TIMING OF EMPLACEMENT OF RARE EARTH ELEMENTS (REE) IN FOSSIL APATITE, FOSSIL LAKE, OREGON

Field studies of one of the most famous and prolific Pleistocene sites in the Pacific Northwest, Fossil Lake, Oregon reveal at least eight exposed thin (< 0.5 m), rhythmic, fining-upward depositional packages, grading from basaltic basal conglomerate to lacustrine claystone, most separated by disconformities. Tephrachronologic correlations indicate that deposition of lacustrine sediments spanned an interval from more than 670 to less than 23 ka, both the Rancholabrean and upper Irvingtonian NALMAs, a much longer span than previously believed. Rabbit skeletons were found in a burrow at the top of Unit 8 (the “Salmonid layer”), the uppermost unit of the section that also contained numerous articulated salmonid skeletons. The salmonid remains occur at a tilt-corrected elevation about 200 m below that of the proposed Pleistocene lake outlet; thus the salmonid fossils must have been deposited in sediments about 200 m below the lake surface. After the salmon died, the lake became isolated and largely dried up before occupation by the rabbit. Most REE in fossils are introduced soon after death of the organism. REE signatures and concentrations were similar in both salmon and rabbit fossils. This indicates that the REE signature is independent of bone material type, diet or species and is solely dependent on the availability for exchange in the pore waters. The similarity of the REE signatures of the rabbit in the burrow and the associated salmonid from the same unit have implications for timing of emplacement as well as the dependency of the signature on the geochemistry of the surrounding pore waters. To constrain timing of REE emplacement in the rabbit and the salmonid, both were 14-C dated. Differences in the dates can be used to limit the amount of time required for emplacement of REE and duration of the fossilization process at Fossil Lake, Oregon.