U-SERIES GEOCHRONOLOGY OF SPELEOTHEMS ACROSS THE EDWARDS PLATEAU AND IMPLICATIONS FOR REGIONAL PALEOCLIMATE DURING THE LATE PLEISTOCENE-HOLOCENE IN TEXAS

The central Texas Edwards Plateau, built on Lower Cretaceous marine carbonates, is one of the largest karstic regions of the U.S. and is host to numerous cave systems. The Plateau is located in a transitional climate zone, with a large rainfall gradient from east to west (80 to 55 cm/year). Paleoclimate reconstructions here may provide context for model analysis of this climatically sensitive region. U-series disequilibrium has been used to determine the time period of growth for 21 speleothem samples from ten caves across the Plateau in order to gain insight into the regional climatic history during the late Pleistocene and Holocene (350 ka to present). The study area covers ~250 km west from the Balcones fault zone at the eastern edge of the Plateau. Previous studies of the timing of speleogenesis, based on stream incision rates, estimate that caves of the western Plateau developed from 4.9 to 3.9 Ma.

Maximum ages for the speleothems range from Holocene to >350 ka. Almost all of the speleothem samples from the eastern part of the Plateau have a maximum age of about 70 ka or less. In contrast, all speleothem samples from the western part of the Plateau have prolonged growth periods that are well beyond 70 ka, with four out of nine samples having maximum ages that exceed upper limit of the dating method (i.e., ~ 350 ka). Such significant differences in the length of the growth periods of speleothems from west to east across the Plateau may result from a number of factors, including: 1) biases in sample availability, 2) temporal variations in paleoclimatic patterns across the Plateau, whereby the western portion was wetter earlier in the Pleistocene, 3) progressive evolution of the karst system, whereby development of vadose zone flow paths, overlying soils, surface connections, and cave ventilation processes controlled drip rates and speleothem deposition.