COMMON ERA SEA-LEVEL, CHESAPEAKE BAY, U.S. ATLANTIC COAST

Proxy based relative sea-level (RSL) reconstructions extend our knowledge of sea-level history beyond the instrumental period and have helped identify spatial and temporal differences along the U.S. Atlantic Coast throughout the Common Era (CE). This paper presents a new RSL CE record utilizing foraminifera from microitdal (<0.4m) salt marshes in the Chesapeake Bay region. Motivation for this work stems from previous CE studies in New Jersey and North Carolina and due to recent rates of change approximately twice the global average at ~3-5mm/yr as revealed by tide-gauge measurements for this region. These high rates largely reflect an ongoing eustatic input coupled with ground subsidence associated with glacial isostatic adjustment since the termination of the Laurentide Ice Sheet combined with sediment compaction and ground water extraction.

Modern surface data of foraminiferal assemblages from salt marshes located at the Smithsonian Environmental Research Facility, MD show a strong relationship with tidal level. Faunal zones dominated by Jadammina macrescens are restricted to the upper tidal frame around mean higher high water confirming their suitability as paleo sea-level indicators. An extensive stratigraphic survey revealed thick sequences of organic salt-mash peat ideally suited for proxy based reconstructions from which a 6.1m core was selected for fossil foraminifera at high (1-5cm) resolution. Estimates of paleo marsh elevation were derived using a modern training set of 60+ surface samples and subtracted from surveyed altitude to provide sea-level index points constrained by a composite chronology combining 15 AMS radiocarbon dates with short-lived radionuclides (²¹⁰Pb and ¹³⁷Cs) within a Bayesian temporal framework. The proxy reconstruction reveals sea-levels increasing ~1.6 mm/yr for much of the late Holocene before rates similar to that of observational period are recorded for the past ~150 years.