Doctor of Philosophy
Environment and Sustainability
Moser, Katrina A.
Longstaffe, Fred J.
Lake sediments integrate signals from the catchment, atmosphere and water column, offering a unique window through which to view changes in the carbon cycle. Carbon dynamics in lakes are changing due to nitrogen loading and anthropogenic climate warming (ACW), threatening the water quality of lakes. This thesis identifies how the carbon dynamics of lakes have responded to anthropogenically-driven forcings by comparing pre- and post- AD 1850 records preserved in lake sediments. First, the carbon dynamics of Barry Lake (Ontario, Canada), a low-elevation temperate lake, are investigated. Effective moisture (the net of water inputs and evaporation) is reconstructed using the carbon and oxygen isotope compositions of marl, carbon isotope compositions of total organic carbon and hydrogen isotope compositions of the n-alkane C17. The isotope compositions and abundances of biomarkers are used to reconstruct changes in organic matter (OM) source and methane oxidation. These reconstructions reveal that current levels of primary production in Barry Lake are unprecedented during the last ~900 years, likely due to land use change and, perhaps, ACW. Furthermore, inferred methane oxidation and OM source, but not primary production, are closely tied to changes in effective moisture. This finding suggests that future changes in aridity driven by anthropogenic climate change may alter the sources of carbon to sediments and affect how methane is recycled by temperate lakes. A second goal of this thesis is to characterize sources of organic matter to four cold, high-elevation alpine lakes (Uinta Mountains, Utah, United States). Analyses of modern terrestrial and aquatic samples provide a baseline to improve interpretations of stable isotopes in alpine lake sediments. A database of carbon and hydrogen isotope compositions of n-alkanes extracted from vegetation surrounding the lakes is used to estimate the carbon isotopic composition of ancient carbon dioxide (δ13CCO2) and hydrogen isotopic composition of ancient precipitation (δ2Hprecip). Finally, a comparison of Barry Lake with the Uinta Mountain lakes reveals that both systems have become more productive in the last 50-100 years. In summary, this thesis identifies recent changes in the carbon dynamics at Barry Lake and advances the ability of researchers to interpret proxies in alpine environments.
Summary for Lay Audience
Lake scientists analyze sediments, or lake mud, to reconstruct environmental conditions. To do this, they first assign ages to each layer of sediment deposited at the bottom of the lake. Next, they measure the chemical, physical and/or biological characteristics of the sediment. In the same way that paleontologists search for the bones of ancient fish to infer whether a landscape was once covered with water, lake scientists use microfossils, chemical analyses and physical characteristics of the sediment to make educated guesses about past environmental conditions. By marrying the ages of sediment layers with these inferences about past environments, lake scientists reconstruct long-term records of variables such as temperature and precipitation. These techniques are used in this thesis to reconstruct environmental change at Barry Lake, a small lake in southern Ontario (Canada). It is determined that European settler-colonizers, rather than natural changes in aridity, likely initiated an unprecedented increase in algal growth at Barry Lake upon their arrival in AD ~1850. It is also uncovered that methane recycling by the lake and, to a lesser degree, the type of vegetation entering the lake, are associated with changes in aridity (i.e., dryness). This finding suggests that, at Barry Lake, increases in aridity may alter the sources of carbon to lake sediments and affect how methane, a greenhouse gas, is recycled by the lake. This thesis also presents a database of chemical signatures from modern sources of organic matter (OM) (e.g., plants and algae) that are expected to occur in the sediments of alpine lakes in the Uinta Mountains, Utah (USA). This database will improve the ability of scientists to determine which OM sources have contributed most to sediments in understudied alpine regions. Reconstructing such sources is vital for determining whether lake sediments are recording environmental conditions within or outside of the lake. Without this information, it is difficult for lake scientists to accurately reconstruct paleoenvironmental change. Finally, Barry Lake is compared with the Uinta Mountain lakes. Recent increases in algal growth are detected in both systems. This finding is concerning because rapid algal growth can degrade water quality.
Doyle, Rebecca M., "Reconstructing carbon dynamics of alpine and temperate zone lakes using stable isotopic analysis" (2020). Electronic Thesis and Dissertation Repository. 7515.
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Available for download on Tuesday, December 20, 2022