3D MODELING OF DIATOM FOSSILS REVEALS NUTRIENT BURIAL AND SEDIMENTATION RATE FLUCTUATIONS IN HERD LAKE, IDAHO, USA

A new method for estimating bio-volume of fossil diatoms is presented here. The samples used to develop this method were recovered from varved (annually layered) sediments in Herd Lake, Idaho. The varved nature of this core allows for precise timing of diatom burial at annual resolution. The Herd Lake diatom assemblage is comprised of 16 species contained within 13 genera, dominant taxa being Stephanodiscus niagarae and S. parvus (over 90% of the total population). These species are reconstructed in 3D computer aided design software (AutoCAD) which allows for calculation of the volume of individual diatoms. Diatoms are single celled algae that contain a silicon dioxide (SiO₂) outer cell wall (valve). The valves of diatoms fossilize readily and comprise a significant portion of lacustrine sediments especially in diatomite and diatomite-forming lakes. When diatoms fossilize they remove silica and oxygen from a lake’s available nutrient load and transfer it to sediment. As such, 3d models of the valves allow diatom-induced burial of silica and oxygen to be quantified on a species-specific basis. This method reveals the biological contribution to the sediment load and nutrient removal from water in the Herd Lake Basin.

Herd Lake formed ~362 CE when a landslide blocked a valley. There are currently ~11 meters of sediment in the lake and the maximum water depth is ~11 meters. Calculating the diatom-induced sediment load will enable us to estimate how much influence diatoms have had upon the timing of basin infill. Initial estimates are that the diatoms have contributed half of the sediment load and will have effectively cut Herd Lake’s life time in half. The method devised here greatly informs lake ontogeny models. Further deployment of this new method will inform age models of sediment and rock cores containing large diatomite sections such as those from deep marine cores from the Southern Ocean and long lacustrine cores from African Great Lakes. This will greatly improve our understanding of the global and regional silica-cycles and will inform oxygen-cycle models. Further experiments will need to be conducted to calculate if diatom fossilization negates the oxygen output by diatoms due to their photosynthetic activity.