Erich Osterberg and I spent the past week along the ice margin in Thule, Greenland, working with a variety of methods: lake coring, cosmogenic nuclide dating of glacial erratic boulders, and radiocarbon dating of fossil vegetation. This work will ultimately become part of my thesis, and was a great way to investigate the Thule area from a variety of angles. Our goal is to study the Holocene (the most recent ~10,000 years) climate history of Thule, and in particular a period of warm climate ~6,000 years ago (called the Holocene Climatic Optimum).
We collected sediment cores from two lakes near Thule. We can investigate various parameters within the lake sediment (for example, the amount of organic material, the size of the sediment, etc) and make inferences about climate of the past. We took cores from two very different lakes in close proximity. The first lake was fed by a large inflow, and therefore has likely experienced a high sedimentation rate throughout the Holocene; accordingly, our 1-m length corer wasn’t sufficient to reach the beginning of the sediment record. The second lake did not have an inflow, and therefore has likely experienced a low sedimentation rate. The cores we collected from this lake were very short, suggesting that the entire Holocene history of the lake is preserved in only a few tens of cm of sediment. We brought home four cores in all, and will analyze and interpret them over the coming year.
To get an idea of the timing of ice margin retreat, we collected samples for cosmogenic nuclide dating in many areas across the landscape near Thule. When an ice sheet retreats (like at the end of the last ice age), it drops large boulders on the landscape. As these boulders are uncovered, they become exposed to the sky and are bombarded by high-energy cosmic radiation from outside our solar system; this high energy radiation causes the formation of beryllium-10, a rare isotope that is not formed through other mechanisms on Earth’s surface. If we know the rate at which beryllium-10 is produced in rocks (only about 5 atoms per gram of quartz per year), and we can quantify the present-day abundance of beryllium-10, we can calculate the time since exposure (i.e. the time that the ice margin last retreated from this area). During our last week in Greenland, we sampled the tops of ~25 large boulders, spread across the land surface. We’ll use this data to determine the timing, and maybe even the rate, of ice sheet retreat at the end of the last ice age.
Finally, our third mission was to collect samples for radiocarbon dating, which allows us to determine the age of fossilized organic material. At several points during the Holocene, the climate warmed and the ice margin probably retreated upward from its present-day position. A receded ice sheet would have allowed vegetation to colonize the previously-covered areas. When climate cooled again, the ice margin would have re-advanced and covered the vegetated surface. We explored several areas of the ice sheet margin near Thule, and found fossilized organic material preserved in two locations. In some cases, this material was melting directly out of the ice. We’re guessing that this material may date to the Holocene Climatic Optimum or the Medieval Warm Period, and hopefully we’ll know by this fall or winter.
All in all, we had a very successful week of work along the Thule margin. We also took some time for fun. On my birthday (July 17th), Erich took me to “D-Launch”, an abandoned missile launch silo on Thule Air Base. Most of the infrastructure is underground, in the permafrost zone, and so all of the rooms are filled halfway with ice. We had to climb down ladders with headlamps to get in, and totally creeped ourselves out while we poked around.
Alas, my 2011 field season has sadly come to an end. Thanks for following the progress of the Dartmouth Thule crew! We’ll continue to post updates as we get data.