POST-GLACIAL ENVIRONMENTAL RECONSTRUCTION FROM SEDIMENT GEOCHEMISTRY, BELLS LAKE, NORTHERN IDAHO

Lake sediment geochemistry provides a valuable archive of past environments, including geomorphic processes, catchment weathering, ecosystem productivity, and paleoclimate. Bells Lake is a 6.3 ha lateral lake situated on the floodplain of the St. Joe River formed in the advancing delta of the St. Joe River into an arm of Lake Coeur d'Alene (CdA). We recoverd a continuous 15-m core using a Livingstone corer before stiff clayey sediments prevented further coring. The core base dates to 15.2 ka, shortly post-dating the last Missoula Flood. A chronology was built from seven radiocarbon dates and three tephras (including 148 cm of Mazama tephra; 7.6 ka).

The sediment consisted of black organic-rich sand mud (0-6.1 ka), silty mud (6.1 ka-8.0 ka), sandy mud (8.0-8.5 ka), muddy sand (8.5-11.6 ka) and clayey-silty mud (11.6-15.2 ka). A major change in limnological conditions at 6.1 ka is interpretted as the isolation of Bells Lake from Lake CdA. The thick Mazama tephra likely contributed to the rapid formation of a gun-barrel delta that extended into Lake CdA. The absence of gravel suggests that the river never migrated to the present lake location.

During the early Holocene, an increase in detrital geochemical proxies (Al, K, Fe, and Ti) and high sedimentation rate (3.72 mm/yr) reveal higher terrestrial input, high productivity, and warm and humid conditions. These changes likely resulted from intensified weathering conditions and high surface runoff with the latitudinal migration of the northern Hemisphere westerlies and diminishing of the continental glaciers. Occasional flooding might have been responsible for the deposition of fine sand layers at 11.6-11.2 ka. The 8.2 ka event as identified in the north Atlantic is visible by a sudden drop in the detrital components and magnetic susceptibility, probably pointing to reduced weathering during a cold and arid phase.

The Younger Dryas (12.9-11.7 ka) is clearly recorded by redox proxies (e.g., Mn/Fe) and three weathering indices, a decrease in TOC, and increased clays. This suggests increases in oxygenation (from lower lake levels or increased wind) and increased fine detrital input (possibly from glacial expansion). Overall, this high-resolution record demonstrates major post-glacial events not yet identified in studies from this region.