Australia: The Land Where Time Began
Cooling at the Eocene-Oligocene Boundary
The general trend during the Cainozoic towards a colder climate was not continuous, occurring in phases alternating with warmer phases. On the boundary of the Eocene and Oligocene, about 35 Ma, there was a pronounced cooling. About 15 Ma, in the Middle Miocene there was another cooling. At this time there was also a drop of the global sea level, indicating that the southern ice cap had formed. The world was in a ice age regime from that time on, about 6 Ma being the time of the next bout of cooling, with an associated drop in sea level as the volume of water locked up in ice increased greatly.
This sea level drop led to the stranding of the Mediterranean Sea, which evaporated dry and then refilled several times until about 5.3 Ma in the Pliocene when it filled as a result of a warmer phase that lasted until about 3 Ma. At this time the northern ice cap formed and the ice age was in full control of the climate. As the size of the ice caps varied with the alternating glacials and interglacials (interstadials) the sea rose and fell in line with the freezing and melting of the ice. Glacial phases, when the sea level was low, were the times when some of the best known drowned valleys in Australia formed as rivers cut down through the rocks on their way to the sea as the gradient was increased. Some examples of drowned valleys along the east coast of Australia that formed during the ice age are Sydney Harbour and Broken Bay, among others.
In his book, The Eocene-Oligocene Transition, Donald R. Prothero presents evidence that the cooling at the end of the Eocene began much earlier, in the Middle Eocene. According to Prothero, based on isotope studies of benthic and planktonic marine foraminifera, the Middle Eocene cooling affected marine organisms, the response of many showing clear indications of cooling, though others don't show the same response. The cooling signals from the foraminifera at the Early-Middle Eocene transition is mixed, some indicating cooling, others not. However, at the Middle-Late Eocene transition the indications are different. It was a major extinction event, especially for warm water marine species. There were complex changes to the benthic and planktonic foraminifera throughout the 12 million years of the Middle Eocene.
At the beginning of the Middle Eocene there was a drop of several degrees at high latitudes and in the water flowing along the bottom from high latitudes to the mid Atlantic and Gulf of Mexico about 51 Ma-(planktonic foraminiferan Zone P8) (Anne Boersma, Isabella Premoli Silva & Nick Shackelton, 1987). Little cooling is thought to have occurred in the near-surface waters, especially in equatorial regions. Equatorial upwelling, with associated abundant siliceous sediment in the Atlantic, as a result of the warm tropical surface water expanding towards the pole. The stagnant water conditions of the Early Eocene were being destabilised.
The Atlantic became more intensely stratified by temperature about 47 Ma (planktonic foraminiferan Zone P11), not long after the start of the Middle Eocene. There was a cooling of about 3o C of the bottom water and the temperate surface water. The result was a significant gradient between the bottom and the surface water. Planktonic foraminiferans diversified greatly to occupy the many water temperature regimes that became available. There were earlier tropical species as well and newly evolved cold-adapted species. The waters of the North Sea were warm, which allowed the spread of warm-water Tethyan foraminiferan species to expand much further north. It was at this time that the foraminiferans display their highest level of provincialism, as a result of the oceans being subdivided into smaller Biogeographic subprovinces by the vigorous circulation patterns.
The Middle Eocene is seen to be a complex pattern of cooling and the response of the biota to the changes. The first decline in the greenhouse conditions of the Early Eocene occurred at the beginning of the Middle Eocene. Cold water sinking around Antarctica and moving north along the sea floor destabilised the previously sluggish ocean currents. The oceans became highly stratified with the formation of surface and deeper waters. The early Middle Eocene, despite the extinction of a few tropical species, was a time of peak diversity of foraminiferans and coccolithophorids as a result of the formation of a variety of depth- and temperature-stratified habitats and increased provinciality.
There was a dramatic cooling at the end of the Middle Eocene that affected the oceans as far as the mid latitudes. Many of the warmth-adapted planktonic species, particularly the foraminiferans and coccolithophorids, went extinct. A wide variety of benthic foraminiferans and molluscs, including the large nummulitids, succumbed when the tropical Tethyan belt cooled and narrowed. In the oceans the extinction event that is usually associated with the end of the Late Eocene, actually occurred at the end of the Middle Eocene. The microfossils and molluscs that were so diverse at the end of the late Middle Eocene never regained that diversity. The echinoids appear to be the only organisms that didn't suffer the same fate (Prothero, 1994).
The changes at the beginning and at the end of the Middle Eocene that are seen in the terrestrial flora appear to correlate with the evidence of cooling obtained by the oxygen isotope record for the oceans. On land, the evidence at the start Early-Middle Eocene transition is of only slight cooling and drying. There is significant evidence from the land of cooling by about 7-11o C (11-15o F) at the Middle to Late Eocene transition. Evidence comes from deposits in northwestern North America, and in both Europe and America, loss of tropical elements. Worldwide, by the end of the Middle Eocene tropical rainforests were declining.
There was a severe extinction event in Asia and North America at the Middle-Late Eocene transition that wiped out many archaic mammals and arboreal mammals of the forests of the Early and Middle Eocene. This extinction event wa comparable to the extinctions at the same time in North America at the close of the Uintan.
In Europe, the end of the Middle Eocene was different from the extinction event in North America or Asia, it was not a major extinction event (Hooker, 1992). In Europe in the Late Eocene animals such as Palaeotherium and Plagiolophus, that were leaf-eating animals about the size of a modern tapir. The Theridomyidae, that were browsers, replaced the Paeudosciuridae, that were fruit eating rodents. The replacing species had teeth better designed for browsing coarser leaves, having well-developed crests. There was a drastic reduction of arboreal mammals (especially primates) and the insectivorous mammals were reduced. There was an increase in large ground mammals and browsing herbivores. At the end of the Middle Eocene, the European hoofed mammals went extinct. These extinctions in Europe parallel those in North America, though they were much less severe than those in North America.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|