Application of natural and artificial fallout radionuclides to determining sedimentation rates in New Zealand lakes

Abstract

Sedimentation rates have been estimated for New Zealand Lakes Rotoiti, Taupo, Ngahewa, Tikitapu, Okareka, Pukaki, Rotorua, Waikaremoana, Wakatipu, Tekapo, and Ohau, and also the Falls Dam, Central Otago, the Pauatahanui inlet (Wellington), and Lake Vanda (Antarctica). Sedimentation rates were able to be calculated using the 210Pb and 210Po methods in 26 out of 28 cores. For the 137Cs method (shown equivalent to the previous methods) calculable rates were obtained in 13 out of 19 cores. An age estimation was always possible by one of the methods. Values of 147Pm and 144Ce showed that most surface layers were undisturbed, hence the hypothetical mixing often invoked to explain 137Cs profiles lacking the expected 1965 peak is incorrect for most of these cores. Deficits in surface layer 210Pb caused by 222Rn loss did not affect the estimates of sedimentation rate. One core contained a record dated back to 1800 A.D. Deposition rates of 210Pb depended on rainfall, not river input. Some local geothermal activity emits 222Rn and deposits more 210Pb than usual for New Zealand lakes, but otherwise absolute 210Pb levels are generally very low compared to other studies. Deposition rates varied from a low of 0.11 kg m‐2 yr‐1 (Lake Tikitapu) to a high of 28 kg m‐2 yr‐1 (one Lake Pukaki core).