Lithium isotopic composition of benthic foraminifera: A new proxy for paleo-pH reconstruction

Roberts, Jenny; Kaczmarek, K.; Langer, Gerald; Skinner, Luke C; Biȷma, Jelle; Bradbury, Harold J.; Turchyn, Alexandra V.; Lamy, Frank; Misra, Sambuddha

doi:10.1016/j.gca.2018.02.038

Cited by 52 publications

(59 citation statements)

References 66 publications

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“…It has been suggested (Coogan et al, 2016) that there might be a strong control by pH as the Δ 7 Li LMC-fluid determined at pH = 8 (Δ 7 Li LMC-fluid = -2.5±1.5‰) is significantly higher relative to that at pH = 7 (Δ 7 Li LMC-fluid = -8±1‰). Similarly, an influence of pH on the δ 7 Li of biogenic LMC such as benthic foraminiferal tests has also been proposed (Roberts et al, 2018). A speleothem study by Pogge von Strandmann et al 2017measured a field-based fractionation factor of Δ 7 Li LMC-fluid = -3±2‰, in good agreement with experimental results at pH = 8 (Marriott et al, 2004a).…”

Section: 1mentioning

confidence: 74%

The effects of diagenesis on lithium isotope ratios of shallow marine carbonates

et al. 2020

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In this study, we present new data on the δ 7 Li values and Li/(Ca+Mg) ratios of carbonate cores from the Great Bahama Bank (Clino, Unda), a deep water core off of the bank top (ODP Leg 166 Site 1007), and the coralline Key Largo Limestone. We use these samples to evaluate the influence of meteoric diagenesis, marine burial diagenesis, and dolomitization on the Li isotope system in carbonates. We find that recrystallization of aragonite to low-Mg calcite in the presence of meteoric fluids results in a systematic decrease of the Li/(Ca+Mg) ratio in Clino, Unda and Key Largo samples, due to the lower Li/(Ca+Mg) ratio in meteoric fluids compared to seawater. For Li isotopes, we observe that the δ 7 Li of meteorically altered low-Mg calcite is +22.0±3.8‰ (n=28, 1σ), which is coincidentally similar to the original aragonite-rich sediments (+22±1‰ in the Bahamas, +18±1‰ in Key Largo), but with a larger variability (from +15 to +27‰). We interpret these features as reflecting the overprinting of primary Li during meteoric alteration with a highly variable isotope signature that may be controlled by a combination of local porewater and/or global climatic conditions; in either case, meteoric diagenesis produces isotopic signatures that are unrelated to seawater composition. In contrast, marine burial diagenesis and dolomitization of Clino and Unda sediments under "fluid-buffered" conditions result in Li isotope composition that is similar (+30.2±1.5‰, n=36, 1σ) to modern seawater (+31‰). For Site 1007, the δ 7 Li values range between +23‰ and +31‰. We interpret this range as reflecting a combination of varying diagenesis style (fluid to sediment-buffered) and varying contribution of calcite derived from pelagic sediments, with distinct isotopic composition due to primary mineralogy. Altogether, our results show that diagenesis does not invalidate the use of bulk carbonates for deriving Li isotope paleo-records, but the reliability of past carbonates as recorders of seawater δ 7 Li values will depend on carefully characterizing their diagenetic history.

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Section: 1mentioning

confidence: 74%

The effects of diagenesis on lithium isotope ratios of shallow marine carbonates

et al. 2020

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“…However, pH can also affect the Li speciation in solution (Bogatko et al, 2013) and a recent study attributed a relationship between δ 7 Li and pH in foraminifera to changes in Li speciation (Roberts et al, 2018). The aqueous speciation of Li is not well defined but Li can be considered to be a very weak acidic cation with aqua (e.g.…”

Section: Ph Dependencementioning

confidence: 99%

Experimental constraints on Li isotope fractionation during clay formation

Hindshaw

Tosca

Goût

et al. 2019

Geochimica et Cosmochimica Acta

134

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Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth system models. This study refines our understanding of isotope fractionation during clay formation with essential implications for the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopic fractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions were all enriched in 6 Li compared to the initial solution. On average, the exchangeable solutions had δ 7 Li values 7 lower than the initial solution. The average difference between the residue and initial solution δ 7 Li values (∆ 7 Li residue−solution) for the synthesised layer silicates was-16.6±1.7 at 20 • C, in agreement with modelling studies, extrapolations from high temperature experimental data and field observations. Three bonding environments were identified from 7 Li-NMR spectra which were present in both bulk and residual solid 7 Li-NMR spectra, implying that some exchangeable Li remains after treatment with ammonium chloride. The 7 Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the 7 Li-NMR data with mass balance constraints we could calculate a fractionation factor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are-21.5±1.1 ,-0.2±1.9 and 15.0±12.3 for the octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1σ). The bulk fractionation factor (∆ 7 Li bulk−solution) is dependent on the chemistry of the initial solution. The higher the Na concentration in the initial solution the lower the bulk δ 7 Li value. We suggest this is due to Na outcompeting Li for interlayer sites and as interlayer Li has a high δ 7 Li value relative to octahedral Li, increased Na serves to lower the bulk δ 7 Li value. Three experiments conducted at higher pH exhibited lower δ 7 Li values in the residual solid. This could either be a kinetic effect, resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in the residual solid and/or a change in Li speciation in solution. This study highlights the power of 7 Li-NMR in experimental studies of clay synthesis to target site specific Li isotope fractionation factors which can then be used to provide much needed constraints on field processes.

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“…In this case, different types of carbonates may reflect various 'vital effects', that is, various degrees of Li isotope fractionation upon their formation. Additional data would be required to further test this hypothesis and to better understand these 'vital effects' and their origins (Misra and Froelich 2012, Dellinger et al 2018, Roberts et al 2018.…”

Section: Assessing Coral LI Isotope Fractionationmentioning

confidence: 99%

Lithium Isotope Composition of Marine Biogenic Carbonates and Related Reference Materials

Bastian

Vigier

Reynaud

et al. 2018

Geostandard Geoanalytic Res

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In this study, the accuracy and the precision corresponding to Li isotopic measurements of low level samples such as marine and coastal carbonates are estimated. To this end, a total of fifty‐four analyses of a Li‐pure reference material (Li7‐N) at concentrations ranging from 1 to 6 ng ml−1 were first performed. The average δ7Li values obtained for solutions with and without chemical purification were 30.3 ± 0.4‰ (2s, n = 19) and 30.2 ± 0.4‰ (2s, n = 36), respectively. These results show that the chosen Li chemical extraction and purification procedure did not induce any significant isotope bias. Two available carbonate reference materials (JCt‐1 and JCp‐1) were analysed, yielding mean δ7Li values of 18.0 ± 0.27‰ (2s, n = 6) and 18.8 ± 1.8‰ (2s, n = 9), respectively. Small powder aliquots (< 15 mg) of JCp‐1 displayed significant isotope heterogeneity and we therefore advise favouring JCt‐1 for interlaboratory comparisons. The second part of this study concerns the determination of δ7Li value for biogenic carbonate samples. We performed a total of twenty‐nine analyses of seven different tropical coral species grown under controlled and similar conditions (24.0 ± 0.1 °C). Our sample treatment prior to Li extraction involved removal of organic matter before complete dissolution in diluted HCl. Our results show (a) a constant δ7Li within each skeleton and between the different species (δ7Li = 17.3 ± 0.7‰), and (b) a Li isotope fractionation of −2‰ compared with inorganic aragonite grown under similar conditions. Comparison with literature data suggests a significant difference between samples living in aquaria and those grown in natural conditions. Finally, we investigate ancient (fossil) carbonate material and foraminifera extracted from marine sedimentary records. Different leaching procedures were tested using various HCl molarities. Results indicate that carbonate preferential dissolution must be carried out at an acid molarity < 0.18 mol l−1. Possible contamination from silicate minerals can be verified using the Al/Ca ratio, but the threshold value strongly depends on the carbonate δ7Li value. When the silicate/carbonate ratio is high in the sediment sample (typically > 2), contamination from silicates cannot be avoided, even at low HCl molarity (⪡ 0.1 mol l−1). Finally, bulk carbonate and foraminifera extracted from the same core sample exhibited significant discrepancies: δ7Li values of foraminifera were more reproducible but were significantly lower. They were also associated with lower Sr/Ca and higher Mn/Ca ratios, suggesting a higher sensitivity to diagenesis, although specific vital effects cannot be fully ruled out.

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Lithium isotopic composition of benthic foraminifera: A new proxy for paleo-pH reconstruction

Cited by 52 publications

References 66 publications

The effects of diagenesis on lithium isotope ratios of shallow marine carbonates

The effects of diagenesis on lithium isotope ratios of shallow marine carbonates

Experimental constraints on Li isotope fractionation during clay formation

Lithium Isotope Composition of Marine Biogenic Carbonates and Related Reference Materials

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