Vertical accretion in a Southwest Louisiana Spartina patens brackish marsh has not kept pace with coastal submergence during the past three decades. The rate of accretion determined from 137 Cs dating and the use of artifical marker horizons averaged 0.8 cm yr (super -1) , whereas coastal submergence obtained from tide-gauge data averaged 1.2 cm yr (super -1) . Conversion from marsh to open-water bodies closely paralleled the increase in coastal submergence. If the resulting aggradation deficit continues, the marsh will likely complete its transformation to an open bay in less than 40 years. In view of recent projections of increasing sea-level rise the results presented may foreshadow a more widespread phenomenon.--Journal abstract.