Australia: The Land Where Time Began

A biography of the Australian continent 

Timeline of boundaries from the Palaeocene to the Miocene

Transitions

55 Ma    Palaeocene-Eocene

50 Ma    Early-Middle Eocene

38-37 Ma    Middle-Late Eocene

36-35 Ma    Middle-Late Eocene

34 Ma     "Terminal Eocene Event"

33 Ma    Early Oligocene deterioration

30 Ma    Middle Oligocene marine regression

23 Ma     Oligocene-Miocene transition

Palaeocene-Eocene

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).

Early-Middle Eocene

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).

Middle-Late Eocene

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).

Oligocene-Miocene transition

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).

Sources & Further reading

1. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
2. Donald R. Prothero, The Eocene-Oligocene Transition-Paradise Lost, Columbia University Press, New York, 1994

Links

The Late Eocene-Oligocene extinctions

• Aridification of Australia
• Glacial Maximum
• Runaway Greenhouse
• Ice Ages
• Indian Ocean Dipole-IOD
• Late Carboniferous Glaciation
• Carboniferous Glaciation
• Precambrian Ice Age
• Early Palaeozoic Icehouse
• Pleistocene Ice Age
• Aerobacter spp. and cloud formation
• Eocene Australia
• Oligocece Australia
• Terminal Eocene Event