BIOSEDIMENTS AND BIOFILMS 293 Figure 14 Schematic diagram of biomarker formation from the degradation of biological organic material As the organic material is heated over millions of years, functional groups are lost from the molecular structure In some cases, the remaining structure may still provide clues to the organism from which the organic material was derived Adapted from original unpublished illustration by J Brocks, with permission are distributed throughout the cosmos In other words, they are a kind of molecular fossil that can: (1) provide evidence of life; and (2) provide information about the nature of those life forms, even in the absence of recognizable fossils Figure 14 describes the formation of biomarker molecules as organisms degrade during burial and heating The oldest known biomarkers are from organic material in the 2.7 Ga Fortescue Group of the Pilbara region, Western Australia The molecules were identified as originating from the group of organisms known as eukaryotes (see Precambrian: Eukaryote Fossils) (Figure 1) Chemical Fossils Microbial activity can be traced, in some instances, by the chemical signatures left behind in rocks However, at present, our ability to detect these signatures may exceed our ability to interpret them We know that microbes act as agents of dispersion, concentration, or fractionation of different chemical components and, in doing so, may impart specific chemical signatures upon their environment However, we also know that similar patterns may be produced by abiological mechanisms In the following paragraphs, we investigate some of the chemical signatures that can be used to trace biological activity, and some of the proposed problems with their interpretation Biological isotopic fractionation signatures form when organisms discriminate between light and heavy isotopes of elements, such as carbon, hydrogen, nitrogen, and sulphur Both the light and heavy isotopes are involved in abiotic reactions, whereas organisms that utilize carbon compounds in their metabolic reactions almost always prefer the lighter 12C isotope The carbonaceous remains of the organisms are enriched in 12C, whereas the source material from which they derived their carbon is enriched in 13C Carbon-bearing minerals (e.g., calcium carbonate or limestone) that precipitate from the residual 13 C-enriched material will acquire the heavy carbon signature Other known biological fractionation processes involve the preferential assimilation of 14N over 15 N, 32S over 34S, and hydrogen over deuterium Thus, isotopic analyses of organic matter and their host mineral deposits can yield evidence of biological activity through fractionated C, N, S, and H signatures Carbon isotope fractionation patterns have been found in Early Archaean carbon-bearing rocks, such as the Strelley Pool Chert and Dresser Formation (Pilbara, Western Australia) and the Buck Reef Chert (Barberton, South Africa) Carbonaceous material (biogenic material?) with low values for 13C/12C from these formations may have derived from the biological fractionation of carbon isotopes However, Fischer–Tropsch synthesis has been proposed as an alternative mechanism for carbon isotope fractionation Fischer–Tropsch synthesis is a chemical reaction process involving carbon that is thought to occur in the mantle in the presence of certain catalytic compounds, such as Fe and Mn Because Fischer–Tropsch synthesis is a high temperature process, and requires the presence of certain compounds, the geological setting of a fractionated C isotope signature may provide clues to differentiate Fischer–Tropsch-type occurrences from biological occurrences However, the distinction is not always easily made and controversy persists with regard to the biological interpretation of light carbon signatures in Early Archaean rocks Beyond the morphological fossil record, which extends back to around 3.5 Ga, evidence for life may only be found via chemical fossils in ca 3.8 Ga rocks from Greenland These rocks are too metamorphosed to contain any morphological remains However, carbon within the rocks is enriched in the light isotope, 12C This enrichment may have resulted from the biological fractionation of carbon, and has