• journal_title：Bulletin de la Societe Geologique de France
• Contributor：Enrique Villas ; Emmanuelle Vennin ; José Javier Álvaro ; Wolfgang Hammann ; Zarela A. Herrera ; Eduardo L. Piovano
• Publisher：Societe Geologique de France
• Date：2002-
• Format：text/html
• Language：en
• Identifier：10.2113/173.6.569
• journal_abbrev：Bulletin de la Societe Geologique de France
• issn：0037-9409
• volume：173
• issue：6
• firstpage：569
• section：Articles

A new approach explaining the main forcing factor of Hirnantian glaciation is proposed herein. It follows the models associating occurrences of continental glaciations with periods of low atmospheric CO₂ levels. The accumulation of great volumes of carbonates during pre-Hirnantian late Ordovician, in regions where these deposits were previously absent, is suggested as a major sink of atmospheric CO₂. This would have caused an important lowering of the average temperature in the early Hirnantian, after CO₂ values had attained a certain threshold. This process was maintained by other positive feedbacks, such as the short-term carbonate weathering CO₂ sink. An increase of the direct flux of CO₂ from the atmosphere to the oceans by means of dissolution would have been driven by the enhancement of carbonate deposition. The great inundation of the low latitude Laurentia craton during Cincinnatian times and the establishment of a temperate-water carbonate sedimentation on the North Gondwana margin during pre-Hirnantian Ashgill allowed the burying of more than 840 × 10¹⁵ kg (1.9 × 10¹⁹ mol) of dissolved CO₂. This mass is equivalent to nearly 350 times the present values of atmospheric CO₂. This is important enough to have greatly altered the equilibrium between the CO₂ dissolved in the oceans and the partial pressure of CO₂ in the air, eventually causing an important reduction of the latter. The new model also offers a simple explanation for the end of the glaciation after a short time-span. Glacioeustatic lowering of the sea level, concomitant with the glaciation, would have stopped the extra-sedimentation of carbonate due to the retreat of the oceans from the platforms, closing this CO₂ sink. Pre-glacial CO₂ levels would then recover, due to volcanic and metamorphic CO₂ outgassing. After subsequent melting of the ice cap, oceanic circulation did not recover pre-Hirnantian Ashgill strength, resulting in a strong stratification of ocean waters and precluding the recovery of an extensive carbonate deposition. The well-known positive shift in the δ¹³C at the base of the Hirnantian is assumed to have been caused by weathering and dissolution of carbonates, relatively enriched in ¹³C, during the glacioeustatic regression and exposure of the platforms.