Sudden enhancement of sedimentation flux of 210Pbex as an indicator of lake productivity as exemplified by Lake Chenghai

Abstract

The fundamental assumption of 210 Pb sediment dating is the stable flux of 210 Pb ex , which was derived from atmosphere and then transferred into sediments via lake water. When the sedimentation rate is relatively constant, the 210 Pb ex activity in sediments will be exponentially reduced with sedimentation age. 210Pbex in lake water is incorporated into sediments mainly via organic particulates. If the sedimentation flux of organic matter in lake water is suddenly in-creased, 210 Pb ex will be significantly deposited and then transferred into sediments. On the one hand such sudden purification effect is obviously unfit for the fundamental assumption of 210 Pb dating; on the other hand, the sudden enhancement of 210 Pb ex flux would be indicative of the conspicuous variation of primary productivity of lake water. This problem will be discussed in accordance with the variation trend of 210 Pb ex in the vertical profile of recent sediments of Lake Chenghai, Yunnan Province. The sediment core was collected from the deep-water area of Lake Chenghai in June 1997. The vertical profile of 137Cs activity is characterized by a tree-peak pat-tern. This profile gave reliable ages, and also showed the stability of sediment accumulation in the recent ten years. The vertical profile of 210 Pb ex activity displays a specific distribution of peaks, and is similar to the vertical profile of Corg. This phenomenon seems to be related to the mecha-nism of constraining the transfer of 210 Pb ex into lake sediments. The average atomic ratios of H org /C org and C org /N org in Lake Chenghai sediments are 5.51 and 7.04, respectively, indicating that the organic matter was predominantly derived from the remains of endogenic algae. In terms of the three-stage evolutionary characteristics of organic matter in sediments, i.e., “deposition- de-composition-accumulation”, the sedimentation fluxes ( F (C org )) of organic carbon (C org ) since 1970 were calculated by modeling. The sedimentation fluxes of 210 Pbex ( F ( 210 Pb ex )) in different years display good synchronous relations with the sedimentation flux of organic carbon ( F (C org )), es-pecially in the years of 1972─1974 and 1986─1989. The variation of F (C org ) led to the variation of F ( 210 Pb ex ); the variation of F ( 210 Pb ex ) reflects, to some extent, the historical variation of lake pro-ductivity.