MISSISSIPPIAN CLIMATE IN THE LIGHT OF OXYGEN AND CARBON ISOTOPES OF CONODONTS AND CARBONATE ROCKS

The Mississippian is a time of climatic change from Devonian greenhouse to Pennsylvanian icehouse conditions. After a Frasnian temperature maximum and a first cooling step at the Frasnien/Famennian boundary (Joachimski and Buggisch, 2002), first glacial deposits are recorded from the Amazon Basin, Brazil, at the Devonian/Carboniferous boundary (Caputo 1985). The pronounced positive δ¹³C excursion during the Tournaisian Late crenulata-isostichia conodont Zone (Mii et al. 1999, Buggisch 2001, Saltzmann 2002) gave rise to the assumption of the built up of a Tournaisian ice cap, although no glacial deposits are known from this time interval. Ice rafted debris and tillites are reported from Visean and Serpukhovian rocks with a maximum during Bashkirian.

The carbon isotopic composition of whole rock carbonates and the oxygen isotopic composition of conodont apatite from late Devonian to early Pennsylvanian sections in Europe and Laurentia were measured in order to reconstruct variations in carbon cycle, marine palaeotemperatures, and ice volume during the Mississippian. The measured conodont apatite δ¹⁸O values reveal two positive shifts with mean amplitudes of 1.5 to 2 ‰ during late Tournaisian and 1 ‰ during Serpukhovian (Lochriea multinodosa conodont Zone). Both parallel positive shifts in the carbonate carbon isotopic composition. The Tournaisian up to +6.5 ‰ and the Serpukhovian up to +5 ‰ shifts in carbonate δ¹³C have been interpreted as consequences of enhanced organic carbon burial rate resulting in a decrease in atmospheric CO₂ concentration. Climatic cooling as a potential consequence of lower atmospheric CO₂ concentration is confirmed by the conodont apatite δ¹⁸O records, which is interpreted as consequence of a waxing ice-cap and a cooling of low-latitude surface waters.

Buggisch, W. (2001), GSA Earth System Processes – Global Meeting, abstracts; Edinburgh. Caputo, M.V. (1985), Palaeogeogr., Palaeoclimat., Palaeoecol. 51, 291-317. Joachimski, MM. & Buggisch, W. (2002), Geology, 30, 711-714. Mii, H., Grossman, E.L. and Yancey, T.E. (1999), Geol. Soc. Amer. Bull. 111, 960-973. Saltzmann, M.R. (2002), Geol. Soc. Amer. Bull. 114, 96-108.