TEPHRA . . . BREAKFAST OF DIATOMS

The database developed by the U.S. Geological Survey Tephrochronology Project contains valuable paleontological information in addition to detailed geochemical and petrographic information on tephra. Since 1990, the presence of diatoms, radiolaria, pollen, phytoliths, foraminifers, and other microfossils has been noted in the description of each sample submitted for analysis. In some cases, microfossils were identified to the generic and species level. These data provide an additional facet to the database, broadening its applicability to biochronostratigraphic and paleoecological studies.

One problem encountered in lacustrine paleoclimatic studies in volcanic areas is that geologically rapid influx of tephra can induce changes in the abundance and composition of diatom floras that are analogous to changes due to climate forcing. The most common response in eutrophic systems dominated by planktonic taxa is the replacement of the genera Stephanodiscus and Aulacoseira by the obligate planktonic genus Fragilaria. The direct association of diatoms and tephra in a variety of trophic settings provides a chronological framework and a detailed synoptic view of short-term changes in both macro- and micronutrients over multiple lacustrine ecotones.

The presence of hundreds of Neogene and Quaternary tephra layers across the Great Basin provides a tool for the study of the paleobiogeography and evolution of freshwater diatoms. These tephra layers provide interbasinal time lines that have been used to help constrain the distribution and first and(or) last occurrences of the genera Actinocyclus, Thalassiosira, Cyclotella, Mesodictyon, Cyclostephanos, and Stephanodiscus.