DIATOMS AND PHOSPHORUS: RECONSTRUCTING CONVECTIVE LAKE MIXING AND DROUGHT CYCLES OF HERD LAKE

Herd Lake is located roughly 35 miles south of Challis, Idaho in the Salmon River basin. The lake formed during a three kilometer-long landslide event that dammed a river valley approximately 2,500 years ago. Herd Lake exhibits very high productivity and sediment burial rates resulting in nearly continuous, thick, seasonal laminations (varves) in the sediment record for nearly the entire history of the lake. A 53-cm surface core was collected in 2011 and subsampled by individual lamellae, producing a sub-annually resolved sediment record extending back to AD1923. In 2013, a 10.99-meter core was collected from the lake and was sub-sampled by u-channel and again sub-sampled down-core by individual lamellae. We analyzed a suite of indicators from both cores, including fossil diatom assemblages, elemental concentrations, and detailed phosphorus geochemistry. Throughout both cores, high concentrations of fossil diatoms were preserved; these samples were characterized by two species of planktonic diatoms distinguished by vastly different valve sizes.

Over the last century, diatom productivity in Herd Lake appears closely tied to the timing and intensity of drought cycles in the region; when regional effective moisture increased, total diatom productivity tended to increase as well. We also found that burial of oxidized phosphorus correlated with periods of higher relative abundance for the large-diameter planktonic diatom species, suggesting that these two indicators are closely associated with seasonal variability in the intensity of convective lake mixing. Here we intend to expand the results from the short core to the longer record from Herd Lake and use the same tools to reconstruct the past climate conditions at a lower resolution.