Towards a coupled δ18O sugar and δ2H n-alkane approach in paleoclimate research

The oxygen and hydrogen isotopic composition of (hemi)cellulose and leaf wax-derived lipids, respectively, are increasingly used in paleohydrological and -climate reconstructions. However, previous studies found it challenging to disentangle the effects of past changes in δ18O and δ2H of precipitation (δ18Oprec and δ2Hprec, respectively) and changes due to evapotranspirative enrichment of leaf water. In this dissertation, a possible solution for the above given constraint is presented, namely a coupled δ18O sugar and δ2H n-alkane biomarker approach. Sugar biomarkers were extracted hydrolytically from plant and soil samples and measured for δ18O using gas chromatography-pyrolysis-isotope ratio mass spectrometry (GC-Py-IRMS). Prior to coupling of the δ18O sugar and δ2H n-alkane results, the recently developed method for compound-specific δ18O analyses of the hemicellulose-derived sugars had to be validated. First δ18Ohemicellulose results obtained from topsoils investigated along two climate transects (Norway and Argentina) enabled detecting and evaluating climate variables that are influencing the isotopic imprint in sugar biomarkers. Accordingly, the sugar biomarkers reflect the δ18O isotopic composition of precipitation altered by evapo(transpi)rative 18O enrichment of leaf and soil water. Both the results of modeling analyses of δ18Oleaf water and a climate chamber experiment corroborate this interpretation. Furthermore, these studies suggest that the degree of evapotranspirative 18O enrichment is most rigorously controlled by relative air humidity (RH), whereas temperature is of minor importance. A coupled δ18Osugar-δ2Hn-alkane approach on topsoils was applied for the first time to the above mentioned Argentinian climate transect. Based on the premise that the sugar and n-alkane biomarkers are primarily leaf-derived, δ18Oleaf water and δ2Hleaf water were reconstructed and used to assess the deuterium excess of leaf water. The calculated deuterium excess proved to be a suitable proxy for RH, revealing a systematic trend towards more negative values in the southern, more arid part of the transect. Using a Péclet modified Craig-Gordon model allows reconstructing biomarker-based RH values which correlate significantly with the actual RH values along the transect. Likewise, δ18Oprec and δ2Hprec can be calculated using the coupled biomarker approach and the Craig-Gordon model; they, too, correlate significantly with the actual δ18O and δ2H values of modern precipitation, thus validating the suggested coupled δ18O-δ2H biomarker approach. Finally, a first paleoclimatic application of the δ18Osugar-δ2Hn-alkane approach to a lacustrine archive is presented for Lake Panch Pokhari, Nepal. The established 16 ka δ18Oprec and δ2Hprec records reflect well the variability of the Indian Summer Monsoon (ISM) and the established deuterium excess record of lake water allows reconstructing the evaporation history of Panch Pokhari Lake. Conclusively, the newly developed and validated coupled δ18O-δ2H biomarker approach presented in this dissertation offers great and intriguing potential for more quantitative paleoclimate research, both on terrestrial and lacustrine sedimentary archives.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Tuthorn, Mario

Contributors dc:contributor

• Zech, Michael

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1739/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1739