Refining Chronologies by new Radiocarbon Dating Methods

Principal Investigators: T. Dunai, J. Rethmeyer

In this project, new radiocarbon dating methods will be used a) to refine and extend sediment chronologies and palaeo-environmental records by compound-specific 14C analysis and b) to determine ages of relatively young cave sediments by burial dating utilizing in-situ 14C.

  1. The establishment of reliable sediment chronology is one of the most fundamental requirements for palaeo-environmental investigations based on aquatic sediments. Due to the lack of terrestrial macrofossils in several lacustrine sediment records investigated in the first CRC period, some sediment chronologies or larger sections were based on AMS 14C ages of carbonates and bulk sedimentary organic matter, both of which may be strongly biased by local reservoir effects, i.e. incorporation of fossil/old organic matter from bedrocks or other sources not in equilibrium with atmospheric 14CO2. The aim of this project will be to improve and extend available chronologies of the African lakes investigated in the last and new funding period using compound-specific 14C analysis (CSRA) of terrestrial plant-derived lipids. These molecules can be isolated from sedimentary organic matter by preparative gas chromatography. Prerequisite for CSRA is a detailed analysis of the distribution of different lipid compound classed in the sedimentary record to identify molecules derived from terrestrial plants (e.g. long-chain n-alkanes and n-carboxylic acids), which are present in sufficient quantities for CSRA. A further objective is to obtain additional and more specific information for vegetation and hydrological changes in continental Africa. This will be achieved by combining the results of the lipid analysis for CSRA with compound-specific stable carbon and hydrogen isotope analyses of terrestrial plant-derived compounds. The results will be compared with extant marine data sets to obtain information on climatic effects of land-ocean interactions.

  2. Currently, young cave sediments (<100 ka) cannot be dated by burial dating utilizing cosmogenic nuclides. This inability is due to the long half-lives of the nuclides used (10Be and 26Al, with T½ = 1.39 Ma and 0.7 Ma, respectively; the method is based on the ability to resolve the modification of the 10Be/26Al-production ratio by radioactive decay after sample burial). 14C has a much shorter half-life (5,730a), which should enable us to date burial histories of a few thousand to about 30 ka. ‘Traditional’ burial dating relies on the assumption that the production ratio of 10Be/26Al is constant and known. This fundamental assumption is, however, in most cases probably not valid for 14C/10Be or 14C/26Al production ratios, i.e. 14C reaches saturation quickly (~30 ka), while 10Be and 26Al concentrations can increase over several Ma. In areas with low erosion rates, i.e. in arid and semi-arid environments, the assumption of constant production ratio might be replaced by assuming saturation of 14C that could be verified by determination of the 10Be-concentration. If the assumption of saturation is valid, young cave sediments, and human artefacts carried into caves, could be dated via the remaining 14C abundance. In situations where the altitude of the potential source areas of the sediments/artefacts are known - relevant for the determination of the saturation concentration - these could, in Principal, be dated for burial durations in the order of 5 to 50 ka however dependent on the analytical blank and knowledge of the source areas (i.e. elevation). In this project we propose to develop the novel concept in situ 14C burial dating, verify it and apply to pertinent settings in the SFB.

The service component for the CRC provided by this project includes sample preparation (chemical cleaning steps, graphitisation and target preparation), method adaption/optimisation for difficult sample materials like badly preserved bones as well as data processing and evaluation of AMS 14C analyses performed by all CRC projects.

 

 

 

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RWTH Aachen University

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