Application of stable carbon isotopes for reconstructing salt-marsh floral zones and relative sea level, New Jersey, USA

We investigated use of δ ¹³C in bulk organic sediment to define the botanical origin of samples preserved in coastal sediment as a means to reconstruct relative sea level in New Jersey, USA. Modern transects at three sites demonstrated that low and high salt-marsh floral zones dominated by C ₄ species (Spartina alterniflora and Spartina patens) were associated with sediment δ ¹³C values between -18.9‰ and -15.8‰ and occurred from mean tide level (MTL) to mean higher high water (MHHW). Brackish transitional settings vegetated by Phragmites australis with Iva fructescens and Typha sp. (C ₃ species) and freshwater upland samples (C ₃ species) were characterized by bulk sediment δ ¹³C values of -27.0‰ to -22.0‰ and existed above MHHW. Parallel transects at one site suggested that intra-site variability was not discernible. The utility of δ ¹³C values for reconstructing relative sea level in New Jersey is limited by an inability to differentiate between brackish sediments related to sea level and freshwater upland samples. To facilitate this distinction in a 4.4m core, we used a multi-proxy approach (δ ¹³C values with presence or absence of agglutinated foraminifera) to recognize indicative meanings for four sample types. Sediment with δ ¹³C values greater than -18.9‰ was derived from a vegetated salt-marsh and formed between MTL and MHHW. Sediment with δ ¹³C values less than -22.0‰ and containing agglutinated foraminifera formed in a brackish transitional zone between MHHW and highest astronomical tide (HAT). This is the narrowest elevational range of the four sample types and most precise sea-level indicator. Sediment with δ ¹³C values less than -22.0‰ and lacking foraminifera can only constrain the upper bound of former sea level. Samples with intermediate values (-22.0‰ to -18.9‰) formed between MTL and HAT. Using these indicative meanings and radiocarbon dates, we suggest that a transition from brackish to salt-marsh δ ¹³C values recorded in the core took approximately 350 years (from 1800 to 1450cal a BP).