USE OF RARE EARTH ELEMENT SIGNATURES IN FOSSIL VERTEBRATES FROM THE UPPER CRETACEOUS PIERRE SHALE, CENTRAL SOUTH DAKOTA, TO DETERMINE PALEOENVIRONMENTAL CONDITIONS

Rare earth element concentrations were measured in mosasaur bones collected from five members (Sharon Springs, Gregory, Crow Creek, DeGrey, and Verendrye) of the upper Cretaceous Pierre Shale at localities near the Missouri River in Brule, Buffalo, Hughes and Hyde counties. Fossils from each member of the Pierre Shale have been shown to have different REE signatures that correspond to other fossils from the same member. REE patterns can indicate microenvironments of deposition and, on a larger scale, reveal fossil provenience.

Differences in REE signatures among members can be interpreted as related to differences in mixing of oxic, shallow seawater and anoxic, deep waters. If differences in mixing are interpreted as depth differences, the Sharon Springs Member was deposited in deep, anoxic water, with gradual shallowing through the Sharon Springs. The Gregory was deposited in shallow coastal water, and the overlying Crow Creek, DeGrey, and Verendrye Members in progressively deeper marine waters. These interpretations are generally consistent with those based on faunal diversity and lithological interpretation of the members.

REE data have typically been displayed as NASC-normalized values for individual elements. The REE patterns define a “signature” for each specimen. These signatures can be interpreted as “fingerprints” for their units. REE signatures reflect the composition of early diagenetic waters and therefore can be used for paleoenvironmental interpretation.

We propose using triangular or ternary diagrams for visualizing and interpreting variations in REE patterns in both vertebrate and invertebrate samples. Representative light (Nd), middle (Gd) and heavy (Yb) REE are at the vertices of the triangle. The ternary diagram allows the basic shape of the REE pattern to be represented. These ternary diagrams can then be compared with those of natural waters to infer the original paleoenvironmental conditions and depth of the water mass.