Variations in lignin content deposited in the Congo fan and its potential for oxidative degradation

The deposition of terrigenous organic carbon (tOC) in offshore deep-marine settings has traditionally been viewed as inconsequential for organic carbon burial. However, it has been shown that deep-sea sediment flows, turbidity currents, can contribute significantly to the burial of tOC. Elemental, isotopic, and molecular data were obtained on sediment samples from three areas within and adjacent to the Congo Deep-Sea Fan. The elemental, organic geochemical, and isotopic data agree well with previous studies from the Congo Fan, which show that terrigenous organic matter from the Congo River extends seaward in the axis of the submarine canyon to abyssal depths. Using advanced solid-state 13C NMR and TMAH thermochemolysis data we verify that a significant amount of lignin is exported to the canyon (∼14 % wt. lignin) and the distal lobe (∼16 % wt. lignin) sediments. The basin plain contains no detectable lignin but does show the presence of terrigenous long-chain fatty acids having an even carbon number predominance. Following a laboratory oxidation experiment on sediments from the distal lobe for 6 d there was an organic carbon mass loss of 59.8 % and the solid-state 13C NMR spectrum shows a major reduction in peaks associated with carbohydrate-like and lignin molecules and a relative increase in aliphatic molecules. This shows that terrigenous lignin molecules can be remineralized to CO2 and/or altered to structures that no longer resemble that of lignin through oxidative degradation processes. This would have a potentially significant implication on what is traditionally viewed as autochthonous marine organic matter.