A LATE-QUATERNARY PALEOENVIRONMENTAL RECORD FROM PARKER'S PIT, A NATURAL TRAP IN THE SOUTHERN BLACK HILLS OF SOUTHWESTERN SOUTH DAKOTA, USA

Parker’s Pit is a cave located in the southern Black Hills of SW South Dakota, a mountainous area surrounded by short-grass prairies of the western Great Plains. The modern entrance is ~ 45 cm wide and 1 m long, with a 13.8 m vertical drop to the cave floor. Hence, the cave is a natural trap for animals, and a debris cone containing sediment and faunal remains has formed beneath the entrance. Over at least the past 15,000 years, runoff episodically poured into the pit and transported sediment and bones down a 6-m-long ramp to a flat area that is a depositional zone. Recent excavations to a depth of 1.5 m in the depositional zone revealed sheetwash consisting of laminated sandy and fine-grained sediment mostly derived from soils eroded off the landscape surrounding the cave. Hence, the package of sediment stored in the depositional zone is an archive of soil organic matter used here to reconstruct late-Quaternary bioclimatic change in the Black Hills.

Based on the stable carbon isotope (δ¹³C) values of organic matter (OM) in the strata, the plant community of the Black Hills was strongly dominated by C₃ vegetation before ca. 14,150 B.P. During that time, C₄ vegetation contributed only 7-13% of the OM to sediments deposited in the cave, and the mean July temperature (MJT) ranged from 17.0 to 17.7° C. Between ca. 14,150 and 11,300 B.P., C₃ vegetation continued to dominate the plant community (C₄ grasses contributed only 20-41% of the OM), though warming was underway, with MJT steadily increasing from 18.3 to 20.3° C. A major shift in δ¹³C values occurred between ca. 11,300 and 8900 B.P. and continued until the late Holocene, with C₄ grasses contributing 60 to 72% of the OC and MJT ranging from 21.7 to 23.3° C (the Altithermal). A C₃-dominated plant community was reestablished after ca. 4350 B.P. and MJT declined (19.6-20.2° C).

As a natural trap, Parker’s Pit provides a representative sample of animals that were small enough to fit through the narrow opening of the cave. Initial analysis and dating of the fauna suggests turnover that generally supports the nature of bioclimatic change inferred from the δ¹³C data. Further reconciliation of observed changes in the fauna in relation to δ¹³C changes observed in sediments presents an opportunity to integrate records of faunal change with independent climatic data for the late Quaternary of the southern Black Hills.