Distribution and biogeochemistry of sedimentary humic substances in the St. Lawrence Estuary and the Saguenay Fjord, Québec

The concentrations of sedimentary humic substances (HS) were measured in the St. Lawrence marine system, Québec (Canada), including the Saguenay Fjord. Concentration ranges for humin, humic acids and fulvic acids were 7–49, 1–19, and 0.5–6 mg per gram of sediment (dry weight), respectively. HS always represented the dominant component of sediment organic matter (SOM) (i.e., 52–100%) for which humin, humic acids, and fulvic acids accounted for 38–88%, 4–28%, and 2–14%, respectively. These results indicate that most HS and an important part of SOM were associated with the mineral matrix and thus defined as humin. HS concentrations varied slightly by site location and depth but major changes in concentration in the fjord were caused by episodic events such as floods and landslides. In the fjord, sedimentary HS were mostly terrigenous and molecules such as lignin likely contributed to the high proportions of HS. In the St. Lawrence Estuary, an important input of marine particulate organic matter and favorable conditions for its degradation seem to have promoted the mineralization of the labile SOM before its burial. The less labile SOM was buried in sediments where humification and stabilization appeared to occur. Such processes could explain the lower SOM concentrations but greater humin proportions with increasing distance downstream and sediment depth. Estimations based on HS profiles suggest that 25% of the surface SOM will be preserved in deeper layers (i.e., >1 m), principally in the form of humin. Sorption of organic matter onto mineral surfaces might have contributed to SOM preservation and humin stabilization in sediments. DRIFT spectroscopy suggests that carboxylate groups may mediate this sorption.