Characterization of organic matter in surface sediments of the Mackenzie River Basin, Canada

The particulate organic matter in < 63 µm surface sediments from the Mackenzie River and its main tributaries was studied using Rock-Eval pyrolysis and organic petrology. The organic matter in the sediments is dominated by refractory residual organic carbon (RC) of mainly terrigenous nature, as indicated by abundant inertinite, vitrinite, and type III kerogen. Sediments from the tributaries contained significantly more algal-derived organic matter than from the main channel of the river, highlighting the importance of low-energy system dynamics in the tributaries, which allows modest algal production, more accumulation, and better preservation of autochthonous organic matter. This is particularly true for tributaries fed by lacustrine systems, which showed the highest S1 and S2 fractions, and consequently higher total particulate organic carbon (POC) in the basin. Organic petrology of the sediment samples confirms abundant liptinitic materials (i.e., fat-rich structured algae, spores and pollen, cuticles, and resins). Forest fire and coal deposits are also confirmed to contribute to the basin. Assuming that suspended and fine surfacial sediments have a similar OC composition, the Mackenzie River is estimated to deliver a total POC flux of 1.1 Mt C/yr to its delta, of which 85% is residual carbon with liptinitic OC (S1 + S2) and S3 accounting for another 9% and 6%, respectively.