Mud‐associated organic matter and its direct and indirect role in marsh organic matter accumulation and vertical accretion
G. Mariotti, Tracy Elsey‐Quirk, Grayton Bruno, Kendall Valentine
Abstract
Abstract In situ plant production is often assumed to be the major contributor to organic matter (OM) accumulation and vertical accretion in tidal marshes. Here, we evaluate the contribution of mud‐associated OM in salt and brackish marshes in Louisiana. Based on 14 soil cores, the OM content of the mud fraction—i.e., any material smaller than 64 μ m—was 17% ± 7% for the salt marshes and 28% ± 14% for the brackish marshes. This remains nearly uniform over the top 35 cm depth, suggesting that this material is deposited contemporaneously with the mud. The dry bulk density of the mud (300–450 kg m −3 ) is also much lower than what was estimated using a previously proposed two‐constituent mixing model (1990 kg m −3 ). To reconcile this discrepancy, we developed a modified mixing model that includes mud OM and differentiates sand as a separate constituent with its high dry bulk density. The model estimates that mud contributes to ~ 60% of the total marsh vertical accretion in Louisiana, considerably higher than the ~ 14% estimated with the two‐constituent mixing model. The result, which is a direct consequence of the relatively high porosity of mud, highlights that mud deposition is crucial for the accretion of microtidal marshes. Further, the model estimates that the mud OM constitutes ~ 60% of the total soil OM, emphasizing that in situ plant production is not the only—and, in minerogenic marshes, not the major—contributor to OM accumulation.