RADIOMETRIC DATING OF SEDIMENT FROM LAKES OHAU AND TEKAPO, NEW ZEALAND
Lacustrine sediment incorporates atmospheric 210Pb, termed excess or unsupported, that decreases with depth due to radioactive decay, hence the 210Pb dating method. There is a background of 210Pb, supported by 226Ra (via 222Rn) within the sediment, to deduct from the total measured 210Pb. Ideally, each sediment sample would be completely decomposed and the total 210Pb and 226Ra, i.e. background, measured. However, 210Pb activities in New Zealand sediments are low, 5-10% of continental sites so the difference between unsupported and supported 210Pb is relatively small, making 210Pb dating difficult. Our approach is to leach meteoric 10Be, excess 210Pb, and a minimum of supported 210Pb from the sediment and then leach more strongly to obtain the supported 210Pb profile and assess the sediment solubility. Although the sediment composition is similar throughout the core, the supported 210Pb background varies with depth and is partly dependent on the sediment solubility.
We find that the excess 210Pb/10Be value decreases more systematically with depth than the excess 210Pb because using the ratio of two isotopes input from the atmosphere tends to compensate for influxes of sediment that are different in composition. The 10Be will not decay perceptibly over a period of ~ 120 years (the limit for 210Pb dating) so it would not decrease significantly with depth if the sedimentation rate were uniform. In the cores from Lakes Ohau and Tekapo, the 10Be profiles have gradients and regular fluctuations that are not due to 11-year solar cycles but they may be climate related. Overall, 10Be is useful in the study of lacustrine sediment in the short as well as long term and ideal for dating using the inventory.