Taphonomy of very ancient microfossils from the ～3400Ma Strelley Pool Formation and ～1900Ma Gunflint Formation: New insights using a focused ion beam

摘要

Focused ion beam (FIB) milling permits the accurate extraction of ultrathin (c. 100. nm) cross sectional lamellae from microfossils found in geological thin sections. Subsequent TEM analysis of these lamellae can provide unique insights into the ultrastructure, chemistry and taphonomy of Precambrian microfossils at the micrometer to nanometer scale. Combining serial FIB milling with SEM imaging extends this capability to three dimensional (3D) tomographic reconstruction and visualization of Precambrian microfossils, revealing information not available in light microscopy.Here we apply these techniques to two iconic silicified microfossil assemblages, from the ～3400. Ma Strelley Pool Formation of Western Australia and the ～1900. Ma Gunflint Formation of Canada. All the examined microfossils have carbonaceous walls surrounded by pure silica. Impregnation of microfossil walls by nano-grains of silica is common, together with variable degrees of wall displacement and replacement by silica. All microfossils are rigidly preserved in 3D and show little or no folding or compression. However, there are also notable differences in taphonomic preservation. Our examples of the spheroidal Gunflint microfossil Huroniospora showed the highest fidelity of preservation with a continuous carbonaceous wall fossilized by spheroidal nano-silica grains that resemble those found on bacterial surfaces in modern silicifying hot-spring environments. The nucleation of these silica nano-spheres on the microfossil walls has induced an artificial 'saw-tooth-like' ridged wall texture that may subsequently hinder species-level identification. The Strelley Pool microfossils in comparison show a lower fidelity of preservation with small parts of the microfossil walls completely replaced by silica, plus extensive recrystallization of spheroidal silica nano-grains to angular micro-quartz. Our examples of the sheath-like filamentous Gunflint microfossil Siphonophycus showed the lowest fidelity of preservation with many gaps in the carbonaceous walls and significant redistribution of carbon by recrystallizing silica grains. A model is presented to explain these observations.Criteria for distinguishing highly probable microfossils from non-cellular carbonaceous microstructures (e.g., botryoids and grain coatings) using FIB-based imaging are put forward for the first time here, using examples drawn from the Strelley Pool Formation and comparisons with younger Gunflint material.The combined in situ techniques of FIB-TEM and FIB-SEM nano-tomography potentially provide a wealth of new nano-scale information regarding the biogenicity, antiquity and taphonomy of Precambrian microfossils. However, the destructive nature of both techniques makes their application to unique palaeontological specimens problematical.