Refining estimates of salt marsh carbon sequestration and its dependencies
Carbon sequestration is one of the important ecosystem services provided by coastal wetlands. We would like to understand why the rate of sequestration differs among estuaries and how it will change with rising sea level.
Carbon sequestration can be derived from knowledge of the sediment carbon concentration and the mineral accretion rate. Estimates made by the marsh equilibrium model (MEM) for marshes differing in tidal dynamics and climate range from about 40 to 120 g C m-2 yr-1. The lowest rates are from the Plum Island estuary in Massachusetts and the highest from the Apalachicola estuary in Florida. Both marshes have peat soils varying from about 40-50% organic matter near the surface to about 20% below the root zone. The stable carbon concentration below the root zone is a product of two proximate factors: the input rates of refractory carbon and mineral matter. Mineral input is a function of tidal dynamics and the concentration of suspended mineral matter. The input rate of refractory carbon is primarily a function of three variables: primary production, partitioning, and the concentration of refractory organic matter in plant tissue. Primary production is a nonlinear function of the rate of sea-level rise. A utilitarian but less informative method of estimation of carbon sequestration uses an empirical relationship between sediment bulk density (D) and sediment organic matter concentration, measured as weight loss upon ignition (LOI). A meta-analysis of this dependency of the form D=a(LOI)b explains 74% of the variability in 1400 samples from a wide variety of estuaries. Application of this equation to wetlands with 20% LOI below the root zone and with a vertical accretion rate of 0.24 cm/yr gives an estimate of 74 g C m-2 yr-1 for carbon sequestration. As sea-level rise accelerates, some marshes will experience higher rates of productivity and carbon sequestration, but continued acceleration leads inevitably to marsh collapse.