Doctor of Philosophy (PhD)
Geology and Geophysics
Major Earth system perturbations in the deep past and today are recorded in sulfate sulfur and oxygen isotopes, as examined here in three cases. (1) Sedimentary sulfate record of the “Marinoan Oxygen-17 Depletion” (MOSD) event, implies ultra-high CO2 atmosphere at ~635 Ma after global glaciation. MOSD duration is constrained by correlating its most complete record to radiometric dates. Barium sulfate layers in South China sediments show the MOSD in lower layers but persistently absent up section. Carbon-13 correlation locates the MOSD within dated intervals from other sites, yielding a 0 - 0.99 Myr duration. Thus, sedimentary constraint on this non-steady-state Earth system response can underpin future work such as models. (2) Pristine natural baselines of riverine sulfate flux and sulfur-34 isotope composition (δ34S) cannot be directly measured, thus leaving anthropogenic impact unconstrained. Exhaustive source compilation and multi-year monitoring of the Mississippi River is used to quantify its natural and anthropogenic sulfate flux and δ34S. We show that, since before industrialization to the present, Mississippi River SO42− has increased in flux from 8 to 28 Tg SO42− yr−1, and in δ34S from −11 to −3‰, reflecting an impressive anthropogenic and bedrock footprint. (3) Long-term heavy anthropogenic secondary atmospheric sulfate (SAS) deposition promotes acidification. Decade-long delay in riverine sulfate response because of SAS soil retention complicates assessments of direct SAS contribution. Higher direct SAS, as we expect in China versus the United States, should appear as higher river sulfate oxygen-17 isotope composition (Δ17O) because SAS has Δ17O > 0‰, while all other sources have Δ17O ≤ 0‰. Our two years of monitoring show that Yangtze and Mississippi mean riverine sulfate Δ17O are −0.09 ±0.05‰ and −0.15 ±0.05‰, respectively, with p < 0.0006. The calculated direct SAS component in the Yangtze and Mississippi is 11 ±9% and 3%, respectively. Sulfate Δ17O is affirmed as sensitive gauge for direct SAS contribution.
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Killingsworth, Bryan Alan, "Untangling Earth System Responses Recorded in Sulfate's Sulfur and Oxygen Isotopes at the Dawn of Multicellular Life and Today" (2014). LSU Doctoral Dissertations. 2397.