STABLE ISOTOPIC TRENDS IN 23-16 KA ARKANSAS STALAGMITE

We present stable isotopic data from a stalagmite from Cosmic Caverns, NW Arkansas, that has been dated by U/Th disequilibrium thermal ionization mass spectrometry (TIMS) to 22,800 - 15,700 year BP. Speleothems spanning this time interval are rare from the North American mid-continent, likely because cold and/or dry conditions minimized speleogenesis. The 22 cm-long cylindrical stalagmite (identified as CS-04-01) is composed of dense, optically-clear and finely-crystalline calcite, and exhibits stable isotopic variability that may reflect changing paleoclimatic conditions following the Last Glacial Maximum (LGM).

The chronology of CS-04-01 is derived from three TIMS dates (22,800 +/- 460, 21,050 +/- 280, and 16,250 +/- 400 yr BP). Carbon isotopic ratios in CS-04-01 average -9 per mil (PDB) between 22,800-21,600 years BP at which time they decrease sharply (over an estimated 300 years) to and remain at an average of -10.5 per mil. This decrease in carbon isotopic ratios could reflect a shift toward a denser forest covering over the cave, or it could also have been driven by a reduction in the contribution of bedrock carbon to the stalagmite.

Oxygen isotopic ratios decrease from approximately -4.3 to -5.3 per mil between 21,800 and 20,800 year BP, and then steadily rise to -3.8 per mil at 15,700 years BP. Neither the seasonality of precipitation nor the relationship between air temperature and the oxygen isotopic composition of precipitation is known for this region during the LGM. However, assuming that modern relationships apply, these elevated 18O/16O ratios could reflect significant (~4 degrees C) regional mean annual warming over this 5000-year interval.