Abstract
Over the past years, a new proxy for the reconstruction of continental paleotemperature and past soil pH has been developed based on branched glycerol dialkyl glycerol tetraethers (GDGTs). Branched GDGTs are membrane lipids of as of yet unknown bacteria that thrive in peat and soils. Their molecular structure can vary
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in the number of methyl branches and cyclopentane moieties that are attached to their tetraether backbone. Their relative distribution in soils is determined by the annual mean air temperature (MAT) and pH of their living environment, and can be quantified using the Methylation of Branched Tetraether (MBT) index, and the Cyclisation of Branched Tetraether (CBT) index. Branched GDGTs are transported with soil organic matter by rivers and deposited in marine coastal regions, where down-core application of the so-called MBT-CBT proxy may yield an integrated paleoclimate record for the river basin. Although the MBT-CBT proxy seems to be a promising tool to obtain information about continental climatic changes in the past, the validity and applicability of the proxy need further testing.
The influence of the two main environmental controls on the distribution and abundance of branched GDGTs in soils (MAT and soil pH) was, therefore, tested in a variety of natural settings. Geothermally heated soils surrounding hot springs were used to determine the influence of changes in temperature, whereas the impact of changes in soil pH was tested in long term pH manipulated field plots. Analysis of branched GDGTs in soils from an altitude transect along a mountain slope showed that MAT and soil pH are indeed the two main controls on the distribution of branched GDGTs in a setting with many varying environmental factors.
Furthermore, MBT-CBT-based MAT estimates were tested on bias towards a specific season, but no influence of seasonality could be ascertained. When verifying to what extent the branched GDGT distribution in fjord sediment reflects that of the nearby continent it was discovered that branched GDGTs can also be produced in situ in the marine environment. This lead to the recommendation to only use the MBT-CBT proxy at sites that receive substantial amounts of soil organic matter relative to the amount of marine organic matter, i.e. close to river mouths. Information about the possible producers of branched GDGTs was derived from the identification of intact polar (i.e. with head group still attached; IPLs) branched GDGTs in a Swedish peat. Changes in head group distribution and their concentration with depth hint towards Acidobacteria as their likely producers. Based on all available soils, the proxy was revised and subjected to statistical tests to derive the best equation to calculate MAT based on the distribution of branched GDGTs. Finally, the MBT-CBT proxy was applied on a loess-paleosol sequence in China, which resulted in a continental paleotemperature reconstruction covering the last 34 kyr. The trends and timing of temperature changes are in agreement with other continental climate records.
The data presented in this thesis show that temperature and pH are indeed the main factors controlling the distribution of branched GDGTs, but that absolute temperatures generated by the MBT-CBT proxy must be interpreted with care.
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