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A mixing model derived from first principles describes the bulk density ( BD) of intertidal wetland sediments as a function of loss on ignition (LOI). The model assumes that the bulk volume of sediment equates to the sum of self-packing volumes of organic and mineral components or BD = 1/[LOI/k(1) + (1-LOI)/k(2)], where k(1) and k(2) are the self-packing densities of the pure organic and inorganic components, respectively. The model explained 78% of the variability in total BD when fitted to 5075 measurements drawn from 33 wetlands distributed around the conterminous United States. The values of k(1) and k(2) were estimated to be 0.085 +/- 0.0007 g cm(-3) and 1.99 +/- 0.028 g cm(-3), respectively. Based on the fitted organic density (k(1)) and constrained by primary production, the model suggests that the maximum steady state accretion arising from the sequestration of refractory organic matter is <= 0.3 cm yr(-1). Thus, tidal peatlands are unlikely to indefinitely survive a higher rate of sea-level rise in the absence of a significant source of mineral sediment. Application of k(2) to a mineral sediment load typical of East and eastern Gulf Coast estuaries gives a vertical accretion rate from inorganic sediment of 0.2 cm yr(-1). Total steady state accretion is the sum of the parts and therefore should not be greater than 0.5 cm yr(-1) under the assumptions of the model. Accretion rates could deviate from this value depending on variation in plant productivity, root: shoot ratio, suspended sediment concentration, sediment-capture efficiency, and episodic events.