Extending the analytical window for water-soluble organic matter in sediments by aqueous Soxhlet extraction

Dissolved organic matter (DOM) in marine sediments is a complex mixture of thousands of individual constituents which are involved in various biogeochemical reactions and serve as substrates for benthic microbes. The bioavailability of DOM compounds is controlled by their size and reactivity. Knowledge of the molecular composition of DOM is a prerequisite for a comprehensive understanding of the biogeochemical processes in sediments. In this study, interstitial water DOM was extracted with Rhizon samplers from a sediment core from the Black Sea and compared to the water-extractable organic matter fraction (<0.4 µm) obtained by Soxhlet extraction from the corresponding sediments. This method mobilizes labile particulate organic matter as well as DOM that is adsorbed to mineral surfaces or bound in complexes and polymeric aggregates. After solid phase extraction (SPE) of DOM, samples were analyzed for the molecular composition by Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry (FT-ICR MS) with electrospray ionization in negative ion mode. On average, 63% of the dissolved organic carbon (DOC) in the interstitial water DOM was amenable to SPE. FT-ICR mass spectra were predominated by compounds consisting of C, H and O. Soxhlet extraction yielded up to 4.35% of the total sedimentary organic carbon, which is more than 30-times the organic carbon content of the interstitial water. However, the Soxhlet extracted organic matter was less amenable to SPE (