Reconstruction of a regional, 12,000-yr silica decline in lakes by means of fossil sponge spicules
A newly developed technique for reconstructing past dissolved reactive silica (DRSi) concentrations in lakes demonstrated an unexpected, regional decline in DRSi in northern Wisconsin lakes over the Holocene. Previously, measurements of lake DRSi, an important nutrient for certain phytoplankton, have been limited to time periods for which direct observations were available, typically years to decades. The new paleolimnological technique is based on the sizes of siliceous sponge spicules preserved in lake sediments; it shows that DRSi declined gradually, but significantly, over the past 12,000 yr in each of eight lakes cored (geometric mean decline of 35-fold; range 4.1–270-fold). Mass balance considerations show that the decline could be caused by decreases in groundwater DRSi concentration or groundwater inseepage rates, by increases in net Si sedimentation, or by a combination of these factors. Although we cannot rule out any of these possible mechanisms, the most likely seems to be decline in groundwater DRSi concentration due to differential weathering of the silicate glacial tills. With appropriate calibration, paleospicule techniques may be useful in assessing long-term silica dynamics in other regions.