Early occurrence of sagebrush steppe, Miocene (12 Ma) on the Snake River Plain

Pollen analysis of soils overlain by welded tuffs of the Yellowstone hotspot provides a radiometrically-dated chronology of the vegetation 12–7 Ma for central Snake River Plain. The welded tuffs, produced as the Yellowstone hotspot migrated eastward, overlie soils containing perfectly-preserved pollen, occasionally containing cytoplasm. The pollen assemblage is dominated by Artemisia (sagebrush; ave. 32%, range 11–61%) and Poaceae (grass; ave. 21.3%, range 4–55%). This assemblage is very similar to that found in the area today, except the welded-tuff samples contain low percentages (< 5%) of arboreal taxa not found in the current flora: Podocarpus, Cedrus, Taxus, Aesculus, and Ulmus. The values of Artemisia and Poaceae are much higher than those found in samples of the same age (12–7 Ma) from the Pacific Northwest (which are dominated by tree pollen) and the Great Salt Lake basin (which are dominated by pine [Pinus], and greasewood [Sarcobatus]). Furthermore, the welded-tuff samples from the Snake River Plain contain a diverse herbaceous taxa characteristic of modern sagebrush steppe, and a diverse fungal spore assemblage including the root symbiont Glomus and the dung fungus Sporormiella. These results show that sagebrush steppe was widespread on the western Snake River Plain 12 Ma, at least 5 Ma earlier than previously recognized.