Equatorial sea surface temperature seasonality in the Mississippian (carboniferous) derived from brachiopod shell calcite

The stable isotope composition (δ18O and δ13C) of biogenic (low-magnesium) calcite is commonly used as a palaeoenvironment proxy. These proxies are used to help understand the palaeotropical environment in the Brigantian to help elucidate the timing of the onset of the Late Palaeozoic icehouse. However, to validate isotope data there needs to be an understanding of the environment in which the constituent organisms lived and the preservation of their calcitic shells must be assessed to ensure resultant data are reliable. This study focussed on shell beds at two localities on the Derbyshire carbonate platform that expose (Brigantian) age limestones deposited in open water, inner to mid-ramp settings. Palaeoecological analysis of the shell beds revealed a high dominance (but low diversity) of Gigantoproductus species (Brachiopoda). Specimens of this genus were collected from these localities and preservation analyses were conducted. This includes ultrastructure analyses via scanning electron, cathodoluminescence microscopy (SEM and CL), and measurement of trace element composition using laser ablation with inductively coupled plasma mass spectrometry (LA ICP-MS). Some species of Gigantoproductus have thick valves, allowing geochemical sampling to be conducted at high resolution across the growth lines of the shells. SEM and CL analyses commonly revealed fine ultrastructure detail preserved in the non-luminescent calcite of the prismatic shell layer. Specimens showing the best-preserved ultrastructure and dull or non-luminescence underwent analysis of trace element abundance (primarily Mn, Fe, Mg and Sr), before five were ultimately selected for δ18O and δ13C analysis. The range in δ18O values suggest a ~7oC seasonal range in late Brigantian shallow sea water temperatures with average temperatures of ~20oC suggesting the onset of the Late Palaeozoic icehouse had begun prior to the latest Brigantian. Overall, this research shows the success of rapid colonisation mechanisms adopted by Gigantoproductus species at the study site, demonstrates the importance of a multi-phased preservation screening approach, highlights the heterogeneity in preservation of the biogenic shell material and contributes robust δ18O data for equatorial Brigantian waters to the literature.