DISTINGUISHING KEROGENS FROM ABIOTICALLY PRODUCED CARBONACEOUS MATERIAL: LIMITATIONS OF RAMAN SPECTROSCOPY

How can we recognize evidence of the earliest, most primitive forms of life on our planet and on other planets? To identify minute samples as primitive fossil material, it would be useful to be able to unambiguously recognize kerogens, a mixture of molecules that are residues from once-living organisms, i.e., biogenic material. Kerogens consist of many different, typically aromatic (i.e., benzene ring-containing), C-O-H molecules. Such molecules are classified as "organic" in traditional chemical nomenclature. (Note: Not all molecules that are organic in structure are biogenic by formation process.) There is a large, published database of Raman spectra of organic compounds to which spectra obtained from purported biogenic materials (e.g., fossils) theoretically could be compared. In practice, however, Raman spectra of kerogens show only the "generic spectral features" indicative of discontinuous arrays of condensed benzene rings. Most prominent are two very wide, first-order Raman bands that peak at ~1360 and ~1600 Dcm^-1. These bands reflect only the most fundamental level of the molecular structure of kerogen and are not specific to individual organic molecules. Nor do typical kerogen spectra show any other bands characteristic of additional chemical functional groups. Further confounding the definitive identification of kerogen in geologic materials is the fact that poorly ordered carbonaceous materials also can arise through non-biological processes, such as heating in situ of organic or inorganic compounds, metamorphic mobilization of pre-existing carbon compounds, and high-temperature precipitation from hydrothermal solutions. Unfortunately, most of those abiogenically produced carbonaceous materials have Raman spectra that are indistinguishable from those of kerogens. Thus, a Raman spectrum cannot definitively identify "kerogen", but only "disordered carbonaceous material." Obviously, the fact that a sample consists of "disordered carbonaceous material" is not sufficient to assure its identity as biogenic carbonaceous material. Thus, the recent declaration by Schopf, et al. (2002), that Raman spectroscopy can identify a kerogenous signature (and, thus, a signature of past life), is erroneous.