VARIABLY PRESERVED ORGANIC MATTER FROM EOARCHEAN IRON FORMATIONS

Organic matter in sedimentary rocks is generally assumed to be biological in origin. However, the origin of organic matter in Eo- and Paleoarchean sedimentary rocks cannot simply be assumed biological because of the need to demonstrate the existence of life at that time. In addition, sedimentary rocks older than 3.2 Ga have generally suffered a protracted metamorphic history and the possibility of remobilized younger biological or non-biological organic matter needs to be evaluated. These challenges might be surmountable through the use of correlated micro-analytical techniques that can yield elemental, isotopic, and molecular characteristics on the nature of individual microscopic particles. An approach can thus consist in establishing a database of the collective geochemical characteristics of unambiguously biological and non-biological organic matter and comparing these datasets with organic matter from the oldest sedimentary rocks on Earth. This report presents the geochemical characteristics of a selection of microscopic particles of organic matter from Eoarchean iron formations from the Nuvvuagittuq and Isua Supracrustal Belts and the Akilia Association and attempts to directly compare it to that from mantle peridotites and from Proterozoic microfossils. The presence of trace elements essential to life (CHNOPS) was detected in all the above-mentioned sources of organic matter. The range of stable carbon isotope compositions overlaps between all kinds of organic matter as well. Molecular bonds with carbon atoms in organic matter so far do not appear to show distinct characteristics between sources of organic matter, but more work is needed in this area, in particular on organic matter from mantle peridotites. The mineralogical context of these occurrences can also be identical, for example with associations between organic matter and apatite having been found in all the above rock types. The sedimentary context of deposition may thus remain a key to distinguishing biological from non-biological organic matter, but this perspective becomes a circular argument. Therefore, the establishment of unambiguous criteria to distinguish biological from non-biological organic matter still awaits solid geochemical support.