Isotope Study of Depositional Environment in East Queen Maud Land Antarctica

Firn cores from East Antarctica were analyzed regarding their depth hoar level, grain size, density, tritium concentration, δD and δ18O value so as to find the age of the firn cores and to study post-depositional changes of isotopic composition. In the 1.2m deep firn core collected at γ 1 Station, a clear stratigraphic boundary was observed at the 26cm depth. Above the boundary grain size of firn is small (2.5) and depth hoar is poorly crystallized (DHL=1). On the contrary, below the boundary the grain size is large (4-5.5) and depth hoar is well crystallized (DHL=3-4.5). This stratigraphic observation suggests the absence of semidimentation for a considerably long period. Tritium concentration in the firn above the boundary is 18-25TU, although less than 8TU below the boundary. Tritium concentration in the upper layer is almost equal to that in the present snow observed at Mizuho Station. From these results, it can be inferred that the lower layer was formed before 1950 prior to the nuclear test. The d-parameter changes from 11 to 18 and shows a good correlation with depth hoar level. This change in d-parameter suggests that in a layer lower than 30cm, 25-40% of firn is vaporized during post-depositional firnification. Tritium concentration in 10m deep firn core obtained at the Advance Camp Station ranges from 8TU to 301TU. From the surface to the 1.4m depth, the tritium content is 54-80 TU and increases rapidly up to 301TU (maximum) at 2.6m. Then, it decreases gradually to 17TU until the 4m depth, and is almost constant at a depth lower than 4m. This vertical profile reflects the temporal variation of tritium concentration in precipitation. The firn at 2.6m, having the maximum tritium concentration, is inferred to have been deposited in 1966. The accumulation rate at the Advance Camp Station is estimated to be 14cm/year