Study of Johnsons Glacier (Livingston Island, Antarctica) by means of shallow ice cores and their tephra and by analysis of 137Cs content



With the aim of monitoring the dynamics of the Livingston Island ice cap, the Departament de Geodinàmica i Geofísica of the Universitat de Barcelona began yearly surveys in the austral summer of 1994-95 on Johnsons Glacier. During this field campaign 10 shallow ice cores were sampled with a manual ve rtical ice-core drilling machine. The objectives were: i) to detect the tephra layer accumulated on the glacier surface, attributed to the 1970 Deception Island pyroclastic eruption, today interstratified; ii) to verify wheter this layer might serve as a reference level; iii) to measure the 137Cs radio-isotope concentration accumulated in the 1965 snow stratum; iv) to use the isochrone layer as a mean of verifying the age of the 1970 tephra layer; and, v) to calculate both the equilibrium line of the glacier and average mass balance over the last 28 years (1965-1993). The stratigraphy of the cores, their cumulative density curves and the isothermal ice temperatures recorded confirm that Johnsons Glacier is a temperate glacier. Wind, solar radiation heating and liquid water are the main agents controlling the ve rtical and horizontal redistribution of the volcanic and cryoclastic particles that are sedimented and remain interstratified within the glacier. It is because of this redistribution that the 1970 tephra layer does not always serve as a very good reference level. The position of the equilibrium line altitude (ELA) in 1993, obtained by the 137Cs spectrometric analysis, varies from about 200 m a.s.l. to 250 m a.s.l. This indicates a rising trend in the equilibrium line altitude from the beginning of the 1970s to the present day. The varying slope orientation of Johnsons Glacier relative to the prevailing NE wind gives rise to large local differences in snow accumulation, which locally modifies the equilibrium line altitude. In the cores studied, 137Cs appears to be associated with the 1970 tephra layer. This indicates an intense ablation episode throughout the sampled area (at least up to 330 m a.s.l), which probably occurred synchronically to the 1970 tephra deposition or later. A rough estimate of the specific mass balance reveals a considerable accumulation gradient related to the increase with altitude.

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