SEASONAL PROFILING IN A MISSISSIPPIAN GIGANTOPRODUCTID SHELL OF PALAEOEQUATORIAL BRITAIN

Gigantoproductus okensis is a large, thick-shelled species which has been collected from bioclastic packstone/grainstone deposited under tropical shallow marine conditions in the upper Viséan Monsal Dale Limestone of Derbyshire. In longitudinal section, this brachiopod shows about 20 growth lines indicating a reduction of growth caused by regular perturbations in the environment, suggesting a life span of 20 years in agreement with the expected survival rates of living brachiopods. To constrain the age of the brachiopod with spiral deviation analysis, the ventral valve has been processed to detect minima of deviations from the logarithmic spiral. About two growth lines occur between the minima and assuming a maximum growth length of 100 growth units, the fitted logistic curve provides an estimated age at death of about 20 years. The shell was sampled for geochemical analyses consecutively at 121 points across the growth lines along the preserved and mostly non luminescent shell interior. Isotope variation shows that growth and geochemistry is periodical, showing regular oscillation of about 1.1‰ for δ180 and 0.5-1 for δ13C. δ180 and δ13C periodicity were correlated with growth lines and Fourier analysis demonstrates that this periodicity is annual in form and thus seasonal, though other types of periodicity were also present. The seasonal oscillation for δ180 in- dicates sea surface temperature (SST) variation of ~ 5-6 ºC. Therefore, the study of this gigantoproductid shell revealed a high seasonal isotopic variation which may have been related both to strong monsoon circu- lation, and to the growth of ice sheets of limited extent on Gondwana.