SURVEY OF THINOLITIC TUFA TOWERS IN THE MONO BASIN, CALIFORNIA, USA

Pleistocene tufa towers, representing sublacustrine springs, can be found at elevations from near the present lake level (~1945 meters, the lowest elevation tufa tower we visited is 1966 meters) to at least 2070 meters in the Mono Basin. We present here observations made from a field trip in 2007 by two of the authors (SRH and NGH). The towers found above 2030 meters all have abundant thinolite texture, indicating cold water (<6°C) during precipitation of the parent mineral, ikaite, that lasted long enough (seasonally?) to allow replacement of this unstable phase by calcite. In addition to the thinolite texture, centimeter- to decimeter-scale dropstones are observed cemented within the towers, attesting to at least seasonally cold conditions. Previously published radiocarbon ages and elevations of tufa towers (Benson et al., 1990, PPP) found that the highest elevation tufas (above 2070 meters) had a limited range of apparent ages, between 11.8 and 14.1 C-14 kyr BP (six of eight are between 12.0 and 13.4 C-14 kyr BP). We find the relatively limited age range somewhat surprising, though promising. Based on radiocarbon dating of Holocene tufa at Mono Lake, it might be expected that there could be variations of 2000-5000 years in the reservoir (hard water) effect based on variations in the lake and spring waters and the mixtures between them. Additionally, the thinolite texture is quite porous so modern carbon contamination might be expected to be an issue, increasing the possible apparent age range. However, the observed range seems to imply that any complications with the radiocarbon system are limited to hundreds, not thousands, of years. New high precision U-Th ages on the cleanest coatings on tufa towers (Wang et al., this session) combined with radiocarbon measurements on sequentially dissolved tufa shows promise for revealing the variations in reservoir ages in the Pleistocene Mono Basin.