THE AGE AND HYDROLOGICAL HISTORY OF BLUE LAKE, SOUTH AUSTRALIA

Three sediment cores from the Blue Lake, a groundwater fed lake of vol canic origin in South Australia, have been investigated using a range of chemical and isotopic parameters. The C-14 activity of both the ino rganic and organic carbon fractions of the sediment decreases monotoni cally with depth. The rate of change with depth is greater for near-su rface samples with an apparent hiatus in sedimentation rate at about 7 000 yr B.P. Estimates of age for the precipitated authigenic carbonate , after correction for dilution with dead carbon from the groundwater, agree well with calculated ages from the organic carbon fraction of t he sediment. We suggest the lake is much older than previously propose d using other dating techniques. Variations in the delta(13)C and delt a(18)O composition of the authigenic carbonate reflect different resid ence times of dissolved inorganic carbon and water in the lake caused by changes in the lake level. During periods of hydrologic steady-stat e, it is suggested that relative changes in the temperature of the lak e can be seen in delta(18)O changes in authigenic carbonate. Blue Lake has been undergoing sedimentation for at least 28,000 years, includin g two lengthy periods of hydrologic steady state. The lake, for a larg e proportion of its existence, was much shallower while for the last 7 000 years has maintained a level close to the present one. The influen ce of pumping from the lake for urban water supply during this century is reflected in the isotopic composition of carbonate in the sediment .