Study of geological history is frequently based on proxy signals, such as the isotopic composition of past sea water, with marine 87Sr/86Sr reflecting tectonic activity, oxygen isotopes climatic oscillations, particularly during the Pleistocene and Cenozoic, and d13C signal mirroring the vagaries of the carbon cycle. In addition, high resolution isotope curves can provide a tool for stratigraphic correlation and dating, that (at least on a regional scale) can attain or exceed the capabilities of biostratigraphy.
The applicability of the isotope approach, particularly at high resolution is, however, dependent on the availability of suitable carrier phases that not only record marine signals, but are also resistant to post-depositional alteration. For the Paleozoic, the internal "secondary" layers of articulate brachiopods are the material of first choice, having shells composed of the most stable marine carbonate phase, low Mg calcite. Yet even these have to be tested for post-depositional overprint. The most widely utilized approch has been cathodoluminescence (CL), but this technique is equivocal, since the manganese content of modern unaltered brachiopods can be high enough to cause CL, while diagenetic calcites, on the other hand, may be non-luminescent. Furthermore, CL provides no direct evidence for recrystalization, let alone for exchange of chemical or isotopic species. It is a qualitative method for detection of "some" manganese and/or iron enrichment in the shells.
In our study of isotope stratigraphy, based on Carboniferous brachiopods from western and central Europe, CL and optical criteria were complemented by trace elements (Mn, Fe, Mg, and Sr) determined by ICP and PIXE. A total of 297 Tournaisian to lower Moscovian brachiopods has been analyzed for d18O and d13C values, with 175 lower Carboniferous samples measured also for their 87Sr/86Sr. A set of 40 Sr isotope measurements on middle Carboniferous shells is still in progress. The ICP-trace element data exist for 259 samples and 55 of these were measured also by PIXE. No correlations has been observed between isotope and trace element data. This suggests either a closed system diagenesis or alteration in a multitude of diagenetic environments. Alternatively, the magnitude of a diagenetic isotope shift - if any - might have been of the same order as natural isotopic variations. To test these alternatives, we analyzed two or three coexisting brachiopods for 62 stratigraphic levels (i.e. from the same bedding plane) and compared the differences in their Mn and Sr contents to differences in d18O, the latter being more susceptible to diagenetic alteration than the 87Sr/86Sr and the d13C signals. For 80% of the cosets, the difference in d18O was < 2 ”, comparable to variations in modern brachiopods from a single location. In general, Mn enrichment seems to be a more sensitive indicator of diagenetic alteration than is Sr depletion. Although the trace element data are not an unequivocal criterion for sample alteration, they are useful for identification of samples with shell chemistry close to that of unaltered modern counterparts. They are therefore helpful in weighing the reliability of isotopic data.
The isotope curves - constructed from samples with trace element signatures comparable to modern brachiopods - show higher order oscillations in the X*106 range that are superimposed on broader lower order trends. The onset of the major phase of Permocarboniferous glaciation coincides with a well-documented upper Visean to lower Serpukhovian d18O shift off some 5 ”. The preceding upper Visean d18O decline conicides with a similar drop in the d 13C record. A second prominent drop in d18O may exist at the Mississippian/Pennsylvanian transition, but the data are sparse or from partially altered samples. The geological meaning of these higher order wiggles is not yet deciphered. The 87Sr/86Sr record is characterized by a steep gradient from the Hastarian to the lower Chadian. For this interval, the stratigraphic resolution of the 87Sr/86Sr curve is better than that of biostratigraphy. In contrast, the chemostratigraphic resolution for the mid Visean is poor.