Mud‐associated organic matter and its direct and indirect role in marsh organic matter accumulation and vertical accretion

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 μm-was 17% AE 7% for the salt marshes and 28% AE 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.

Fetch and distance from the bay control accretion and erosion patterns in Terrebonne marshes (Louisiana, USA)

“…In the inner marshes, organic deposition is more important, indicating that mineral sediment is less available as we move inland. This is consistent with findings from Mariotti et al (2020), as they measured an organic matter content of 28±14% in internal brackish marshes compared to 17±7% in salt marshes.…”

Coastal Wetland Resilience, Accelerated Sea‐Level Rise, and the Importance of Timescale

“…While this organic contribution can make a difference when relative elevation is optimal for plant growth (Figure 2), even then there is a limit to primary production and, hence, biogenic accretion (Morris et al., 2016). Furthermore, recent work has shown that even in organic‐rich marshes, mud is a key constituent driving vertical accretion (Mariotti et al., 2020).…”

Rapid tidal marsh development in anthropogenic backwaters

“…Marsh sustainability therefore requires that organic and inorganic sediment accretion keep pace with the compounding impacts of sea level rise and sediment autocompaction (Cahoon et al, 1995). Emergent low marsh can quickly build elevation as marsh grasses increase friction, reduce current velocities, and hasten the trapping of allochthonous suspended sediment (Bouma et al, 2005) and suspended particulate organic material (Mariotti et al, 2020). In addition to trapped sediment, marsh grass growth produces autochthonous vegetative material mainly via its roots, further contributing to marsh platform elevation.…”