Characterization of a pyritized fossil pollen cone from Clarkia, Idaho

Abstract

The Miocene Lake deposit in Clarkia, Idaho is composed of highly fossiliferous silty clays. These rocks were formed in an anoxic and likely toxic deep lacustrine environment which facilitated the fossilization of plant material with a high amount of morphological detail. Many of the plant compression fossils exhibit exceptional preservation, including the preservation of original biomolecules. Pyritized plant specimens are much less common and the details of plant pyritization at this site are not yet fully understood. Pyritized male pollen cones from Taxodium dubium provide an opportunity to explore the processes of pyrite preservation in the Clarkia deposit.One specimen was examined using scanning electron microscopy (SEM) and energy-dispersive x-ray analysis (EDS). The only pyrite textures observed on this specimen were framboids and framboidal microcrystals. Pyrite was found in abundance on the surface of the pollen cones and to a much lesser extent on the surface of the stem. Underneath the pyrite coating, both the cones and stem were carbon-rich, indicating organic preservation. Some pyrite crystals were found in the interior of the stem, but likely originated from the exterior and developed through cracks or tears in the fossil. Pyrite crystals did not form inside the cell walls of the stem. The decaying specimen likely provided a scaffold that could facilitate the formation of pyrite upon the specimen surface. Gypsum crystals found in close association with pyrite likely formed as a product of oxidation after the specimens were taken from the rock.