UC Irvine

Three sediment cores from the Blue Lake, a groundwater fed lake of volcanic origin in South Australia, have been investigated using a range of chemical and isotopic parameters. The 14C activity of both the inorganic and organic carbon fractions of the sediment decreases monotonically with depth. The rate of change with depth is greater for near-surface samples with an apparent hiatus in sedimentation rate at about 7000 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 the sediment. We suggest the lake is much older than previously proposed using other dating techniques. Variations in the δ13C and δ18O composition of the authigenic carbonate reflect different residence times of dissolved inorganic carbon and water in the lake caused by changes in the lake level. During periods of hydrologic steady-state, it is suggested that relative changes in the temperature of the lake can be seen in δ18O changes in authigenic carbonate. Blue Lake has been undergoing sedimentation for at least 28,000 years, including two lengthy periods of hydrologic steady state. The lake, for a large proportion of its existence, was much shallower while for the last 7000 years has maintained a level close to the present one. The influence of pumping from the lake for urban water supply during this century is reflected in the isotopic composition of carbonate in the sediment. © 1995.