Air photo reconnaissance of exposure dating sites in and around the Larsen B Embayment

Part of NSF IPY proposal: IPY in the Antarctic Peninsula -- Ice Shelves, Oceans, Climate, and Biota

Inquiries about this page to: Greg Balco, balcs@bgc.org

Scatter of erratic cobbles on a recently deglaciated surface in the
Ford Ranges of
Marie Byrd Land, coastal West Antarctica.

A brief introduction.
Granite erratic at Mt. Byrd, Ford Ranges.

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 at right 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. It turns out that there's a surprising amount of promising-looking rock outcrop in the Larsen B embayment.

On the map at right, the dark circles indicate areas where we've identified promising exposure-dating sites from low-elevation air photos. Clicking on these circles will link to the actual air photos farther down on this page.

The grey circles and boxes are areas that we've examined on satellite images and verified that there are significant ice-free areas adjacent to the major glaciers, but have not yet looked at on oblique air photos. An example Landsat image of rock outcrops near the Sjogren Glacier (indicated by the grey box on this image) is also farther down on this page.

Drygalski Glacier looking west. Many suitable outcrops near bottom of glacier indicated by arrows.

U.S. Navy photo TMA 2143, 312R

Hektoria Glacier looking E. Small outcrops throughout glacier.

U.S. Navy photo TMA 2153, 284L

Closer view of Hektoria Bluffs near bottom of glacier (indicated by 'A' in image above). Ice shelf breakup has made this outcrop accessible by boat.

U.S. Navy photo TMA 2157, 66R

Crane Glacier, looking east (down-glacier). Large, low-angle, ice-free area at mid-glacier. Extremely promising-looking site.

U.S. Navy photo TMA 2153, 312L

Lower Crane Glacier, looking west (up-glacier). Upstream arrow shows outcrops in image above. There are also a number of promising-looking sites on the south side of the lower glacier, indicated by the downstream arrow.

U.S. Navy photo TMA 2157, 50L

Melville Glacier looking west (up-glacier). Promising outcrops at mid-glacier.

U.S. Navy photo TMA 2157, 46L

Glaciers near Melville Glacier are named after the crew of the Pequod. This is Starbuck Glacier looking west (up-glacier). Promising outcrops at mid-glacier.

U.S. Navy photo TMA 2157, 38L

More Pequod crew members. This is Flask Glacier looking east (down-glacier). The relatively low-angle outcrop at the south side of the glacier mouth is a likely site.

U.S. Navy photo TMA 2157, 33R

Mouth of Leppard Glacier looking east (down-glacier). The nunatak at the south side of the glacier mouth is not only bordered by what appears to be a moraine actively forming on the ice surface at present, but shows a horizontal color change halfway up it's ice-proximal slope that suggests a moraine recording a former glacier highstand. This is a little easier to see on the full-size image.

U.S. Navy photo TMA 2157, 25R

Upper Leppard Glacier looking west (up-glacier). Relatively small outcrops throughout glacier.

U.S. Navy photo TMA 2157, 25L