Analysis and Characterization of Organic Carbon in Early Holocene Wetland Paleosols using Ramped Pyrolysis 14C and Biomarkers

Abstract:

Radiocarbon analyses are a key tool for quantifying the dynamics of carbon cycling and storage in both modern soils and Quaternary paleosols. Frequently, bulk ¹⁴C dates of paleosol organic carbon provide ages older than the time of soil burial, and ¹⁴C dates of geochemical fractions such as alkali and acid extracts (operationally defined as humic acids) can provide anomalously old ages when compared to coeval plant macrofossil dates. Ramped pyrolysis radiocarbon analysis of sedimentary organic material has been employed as a tool for investigating ¹⁴C age spectra in sediments with multiple organic carbon sources. Here we combine ramped pyrolysis ¹⁴C analysis and biomarker analysis (lignin-phenols and other cupric oxide products) to provide information on the source and diagenetic state of the paleosol organic carbon. We apply these techniques to immature early Holocene brackish wetland entisols from three sediment cores in southeastern Louisiana, along with overlying basal peats. Surprisingly, we find narrow ¹⁴C age spectra across all thermal aliquots from both paleosols and peats. The weighted bulk ¹⁴C ages from paleosols and overlying peats are within analytical error, and are comparable to independently analyzed ¹⁴C AMS dates from charcoal fragments and other plant macrofossils from each peat bed.

Our results suggest high turnover rates of carbon in soils relative to input of exogenous carbon sources. These data raise broader questions about processes within the active soil and during pedogenesis and burial of paleosols that can effectively homogenize radiocarbon content in soils across the thermochemical spectrum. The concurrence of paleosol and peat ¹⁴C ages also suggests that, in the absence of peats with identifiable plant macrofossils, ramped pyrolysis ¹⁴C analyses of paleosols may be used to provide ages for sea-level indicators.