Stratigraphic and paleoenvironmental variations in Rare Earth Element (REE) composition and crystallography of fossil bioapatite

Rare Earth Elements (REE) and unit cell dimension (UCDA) analyses of fossil bioapatite from Cretaceous, Tertiary and Pleistocene age vertebrates were used to identify and interpret changes occurring in the paleoenvironment over time and for stratigraphic correlation. REE composition and the UCD in bioapatite are dependent upon the composition of diagenetic waters, concentrations of REE and other species incorporated into bioapatite during permineralization. REE signature and UCD differences in bioapatite identify distinct intervals in lithologic units. These differences reflect changes in the depositional environment, and can be used to interpret paleoenvironments. Because these distinct intervals represent an averaging of periods of certain depositional environments, they can be correlated over significant areas. Fossil vertebrate samples were obtained from the Upper Cretaceous, Campanian and Maastrictian deposits of the Pierre Shale, from the Sharon Springs, Gregory, Crow Creek, DeGrey and Verendrye members. These samples were collected in an area between Chamberlain and Pierre, South Dakota, at localities along the banks of the Missouri River. REE signatures and UCD variations were found to be consistent within individual lithostratigraphic units, but are significantly different between these units. Variations in REE signatures and UCD in the Pierre Shale represent changes in the depositional environment and have been used successfully to interpret events during diagenesis. Specific REE signatures and UCD have been shown to extend laterally and act as markers for their units. These markers can be used to discriminate among units for purposes of stratigraphic correlation in the Pierre Shale. Analysis of REE in fossils from the Pleistocene Fossil Lake, Oregon, also revealed that REE signatures are laterally constant over many km, but differ among the thin lithostratigraphic units. Paleoenvironmental interpretations from REE analysis were consistent with those from other geologic data. Preliminary research was conducted in the fluvial deposits of the Upper Cretaceous Judith River and Lance Formations and the Tertiary deposits of the Fort Union Formation in Wyoming. Unique REE signatures and UCD variations were found for each tested lithostratigraphic unit. REE and UCD analyses resulting from this research provides a finer scale of resolution for stratigraphic correlation and a proxy for paleoenvironmental interpretation

Lithostratigraphy, tephrochronology, and rare earth element geochemistry of fossils at the classical Pleistocene fossil lake area, South Central Oregon

A B S T R A C T One of the most famous fossiliferous Pleistocene sites in the Pacific Northwest is Fossil Lake, Oregon. Until recently, fossil collections from the area were not stratigraphically controlled, owing to the lack of a detailed stratigraphic and chronologic framework. Our field studies reveal at least nine exposed thin rhythmic fining-upward depositional packages, most separated by disconformities. Analysis of interbedded tephras reveals that the Rye Patch Dam (∼646 ka), Dibekulewe (∼610 ka), Tulelake T64 (∼95 ka), Marble Bluff (47 ka), and Trego Hot Springs (23.2 ka) tephra layers are present in the section, indicating deposition from more than ∼646 ka to less than 23 ka, which includes both the late Irvingtonian and Rancholabrean North American land mammal ages, a much longer time span than previously believed. Bones analyzed from eight of the defined units have distinctly different rare earth element (REE) signatures. Fossils obtain REE during early diagenesis, and signatures are probably closely related to lake water compositions. REE signatures in fossils from lower packages suggest uptake from neutral pH waters. In contrast, REE signatures become increasingly heavy REE-enriched up-section, with positive Ce anomalies in the upper units. REE signatures in fossils from the upper units are very similar to waters from modern alkaline lakes, such as Lake Abert, Oregon, suggesting diagenetic uptake in increasingly alkaline and saline waters. These REE changes suggest increasing aridity up-section, a contention reinforced by the habitat preferences of the terrestrial vertebrates preserved