Characterisation of stratigraphy and palaeoceanography using graptolites: exploring new concepts in the Aeronian (Silurian) of Wales, Scotland and Northern Ireland

Graptolites, extinct macrozooplankton of the phylum Hemichordata were a major element of the Early Palaeozoic seas. They are here investigated from Wales, Scotland and Northern Ireland and their use beyond biostratigraphy is considered. The leptotheca and convolutus biozones (Aeronian; Silurian) are characterized and correlated across Wales, Scotland and Northern Ireland, and in Scotland subdivision of the convolutus Biozone was possible through identification of Cephalograptus cometa extrema. In the Irish successions, previously unrecognized morphological intermediates within the established Petalolithus-Cephalograptus lineage were recognised and in Wales and Scotland, ‘pre-maxiculus’ morphotypes of Coronograptus are suggested. At all three localities Campograptus lobiferus is the dominant species with intraspecific variation recorded; the type material of this species from Scotland is shown to be unusually large. Use of X-ray images has enhanced biostratigraphy by enabling more specimens and taxa to be recorded, shown how graptolite number varies microstratigraphically and demonstrated that rhabdosomes are systematically aligned. The number and thickness of Aeronian ‘anoxic’ units varies within Wales, and between the Welsh and Scottish successions, indicating diachroneity of anoxia. A schema of microfacies types is constructed for the leptotheca Biozone anoxic unit in Wales and is applied to the Welsh convolutus Biozone anoxic units and to the gregarius to convolutus Biozone strata in Scotland. Facies with less clastic input and no burrows are associated with the best preserved graptolites, with pyrite formation being influenced by variations in palaeoproductivity and clastic input. Levels with well-preserved graptolites are not geochemically distinct, but differences in the major elements indicate provenance differences between the leptotheca and convolutus biozones. A reproducible &delta ^{13}C signal is obtainable from small amounts of carbon from graptolite periderm and periderm &delta^{13}C is different but not consistently so to whole-rock &delta^{13}C. Initial results suggest that periderm &delta^{13}C is not affected by physiology, life habit or metamorphic grade and that for chemostratigraphic study little regard is needed for species composition.