RADIOMETRIC DATING OF SEDIMENT FROM LAKES OHAU AND TEKAPO, NEW ZEALAND

In this paleoclimate study we employed ¹⁰Be to correct for sedimentological variation in the excess ²¹⁰Pb used to obtain sediment accumulation rates. ¹³⁷Cs (half-life = 30.2 years), from atmospheric nuclear weapons testing, and ²¹⁰Pb (half-life = 22.3 years), produced by the decay of atmospheric ²²²Rn gas, are widely used for dating sediment. Using our accelerator mass spectrometer, we measure the cosmogenic isotope ¹⁰Be (half-life = 1.36 x 10⁶ years) and use it as an adjunct for the ²¹⁰Pb method and to identify changes in sedimentation rate. ²¹⁰Pb, via ²¹⁰Po, and ¹³⁷Cs, were measured using alpha and gamma spectrometry, respectively.

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.