Studies on Tropical Palaeo-variation in Climate and Cosmic Ray Influx : Geochemical data from stalagmites collected in Tanzania and northern South Africa

Abstract: The main aims of this project were to contribute to the knowledge about tropical climate variations, and to investigate the possibilities of obtaining cosmic ray influx data from stalagmites.Stalagmites from Tanzania and northern South Africa were palaeoclimatologically analysed.U-series dating, combined with previously published 14C-data (Goslar et al. 2000), shows that one of the Tanzanian stalagmites was precipitated during the latter half of the latest glacial, between approximately 39 and 27 ka. Results from stable isotope analysis (δ13C and δ18O) were interpreted as indicating millennial-scale fluctuations in atmospheric CO2-level during that time. These fluctuations show a pattern which is similar to that of the δ18O records from the GRIP and Byrd ice-cores – suggesting that the stalagmite record captures a global climate signal.U-series dating shows that the largest of the South African stalagmites covers most of the last 25 ka. The δ13C- and δ18O-data indicate millennial-scale variation in the South African climate throughout the time of the formation of this stalagmite. Because of similarities to Antarctic records (Steig et al. 2000), the driving force for these variations was suggested to be atmospheric circulation changes associated with change in the Southern Hemisphere circumpolar westerly wind vortex. Like the Tanzanian results, these South African data lend further support to the theory about global persistent millennial-scale climate-fluctuations.A beryllium isotope study was then performed on the two stalagmites described above. This study assesses, for the first time, the potential of the cosmogenic isotope 10Be as a tool in stalagmite studies. As a control, 9Be-analysis was also made on each 10Be-sample.The aim of the study was to test the idea that stalagmites could be a new source of well-dated and directly climate-proxy synchronized information about past variations in cosmic ray influx intensity – i.e., a source that would serve as a new tool for general improvement of the knowledge about past variations in cosmic ray influx, and that could possibly also provide further clues to whether such variations can affect Earth’s climate.The main excursions/fluctuations in the 10Be-and 9Be-data from these stalagmites do not coincide. Hence, the excursions in 10Be-content may be interpreted as indicating excursions in cosmic ray influx. The 10Be-data show three pronounced peaks at around 38, 35 and 21 ka respectively; plausibly corresponding to the peaks previously observed at approximately 39, 32 and 23 ka in cosmogenic isotope data from sediments and ice.In glacial parts of the stalagmite material, the 9Be-concentration fluctuates on a millennial-scale; inversely synchronized with fluctuations in δ13C-data from the same material. This indicates that 9Be could, apart from being a necessary control for 10Be-results, also function as a palaeoclimatological proxy in stalagmite studies.In order to improve dating facilities for stalagmites, a procedure for high-precision measurements of U and Th isotope ratios was also developed, using an IsoProbe® Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS). The 230Th/232Th ratio for the thorium concentration-standard Alfa Th was determined to 7.96 (±0.012) × 10-7 (2σ), and the δ234U obtained for the uranium standard SRM950a was 18.4 ±0.6(2σ). The thorium results show, that even extremely biased isotope ratios, in low-concentration samples, can be measured with remarkably good precision. The quality of each sample measurement can also easily be evaluated. However, due to repeated technical problems with the instrument, it has not yet been possible to use the developed procedure for routine analysis and dating of samples.