Ni isotope fractionation factor as a function of calcite growth rate and implications for natural systems.

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Nickel isotopes

Naturally occurring Ni is composed of five stable isotopes (58, 60, 61, 62, and 64) with 58Ni being the most abundant (68.08%). Ni isotope compositions are conventionally reported relative to the U.S. National Institute of Standards and Technology Standard Reference Material 986 (NIST SRM 986) international standard. While most past work on Ni isotopes has centred on cosmochemistry, Ni isotope compositions for silicate Earth, sediments, and water samples have also been characterized. This study focuses on existing oceanic Ni data.

Nickel in the ocean

Nickel in the ocean exhibits nutrient-like behavior as its dissolved concentration ranges from of 2 nmol/kg at the surface to 12 nmol/kg at greater depths with an estimated average of about 8 nmol/kg (Sohrin and Bruland, 2011). It is a bioessential trace metal, and 10–60% of Ni in coastal and open-ocean is complexed with organic ligands (Donat et al., 1994; Saito et al., 2004). In seawater, the dominant Ni species is Ni2+, the only stable oxidation state over the pH range of most natural waters.
The global marine Ni budget is controlled by the input of dissolved Ni from river water, dissolution in the oceans of riverine and atmospheric transported particulate material, Ni scavenging by marine minerals sedimentation, and geothermal activity (Sclater et al., 1976; Li et al., 2003; Jeandel and Oelkers, 2015). The main sink of Ni in the ocean is ferromanganese crust.
The isotopic composition of seawater was determined by Cameron and Vance (2014) who reported values of δ60Ni = 1.44 ± 0.15. Interestingly, the isotopic composition of Ni in the oceans is nearly homogeneous across all ocean basins. This can be explained by the significantly longer residence time of Ni, about 30,000 years (Cameron and Vance, 2014), compared to the mixing time for global oceans of about 2,000 years (Jenkins, 2003). Along different depths in a study of the Black Sea, variation in isotopic composition of Ni is very substantial, at about 1.5‰, requiring very large isotope effects associated with the sources and sinks of Ni (Vance et al., 2016).
Rivers, the main input of Ni into the ocean, are isotopically heavier than the crust from which they originated, meaning that light Ni isotopes are probably retained during soil formation (Ratié et al., 2015). They been determined to have an annual δ60Ni average of +0.80‰ and to show significant seasonal variability between +0.29 and +1.34‰ (Cameron and Vance, 2014). It should be noted that while compared to the crust, rivers provide isotopically heavy inputs into oceans, Ni in oceans is about 0.6‰ heavier than Ni in rivers.
Based on isotopic mass balance considerations, a source of isotopically heavy Ni in modern oceans is missing. (Cameron and Vance, 2014). It has also been speculated that the effects of Ni sorption onto Fe/Mn oxide surfaces, which is a major sink of Ni in the ocean (Peacock and Sherman, 2007), could explain, at least in part, the isotopic composition of seawater since it favors enrichment of lighter isotopes (Wasylenki et al., 2015; Gueguen et al., 2018). Alternatively, the source of heavy Ni could be balanced by an oceanic sink enriched in light Ni isotopes. It has been suggested that this sink could be sulfides associated with anoxic or suboxic marine sediments that are enriched in organic matter (Gueguen et al., 2013; Hofmann et al., 2014). However, the role of organic-rich sediments in Ni isotope mass balance is unclear at this time as the bulk δ60Ni of organic-rich sediments displays a large range from 0.28‰ to 2.5‰ (Porter et al., 2014).
Table 2 shows a compilation of the data on Ni fluxes into and out of the ocean from with the addition of Ni isotope data from several publications. These data, which represents the known information on the Ni budget in the ocean is schematically represented in Figure 8.

The motivation for studying Ni isotopic fractionation during its interactions with calcite

Isotope exchange reactions between calcium carbonates and seawater are important in paleo-reconstructions including determination of past temperature, atmospheric CO2, and oceanic pH from oxygen, carbon, and boron isotopes, respectively. In paleo oceanographic research proxies are continuously evolving and improving. Simultaneously new proxies are being studied and developed through water column analysis, surface sediment analysis, and through laboratory experimentation. When Ni substitutes for Ca2+ in calcite as octahedral Ni2+ the Ni isotopic composition (in sedimentary calcite) can provide valuable information on the chemical composition, pH, and pCO2.
If the removal of Ni from the dissolved phase in the surface ocean is associated with isotopic fractionation, then Ni isotopes may be able to yield constraints on precise biogeochemical processes involved at the time of mineral formation. This is due to its strong complexation with organic ligands and its slow water exchange rate (table 1); out of the first row transition metals, Ni has the most inhibiting enthalpy of dehydration at -513 kcal/mol (Baes and Mesmer, 1976).
Another advantage of Ni over other divalent metals is that during its incorporation in calcite it undergoes fewer processes that cause fractionation (i.e. redox or change of coordination number) and thus the main mechanism causing fractionation will be the distribution within species. This distribution within species will reflect in the Ni2+ composition since it is the only species of Ni that can exchange into the calcite lattice. The isotope fractionation among aqueous species will, therefore, be reflected in the composition of free Ni and consequently in the fractionation between Ni in the solid and the reactive fluid.
The speciation of a metal in solution affects its kinetic and thermodynamic properties. It also affects the isotopic composition of the aqueous metal and the metal incorporated from the solution into a solid. This study provides new insights into the parameters controlling isotope fractionation of Ni during its interactions with calcite and provides new tools to reconstruct paleo-environmental conditions based on the composition of the Ni isotopes recorded in carbonate sediments.

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Scope of this thesis

Only a few experiments have investigated the adsorption of Ni on the calcite surface and its coprecipitation with this mineral. However, to our knowledge, no isotope data has been published on Ni isotope fractionation that is linked to these processes. The aim of this thesis is to fill this gap and also characterize the mechanisms controlling Ni isotope fractionation that are driven by interactions of this element with calcite. It is expected that the extent of adsorption of Ni on calcite and coprecipitation with calcite can reveal information on the chemical composition of the reactive fluid from which Ni came into contact with this mineral. Results of this thesis will provide important insight into several key questions:
• Does the speciation of the solution affect the fractionation of Ni during adsorption on calcite?
• What is the mechanistic cause of Ni isotope fractionation during adsorption and coprecipitation on/with calcite?
• Is there a link between calcite precipitation rate and the extent of Ni isotope fractionation?
• Can Ni incorporated into calcite be used as a proxy for paleo oceanic conditions?
• Does Ni incorporated in calcite play a significant role in the oceanic Ni isotope budget?
In an attempt to answer these questions, two experimental studies have taken place. Each is presented in a different chapter of this thesis. The first study (chapter 3) is devoted to the investigation of Ni isotope fractionation during adsorption on the calcite surface as a function of pH. This study was motivated by the search for a heavy source or light sink of Ni in the ocean in order to better constrain the Ni global oceanic isotope budget. The second study (chapter 4) investigates Ni isotope fractionation associated with its coprecipitation with calcite. The aim of this study is the development of a new proxy to determine paleo oceanic chemical composition (pH, pCO2, saturation state with respect to calcite). To evaluate this new proxy, experiments were designed to quantify Ni isotope fractionation as a function of the precipitation rate of calcite at 1 atm pCO2 and 25oC.
Les carbonates sont des roches sédimentaires trouvées dans pratiquement tous les systèmes marins et caractérisés par la présence de l’ion carbonate (CO32-) dans leur structure. Le carbonate de calcium (CaCO3) est le carbonate le plus abondant à la surface de la Terre, avec deux polymorphes principaux, la calcite et l’aragonite. La calcite a une forte affinité pour les métaux divalents (Davis et al., 1987; Zachara et al. 1991; Lakshtanov et Stipp, 2007). Les principales voies d’interaction des métaux avec la calcite sont leur adsorption à la surface du minéral et leur incorporation dans le réseau cristallin. Les processus de sorption et d’incorporation sont étroitement liés puisque l’adsorption est une étape intermédiaire nécessaire pour toutes les réactions d’incorporation et que les deux mécanismes sont contrôlés par les mêmes facteurs chimiques (Curti, 1999). Cela fait de la calcite un puits pour des éléments divalents tels que Cd, Zn, Mn, Fe, Cd, Mg et Sr. (Wilkinson et Algeo, 1989; Tipper et al., 2006; Krabbenhöft et al., 2010; Pearce et al., 2015) qui peut avoir un impact important sur la mobilité et les cycles géochimiques des métaux traces dans les milieux aquatiques (Van Capellen, 1993; Villegas-Jimenez, 2009; Martin-Garin et al., 2003).

Table of contents :

Chapter 1. Introduction
1.1 Carbonates and their role regulating the ocean’s pH
1.1.1 CO2 and the carbonic acid system in the ocean
1.1.2 Determination of seawater conditions in the past through proxies
1.2 Calcite growth from solution
1.2.1 Calcite
1.2.2 Thermodynamic principles of mineral formation
1.2.3 Reactions at the calcite surface
1.2.4 Mechanism of divalent metal incorporation
1.3 Isotope fractionation during mineral growth
1.3.1 Isotope notation
1.3.2 Isotope fractionation
1.3.3 Equilibrium fractionation
1.3.4 Equilibrium isotope fractionation during adsorption
1.3.5 Kinetic fractionation
1.3.6 Significance of the water exchange rate
1.3.7 Implications for paleo reconstructions
1.4 Nickel
1.4.1 Nickel isotopes
1.4.2 Nickel in the ocean
1.4.3 The motivation for studying Ni isotopic fractionation during its interactions with calcite
1.5 Scope of this thesis
Chapitre 1b. Introduction générale
Chapter 2. Experimental and analytical methods
2.1 Starting materials
2.1.1 Reagents
2.1.2 Calcite
2.1.3 Aqueous Nickel stock solution
2.2 Experimental methods
2.2.1 Reactors
2.2.2 Thermodynamic calculations
2.2.3 Adsorption Experiments
2.2.4 Coprecipitation Experiments
2.3 Experimental protocol
2.3.1 Adsorption experiments
2.3.2 Coprecipitation experiments
2.4 Chemical analysis
2.4.1 Alkalinity and pH
2.4.2 Ca and Ni concentration measurement
2.4.3 Isotope analysis
Chapter 3. Ni isotope fractionation during adsorption on the calcite surface
3.1 Introduction
3.1.1 Ni isotope imbalance in the ocean
3.1.2 Ni adsorption on calcite
3.1.3 Isotope fractionation during adsorption
3.2 Materials and methods
3.2.1 Materials
3.2.2 Thermodynamic calculations
3.2.3 Adsorption experiments
3.2.4 Determination of Ni concentration in the solutions and solids.
3.2.5 Determination of the isotopic composition of Ni in aqueous solutions
3.2.6 Determination of the isotopic composition of Ni in the solids
3.3 Results
3.3.1 Ni aqueous speciation
3.3.2 Adsorption isotherm
3.3.3 Adsorption as a function of time
3.3.4 Adsorption as a function of pH
3.4 Discussion
3.4.1 Comparison with previous results
3.4.2 Isotopes
3.4.3 Interpretation of fractionation factors
3.5 Conclusion
Chapter 4. Ni isotope fractionation during its coprecipitation with calcite
4.1 Introduction
4.1.1 Divalent metals as proxies of past seawater composition
4.1.2 Theoretical background
4.2 Methodology
4.2.1 Experimental set up
4.2.2 Sampling
4.2.3 Thermodynamic calculations
4.2.4 Chemical analysis
4.2.5 Calcite growth rates and Ni partition coefficients
4.2.6 Isotope Analysis
4.3 Results
4.3.1 Mineralogy and composition of the precipitated phases
4.3.2 Chemical composition of the reactive fluids
4.3.3 Ni partition between calcite and fluid DNi
4.3.4 Ni isotope fractionation during calcite precipitation
4.4 Discussion
4.4.1 Ni distribution coefficient between calcite and solution
4.4.2 Control of Ni isotope composition in calcite
4.4.3 Ni isotope fractionation factor as a function of calcite growth rate and implications for natural systems.
4.5 Conclusions
Chapter 5. General conclusions
5.1 Conclusions
5.2 Suggestions for future research
Chapitre 5b. Conclusions générales

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