EAR-PF: What drove localized pyrite formation and taphonomic bias in the fossil record?

Dr. Kelsey Moore has been awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out research and education activities at Johns Hopkins University under the mentorship of Professors Emmy Smith and Maya Gomes. For the first 2-3 billion years of Earth history, life was dominated by microbes—simple organisms like bacteria and early unicellular eukaryotes.

This early life played a fundamental role in in setting life on its evolutionary course and in driving the evolution of our atmosphere and our planet. To learn about this ancient life, how it evolved, and how it interacted with the early Earth, we turn to the fossil record. However, this record is biased because the processes that facilitate fossilization require special circumstances that often selectively fossilize only certain organisms.

Fossilization of soft, microbial organisms is especially rare. Luckily, some minerals, like pyrite (FeS2), did preserve a record of ancient soft-bodied microbes. An important example is an assemblage of pyritized Obruchevella, cyanobacterial fossils preserved in the Neoproterozoic Ikiakpuk Formation.

These are fossils of photosynthetic bacteria that thrived in the aftermath of a global glacial event—the Sturtian glaciation of the Snowball Earth event. But how these cyanobacteria interacted with the environment and became fossilized remains unclear. This project seeks to better understand the microbial biosphere in the aftermath of this extreme climatic event, how it coped with environmental stresses, and how it became fossilized.

It is possible that the cyanobacteria may have played a role in their own fossilization. Modern cyanobacteria produced sulfur-rich organic compounds in responses to environmental stresses and these compounds may have contributed to the formation of pyrite and fossilization in the past. This project will test this hypothesis by conducting fossilization experiments with living organisms that are similar to the fossils.

These experiments will be paired with in-depth analysis of the Ikiakpuk Formation and the pyritized fossils that it contains. The aim of this work is to determine how the organisms became fossilized and what biological and abiotic factors contributed to pyrite formation. With these insights, it may be possible to paint a more complete picture of the shallow marine environments after this global glaciation, the microbial communities that thrived in the aftermath of the glaciation, and how those microbes evolved and coped with environmental stresses.

While different models for pyritization have been suggested, little attention has been given to the organic compounds produced by the organisms being fossilized and their role in sulfur cycling and iron sulfide nucleation. In particular, the fossils preserved by pyrite in the Ikiakpuk Formation are similar to modern cyanobacteria that produce sulfated polysaccharides, organosulfur compounds that may play a key role in localized pyrite formation.

To address this, this project will test the contribution of organosulfates to local pyritization. Through a combination of taphonomy experiments and fossil analysis of pyritized fossil assemblages, this study seeks to determine (1) whether or not organosulfates can be used as a sulfate source for MSR, (2) whether or not this specific localized sulfate source can account for localization of pyritization and preferential preservation of some organisms over others, and (3) whether organosulfur imparts a characteristic sulfur isotope composition in fossil pyrite.

This work will take place at Johns Hopkins University in collaboration with Professors Emmy Smith and Maya Gomes, as well as external collaborators Sara Pruss (Smith College) and Francis Macdonald (University of California at Santa Barbara). Experiments with modern microbes will help constrain how microbial biogeochemical makeup, nutrient cycling, and ecological interactions drive fossilization, taphonomic bias, and sulfur isotope fractionation.

Analysis of analog fossil assemblages then provides a means of applying these findings to the fossil record and testing hypotheses related to organism diversity and abundance as they relate to taphonomic bias. These combined analyses also provide a means of applying sulfur isotope fingerprints to test the application of a pyritization model that accounts for organosulfates to the rock record.

The combination of experimental taphonomy and fossil analyses provides a novel approach to gain insight into ancient microbial communities, seawater chemistry, and the global biosphere beyond the information offered by a single fossil assemblage. This is especially important as we attempt to understand the evolution of environments and the biosphere following a global glacial event like the Sturtian Glaciation.

More broadly, this work will inform how we interpret pyritized fossil assemblages in the rock record during other intervals in Earth history and reveal what the isotopic signatures can tell us about ecology, cell physiology, and preservation of organic matter.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.