Li/Ca of modern brachiopods: A potential paleoseawater thermometer

Abstract

Lithium in marine carbonates may hold key information about their temperature of formation and thus of the thermal history of ambient seawater. Here, we present Li/Ca, Mg/Ca, and Sr/Ca results of modern rhynchonelliformean brachiopods from high to low latitudes, shallow to deep and warm to cold seas covering the world's oceans. Craniids as a group incorporate Li into their shell at a faster rate than any other brachiopod group, and due to their paucity will not be considered contributing material. The primary layer calcite of rhynchonelliformean brachiopods, due to the highly variable nature of its Li content, is deemed unsuitable as a proxy. In contrast, the Li/Ca ratios of fibers of the secondary layer within Terebratulida and Rhynchonellida, in conjunction with the innermost calcite of the Thecideida, appear to be free of major influence by growth rate, water depth, seawater pH and salinity. We compiled Li/Ca thermometers based on: 1) second-layer fiber-calcite Li results [2LB, NG = 57], and 2) with additional Li/Ca results of thecideide calcite [2LBT, NG = 60]. The proposed Li/Ca-brachiopod seawater thermometers (linear 1 and 2), which are independent of seawater chemistry, are as follows: T°C = 32.85 (±0.52) – 0.670 (±0.014) Li/Ca2LB (μmol/mol) r2 = 0.978 (1) T°C = 34.73 (±0.37) – 0.716 (±0.010) Li/Ca2LBT (μmol/mol) r2 = 0.987 (2) Brachiopods are important biocalcifiers especially during the Paleozoic, and thus, the proposed Li/Ca thermometer may be an invaluable supplementary tool to the oxygen and clumped isotope thermometers in characterizing its seawater and climate history.