Geological configuration of Wallacea

12 The contribution of sea level changes

Changes in sea level have also contributed to the relationships and proximity of the islands of Indonesia and the north coast of Australia, and thus may have contributed to the dispersal of animals and plants between islands. During glacial periods the continental margins were exposed to levels as low as 200 metres below present. In the glacial period toward the end of the Pleistocene (Last Glacial Maximum [LGM], c. 12,000 years ago), sea levels were approximately 120 metres below present. At this time north Australia was joined to New Guinea and Lake Carpentaria formed in the area between the (present day) Top End and Cape York regions. It is thought that the land area was covered with savanna, although gallery forests would have lined the rivers. On the Sunda Shelf Borneo, Sumatra, Java and Bali were linked by dry land and were joined to the Malay Peninsula, thus forming one large landmass.

Currently we live in an interglacial epoch, the Recent or Holocene, in which sea levels are relatively high. Multiple lines of evidence indicate that parts of the continental shelf of northern Australia were inundated as sea levels rose after the LGM, and there is some oral history amongst Indigenous people of flooding by the sea in the ancient past. Because sea level was approximately 130 metres below present level, the Australian continent during the LGM was one-third larger than now. At times on the very flat continental margins, the incursion of the sea may have been as rapid as two metres per day, thus each tide would have been higher than the last. These events would have been noticed by coastal peoples, but would also have forced animals to vacate their territories and move inland, and would have resulted in major redistribution of vegetation communities.

The region of Wallacea between the Sunda and Sahul shelves.
The region of Wallacea between the Sunda and Sahul shelves.

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