Lake Carpentaria

Australia: The Land Where Time Began

Lake Carpentaria

Bore hole cores from the Gulf of Carpentaria have revealed the presence of a shallow lake, up to 10 m deep, and covering an area of about 29,000 km², existed in the Carpentaria Basin after the sea level dropped, between about 35,000 and 12,000 BP. The climate was in a wetter phase between about 35,000 BP and 26,000 BP. At the onset of the build up to the glacial maximum of the last glacial phase the sea levels were dropping and the monsoon pattern of the region began to become irregular then stopped completely as the glacial maximum approached.

Prior to the about 35,000 BP the Fly and Strickland Rivers of New Guinea flowed south into the Carpentaria Basin across the Torres Strait land bridge from a point upstream of their junction with the Oriomo River. At about 35,000 BP an uplift changed the flow of the Fly and Strickland Rivers so that they flowed eastward instead of south. The present day rivers around the Gulf, that now flow to the Gulf, flowed into the Carpentaria Basin at the time of the Carpentaria Lake. Floodplain swamps now exist around the edges the Gulf, but when the lake was present these would have been greatly extended by the addition of large areas of swamps the formed around the lake in the middle of the basin.

It has been found that the northern margin of the Great Artesian Basin (GAB) extends into the Gulf of Carpentaria. Springs from the GAB are visible in the intertidal zone at low tide, but there are also some below the low tide mark. Where these occur on the bed of the Gulf the local prawn fishermen call them wonky holes, places where fine sediment is stirred up by the freshwater springs. Still others occur further out in the Gulf in deeper water.

The lake occupied the lower middle of the region. The springs would have been on the southern edge of the lake. Some believe they may have been mound springs. Pollen analysis of the cores indicates that the flora of the area hasn't changed significantly over the last 35,000 years, the vegetation being similar to that of the present-day black soil plains to the south of the Gulf. The lower parts being vegetated by sedge grass communities, and aquatic species in the swamps. The basin was mainly grassy savanna, with some woodlands of eucalypts, cypress and casuarinas. The evidence of vegetation of this type across the northern parts of Australia over the past 40,000 years is inconsistent with the postulated presence of widespread rainforest across the north. It is believed that there were probably riverine vine forests along the courses of the rivers, and that they probably extended further south, beyond the zone of the present monsoonal belt, during the wetter phase that occurred between 35,000 and 26,000 BP

When sea level began to rise after about 12,000 BP, it began having an effect on the lake, a permanent open connection to the sea forming, though between about 12,000 to 8,000 BP it apparently remained as a low salinity water body, partly because the Arafura sill allowed only limited exchange with the ocean, and partly because of the monsoonal conditions with tropical cyclones dumping large amounts of rain on the catchments that drained into the lake. The present Torres Strait was formed as the land bridge between the Australian mainland and the previously connected New Guinea was completely flooded, after which the lake was swamped by the rising sea. Unlike many Australian lakes, Lake Carpentaria never became a dry playa at any time throughout its existence.

The sediment cores also record the climatic changes that affected the Australian continent, as well as the global climatic swings. The cores demonstrate the regular occurrence of dry conditions, as indicated by layers of wind-blown sand and dust, about every 2250 years, each lasting about 600 years. Coinciding with the Younger Dryas Event of the Northern Hemisphere, one of the dry, windy phases occurred between about 11,375 and 10,430 BP. This was a cold period that lasted almost 1000 years. In this phase, the Northern Hemisphere, as well as New Zealand, glaciers advanced and the climate was severe in Europe. At this time there is evidence of aridity in the northeastern part of Australia. The dunes of the central desert areas were reactivated.

These drier, colder phases are also recorded by oxygen isotope analysis of ice cores from Greenland and Antarctica. Changes in the foraminiferal faunas of the deep sea also occur at these times. More substantiating evidence for these cycles has been found in sediment cores from the Great Salt Lake in Utah. Still more evidence was found in coral growth from Barbados. All the available evidence points to the climatic events being on a global scale.

In Australia, the 600 year long droughts were followed by about 1500 years of wetter conditions. It has been suggested that these dry periods may be the result of strengthening of the eastern trade winds over northern Australia, shifting the Intertropical Convergence Zone to the north that would result in failure of the monsoon over the Gulf and simultaneously a strengthen of the Southern Oscillation over the Pacific. It is generally believed that this cyclical wet and dry phases are probably caused by cycles of sunspot activity. (White, 2000)

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