FLY ASH IN RAVINE ALLUVIUM: MAPPING THE HOLOCENE-ANTHROPOCENE DIVIDE IN THE LSRB

Ravine fans in the Le Sueur River Basin (LSRB) in southern Minnesota preserve the record of early Holocene and modern anthropogenic disturbances. Seventy meters of base-level drop at the end of the last glaciation initiated millennia of incision that continues on the LSRB today. Onto this template of on-going incision, Euro-American land clearing and drainage of previously stable upland prairie and wetlands in the mid-1800s may have further increased erosion rates in the basin. Ravines, first-order channels that link low-gradient uplands with the deeply-incised channel network, likely experienced changes in erosion rates over time from both of these impacts, with the erosional history preserved in alluvial fans at the mouths of ravines where they terminate on fluvial terraces. We take advantage of these fan deposits to determine the fluvial response of upland agricultural land conversion on steep first-order drainages. Fly ash from coal combustion was used as a stratigraphic marker in fan deposits recording an event (Western settlement) in the fan’s history—its source being the nearby city of Mankato where railroads traversed the region starting in 1868. Bulk soil samples collected at 10, 20, 40, 100, and 200 centimeter depths on six fan surfaces as well as in any incised channels dissecting the fan surface were analyzed for fly ash presence and abundance. Initially, all samples were processed to obtain the magnetic fractions, then observed under a transmitted and reflected light microscope to confirm or deny the presence of fly ash. EDS analysis on the scanning electron microscope distinguished fly ash grains from other natural magnetic grains, which differ in shape and elemental composition. Basin average fly ash thickness was 135 centimeters ranging from 95 to 157 centimeters, which yields a deposition rate of 0.91 cm/yr with greater fly ash density correlating with proximity to its source. Elsewhere in the region, Lake Pepin experienced a ten-fold increase in sediment deposition as a result of land use change. We believe further research will confirm regional deposition trends in ravine post-settlement alluvium.