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Development of new analytical techniques for measuring radiosulfur in terrestrial samples to understand the present-day and Archean atmospheric sulfur cycles


Sulfur, the 10th most abundance element in the universe, has variable valence states (from -2 to +6) and therefore actively participates in a host of biogeochemical processes in nature. Since the earliest geological record of the primitive Earth, sulfur has been ubiquitous and has played an important role in the evolution of life and the ability to track its origin. In the present-day Earth, interest in the terrestrial sulfur cycle predominantly stems from anthropogenic influences on the atmospheric sulfur budget and the key role of sulfate in affecting climate. To understand a wide range of physical, chemical, and biological processes involved in the aforementioned topics, measurements of quadruple stable sulfur isotopes (32S, 33S, 34S, and 36S) in terrestrial samples (e.g., rocks, ice cores, aerosols) have been utilized. However, cosmogenic 35S, the only radioactive sulfur isotope with an ideal half-life (~87 days) for tracking atmospheric, hydrological, and biogeochemical processes, is seldom measured because of its low-energy decay, low abundance in nature, and associated analytical difficulties. In this dissertation, new analytical techniques were developed for accurate quantification of 35S in atmospheric, cryospheric, and hydrospheric samples using an ultra-low-level liquid scintillation spectrometer (chapters 2 and 3). Based on these newly developed analytical methods, I measured 35S in varying natural samples collected around the Northern Hemisphere to explore the use of 35S in the Earth, atmospheric, and planetary sciences. I demonstated that 35S is a highly sensitive tracer for quantifying the downward transport of high-altitude air masses and gas-to-particle conversion rates of sulfur in the terrestrial atmosphere, and provided new insights into atmospheric vertical mixing (from the boundary layer to stratosphere) and sulfur chemistry over the Pacific Rim, Himalayas and Tibetan Plateau (chapters 4-8). Using simultaneous measurements of all five sulfur isotopes (32S, 33S, 34S, 35S, and 36S) in the same sulfate aerosol samples, I discovered two distinct mass-independent sulfur isotope effects in the present-day atmosphere, which points to previously unrecognized areas for understanding the fundamental chemical physics of sulfur isotopic mass-independent fractionation and the earliest sulfur cycle on Earth during the appearance and evolution of early life (chapters 9 and 10).

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