Skip to content

Why are there no exposure-age data from glaciated Mexican volcanoes?

November 8, 2019

In a previous post a while ago, I used this diagram, which is a fairly famous (at least among glacial geologists) representation of the changes in the extent of mountain glaciers worldwide between the last glacial maximum (LGM) about 20,000 years ago and the present time, and is generally thought to have first been put together by the late glacial geologist Steve Porter.

This particular version appears in an article in Scientific American by Wally Broecker and George Denton. It shows a topographic profile down the North American Cordillera that forms the eastern edge of the Pacific Ocean, and highlights the difference between the equilibrium line altitudes of modern glaciers (red) and glaciers at the LGM (blue). I was using the diagram to highlight the impressive amount of cosmogenic-nuclide exposure age data that have been collected from glacial deposits worldwide and are tabulated in the ICE-D:ALPINE database; the blue dots added below show the locations of exposure-dated alpine glacial landforms in the mountains of North and South America as well as the same sector of Antarctica.

There are a lot of measurements here. Researchers who collect these data have focused for many years on visiting glaciers across the entire range of latitude, with the goal of learning about latitudinal variations in temperature and heat transport during the LGM and major late-glacial climate changes like the Younger Dryas and the Antarctic Cold Reversal. Nearly all the areas that Denton and Broecker specifically highlighted in this figure, as areas where prominent LGM-to-present glacial deposits could be found, have been visited and thoroughly studied.

With one exception. The “Mexican Volcanoes.” In fact, there is a pretty big gap between southern California (the southern end of the Sierra Nevada in the image) and Colombia (beneath the “America” in “Central America”). There are at least a few measurements that exist from this range of latitude that I know about that are not at the moment included in the database (mainly because of growing pains having to do with dynamically calculating chlorine-36 exposure ages); they are from Costa Rica and described in this dissertation. However, the reason that “Mexican Volcanoes” appear as a specific callout in this figure is because there are glacial deposits there, and they were fairly well studied in the 1980’s by Klaus Heine and others, as described, for example, in this paper.  But no one seems to have been there for exposure-dating purposes. Get to it, people! This is the only significant gap left in the Porter transect. It is easy to see why there are very few exposure ages on glacial deposits from, say, trans-Baikalia or interior Yukon, but the Mexican volcano sites are near major cities that you can drive to, in your own car, in only a few days’ drive from major cosmogenic-nuclide labs in the US. According to the Heine papers, there are enough radiocarbon dates on some of the moraines that they could potentially be used as production rate calibration sites, which could be useful. And if you are interested in tropical glaciers, the radiocarbon dates in that paper indicate that there are late-glacial moraines and ~30-35 ka moraines, but possibly not any standard-issue 18-25-ka-LGM moraines. Is this really true? Are there boulders enough to figure it out? You could find out.



No comments yet

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: