Relationship between organic matter and mercury in recent lake sediment: The physical–geochemical aspects

This study addresses the physical geochemical aspects of the relationship between Hg and organic matter in recent sediment from eutrophic lakes in central Alberta, Canada. The types of organic matter in the sediment are classified based on their degree of thermal degradation and their petrographical characteristics. This study uniquely applies the methods conventionally used in petroleum geosciences (Rock-Eval^® analyses and organic petrology) to investigate the relationship between various types of organic matter and the concentration of Hg in sediment.The results show that the total organic carbon (TOC) in sediment represents the sum of various organic compounds, which may play a completely different role in the distribution and accumulation of Hg. Strong correlations between TOC and the concentration of Hg in the studied sediment arise mainly from the thermally labile portion of organic matter released during pyrolysis under 300 °C. These compounds primarily consist of easily degradable algal-derived lipids and various pigments, which are petrographically described as soluble organic matter (SOM). The preserved SOM in sediment is commonly entrapped within the cell walls of phytoplankton and also appear as surface coating on sediment particles. The strong affinity between Hg and SOM is due not only to its chemical reactivity, but also to the physical characteristic of these labile compounds. The SOM may provide a substrate with enormous surface area by concentrating on the finer sediment size fractions and potentially acting as a “concentrator” for Hg and other organic-associated elements. Lastly, the quantity of the SOM has been calculated as an “elemental concentrator” portion of the TOC, which plays the most important role in the distribution of Hg in sediment.