Mountain building-enhanced continental weathering and organic carbon burial as major causes for climatic cooling at the Frasnian– Famennian boundary (c. 376 Ma)? O. Averbuch, 1 N. Tribovillard, 1 X. Devleeschouwer, 2 L. Riquier, 1 B. Mistiaen 3 and B. van Vliet-Lanoe 1 1 UMR 8110, Processus et Bilans en Domaines Se´dimentaires, Universite´ de Lille 1, 59655 Villeneuve d’Ascq ce ´dex, France; 2 Service ge ´ologique de Belgique, rue Jenner 13, Bruxelles, Belgium; 3 UMR 8014, Pale´ontologie et Pale ´ogeographie du Pale´ozoique, FLS et ISA, 13 rue de Toul, 59046 Lille ce ´dex, France Introduction In recent years, much concern has risen about the long-term climatic effects of mountain building (e.g. Ruddiman, 1997). Uplift of continen- tal crust has been suggested to be a significant mechanism for long-term climatic cooling through depletion of atmospheric CO 2 content (and the associated greenhouse effect) by in- creased silicate weathering (e.g. Ray- mo, 1991; Franc¸ ois et al., 1993; Gaillardet et al., 1999; Kump et al., 1999) and organic carbon burial into marine sediments (France-Lanord and Derry, 1997; Franc¸ois and Godde´ris, 1998). Mountain building has also been considered to have a strong effect on oceanic and atmospheric circula- tions and thereby alter heat transfer on the Earth’s surface (e.g. Rind et al., 1997). These problems have so far largely been addressed with regard to Cenozoic global cooling and their relationships with the uplift of the Alpine–Himalayan belt. Late Palaeozoic times (Late Devo- nian–Carboniferous–Permian), were, however, probably one of the most intensively orogenic periods of the Phanerozoic due to the multistage accretion of continental blocks, lead- ing to the formation of the Pangea megacontinent during Permian times. Within the Upper Palaeozoic orogenic system, the Acadian–EoVariscan belt resulted in Late Devonian times from the incipient collision of different lith- ospheric plates, i.e. Laurussia (Old Red Sandstone Continent), Gondw- ana, Kazakhstan and Siberia due to the closure of intervening oceanic basins. Crustal deformation at the margins of these major blocks induced the uplift of large continental areas extending from Central Asia to South America (including the Urals, Central and Western Europe, Northern Africa and Northern America). This period is also known to have experienced drastic environmental changes both in terrestrial and oceanic domains with major crises of the biosphere (e.g. the Frasnian-Famen- nian and the Devonian-Carboniferous mass extinctions) and the onset, in Famennian times, of glaciations upon Southern Gondwana (e.g. Caputo, 1985; Isaacson et al., 1999; Scotese et al., 1999; Streel et al., 2000). Cold- water oceans (e.g. Copper, 1986), sea-level changes (e.g. Hallam and Wignall, 1999), oceanic anoxia (e.g. Joachimski and Buggisch, 1993), ocea- nic eutrophication (Caplan and Bus- tin, 1999; Murphy et al., 2000) as well as asteroids impacts (e.g. McGhee, 2001) have been considered as inde- pendent driving mechanisms for the Late Devonian mass extinctions. In- creased continental weathering has been proposed as a possible source for an atmospheric CO 2 depletion in Late Devonian times but was only restricted to the effects of the devel- opment of vascular land plants (e.g. Algeo et al., 1995). Enhanced erosion and weathering due to the major Acadian–EoVariscan orogeny have so far not been considered except at a local scale (Loevezijn, 1989; Peter- hansel and Pratt, 2001). In this regard, we review here the environmental changes observed in the stratigraphic record at the Frasnian–Famennian transition and explore their possible relationships with the development of a major orogenic system: the Late Devonian Acadian–EoVariscan mountain belt. Environmental changes at the Frasnian–Famennian boundary The Frasnian–Famennian boundary (FFB) in Late Devonian times (c. 376 Ma following Tucker et al., 1998) has long been recognized as marking a major environmental crisis (one of the five largest of the Phan- ABSTRACT The Late Devonian was a period of drastic environmental changes, as exemplified by a major biotic crisis at the Frasnian– Famennian boundary (FFB) and the onset in Famennian times of glaciations across southern Gondwana. Worldwide evidence for the coeval development of the major Acadian–Eovariscan belt led us to propose a model relating the Late Frasnian–Famennian environmental perturbations to extensive continental uplift through two atmospheric CO 2 -depleting mechanisms: (1) the intensification of silicate weathering on the continental areas as attested by a major rise in the 87 Sr ⁄ 86 Sr composition of sea water at the FFB; and (2) the massive burial of organic carbon (Kellwasser events) in partially confined basins due to the collisional-induced reduction of equatorial oceanic communica- tions between the Palaeotethysian and Panthalassic oceans. This process is also suggested to have been controlled by an important primary productivity connected to an increased nutrient availability triggered by the enhanced continental run-off. Terra Nova, 17, 25–34, 2005 Correspondence: O. Averbuch, PBDS, Sci- ences de la Terre, Universite´ de Lille 1, 59 655 Villeneuve d’Ascq ce´dex, France. Tel.: +33 (0)3 20 33 71 30; fax: +33 (0)3 20 43 41 19; e-mail: olivier.averbuch@univ- lille1.fr Ó 2005 Blackwell Publishing Ltd 25 doi: 10.1111/j.1365-3121.2004.00580.x