LATE HOLOCENE SEA-LEVEL AND CLIMATE CHANGE IN THE WESTERN ATLANTIC OCEAN

High-resolution sea-level data, extending beyond the span of instrumental observations, are necessary to analyse the response of the sea surface to natural climatic changes and to date the onset of ongoing sea-level rise recorded by mid-latitude Atlantic tide gauges. This study establishes three high-resolution sea-level records, spanning the past 1200 years, from Wells and Machiasport (northern Gulf of Maine, USA) and Chezzetcook (Atlantic coast of Nova Scotia, Canada). Sea-level changes are reconstructed from foraminiferal and chronological analyses of salt-marsh peat sequences. The relationship between foraminiferal distributions and elevation on the modern salt marshes is quantified by regression analyses and applied to fossil foraminifera in cores to reconstruct former heights of the salt-marsh surface relative to tide levels. The peat sequences are dated by ²¹⁰Pb, ¹³⁷Cs, ²⁰⁶Pb/²⁰⁷Pb and AMS¹⁴C. We use pollen markers and historical archives to date the onset of land clearing subsequent to European settlement in the late 18^th century. We corrected for palaeo-tidal changes in the Gulf of Maine and the Bay of Fundy and for the effects of autocompaction. The latter was achieved by comparing the chronology with a series of AMS¹⁴C dates on basal peat. Finally, we detrended the sea-level records to account for long-term glacio-isostatic subsidence by selecting the nearest tide-gauge record to Wells (at Portland) as a baseline. The calculated errors on former mean tide level positions are ±20 cm for the Maine sites and ±7 cm for Chezzetcook. Twentieth century data reconstructed from the salt-marsh sequences compare well with nearby tide-gauge observations. Results of this study suggest that, during the past millennium, steric change has been an important contributor to sea-level variability in the western Atlantic on decadal to centennial timescales. Rates of 20^th century sea-level rise in the western Atlantic are unprecedented in the past millennium.