In moisture-limited environments, meteoric precipitation is the primary control of speleogenesis. We have investigated the growth histories of two stalagmites, 117 & 118, from C -126 Cave, Cape Range, Western Australia (22.1°S, 113.9°E) in order to reconstruct the timing of wet and dry periods during the Holocene and latest Pleistocene. Both stalagmites were dated by 238U-234U-230Th thermal ionization mass spectrometry (TIMS) at the University of New Mexico Radiogenic Isotope Lab. Sample 117 is a 78 cm-tall clean stalagmite with banding alternating between milky and clear calcite, and with some detrital layers in the bottom 12 cm. Five TIMS dates from this stalagmite suggest continuous growth from 10,100 ± 260 to 5,450 ± 860 yr BP. These results indicate that the early to middle Holocene may well have seen precipitation levels higher than those of the last c. 4.5k years. These results provide the first firm indications of Holocene precipitation changes for this part of Australia.
Stalagmite 118 is 48 cm tall and also composed of clear calcite, but detrital layers are common from bottom to top that introduced uncertainty (via low measured 230Th/232Th ratios) into its radiometric dates. The four dates obtained for 118 range from 22,420 ± 2,160 to 14,970 ±610 yr BP, with the large error due primarily to the initial Th correction, for which we used the average crustal silicate ratio of 230Th /232Th of 4.4±2.2 ppm. Attempts to construct isochrons in order to better constrain the initial Th ratios were unsuccessful. Nonetheless, these ages, coupled with an absence of visible hiatuses, suggest continuous growth in both stalagmites. These stalagmites are from a region south of the northwest Australian summer monsoon region, and thus it may not be surprising that these stalagmites do not appear to coincide with other regional records of monsoon moisture that suggest a significant increase in monsoon moisture at ~15,000 yr BP. However, they are also at variance with optically stimulated luminescence dates obtained from desert dunes in the Cape Range region, which indicate strong aridity at the time of the Last Glacial maximum.
Kristyn Rodzinyak, ’09 Colorado Springs, CO
Majors: Geology, Chemistry
Rhawn F. Denniston
University of New Mexico
University of Western Australia
Western Australian Museum
Sponsor: Rhawn Denniston