Paleosol Records of Middle Miocene Climate Change

Abstract

The middle Miocene thermal maximum (~16 Ma) was a period of global climate unusually warm and wet for the Neogene and is of interest as a paleo-analog for future climate change from anthropogenic global warming. In Australia, paleosols of the Oligocene-Miocene Etadunna and Pliocene Tirari Formations formed in arid palaeoclimates and include pedogenic gypsum. The Middle Miocene paleosol has shallow calcareous nodules and stout root traces suggesting vegetation like dry woodland. Comparable mallee vegetation now grows no closer than 1200 km to the southwest, so middle Miocene warm-wet climate enabled range extension of mallee and woody thickening of plants in the Australia outback. There is no evidence in the outback of middle Miocene rain forest, which may have expanded its range to form kaolinitic Ultisols near Sydney, Mudgee, and Gulgong, in New South Wales. Nor is there evidence so far inland of swamp woodlands and heaths like those producing brown coals in the Latrobe Valley, Victoria.

In Argentina, the Santa Cruz Formation, in addition to its rich fossil vertebrate assemblages, contains a similar paleosol record of middle Miocene climate change. The early Miocene in the Santa Cruz Formation is characterized by a thick sequence of weakly to strongly developed grassland paleosols (Orthents and Ustolls) as well as paleosols with root traces and profile forms of open shrubland vegetation (Cambid). The middle Miocene thermal maximum is characterized by paleosols (Udepts, Udalfs) developed under open woodland-shrubland vegetation, during a period of climatic warmth and humidity. The late middle Miocene is characterized by a greater diversity of pedotypes, all suggestive of an arid and cooler environment (Argids, Cambids, and Ustepts).

Middle Miocene soil maps compiled for this study show extension of tropical soils (Oxisols, Ultisols) into northern and southern mid-latitudes, accompanied by thermophilic flora and fauna. Peats, lignites, and Histosols of wetlands are also more abundant at higher latitudes, especially in the northern hemisphere, during the middle Miocene. The expansion of such soils is an expected result of greater precipitation associated with higher water vapor content of a warmer atmosphere during the Middle Miocene.

This dissertation includes previously published co-authored material in chapter II.