Incorporation and elimination of molecular oxygen signal in sulfate: Experiments and modeling

地质历史时期大气氧的稳定同位素组成传递了丰富的地球系统历史信息。然而，已知很少有化合物能直接可靠地记录氧同位素组成。硫酸盐是一种不稳定的氧阴离子，能够形成弱溶性矿物质，是迄今为止唯一可行的载体，从中可以检索到遥远过去的直接大气氧信号。尽管如此，氧信号最初是如何被吸收，后来被去除，或最终保留在硫酸盐中的，目前尚不清楚。早期的研究和研究者？初步结果表明，黄铁矿氧化生成硫酸盐过程中产生的中间和最终亚砜阴离子中的氧都是由水和氧贡献的。这些早期的结果也表明，目前对硫化物氧化过程的机理认识明显不足。只有通过对硫氧化还原循环动力学的研究，才能充分发挥氧气档案的潜力。在这个项目中，研究者将从理论上和实验上开发和测试一套非平衡动态模型，用于硫化物氧化过程中氧信号的吸收和异化硫酸盐还原过程中氧信号的消除。预计本研究将提供一种新的矿物表面和溶液中硫化物氧化成硫酸盐的机理解释；2)解释竞争氧化和交换途径的内在动力学；3)统一许多化学变量，这些变量似乎是硫酸盐中最终氧气分配的原因。至少有五个模型预测将通过量化氧化和交换反应的速率来进行测试，这些反应反映在亚砜阴离子的氧同位素组成中。实验将利用三氧同位素标记的水，通过它可以测量分配和分馏幅度的细微变化。虽然这项研究最初是为了了解大约6.35亿年前在世界范围内的冰川后沉积物中发现的神秘的非质量依赖硫酸盐氧-17枯竭，但结果将为一类涉及一组很少达到平衡的动态同位素分馏过程的科学问题提供一个范例解决方案。该项目将由一名博士后研究员和一名研究生领导，并协助不同种族和文化背景的本科生和高中生。研究结果将在初级和高级本科班以及两所当地高中传播。将开发两个教学模块来传授科学家？古大气特征矿物档案发掘工作。

The stable isotope composition of atmospheric Oxygen in the geological past conveys rich information on the history of Earth system. Few compounds, however, are known to record oxygen isotope composition directly and reliably. Sulfate, being a non-labile oxyanion and capable of forming weakly soluble minerals, is thus far the only viable carrier from which direct atmospheric oxygen signals in the distant past can be retrieved. Nonetheless, how the oxygen signal is initially incorporated, later removed, or finally retained in sulfate is not clear. Earlier studies and the investigator?s initial results indicate that both water and oxygen contribute to the oxygen in intermediate and final sulfoxyanions generated during pyrite oxidation to sulfate. These earlier results also revealed that the current mechanistic understanding of the sulfide oxidation process is evidently inadequate. The full potential of this oxygen archive can only be attained through a study focusing on the kinetics of sulfur redox cycling. In this project, the investigator sets to develop and test, both theoretically and experimentally, a set of non-equilibrium, dynamic models for the oxygen signal incorporation during sulfide oxidation and for the oxygen signal elimination during dissimilatory sulfate reduction. It is expected that the study will 1) present a new mechanistic interpretation of sulfide to sulfate oxidation on mineral surface and in solution; 2) account for the intrinsic dynamics of competing oxidation and exchange pathways; and 3) unify the many chemical variables that appear to be responsible for the final oxygen apportionment in sulfate. At least five model predictions will be tested by quantifying rates of oxidation and exchange reactions as reflected in the oxygen isotope composition of sulfoxyanions. The experiments will utilize three-oxygen-isotope labeled water by which subtle changes in apportionment and in the magnitude of fractionation can both be measured. Although this study is initially driven by an interest in understanding a mysterious non-mass-dependent sulfate oxygen-17 depletion found in world-wide post-glacial deposits at about 635 million years ago, the result will offer an exemplary solution to a class of science problems that involve a set of dynamic isotope fractionation processes that rarely reach equilibrium. The project will be led by a postdoctoral researcher and a graduate student along with assisting undergraduate and high-school students of diverse ethnic and cultural backgrounds. Research results will be disseminated in introductory and upper-level undergraduate classes and in two local high schools. Two teaching modules will be developed to convey scientists? efforts in exploring mineral archives for ancient atmospheric signatures.