A weathering hypothesis for glaciation at high atmospheric pCO(2) during the Late Ordovician

New paired carbonate and organic-carbon isotope analyses from Nevada, USA, together with a consideration of the effects of mountain-building and ice-s heet coverage of the continents on atmospheric pCO(2), lead to a new hypoth esis for the cause of the Late Ordovician glaciation. We suggest that the T aconic orogeny, which commenced in the late-middle Ordovician, caused a lon g-term decline in atmospheric pCO(2) through increased weatherability of si licate rocks. Ice-sheet growth was triggered when pCO(2) decreased to a thr eshold of similar to 10x present atmospheric level and proceeded by positiv e ice-albedo feedback. In the midst of glaciation, atmospheric pCO(2) began to rise as continental silicate weathering rates declined in response to c overage of weathering terrains by ice sheets. Ar first, this enhanced green house effect was overcompensated for by ice-albedo effects. Ultimately, how ever, atmospheric pCO(2) reached a level which overwhelmed the cooling effe cts of ice albedo, and the glaciation ended. The isotope results can be int erpreted to indicate that atmospheric pCO(2) rose during the glaciation, co nsistent with other proxy information, although alternative interpretations are possible. The large, positive carbonate isotope excursion observed in Late Ordovician rocks around the world is explained as the expected respons e to increased carbonate-platform weathering during glacioeustatic sea-leve l lowstand, rather than as a response to increased organic-carbon burial. ( C) 1999 Elsevier Science B.V. All rights reserved.