Pliocene Era

THE PLIOCENE EPOCH is the uppermost subdivision of the Tertiary period (65.5 to 2.588 million years ago), and represents a geological stage from about 1.806 to 5.332 million years ago. Although the Pliocene was generally warmer than the present, this epoch is characterized by pronounced climatic oscillations that ultimately led to the characteristic cooling of the late Quaternary glacial-interglacial cycles. Pliocene climate data are inferred from oxygen isotope, dust, microfossil, and in some cases pollen data from cores collected under the flag of the Ocean Drilling Program (ODP), as well as terrestrial deposits. These records have allowed climatologists to refine the absolute chronology of the Pliocene epoch, and provide a continuous climatic record of global ice volume, sea surface temperatures, aridity, and terrestrial vegetation patterns.

The first Pliocene cooling event is documented at 4.5 million years ago, and was followed by variable, but persistent reductions in temperature after 3.6 million years ago. A brief period of warmth followed until 3.5 million years ago, at which time a second cooling event took place. A well-characterized mid-Pliocene warm period dates to approximately 3.3 to 3.15 million years ago, and is followed by the return to progressive cooling that culminated in the arrival of early northern hemisphere glacial-interglacial cycles about 2.75 million years ago. Significant growth of ice sheets did not begin in Greenland and North America until approximately three million years ago, following the formation of the Isthmus of Panama. Many agree that this final Pliocene cooling period set the stage for strongly developed glacial events of the Pleistocene (1.8 million to 11,550 thousand years ago) and thus represents a climatic stage that is most relevant to the climates of late Tertiary and early Quaternary.

The contemporary significance of the mid-Pliocene warm period lies in its utility as a model for future scenarios of global warming. This is because continental distributions and climate-indicative plant taxa are thought to have been very similar to today. Members of the Goddard Institute for Space Studies (GISS) and the PRISM (Pliocene Research, Interpretations and Synoptic Mapping) group have exploited these paleofeatures in their efforts to model global Pliocene climate and vegetation distributions. Average mid-Pliocene global sea levels are modeled at 33 to 82 ft. (10 to 25 m.) higher than today, due to reduced Greenland and Antarctic ice cover, while sea surface temperatures were approximately 6.5 degrees F (3.6 degrees C) warmer than at present day. Mid-Pliocene climate simulations generally indicated increased surface air temperatures, particularly during the winter, and increased annual rainfall, evaporation, and soil moisture. Pollen records from land-based cores are less chronologically accurate, but consistent with a 7–18 degrees F (4–10 degree C) warmer northern hemisphere climate, coupled with higher continental moisture levels. This is especially evident in high latitude regions such as the Arctic.

The PRISM group has used fossil and pollen data to document vegetation patterns across the globe during the mid-Pliocene warm period. Their work indicates extensive conifer and mixed forests in the mid-Pliocene Arctic, and generally more northerly distributions [Page 797]of the mixed deciduous forests of eastern North America. Interior North America was likely to be moister, and warmer than today, with evidence of lakes in southeastern California, Arizona, and Utah. Northern Europe was warmer and wetter, with a greater abundance of swamps and wetland areas. Little information exists about Central and South America, but the limited numbers of pollen studies are consistent with GISS climate models suggesting a warmer, wetter climate, with a greater abundance of steppe and prairie vegetation. The Australian mid-Pliocene warm period is poorly documented, but it is thought to be wetter than today, with broader distributions of forest flora. Regions of Antarctica were significantly warmer than today, so increased exposure of soils supported the presence of mixed beech forests.

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