other research

Holmes and I made a quick trip to the sloth site friday with Dr. Art Bettis, Department of Geoscience, University of Iowa.  Our goal was to collect a sand sample for optically stimulated luminescence (OSL) dating.  The radiocarbon data from the cores we collected last fall were inconclusive and hint that the sloths may be a lot older than we thought. OSL will give us the answer.   Sloth veteran Will Mott drove over from Council Bluffs to operate the bobcat and offer his usual extraordinary assistance.

Quartz sand particles  have tiny cracks and imperfections in their crystal structure that trap electrons emitted by radioactive elements in the surrounding sediment.  The longer they are buried the more electrons they trap.  Sunlight resets the “clock” so keeping the sample dark is essential.  Accuracy is about plus or minus 10-15%.  Expect a date in a couple of months. . . . Dave

Dave and I  met Dr. Art Bettis, Department of Geoscience, University of Iowa,  and Dr. Adel ‘Eddie’ Haj and his graduate assistant  Harold Ray,  Department of Biology and Earth Science, University of Central Missouri, at 8:00 AM, November 23, 2010 in Shenandoah and drove to the Tarkio Valley site to core the sloth locality with a Giddings trailer- mounted rig. AM temperatures started at 16 degrees and stayed below freezing all day. Frozen farm fields made access easy despite the previous week’s heavy rainfall.

The first 3″ core was taken upstream from the site on the southeast bluff overlooking the Tarkio, about 10 meters downstream from the northeast corner of the Athen’s property. This well produced a 40 foot core which was augmented by a five-foot, auger sample into underlying pre-Illinoian till. After completion, the rig was moved to the northwest bank of the Tarkio on the old Tiemann property (now Gary Peregrine’s farm).  We started drilling about 15 feet from the north face of the sloth excavation pit, but the effort ended prematurely when the core barrel became blocked by a rock. A second core was initiated about 15 feet downstream from the first. This core extended to 35 feet and bottomed out in a sand deposit that kept refilling the hole, stopping deeper exploration. This core undoubtedly penetrated the sloth-bearing level which lies about 24 feet below the surface of the field.

 Harold will describe and interpret the cores for his master’s thesis at UCM. It is believed that the Athen core sampled the valley wall of the Tarkio and that the Peregrine core penetrated valley fill deposits of the DeForest Formation. It is  not clear yet if the sloth-bearing deposits are associated with the older wall deposits or the younger fill deposits. Age differences are substantial.

Before we left, Bob Athen produced his latest bone discovery.  According to Greg McDonald, it appears to be the proximal end of a sloth metapodial. It is not complete though and Greg currently has the specimen to study for a potential match.  The specimen came from the creek bed in  the excavation area.

Holmes

The radiocarbon test on the humic acid Tom Stafford, Stafford Research, Inc.,   extracted from inside the Paramylodon bone came back this week: 5434 to 5305 years before present.  We had assumed the humic acid entered the bone canal system after death but clearly there is recent contamination. We planned to do the same test on the Megalonyx astragalus but given this result have decided the money can be better spent.  Any hope of linking the Paramylodon, which was found in a gravel deposit a short distance downstream from Megalonyx site, to our trio now rests on an analysis of rare earth elements deposited immediately post mortem , or finding more bones in situ.

Tests on the Megalonyx are more hopeful– Robert Feranec, Curator of Vertebrate Paleontology, New York State Museum,  managed  to extract a small amount of collagen from a molar  while he was assisting Alex Bryk, Penn State,  with a stable isotope analysis.  Bob had to do 7 separate extractions, but he got plenty.  To his eye, the collagen “looks fine. ” It weighed about 2.5mg. That’s not a lot, but enough. An AMS radiocarbon date should be available from Woods Hole  next month. . . . Dave