Contributions of organic and inorganic matter to sediment volume and accretion in tidal wetlands at steady state

Abstract

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.

Key Points

• Sediment bulk density can be largely explained by its organic matter concentration

• Bulk volume approximates the summed volumes of the mineral and organic fractions

• Upper bound of steady‐state accretion defines sustainability of coastal marshes under SLR