Sulfur Cycling Through the Neoproterozoic Interglacial Interval: An Integrated Field and Geochemical Study of the Abenab Subgroup in Northern Namibia

新元古代晚期见证了地球历史上最极端的气候事件之一，其中可能包括两次雪球冰期（斯图亚特冰期和马里诺冰期）。最近，相当多的注意力集中在测试雪球地球假说的有效性，用放射性测定法测定冰川沉积物的年代，以及了解新元古代中期全球变冷的地质条件。然而，在斯图亚特冰期和马里诺冰期之间沉积的沉积物的地球化学方面，人们只做了有限的工作，尽管这些沉积物在这个动荡的气候时期保存了新元古代地球系统最直接和连续的记录。这一建议代表了对ca.55 Myr间冰期全球硫循环动力学的无与伦比的研究，同时使用了碳酸盐岩伴生硫酸盐（CAS）的硫和氧同位素组成以及来自纳米比亚北部Abenab亚群沉积物的沉积黄铁矿的硫同位素组成。该提案的主要目标是限制Sturtian冰川事件后的海洋化学，研究新元古代中期海洋氧化还原与碳和硫的地球化学循环之间的关系，并测试关于Marionan冰川可能触发机制的假设。近年来，随着CAS硫同位素组成的研究进展，对前寒武纪外源硫旋回的认识有了显著提高。通常，这些研究工作通过在一个地点的单一地层剖面评估CAS的硫同位素组成来表征给定时期的硫同位素演化。在这个提议中，采取了一种不同的方法。利用垂直于古海岸线的多断面CAS和黄铁矿的硫同位素组成，评价间冰期海水硫酸盐的硫同位素演化。最近的研究表明，在马里诺冰期之后，形成了较大的横向d34硫硫酸盐梯度（Hurtgen et al., 2006）。这些结果对重建新元古代34Ssulfate记录和确定海相硫酸盐储层的发育时间具有重要意义。本项目将重点研究CAS d34S和d18O数据的时空变异性，以建立CAS保存的同位素组成的本征散点，评估成岩作用和海平面对海洋硫同位素组成的影响，并确定横向d34S梯度是间冰期的特征还是冰川后海洋的典型特征。串联d34S-d18O数据将用于比较Sturtian和Marinoan冰期后的海洋学条件，并约束冰期开始的触发机制。该项目的结果还将确定d34 S（硫酸盐-黄铁矿），并有助于确定连接海洋氧化还原和碳和硫的地球化学循环的复杂反馈集，以及这些反馈如何影响新元古代中期的气候。更广泛的影响：(1)该项目将为一名研究生提供实地研究技术和分析同位素地球化学方面的培训，并为该学生提供与其他机构同事合作的机会。(2)该项目产生的数据将成为一个新网站的核心，该网站致力于存储所有由公共资助项目产生的新元古代硫数据。将鼓励该领域的其他工作人员贡献其已发表的数据，以便以易于查阅和一致的方式存档。(3)为了吸引代表性不足的年轻人学习地球科学，PI将参与EXCITE项目，该项目将西北大学的人才发展中心与当地学校联系起来，以识别有科学倾向的少数民族学生，并支持他们在高中期间发展数学和科学技能。PI将招募一名EXCITE学生参与该项目。

The late Neoproterozoic witnessed among the most extreme climatic events in Earth history, including perhaps two snowball glaciations (Sturtian and Marinoan). Considerable attention has been focused recently on testing the validity of the snowball Earth hypothesis, radiometrically dating the glacial deposits, and understanding what geological conditions were responsible for the onset of global cooling in the middle Neoproterozoic. However, only limited work has been done on the geochemistry of sediments deposited between the Sturtian and Marinoan ice ages, despite the fact that these preserve the most direct and continuous record of the Neoproterozoic Earth system during this tumultuous climatic epoch. This proposal represents an unparalleled investigation into dynamics of the global sulfur cycle during the ca.55 Myr interglacial interval, using both the sulfur and oxygen isotope composition of carbonateassociated sulfate (CAS) and the sulfur isotope composition of sedimentary pyrite from sediments of the Abenab Subgroup in northern Namibia. The principal objectives of the proposal are to constrain ocean chemistry in the aftermath of the Sturtian glacial event, examine the relationship between ocean redox and the geochemical cycles of carbon and sulfur for the middle Neoproterozoic, and test hypotheses regarding possible trigger mechanisms for the Marionan glaciation. In recent years, significant improvements to the understanding of the Precambrian exogenic sulfur cycle have been made possible by advances involving the sulfur isotope composition of CAS. Typically, these research efforts characterize sulfur isotope evolution for a given time period by assessing the sulfur isotope composition of CAS through a single stratigraphic section at one locality. In this proposal, a different approach is taken. The sulfur isotope evolution of interglacial seawater sulfate will be evaluated using the sulfur isotope composition of CAS and pyrite in multiple sections along a transect perpendicular to the paleo-shoreline. Recent work has shown that large lateral d34Ssulfate gradients developed in the aftermath of the Marinoan glaciation (Hurtgen et al., 2006). These results have important implications for the reconstruction of the Neoproterozoic 34Ssulfate record and the timing of the growth of the marine sulfate reservoir. This project will focus on both the temporal and spatial variability of CAS d34S and d18O data to establish the intrinsic scatter in isotopic compositions preserved in CAS, evaluate the effects diagenesis and of sea level on marine sulfur isotope composition, and determine whether lateral d34S gradients were a feature of interglacial times or if they were typical only of the post-glacial ocean. Tandem d34S-d18O data will be used to compare the oceanographic conditions following the Sturtian and Marinoan glaciations and to constrain the trigger mechanisms for the onset of glaciation. The results of this project will also establish d34 S(sulfate-pyrite) and help to ascertain the complex set of feedbacks that link ocean redox and the geochemical cycles of carbon and sulfur and how these feedbacks may have operated to influence climate in the middle Neoproterozoic.Broader Impacts: (1) This project will provide training to one graduate student in field research techniques and analytical isotope geochemistry and an opportunity for that student to work collaboratively with colleagues from another institution. (2) The data generated from this project will be the nucleus of a new website devoted to the storage of all Neoproterozoic sulfur data generated through publicly funded projects. Other workers in the field will be encouraged to contribute their published data to be archived in an easily accessed and consistent manner. (3) In order to attract under-represented youth to the Earth Sciences, the PI will participate in the Project EXCITE program, which links Northwestern's Center for Talent Development with local schools to identify scientifically-inclined minority students and support the development of their math and science skills through high school. The PI will recruit an EXCITE student to participate in this project.