U-TH DATING OF FOSSIL-ASSOCIATED CAVE CALCITES FROM SOUTHEASTERN ALASKA

On Your Knees Cave in southeastern Alaska preserves a rich source of paleontological information from the most recent glacial and post-glacial periods. Bone material older than the 45,000 year limit of radiocarbon dating is also found in the cave, and although potentially important, the inability to directly date this material presents difficulty in understanding older parts of the record. We demonstrate that in particular favorable circumstances, U-Th dating of speleothem material in stratigraphic association with bone material can place constraints on the chronology of the older portion of the paleontological record in On Your Knees Cave.

A large speleothem and sediment complex was discovered in the cave, which illustrates our point. The complex consists of a large stalactite which fell and landed on its side, was subsequently buried by gravel and cobble-sized fluvial sediment including a fragment of bear scapula, and was then covered by the calcite growth of a large stalagmite. The large size (several kg) and relatively pristine condition of the stalactite argues that it fell and was unlikely transported any significant distance (if at all). We have used high-precision ICP-MS methods to date the oldest and youngest material from both the stalactite and stalagmite, which may constrain the age of the sandwiched bone material provided the bone has not been reworked from older sediments. Although possible, the relatively good condition of the fragile scapula argues against significant reworking.

The inner (oldest) and outer (youngest) sub-samples of the stalactite yielded ages of 137,900 ± 2,100 and 137,900 ± 1,500 years, respectively, illustrating that this formation grew relatively quickly and then detached from the cave ceiling around 138,000 years ago. The lower (oldest) and upper (youngest) sub-samples of the stalagmite yielded ages of 57,260 ± 720 and 53,300 ± 450 years, respectively. We therefore conclude that the bear scapula is most likely between 138,000 and 57,000 years old. Three other flowstone fragments recovered from sediments within the cave yielded basal ages of 132,800 ± 2,000, 133,800 ± 1,500, and 185,800 ± 2,800 years in age. The apparent non-random groupings of these ages into periods of early interglacial times and a warmer phase of glacial time likely have climatic implications.