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

Abstract

New paired carbonate and organic-carbon isotope analyses from Nevada, USA, together with a consideration of the effects of mountain-building and ice-sheet coverage of the continents on atmospheric pCO 2, lead to a new hypothesis for the cause of the Late Ordovician glaciation. We suggest that the Taconic orogeny, which commenced in the late-middle Ordovician, caused a long-term decline in atmospheric pCO 2 through increased weatherability of silicate rocks. Ice-sheet growth was triggered when pCO 2 decreased to a threshold of ∼10× present atmospheric level and proceeded by positive ice-albedo feedback. In the midst of glaciation, atmospheric pCO 2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets. At first, this enhanced greenhouse effect was overcompensated for by ice-albedo effects. Ultimately, however, atmospheric pCO 2 reached a level which overwhelmed the cooling effects of ice albedo, and the glaciation ended. The isotope results can be interpreted to indicate that atmospheric pCO 2 rose during the glaciation, consistent 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 response to increased carbonate-platform weathering during glacioeustatic sea-level lowstand, rather than as a response to increased organic-carbon burial.