Geochemistry of lignin biomarkers in marsh environments of the Georgia coast
Loftis, Kathy Mae
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Lignin biomarkers and their stable-carbon isotopic compositions have been used to detect vascular plant inputs in marine sediments, as well as to quantify the relative contribution of plants using either the C3 or the C4 photosynthetic pathway. However, δ13C values of lignin biomarkers in environmental sediment samples exhibit disparate trends among individual compounds (Goñi et al., 1997; Goñi et al., 1998). It is unclear if either the temporal variability of lignin δ13C values in source plants, or the effect of degradation on the compound-specific isotopic compositions of lignin derivatives, may result in the observed variability. We set out to address the underlying causes. The organic matter dynamics in salt marsh environments were investigated by following temporal variations of bulk organic matter parameters, lignin-derived phenols and their compound-specific stable carbon isotopic compositions in salt marsh plants and sediments. The δ13C values of lignin-derived phenols in the salt marsh plants did not exhibit clear temporal trends, largely due to the considerable variability in δ13C values during the study period. Different processes controlled organic matter dynamics at the two marsh sites. Sediments at a creek-bank were dominated by inputs of vascular plant matter from both C3 and C4 organic matter sources, which are deposited during periods of high river discharge. They are subsequently degraded, altering the geochemical signals in the sediment. At a high-marsh site sediments were composed predominately of C3 vascular plant inputs, most likely from J. roemarianus. Our next study assessed the impacts of plant material decomposition on geochemical characteristics including molecular isotopic signals. Leaves from two plants, Spartina alterniflora and Juncus roemarianus, were separately incubated under three different environmental conditions: in constantly oxic seawater, in surface (0-5 cm) and subsurface (>10 cm) sediments of a salt marsh in Sapelo Island, Georgia, USA. Our experimental results showed that the extent of decomposition and the variation in the δ13C values of lignin phenols differed for each plant and environment, highlighting the complexity in predicting the effects of degradation on the δ13C value of lignin phenols. We then present a mixing model that incorporates multiple parameters, with the goal more accurately estimating the combination of end-members to a mixture. We show that multi-parameter mixing models require the use of parameters that ensure mass conservation, which complicates the use of elemental ratios and δ values in mixing models. In a reanalysis of literature data on organic matter cycling, we investigated the impact of parameter choices and treatment on the interpretation of the observational data. Finally we conducted an in situ incubation experiment in a natural salt marsh environment to investigate how the bacterial communities in surface (0 – 5 cm) and subsurface (>10 cm) sediments vary during decay of S. alterniflora. The bacterial community on S. alterniflora litter in surface sediments did not change significantly over the course of the study. On the other hand, the bacterial community on litter in subsurface sediments changed significantly over the course of the study. Compared to the ambient sediments, the communities on the litters in both surface and subsurface sediments were less diverse.