Diatom based climate reconstruction for the western Great Lakes region

Background/Question/Methods

Pollen-based studies indicate that mesic (less drought tolerant) taxa increased at the prairie-forest border and throughout the western Great Lakes region ~700 years ago.The timing of mesic taxa expansion corresponds with the Little Ice Age (LIA) found across much of the northern hemisphere. However, there are few climate records that are independent of vegetation in this region and inconsistencies between studies and research sites remain problematic. We analyzed lake sediment from a transect of shallow water cores from Cheney Lake, WI. Cheney Lake is a relatively small (<10 ha), climate responsive seepage lake with a deep basin (6.4 msimple bathymetry, surrounded by jack pine forest with no nearby buildings or roads. Diatom samples were analyzed at 0.5 cm resolution from a core in ~50cm of water with several sand lenses. A minimum of 300 valves per sample were counted to obtain a sub-decadal history of the diatom species assemblage in Cheney Lake.  Modern surface-sediment samples were also collected along multiple transects to assess diatom species diversity along a water depth gradient to reconstruct past lake levels using the modern analogue (MAT) and weighted-averaging (WA-PLS) techniques. Each of the surface sediment transects consisted of multiple replicates at a resolution of 1m to a total depth of 6m.   

Results/Conclusions

Current data suggest that littoral zone habitats shifted abruptly approximately 1500 (57.5 cm) and 5500 (90 cm) years ago to relatively shallow conditions. Diatom species assemblage data suggest that available light and low water depth severely limited the growth of benthic motile and attached diatom species. Little diatom material was retrieved at depths below 57cm in the sediment core; however silica is still readily available throughout the system as evidenced by abundant fresh-water sponge spicules and Chrysophyceae cysts. Little variance in diatom species assemblages after 1500 cal BP suggests that extended droughts of this severity were absent. These results are consistent with results of loss on ignition that suggest a long-term increase in moisture and more climate variability after 1500 cal BP.