Organic preservation of vase‐shaped microfossils from the late Tonian Chuar Group, Grand Canyon, Arizona, USA

Vase‐shaped microfossils (VSMs) are found globally in middle Neoproterozoic (800–730 Ma) marine strata and represent the earliest evidence for testate (shell‐forming) amoebozoans. VSM tests are hypothesized to have been originally organic in life but are most commonly preserved as secondary minerali...

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Published in:	Geobiology Vol. 21; no. 3; pp. 290 - 309
Main Authors:	Tingle, Kelly E., Porter, Susannah M., Raven, Morgan R., Czaja, Andrew D., Webb, Samuel M., Bloeser, Bonnie
Format:	Journal Article
Language:	English
Published:	England Wiley Subscription Services, Inc 01-05-2023 Wiley-Blackwell
Subjects:	Absorption spectroscopy amoebozoa Analytical methods Arizona Bitumen Canyons Chuar Group Colour Cretaceous Electron microscopy Fossils Iron Light microscopy Microscopy Neoproterozoic Organic matter organic preservation Preservation Pyrite Raman spectroscopy Scanning electron microscopy Shales Silica Spectrum analysis Spectrum Analysis, Raman - methods Sulfur Sulphur testate amoebae Tonian vase‐shaped microfossils Wavelength Arizona Grand Canyon United States > US testate amoebae Tonian vase-shaped microfossils Neoproterozoic Chuar Group organic preservation amoebozoa pyrite
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Summary:	Vase‐shaped microfossils (VSMs) are found globally in middle Neoproterozoic (800–730 Ma) marine strata and represent the earliest evidence for testate (shell‐forming) amoebozoans. VSM tests are hypothesized to have been originally organic in life but are most commonly preserved as secondary mineralized casts and molds. A few reports, however, suggest possible organic preservation. Here, we test the hypothesis that VSMs from shales of the lower Walcott Member of the Chuar Group, Grand Canyon, Arizona, contain original organic material, as reported by B. Bloeser in her pioneering studies of Chuar VSMs. We identified VSMs from two thin section samples of Walcott Member black shales in transmitted light microscopy and used scanning electron microscopy to image VSMs. Carbonaceous material is found within the internal cavity of all VSM tests from both samples and is interpreted as bitumen mobilized from Walcott shales likely during the Cretaceous. Energy dispersive X‐ray spectroscopy (EDS) and wavelength dispersive X‐ray spectroscopy (WDS) reveal that VSM test walls contain mostly carbon, iron, and sulfur, while silica is present only in the surrounding matrix. Raman spectroscopy was used to compare the thermal maturity of carbonaceous material within the samples and indicated the presence of pyrite and jarosite within fossil material. X‐ray absorption spectroscopy revealed the presence of reduced organic sulfur species within the carbonaceous test walls, the carbonaceous material found within test cavities, and in the sedimentary matrix, suggesting that organic matter sulfurization occurred within the Walcott shales. Our suite of spectroscopic analyses reveals that Walcott VSM test walls are organic and sometimes secondarily pyritized (with the pyrite variably oxidized to jarosite). Both preservation modes can occur at a millimeter spatial scale within sample material, and at times even within a single specimen. We propose that sulfurization within the Walcott Shales promoted organic preservation, and furthermore, the ratio of iron to labile VSM organic material controlled the extent of pyrite replacement. Based on our evidence, we conclude that the VSMs are preserved with original organic test material, and speculate that organic VSMs may often go unrecognized, given their light‐colored, translucent appearance in transmitted light.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 USDOE DE‐AC02‐76SF00515
ISSN:	1472-4677 1472-4669
DOI:	10.1111/gbi.12544