Abstract

Selected surface sediment samples (sediment depth 0–10 cm) collected in the Niagara basin (western Lake Ontario) were chosen for organic matter characterization using organic petrological and geochemical techniques normally reserved for the study of coal, petroleum source rock, and crude oil. The study was performed to assess the effectiveness of these techniques in discriminating between natural and anthropogenic organic matter and to determine the extent of organic pollution of the lake sediments. The organic carbon contents of the samples varied from 1.5 to 3.5%, with upper layer (0–5 cm) samples having higher total organic carbon values than those from the lower layer (5–10 cm) at each site. Relatively low Rock Eval hydrogen indices and high oxygen indices are consistent with dominance of the pyrolyzable organic matter by recent input. This is supported by the pyrolysis–gas chromatography/mass spectrometry results. On the molecular level, the pyrolyzates are complex mixtures of (predominantly short-chain) aliphatic hydrocarbons, simple phenols, abundant organonitrogen compounds, and simple monoaromatic and polyaromatic hydrocarbons, with only minor variation between samples. The organic petrologic results indicated a greater diversity, showing a predominance of amorphous organic matter in some samples and of coaly debris and combustion residues in others. Because the latter type of organic matter typically has low pyrolysis yields, the chemical techniques employed understated its importance. Its presence is indicative of direct anthropogenic input to the lake sediment. The amorphous organic matter is at least in part the result of anthropogenic perturbation of the ecosystem, being the subfossil remains of degraded algal and bacterial biomass whose growth had been stimulated by artificially high nutrient flux to the lake waters, particularly during the past century. The results of the study show the benefits derived from the tandem application of organic geochemical and petrologic techniques to environmental problems.