Lake Helena Charcoal and Macrofossils as Indicators of Paleoclimate

Abstract

Lake Helena formed in 1912 when the Upper Missouri dam system, particularly Hauser Dam, flooded a preexisting wetland fed by Silver, Tenmile, and Prickly Pear Creeks. Wetlands and lake basins act as long-term sinks for organic and inorganic material, and their low-oxygen sediments accumulate layered deposits that hold thousands of years of environmental history. Sediment cores from Lake Helena, containing the ~8,000 yr-old Mazama Ash, indicate that lake sediments provide a record of Holocene paleoecology. This study examines how macroscopic charcoal and macrofossils reveal past fire and vegetation patterns that act as indicators for climate-driven environmental change. Macroscopic charcoal is produced during local forest fires and transported by wind or water into lake sediments, providing evidence of fire frequency and fuel type (wood versus grass). Plant macrofossils are preserved as leaves, seeds, needles, and other identifiable remains, representing past vegetation communities and shifting in response to climate. The shape and size of mineral grains may also indicate landscape processes such as sediment mobility. Previous cores have been analyzed to produce a charcoal record of variable fire regimes, preliminary macrofossils, and diatom stratigraphy. Using cores obtained in 2025, analysis of charcoal and macrofossil abundance allows us to reconstruct how fire and vegetation varied through time and how these changes reflect paleoclimate trends. This work provides insights into the natural processes that have shaped the Helena valley over the past 10,000 years and helps us understand how present-day ecosystems respond to climate change.