Chapter 38 - The Significance of Carbonaceous Matter to Understanding Life Processes on Early Earth

Abstract

Carbonaceous material (CM) found in metamorphosed Archean rocks may represent the altered remains of ancient (i.e., syndepositional) organisms, the product of abiologic synthesis, migrated bitumen from younger rocks, or the degraded remnants of modern endolithic microorganisms. Here, a general summary is presented of CM formation processes that can generate abiotic CM, followed by a description of three well-known controversies on the origin of Archean CM. These include CM found in the 3.9Ga Akilia association in southern West Greenland, the 3.7Ga Isua supracrustal belt (ISB)of southern West Greenland, and the 3.5Ga Apex chert, Pilbara, Western Australia. Based on these controversies, a set of criteria are presented for identification of biogenic CM in Archean rocks. Biogenic CM should be syngenetic with the rock (e.g., be completely enclosed in a microcrystalline matrix, have a degree of structure in line with the metamorphic grade) and should display compositional characteristics (e.g., δ13C, heteroatoms such as N, P, S) in line with a biologic source material.