PLEISTOCENE DIATOMS FROM THE 602, 000 YR B.P. LAVA CREEK B ASH

Abstract

A lacustrine deposit in central Texas contains the southernmost known locality of the 602,000 yr old Lava Creek B volcanic ash, which came from the most recent eruption of the Yellowstone Caldera in Wyoming. Diatoms were concentrated in the basal 10 cm of the ash, which represents the initial ashfall. Subsequent sediments, which are increasingly less diatomaceous, contain a mixture of ash, silt and sand that was gradually washed into the lake. The diatoms preserved in the initial ashfall include both planktonic and benthic species typical of a cool, alkaline, moderate to high conductivity, slow-flowing stream or lake. Cells of some of the filamentous species were still attached to each other in chains of 10–15 individuals, and long, slender frustules measuring 180 × 5 μm were preserved intact. Because the Lava Creek B ash was ejected from a caldera lake, transport of diatoms from the crater lake with the ash is possible; however, the large initial volume of hot ejecta, the localization and concentration of diatoms in the lower part of the ash, the occurrence of linked chains of diatom frustules, and the preservation of long, slender forms argue against transport of the diatoms within the Wyoming ash. The absence of diatoms in the lacustrine sediments below and above main ashfall suggests that the ash changed the nature of of the surrounding chemical microenvironment to either induce the growth of diatoms in an otherwise unfavorable habitat, or preclude diatom dissolution in what had been, prior to the ashfall, an environment in which diatoms were not preserved. The diatoms encapsulated in the ash provide a snapshot of the diatom population living in the lake at and immediately after the time of the eruption.