The Sedimentary Si/Al Ratio as an Indicator of Diatom Productivity Changes

Elsa Peinerud Division of Applied Geology, Luleå University of Technology, 971 87 Luleå, Sweden

The Si/Al ratios in sediments depend upon the Si/Al ratio of the settling suspended matter. Where the suspended matter
is dominated by detrital particles, i.e. physically eroded soil or bedrock particles, the Si/Al ratio will be close to the ratio in
soil or bedrock within the catchment. Another important phase for suspended Si is diatoms. High suspended matter Si/Al
ratios caused by intense diatom production lead to high sedimentary Si/Al ratios. In many lakes, diatom frustules constitute a considerable part of the sediments. If the diatom production in the water column changes, the Si/Al ratios in the suspended matter, and later on in the sediment, will change. The correlation between the Si/Al ratio and the number of diatom valves in sediments can be established by counting diatom valves under the microscope.

In Sakajärvi, a lake in the Kalix River catchment in northern Sweden, the Si/Al ratio in the sediment declines from about 16 at the depth of approximately 30 cm to between 4 and 6 in the uppermost layers. This lower value is close to the Si/Al ratio of the local till, representing detrital matter. Supposing the only additional silica-containing phase is diatom remains,
this decrease implies a dramatic reduction of the diatom productivity. The decrease corresponds in time to the onset of industrialization. As a consequence of the adjacent mining activities, there is likely to have been a change in lake chemistry. Enhanced concentrations of several heavy metals and dissolved nitrogen could have affected the diatom community and led to a lowered productivity. In the sediments of Kutsasjärvi, an unpolluted lake in the same area, the Si/Al ratio is higher (around 24) and with only insignificant vertical variations, suggesting a diatom production of undiminished extent.