Australia: The Land Where Time Began
Timeline of boundaries from the Palaeocene to the Miocene
A time when the atmosphere was warm, ocean currents were sluggish and almost the full water column of the oceans was warm with high saline levels. The warm oceans generated a balmy climate, even to high latitudes. A suggestion for the warmth of the climate is that it resulted from high levels of CO2 that was produced by volcanoes along mid-ocean ridges, as spreading was active at this time, and there wqere massive flood basalt eruptions in the North Atlantic. It has been suggested that Mediterranean Tethys may have made a contribution to the warming of the climate (Johnson, 1994).
It is believed a 'reverse greenhouse' may have begun at the close of the Early Eocene, and that carbon reservoirs, in the form of mostly coal deposits, took much of the CO2 out of the atmosphere. It is uncertain what caused this event, most of the rapid spreading of the Early Eocene had by now ceased, reducing the addition of the more CO2 to the atmosphere. At the time when CO2 would have been leaving the atmosphere, oxygen isotopes indicated a slight cooling, and in the oceans silica precipitated out of solution as the water cooled. Mountain glaciers began to form on the Antarctic Peninsula (Johnson, 1994).
The initial formation of Antarctic glaciers appears to coincide with the most severe extinction event in the Cainozoic and a further cooling of the climate. Increased amounts of moisture are believed to have been allowed to fall as snow over the South Pole as rapid rifting pushed Australia away from Antarctica. Between 36 and 35 Ma there were possibly 4 impacts , but it is believed they had minimal effects on extinctions or the climate. The circum-Antarctic current had started to form as shallow water circulation between Antarctica and the South Tasman Rise (Johnson, 1994).
"Terminal Eocene Event"
About 34 Ma, this event (it has been called a non-event) caused only minor extinctions of planktonic foraminiferans, few other organisms apparently being affected (Johnson, 1994).
Early Oligocene deterioration
About 33 Ma there is good evidence of a massive cooling, increased Antarctic glaciation. There was also deep ocean erosion from cold bottom waters flowing from the Arctic to the North Atlantic. Increasing circum-Antarctic circulation led to increasing flows. It was at this time that the mass extinctions that are usually referred to a Terminal Eocene Event actually occurred (Johnson, 1994).
Middle Oligocene marine regression
About 30 Ma the largest sealevel drop of the Cainozoic occurred, believed to have been caused by major glaciation on Antarctica. There was further ocean cooling and high levels of deep sea erosion. The extinction event was not as extensive as some others, probably because the fauna were already cold-adapted to some degree (Johnson, 1994).
At about 23 Ma, a pulse of glaciation in Antarctica occurred at the end of the Oligocene, thought to probably be the result of completion of sufficient opening of the gap between Antarctica and South America, allowing the circum-Antarctic Current to increase through the Drake Passage to the south of the tip of South America. During the Early Miocene the global climate began to warm, accompanied by increasing biodiversity. In the Middle Miocene the modern phase of Antarctic glaciation began, and Earth has been considered to be in an "icehouse" since then, the warm periods merely being comparatively short interludes in an overall ice age (Johnson, 1994).
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|