The Rhynie Cherts: An Early Devonian Ecosystem Preserved by Hydrothermal Activity

The Rhynie cherts contain a remarkable early Devonian terrestrial to freshwater biota preserved in siliceous sinter by the action of a precious-metal-bearing hot spring system. Arthropods, vascular and non-vascular plants, algae, fungi and cyanobacteria are present. Preservation ranges from perfect 3D cellular permineralization to compacted coalified films, and can be related to both silicification processes and stages of biological and physical degradation of the plants at the time of silicification. Plants occasionally have original subaerial vertical axes preserved in growth position, and rhizomes bearing rhizoids. The plant litter of the substrate is also partly silicified. Silicification of organic material took place in hot spring pools, by surface flooding of areas with growing plants, and by permeation of the substrate. Sinters recognized include botryoidal geyserite typical of vent margins, and laminated sinter comparable with that of modern sinter terraces. Massive, vuggy, brecciated and nodular sinter textures are also present. At the microscopic level, textures associated with filamentous elements of the biota, and with the preservation of plants, closely match those present in modern sinters. Oxygen isotope and organic geochemical data from the Rhynie cherts indicate a temperature of 90-120 degrees C. This is apparently greater than the temperature at which elements of the biota were preserved and represents subsequent shallow burial in the hot spring system. The range of temperature and chemistry present at the surface provided high local environmental gradients. Current work attempts to identify thermophilic elements of the biota and document environmental zonation of biota relative to hot spring vents.