Thursday, October 13, 2011

Orbital cycles, Australian lake levels, and the arrival of aborigines

ResearchBlogging.orgAustralia is a dry country. It is so dry, that the largest drainage basin on the continent has rivers that only occasionally carry water, and drains into a salt pan. Imagine if the Missouri only flowed every third year, or if the Zambezi was a generally a sand-filled channel that crossed a nondescript cliff at what we know as Victoria Falls.

Admittedly, the Lake Eyre basin is smaller than either of these drainages, but only slightly. But for the last 150 thousand years, it has told geologic tales which rival the best Swahili stories or Souix legends. It describes the movement of the Earth against the stars, and the coming of the first people to Australia.

The reason it can tell these stories is that, as a closed basin, the water level of lake Eyre varies dramatically with the amount of water flown in from its major tributaries. So, although the part of central Australia around the lake and the southern part of the drainage is a desert, tropical rainfall in the northern rivers fills it occasionally today, and has in the past allowed a lake many times larger than the current lakebed to exist. Magee et al. (no relation), have carefully and painstakingly reconstructed the history of the lake level over time, and it tells a fascinating tale of alternating floods and aridification over the last 150 thousand years.

What they found is that there have been five periods where a large, permanent lake replaced the current playa. Comparing the lake record to the changes in the Earth’s orbital tilt and eccentricity shows that the lake filling is consistent with wetter conditions- and a more powerful Australian monsoon, being correlated with high sea levels, low ice mass, and high northern hemisphere sunshine.

The exact reasons for this are not discussed in great detail. One is that the outflow from the Asian winter monsoon might be pushing moist tropical air towards Australia more than Australia’s modest monsoon sucks air in. Another point (made mostly in related, referenced publications) is that the warm sea north of Australia- the Gulf of Carpentaria- is shallow, and during times of low sea level, was land. So the northern edge of the Lake Eyre basin was a thousand kilometers from the sea instead of 150, due to the retreat of the Gulf shoreline.

But the other big feature is that the lake-filling events that occurred after 50,000 years ago were much smaller than those which occurred before. Climactically, the conditions 10,000 years ago should have been the same as the conditions 115,000 years ago. But the lake was only a fraction of the size. The authors find no natural causes which can explain this. So they suggest that the aridity starting around 50,000 years ago is related to the reduction in forest and increase in grasslands which occurred at this time. This vegetation change was a result of a huge increase in the frequency of fire in central Australia, which allowed fire-adapted plants to prosper at the expense of moisture-retaining forest. The increase in fire at this time is generally associated with the arrival of the first people on the Australian continent. IT is known that of Australia’s megafauna went extinct at this time, but Magee et al. (2004) show that even the tropical rains were effected by human migration, with drastic changes to the continent’s largest river basin.

Magee, J., Miller, G., Spooner, N., & Questiaux, D. (2004). Continuous 150 k.y. monsoon record from Lake Eyre, Australia: Insolation-forcing implications and unexpected Holocene failure Geology, 32 (10) DOI: 10.1130/G20672.1

p.s. A few Gene Expression commenters asked a month ago if I could summarize this paper. I hope this helps.


Chris Phoenix said...

Holy $#!*. So the aborigines' ancestors may have desert-ified a whole continent?

Compare and contrast with logging in the Amazon.

Of course, there's the correlation/causation thing. Maybe the aborigines arrived because of a climate change that also caused more lightning storms and burned off the forests. Or something like that.

But it's still damn spooky.

C W Magee said...

Not the whole continent: the main aridification event was the transition from NZ-style rain forest to semiarid in the Miocene, as the continent drifted north and global temperatures fell. But for east central Australia, it looks likely that the introduction of anthropogenic burning make have given the current pyrophilic biota a leg up in the tropics.

Anonymous said...

We all know that already. Loss of Megafauna, replacement of flora by grasslands and xerophytes, the aridity of Australia and the coming of the Aborigines are all connected.

The Central Aborigines are still doing that today, burning grasslands and the xerophytic plants, despite having no imperitive to do so. Hunting and gathering here is replaced by supermarkets and fast food.

C W Magee said...

Another interesting question is whether or not global warming will bring us a stage 5 style (smallish) permanent lake. Gavin?

Jim Baerg said...

So if humans in Australia started suppressing fire rather than encouraging it, the continent would green up to some extent?