Controls on upper Viséan (Carboniferous) depositional environments in Ireland
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The lithostratigraphy and fauna of Carboniferous sedimentary sequences in northwest and southwest Ireland were investigated to reconstruct how and why palaeoenvironments were changing throughout the upper Viséan and Serpukhovian time interval. Spectral analysis of a sequence in northwest Ireland indicates an influence of the precession and obliquity Milankovitch cycles on sedimentation in the early Asbian. The development of palaeokarsts in southwest Ireland, and high-frequency lithological variation in the northwest, indicate rapid sea-level fluctuations in the late Asbian and Brigantian, which are attributed to glacioeustasy. Stable isotopic analyses of biogenic apatite indicate cooling of local sea-surfaces by ~6ºC from the basal Asbian to the mid-Brigantian. Isotopic curves are compatible with the 3rd order sea-level curve identified in this work, suggesting that global climate and glaciation was the primary driving force of palaeoenvironmental change in Ireland during this time. It is proposed that the climate began deteriorating (heralding the onset of an episode of the Carboniferous glaciation) at the base of the Asbian before stabilising (in a glaciated state) in the mid- to late-Brigantian. Comparative, optical and Laser Raman investigations of (tandem) ichthyolith and conodont apatite samples suggest that ichthyoliths are prone to diagenetic lowering of their ¿18O values. However, discrepancies between the values of conodonts and altered ichthyoliths appear relatively consistent, and therefore are potentially correctable within regions. A suite of conodont species was recovered and their biostratigraphical and palaeoenvironmental significances are discussed. An evolutionary lineage of the genus Lochriea was identified in northwest Ireland that is compatible with published work, although the recognition of irregular and transitional forms suggests some reconsideration of the taxonomy is necessary. The first appearance of L. ziegleri and L. cruciformis suggest that the base of the L. ziegleri Biozone (proposed base of the Serpukhovian) correlates with the late Brigantian P2a ammonoid Biozone in Ireland.
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