Date of Award
Doctor of Philosophy
Maria E. Uhle
Claudia Mora, Joseph Kelley, Jake Weltzin
Bulk carbon isotopes were previously used as a tool to differentiate between C3 and C4 plant communities in ancient salt-marsh deposits (Chmura and Aharon 1995). However, the bulk carbon values reflect organic matter contributions not only from salt-marsh plants, but also from algal bacterial inputs, as well as allochtonous terrestrial organic matter. The introduction of gas chromatography-isotope ratio mass spectrometery (GC-IRMS) allows for the separation of algal vs. higher plant contributions to the sediment pool. In the present study, this technique is applied to a core and modern plant samples collected from two Maine salt-marshes. We sampled 10 plant species common to Maine marshes including Spartina alterniflora, Spartina patens, Juncus Gerardi, Solidago sempervirens, Salicornia europa, Potentilla anserina, Atriplex patula, Plantago maritima, Suaeda maritima, and Limonium nashii. Isolation of two n-alkanes (C-27 and C-29) present in marsh plant samples allows for the reconstruction of past plant communities present in a Machiasport, Maine salt-marsh core. Observed δ13C values for the C-27 and C-29 homologues are ~ 10‰ more negative than what would be expected using bulk isotopic analysis. For the C-27 and C-29 homologues there is a 3-10‰ separation in carbon isotopic composition between C3 and C4 species. Carbon isotopic values down core show a transition from a C4 dominated planted community to a C3 dominated community, attributable to the migration of lower marsh zones onto higher marsh zones due to documented late Holocene sea-level rise (Gehrels et al., 1996). The two homologues used have the potential to provide paleoenvironmental information in future core work, where the sea-level variable will be controlled.
Sources of sedimentary organic matter to a Morse River, Maine (USA) salt marsh over the last 3390+/-60 RCYBP are determined using distribution patterns of n-alkanes, bulk carbon isotopic analysis, and compound-specific carbon isotopic analysis. Marsh foraminiferal counts suggest a ubiquitous presence of zone 1B deposits, implying that the deposits were laid down ~0.2m to 0.5m above mean high water. Distributions of n-alkanes show a primary contribution from higher plants, confirmed by an average ACL value of 27.5 for the core sediments, and CPI values all >3. Many sample depths have a maximum abundance at the C25 alkane. The ACL value for the average of 10 common marsh species is 29.1. Salicornia europa has a similar n-alkane distribution to many of the salt marsh sediments, and we suggest that it is either an important source to the biomass of the marsh through time, or that another unidentified higher plant source is contributing heavily to the sediment pool. Bacterial degradation or algal inputs to the marsh sediments appear to be minor. Compound specific carbon isotopic analyses of the C27 alkane are on average 7.2‰ depleted relative to bulk values, but the two records are strongly correlated (R2 = 0.87), suggesting that marsh plants are “swamping” the bulk carbon isotopic signal. Our study underscores the importance of using caution when applying mixing models of plant species to salt marsh sediments, especially when relatively few plants are included in the model.
A record of late Holocene sediment, foraminiferal, compound specific carbon isotope, and organic geochemical stratigraphy is presented for a series of radiocarbon-dated cores from four Maine, USA salt marsh sites. Though we present some evidence for correspondence between foraminiferal records, that indicate mean high water, and carbon isotope values, that record plant community fluctuations (p<0. 05), there are also significant excursions in isotope values within specific foraminiferal zones. These excursions are not predictable, do not correspond between field sites, and cannot be related with any confidence to environmental variables. We suggest that salt marshes in Maine undergo a major change in water level, photosynthetic plant community, or sediment influx on average at least once every 288 years, and probably much more frequently. We question the recent use of salt marshes as recorders of ancient climate change and urge caution in the interpretation of marsh pant community records.
Average fractal dimensions (D) are calculated for Maine’s four coastal compartments using a GIS approach and digitized USGS 7.5 minute series topographic quadrangle maps. The D values indicate relatively little complexity for the southwest coastal compartment (Avg. D=1.11), higher complexity for the south-central compartment (Avg. D=1.35), and intermediate complexity for the north-central compartment (Avg. D=1.23). Our analysis suggests that the northeastern compartment should be further divided into two sub-compartments (Cobscook Bay and Non-Cobscook Bay), which have Average D values of 1.37 and 1.18 respectively. Subdivision of the northeast coastal compartment is also supported by the geologic makeup of the region. Statistical tests show that all of the geologically-different coastal compartments can be discriminated in terms of D at the 95% confidence level, while the geologically similar compartments (south-central compartment and Cobscook Bay sub-compartment) cannot be statistically distinguished. Further research along previously glaciated shorelines should be carried out to build upon our results.
Tanner, Benjamin Ryan, "C3/C4 Variations in High Salt Marsh Sediments: An Application of Compound Specific Isotopic Analysis of Lipid Biomarkers to Late Holocene Paleoclimatic Research. " PhD diss., University of Tennessee, 2005.