The lithium and magnesium isotope signature of olivine dissolution in soil experiments

Highlights

• Enhanced weathering experiments mimic geoengineering methods.

• Crushed olivine is added to a soil core, and compared to a control core.

• Lithium and Magnesium isotopes are measured in solids and drip waters.

• Both isotopes are strongly affected by retardation by the soil exchangeable complex.

Abstract

This study presents lithium and magnesium isotope ratios of soils and their drainage waters from a well-characterised weathering experiment with two soil cores, one with olivine added to the surface layer, and the other a control core. The experimental design mimics olivine addition to soils for CO₂ sequestration and/or crop fertilisation, as well as natural surface addition of reactive minerals such as during volcanic deposition. More generally, this study presents an opportunity to better understand how isotopic fractionation records weathering processes. At the start of the experiment, waters draining both cores have similar Mg isotope composition to the soil exchangeable pool. The composition in the two cores evolve in different directions as olivine dissolution progresses. Mass balance calculations show that the water δ²⁶Mg value is controlled by congruent dissolution of carbonate and silicates (the latter in the olivine core only), plus an isotopically fractionated exchangeable pool. For Li, waters exiting the base of the cores initially have the same isotope composition, but then diverge as olivine dissolution progresses. For both Mg and Li, the transport down-core is significantly retarded and fractionated by exchange with the exchangeable pool. This observation has implications for the monitoring of enhanced weathering using trace elements or isotopes, because dissolution rates and fluxes will be underestimated during the time when the exchangeable pool evolves towards a new equilibrium.

Introduction

Chemical weathering of continental silicate rocks supplies alkalinity and cations that drive carbonate formation and provides nutrients to the ocean to fuel primary productivity and organic carbon burial (Berner, 2003; Ebelmen, 1845). Because these are the primary processes removing CO₂ from the atmosphere on timescales >10⁵ years, weathering is a key process driving climate change over geologic time (Berner et al., 1983; Colbourn et al., 2015; Godderis et al., 2013). A climate-driven feedback may make silicate weathering a dominant climate-moderating process, helping to explain the long-term stability of Earth's climate (Walker et al., 1981). However, the strength of this feedback is disputed, in part because supply of weatherable material (high activity primary silicates) also exerts an important control on weathering fluxes (Goudie and Viles, 2012; Pogge von Strandmann et al., 2017a; Pogge von Strandmann et al., 2017c; Raymo et al., 1988; West et al., 2005).

Despite considerable research, the fundamental processes controlling weathering, and ultimately the evolution of atmospheric pCO₂, are still poorly known. Understanding the relative roles of climate vs. material supply in determining weathering fluxes is challenging because the weathering mechanisms that operate at soil profile scales remain difficult to resolve. Moreover, it has proven difficult to relate soil-scale processes to those that operate at catchment and global scales, given the heterogeneities in supply of fresh primary rock, mineral saturation, pH, mineral surface area, reactivity and temperature in natural environments (e.g., Gislason et al., 2009; Maher and Chamberlain, 2014; Stallard and Edmond, 1983; Stefansson and Gislason, 2001; West et al., 2005). Laboratory experiments provide insight into mineral weathering rates and their dependence on many of these parameters, but rates calculated from laboratory experiments are generally several orders of magnitude higher than those observed in natural environments (White and Brantley, 2003), suggesting that the underlying mechanisms may also differ. To address this discrepancy, experiments emulating the inherently complex processes operating in the natural environment are necessary.

The uncertainty in understanding weathering rates in natural systems has further implications for understanding the potential for “enhanced weathering” as a strategy for removing anthropogenic CO₂ from the atmosphere (Andrews and Taylor, 2019; Schuiling and Krijgsman, 2006; Taylor et al., 2015). Proposals for enhanced weathering include spreading finely-ground silicate and carbonate minerals on land surfaces (Hartmann et al., 2013; Koehler et al., 2010; Renforth, 2012; Schuiling and Krijgsman, 2006), in coastal environments (Hangx and Spiers, 2009; Schuiling and de Boer, 2010), and in the ocean (Koehler et al., 2013; Renforth et al., 2013). In each case, the aim is to increase the amount of mineral dissolution and associated CO₂ drawdown. However, without precise knowledge of weathering rates, it is not yet possible to determine whether enhanced weathering would be a net sink of CO₂, given the energy required for rock grinding and transportation (Moosdorf et al., 2014; Renforth, 2012).

A relatively new development in the study of weathering is the use of “non-traditional” stable isotopes. A number of these systems are fractionated by weathering processes and therefore have the potential to trace weathering, and even determine weathering fluxes. These tools could offer significant advantages over approaches that require several simultaneous measurements (e.g. multiple elemental concentrations, runoff rates, surface area, etc.). Magnesium isotopes (δ²⁶Mg) are such a potential tracer of weathering processes, useful because Mg is a direct part of the long-term carbon cycle (Berner et al., 1983). Magnesium is present in both silicate and carbonate rocks, and these components typically have significantly different isotope ratios (Huang et al., 2013; Li et al., 2010; Saenger and Wang, 2014). This difference means that the δ²⁶Mg of rivers is controlled both by lithology and by additional, smaller fractionation during uptake of mostly heavy Mg during formation of secondary minerals (Liu et al., 2014; Opfergelt et al., 2014; Opfergelt et al., 2012; Pogge von Strandmann et al., 2008; Pogge von Strandmann et al., 2012; Ryu et al., 2016; Tipper et al., 2010; Tipper et al., 2008; Tipper et al., 2006b; Wimpenny et al., 2011; Wimpenny et al., 2014).

Lithium isotopes (δ⁷Li) also show promise as an isotopic tracer of weathering, unusual in being dominantly controlled by silicate weathering. The Li isotope ratio is unaffected by plant uptake and primary productivity (Lemarchand et al., 2010; Pogge von Strandmann et al., 2016), and negligibly influenced by weathering of carbonates, even in carbonate-dominated catchments (Kisakürek et al., 2005; Millot et al., 2010; Pogge von Strandmann et al., 2017b). The δ⁷Li of silicate rocks comprises a narrow range (δ⁷Li_{continental crust} ~ 0.6 ± 0.6‰ (Sauzéat et al., 2015), δ⁷Li_basalt ~ 3–5‰ (Elliott et al., 2006)) relative to that in rivers (2–43‰, global mean ~ 23‰ (Dellinger et al., 2015; Huh et al., 1998; Murphy et al., 2019; Pogge von Strandmann et al., 2006)). The high variability in rivers is caused by preferential uptake of ⁶Li into secondary minerals formed during weathering, driving residual waters isotopically heavy (Huh et al., 2001; Huh et al., 1998; Kisakürek et al., 2005; Lemarchand et al., 2010; Liu et al., 2015; Millot et al., 2010; Pistiner and Henderson 2003; Pogge von Strandmann et al., 2010; Pogge von Strandmann et al., 2006; Pogge von Strandmann and Henderson 2015; Pogge von Strandmann et al., 2014; Vigier et al., 2009; Wimpenny et al., 2015; Wimpenny et al., 2010). Dissolved Li isotope ratios are therefore controlled by the ratio of primary mineral dissolution (supplying low, rock-like, δ⁷Li to solution), relative to secondary mineral formation (preferentially removing ⁶Li and therefore leading to high dissolved δ⁷Li). This balance means that dissolved Li isotopes trace what is often referred to as the congruency of silicate weathering (where congruent weathering features a high ratio of primary mineral dissolution to secondary mineral formation) (Bouchez et al., 2013; Dellinger et al., 2015; Misra and Froelich, 2012; Pogge von Strandmann et al., 2010; Pogge von Strandmann and Henderson, 2015).

In this study, we examine Mg and Li isotope ratios from a well-characterised soil weathering experiment (Renforth et al., 2015). The aims of this study are (i) to better understand the processes that affect dissolved Li and Mg isotope compositions, (ii) to assess whether these isotopic tracers are useful for determining weathering processes and rates, including in enhanced weathering applications, and (iii) to compare isotopic methods with a more conventional approach to calculating weathering rates using elemental concentrations and ratios (Renforth et al., 2015).