THE ISOTOPIC RECORD OF OCEAN CHEMISTRY AND DIAGENESIS PRESERVED IN NONLUMINESCENT BRACHIOPODS FROM MISSISSIPPIAN CARBONATE ROCKS, ILLINOIS AND MISSOURI

Fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew, owing to their stable mineralogy and their presence in mar ine strata throughout Phanerozoic time. The utility of these fossils f or preserving ocean-water compositions is evaluated through analyses o f multiple specimens that grew in contemporaneous sea water at individ ual stratigraphic horizons in the Mississippian-age Burlington-Keokuk Formation of mid-continent North America. Significant variations in Sr , C, and O isotopic compositions are observed in contemporaneous, non- luminescent shells, indicating that apparently pristine brachiopod she ll material has been altered by post-depositional processes of diagene sis. The least-altered compositions at each horizon are used to estima te secular variations of Sr-87/Sr-86 in Early Mississippian ocean wate r. The estimated rates of change of ocean-water Sr-87/Sr-86 are compar able in magnitude to the rapid Sr isotopic changes documented for Ceno zoic ocean water, for which high-frequency glacial cycles are inferred as a driving mechanism. The rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the adva ncement of high-resolution reconstructions of paleo-ocean chemistry.