The Use of Biomarker and Stable Isotope Analyses in Palaeopedology / Reconstruction of Middle and Late Quaternary Environmental and Climate History, with examples from Mt. Kilimanjaro, NE Siberia and NE Argentina
Palaeosols are important terrestrial archives for the reconstruction of the Quaternary landscape and climate history. In order to derive reliable information from these archives about sedimentation, vegetation and climate history, various methods and proxies are traditionally applied, e.g. texture analysis, numeric dating methods and mineral analysis. The aim of this dissertation is to evaluate the potential of biomarker and stable isotope analyzes. Specifically, I focused on plant leaf wax-derived n-alkanes, amino acid enantiomers, stable carbon and nitrogen isotopes (d13C and d15N) in bulk soil organic matter (SOM) and on compound-specific isotope analysis (CSIA) of n-alkanes. The respective methods were partly optimized and then applied in multi-proxy analytical approaches to three selected palaeosol records, representing different ecological environments. In all three study areas, long-chain n-alkane ratios, nC31/nC27 and (nC31+nC29)/nC27, respectively, proved to be straightforward biomarker proxies for the reconstruction of the terrestrial palaeovegetation at plant community level (especially grasses and herbs versus trees). Short- and mid-chain n-alkanes (nC17–nC19 and nC20–nC25, respectively) were successfully used for detecting algal- and aquatic macrophyte-derived organic matter (OM) in the sediment core Arg. D4. Amino acid enantiomers as nitrogen (N) biomarkers allowed a further characterization of the SOM in the Tumara Palaeosol Sequence: On the one hand, the depth functions of D/L-aspartic acid (Asp) and D/L-lysine (Lys) could be roughly described by exponential fits, reflecting SOM aging. On the other hand, brown interglacial/-stadial palaeosols generally revealed higher D/L-ratios than dark gray glacial palaeosols. This finding suggests that D/L-aspartic acid and D/L-lysine may serve as palaeotemperature proxies. In the Arg. D4 record, d13C varied in a wide range (from -30.1‰ to -17.4‰), indicating C3-C4 vegetation changes and hence allowing a reconstruction of the palaeovegetation. However, the natural abundance of 13C was no straightforward proxy for the interpretation of the palaeosol sequences on Mt. Kilimanjaro and in the Tumara Valley. There, the interpretation of smaller d13C variations – assumed to be independent of C3-C4 vegetation changes – needed multi-proxy analytical approaches for disentangling the various possibly influencing environmental factors: On Mt. Kilimanjaro, d13C is higher in palaeosols, which developed under ericaceous vegetation (~ -25‰) compared to those developed under tropical montane forests (~ -27‰), suggesting that such vegetation changes are responsible for the observed d13C pattern. In the Tumara Palaeosol Sequence, d13C correlates negatively with total organic carbon (TOC) and TOC/N. As both parameters may serve as proxies for SOM decomposition, it is assumed that degradation processes have contributed significantly to this d13C record. Furthermore, also changing water stress conditions for the plants could have played a crucial role for d13C in the Tumara Palaeosol Sequence. In contrast to d13C, d15N in the Tumara Palaeosol Sequence does not correlate with any of the other SOM characterizing parameters (TOC, TOC/N and d13C). Although other processes than SOM decomposition like (i) denitrification, (ii) N fixation, (iii) N losses by frequent fire events, and (iv) changes in the atmospheric 15N deposition are discussed as factors contributing to an open N cycle, d15N in the Tumara Palaeosol Sequence seems not to be a straightforward proxy. The compound-specific d13C analysis (CSIA) of n-alkanes was optimized and applied to selected samples from the Arg. D4 record. The highly significant correlations of the compound-specific isotope results with bulk d13C corroborate the reliability of the d13C vegetation proxy. Furthermore, the increasing d13C amplitudes from nC27 to nC33 validate the origin of these biomarker molecules, with nC27 and nC29 mainly deriving from C3 trees and shrubs and nC31 and nC33 mainly deriving from C3 or C4 grasses and herbs. Eventually, the multi-proxy analytical approaches (including the innovative biomarker and stable isotope analyzes) enabled detailed reconstructions of the Middle and Late Quaternary palaeoenvironmental changes in the three study areas: Accordingly, the deep black palaeosols on the southern slopes of Mt. Kilimanjaro reflect periods of climatic deterioration during the Last Glacial Maximum (LGM) and the Late Glacial, which coincided with a descent of the ericaceous vegetation belt. The palaeopedologic findings from the Tumara Palaeosol Sequence suggest that the dark gray and brown stratigraphic units of this record describe alternating glacial and interglacial/-stadial periods during the last ~240 ka. The stratigraphic units of the Arg. D4 record were correlated with climatic events on the Bolivian Altiplano and discussed in terms of an intensified palaeo-South American Summer Monsoon (SASM).
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