SOIL CHRONOFUNCTIONS AS A GEOCHRONOLOGIC TOOL: APPLICATION TO MAPPING COMPOSITE FLUVIAL TERRACES, FOUR MILE CREEK, OHIO

Determining the age of fluvial terraces is vital to reconstructing the depositional history of a stream system and establishing modern flood and erosion risk. Cyclic climate change during the Holocene caused alternating periods of incision and aggradation in midwestern US watersheds. Traditionally, terrace scarps define the boundary between geomorphic surfaces of different ages within a river valley. However, if a stream aggraded to the same elevation multiple times, distinct periods of overbank deposition would be preserved without clear terrace scarps. In this scenario, the landscape is relatively flat across a ‘composite fluvial terrace’, requiring a novel mapping strategy. Calculable changes to geochemistry over time can be used to infer the age of soils on terrace surfaces and map the spatial distribution of time-stratigraphic overbank deposits. In this study, a portable XRF was used to measure geochemistry of nine radiocarbon-dated soil pedons on fluvial terraces along Four Mile Creek, Ohio spanning an age range from 50 to ~17,000 cal yrs BP. Parent material samples collected from the study area were analyzed to quantify initial geochemistry. Chronofunctions were calculated from simple linear regression models of weathering index changes over time, including Fe/Ca, Zr/Ca, and Ti/Ca, at multiple soil depths. Fe/Ca models returned R² values between 0.69 and 0.97 and had the lowest uncertainties compared to Zr/Ca and Ti/Ca models. Samples collected at 20-30 cm depth resulted in the highest R² values, between 0.93 and 0.97, compared to samples collected at 0-10 and 60-70 cm. These relationships were then used to infer soil age from soil geochemistry at 388 locations in a portion of the Four Mile Creek valley. Overbank deposits were delineated based on inferred soil age and a surficial geologic map was created. No deposits were mapped with an age between 2,500 and 5,000 years, suggesting a period of incision within the watershed. The boundary between soils < 500 years old and older soils is distinct, so the 500-year floodplain was delineated. Results support the use of this field-based technique to map time-stratigraphic fluvial overbank deposits at a high resolution. This method can improve our understanding of the depositional history of a stream system and inform zoning by determining modern flood and erosion risk.