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 distribu tions 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.

The cause of the mid-Pliocene warming is uncertain, but some combination of CO2 increase and change in ocean heat transport may have been responsible. Carbon isotopic data from deep-sea microfossils, coupled with GISS climate models, support the increased strength of thermohaline circulation during the mid-Pliocene, particularly with respect to North Atlantic deep water production. However, simulations where CO2 is the single variable show that the proposed, realistic patterns of mid-Pliocene oceanic heat transports would only have been possible at CO2 levels greater than 1,200 ppm. There is no evidence supporting such elevated CO2 excursions, but some workers suggest that even the predicted minor increases up to 380 ppm, in combination with altered ocean heat transport, may have been enough to catalyze mid-Pliocene warming.

Early Pliocene fauna was transitional, favoring grazers over browsers, as grasslands and savannas expanded in central North America and Africa, thereby replacing woodlands and their associated fauna. Charismatic Pliocene fauna included mammoths, mastodons, camels, and hippopotamus in the mid-Northern Hemisphere latitudes, while large turtles and marsupials were found in the southern hemisphere. Pliocene high-Arctic fauna was primarily Eurasian, characterized by now extinct species of beavers, badgers, deer, and caniids, the presence of which is consistent with mixed-evergreen forest vegetation. The Pliocene deposits of eastern North America revealed mostly Eurasian fauna, most notably new species red panda.

Pliocene Africa, prior to 2.8 million years ago, was wetter than today, as evidenced by deposits of mangrove swamps and tropical forests, which retreated southward as desertification intensified. The western Sahara desert likely formed after 2.8 million years ago. The Pliocene is a particularly important time for the evolution and diversification of hominids. The aridity-humidity cycles that were related to the late Pliocene glacials-interglacials in the northern hemisphere in the Pliocene climate of Africa may have shaped hominid evolution by creating cyclic opportunities for species extinction and innovation.

sEE ALsO: Global Warming; Pleistocene Era; Quartenary Era; Tertiary Climate.

bibliography. M.A. Chandler, D. Rind, and R.S. Thompson, "Joint Investigations of the Middle Pliocene Climate II: GISS GCM Northern Hemisphere Results," Palaeogeogra-phy, Palaeoclimatology and Palaeoecology (v.9, 1994); E.S. Vrba, et al., Paleoclimate and Evolution, with Emphasis on Human Origins (Yale University Press, 1995); J.H. Wrenn, J.-P. Suc, and S.A.G. Leroy, The Pliocene: Time of Change (Publishers Press, 1999).

Jarmila Pittermann University of California, Berkeley

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