Biotic, geochemical and environmental changes through the early Sheinwoodian (Wenlock, Silurian) carbon isotope excursion (ESCIE), Buttington Quarry, Wales
Abstract Palaeontological, sedimentological and geochemical data (particularly from LA-ICPMS analyses of pyrite) from a 12.4 m section through the lower Sheinwoodian Trewern Brook Mudstone Formation of Buttington Quarry, Wales are combined to reveal the environmental changes taking place during the early Wenlock Epoch of the Silurian, during the early part of the ESCIE (early Sheinwoodian carbon isotope excursion). The uppermost Cyrtograptus murchisoni through to lower Monograptus riccartonensis biozones show increased nutrient levels and peaks in palynomorph and graptolite abundance. At this time graptolite diversity globally was declining dramatically, possibly associated with metal (particularly zinc) concentrations attaining toxic levels and probably with the base of the euphotic zone being higher in the water column (resulting from blooms of phytoplankton) causing those graptolites that specialized in feeding on phytoplankton that were outcompeted in this diminished volume of illuminated surface waters to decline in numbers and eventually become extinct. Correlation of the graptolite and conodont biozonations enables the levels of the main stratigraphically higher Ireviken Event Datum Points to be proposed for the Buttington section and this in turn allows events recognised elsewhere (intervals of malformed palynomorphs and re-organization of polychaete faunas, together with an oceanic anoxic event) to be correlated with the lower part of the studied Buttington section. The commencement of the OAE, identified on Gotland at Ireviken Event Datum Point 4, appears to correlate rather well with a rapid transition, marking the top of the Butterley Mudstone Member at Buttington, from heavily bioturbated mudstones to laminated graptolitic mudstones. Surprisingly, the most rapid change in δ13Ccarb values within the Buttington section (from 0.6‰ at 8.19–8.25 m to 2.09‰ at 8.50–8.55 m) is not associated with any major faunal, palynological abundance or geochemical changes – these are all coincident with the initial rise in δ13Ccarb values, up to about the 5 m level in the Buttington section. Another unexplained feature of the graptolite record is a sudden change from Monograptus-dominated to Pristiograptus-dominated assemblages about half way through the studied section.