Isotopic records of brachiopod shells from the Russian Platform — evidence for the onset of mid-Carboniferous glaciation

Abstract

We performed isotopic analyses of Carboniferous brachiopod shells from the Russian Platform to examine global and regional environmental change along the western and eastern margins of Laurussia during the formation of Pangea, and specifically to examine the isotopic evidence for the onset of mid-Carboniferous glaciation. Shell preservation was evaluated from shell microstructure, cathodoluminescence, trace element content and isotopic comparison with matrix material. Most interior nonluminescent (NL) prismatic shell appears to be chemically well preserved as indicated by low to undetectable Si, Al, Fe and Mn contents. With minor exception, NL shell δ 13C and δ 18O values are higher than those of corresponding cements and matrix. The δ 13C record for the Russian Platform clearly shows a 3‰ increase (from 2.4±0.7‰ to 5.5±0.6‰) in the Serpukhovian or Early Bashkirian. This shift was first reported by Popp et al. [Popp, B.N., Anderson, T.F., Sandberg, P.A., 1986. Brachiopods as indicators of original isotopic compositions in some Paleozoic limestones. Geol. Soc. Am. Bull. 97, 1262–1269.], but had not been documented for a single region. Widespread occurrence of high δ 13C values in the Late Carboniferous support the interpretation of Popp et al. with regard to this shift as a record of increased burial of organic carbon. North American sections show a mid-Carboniferous δ 13C shift of only ∼1.5‰. We hypothesize that the reduced δ 13C shift reflects enhanced upwelling on the epicontinental seas of North America after the closing of the seaway between Laurussia and Gondwana. The δ 18O record for the Russian Platform shows a 1.8‰ increase in the mid-Carboniferous correlative with increased occurrence of glacial sediments and a drop in sea level. As a first approximation, ice volume calculations suggest that ∼0.7‰ of the mid-Carboniferous δ 18O shift is due to changes in seawater δ 18O, and ∼1.1‰ is due to 5°C cooling. Concurrent positive δ 13C and δ 18O shifts provide evidence for a relationship between mid-Carboniferous glaciation and burial of organic carbon, presumably through changes in atmospheric CO 2 levels.