Equatorial sea surface seasonality of the Mississippian (Early Carboniferous) derived from Brachiopod shell calcite

The monsoon rainfall seasonality of the palaeoequatorial Mississippian (L Carboniferous) has been deduced from fossil plant morphology. However little is known about palaeoequatorial Mississippian sea surface temperature (SST) seasonality. In particular no numerical values are available for SST seasonality to feed into General Circulation Models (GCMs) of the Mississippian greenhouse to icehouse transition, a period of global climatic significance. The main goal of this research is to derive temperatures for palaeoequatorial Mississippian seawater seasonality through sclerocronology and isotope profiling of fossil shells. Little is known presently of SSTs in the Carboniferous and this type of interannual information is vital so that tie points can be placed within GCMs for wider estimates of SST seasonality. Preliminary data from one shell indicates SST seasonality of ~ 5-6ºC (over a 20 year period). This is 1-2ºC greater than the seasonal temperature variation at the present day equator in both the Pacific and Atlantic. A Gigantoproductid. sp. shell collected from Derbyshire, sectioned longitudinally highlighting the growth of the shell and bioclastic carbonate matrix. found that even after diagenesis primary variation is preserved, this has far-reaching effects for isotope proxy studies, since much material is diagenetically altered and A secondary goal is to investigate the possibility of a ‘vestigial signal' in altered brachiopod shell preserving the pattern of a primary signal if not primary values. If it is therefore presently deemed unsuitable for palaeoclimate research. In a pilot study a large brachiopod shell (Gigantoproductus) was sampled consecutively across annual growth bands, and δ18O and δ13C (interpreted as temperature and productivity) was found to correlate with growth as an annual rhythm (winter-summer). The same periodicity is also preserved superimposed on diagenetic values of δ18O and δ13C as a ‘vestigial signal' within an altered part of the shell.