Pyrite morphology and sulfur isotopes refine taphonomic models for the 2.1 Ga Francevillian biota, Gabon

Abstract

Pyritization is a key taphonomic process that preserves some of Earth's oldest fossils. It is influenced by various factors such as organic matter type, the availability of iron and sulfur, and sedimentation rates. In this study, we analyzed pyritized biotic and abiotic structures from 2.1 Ga deposits in Gabon's Francevillian Basin, to reconstruct their taphonomic pathway at the micron scale. Using secondary ion mass spectrometry and scanning electron microscopy, we examine sulfur isotope compositions, pyrite morphology and grain size within individual fossils and compare them to abiotic pyritic concretions from the same stratigraphic level. Our results reveal differences in pyrite grain size and sulfur isotope composition between fossils and concretions. More importantly, chemical and morphological variations are observed within individual fossils, likely due to distinct reactive environments for pyrite mineralization, linked to organic matter, sulfate and iron availability during early diagenesis. This remarkable variation in pyrite morphology and δ³⁴S values in the fossilized specimens, indicates that they were compositionally more complex than the substrate that formed the homogeneously pyritized concretions. This well-preserved ecological window represents an exceptional record of the earliest multicellular life forms on Earth.

Fig. 1

Pyritization patterns in fossils and abiotic concretions. (a) Lobate fossils are composed of a central dome and radial fabric at the edges. b. Abiotic concretions show millimetric pyrite grains. (c,d) The center of the dome contains coagulated pyrite grains and many fractures. (e,f,g) The interface between the dome and the radial fabric is marked by a change in pyrite grain size and distribution. (h) Small euhedral pyrites of around 30 μm in size (i) are found in radial fabrics. j. A clear pyrite size evolution is noticed in the dome, going from the center outwards, with pyrite reaching 300 μm at the border (k). (l,m,n) Pyrite grains in the concretion are massive with no specific shapes.

Fig. 2

Sulfur isotope data on fossils and abiotic concretions. Spatial δ³⁴S analyses on two lobate fossils (a,c), and related histograms showing the distribution of these isotopic data (b,d). Spatial δ³⁴S analyses on the abiotic concretion (e), and corresponding histogram (f). Note that the δ³⁴S data are heterogeneous within the fossils, but homogenous in the abiotic concretion.

Fig. 3

Boxplots of δ³⁴S variability between fossils and abiotic concretions and within fossils. Values of δ³⁴S vary between the vertical and longitudinal transects of two lobate pyritized fossils and between the dome and the radial fabric within each fossil. These δ³⁴S values are also different from those observed on a diagonal transect in the abiotic concretion.

Fig. 4

Evolution of the sulfur isotopic fractionation (rate, duration) during the pyritization of the lobate fossils and concretions.

Fig. 5

Conceptual pyritization model for the lobate fossils. (a) The lobate fossil is composed of a central dome and radial fabrics at the edges. (b) After death, decay proceeds releasing organic material and leading to the reduction of sulfates from seawater into sulfides. (c) Sulfides react with available iron leading to pyrite nucleation in radial fabrics and the center of the dome. (d) Pyrite nucleation continues exclusively where organic matter is not fully consumed (to reduce sulfates), mainly at the border of the dome, but this pyrite has a different isotopic composition from the previously precipitated one since light sulfur has been already consumed. (e) Pyrite growth occurs where organic material is still available at the outermost part of the dome (where iron from the environment is also available). The growth of pyrite at the borders eventually isolates the pyrite at the center of the dome from the external environment, prohibiting its further growth and resulting in an increase of pyrite grain size from the center going outward of the dome.