Early diagenesis and organic matter preservation — a molecular stable carbon isotope perspective

Through a new development in stable isotope capability, gas chromatography coupled to a stable isotope ratio mass spectrometer (GC-C-IRMS), the reaction pathways of organic matter diagenesis can potentially be traced by analysis of the isotopic compositions of individual molecular components. In this study, changes in bulk stable carbon and nitrogen isotope compositions, amino acid contents and carbon isotope compositions of individual amino acids are reported for seagrass and sediment degradation experiments using isotopically resolvable organic substrates. Seagrass showed slight enrichments in δ 15N (+0.4‰) with little change in δ 13C following 4 weeks of decomposition. During that period, the identifiable amino acid content decreased by ∼ 50% for each amino acid. Mixtures of marine sediment with the same seagrass showed enrichments in 15N (+2.3‰) with associated depletions in carbon (−2.8‰) isotopic compositions over the same time span. These changes are attributed to hydrolysis, deamination, decarboxylation and condensation reactions. Control experiments on the sediments without added fresh seagrass showed no change in isotopic content. At the molecular level, using GC-C-IRMS, certain amino acids in the seagrass (e.g., Glu, Ala, Val, Gly) are seen to decrease in 13C content during decomposition whereas others remained constant (e.g., Leu, Ile, Pro) or became increasingly enriched in 13C (Asp). The molecular isotope approach indicates that the process of early diagenesis, leading to the eventual residual materials which are preserved, is significantly more complex than simple breakdown and loss. A portion of the organic matter consequently preserved in organic-rich deposits can be attributed to new production during early diagenetic stages.