Early diagenesis of organic matter in marine sediments: progress and perplexity

Early diagenesis is the combination of biological, chemical, and physical processes which change the quantity and composition of the organic matter in the upper several hundred meters of marine sediments. During early diagenesis, from 30% to more than 99% of the organic matter deposited on the sediment surface is remineralized by sediment organisms. Rates of organic matter oxidation or oxidant consumption have now been measured in a variety of sediments and range over at least three orders of magnitude, from 20 mmol C m −2 year −1 to 60 mol C m −2 year −1. High rates of organic matter oxidation are associated with high rates of sediment accumulation, organic matter influx to the sediment surface, and organic matter burial. Some patterns in the decomposition rates of different organic substances have emerged. Small, dissolved molecules (e.g. acetate) are remineralized much more rapidly than the biopolymers in fresh algal detritus, which in turn break down faster than refractory macromolecules such as lignins and melanoidins. Much of the organic matter in sediments consists of macromolecules of uncertain origin and composition which resist decomposition. However, the specific compound classes which have been measured in sediments (e.g. hydrolyzable amino acids and carbohydrates, fatty acids and hydrocarbons) often decompose as slowly as TOC (total organic carbon) in the upper 1 m of coastal sediments. In many sediments, a substantial fraction of the organic matter deposited on the sediment surface is not remineralized during early diagenesis. This preservation of organic matter in sediments remains partly a puzzle. One key process could be the incorporation of organic compounds into refractory geomacromolecules or humic substances. However, detailed information on the composition and structure of geomacromolecules and concrete evidence for a mechanism of geomacromolecule formation remain elusive. An alternative hypothesis is that much of the organic matter preserved in sediments is refractory when deposited and survives diagenesis with little change.