Chronology of late Holocene relative sea-level change in Boston Harbor

Abstract

We use a core of salt-marsh sediment from Boston Harbor (Massachusetts, USA) to evaluate the sensitivity of late Holocene relative sea-level (RSL) reconstructions to the pre-treatment, graphitization, and upcore distribution of radiocarbon dates. The 87 radiocarbon dates from 58 unique depths in the 4.2-m long core show that ages from plant macrofossils (principally rhizomes and stems of Spartina patens and Distichlis spicata) are insensitive to pre-treatment and graphitization regimes. Statistical resampling from the pool of radiocarbon dates generated many plausible chronologies and demonstrates that the precision of age-depth models increases as dates are added, but with diminishing returns. Estimated sample ages are not systematically biased by the density of dates. Identification of periods with slower sedimentation requires a higher density of dates than identifying periods with faster sedimentation. These results suggest that RSL variability reconstructed among sites and regions is unlikely to be the result of choices in the preparation and selection of samples from salt-marsh sediment for radiocarbon dating. We reconstructed paleomarsh elevation using a Bayesian transfer function trained on the observed relationship between salt-marsh foraminifera and tidal elevation in 212 surface-sediment samples. This model utilized informative prior information from bulk-sediment δ¹³C values. During the past ∼4200 years RSL rose by ∼4.2 m in Boston Harbor at a rate of ∼0.9 mm/yr until the early 20th century when the rate increased to ∼3 mm/yr, which is consistent with estimates of glacio-isostatic adjustment and historic tide-gauge measurements in Boston Harbor.