RARE EARTH ELEMENT ANALYSIS (REE) OF FOSSIL VERTEBRATES IN THE PIERRE SHALE: PALEOENVIRONMENTAL CONDITIONS

Concentrations of rare earth elements (REE) were measured in vertebrate fossils from five members (Sharon Springs, Gregory, Crow Creek, DeGrey, and Verendrye) of the upper Cretaceous Pierre Shale in central South Dakota. REE signatures within fossil apatite, records the early diagenetic environment and are dependent only on the composition of the original waters in which fossilization occurred. Therefore these stable REE signatures provide insight into the original composition of the water mass. REE in modern waters show systematic variations as a function of redox, pH, sorption, and alkalinity, which affect the concentration and availability of REE in the water mass. REE signatures and their stratigraphic variations in fossil materials can be used to interpret variations in paleoenvironmental conditions by comparisons of REE signatures in fossils with modern waters. Such variations may be best visualized by plotting normalized concentrations of representative light (Nd), middle (Gd) and heavy (Yb) REE in ternary diagrams. This allows the basic shape of the REE signature to be represented. REE signatures in Pierre Shale fossils can be related to differences in mixing of oxic, shallow seawater and anoxic deep waters. If the REE trends are interpreted as being related to ocean depth, then the lower part of the Sharon Springs Member was deposited in relatively deep,anoxic waters. The REE results for the upper Sharon Springs member is consistent with decreased depths and regression and the overlying Gregory and Crow Creek Members were apparently deposited in even shallower waters. The overlying members(DeGrey and Verendrye) were deposited in progressively deeper water. These results are generally consistent with previous paleoenvironmental conclusions based on analyses of biofacies, sedimentology and eustatic sea level curves.