The slow rise of The Andes

The Andes is the world’s longest continental mountain range and the highest outside Asia, with an average elevation of 4000 metres. The question of how quickly the mountains reached such heights has been a contentious one that University of Michigan paleoclimatologist Christopher Poulsen and graduate student Nadja Insel working with Todd Ehlers of the University of Tuebingen in Germany, believe they may now have settled with a new interpretation of isotopic data. Their work suggests that the rise of the Andes was a very gradual process.

Poulsen’s uplifting work suggests that previous interpretations of the evidence have misconstrued changes in oxygen isotope ratios as being due to a rapid rise of the mountain range whereas the more likely explanation is that the changes are due to shifts in ancient climate.

“In the modern climate, there is a well-known inverse relationship between oxygen isotopic values in rain and elevation,” Poulsen explains.
“As a rain cloud ascends a mountain range, it begins to precipitate.
Because atoms of oxygen-18 are more massive than those of oxygen-16, it is preferentially rained out. Thus, as you go up the mountain, the precipitation becomes more and more depleted in oxygen-18, and the ratio of oxygen-18 to oxygen-16 decreases.” Geologists use the ratio of these isotopes, preserved in rock, to infer past elevations and so the rate of rise of a mountain range.

“If the ratio decreases with time, as the samples get younger, the interpretation would typically be that there has been an increase in elevation at that location,” Poulsen adds. He points out that that is the precise conclusion drawn by a series of papers on the uplift history of The Andes published over the past four years. On the basis of oxygen isotope ratios determined by analysis of carbonate rocks, the authors of those papers suggested that the central Andes rose about 2500 to 3500 metres in a mere three million years, Other geologists had assumed that the rise to those heights took place over tens of millions of years.

Unfortunately, elevation is not the only thing to disturb oxygen isotope ratios in precipitation. “It can also be affected by where the vapour came from and how much it rained,” says Poulsen. “More intense rainfall also causes oxygen-18 to be preferentially precipitated.” He and his colleagues were skeptical of the rapid-rise scenario, and so performed climate modelling experiments to investigate whether something other than altitude might have given rise to the shift in ratio observed in carbonate deposits.

Andes: Credit to http://www.flickr.com/photos/atyt/

“The key result in our modelling study is that we identified an elevation threshold for rainfall,” Poulsen says. “Once The Andes reached an elevation greater than 70 percent of the current elevation, the precipitation rate abruptly increased. In our model, the increased precipitation also caused the ratio of oxygen-18 to oxygen-16 to significantly decrease. Our conclusion, then, is that geologists have misinterpreted the isotopic records in the central Andes. The decrease in the ratio is not recording an abrupt increase in elevation; it is recording an abrupt increase in rainfall.”

This conclusion is backed up by geochemical and sedimentological data, Poulsen said. “There is evidence that the central Andes became less arid at the same time that the isotope records show a decrease in the ratio of oxygen-18 to oxygen-16.”

Links

Christopher Poulsen

Science Express, 2010, online

Exposed stumps and global warming

Tree stumps at the foot of glaciers in western Canada show that glaciation there is at a 7000-year low. The revelations could provide new insights into the accelerated rates at which these ancient rivers of ice are shrinking because of climate change.

Geologist Johannes Koch of The College of Wooster and colleagues have found deceptively fresh and intact tree stumps beside the retreating glaciers of Garibaldi Provincial Park, which lies about 60 km north of Vancouver, British Columbia. What puzzled Koch most about these stumps was just how long ago the glaciers entombed this forest in ice. Fortunately, he had access to sophisticated radiocarbon dating technology and determined that the trees first entered the big freeze a staggering 7000 years ago.

Dr. Johannes Koch

Dr. Johannes Koch

The stumps were in very good condition sometimes with bark preserved, explains Koch, who presented the results at the Geological Society of America Annual Meeting in Denver on October 31. Their pristine condition could best be explained if the stumps had spent many millennia trapped under dozens of metres of ice. Astoundingly, all the stumps were still rooted to their original soil and location.

These stumps really indicate when the glaciers overrode them, and their kill date gives the age of the glacier advance, Koch adds. They also give us a span of time during which the glaciers have always been larger than they were 7000 years ago, until recent warming released the stumps from their icy tombs.

Overlord Glacier is 7000 years old (Photo courtesy of Johannes Koch)

Overlord Glacier is 7000 years old (Photo courtesy of Johannes Koch)

If that were the only evidence, then the story of Koch’s stumps would not be quite so significant. However, he also compared the kill dates of the trees in the southern and northern Coast Mountains of British Columbia and those in the mid- and southern Rocky Mountains in Canada to similar records from the Yukon Territory, the European Alps, New Zealand and South America, as well as to the age of Oetzi, the prehistoric mummified alpine Iceman found at Niederjoch Glacier, and similarly well-preserved wood from glaciers and snowfields in Scandinavia. The radiocarbon dates seem to be the same around the world.

It’s important to note that there have been many advances and retreats of these glaciers over the past 7000 years, but no retreat has pushed the glaciers back so far upstream as to expose these trees. The age of the tree stumps gives new emphasis to the well-documented before and after photographs of retreating glaciers during the 20th century. It seems like an unprecedented change in a short amount of time, Koch adds, From this work and many other studies looking at forcings of the climate system, one has to turn away from natural ones alone to explain this dramatic change of the past 150 years.

Further reading

Alpine Glaciers in Western Canada approach their smallest size of the past 7000 years
http://gsa.confex.com/gsa/2007AM/finalprogram/abstract_127930.htm

Suggested searches

glaciers
global warming
climate change

Red rocks date

The radioactive decay of the rust-coloured mineral monazite can help scientists synchronise their geological clocks thanks to work carried out at New York’s Vanderbilt University. Microscopic crystals of the material act as tiny clocks allowing a date to be stamped on rich ore deposits in rock formations altered by high-temperature fluids.

Monazite crystals contain thorium, which is radioactive, Vanderbilt’s John Ayers explains. Thorium decays into lead. So, by measuring the ratio of thorium to lead we can date a crystal’s age. What we have shown is that monazite dissolves readily in high-temperature fluids, resetting its radioactive clock. This means it can then be used to date areas altered by hydrothermal activity. Such a geological timer is important because geologists have difficulty detecting and characterizing areas that were affected by hydrothermal action millions of years ago. The fluids involved have usually disappeared and the minerals they leave behind can be hard to identify and cannot be used to determine the date when they were altered.

Ayers and Loflin scanning microscopic monazite crystals (Credit: Vanderbilt University)

Ayers and Loflin scanning microscopic monazite crystals (Credit: Vanderbilt University)

Ayers and Calvin Miller of Vanderbilt working with Mark Barton from the University of Arizona, Tucson, and Christopher Coath from the University of Bristol, UK, have confirmed the date of monazite crystals more than a half kilometre from the Birch Creek intrusion in the White Mountains of eastern California and found that they agree with the 500 million year age of the older sedimentary layer. The rock’s oxygen isotope ratios differed markedly from those of crystals within the granite itself and within 500 metres of the area. In contrast, they report that the age and the isotopic composition of the crystals within 500 metres of the intrusion have dates and isotope ratios identical with those within the granite.

Monazite’s characteristics mean it could be used to date plutons, areas surrounding intrusions of molten rock that solidify below the surface, which could help in the location of rich metal-ore deposits. The crystals could also be useful in dating earthquake fault zones providing clues as to when the last quake occurred on a particular fault. Additionally, the crystals could provide dates for ultrahigh pressure terrains, the geological areas created when continents collide.

The Bird’s eye view of Birch Creek Pluton, California, and computer enhanced image (Credit: Patrick Kozak)

The Bird’s eye view of Birch Creek Pluton, California, and computer enhanced image (Credit: Patrick Kozak)

Ayers’ and his colleagues’ results confirm the validity of monazite crystals in identifying and obtaining dates for previously untenable rocks altered by hydrothermal events. However, geologists will have to reinterpret results based on the assumption that monazite was not affected by hydrothermal events.

Worm’s eye view without enhancements (Credit: Miranda Loflin)

Worm’s eye view without enhancements (Credit: Miranda Loflin)

Further reading

Dr. John C. Ayers
http://www.vanderbilt.edu/ees/johncayers