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Molecular Analysis of Flood Deposits from the Tennessee River Valley: Insights into Carbon Cycling and Dating


Emily Blackaby, SA- 8483, Owen Craven, SA-8485, William Hockaday, Steve Forman, Gary Stincomb, and Stewart Lance


Introduction


The middle Tennessee River Valley contains both his- toric and prehistoric (>AD 1600) flood deposits. Stratigraphic sequences of stacked flood deposits that often bury soils provide new insight to the timing, frequency, and magnitude of flood event prior to and after European colonization. When dating stacked flood deposits, it is common to use radiocarbon ages of the bulk organic material. However this may be problematic as bulk 14C radiocarbon ages are a weighted average of organic matter from before and after the flooding event. Floodwaters might transport organic matter from materials such as plants, soils, alluvium, and bedrock—each having potentially substantial inbuilt age derived from their inherent residence times on the landscape prior to the flood events. Bulk organic matter from flood deposits is also likely to include carbon that was fixed from the atmosphere after the flood event, by riparian vegetation. Thus, bulk 14C radiocarbon ages of humates or even plant tissues may not be representative of the flood timing.


Our approach to determining the best material to date for the purpose of constraining the timing of past flood events is to compare multiple complimentary methods. This project will use molecular spectroscopy to characterize the organic matter in floodplain deposits, and then chemically isolate various fractions of the bulk carbon (e.g. charcoal, lignin, lipids) for 14C analysis. We will then compare the 14C age of the organic fractions to the depositional age of silica, determined by opti- cally stimulated luminescence.


Methods


Three sites along the Tennessee River were chosen for this study: Right Bank Canal Site (T003a), Left Bank Canal Site (T003b), and the Bond Site (T007) (Figure 1).


Ten samples were taken from three sites. Each sample was dated using optically stimulated luminesce (OSL) at Baylor University. The molecular composition of each sample was measured using solid-state 13C nuclear magnetic resonance spectroscopy (NMR). The integrated NMR peak areas, which quantify the molecular structures, were used in the molecular mixing model (MMM) described by Baldock et al, 2004. This model estimates the weight percent of common organic molecu- lar classes such as charcoal, lipids, and proteins.


Results and Discussion:


The OSL ages of the samples ranged from 100 to 2640 years old (personal conversation, Steve Forman), and NMR


58 TPG • Jul.Aug.Sep 2018


Figure 1. A) The Right Bank and Left Bank Canal Sites. The Left Bank Canal Site is located slightly downstream on the inside of a large meander bend. B) The Bond Site is located approximately 10 miles upstream from the Right Bank and Left Bank Canal sites.


analysis revealed that the samples were charcoal dominated. Charcoal comprised 35.5 to 56.0% of the samples and was most prominent in the older samples. Younger samples, while still charcoal dominated, tended to display more molecular diversity. The charcoal fraction increased in weight percent with depth at all three sites (Figure 2 on page 59) which may indicate that it is more chemically stable than other molecular classes and less likely to degrade through time. This resistance to degradation may also indicate that the charcoal fraction could have been on the landscape much longer before deposi- tion, making it possible that any radiocarbon date on the bulk organic fraction may be significantly skewed. We are currently in the process of measuring the radiocarbon ages of the bulk organic matter as well as isolated charcoal fragments.


Conclusion


The flood deposit paleosols were charcoal dominated and showed an increase in charcoal by weight percent with depth. Due to the chemical stability of the charcoal fraction, it is likely that the increase in charcoal weight percent is due to its resistance to degradation as this trend was observed at all three sites. The stability of charcoal provides charcoal with a long residency time that may skew bulk radiocarbon dates.. Further work is currently being conducted to compare OSL, bulk radiocarbon, and isolated charcoal radiocarbon ages in


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