https://doi.org/10.1177/0959683619862028 The Holocene 1–15 © The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0959683619862028 journals.sagepub.com/home/hol Introduction Today’s global mean sea level (GMSL) determined from tide gauges, satellites, and geophysical models is rising by approxi- mately 3.0 (Hay et al., 2015) to 3.2 mm yr -1 (Lyu et al., 2014) and these rates are higher than rates of 1.2 mm yr -1 for the last century (up to 1990) (Mitrovica et al., 2015). Nerem et al. (2018) estimated from 25 years of satellite data that climate-change-driven accelera- tion of GMSL was over 0.084 ± 0.025 mm yr -1 . Other studies sug- gest that the anthropogenic temperature-related signal has influenced GMSL since 1970 (Slangen et al., 2016) and over the 20th century (Kopp et al., 2016) but the causes of sea-level rise (SLR) acceleration since 1990 remains unclear (Davis and Vino- gradova, 2017). This could be due in part to potential biases in long tide gauge records (Thompson et al., 2016). Significant regional variability in sea level along any coastline is due to a number of factors including glacio-isostatic adjustment (GIA) since the last glacial maximum (LGM, ~22 ka (kilo-annum); Peltier et al., 2015), ocean circulation dynamics, and regional subsidence or uplift. Complicating our understanding of sea level and contradicting views that pre-20th century late-Holocene sea level was invariant (Nicholls and Cazenave, 2010), there is evidence for decadal to centennial sea-level variability of 10s of centimeters during the last few millennia although causes remain uncertain (Cronin et al., 2014). As a consequence, detection of acceleration in the global rate of SLR and understanding future SLR and coastal inundation from models (e.g. Lentz et al., 2016) hinges on a firm understanding of the baseline sea-level variability obtained from tide gauges and satellites augmented by paleo-sea-level reconstructions from tidal marshes and other coastal paleo- records (Haigh et al., 2014). In this study, we analyze Holocene relative sea-level rise (RSLR) records from tidal marshes on the Potomac and Rappah- annock River estuaries in the central Chesapeake Bay region, east- ern United States. The Chesapeake Bay is the largest estuary in the United States, with shores bordering the states of Virginia and Maryland, and the District of Columbia. Our primary objectives are (1) to establish a baseline pre-anthropogenic regional sea-level record using physical, biological, and geochronological records of Holocene tidal marsh deposits; (2) to examine marsh evidence that a decrease in the late-Holocene regional SL rate along the eastern US coast reflects global processes (i.e. glacio-eustasy); and (3) to compare Holocene rates to accelerated rates based on instruments and modeling for the 19th and 20th centuries. Holocene sea-level variability from Chesapeake Bay Tidal Marshes, USA Thomas M Cronin, 1 Megan K Clevenger, 2 Neil E Tibert, 2† Tammy Prescott, 2 Michael Toomey, 1 J Bradford Hubeny, 3 Mark B Abbott, 4 Julia Seidenstein, 1,5 Hannah Whitworth, 1 Sam Fisher, 4 Nick Wondolowski 4 and Anna Ruefer 1 Abstract We reconstructed the last 10,000 years of Holocene relative sea-level rise (RSLR) from sediment core records near Chesapeake Bay, eastern United States, including new marsh records from the Potomac and Rappahannock Rivers, Virginia. Results show mean RSLR rates of 2.6 mm yr −1 from 10 to 8 kilo-annum (ka) due to combined final ice-sheet melting during deglaciation and glacio-isostatic adjustment (GIA subsidence). Mean RSLR rates from ~6 ka to present were 1.4 mm yr −1 due mainly to GIA, consistent with other East Coast marsh records and geophysical models. However, a progressively slower mean rate (<1.0 mm yr −1 ) characterized the last 1000 years when a multi-century-long period of tidal marsh development occurred during the ‘Medieval Climate Anomaly’ (MCA) and ‘Little Ice Age’ (LIA) in the Chesapeake Bay region and other East Coast marshes. This decrease was most likely due to climatic and glaciological processes and, correcting for GIA, represents a fall in global mean sea level (GMSL) near the end of Holocene Neoglacial cooling. These pre- historical climate- and GIA-driven Chesapeake Bay sea-level changes contrast sharply with those based on Chesapeake Bay tide-gauge rates (3.1–4.5 mm yr −1 ) (back to 1903). After subtracting the GIA subsidence component, these rates can be attributed to long-term (millennial) global factors of accelerated ocean thermal expansion (~1.0 mm yr −1 ) and mass loss from alpine glaciers and Greenland and Antarctic Ice Sheets (1.5–2.0 mm yr −1 ). Keywords Chesapeake Bay, foraminifera, Holocene, sea level, tidal marsh, US East Coast Received 6 November 2018; revised manuscript accepted 10 May 2019 1 Florence Bascom Geoscience Center, US Geological Survey, Reston, VA, USA 2 University of Mary Washington, Fredericksburg, VA, USA 3 Salem State University, Salem, MA, USA 4 University of Pittsburgh, Pittsburgh, PA, USA 5 Department of Geosciences, University of Massachusetts, Amherst, MA, USA † Deceased. Corresponding author: Thomas M Cronin, Florence Bascom Geoscience Center, US Geological Survey, Reston, VA 20192, USA. Email: tcronin@usgs.gov 862028HOL 0 0 10.1177/0959683619862028The HoloceneCronin et al. research-article 2019 Research paper