Coastal marsh loss in the United States during the past 25 years has occurred primarily in Louisiana due to waterway engineering, reduced sediment delivery, and erosion by storms. In a recent (1996-2000) USGS study of terrestrial carbon flux in the Mississippi River Deltaic Plain (MRDP), soil/sediment organic carbon (SOC) mass storage and SOC sequestration rates were measured at selected core sites. Using land-loss estimates for the MRDP from 1978 through 2000 and projected land-loss estimates from 2000 through 2050 (Louisiana Coastal Area Ecosystem Restoration Study, U.S. Army Corps of Engineers), SOC-loss estimates for the surface meter of soil/sediment were derived using SSURGO-linked pedon and MRDP core data. For the MRDP, the SOC loss from 1978-2000 was 10.4 percent of the 1978 inventory (717.5 Tg) equivalent to a 0.5 percent annual loss rate (2,540 g m^-2 yr^-1). An additional 68 Tg of SOC is projected to be lost through coastal erosion during the next 50 years (0.2 percent annual loss rate, 1,101 g m^-2 yr^-1). SOC sequestration rates for MRDP cores were derived from isotopic ¹³⁷Cs and ¹⁴C data. The median rate using ¹³⁷Cs is 265 g m^-2 yr^-1; using ¹⁴C the median rate is 170 g m^-2 yr^-1. These rates were applied to the MRDP to estimate a range of regional SOC sequestration and net SOC loss/gain during the 1978-2050 assessment period. The estimated 43-67 Tg sequestration during 1978-2000 partially offset the SOC erosional loss of 75 Tg to produce a net loss of 8-32 Tg. A projected 87-135 Tg sequestration during 2000-2050 would offset the projected loss of 68 Tg to produce a net gain of 19-67 Tg. The reduction in SOC-loss rate from 0.5 to 0.2 percent per year suggests that the MRDP would switch from SOC source to sink. Potential mitigation strategies could further increase SOC storage.