Geochemistry of recent and fossil brachiopod calcite of Megathiris detruncata (Terebratulida, Megathyrididae): A modern baseline study to trace past environmental conditions

Abstract

Modern and fossil calcareous shells of the terebratulide brachiopod Megathiris detruncata have been investigated for stable oxygen and carbon isotope and elemental (Mg, Sr, Mn, Fe, Ba) compositions to assess the possibility of using this species to trace past environmental conditions. Modern shells were collected from the Mediterranean Sea and the Atlantic Ocean, while fossil specimens were sampled from late Eocene to Pleistocene deposits in Europe. We mainly analysed bulk shells (i.e., primary and secondary layers together) but, in some cases, their single median and lateral septa, ventral and dorsal valves. The oxygen and carbon isotope compositions of the modern shells indicate that M. detruncata mineralizes the bulk of its calcite shell in equilibrium with seawater. In addition, their Mn, Ba, and Fe concentrations are low with few exceptions from the Atlantic Ocean where high Fe/Ca ratios can probably be related to locally enhanced Fe availability. The Sr/Ca ratio ranges narrowly between 1.4 and 1.7mmol/mol, while the Mg/Ca ratio has larger variability from about 23 to 45mmol/mol. The δ18O derived temperatures do not correlate with Mg/Ca, suggesting that Mg is likely affected by greater vital effects. Physiological control on biomineralization might also explain the somewhat larger spread of δ13C between different valves of the same individual and will require further testing by in situ analyses.The fossil M. detruncata shells have a compositional range much larger than modern specimens, with large intra-shell and intra-site variabilities indicative of partial or full alteration. Altered fossils, characterized by low δ18O and δ13C values, low Mg/Ca and Sr/Ca, and high Mn/Ca and Fe/Ca ratios are often associated with clear signs of re-crystallization and/or secondary calcite precipitation within the shells.We used fossil samples without obvious recrystallization features and chemical and isotopic compositions similar to modern specimens to discuss environmental parameters such as temperature and δ18Oseawater at the given time. We recognized variable local controls on seawater isotopic compositions in the Paratethys from the Oligocene until the Middle Miocene, and we propose somewhat warmer habitats for the M. detruncata in the Aquitaine Basin in southwestern France during the Late Oligocene. Isotope variation recorded in the Plio-Pleistocene Mediterranean shells are instead best explained by changing glacial-interglacial conditions.