Late Quaternary environmental and climatic changes in central Europe as inferred from the composition organic matter in annually laminated maar lake sediments

摘要

Geochemical (elemental analysis, bulk delta(d1)3 C-TOC, thermal degradation techniques, molecular analysis of n-alkanes) and microscopic investigations (maceral analysis, vitrinite reflectance) were performed (1) to characterize the organic matter (OM) in sediments from two maar lakes in the Westeifel volcanic field, Germany, and (2) to discuss paleoenvironmental processes which have controlled the accumulation and preservation of OM. The annually laminated sediments show a wide range of organic carbon contents between 0.3 and 21.4% TOC and Hydrogen Index values between 80 and 501 mg HC/g TOC. Well-developed laminations mirror suboxic to anoxic bottom waters. Diagnostic compounds in the pyrolysates and microscopic analysis permit discrimination between the principal OM sources, and their variations over time provide important evidence for the reconstruction of the environmental histories of the lakes and their watershed. The significance of terrigenous OM is reflected by the occurrence of methoxyphenols in the insoluble as well as mid- and long-chain n-alkanes in the soluble OM which are indicative of land plant or macrophyte input to the lakes. The relative abundance of certain homologues together with the amount of terrigenous macerals can be used to show differences in higher plant input and vegetation changes through time. During the Pleniglacial and Younger Dryas period, high-reflecting reworked OM was introduced by soil erosion in an environment with relatively open vegetation. The alkylphenols in the pyrolysates originate partly from the microbial reworking of proteinaceous tyrosine moieties and/or reflect a direct contribution from cyanobacteria. We assume that variations in the carbon isotopic composition of OM are mainly due to changes in the lake's primary productivity with the exception of the Pleniglacial. Various forms of land use are reflected by increasing amounts of terrigenous particles as well as chemical plant markers from 500 varve years B. P. until present. The organic geochemical and microscopic data correlate well with other proxy information about climate-dependent fluctuations in the ecosystem of the studied sites.