Abstract
Increasing our knowledge about the global climate system will help society to mitigate anthropogenically induced global climate change and its impacts on the environment. Information on past climates from before the instrumental data era comes from proxies (i.e. marine and lake sediments, corals, tree rings), or indicators of past climate.
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The research contained in this thesis focuses on the use of organic biomarkers in the form of fossilized sedimentary lipids as proxies. The preserved distribution of fossilized sedimentary lipids in the sediments can reflect past environmental conditions (i.e. temperature). The proxies examined within this thesis are based on two different types of sedimentary lipids that are biomarkers for temperature, long-chain alkenones and glycerol dialkyl glycerol tetraethers (GDGTs). Most of the research contributing to this thesis was performed on sediments from the Baltic Sea, where sedimentary long-chain alkenones, isoprenoid GDGTs (isoGDGTs), hydroxylated GDGTs (OH-GDGTs), and branched GDGTs (brGDGTs), were applied to examine environmental changes in the region as well as the success and limitations of these proxies under the changing environmental conditions that occurred in the Baltic Sea over the Holocene. In this thesis, sea surface temperature (SST), indicators for anoxia and biomarkers for cyanobacteria (bloom forming microorganisms that when they die and sink to the bottom their decomposition consumes oxygen in the bottom waters) from the Baltic Sea were compared and showed that climate is the main controller of low-oxygen conditions in the basin.Also, SST estimates were compared to a reconstruction of human population from northern Europe and the relationship demonstrated the impact climate had on human population size and economy from 7,500-3,500 BP in the Baltic Sea region.Next, several OH-GDGT based indices were applied to sedimentary records in the central and southern regions of the basin and revealing that OH-GDGTs are a promising proxy for estimating SST in the Baltic Sea. Another study in this thesis demonstrated that the distribution of long-chain alkenones co-vary with salinity changes in the Baltic Sea, both regionally and in the past, suggesting the distribution of long-chain alkenones could be used for paleosalinity studies.The last study on Baltic Sea sediments in this thesis examines the provenance of brGDGTs in two different regions (the central and southern regions) in the basin and showed that the provenance changed when the basin was reconnected to the North Sea transforming from a freshwater lake to a brackish body of water. Despite the changes in brGDGT provenance over the Holocene, continental temperatures were reconstructed by applying appropriate calibrations during the different phases in the Baltic Sea history. In the Tagus River basin on the Iberian Peninsula, a re-examination of the provenance and distribution of brGDGTs from a previous study was performed using enhanced chromatography methods to determine if a new calibration could improve temperature estimates in the area. The distribution of brGDGTs in past sediments were also examined by analyzing four sediment cores in a transect from the river to off the continental shelf to determine if present day trends held true over the Holocene.
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