We compared the use of delta C-13 values and C:N ratios from salt-marsh sediments to reconstruct relative sea level (RSL) with an established approach using foraminifera. Analysis of bulk-organic sediment and plant samples collected along transects at two sites in North Carolina, USA demonstrates that sediment delta C-13 values can be used to distinguish between Spartina alterniflora-dominated low marsh (C photosynthetic pathway, delta C-13 values from -17.6 parts per thousand to 16.1 parts per thousand) and Juncus roemerianus-dominated high marsh (C-3 photosynthetic pathway, delta C-13 values from -28.2 parts per thousand to -21.8 parts per thousand) environments. Juncus roemerianus plants undergo little decompositional change in delta C-13 (average 0.8 parts per thousand), although there is a clear difference between Spartina alterniflora tissue and bulk-organic sediments (approximately 4 parts per thousand). C:N ratios on bulk-organic sediment from freshwater upland and salt-marsh environments converge during early diagenesis, rendering them of little use in reconstructing RSL. The utility of delta C-13 values as a sea-level indicator is limited by the elevational range of C-4 plants, making it difficult to recognize salt-marsh subenvironments and improve the precision of RSL reconstructions. Furthermore, Juncus roemerianus-dominated high marsh and freshwater upland sediments cannot be adequately distinguished with delta C-13 values.
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