Amorphous organic matter — Experimental data on formation and the role of microbes

The origin of microscopically-amorphous organic matter (AOM) is problematic. It has been extensively studied because it is the dominant kerogen constituent in petroleum source rocks. Although microbes are widespread in natural environments, they are commonly associated only with marine AOM derived from phytoplankton. In this study, we have selected terrestrial and marine samples with various microbial inputs in order to decipher the role of microbes in AOM composition. A specially-tailored laboratory device has been used for determining the effect of oxygen- and light-depleted conditions on recent microbial mats for a duration of three years. This experiment aimed at reproducing conditions existing in nature at the water–sediment interface.This research has permitted the characterization of AOM according to its biological origin. Two different types of AOM have been observed, i.e., gelified and granular types. They are related respectively to microbial reworking of terrestrial fragments and primary microbial populations. Moreover, bacterial bodies constitute the ubiquitous, strongly fluorescent material, whereas extracellular polymeric substances (EPS) surround bacteria and show a weaker fluorescence. Consequently, this study on modern OM has unravelled the amorphization process of specific organic particles leading to AOM classically encountered in fossil sediments. This has considerable implications for palaeoenvironmental reconstructions associated with the origin and preservation of OM.

► Experimental diagenesis demonstrates the role of microbes on replaced by AOM formation.
► Gelified AOM replaced by derives from microbial reworking, granular AOM replaced by has a primary microbial populations.
► Weakly to non-fluorescent AOM could be related to extracellular polymeric substances, well-preserved EPS.
► AOM produced in anoxic and dark conditions increases by 15%+ over natural conditions.