Sources and early diagenesis of lignin and bulk organic matter in the sediments of the Lower St. Lawrence Estuary and the Saguenay Fjord
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Elemental and molecular organic matter concentrations were analyzed in sediments from the Lower St. Lawrence Estuary and the Saguenay Fjord in order to evaluate the historic evolution of pulp and paper mills solid-waste inputs in the system in the last decades and the relative reactivities of lignin and bulk organic materials in coastal sediments. A qualitative estimation of vascular plant sources to the Saguenay Fjord shows that the sedimentary terrigenous plant material is comprised predominantly of gymnosperm woods. In the deeper sediment horizons of the upper Saguenay basin, low intensive lignin parameters (C/V and S/V) and high percentages of lignin to total sedimentary organic carbon (> 20%) all indicate elevated concentrations of woody gymnosperm tissues unprecedented in coastal sediments and directly related to the intense activity of the region's pulp and paper industries. The increased control on solid organic wastes from industrial effluents into the Saguenay fiver in the late 1980s to early 1990s is clearly apparent from increasing intensive parameter values and decreasing lignin fractions to the total sedimentary organic carbon ( 6-8%) in the upper basin surface sediments. Elemental and molecular analyses of fjord sediments, all indicate that most of the solid-phase discharge of lignified material by the pulp and paper industry is deposited rapidly close to the mouth of the river without reaching the downstream basins. In the St. Lawrence Estuary, intensive lignin parameters indicate that gymnosperm tissues are a major component of the sedimentary vascular plant material but with a significant fraction composed of angiosperm and nonwoody tissues. These latter types of organic tissues are particularly important components of terrigenous material in sediments deposited prior to the 1910-1920s. Acid/aldehyde ratios in most cores studied do not indicate clear-cut oxidative degradation of lignin material prior to its introduction in the aquatic system. The only exceptions are the two estuarine cores, where slightly elevated acid/aldehyde ratios relative to the range for fresh vascular plant tissues, might indicate mild aerobic fungal degradation of the sedimentary lignin material. Organic carbon, total nitrogen, organic phosphorus and lignin derived phenols all exhibited decreasing concentrations with core depth in the sediments of the Lower St. Lawrence Estuary. First-order degradation rate constants for all four chemical categories ranged between 0.02-0.05 yr-1. The order of apparent reactivity among the different organic compounds is TN C(org) > P(org) > lignin at the head of the Laurentian channel and lignin TN = C(org) > P(org) further downstream. The surprising diagenetic selectivity observed at the upstream station is probably due to a higher flux of fresh, labile organic matter that reaches the sediment-water interface and degrades preferentially to more refractory materials such as lignin. Further downstream, little diagenetic selectivity was observed below the sediment-water interface indicating an overall refractory nature of the sedimentary organic matter. Finally, the differences in reactivity observed between C(org) and P(org) at both stations contradict earlier assumptions that no fractionation occurs between organic carbon and phosphorus during anaerobic degradation.
author list (cited authors)
Louchouarn, P., Lucotte, M., Canuel, R., Gagn, J., & Richard, L.
complete list of authors
Louchouarn, Patrick||Lucotte, Marc||Canuel, René||Gagné, Jean-Pierre||Richard, Louis-Filip