1. Organic Matter Preservation In Marine Environments: Insights From A Study Of Marine Dissolved Organic Matter (DOM) Composition, Age And Source
Composition and formation mechanisms of refractory marine DOC have been equated to those of humic substances; humic substances may be produced as intact, heterogeneous biochemicals that undergo post-production modification to yield structures that cannot be easily recognized and traced to a particular source compound. Alternatively, biochemicals from different sources, formed at different times and modified by distinct processes, may associate through molecular interactions to render their total structure unrecognizable. These two formation pathways can be distinguished through Δ14C measurements. In the first case, constituents isolated from humic substances will have similar Δ14C signatures because they were produced contemporaneously. In the second case, individual constituents may have unique Δ14C signatures due to their different histories in the marine environment. Data presented in Figure A favor the second model of “humic substance” formation, in which a refractory lipid core appears to be associated with and modified by labile, polar biochemicals (Figure B). Results identify both structural features that facilitate DOC preservation in marine environments on long timescales and molecular interactions that could allow transient accumulation of bioavailable compounds in the upper ocean.
- 313‰
- 96‰
19±24‰
Refractory
lipids
Sugar Fractions
Bulk Extract
A. The chemical fractionation scheme for DOC isolated by either ultrafiltration (as shown here) or solid phase extraction (SPE). 1H NMR spectra are included to highlight major functional groups in each fraction and radiocarbon (Δ14C) values for relevant fractions are shown for reference.
B. The cycling of labile, young dissolved compounds (e.g., sugars) and refractory DOC (e.g. certain lipids) as deduced from both the isolation scheme and Δ14C data (A) presented in this study. Multiple, dotted arrows identify processes that occur on long timescales. The refractory lipid in marine environments may be autochthonous or allochthonous, and is shown schematically to be associated with labile carbohydrates through an ester bond.