LAKE CIRCULATION INFLUENCED BY SOLAR ACTIVITY?

Diatom communities often undergo seasonal succession driven by changes in their aquatic environment. We studied seasonal diatom dynamics in temperate and boreal lakes and their significance for interpretations of paleoclimate from fossil diatom assemblages. A sediment trap study with bi-weekly resolution was performed in 4 boreal lakes located along an elevation gradient in the Laurentian Mountains, eastern Canada. The insights gained from these process-based studies were applied on fossil diatoms preserved in a sediment record from one of the lakes, lac du Sommet. The relative abundance of the dominant fossil diatom Fragilaria virescens var. exigua was highly correlated to diatom flux, which was correlated to colder July temperature and wind velocity in June, suggesting that diatom growth is favoured by an unstable water column. Modern occurrence of F. virescens var. exigua was associated with strong circulation of the water column and high Si:SRP ratio. A strong correlation between the diatom-inferred circulation of the water column, F. virescens var. exigua and diatom flux with reconstructed number of sunspots (Solanki et al. 2004; r = 0.64; r = 0.67; r = 0.62, p < 0.0001) during the Holocene might suggests that our record is forced by solar activity. In addition, relative abundance of F. virescens var. exigua shows a ~2100-year cycle known as Hallstattzeit cycles in delta 14C data. These changes are independent of maturing of the lake or vegetation changes as investigated by diatom-based dissolved organic carbon reconstructions, LOI analysis and the analysis of fossil pollen. These proxies and chironomid-inferred water temperature seem to respond to local climate warming caused by the rerouting of the meltwater stream from the Laurentian Ice Sheet following the collapse of the ice dome in the Hudson Strait region around 8.2 ka BP. Except for this increase, diatom-inferred DOC and sediment organic matter content (measured as LOI) and chironomid-inferred water temperature showed only little variation. Inferred DOC is not significantly correlated to the number of sun spots. We conclude that sediment trap studies of sub-annual resolution can be a valuable addition to the surface-sediment calibration data sets in paleolimnological studies.