Carbon and oxygen stable isotopes of selected Cenomanian and Turonian rudists from Egypt and Czech Republic, and a note on changes in rudist diversity

The stable isotope record and diversity of Cenomanian and Turonian rudists were compared for areas located at low paleolatitudes (10 degrees N - Northern Egypt; 5 samples; 13 analyses) and middle paleolatitudes (35 degrees N - Czech Republic, Bohemian Cretaceous Basin, BCB; 4 samples; 7 analyses). The oxygen isotope data indicate that the delta O-18 value of seawater was not identical for both areas. Moreover, local variations in seawater delta O-18 in the range of at least +/- 1 parts per thousand are probable in the shallow water near-shore environment where the rudists lived. Supposing a value of -1 parts per thousand VSMOW for contemporaneous seawater, the paleotemperature can be calculated as being between 24.5 and 31.5 degrees C for the Late Cenomanian and between 32.9 and 34.7 degrees C for the Early Turonian in the BCB. The delta O-18 seawater value for the northern part of Egypt was probably slightly lower. Calculation using a seawater value of -1.5 parts per thousand VSMOW gives temperatures for the Egypt in the range of 31.0 to 38.8 degrees C for the Late Cenomanian and 35.5 to 41.2 degrees C for the Turonian. The sclerochronological isotope profiles within one shell show changes of the calcite delta O-18 value, probably reflecting seasonal changes, with the largest observed within-shell temperature shift corresponding to 8 degrees C. The analyzed pilot sample set shows that the BCB and Egyptian rudists contain valuable paleoenvironmental information and that a detailed isotope study is needed. Rudist generic diversity of both areas was also compared. The Cenomanian and Turonian rudist assemblages from Egypt and the BCB show similar generic diversity, and also display a similar marked diversity decrease across the Cenomanian/Turonian boundary interval. It is important to note that both areas have similar diversity on the generic and partially species level during the Cenomanian and Turonian.