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Not exposure-dating at the western Antarctic Peninsula

May 19, 2010

The basic concept of exposure-dating of glacial deposits is that glaciers and ice sheets quarry rock at their beds where it has never been exposed to the cosmic-ray flux, then transport it to their margin and deposit it. Where there is ablation at an ice margin, this englacial debris melts out of the ice and becomes exposed to cosmogenic-nuclide production. Subsequently the exposure age of this debris can tell us the length of time since the ice margin occupied the site, which enables us to reconstruct ice sheet changes over time. This method works extremely well in most of Antarctica — and is now responsible for most of what we know about LGM-to-Holocene Antarctic ice sheet change — because once fresh debris is exposed on a rock surface by ice surface lowering, it is generally not covered by snow (because Antarctica is mostly a cold and windy place where most ice-free areas see very little snowfall or accumulation) or disturbed by geomorphic processes (because bioturbation, aqueous weathering, and soil creep are largely absent in Antarctica). This allows us to go to nunataks that stick out of the middle of the ice sheet, collect glacially transported debris at various elevations, determine its exposure age, and put together a history of ice surface lowering. Here is an example from the Ford Ranges of Marie Byrd Land (described in a paper by John Stone):

The red squares show the location of exposure-age samples and the plot at left shows the exposure-age/elevation relationship. The important thing here is that these nunataks are bare of snow or soil cover. Once glacial debris is deposited by ice retreat from a particular surface elevation, it stays put and accurately records the deglaciation history.

So that’s why exposure-dating works so well in most of Antarctica. I spent much of last winter on a ship near the Antarctic Peninsula trying to carry out a similar study on the east side of the Peninsula. This is an area where we have very little information about the actual LGM-to-present deglaciation chronology: only the very broad outlines are know from radiocarbon-dated marine sediments. It should be possible to do a much better job with exposure-dating, because the geomorphology of the east side of the Peninsula is similar to the rest of Antarctica in that  ice-free nunataks separate major outlet glaciers: as the surface of these glaciers lowered, glacial debris should have been preserved more or less undisturbed. This view looks east from the center of the Antarctic Peninsula over the east-side outlet glaciers:

Once you get off the summit ice cap, there are many ice-free areas where we can reasonably expect exposure-dateable glacial debris to be preserved.  So this project should go very much like other, mostly pretty successful, exposure-dating efforts in Antarctica.

The west side of the Peninsula is totally different. This is also an area where we would really like to be able to learn about the deglaciation chronology through exposure dating. Again, the overall deglaciation chronology is known in broad terms from marine radiocarbon dates, but there is very little information about the history of ice thickness change, which of course is the important thing from the point of view of sea-level impacts. However, the west side of the Peninsula is notable because of all the world’s glacial landscapes, it is the one where we have the least chance of using exposure dating to learn anything about past glacier change.

Here is what the west side of the Peninsula looks like:

This is cruise-ship Antarctica, where massive glaciers calve into crystal blue water filled with frolicking penguins. From the perspective of LGM-to-present ice sheet history, this landscape is deglaciated. The marine geology shows very clearly that all the open water visible in this image was filled with thick flowing ice at approximately the LGM. However, from the perspective of exposure-dating, almost nothing in this landscape is exposed. Nearly all rock surfaces are covered with tens to hundreds of meters of ice:

This is a consequence of the nearly unique climate in this region: the west side of the Peninsula is exposed to the southern hemisphere westerlies and receives large amouts of precipitation, but the temperature is cold enough that much of the snow and ice accumulation can remain frozen to rock surfaces. There is almost nowhere else in the world where a landscape this rugged is so comprehensively ice-covered. The only rock surfaces that are not ice-covered are nearly vertical, and these vertical faces are calving rock almost as fast as the adjacent glaciers are calving ice. Sure, maybe that is a little bit of an overestimate, but we observed daily rockfall on many of these faces. Here is a large fresh rockfall that postdates the snow that fell 24 hours before this photo was taken:

It is pretty clear that even if some glacial deposits were present on these near-vertical faces at the time the ice sheet surface lowered through this landscape, they’re not there now.

The only relatively flat ice-free areas in this landscape are small islands and peninsulas very close to sea level:

And, as evidenced by the granite erratic in the center of the photo, these sites do preserve glacial deposits that record occupation of this fjord by through-flowing ice. The difficulty is that these tiny ice-free sites are  fundamentally no different from nearby sites that are covered by ice caps:

so there is no way to know whether the exposure age of an erratic on this tiny bit of rock records the time that major fjord glaciers retreated from their LGM positions (which we would like to know) or the time that the last 20-meter-thick bit of ice slid off the outcrop in an unusually warm summer (which would be sort of interesting, but not very relevant to the main problem). So to summarize, even though the western Antarctic Peninsula is deglaciated in a sense, there are few if any rock surfaces that were permanently exposed by this deglaciation. Those rock surfaces that are likely to have been exposed since deglaciation are nearly vertical and demonstrably disintegrating at extreme rates. This is one of the few glacial landscapes in the world where cosmogenic-nuclide exposure-dating is unlikely to help make any progress on understanding past glacier and ice sheet change.

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