Late Miocene climate and life on land in Oregon within a context of Neogene global change

Abstract

Clarendonian (12 Ma) fossil soils, plants, molluscs, fish, and mammals of eastern Oregon allow reconstruction of late Miocene paleogeography, paleoclimate, and paleoecology on land between the global thermal maximum of the middle Miocene (16 Ma) and global cooling and drying of the late Miocene (7 Ma). Six different pedotypes of paleosols recognized near Unity and Juntura allow reinterpretation of local mammalian paleoecology. Fossil beavers dominated gleyed Entisols of riparian forest. Abundant camels and common hipparionine horses dominated Alfisols of wooded grassland and grassy woodland. Diatomites overlying mammal-bearing beds have bullhead catfish [ Ictalurus ( Ameiurus ) vespertinus ], as well as fossil leaves dominated by live oak ( Quercus pollardiana ). Fossil plants and soils of Unity and Juntura are most like those of grassy live oak woodland and savanna on the western slopes of the Sierra Nevada in northern California today. Fossil plants and soils indicate mean annual temperature of 12.9 (7.7–17.7) °C and mean annual precipitation of 879 (604–1098) mm. Miocene paleoclimatic changes in eastern Oregon show no relationship to changes in oxygen isotopic composition of marine foraminifera, usually taken as an index of global paleoclimatic change. Mismatch between land and sea paleoclimatic records is most likely an artefact of global ice volume perturbation of oxygen isotopic values. Instead, Miocene paleoclimatic change in eastern Oregon parallels changes in carbon isotopic composition of marine foraminifera, presumably through fluctuations in greenhouse gases.