TEPHRA ANALYSIS OF SEDIMENTS FROM MIDGE LAKE (SOUTH SHETLAND ISLANDS)AND SOMBRE LAKE (SOUTH ORKNEY ISLANDS), ANTARCTICA

Lake sediment cores from Midge Lake, Livingston Island, South Shetland Islands and Sombre Lake, Signy Island, South Orkney Islands were anal ysed for volcanic tephra using light microscopy and magnetic susceptib ility. Cores were dated using published C-14 and Pb-210 chronologies. Electron probe microanalyses of discrete tephra glass shards were unde rtaken to characterise the tephra geochemically in order to identify p ossible source volcanoes and refine tephrochronological data for the r egion. Results identified five tephra horizons in a core from Midge La ke. Four of these tephra at 3-4 cm, 8-9 cm (c. 450 yr BP), 15-16 cm (c . 755 +/- 1.05 yr BP) and 21-22 cm (c. 1340 +/- 100 yr sp) consisted o f sodic basaltic to basaltic-andesitic glasses, containing abundant la bradoritic feldspar inclusions, and a single 'acidic' tephra was found at 2-3 cm. Seven tephra horizons were identified in the Sombre Lake c ore including three basaltic tephra at 3-9 cm (30 +/- 4 yr BP to 125 /- 25 yr BP), 31-32 cm and 44-46 cm (1325 +/- 50 C-14 yr BP) and four acidic tephra at 21-22 cm and 24-25 cm, 33-36 cm (c. 1021 C-14 yr BP) and 54-56 cm(c. 1450 C-14 yr BP). These are the first tephra to be ide ntified from the South Orkney Islands. Geochemical and grain size anal ysis indicated that the analysed Midge Lake tephra were derived from t he Quaternary Deception Island volcano. Smaller grain sizes, congruent geochemical data and prevailing wind directions also indicate this vo lcano as the Likely source of Sombre Lake tephra. Results highlight th e importance of establishing geochemical consistency between tephra de posited across wide geographical areas, during apparently synchronous time periods, if they are to be used in a regional tephrochronology.