Paleoclimate and Hydrology Reconstructed from Multiple Sediment Cores from Lake Helena

Abstract

Lake Helena was created when the Holter Dam was built across the Missouri River in 1911. Prior to that, the lowest part of the valley was likely a dynamic wetland environment responsive to climate change and precipitation in particular. Sediment cores containing the ~8000 yr-old Mazama Ash indicate that the lake sediments host a record of Holocene paleoecology and hydrology. While there are numerous paleoclimate records from around Montana, few are located near the mountain-prairie ecotone. We aim to discover if a series of lake sediment cores, obtained along a depth transect from the edge of Lake Helena can be correlated with each other and provide a record of climate and hydrological change through the Holocene. From previous cores, we have completed a charcoal record of fluctuating fire regimes as well as preliminary assessments of macrofossil and diatoms stratigraphy. Using new cores obtained in 2025, we will perform similar analyses of charcoal, macrofossils, and diatoms. Radiocarbon dating will allow us to determine if records such as fire frequency and shifts in plant communities are correlative between multiple cores along the transect. Particular attention will be paid to the macrofossil record in the core. A reference library of present-day plants (seeds, flowers, stems), growing along topographic moisture gradients, was collected in fall of 2024. This collection will assist us with identification and interpretation of the plant macrofossils preserved in the cores. Ongoing analyses of diatoms, sediments, and mineralogy provide supplementary records to aid in our paleoclimate reconstruction of the Helena Valley.