LESSONS LEARNED FROM A RECYCLED LANDSCAPE: BOULDER AGES IN EAST GREENLAND

Significant insights about glaciation derive from glacial sediments, and Joe Hartshorn brought those lessons to New England. However, those sediments have to be derived from older sediments or from bedrock. We present insights about glacial erosion from a study of moraine chronology in East Greenland. As often happens in polar landscapes, cosmogenic dating of moraines yielded apparent landform ages that are older than expected. Moraines formed during the Little Ice Age yield ages up to 3,000 yr old. Moreover, this problem occurs to a greater degree on older moraines. Here, we use these data to make inferences about the style of glacial erosion in East Greenland, and we apply these insights to New England.

Numerical modeling reveals the relationship between pre-exposure duration and the depth of glacial erosion. Before glaciation, landscapes accumulate cosmogenic nuclides. Glaciation removes part or all of these accumulated nuclides. On cumulative density plots of cosmogenic exposure dates, the depth of glacial erosion controls the curvature of the plot, whereas the duration of pre-exposure controls the range of the observed exposure dates. Inverse modeling of cosmogenic exposure dates in East Greenland suggest effective erosion depths up to a few meters, indicating the landscape is not reset.

In New England, on-going cosmogenic exposure dating may likewise produce data on effective erosion depth. For a dated feature, any scatter that exceeds the analytical uncertainty may indicate an inherited component in the exposure dates, and thus incomplete glacial resetting of the landscape. Recent exposure dates from the Martha's Vineyard moraine have a structure very similar to that of our exposure dates from East Greenland. We take this similarity as evidence of a strong inherited component in the cosmogenic exposure dates from the Martha's Vineyard moraine, though we also see evidence of some post-depositional erosion. We anticipate that further exposure dating in New England will produce other, similar data sets, perhaps allowing us to determine the pattern of glacial erosion in New England.

Our work thus parallels the seminal studies of Joe Hartshorn, who examined the glacial record in New England in light of his prior work in Greenland.