M.A. Theses: Thomas A. Brown, 1991 (MSc)

10Be Production Rate Variations Over the Last Twenty Thousand Years as Recorded in a Lake Sediment

Records of the past variations of the global average 10Be production rate have been derived from measured concentrations of 10Be in two lake sediment cores. Since cosmogenic radioisotope production rates are dependent on the geomagnetic field strength, the derived 10Be records provide information on the constancy of this factor as well as the production rates of other cosmogenic radioisotopes such as 14C.

The 10Be concentrations of the lake sediments were determined by accelerator-based mass spectrometry. An age-depth chronology for the cores was determined using the 14C dating technique. The chronology showed that the records provided by the sediment cores spanned the last 20ky b.p. The current lack necessitated the use of reasonable assumptions concerning the variations of the 35-40N latitude-belt-averaged precipitation rate during the calculation of the global average 10Be production rates.

Overall, the derived records show that the long-term variation of the 10Be production rate did not exceed 40% over the last 20 ky b.p. Under one assumption, the 10Be production rate record obtained for the Holocene period correlated with the 14C production rate variations implied by the atmospheric 14C concentration record and the available paleomagnetic data. Under an alternative assumption, the derived Holocene period record was consistent with the conclusions of a recent study of 10Be concentrations preserved in a polar ice core. Significantly, both records show that the long-term variations of the 10Be production rate did not exceed 15% during the 20 to 13 ky b.p. period.

The results obtained in this study suggest that appropriately chosen lake sediments are viable sources of records of the past variations of the 10Be production rate. At this time, the certainty of the derived production rate records appears to be limited by the general lack of knowledge of past global precipitation distributions.