61st Annual Report on Research 2016

Main Activities

• Detailed petrography, morphological analysis and geochemical analysis of Green River Formation stromatolites
• Continued petrography and morphological analysis of Rossport Formation and Shakopee Formation stromatolites
• Analysis of the relationship between microstructure, form, and pore space distribution
• Conference presentations by students
• Two senior theses

Main Findings

• Macroscopic morphology is not well-correlated with microtexture
• The vast majority of stromatolites from all study areas are composed of precipitated carbonate textures – grumeaux texture (clotted microstructure) and isopachous crusts (radial fibrous microstructure). Trapping-and-binding of grains is rare.
• The size and distribution of pores in stromatolites is highly variable. Some variation is attributable to original texture. Some porosity is attributable only to secondary processes that do not correlate with original texture.
• Trace element geochemistry, in the well-preserved and least-altered stromatolites of the Green River Formation, does not predict microtexture or carbonate fabric, suggesting that bulk water chemistry is not the main factor controlling carbonate textures.

Narrative

Following fieldwork (summer 2015), students Lindsey Reiners and Tanner Eischen added the Eocene-age stromatolites of the Green River Formation to our dataset. By comparing the older Proterozoic (Rossport) and Paleozoic (Shakopee) with younger (Green River), we concluded that precipitated carbonate is the dominant building block for all these stromatolites. Older stromatolites present a greater proportion of rapidly precipitated textures (e.g., isopachous crusts), and younger show occasional evidence of slower lithification (e.g., trapped-and-bound textures), but the three study areas are remarkably similar at the microstructural scale, despite differences in age and morphology.

Reiners and Eischen continued this work throughout the academic year, further examining stromatolites of the Sibley and Shakopee formations. Their research included detailed petrographic analyses and comparison of macroscopic form, microtexture, and depositional environment.

At the same time, two students, Zach Martinez and Grant Noennig, undertook senior thesis research to explore the relationship between porosity, permeability, microtexture, and morphology for stromatolites in the Green River Formation. Martinez took a traditional approach, creating serial sections and acetate peels to examine the nature and distribution of pore space. Noennig used x-ray computed tomography (XRCT) for non-destructive analysis of the relationship between pore space and texture. He successfully imaged four stromatolites – the first time, to our knowledge, that a fully lithified, pre-modern stromatolite has been imaged by XRCT. These two students came to broadly similar conclusions: the size and distribution of pore space varies widely within and among stromatolite specimens. Some of this variability is attributable to differences in depositional texture (e.g., textures precipitated with little original pore space remain “tight” through post-depositional alteration), but some is not attributable to intrinsic characteristics of the stromatolites (e.g., dissolution of particular horizons increases porosity and may not follow original fabric type).

During the summer of 2016, students Reiners and Eischen collaborated with students of Dr. Tom Hickson of the University of St. Thomas, also a PRF awardee, to share information, develop a common descriptive vocabulary for stromatolites, and report current research results across projects. The students at St. Thomas traveled to Gustavus Adolphus for a stromatolite workshop and to use our instrumentation. The Gustavus team also traveled to St. Thomas and participated in a summer research symposium, to share in-progress work and brainstorm ideas. Both research groups are now using data and observations to populate a database that will enable information exchange about stromatolite characteristics. The joint workshop revealed striking commonality in the textural and morphological features of stromatolites, despite significant differences in features of the respective study areas.

Also during the summer of 2016, Reiners and Eischen continued work on the Green River stromatolites collected during the 2015 field season, with the aim of assessing a relationship between microstructure and trace element chemistry. They analyzed thin sections, conducted cathodoluminescence imaging, and measured trace element compositions at the lamina scale. The result of these analyses is summarized in the graphic TOC; in short, the trace element geochemistry does not correlate well with microtexture nor with macroscopic morphology. This finding suggests that bulk water chemistry features, such as degree of evaporation, are not the principal driving factor in determining stromatolite texture.

Impact on students

Students Reiners and Eischen were able to synthesize a great deal of information about stromatolites in a way that has not been done before. The overall finding that macroscopic morphology, microscopic texture, and trace element chemistry are not well correlated with each other is new, but not unexpected when interpreted against the backdrop of published work on stromatolites. This pair of students was able to present the results of their work in several venues this year, providing them with substantial local, regional, and national exposure within the scientific community and to prospective graduate schools (both students presently intend to apply to graduate programs this year). Their conferences include:

• The Midstates Consortium on Math and Science Undergraduate Research Meeting (Chicago – Fall 2015)
• The Institute of Lake Superior Geology (Duluth – Spring 2016)
• Abstract submitted for the Geological Society of America Meeting (pending: in Denver, Fall 2016)

Students Martinez and Noennig had the opportunity to complete senior theses on this work. Both students learned valuable techniques, and Noennig’s work represents a novel contribution to the field, as no published XRCT data on pre-modern stromatolites has been published. These research projects represent substantial independent work for these two students. Their thesis titles:

• Grant Noennig: Analysis of stromatolite reservoir potential using computed tomography
• Zachary Martinez: Pore network connectivity in microbialites of the Green River Formation (Wyoming, USA): analogs for petroleum reservoir potential

Impact on faculty

The work of this project has invigorated Julie Bartley’s research agenda and has opened up a host of new collaboration opportunities. Ongoing collaboration with Tom Hickson (University of St. Thomas) has been strengthened, and we foresee numerous additional opportunities to work together. This project has also put Bartley in contact with colleagues from several institutions who work on stromatolites. We expect to have at least three publications arise from this work, and expect that this work will catalyze a future grant proposal, to be written in 2018.