In an earlier post (Reading the Landscape), I described how we study landscape features in northwestern Greenland to try to reconstruct past events. For example, we can use glacial moraines to make inferences about the extent of past re-advances of the ice sheet or its outlet glaciers. This type of work focuses on the larger perspective: the landscape as a whole.
This past week, we’ve shifted out focus to investigating specific areas in much more detail. In particular, we dig pits in the ground and study the sediments we find; we call this studying the stratigraphy. Reading a record of past sediments is similar to reading an ice core: sediments get laid down progressively over time, with the oldest sediments being on the bottom and the youngest sediments being on the top. We can use the sequence of sediments to make inferences about how the area has evolved over the recent geological past, since the end of the Last Glacial Period.
A big excavation project: we dug this 1.5-meter tall section into a stream bank near the fjord in Thule.
Some of the areas we excavated have yielded an interesting story. The sediment on the bottom appears to be mostly silt and contains an abundance of marine shells. On top of that is a thick sequence of nicely layered sands; these layers are sometimes folded or truncated. The top unit in the sequence is an unlayered sandy material with abundant large rocks that appears to be glacial till.
We found shells! This one was embedded in the silty/sandy material ~30 cm from the bottom in the section shown above.
My mission, after returning home from Thule, is to analyze the data from this work and try to unravel the sequence of events. For example, one possible scenario is that relative sea level was higher after the end of the Last Glacial period, depositing the shell-rich and sandy layers in an area that is now 20-30 m above present-day sea level. Later, the glaciers in the fjord re-advanced over the marine sediments and covered the landscape in glacial till. In addition to interpreting the sequence of sediments, we can also use radiocarbon dating to determine the age of the shells and cosmogenic dating to determine the age of the boulders in the glacial till. Using these techniques will allow us to understand how the landscape in northwestern Greenland has evolved during the present interglacial period.