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Microbial Involvement in the Silicification Process: New Evidence from Stratigraphically Diverse Fossils.

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Hudson Hanks

Authors:

Hanks, Hudson¹, Melim, Leslie A.², Hegna, Thomas³, Bellot, Brian⁴, Lerosey-Aubril, Rudy⁵

(1) Western Illinois University, Macomb, IL, USA, (2) Western Illinois University, Macomb, IL, USA, (3) State University of New York at Fredonia, Fredonia, New York, USA, (4) Western Illinois University, Macomb, Illinois, USA, (5) Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA,

Abstract:

Silicification is the process by which the carbonate material of a skeleton is dissolved and replaced with silica, often while preserving fine detail. Although microbial activity has long been suspected to play a role silicification, direct evidence of microbial involvement has been hard to find. Recent studies have found evidence in the form of silicified biofilm and pyrite framboids on fossils from the Cambrian and Permian. The goal of this study was to elaborate on their findings by looking for microbial structures on a variety of silicified fossils from diverse localities.

Carbon-rich textures were found across multiple samples, ranging from smooth ‘ropy’ filaments no more than 2 µm thick, to a densely interwoven ‘crinkly’ texture with strings less than 1 µm thick. On multiple fossils, areas with detrital grains and clays present exhibited a ‘crunchy’ texture at contact points between fossil and sediment. EDS analysis of all observed textures showed a small but distinct carbon spike, not present in the background silica and clearly distinguishable from the very strong carbon signal of modern contamination. Notably, all of these textures also extended into the surrounding silicified matrix, rather being restricted to the fossil. Associated with these carbon-rich features, several samples also contain pyrite framboids, indicative of organic decay by sulfate-reducing bacteria. In some cases, the pyrite was replaced with iron oxide, but retained the distinct framboid morphology.

The carbon-rich textures are interpreted as silicified biofilm, based on the distinctive ropy and crinkly morphology and chemical composition. The presence of identical silicified microbial features on multiple fossil species, and their recurring association with biologically controlled pyrite framboids, supports the working hypothesis that microbial decay is commonly involved in the silicification of carbonate fossils. The extension of silica and silicified biofilm into the surrounding matrix also suggests involvement by organic matter in the matrix, not only that found within the carbonate fossil.