• Harvey Thorleifson, Chair
    Minnesota Geological Survey
  • Carrie Jennings, Vice Chair
    Minnesota Geological Survey
  • David Bush, Technical Program Chair
    University of West Georgia
  • Jim Miller, Field Trip Chair
    University of Minnesota Duluth
  • Curtis M. Hudak, Sponsorship Chair
    Foth Infrastructure & Environment, LLC


Paper No. 1
Presentation Time: 9:00 AM


DITCHBURN, R.G.1, BARRY, B.J.1, GRAHAM, I.J.2, LEVY, R.3, VANDERGOES, Marcus J.4 and ZONDERVAN, A.5, (1)National Isotope Centre, GNS Science, P.O.Box 31-312, Lower Hutt, 05010, New Zealand, (2)Research Office, GNS Science, P.O.Box 31-312, Lower Hutt, 05040, New Zealand, (3)Paleontology, GNS Science, P.O. Box 31-312, Lower Hutt, 05040, New Zealand, (4)Geological Rescources Division, GNS Science, P.O. Box 31-312, Lower Hutt, 05040, New Zealand, (5)National Isotope Centre, GNS Science, P.O.Box 31-312, Lower Hutt, 05040, 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.

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