CHARACTERISTICS AND ORIGIN OF THE LAKES AND PONDS OF CLEARWATER MESA, JAMES ROSS ISLAND, ANTARCTIC PENINSULA

LIRIO J.M.; CORIA SH; LOPEZ SNB; VIGNONI P.A.; KOHLER TJ; KOPALOVÁ, K; CHAPARRO M.A.E.; LECOMTE K.L.; NÝVLT D; MAC CORMAC W.P.

Congreso; 12th International Symposium on Antarctic Earth Sciences (ISAES XII); 2015

James Ross Island is located NE of the Antarctic Peninsula, and has important ice-free areas with numerous lakes and ponds. Clearwater Mesa is located at the SE side of Croft Bay, 250 m above sea level, is 8 km2 in area, and represents a prominent volcanic mesa comprised of James Ross Island Volcanic Group rocks. This meseta has a very flat plateau, with vertical variability of no more than 30 m from the valley floors to the hilltops. The cliff-lined coasts of Clearwater Mesa prevent access to the area from the sea. As a result, more than 50 lakes and ponds unexplored by scientists are present on the mesa. In January 2015, travel by helicopters made a field campaign possible to study this pristine and poorly known area. To document and better understand the physical processes and evolution of the Clearwater Mesa environment, we study the geology, chemistry, and associated flora and fauna of each waterbody in this important site. Each lake and pond was named, and area, perimeter and altitude were obtained using GPS. Additionally pH, conductivity and temperature were measured and depth estimated. Each basin was additionally sampled for biological and hydrochemical parameters. In the biggest lake, named Katerina, a bathymetric map was created, and a core was taken in order to reconstruct environmental history and evolution of the lake, and biological and microbiological content. We found that the lakes and ponds are located within depressions formed by glacial erosion of the ice cap that previously occupied the meseta. A few lakes were found at lower altitude and close to the two glaciers surrounding Clearwater Mesa, located in depressions formed by glacial deposit. The water bodies are shallow and generally fed by snow melt. A NW-SE orientation of the lake?s long axis prevails, similar to the elongated hills on the meseta. Stony rims are very common in the lower and humid surfaces of the depressions, especially in lake and pond coasts. Some ponds are actually inundated stony rim areas, and the biggest lakes used to be shallow open basins, and are no more than 2 m deep. Water bodies are connected by small streams with similar water chemistry, and visible fauna and flora. During the visit, no streams draining the lakes reached the cliffs, and instead disappear underground into a basement made by volcanic rocks. Waterbodies exhibited a gradient in conductivity, with closed basin lakes having the highest values and show white salt deposits on their coasts, evidencing a previous higher level. The observed pH of lakes was much less variable, and was neutral to slightly basic. In the south sector of Clearwater Mesa, the hill altitudes are higher (282 m above sea level at Cerro Pablo), and the valley is U-shaped, with a NW-SE orientation. On the bottom of the valley it is possible to find rounded basalt clasts up to 70 cm in diameter, suggesting the presence of fluvial transport and the existence of previous streams, now absent, that drain the meseta as the ice cap that occupied the meseta melted. The lakes have few clastic inputs, and the bottoms are covered with high biomasses of cyanobacteria and green algae which support the fairy shrimp Branchinecta gaini, useful for paleoenvironmental reconstruction. Previously found only in lake cores from James Ross Island by Bjorck et al. 1996, and supposed to be extirpated from James Ross Island, this species was present in the majority of lakes and ponds studied here, as well as in the Ulu Peninsula during the lake surveys of the last years. At the top of the meseta, the presence of erratic striated boulders of igneous and metamorphic composition, provide evidence that glaciers from the Antarctic Peninsula had previously overrun the meseta. Physical weathering (congelifraction) has been important, and as a result the volcanic rock is split and broken in angular clasts of several cm in diameter. These angular volcanic rocks cover vast areas of the plateau and are material source for the formation of the stony rims very common in Clearwater Mesa. The prevailing SW wind contributes to rock erosion, snow drift accumulation, humidity and flora distribution: the luf-side of the rocks is generally dry and affected by wind abrasion. On the other hand, the lee sides allow snow drift accumulation, is more humid, and the arborescent lichens grow up protected from wind abrasion. Clearwater Mesa has more than 50 lakes and ponds, each of them with a flora typical of the Antarctic Peninsula. Interestingly, Branchinecta gaini, a crustacean thought to be extirpated from James Ross Island, was found on the majority of the waterbodies. Better understanding the evolution of the lakes and associated biota here could help in disentangling the evolution of other areas in Antarctica now covered by ice and improve the environmental interpretation of lakes cores obtained in the area. Furthermore, Clearwater Mesa has been found an important area with high persistence under future climate changes based on the RACER analysis and supported by the expert opinion of our group.