis? Evidence from a high-resolution conodont $\delta^{18}$O$_{PO4}$ record from South China. **Earth and Planetary Science Letters,** 495, 174–184.

418
419 Joachimski M.M., Lai X., Shen S., Jiang H., Luo G., Chen B., Chen J. and Sun Y. (2012) Climate warming in the latest Permian and the Permian–Triassic mass extinction. **Geology,** 40, 195–198.

422
423 Kolodny Y., Luz B. and Navon O. (1983)

424 Oxygen isotope variations in phosphate of biogenic apatites, I. Fish bone apatite – rechecking the rules of the game. **Earth and Planetary Science Letters,** 64, 398–404.

426
427 Kolodny Y. and Raab M. (1988)

428 Oxygen isotopes in phosphatic fish remains from Israel; paleothermometry of tropical
429 Cretaceous and Tertiary shelf waters, **Palaeogeography, Palaeoclimatology, Palaeoecology,**
430 **64**, 59–67.

431
432 Lécuyer C., Amiot R., Trotter J. and Touzeau A. (2013)

433 Calibration of the phosphate $\delta^{18}$O paleothermometer with the calcium carbonate-water oxygen isotope fractionation equations. **Chemical Geology,** 347, 217–226.

435
436 Lécuyer C., Fourel F., Martineau F., Amiot R., Bernard A., Daux V., Escarguel G. and Morrison J. (2007)
438 High-precision determination of ¹⁸O/¹⁶O ratios of silver phosphate by EA-pyrolysis-IRMS
439 continuous flow technique. **Journal of Mass Spectrometry**, 42, 36-41.

440
441 Lécuyer C., Grandjean P., Mazin J.-M. and De Buffrénil V. (1999a)

442 Oxygen isotope compositions of reptile bones and teeth: a potential record of terrestrial and
443 marine paleoenvironments, in: E. Hoch, A.K. Brantsen (Eds.), **Secondary Adaptation to Life**
444 in Water II, University of Copenhagen (Denmark), Geologisk Museum, p. 33.

445
446 Lécuyer C., Grandjean P. and Sheppard S.M.F. (1999b)

447 Oxygen isotope exchange between dissolved phosphate and water at temperatures <135°C: inorganic versus biological fractionations. **Geochimica et Cosmochimica Acta**, 63, 855-862.

450
451 Lécuyer C., Grandjean P., Barrat J.-A., Nolvak J., Emig C.C., Paris F. and Robardet M.,
452 **1998**.

453 δ¹⁸O and REE contents of phosphatic brachiopods: a comparison between modern and lower
454 Paleozoic populations. **Geochimica et Cosmochimica Acta**, 62, 2429-2436.

455
456 Lécuyer C., Grandjean P., O'Neil J.R., Cappetta H. and Martineau F. (1993)

457 Thermal excursions in the ocean at the Cretaceous-Tertiary boundary (northern Morocco):
458 δ¹⁸O record of phosphatic fish debris. **Palaeogeography Palaeoclimatology Palaeoecology,**
459 **105**, 235-243.

460
461 Liang Y. and Blake R.E. (2009)
Compound-and enzyme-specific phosphodiester hydrolysis mechanisms revealed by δ¹⁸O of dissolved inorganic phosphate: Implications for marine P cycling. **Geochimica et Cosmochimica Acta**, 73, 3782–3794.

**Lide D.R. (2005)**

CRC Handbook of Chemistry and Physics 86th Edition 2005-2006. **CRC Press, Taylor & Francis**, Boca Raton, FL 2005, p. 4–79.

**Lightfoot E. and O’Connell T.C. (2016)**

On the use of biomineral oxygen isotope data to identify human migrants in the archaeological record: intra-sample variation, statistical methods and geographical considerations. **PLOS ONE**, DOI:10.1371/journal.pone.0153850.

**Longinelli A., Bartelloni M. and Cortecci G. (1976)**

The isotopic cycle of oceanic phosphate, I. 1976. **Earth and Planetary Science Letters**, 32, 389–392.

**Markel D., Kolodny Y., Luz B. and Nishri A. (1994)**

Phosphorus cycling and phosphorus sources in Lake Kinneret: Tracing by oxygen isotopes in phosphate. **Israelian Journal of Earth Sciences**, 43, 165–178.

**McLaughlin, K., Kendall C., Silva S.R., Young, M. and Paytan A. (2006).**

Phosphate oxygen isotope ratios as a tracer for sources and cycling of phosphate in North San Francisco Bay, California. **Journal of Geophysical Research**, 111, G03003, doi:10.1029/2005JG000079.
487

488 **Northrop D.A. and Clayton R.N. (1966)**

489 Oxygen isotope fractionations in systems containing dolomite. **Journal of Geology**, 74, 174–196.

491

492 **O'Neil J.R., Roe L.J., Reinhard E. and Blake R.E. (1994)**

493 A rapid and precise method of oxygen isotope analysis of biogenic phosphate. **Israeli Journal of Earth Sciences**, 43, 203–212.

495

496 **Pellegrini M., John Pouncett J., Jay M., Pearson M.P. and Richards M.P. (2016)**

497 Tooth enamel oxygen “isoscapes” show a high degree of human mobility in prehistoric Britain.

498 **Scientific Reports**, 6, 34986; doi: 10.1038/srep34986.

499

500 **Pistocchi C., Tamburini F., Gruau G., Ferhi A., Trevisan D. and Dorioz J.-M. (2017)**

501 Tracing the sources and cycling of phosphorus in river sediments using oxygen isotopes:

502 Methodological adaptations and first results from a case study in France. **Water Research**,

503 **111**, 346–356.

504

505 **Rey K., Day M.O., Amiot R., Goedert J., Lécuyer C., Sealy J. and Rubidge B.S. (2018)**

506 Stable isotope record implicates aridification in late Capitanian mass extinction. **Gondwana Research**, 59, 1–8.

508

509 **Sun Y., Wiedenbeck M., Joachimski M.M., Beier C., Kemner F. and Weinzierl C. (2016)**

510 Chemical and oxygen isotope composition of gem-quality apatites: Implications for oxygen

511 isotope reference materials for secondary ion mass spectrometry (SIMS). **Chemical Geology**,
512 **440**, 164–178.

513

514 **Tamburini F., Pfahler V., von Sperber C., Frossard E. and Bernasconi S.M. (2014)**

515 Oxygen isotopes for unraveling phosphorus transformations in the soil–plant system: A review. *Soil Science Society of America Journal*, **78**, 38–46.

517

518 **Touzeau A., Blichert-Toft J., Amiot R., Fourel F., Martineau F., Cockitt J., Hall K.,**
519 **Flandrois J.-P. and Lécuyer C. (2013)**

520 Egyptian mummies record increasing aridity in the Nile valley from 5,500 to 1,500 B.P. **Earth**
521 **Planetary and Science Letters**, **375**, 92–100.

522

523 **Tütken T. and Vennemann T. (2009)**

524 Stable isotope ecology of Miocene large mammals from Sandelzhausen, southern Germany.
525 **Paläontologische Zeitschrift**, **83**, 207–226.

526

527 **Vennemann T.W., Fricke H.C., Blake R.E., O’Neil J.R. and Colman A. (2002)**

528 Oxygen isotope analysis of phosphates: a comparison of techniques for analysis of Ag₃PO₄.
529 **Chemical Geology**, **185**, 321–336.

530

531 **Vennemann T. W. (2012)**

532 Further comparison of methods of oxygen isotope analysis of phosphates and standards use.
533 **IsoPhos 2012 Annual Meeting**, 25–29 June, Ascona, Switzerland.

534

535 von Sperber C., Lewandowski H., Tamburini F., Bernasconi S.M., Amelung W. and
536 Frossard E. (2017)
537 Kinetics of enzyme-catalysed oxygen isotope exchange between phosphate and water revealed
538 by Raman spectroscopy. **Journal of Raman Spectroscopy**, **48**, 368–373.

539
540 **White C.D., Spence M.W., Stuart-Williams H.L.Q. and Schwarcz H.P. (1998)**

541 Oxygen isotopes and the identification of geographical origins: The Valley of Oaxaca versus
542 the Valley of Mexico. **Journal of Archaeological Science**, **25**, 643–655.

543
544 **Zeebe R. E. and Wolf-Gladrow D. (2001)**

545 CO₂ in Seawater: Equilibrium, Kinetics, Isotopes. **Elsevier Oceanography Series**, 65, pp. 346,
546 Amsterdam, 2001.

547
548

549 **Table captions:**

550

551 Table 1: Comparison between measured (δ<sup>18</sup>O<sub>measured</sub>) and predicted (δ<sup>18</sup>O<sub>predicted</sub>) oxygen

552 isotope compositions of silver phosphate samples. The δ<sup>18</sup>O<sub>predicted</sub> values have been calculated

553 by using equation (6) and (12) knowing the temperature T and duration t of isotope exchange

554 reaction along with the initial compositions of dissolved phosphate δ<sup>18</sup>O(PO<sub>4</sub>)<sub>i</sub> and water

555 δ<sup>18</sup>O(H<sub>2</sub>O)<sub>i</sub> reactants. The δ<sup>18</sup>O<sub>measured</sub> values have been obtained by using the analytical

556 protocol presented in sections 3.1 and 3.2.

557
558
559 **Figure captions:**
560
561 Figure 1: pH-dependent speciation of dissolved phosphate in aqueous solution according to
562 Zeebe and Wolf-Gladrow (2001).
563

564 Figure 2: Oxygen isotope composition of dissolved phosphate (or precipitated silver phosphate)
565 as a function of time t (in days) of isotopic exchange for a temperature T of 110°C and a δ¹⁸O
566 of water of +15‰ (blue curve) and for a temperature T of 130°C and a δ¹⁸O of water of -20‰
567 (red curve). After no more than 1 month of isotopic exchange between dissolved phosphate and
568 water, the precipitated silver phosphate crystals reach the isotopic equilibrium with water and
569 have δ¹⁸O values close to +31‰ VSMOW (blue curve) and -7‰ VSMOW (red curve),
570 respectively.

571 For instance, the δ¹⁸O of H₂O(i) of +15‰ VSMOW can be obtained by collecting 1 L
572 of residual water obtained by evaporation at ambient temperature (≈ 20°C) under a laminar flow
573 hood of 50 L of deionized water with an initial δ¹⁸O of -10.5‰ of which the ultimate source is
574the Rhône river. The δ¹⁸O of H₂O(i) of -20‰ VSMOW can be obtained in Lyon, France, by
575collecting frozen atmospheric vapour trapped inside a freezer working at a temperature of -
57680°C.

577
578 Figure 3: Linear regression between the oxygen isotope compositions of measured and
579 predicted δ¹⁸O values of three samples of synthesized silver phosphate. A slope of 1.032±0.029
580 and an intercept of -0.794±0.466 with a R² of 0.999 indicate a good fit between these two
581 variables. The standard error of 0.24‰ measures the robustness of the isotopic composition of
582 the precipitated silver phosphate that can be used as in-house made calibrated material.
583
584

585 **Supplementary File:**

586

587 Table: "Excel calculation sheet" that computes the oxygen isotope composition of the precipitated silver phosphate depending on time t and temperature T of isotopic exchange along with the oxygen isotope compositions of reactants KH₂PO₄ and H₂O at t = 0. The provided example provided shows a possible tuning of the parameters described above to obtain silver phosphate crystals with a δ¹⁸O value (‰ VSMOW) similar to that certified for the NBS 127 barium sulfate international reference.

591
592
593 Description of column assignment: C0-C1 = temperature T; C2-C3 = rate constant k; C4-C5 = time t; C6 = fraction f of isotopic exchange; C7-C8 = oxygen isotope compositions of reactants at t = 0; C9-C10 = isotopic fractionation factor α between dissolved phosphate and water; C11-12 = oxygen isotope composition of dissolved phosphate at equilibrium at any time t.

597
<table><thead><tr><th>Calibrated<br>Material</th><th>δ<sup>18</sup>O(water)<br>(‰ VSMOW)</th><th>Equilibration<br>(days)</th><th>δ<sup>18</sup>O(Ag<sub>3</sub>PO<sub>4</sub>) predicted<br>(‰ VSMOW)</th><th>S.D.</th><th>δ<sup>18</sup>O(Ag<sub>3</sub>PO<sub>4</sub>) measured<br>(‰ VSMOW)</th><th>S.D.</th><th>N</th></tr></thead><tbody><tr><td>CAL1</td><td>7.74</td><td>37</td><td>21.68</td><td>0.54</td><td>21.66</td><td>0.10</td><td>8</td></tr><tr><td>CAL3</td><td>-1.18</td><td>44</td><td>15.06</td><td>0.01</td><td>14.55</td><td>0.35</td><td>10</td></tr><tr><td>CAL2</td><td>-7.42</td><td>44</td><td>9.90</td><td>0.28</td><td>9.53</td><td>0.28</td><td>6</td></tr></tbody></table>
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