Determination of refractory organic matter in marine sediments by chemical oxidation, analytical pyrolysis and solid‐state 13C nuclear magnetic resonance spectroscopy

Abstract

SummarySeeking to quantify the amount of refractory organic matter (ROM), which includes black carbon‐like material (BC), in marine sediments, we have applied a two‐step procedure that consists of a chemical oxidation with sodium chlorite of the demineralized sediments followed by integration of the aromatic C region in the remaining residues by solid‐state 13C nuclear magnetic resonance (NMR) spectroscopy. The efficacy for lignin removal was tested by analytical pyrolysis in the presence of tetramethyl ammonium hydroxide (TMAH). Riverine, estuarine and offshore marine sediment samples were collected from the southwest Atlantic coast of Spain, a site of geological and environmental interest. Measured contents of BC‐like material ranged between 3.0 and 45.7% of the total organic carbon. Greater relative BC contents were found in riverine sediments close to urban areas, which show an elevated input of anthropogenic organic material. The contents of BC‐like material in offshore marine sediments (5.5–6.1%) were similar to those previously reported for these kinds of samples. However, NMR and pyrolysis‐GC/MS of the isolated ROM reveals that abundant refractory aliphatic organic material remains in most of the marine samples after chlorite oxidation. We suggest that this pool of aliphatic carbon may play an important role as a stable carbon pool within the global C cycle.