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

We investigated use of delta C-13 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 delta C-13 values between -18.9 parts per thousand and -15.8 parts per thousand 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 delta C-13 values of -27.0 parts per thousand to -22.0 parts per thousand and existed above MHHW. Parallel transects at one site suggested that intra-site variability was not discernible. The utility of delta C-13 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 4.4 m core, we used a multi-proxy approach (delta C-13 values with presence or absence of agglutinated foraminifera) to recognize indicative meanings for four sample types. Sediment with delta C-13 values greater than -18.9 parts per thousand was derived from a vegetated salt-marsh and formed between MTL and MHHW. Sediment with delta C-13 values less than -22.0 parts per thousand 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 delta C-13 values less than -22.0 parts per thousand and lacking foraminifera can only constrain the upper bound of former sea level. Samples with intermediate values (-22.0 parts per thousand to -18.9 parts per thousand) formed between MTL and HAT. Using these indicative meanings and radiocarbon dates, we suggest that a transition from brackish to salt-marsh delta C-13 values recorded in the core took approximately 350 years (from 1800 to 1450 cal. a BP). Copyright (C) 2011 John Wiley & Sons, Ltd.