Trace metal geochemistry of brachiopod calcite: a new window to the past

• Main Author: Butler, Scott
• Format: Thesis
• Language: English
• Published: 2015
• Subjects: QD Chemistry; QE Geology; New Zealand; Antarc*; Antarctica
• Online Access: https://orca.cardiff.ac.uk/id/eprint/78889/; https://orca.cardiff.ac.uk/id/eprint/78889/1/Scott%2BButler%2BThesis%2B%2BCorrected%2BVersion.pdf; https://orca.cardiff.ac.uk/id/eprint/78889/2/Electronic%20T%20%26%20D%20Publication%20Form.pdf

The Mg/Ca temperature proxy was applied for the first time to seasonality records in calcitic brachiopod shells, using a laser ablation tracking technique on both Recent and fossil material. Seawater temperature calibrations were established using modern Terebratulina retusa from Scottish waters with annual temperature variations ~7 °C, and Liothyrella neozelanica from a water depth transect (168–1488 m) off New Zealand. Comparison of intra-shell Mg/Ca with shell δ18O confirms a temperature control on brachiopod Mg/Ca. Preliminary temperature calibrations are: T. retusa Mg/Ca=1.76±0.27e(0.16 ± 0.03)T, R2=0.75 L. neozelanica Mg/Ca=0.49 ± 1.27e(0.2 ± 0.11)T, R2=0.32. The Eocene-Oligocene Transition (EOT) is the first time during the Cenozoic that Antarctica was permanently glaciated. Mg/Ca and δ18O of brachiopod calcite from two New Zealand sites, one Late Eocene and one Early Oligocene, were sampled to assess seasonality over the EOT. Mg/Ca data identify no statistically significant change in seasonality in this area over the EOT. A 1.3 ‰ positive excursion in δ18O is identified: 0.6 ‰ is attributed to global change in δ18Oseawater and 0.7 ‰ to regional change in δ18Oseawater. Surface currents remained the same around New Zealand across the EOT. Summer Mg/Ca seawater temperatures (~21-24°C) in New Zealand are similar to temperatures derived from TEX86 (Site 1172). The Silurian Ireviken Excursion (~428 Ma) is a worldwide δ13C and δ18O isotopic excursion. Atrypa from Gotland, Sweden was sampled from before and after this Llandovery/Wenlock excursion. Mg/Ca and δ18O data indicate no significant change in seawater temperature over this excursion, against a shift of 1.29 ‰ in δ13C. Relative temperature changes between summer and winter Mg/Ca suggest seasonality of 3.5–5.4 °C. Estimated seawater temperature ~37 °C agrees with a recent clumped isotope study. High temperatures obtained in another study may be attributable to sampling that incorporated the outer side half of the secondary layer.