Abstract
The Hirnantian is an important time in Earth history as it includes one of the major episodes of continental glaciation and marine mass extinction (e.g. Brenchley et alii, 1994). Positive C-isotope excursions and an eustatic lowering of sea level extend from a level near the base of the Normalograptus extraordinarius-N. ojsuensis Zone (lower Hirnantian) to a level within the upper Hirnantian N. persculptus Zone. C-isotope values decline to pre-glacial levels throughout the extent of uppermost Hirnantian strata. At least two discrete and distinct positive shifts of δ13Corg have been observed globally; they may coincide with two pulses of peak glaciation (Melchin et alii, 2003; see also Sutcliffe et alii, 2000).
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