Saturday, September 17, 2011

How long as the Atacama been dry?

“I am flying home from Europe in late August with nothing but a notebook and the 2011 Goldschmidt conference Geology giveaway issue to keep me occupied. Using the old-fashioned method of reading and writing on paper, I will blog my way through the compilation of highlighted geochemistry papers as time allows. These will then be posted via time delay to keep the blog moving while preventing paper burnout.”

ResearchBlogging.orgThe Atacama desert, on the west coast of South America, is the driest desert on Earth. The high Andes mountains block moisture transport from the Amazon basin, and the cold Humboldt current offshore provides little evaporative moisture.

Dunai et al. (2005) attempt to determine whether the hyperarid conditions are ancient (early Miocene) or more recent (late Miocene) by looking at the cosmic ray exposure ages of easily eroded sediment.

Cosmic rays are extremely high energy protons which are generated beyond our solar system (ask an Astronomer for details). The are energetic enough to penetrate the atmosphere and the first few meters of rock when they strike the Earth. When they do hit rock, they can create nuclear reactions between the atoms in the rock. One of the products of these reactions, 21Ne, can be measured using noble gas mass spectrometry. So the amount of excess 21Ne a rock has is proportional to how long it has been close to the earth’s surface, and the cosmic ray flux.

Dunai et al. (2005)’s sample sites were specifically chosen to exclude areas where the outwash from the high Andes east of the desert would erode or cover the local rocks. Only local rainfall could erode the selected areas, so only local, medium elevation, near-shore precipitation (or lack thereof) was relevant to the erosion rates.

Their results show that most of the rocks they sampled have been at or near the surface for 20-30 million years. These are among the oldest exposure ages in terrestrial rocks. The implication is that there has been negligible erosion since that time.

On the other hand, I wish the paper made more of an effort to explain why the results given were not within error of each other. Call me old fashioned, but a data table would be nice as well.

The other question that they ask is which came first, the aridity or the uplift? It is easy to see how uplift causes aridity- the rain shadow gets stronger. How aridity causes uplift is less obvious, and the reference given is not available on this aircraft. But the general idea (based on context) seems to be that with no fluvial input to the subduction trench, it accumulates very little sediment. Without sediment, the rocks are stronger, and can withstand more stress, pushing the mountains higher.

The problem with this conclusion is that it requires knowing the sediment flux from the entire drainage area. Presumably the sediment transport would be controlled mainly by erosion of the high (and higher precipitation) Andes.

Dunai et al. (2005) specifically chose a site that did not record the sediment flux from the eastern, mountainous part of the drainage basin. Instead they chose to focus on local conditions. By excluding the most important potential sediment source, they put themselves in the worst possible position to answer questions about sediment transfer in the rest of the Atacama desert, including total transport to the trench.

Dunai, T., González López, G., & Juez-Larré, J. (2005). Oligocene–Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms Geology, 33 (4) DOI: 10.1130/G21184.1

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