By Dr Brenda Hall, Climate Change Institute, University of Maine
The goal of our project is to examine glacial deposits in and around cirques at Mt. Usborne in order to gain a better understanding of the glacial and climate history of the Falkland Islands.
The Black Tarn, Mt. Usborne, East Falkland
Specific Activities Conducted:
From March 16-19, 2014, a field party of three carried out glacial geologic field work in the Black Tarn area of Mt. Usborne. Field members consisted of Drs. Brenda Hall and Thomas Lowell, both from universities in the United States, and Mr. Antony Smith of Discovery Falklands, who provided logistics and a wealth of local knowledge. The party arrived at a camp site ~750 m from the Black Tarn on March 16 and was able to spend the afternoon carrying gear to the pond and making a bathymetric map (Fig. 1). The map was constructed using a portable depth sounder and by making transects across the lake in an inflatable row boat. Maximum depth recorded was ~10 m. This was less than indicated previously (McAdam and Roberts, 1981, Falkland Islands Journal, p. 23-28) by ~ 3 m, but no deeper area could be found. During this time, the field team also made preliminary observations on the glacial geology surrounding the tarn.
On March 17, the remaining gear was carried to the field site and Drs. Hall and Lowell began coring. A piston coring system was set up from the inflatable boat anchored over the deepest part of the pond. Briefly, this consisted of a polycarbonate tube with a piston that moved up the tube as it was pushed into the mud. The piston provided suction that kept sediments in the tube and allowed recovery. The corer was deployed using a rope. The initial coring drive was successful and a little over one meter of sediment was recovered (Fig. 2). This sediment shows some structure and changes in both color, composition, and grain size and will be the subject of future reports. Without laboratory analysis, it is impossible to say much for certain, but it seems as if the sediments record several wet and dry periods, the timescale of which will become clear as analyses progress. We extruded the core and took subsamples for analysis. We then attempted to take a second meter of core as we had not hit bedrock with the first drive. However, this attempt did not prove successful. We penetrated to two meters depth, but the sediments did not remain in the tube when it was pulled out. This is due mostly to the fact that we needed a different type of equipment than we had with us. The one previous coring trip to the area in the 1970s (McAdam and Roberts, 1981) had retrieved only 45 cm of sediment before meeting refusal, so we had not expected such thick sediment sequences. In the future, bringing a different type of equipment would allow us to recover this sediment and a longer climate record.
On March 18, high winds prevented us from working on the lake. We remained on shore and sampled sediments immediately adjacent to the lake using the same piston core technique. We were able to penetrate nearly three meters and retrieve silt identical to that from our lake core. This core also was subsampled. On March 19, due to increasingly bad weather and the rapidly deteriorating ground on the route out, we packed up camp and returned to Stanley.In general, except for the first day, weather conditions were wet and at freezing. Ground conditions for accessing the site by Land Rover were much worse than anticipated and a function of a rather wet March. Despite these issues, we were able to camp within a short walk of the tarn and were able to meet our scientific goals. We are excited, because there proved to be a lot more to the Black Tarn record than expected based on previous work. Our task now is to analyze the samples, particularly for radiocarbon dating, to begin to assign a timeline to the changes in sediment types that we see in the cores. At present, our best guess is that these cores span time on the order of thousands to tens of thousands of years. Samples have been submitted to the accelerator laboratory for radiocarbon dating, and we expect results in about two months.
Fig. 1. Bathymetry of the Black Tarn obtained by repeat transects with an electronic depth sounder.
Fig. 2. Core BT-14-1, from the Black Tarn, consisting of 1.2 m of sediment. From the base, the sequence consists of gray clay overlain by moss, overlain in turn by a thick layer of tan-gray silt. This is overlain by moss and then by a sticky gray silt layer. The entire sequence is capped by stiff, orange, sandy silt.