Stable carbon isotopes as potential sea-level indicators in salt marshes, North Carolina, USA

We compared the use of δ¹³C 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 δ¹³C values can be used to distinguish between Spartina alterniflora-dominated low marsh (C₄ photosynthetic pathway, δ¹³C values from -17.6‰ to 16.1‰) and Juncus roemerianus-dominated high marsh (C₃ photosynthetic pathway, δ¹³C values from -28.2‰ to -21.8‰) environments. Juncus roemerianus plants undergo little decompositional change in δ¹³C (average 0.8‰), although there is a clear difference between Spartina alterniflora tissue and bulk-organic sediments (approximately 4‰). 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 δ¹³C values as a sea-level indicator is limited by the elevational range of C₄ 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 δ¹³C values.