ISOTOPE STRATIGRAPHY OF THE EUROPEAN CARBONIFEROUS: PROXY SIGNALS FOR OCEAN CHEMISTRY, CLIMATE AND TECTONICS

Abstract

Carboniferous brachiopods from western Europe and the former USSR have been utilized as carriers of isotopic proxy signals for paleoceanography and paleoclimatology during the assemblage of Pangea. This is of particular interest, because this time interval coincides with the transition from greenhouse to icehouse conditions. The δ18O values (3 Ma running means) show an overall δ18O increase from about -7+/-1%% to -3+/-1%% during the Carboniferous. The rise in the δ18O is stepwise, with major shifts in the mid-Tournaisian and at the Visean/Serpukhovian transition. The comparable δ13C record shows a Tournaisian peak at 5%%, a Visean oscillation at about 2.5+/-1%%, a Serpukhovian rise of more than 3%%, a plateau around 5%% during the Bashkirian and Moscovian, a 4%% drop during the Kasimovian, and another rise, of about 3%%, during the Gzhelian. The 87Sr/86Sr record shows an Early Carboniferous decline from ~0.7083 to a mid-Visean minimum of 0.7077, followed by a rapid Serpukhovian rise to a Bashkirian and Moscovian plateau at 0.7087 and a slight decline during the Kasimovian and Gzhelian. The covariance of isotopic trends, and the coincidence with geological phenomena, indicates that the long term variations in ocean chemistry and climate may be coupled to the tectonic evolution of the earth system. The Namurian witnesses the onset of Hercynian orogeny, initiation of the main phase of Carboniferous glaciation, formation of the extensive coal deposits and the rise in 87Sr/86Sr, δ13C and δ18O. All this suggests that a tectonically controlled climate change, resulting in cooling and increased carbon storage in the organic reservoir, was the major triggering factor for all the above phenomena. The late Carboniferous glacial episode was interrupted by a warm interval in the Stephanian. The carbon and oxygen isotope data indicate that this episode may have been restricted to the early Kasimovian, a proposition at odds with the poorly dated record of glacial deposits that are interpreted as indicating a warm climate for the entire Stephanian. In addition, the pattern of short-lived peaks in δ13C and δ18O in the Tournaisian (and possibly Visean) may be interpreted as a reflection of short-lived glacial episodes, a proposition consistent with geologic records.