APPLICATION OF NATURAL AND ARTIFICIAL FALLOUT RADIONUCLIDES TO DETERMINING SEDIMENTATION-RATES IN NEW-ZEALAND LAKES

Sedimentation rates have been estimated for New Zealand Lakes Rotoiti, Taupo, Ngahewa, Tikitapu, Okareka, Pukaki, Rotorua, Waikaremoana, Wak atipu, Tekapo, and Ohau, and also the Falls Dam, Central Otago, the Pa uatahanui inlet (Wellington), and Lake Vanda (Antarctica). Sedimentati on rates were able to be calculated using the Pb-210 and Po-210 method s in 26 out of 28 cores. For the Cs-137 method (shown equivalent to th e previous methods) calculable rates were obtained in 13 out of 19 cor es. An age estimation was always possible by one of the methods. Value s of Pm-147 and Ce-144 showed that most surface layers were undisturbe d, hence the hypothetical mixing often invoked to explain Cs-137 profi les lacking the expected 1965 peak is incorrect for most of these core s. Deficits in surface layer Pb-210 caused by Rn-222 loss did not affe ct the estimates of sedimentation rate. One core contained a record da ted back to 1800 A.D. Deposition rates of Pb-210 depended on rainfall, not river input. Some local geothermal activity emits Rn-222 and depo sits more Pb-210 than usual for New Zealand lakes, but otherwise absol ute Pb-210 levels are generally very low compared to other studies. De position rates varied from a low of 0.11 kg m(-2) yr(-1) (Lake Tikitap u) to a high of 28 kg m(-2) yr(-1) (one Lake Pukaki core).