| Summary: | In addition to the more acknowledged consequences of climate change, such as global warming, the current human-induced increase of CO2 into the atmosphere is also responsible for a change in the chemical composition of seawater. Since 1750, the initiation of the industrial revolution, approximately 50% of the emitted anthropogenic CO2 is taken up by the oceans. These enhanced concentrations of aquatic CO2 is responsible for an increase of the seawater acidity and thus a decrease in pH, leading to ocean acidification. The impact of present-day ocean acidification on the development of future climate change is still not entirely understood. Of key importance, in this matter, is the role of primary producers within the global carbon cycle and underlying feedback mechanisms. Studying past periods of ocean acidification that are characterized by low pH and high atmospheric CO2 levels, are important in unravelling these issues. In this thesis the response of micro-organisms (algae, bacteria and archaea) to high CO2 and low pH levels has been investigated by studying the distribution patterns and the stable carbon isotopic composition of specific biomarkers in present day and past environments. This resulted in the development of a number of new and potentially promising proxies. The distribution of branched glycerol dialkyl glycerol tetraethers, expressed by the Cyclisation of Branched Tetraether (CBT) index, is now shown to record lake water pH and alkalinity and may therefore be suitable for reconstructing lake water chemistry. The strong dependence of stable carbon isotopic fractionation on aquatic CO2 concentrations in several algal species, as well as the application of algal biomarkers in reconstructions of past pCO2 levels during Eocene Thermal Maximum 2, suggests that specific biomarkers from various important phytoplankton groups can be used for the reconstruction of past pCO2 levels. Finally, the stable carbon isotopic composition of isoprenoid glycerol dialkyl glycerol tetraethers, derived from ...
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