The Mg/Ca-temperature relationship in brachiopod shells: Calibrating a potential palaeoseasonality proxy

- Laser ablation ICP-MS Mg/Ca profiles of modern brachiopod shells.
- Consistent Mg/Ca ranges in six contemporaneous Terebratulina valves.
- Liothyrella valves from deeper water depths display lower Mg/Ca.
- Intra-shell Mg/Ca and δ18O profiles confirm temperature control on brachiopod Mg/Ca.
- Brachiopod Mg/Ca profiles have potential as palaeoseasonality indicators.

Brachiopods are long-lived, long-ranging, extant organisms, of which some groups precipitate a relatively diagenetically stable low magnesium calcite shell. Previous work has suggested that the incorporation of Mg into brachiopod calcite may be controlled by temperature (Brand et al., 2013). Here we build upon this work by using laser ablation sampling to define the intra-shell variations in two modern brachiopod species, Terebratulina retusa (Linnaeus, 1758) and Liothyrella neozelanica (Thomson, 1918). We studied three T. retusa shells collected live from the Firth of Lorne, Scotland, which witnessed annual temperature variations on the order of 7 °C, in addition to four L. neozelanica shells, which were dredged from a water depth transect (168-1488 m) off the north coast of New Zealand. The comparison of intra-shell Mg/Ca profiles with shell δ18O confirms a temperature control on brachiopod Mg/Ca and supports the use of brachiopod Mg/Ca as a palaeoseasonality indicator. Our preliminary temperature calibrations are Mg/Ca = 1.76 ± 0.27 e(0.16 ± 0.03)T, R2 = 0.75, for T. retusa and Mg/Ca = 0.49 ± 1.27 e(0.2 ± 0.11)T, R2 = 0.32, for L. neozelanica (errors are 95% confidence intervals).