185 New Advances and Voices in Geobiology (Posters)

This session will focus on new research at the intersection between geologic and biologic processes with special emphasis on novel materials and methods, new field sites, and advances at the intersections of scientific fields.

Tuesday, 21 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Vase-shaped microfossils of the late Tonian upper Walcott Member, Chuar Group, lived in restricted, evaporative environments

Paper number: 185-8
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Vase-shaped microfossils (VSMs) are globally distributed eukaryotic microfossils that appear in the late Tonian. VSMs rarely occur with other eukaryotic microfossils and are most commonly found in (silicified) carbonates – lithologies that form in peritidal settings and generally preserve only prokaryotic microbiotas for most of the Proterozoic. This suggests that VSMs may have occupied different habitats than most other early eukaryotes.

Here, we investigate the mineralogy and petrology of carbonate nodules within the late Tonian (~741–729 Ma) upper Walcott Member of the Chuar Group to gain a better understanding of VSM paleoenvironmental context and ecology. VSMs are found in high abundance in black shale-hosted, organic-rich ferrous dolomite nodules which contain minor quartz, gypsum, pyrite, iron oxide, and aluminosilicate clays. The principal fabric within nodules is sub-mm scale spherulites, a common fabric today associated with microbial mats in tidal flat and terrestrial hypersaline and/or hyperalkaline environments. Spherulites contain acicular crystals with square or pointed terminations, arranged around a locus of amorphous organics or occasionally, a VSM. Other fabric elements include mm-scale continuous and discontinuous laminations of organic matter and rhombic cements within the matrix and VSM test interiors. VSM tests are preserved as siliceous, ferrous dolomite, iron oxide, or rare pyrite casts and may contain a mix of these minerals. In some samples, VSMs have poorly preserved fragments of their original organic test. Stable carbon (δ13C) isotope values in nodule samples (-6.5 – -12.4‰) are generally more negative than Walcott Member bedded carbonates (~2 – -5‰).

We propose that VSMs lived or were deposited in the surface waters of a stratified, restricted, marginal marine environment. VSMs settled down onto microbial mats where they were mineralized syndepositionally. Tests were quickly entombed in carbonate such as dolomite (within tests) and aragonite (spherulites). Upon further evaporation, spherulites were cemented with primary dolomite and subsequently dolomitized, and gypsum formed within cracks post-lithification. These results complement chemostratigraphic and sedimentological evidence for cooler and drier conditions during upper Chuar Group deposition and provide support for the hypothesis that VSMs occupied terrestrial habitats during the Neoproterozoic, as has recently been suggested in ancestral state reconstructions. VSMs may have been among the first eukaryotes able to tolerate the hypersaline and evaporative conditions common at the interface of marine and terrestrial environments.

 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413662

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