Miocene

Miocene The Miocene epoch is a division of Cenozoic time that was first distinguished by Charles Lyell in his famous book Principles of geology (1830) on the basis of the fossil mollusc fauna present in the rocks of the Paris Basin. As the earlier of the two divisions of the Neogene sub-period, it follows the Oligocene epoch and precedes the Pliocene. Today, Lyell's criterion of some 17 per cent of the Miocene mollusc fauna being extant is no longer applied; the Miocene is held to have begun at about 23 Ma and lasted for around 17 million years. It contains three series and six stages, based upon sections in the Mediterranean region. The marine biostratigraphy is based upon microfaunas and floras, notably planktonic foraminifera and radiolaria.

In North America the continental Miocene deposits of the western interior have been apportioned to three stages on the basis of their mammalian faunas, which locally are prolific.

The principal feature revealed in Miocene stratigraphy is a continuation of climatic cooling. By about 15 Ma a major ice cap had accumulated on the Antarctic continent and a permanent ice shelf was beginning to form around its shores. The circum-Antarctic current intensified and expanded. Glaciers made their appearance in the southern Andes. On the other side of the world the first significant ice fields were develop-ing in Alaska, and polar sea ice became permanent. In consequence, by 10 Ma world sea level began to fall.

At the end of the Oligocene the Ural sea had disappeared and the Himalayan uplift was taking place. Eurasia was more or less consolidated except that the Mediterranean–Tethyan region was still in motion; the seas became reduced in area, and by 18 Ma the Mediterranean was separated from the Indo-Pacific seas. Africa and Arabia were now in contact with Eurasia and free access between the two was available to land animals. The Mediterranean also became closed off from the Atlantic. In the late Miocene, desiccation lowered the level of the Mediterranean waters to well below that of the Atlantic and diminished their area by as much as 2000 km². The last stage of the Miocene was influenced by global environmental changes, probably sharp climatic fluctuation which regulated sea levels up and down. A eustatic rise of Atlantic sea level breached the Gibraltar barrier to flood the western Mediterranean evaporite basin.

Miocene littoral and sublittoral deposits on the western side of the Atlantic show the continuation of the transgression begun in earlier times, while the Gulf of Mexico continued to subside under a growing load of sands and silts.

Around 14 Ma there was a sharp increase in volcanic activity in many regions, especially in the Central American and Pacific regions. The Hawaiian hot-spot was notably active, producing basaltic flows and ashes in the islands.

In the oceans, the foraminifera were flourishing as earlier in the Cenozoic, and underwent a successful radiation to reach a peak of species diversity around 15 Ma. Thereafter they went into decline. The calcareous nannofossils, too, steadily declined in diversity until at the end of the Early Miocene only about 35 species are known. They made a short and slight comeback in the Middle Miocene.

The land flora was susceptible to minor fluctuations in temperature, and the cooling episodes were felt even in the tropics. Coniferous forests spread across the high northern latitudes, and grasslands spread in the lower latitudes. Several modern plant families made their first appearance.

In the history of the mammals the Miocene is important for the diversification of the primates around 21 Ma and the emergence of the early hominid Ramapithecus some 7 Ma later. It was around 14 Ma that many land mammals of their kind that had been common in the earlier Cenozoic were lost and new grazing types became abundant. As the climate became cooler, grasses spread and provided habitats for new grazing animals. Towards the end of the Miocene, modern cats and the first elephants arrived on the scene.

D. L. Dineley