Part of NSF IPY proposal: Abrupt Environmental Change in the Larsen Ice Shelf System
Erratics and their exposure ages:
Our goal in this part of the project is to learn about past ice sheet elevations by finding evidence of past ice sheet positions above the present ice surface, and dating it. In order to do this, we need two things: first, some deposit that marks where the ice was in the past; second, some means of determining its age.
The photos below give an idea of what sort of deposits can serve this purpose. In some parts of Antarctica, in particular "dry Antarctica," the parts of the Transantarctic Mountains where significant ablation occurs at many ice margins and therefore large quantities of morainal debris can accumulate, it's common to find large and obvious lateral and terminal moraines that mark former ice sheet positions. Conditions in the Antarctic Peninsula region are nothing like this, however: nearly all glaciated areas are sites of snow accumulation and we expect that prominent moraines will be extremely rare. We expect instead that ice-free surfaces will much more closely resemble those in coastal West Antarctica, where glacially eroded bedrock surfaces are covered by a thin scatter of glacially transported debris. The photo above and right shows an example of this situation from the Ford Ranges, near Sulzburger Bay in Marie Byrd Land.
These erratics -- 'erratic' meaning that the rock type isn't found locally and hence must have been delivered to the site by ice -- tell us that the ice surface must have been higher in the past to have deposited them. The photo at right gives another example from the Ford Ranges. It's a granite cobble on a phyllite nunatak, so we know it's from away, as they say in Minnesota. We can then use cosmogenic-nuclide dating methods to determine the the exposure age of this cobble, which tells us the time since it was first exposed by melting out of the ice margin, that is, the time since the ice surface was at this elevation. Exposure-age dating is generally the most effective way to do this sort of thing, but occasionally one can use other means. For example, one often finds melt pools at the ice margin in which algae grow in the summer -- given an abandoned one of these associated with an past, higher, ice margin, the radiocarbon age of the algae tells you the age of the ice margin.
Can we do this in the Larsen Embayment?
The important thing for the present proposal is that, if we're to have a reasonable chance of doing this in the Peninsula region, two things have to be true: First, we need to find ice-free areas adjacent to the ice whose past elevation we care about (in this case the major glaciers that flow into the Larsen ice shelves), where we can find ice-marginal deposits. Second, these rock outcrops must not be too steep to accumulate erratics and other ice-marginal debris -- in our experience erratics and other dateable material are rarely found on slopes greater than approximately 30-40 degrees. As much of the rock exposed adjacent to the major glaciers in the Antarctic Peninsula consists of ice-marginal cliffs, this is a potentially serious concern.
Given these requirements, in preparation for this proposal we carried out a search of air photos from the U.S. Antarctic Resource Center photo library to find out if there existed suitable ice-free areas for the exposure-dating part of the project. These main project page summarizes the results of this effort.