FOSSIL EUKARYOTES IN TERRESTRIAL PALEOENVIRONMENTS: EXAMPLES FROM NEOPROTEROZOIC TO CAMBRIAN PALEOSOLS AND PALEOSAPROLITES

Paleosols and paleosaprolites formed from weathering of Precambrian crystalline basement are common across the North American craton as well as across other cratons. Petrographic inspection of thin sections from samples obtained from two paleosols (Neoproterozoic Baltic paleosol, Estonia, and sub–Cambrian paleosol, core TR119, southwestern Wisconsin) contain cryptic vermiform features interpreted as evidence of microbial communities, including eukaryotes, living in weathered crystalline basement. The vermiform features are 10-15 μm in diameter, up to 50-100 μm long, smooth-walled, straight to weakly sinuous, and are generally preserved in cracks and microfractures in saprolitized basement, within secondary silica deposits (both chalcedony and chert). UV fluorescence microscopy reveals that some of the Baltic paleosol vermiforms have an organic-rich wall, and, in some cases, an organic-rich interior, each with unique fluorescence. EMP analyses indicate that the Baltic paleosol vermiform features are preserved by secondary mineralization by silica, iron and magnesium, with thin clay coatings on organic(?) walls; FTIR did not verify organic preservation. Vermiforms from the TR119 paleosol have been replaced by illite (or illite-vermiculite), not silica, but otherwise appear morphologically like those of the Baltic paleosol. The morphology of the investigated vermiform features, which are interpreted to be biotic in origin, are different than those of hydrothermal, abiotic vermicular chlorite. Vermiform features in these two paleosols are similar in size and shape to nematodes described from the deep terrestrial subsurface in South Africa and could represent fossil (mineralized) examples of these organisms. Other vermiform and coccoid-like features preserved in siderite and Fe oxyhydroxides are also possible biotic features. These paleosol and paleosaprolite microfossils are important evidence of terrestrial microbial communities that were dependent upon, and spatially controlled by crystalline rock substrates. This study emphasizes the need for careful petrographic and micro-analytical study of materials comprising Precambrian and Cambrian paleosols and paleoweathering surfaces for evidence of early terrestrial communities and for reconstruction of ecosystem functions.