A MULTIDISCIPLINARY APPROACH TO DATING LATE HOLOCENE LAKE SEDIMENTARY RECORDS

Chronologies of climate and human impact on lake sedimentation during the Late Holocene have been established for field sites in California and South Africa using a suite of techniques. The multidisciplinary approach taken has had to overcome mismatches between the period of record and established radiometric dating techniques, as well as problems with low activity samples resulting in considerable analytical uncertainty. The primary focus has been on determining chronological marker horizons using pollen data in combination with documented introductions of exotic species for reasons of health, economic development and environmental management. Down-core trends in inorganic geochemical and magnetic data have been used to identify potential pollution chronologies, or changes in the nature of sedimentation resulting from changes in lake level and/or sediment provenance linked to land-use activity and aperiodic flooding. Organic geochemistry has also been used to establish pollution chronologies based on the recent use of agrochemicals such as DDT. In California the established chronology reveals a switch from minerogenic sedimentation to biological productivity in Pinto Lake during the 1950s without any significant change in the rate of sedimentation. The upper part of the record then preserves catchment flooding events as pulses of minerogenic inwash (which may be linked to El Nino magnitude/frequency). In South Africa, recent human impact has resulted in a significant increase in the delivery of topsoil-derived material to the floodplain wetlands of the Mkuze River, KwaZulu-Natal, as a consequence of changing land-use and increased sensitivity to runoff since the 1960s.