Cretaceous marine invertebrates: A geochemical perspective.

A diagenetic evaluation was performed on marine fossil shell material from Cretaceous sediments of North America, the Arctic, the Antarctic and several localities in Europe. Trace element chemistry, XRD, SEM and stable isotope geochemistry were consistent in their results. Preservation of the original shell material of the low-Mg calcite organisms, brachiopods and belemnites, and the numerous aragonitic organisms was slightly variable with the majority of samples well preserved. Those samples that were altered underwent diagenetic stabilization in both reducing and oxic environments. Using the chemical data from only well preserved fossil shell material, basin paleo-reconstructions showed that from Aptian to Maastrichtian time, the Cretaceous seas were generally aerobic with some dysaerobia evident at the sediment/water interface and in the shallow sediment column. Paleosalinities fluctuated from brackish to normal marine, especially in the Western Interior Seaway of North America and the Paris Basin. The Lower Saxony basin, the Arctic and Antarctic were mainly normal marine with brackish conditions developing on occasion. Paleotemperatures determined from $\partial\sp $O data of preserved aragonite and low-Mg calcite shell material, also showed some variance. The Arctic and Antarctic were coolest, with Campanian/Maastrichtian temperatures about 12 or 13$\sp\circ$C, whereas the Lower Saxony basin and the Western Interior Seaway were slightly warmer, ranging from 11 to 20$\sp\circ$C. The Barremian/Aptian appeared to be the warmest time and a cooling trend was fairly consistent from then on