Refractory organic matter in coastal salt marshes-effect on C sequestration calculations.

The age and ability of salt marshes to accumulate and sequester carbon is often assessed using the carbon isotopic signatures (ΔC and δC) of sedimentary organic matter. However, transfers of allochthonous refractory carbon (C) from the watershed to marshes would not represent new C sequestration. To better understand how refractory carbon (C) inputs affect assessments of marsh age and C sequestration, ΔC and δC of both total organic carbon (TOC), C, and non-C organic matter fractions were measured in salt marshes from four contrasting systems on the North Atlantic coast. To our knowledge, no salt marsh sediment study has considered refractory or allochthonous carbon in carbon budget calculations or the impact on chronologies. Stable and radiogenic isotope data suggest that while TOC was dominated by autochthonous plant inputs, C was dominated by locally recycled or allochthonous C, the delivery of which was controlled by the size and slope of each watershed. Steep-gradient rivers analyzed delivered ΔC-depleted C to their estuarine marshes, while the site located in the low-gradient river was associated with larger C content. Finally, the marsh isolated from riverine input contained the least fraction of TOC as C. Laterally transported C caused only a small offset in ΔC in relation to TOC in low-gradient systems (average ΔC offset was -44.4 and -24.2‰ at each location). However, the presence of allochthonous ΔC-depleted C in sediments of steep-gradient rivers led to large overestimates of the time of organic matter deposition (i.e. apparent age was older than the 'true' time of deposition) (ΔC offset ranged from -170.6 to -528.9‰). Further, reliance on TOC or loss on ignition analyses to calculate C sequestration by marshes might produce overestimates of at least as much as 10 to 20% since neither account for the lateral transport of allochthonous carbon.