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.
