DIATOM AND SEDIMENT GEOCHEMISTRY OF HERD LAKE: A HIGH-RESOLUTION VARVED LAKE RECORD

Herd Lake formed during a landslide event that dammed a river valley approximately 2,500 years ago. The lake is located approximately 35 miles south of Challis, Idaho. Herd Lake exhibits very high productivity and sediment burial rates compared to other lakes in the Salmon River basin. This has resulted in thick, seasonal laminations (varves) in the sediment record for nearly the entire history of the lake. In 2011, a 53-cm surface core was collected and sub-sampled by individual lamellae, producing a sub-annually resolved sediment record extending back to AD1923. We analyzed a suite of indicators from this core, including fossil diatom assemblages, elemental concentrations, and detailed phosphorus and other sediment geochemistry to explore the recent history of the lake basin.

Here we present the 88-year record of downcore changes in these indicators, focusing primarily on the transitions in the diatom assemblages, elemental concentrations, and phosphorus geochemistry. Together our environmental indicators from the lake sediments highlight interesting interactions between the landscape, watershed, and the lake. Correlation in the record between burial of oxide-bound phosphorus and variation in the relative abundance of a large-diameter planktonic diatom species suggests that these two indicators are closely associated with seasonal variability in the intensity of convective lake mixing. The record of percent carbon, percent nitrogen, organic phosphorus, and total phosphorus all display nearly identical downcore patterns, including a long-term trend of increasing nutrient burial over the last century. Using this rich collection of environmental indicators, we compare our high-resolution record to regional and local historical climatological data to explore potential teleconnections that may explain the long-term trends in variability observed.