Australia: The Land Where Time Began

A biography of the Australian continent 

The Southern Ocean formed when the gap between Antarctica and the southernmost margins of the southern continents opened as they began drifting north at the break up of Gondwana. The water of the Southern Ocean is dominated by the Antarctic Circum Polar Current. Local currents and conditions lead to an uneven distribution of marine life around the continent.

Prior to the separation of South America and Australia from Antarctica, cold currents flowing along the Antarctic coast were diverted north to the tropics when they struck the west coasts of South America and Australia, returning south to Antarctica after they had been heated by their passage through the equatorial regions, taking that heat south to warm Antarctica. These warming currents were disrupted by the opening of the ocean between Antarctica and South America and Australia, allowing the polar regions to become a progressively colder closed climatic system. The southern parts of Australia became cooler, and the latitudinal temperature gradients steepened, and the climatic zones became more pronounced. The movement of Australia north gradually moved the central and northern parts of the continent away from the moist westerly winds, and into the region of the drier, warmer subtropical high pressure systems (Bowler, 1982; Bowman, 2000).

As the ocean to the south of the drifting southern continents was widening, allowing the Southern Ocean to form the strengthening circumpolar current, the equatorial oceans were being progressively blocked by the northward movement of the continents of the Southern Hemisphere. As Africa slammed into Laurasia it closed the remaining part of the Tethys Sea, isolating the Mediterranean Sea Basin. The western end of the Mediterranean Basin was closed by the closing of the gap between Spain and Morocco.

The movement of Australia into the north, ploughing into a previously continuous string of islands, making the flow of water between the Pacific and Indian Oceans more restricted.

The final gap to be blocked was that between North and South America, as the Isthmus of Panama rose about 5 million years ago. Throughout the Cainozoic Era the equatorial flow was being progressively weakened as each gap was closed by the drifting continents. 

Permanently ice covered regions are present close to the coast of Antarctica. Around this is a zone of seasonal ice, where ice covers the sea for most of the year, and between this zone and the Polar Front is a zone ice-free sea.

The Polar Front, the point where the cold circumpolar water meets the warm water from the north, had formed  by 22 million years ago. This is a major biological barrier in the circumpolar zone, the water immediately south of the boundary of the northern water, is 2^o C colder.

The Permanent Open Ocean Zone has high winds and rough seas as a result of the strong current and strong winds. The result is that there is deep mixing of the surface waters. It is a region of relatively high nutrients but the primary production is kept below its potential because the deep mixing pushes much of the phytoplankton below the photic zone. The phytoplankton in this zone are mostly very small nannoplankton, such as diatoms and flagellates.

The ecosystems of these zones are connected but are 3 separate ecosystems, not a single ecosystem as is sometimes believed.

Zones of the Southern Ocean

Permanent Open Ocean Zone (POOZ)

Seasonal Ice Zone (SIZ)

Coastal and Continental Shelf Zone (CCSZ)

Southern Ocean plankton

Abyssal Water Circulation

Sources & Further reading

Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003

Links

1. An oceanic 'fast lane' for climate change
2. Large iceberg breaks off Antarctica'a Mertz Glacier