Silica Microphorms in Soils Affected by Hydrothermal Activity on the Yellowstone Volcanic Plateau, USA

Abstract

Many prominent ecological characteristics of Yellowstone derive from its hotspot-induced uplift, including the moderate-to high-elevation terrain, the cool climate and deep snowfall. Heat from the hotspot rises upward and drives Yellowstone's famed geysers, hot springs and mud pots. The major soil-forming factors in the area are volcanic parent rocks -rhyolite and andesite -and lake sediments overlying rhyolite, cold temperatures, and deep snow in winter and low precipitation in summer. The purpose of the work was to study the effect of hydrothermal activitygeysers, hot springs and mud pots on the Yellowstone Plateau post-volcanic soils, developed on rhyolite, lake sediments and andesite. The sampling sites were chosen in areas both affected by hydrothermal activity -by mud pots, active geysers and the field of thermal waters and off the direct hydrothermal effect -Hayden Valley and the Lamar River Valley. Chemical weathering was a major feature of the affected soils. Near active mud pots, at pH 5.1-5.6, active rhyolite weathering resulted in abundant amorphous silica production and sequential thriving of diverse diatom algae. In soils on the lake sediments and close to the geysers, at low pH values (< 4), weathering was moderate and biogenic silica was presented mostly by shells of testate amoebae. The content of diatoms in these soils corresponded with that in the parent lake sediments. Similar conditions were observed for the soils on andesite in the Lamar River Valley. Biogenic silica was also found in the form of phytoliths, well-preserved in the productive grassland of the Hayden Valley, but significantly affected in the soils near active hydrotherms. Hydrothermal activity was a driving force of silicate mineral weathering and resulted in the thriving of diatoms on the plateau.