Geochemistry and mineralogy of southwestern Lake Superior sediments with an emphasis on phosphorus lability

Purpose: The lability of phosphorus (P) in Lake Superior sediments and its relationship with sediment mineralogy and geochemistry remains poorly understood. The purpose of this study is to provide information about southwestern Lake Superior (SLS) sediment P lability and relate this information to the mineralogical composition and geochemical characterization of these sediments. Materials and methods: Samples were collected from riverbed (input), shoreline (input), and lake bottom. Thirteen samples were submitted to P sequential extraction and nine were submitted to mineralogical and geochemical analysis to provide context to measures of P lability. Results and discussion: Sediments were composed of non-clay minerals of low P reactivity (quartz, augite, calcite, dolomite, anorthite, microcline, and magnesite) and minerals with greater P reactivity such as oxides (rutile and hematite) and clay minerals (kaolinite, smectite/vermiculite, chlorite, mica or illite, and interstratified clays). The total P concentrations in lake bottom, riverbed, and shoreline sediment sources were, on average, 1151, 945, and 837 mg kg⁻¹, respectively. However, labile P [extracted by NH₄Cl (P loosely sorbed) and NaHCO₃+Na₂S₂O₄ (P redox sensitive)] represented less than 10% of the total P. More than 80% of P in the sediments was associated with organic compounds and carbonates with low availability to biota. Conclusions: Findings indicate that riverbed, shoreline, and lake bottom sediments contain very low concentrations of P available to aquatic biota due to the presence of minerals with high chemical reactivity with potential to adsorb P, such as poorly crystalized iron oxides, clay minerals like chlorite, and 2:1 clay minerals with hydroxy-aluminum interlayered minerals. These results help explain the high P retention in the lake and suggest that sediments in SLS may act as a long-term sink for P, which helps to explain the very low concentration of P found in the SLS aquatic system.