Algal-silica cycling and pigment diagenesis in recent alpine lake sediments: mechanisms and paleoecological implications

The quality and interpretability of the paleobiological record depends on the preservation of morphological and geochemical fossils. Siliceous microfossils and sedimentary pigments are often cornerstones in paleoecology, although the microbial and geochemical processes conducive to their preservation remain poorly constrained. We examined sediments from an alpine lake in Banff National Park (Alberta, Canada) where diatom frustules are completely dissolved within 50 years of deposition. Diatom dissolution, silica recycling, and diagenetic alteration of algal pigments were investigated, in conjunction with porewater geochemistry and microelectrode profiling of the sediment-water interface. Analysis of sediment trap material showed similar to 90% of biogenic silica (BSi) production is lost prior to burial. Silica flux calculations, based on dissolved silica (as H4SiO4) in pore-waters, show a further similar to 6% of total BSi is returned to the water column from the upper 4 cm of sediments, implying that only similar to 4% of total BSi is permanently archived in sediments. In situ sediment pH and O-2 profiles reveal that aerobic respiration by bacteria fully consumes oxygen by a depth of 4 mm into the sediment, with associated strong pH and redox gradients. During sedimentation and early diagenesis, diatoms undergo loss of extracellular polymeric substances that coat their frustules, promoting silica dissolution and leading to the loss of the microfossil record by a depth of 3.25 cm. Sedimentary pigments similarly undergo rapid degradation, but diatom-related carotenoids persist below the depth of silica dissolution. This work provides new insights on diagenetic processes in lakes, with broad implications for the interpretation of sedimentary proxies for algal production.