Tidal salt marshes are the most productive “Blue Carbon” ecosystem and play a significant role in the Global Carbon Cycle (Mcleod et al., 2011, Chung et al., 2011). Salt marshes account for 75% of the organic carbon (C) found in “Blue Carbon” systems, yet cover less than 1% of Earth’s surface (Hopkinson et al., 2012, Howard et al., 2014). They have a high C storage capacity due to a continuous sediment C accumulation rate (CAR) greater than that of any other “Blue Carbon” ecosystem (Murray et al., 2011, Chmura, 2013, Ouyang and Lee, 2014). However, Global estimates of salt marsh C-stocks and CAR are subject to large uncertainties (Duarte et al, 2013, Chastain et al, 2018). The Delaware Bay (DB) salt marshes have been developing for ~2000 years. When these systems are degraded they become a potential source of C-emissions. 8.85 km2 of salt marsh has converted to open water between 1996-2010 and future losses are estimated to reach 5 km2/yr by 2100 (Partnership for the Delaware Estuary, 2017). Conversion could outpace C storage if the depth of erosion is ≥ the thickness of the marsh sediments (Theuerkauf et al., 2015). Most salt-marsh sediment C-stock assessments are reported within the top 1 m of the sediment column (Ouyang and Lee, 2014), thereby representing ~ 100 years of salt-marsh accumulation as compared to the actual 1-6 m sediment sequences accumulated throughout the life span of most U.S. Mid-Atlantic regional salt marshes (Nikitina et al., 2015, Kirwan et al., 2013, Kemp et al., 2013). We estimate the average thickness of the DB salt marsh sediments is 2.6 m, C-stock is 0.1020 MgC/m2 and salt marsh C-stock loss over the 14 yr period is ~0.9 TgC (3.3MMT CO2 equivalents). As this critical “Blue Carbon” habitat reportedly declines, the resulting CO2 degassing flux has a significant impact on the Global Carbon Budget contributing to climate change and ocean acidification (Cai W-J, 2011). Recognition of this sink-to-source conversion emphasized the need for more accurate stock estimates and risk assessments based on estimates of CO2 emissions from lost and degraded salt marshes (Lovelock et al., 2017). The results show that the DB salt marshes sequester significant amounts of C, suggesting that C-stock assessments focused on the top 1 m of sediment underestimate the total C-stock and potential C-emissions by more than three-fold