Palaeoecological and possible evolutionary effects of early Namurian (Serpukhovian, Carboniferous) glacioeustatic cyclicity

Early Namurian (Serpukhovian, Carboniferous), sedimentary cycles in the Throckley and Rowlands Gill boreholes, near Newcastle upon Tyne, UK, consist of fossiliferous limestones overlain by (usually unfossiliferous) black mudstone, followed by sandstones and often by thin coal seams. Sedimentological and regional geological evidence suggests that the largest are high-amplitude cycles, probably of glacioeustatic origin. 13C (bulk organic matter) delineates marine and non-marine conditions because of the large difference between terrestrial and marine 13C, and indicates that full marine salinity was only intermittent and resulted from glacioeustatic marine transgression superimposed on a background of inundation by freshwater from large rivers, which killed off the marine biota. Palynology suggests that plant groups, including ferns and putative pteridosperms, were affected by changing sea level, and that there is a theoretical possibility of connection between cyclicity and the first appearance of walchiacean conifer-like monosaccate pollen such as Potonieisporites. Long-term terrestrial and marine increasing 13C (organic) may reflect the onset of major glaciation in Gondwana, as there is evidence to suggest that the two are coeval, but no specific mechanism can be suggested to link the trends.