Lithium elemental and isotope systematics of modern and cultured brachiopods:implications for seawater evolution

• Published in: Chemical Geology
• Main Authors: Gaspers, Natalie, Magna, Tomáš, Jurikova, Hana, Henkel, Daniela, Eisenhauer, Anton, Azmy, Karem, Tomašovych, Adam
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2021
• Subjects: Lithium isotopes; Brachiopod calcite; Controlled culturing experiements; Seawater history; Paleo-proxy; Planktonic foraminifera
• Online Access: https://research-portal.st-andrews.ac.uk/en/researchoutput/lithium-elemental-and-isotope-systematics-of-modern-and-cultured-brachiopods(915162f2-fd8b-4466-8374-c7e14cfb286d).html; https://doi.org/10.1016/j.chemgeo.2021.120566; https://research-repository.st-andrews.ac.uk/bitstream/10023/24370/1/Gaspers_2021_Chemical_Geology_Lithium_elemental_isotope_systematics_CC.pdf

Lithium has proven a powerful tracer of weathering processes and chemical seawater evolution. Skeletal components of marine calcifying organisms, and in particular brachiopods, present promising archives of Li signatures. However, Li incorporation mechanisms and potential influence from biological processes or environmental conditions require a careful assessment. In order to constrain Li systematics in brachiopod shells, we present Li concentrations and isotope compositions for 11 calcitic brachiopod species collected from six different geographic regions, paralleled with data from culturing experiments where brachiopods were grown under varying environmental conditions and seawater chemistry (pH–pCO 2 , temperature, Mg/Ca ratio). The recent brachiopod specimens collected across different temperate and polar environments showed broadly consistent δ 7 Li values ranging from 25.2 to 28.1‰ (with mean δ 7 Li of 26.9 ± 1.5‰), irrespective of taxonomic rank, indicating that incorporation of Li isotopes into brachiopod shells is not strongly affected by vital effects related to differences among species. This results in Δ 7 Li calcite–seawater values (per mil difference in 7 Li/ 6 Li between brachiopod calcite shell and seawater) from −2.9‰ to −5.8‰ (with mean Δ 7 Li calcite–seawater value of −3.6‰), which is larger than the Δ 7 Li calcite–seawater values calculated based on data from planktonic foraminifera (~0‰ to ~−4‰). This range of values is further supported by results from brachiopods cultured experimentally. Under controlled culturing conditions simulating the natural marine environment, the Δ 7 Li calcite–seawater for Magellania venosa was −2.5‰ and not affected by an increase in temperature from 10 to 16 °C. In contrast, a decrease in Mg/Ca (or Li/Ca) ratio of seawater by addition of CaCl 2 as well as elevated pCO 2 , and hence low-pH conditions, resulted in an increased Δ 7 Li calcite-seawater up to −4.6‰. Collectively, our results indicate that brachiopods represent valuable archives and provide an ...