Quantitative assessment of changes occurring in organic matter during early diagenesis

Publisher Website
Google Scholar
Abstract
It is perhaps a truism that sedimentary organic geochemical processes can best be quantified if the concentrations of the major organic chemicals are known. While numerous authors have reported the concentrations of one or other compound classes, relatively few studies have accounted for all of the organic carbon in a well-resolved depth sequence of Recent sediments. In the present study, we report a mass balance for organic carbon in 1 cm sections of Recent sediment (0-10 cm) from offshore Peru by measuring the major, operationally defined classes of biochemicals: proteins, carbohydrates and lipids. However, in deeper sediments in the same core (200 cm), the proportion of organic carbon that can be accounted for by present analytical methods is only about 60%, and recalculation of published data shows that this decreases to only a few percent at 100 m depth. We discuss briefly, methods which have proved useful for characterization of the uncharacterized fraction which some workers have termed humin or ‘proto-kerogen’. The smooth and rapid decreases in the proportions of ‘protein’ and ‘carbohydrate ’ carbon and ‘ bound ’ lipids in this core are attributed to diagenetic alteration rather than to variations in input, confirming many previous results. The overall proportions of free lipids, in contrast, did not decrease systematically in the shallowest sediments (0-25 cm). This is not reflected in most previous studies of individual lipids, many of which decrease rapidly with depth in these sediments. We suggest that interactions within the lipids may account for this apparent discrepancy.