Proposed Testate Amoebae Fossils Between Snowball Earth Episodes from Neoproterozoic

Microstructures in metasedimentary rocks from the Neoproterozoic Bonahaven Formation, Scotland: Microconcretions, impact spherules, or microfossils?

Authors:

1. Ross P. Anderson (a)

2. Ian J. Fairchild (b)

3. Nicholas J. Tosca (c)

4. Andrew H. Knoll (d)

Affiliations:

a. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA

b. School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

c. Department of Earth Sciences, Irvine Building, University of St Andrews, St Andrews, Fife KY16 9AL, UK

d. Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA

Abstract:

Microscopic spherules in relatively undeformed mudstones of the Neoproterozoic Bonahaven Formation, Islay, Scotland, are differentiated from their matrix by a sharp micron-scale, smoothly rounded boundary. These elongate spherules were earlier interpreted as hollow bodies filled penecontemporaneously by glauconite and subsequently metamorphosed to phengite, but their origin remains a matter of debate. Spherules observed in thin section are predominantly rounded (~74%) but can exhibit a flat edge or protrusion at one end. In 11% of a sample population, two or more spherules are conjoined. X-ray diffraction indicates that spherule-bearing mudstones consist mainly of muscovite, with variable amounts of kaolin-group minerals and minor iron-chlorites. A range of physical origins for the spherules – including microconcretions or metamorphic microstructures; deposition from the sky as micrometeorites, microtektites/microkrystites, or accretionary volcanic ash particles; and detrital grains – is considered but rejected on distributional, morphological, and mineralogical evidence. Biological origins are considered most likely, especially protistan tests similar to the vase-shaped microfossils found in somewhat older Neoproterozoic rocks. If correct, this provides the first report of eukaryotic life in the Dalradian succession that passes critical tests for biogenicity and new evidence for testate microfossils in post-Sturtian but pre-Marinoan aged rocks.