Reports: UNI256423-UNI2: Improving Interpretations of the S-Isotope Composition of Pyrite with Isotopic Measurements of Individual Pyrite Framboids
Katja M. Meyer, PhD, Willamette University
I. Summary of Research Findings to Date
The goal of this project is to use high-resolution sulfur isotope measurements of pyrite framboids to improve the interpretation of δ34S in the geological record, especially during intervals of ocean redox change. Changes in marine sulfur cycling during intervals of ocean anoxia/euxinia are often observed as widespread, rapid changes in the isotopic composition of sedimentary pyrite (δ34Spyrite) and calcite-associated sulfate (δ34SCAS). However, the underlying dynamics of these changes remain incompletely understood. In particular, δ34Spyrite records often show high variability over short stratigraphic ranges and cannot easily be correlated with other coeval sections worldwide. The difficulty in interpreting sedimentary δ34Spyrite records may result from the presence of both syngenetic and diagenetic sources in a bulk measurement.
Since being funded by PRF, my research group has prepared both rock and lake sediment samples for isotope analysis using SIMS to measure the δ34S composition of individual pyrite framboids that span a range of sizes. Using methods developed in the Fike Lab (Washington University, St Louis), two 2016 summer undergraduate researchers separated pyrite framboids from Fayetteville Green Lake (FGL) sediments from below and above the sulfidic monimolimnion. These mineral separates were embedded in 1” round epoxy disks, polished, and analyzed via SEM and SIMS. While SEM visualization revealed characteristic pyrite framboids in the samples, the relatively low number and their wide spacing made it difficult to locate and measure d34S of pyrites in the SIMS. Subsequently, our main goals during summer 2017 were to: 1. Improve the separation method in order to reduce the number of non-pyrite mineral grains in the separated fractions, and 2. Map the position of framboids on the epoxy samples using the SEM in order to aid locating framboids when the samples are gold-coated and inserted into the SIMS. During summer 2017, two undergraduate research students worked with a suite of FGL sediment samples from a short core from the deep, sulfidic portion of the lake and several samples collected near the chemocline. After separating pyrites and experimenting with methods for imbedding the mineral separates into the epoxy disks, they used two programs developed at the University of Oregon SEM facility (CAMCOR) to map the location of pyrites with respect to several marks used to orient the round sample. This should greatly aid in the visualization and isotope analysis of these pyrites (expected SIMS time: October 2017).
Second, my research group has also investigated the δ34S – framboid diameter relationship in Early Triassic limestones that were deposited following the end-Permian mass extinction interval. Several undergraduates participated in using SEM to measure the diameter and map the position of pyrite framboids on 4 different 1” round thin sections of Early Triassic limestones from the Great Bank of Guizhou, Nanpanjiang Basin, South China. We have generated ~50 δ34S measurements from one sample and have mapped the position of several hundred pyrites for future isotope analysis via SIMS. The initial δ34S are surprising to us: the isotope compositions within a single sample span a range of almost 30 per mil and do not show any relationship to framboid diameter. Furthermore, there is also no δ34S-diameter relationship after removing any framboids that are partially infilled or show signs of post-depositional alteration. We are continuing to generate data from other limestone samples to see if this observation holds true.
II. Impact of the research on field of study and career
To date, this research project has opened new collaboration (with David Fike’s research group, Washington University, St Louis) and has led to networking opportunities with other researchers working on questions related to the production and preservation of δ34S in pyrites and calcite-associated sulfate. There is considerable interest in this work, which is complementary to colleagues’ work on sulfur cycling in modern and ancient microbial mats and studies of the δ34S and depositional environment relationship. My work at Fayetteville Green Lake and conversations with collaborators Bill Gilhooly (UI-PUI) and Chris Junium (Syracuse University) has led us to organizing a field trip and research symposium at FGL (currently applying for funding; symposium is anticipated in summer or fall 2018). I expect this symposium will be an excellent venue for presenting this work and networking. I anticipate this project leading to 2 peer-reviewed publications with undergraduate co-authors after collection and analysis of additional δ34S data.
III. Student mentoring and outcomes
Four students have been supported by ACS-PRF funding to work on this project, and two students have participated in the work informally (through independent study during the academic year). The grant has allowed my undergraduate team to gain research experience that is not otherwise available through our formal curriculum. They also were able to travel to the University of Oregon imaging facility (CAMCOR) to use the SEM. They report that this experience was valuable not only to visualize and map their samples, but also to be exposed to the range of interesting research going on at a major research university. Students participating in this project have presented at the regional Murdock College Science Research Conference (November 2016 and anticipated, November 2017). This regional undergraduate science meeting is an excellent opportunity to expose Willamette students to the breadth of undergraduate research in the Pacific Northwest; students attending this meeting also have access to representatives from major graduate programs in the area. Finally, two undergraduates will present this work at the Geological Society of America Meeting (2017, Seattle) in the session, New Voices in Geobiology. I anticipate that this opportunity will particularly benefit these students who are passionate about careers in academia but unsure of the specific discipline that they want to pursue.