PRELIMINARY ANALYSIS OF LATE PLEISTOCENE AND HOLOCENE DIATOM ASSEMBLAGES IN THE BONNEVILLE BASIN, WESTERN UTAH, USA: BLUE LAKE WARM SPRING

Although the sediments of Lake Bonneville and its successor, the Great Salt Lake have been studied for over a century, there are few records of diatom floras from this area. Blue Lake warm spring is located near the Utah-Nevada border (40.5^o N; 114.0^o W; 1,297 masl) along the former western margin of late Pleistocene Lake Bonneville. The spring is a NaCl-dominated mesohaline brackish (3-10 ^o/_oo) system with an elevated sulfate concentration. Core BL04-4 (822 cm long) was collected near the southwestern margin of the spring (Louderback and Rhode, 2009; L&R 09).

The modern diatom flora is dominated by a diverse suite of benthic species, most of which are motile epipelic and epiphytic, and a few are tychoplanktic. There are few planktic taxa. There are a number of brackish and marine species, and a portion of the flora is endemic. Freshwater assemblages are generally better preserved. The diatoms from two downcore intervals (~44-27 ka and ~13-1 ka) were studied. The older interval is dominated by benthic taxa. Navicula spp. dominate the intervals between ~44-39 and ~37-31 ka; this group is briefly replaced within these intervals by halophilic taxa. The assemblage in the intervening period included freshwater tychoplanktic and heterotrophic benthic taxa. The former group is also abundant at ~28 ka. The younger interval contains an assemblage that more closely approximates the modern Blue Lake warm spring assemblage, with several species that are not present in the older interval. Tychoplanktic fragilarioids are more abundant in the early and late Holocene. The diatoms of the older interval are indicative of Lake Bonneville while those in the younger interval are affected by the waters emanating from the spring system.

The 15,000-year record of vegetation change from core BL04-4 (L&R 09), identifying a local component of the flora affected by water availability at the spring, and pollen representing broader regional climate variability. Because of sampling differences, it is not possible to directly compare the diatom and palynofloras, but there is a qualitative similarity in changes in the abundance of cypreaceae pollen and fragilarioid taxa, both indicative of standing water.

Louderback & Rhode, 2009, 15,000 years of vegetation change in the Bonneville basin: The Blue Lake pollen record: Quat. Sci. Rev. (28) 308-326.