NOVEL APPLICATION OF HIGH-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY TO ARCHEAN PALEONTOLOGY

The Apex chert in Western Australia has been the center of many debates about whether these rocks contain unambiguous evidence of the early biosphere. Originally, these rocks were described to contain cyanobacterial microfossils although later studies have described these features as having a morphology and mineralogy inconsistent with life. Additionally, these rocks contain carbonaceous material (CM) of unknown origin. This CM as been described by some as being of abiotic catalytic origin, and by others as biogenic. Although Raman spectroscopy is not sufficient in and of itself to determine the source of CM, including whether it is biogenic, we recently used Raman spectroscopy in a paragenetic framework to demonstrate that the CM is from at least two separate populations. Here we show using high-resolution transmission electron microscopy (HRTEM) that the CM found in the Apex chert exhibits at least four different microtextures. As different types of CM have different carbon crystallinities, the sources of these microtextures can be determined, revealing CM produced by processes such as abiotic catalytic synthesis, meteoritic input, and biological synthesis. These four populations of CM, combined with the geological history of the area, reveal over a billion years of fluid mixing and CM redeposition. Our study reveals that while care should be taken when interpreting bulk chemical data in samples from the Apex chert, HRTEM can offer the ability to delineate biogenic CM from abiotic CM in Archean sedimentary sequences.