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

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 delta C-13 and delta O-18 values are higher than those of corresponding cements and matrix. The delta C-13 record for the Russian Platform clearly shows a 3 parts per thousand increase (from 2.4 +/- 0.7 parts per thousand to 5.5 +/- 0 61 parts per thousand) 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. Sec. Am. Bull. 97, 1262-1269.], but had not been documented for a single region. Widespread occurrence of high delta C-13 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 delta C-13 shift of only similar to 1.5 parts per thousand. We hypothesize that the reduced delta C-13 shift reflects enhanced upwelling on the epicontinental seas of North America after the closing of the seaway between Laurussia and Gondwana. The delta O-18 record for the Russian Platform shows a 1.8 parts per thousand 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 similar to 0.7 parts per thousand of the mid-Carboniferous delta O-18 shift is due to changes in seawater delta O-18, and similar to 1.1 parts per thousand is due to 5 degreesC cooling. Concurrent positive delta C-13 and delta O-18 shifts provide evidence for a relationship between mid-Carboniferous glaciation and burial of organic carbon, presumably through changes in atmospheric CO2 levels. (C) 2001 Elsevier Science B.V. All rights reserved,