Historical records show a massive decline in salt marsh area (Pendleton et al. 2012), > 50% in many locations, such as sites in Australia (Saintilan and Williams, 2000; Rogers et al. 2006), the British Isles (Baily and Pearson, 2007), and New England, USA (Bertness et al. 2002). These losses are mainly fueled by an underappreciation of the large contributions of salt marsh to maintaining healthy and productive estuaries. Prior to the middle twentieth century, the value of salt marsh primarily depended on its potential for reclamation. Davis (1910) proclaimed that “…[salt marshes] are conspicuous, being generally unutilized for any purpose except for making a small amount of inferior hay, hence they are practically desert places, except where land values are sufficiently high to make it worth while to raise the surface above high tide level for building purposes, or to dike out the tides.” We now view salt marsh as a valuable estuarine habitat because it provides coastal protection from waves (Shepard et al. 2011), erosion control (Neumeier and Ciavola, 2004), water purification (Sousa et al. 2008), fish and bird habitat (Peterson and Turner, 1994; Van Eerden et al. 2005), carbon sequestration (Mcleod et al. 2011), and tourism/recreation (Barbier et al. 2011; Altieri et al. 2012). Salt marsh is also a coastal depositional environment that can accrete vertically over millennial time scales at rates equal to, or greater than, sea-level rise (Gehrels et al. 1996; Ouyang and Lee, 2014). The relatively high accretion rates and resistance of salt marshes to erosion (Mudd et al. 2010) make them valuable sites for preserving records of sea level (van de Plassche et al. 1998; Engelhart et al. 2011; Kemp et al. 2017), storms (Donnelly et al. 2001; Boldt et al. 2010; de Groot et al. 2011), and tsunamis (Morton et al. 2007; Komatsubara et al. 2008) in their sediments. Salt marsh loss and associated services have been pervasive globally, mainly due to the direct (grazing, ditching, pollution, etc.) and indirect (climate change) effects of human activities, resulting in the recent emphasis on restoration, conservation, and management (Lotze et al. 2006; Airoldi and Beck, 2007; Gedan et al. 2009). Although recent focus has been on better understanding of those mechanisms and processes that are related to salt marsh degradation, reviewing salt marsh formation and the different modes of salt marsh expansion will aid efforts aimed at preserving and increasing salt marsh habitat area and extracting climate and tectonic information from their sedimentary records.
