THE PRE-DEPOSITIONAL AGE OF ORGANIC MATTER IN FLOOD DEPOSITS: INSIGHTS TO THE TIMESCALE OF BIOGEOCHEMICAL PROCESSES

This study applies multiple radiometric techniques for establishing the timing of both historic and prehistoric (>AD 1600) flood events in the Tennessee River Valley using three sites near Huntsville, Alabama. It has been common practice to use ¹⁴C radiocarbon dates of bulk organic matter in floodplain soils. However, bulk ¹⁴C ages can be influenced by stable forms of organic matter from the elsewhere in the watershed being redeposited on the flooding surface. This allochthonous organic matter can derive from plant, soil, alluvial, and bedrock sources that can have widely different ages. To determine the best datable organic material for constraining the timing of past flood events, complementary methods were used to age and characterize the floodplain soil organic matter. Radiocarbon (¹⁴C) ages were determined by accelerator mass spectrometry and compared to optically stimulated luminescence (OSL) derived depositional ages of quartz from the same deposit. The ¹⁴C of the bulk soil organic matter was consistently 1 ka older than the OSL-based ages. The age offset between ¹⁴C and OSL is suggestive of a time lag related to biogeochemical or geomorphic controls. Molecular spectroscopy (¹³C nuclear magnetic resonance) revealed that charcoal was the most abundant component of the organic matter at all three sites and increased with depth while the lipid, lignin, protein, and carbohydrate fractions decreased with depth. To better understand the origin of ¹⁴C and OSL age differences, we used a sequentially remove specific biochemical fractions from the bulk organic matter, which could be verified spectroscopically and subsequently radiocarbon-dated. The charcoal fraction at the was calculated to be between 930 to 1085 years older than the OSL-derived quartz ages. The lipid fraction was found to be older than the charcoal fraction and the ¹⁴C date was 1134 to 2838 years older than the OSL ages. Lignin showed the smallest offset and was between 205 years younger to 761 years older than the OSL age. We will discuss the implications of the results for geomorphic controls (soil erosion, transport, deposition) upon the biogeochemical cycling of carbon.