USING RAMAN SPECTROSCOPY OF ANCIENT KEROGEN TO UNDERSTAND THERMAL ALTERNATION AND BIOSIGNATURE PRESERVATION

Fossil organic matter is an important biosignature for life on Earth and elsewhere in the solar system. The preservation and alteration of organic matter in fossils is complex and not fully understood. Understanding the process of fossilization and thermal alteration of organic matter is important when trying to determine biologic origin and structure, especially for microfossils with simple morphologies.

The microfossils studied in this project come from the 2.5 billion-year-old Tsineng member of the Gamohaan Formation from the Transvaal Supergroup, South Africa. Fossils include both broad filaments and coccoids and are preserved in chert layers within a deep-water carbonate depositional setting. Samples come from both outcrop and drill cores, and all come from two localities approximately 80 km apart. They are expected to have experienced similar degrees of thermal alteration based on regional geology. In this project, we use Raman spectroscopy and optical microscopy to study and quantify the variability in preservation of these fossils.

Raman spectra can provide a “fingerprint” of fossil organic matter, or kerogen, composition and thus thermal maturity. Kerogen has two major spectral features, a D band at 1350 cm^-1 and a G band at 1600 cm^-1. As kerogen becomes more thermally mature, the D band narrows and grows taller and the G band shifts to lower wavenumbers. The way these bands shift, caused by changes in intensity of subsidiary peaks, has been used previously to make geothermometers. Most of the literature using Raman spectroscopy as a geothermometer focuses on metamorphosed shales and has not been tested on other lithologies such as chert.

It was expected that the Raman spectra of the organic matter comprising the Tsineng member microfossils should be similar, but there are significant differences between them. The goal of this project is to quantify and evaluate potential causes of the differences. This is important to help understand what factors might cause variability in thermal maturity, and to test whether published geothermometers can be applied to organic matter in chert. Continuing work will also test other lithologies, such as carbonate, to create a universal geothermometer to better understand kerogen as a biosignature on Earth and beyond.