Abstract
Waters with different salt concentrations have a large influence on ocean currents and ocean circulation, both of which have strong implications for global climate change. It is not well-known how salinity changed in the past, and the reconstructions of salinity from ancient environments that do exist are associated with large
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errors. Current estimates of past salinity change rely heavily on oxygen isotope ratios of inorganic material preserved in sedimentary archives. Stable isotope ratios of both water isotopes, oxygen and hydrogen, change throughout the hydrological cycle, making them useful tools for reconstructing past hydrological changes. Freshwater generally has an isotopic signature that is very depleted in deuterium, the heavy isotope of hydrogen, in contrast to saline surface ocean waters that are relatively enriched in deuterium. Thus, there is a strong correlation between isotope ratios of water and salinity. Because ancient water is not readily available or easily preserved, other sources must be employed. Throughout this thesis, the use of hydrogen isotope ratios of algal chemical fossils was explored as a tool for reconstructing salinity of past oceans. Specifically, the focus of this thesis was to gain a more robust understanding of factors that affect the sensitivity of hydrogen isotope ratios of organic compounds to salinity in laboratory culture, modern marine environments and ancient sediments. Long-chain alkenones, which are lipids synthesized by only a select group of photosynthetic algae, found globally in lacustrine to marine environments, were used as the compound of interest. Previously published laboratory culture studies showed enrichment of hydrogen isotope ratios with increasing salinity in a number of different alkenone-producing species, but the data is sparse when it comes to published paleo records and environmental surveys. Controlled laboratory experiments outlined in this thesis helped to determine that high light intensity, alkalinity and calcification do not significantly alter the correlation between hydrogen isotope ratios of alkenones and salinity. Four different modern environments and two down-core records were investigated for their alkenone hydrogen isotope values, which greatly contributed to our understanding of the environmental signature recorded by this tool. The different sensitivity to salinity for different species, which was noted in marine surface sediments from a variety of Northern Hemisphere locations and in Baltic Sea sediments over the last 11,000 years, was a prevalent feature of the datasets. A record of hydrogen isotope ratios of alkenones from the last 150,000 years at the Chile Margin, as well as a comparison with previously generated paleo records, showed a larger sensitivity to salinity in the geologic record relative to culture results, suggesting that salinity changes may have been larger than previously believed. The results of this thesis emphasize the differential response of alkenone-producing species, a factor which can now be better constrained with recent analytical advances. Linking alkenones preserved in sedimentary archives to the species that produced them can greatly improve estimates of past salinity change.
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