Beware intercepts: interpreting compositional ratios in multi-component sediments and sedimentary rocks

The Corganic/Ntotal ratio and the hydrogen index (HI) are commonly used to describe the bulk composition, the sources and the post-depositional alteration of sedimentary organic matter. Data from modern sediments and sedimentary rocks frequently show significant intercepts and strong linear relationships in plots of Corganic vs. Ntotal and of S2 vs. Corganic for many different data sets, demonstrating that the ratios are not constant across the range of organic matter contents. The presence of such intercepts, whose causes are well understood, means that changes in Corganic/Ntotal and HI as %Corganic changes cannot be used to infer different organic matter sources and/or post-depositional alteration or degrees of organic matter preservation of different sediment fractions. Moreover, the strong linear regressions in such x–y plots imply that the organic fraction of sediments and sedimentary rocks in any given setting or stratigraphic section generally has a uniform composition.