83-3 Autochthonous Carbon Accumulation Across Tidal Zones in the Wax Lake Delta, Louisiana

Abstract:

Louisiana is experiencing a land loss crisis. Levees, dams, and other human modifications to the Mississippi River have restricted sediment delivery to the state’s coastal plain. As a result, Louisiana has lost over 25% of its coastal wetlands since the 1930s, an area the size of Delaware. The rapidly prograding Wax Lake Delta (WLD), the product of a river diversion dredged by the U.S. Army Corps of Engineers in 1942, is one of the few areas of the Louisiana Gulf Coast building land. The WLD serves as an iconic field model for studying processes driving delta maintenance and growth, and for evaluating how river diversions can be used as effective tools for coastal restoration. However, the processes driving soil development and their role in maintaining and growing deltaic islands remain understudied, even for the well-researched WLD.

To study soil development in the WLD, we collected 3-m vibracores from 14 sites along a chronosequence of subaerial land development, and sampled dominant plant species at each site. Cores were split, imaged, described, and sampled at regular intervals for dry bulk density (DBD), organic carbon content (%OC), stable carbon isotopes (δ¹³C), radiocarbon (Δ¹⁴C), ²¹⁰Pb activity, and grain size. DBD and OC content were multiplied to calculate OC concentration profiles. Plant samples were measured for δ¹³C. Depth profiles were normalized to the mean lower low water tidal datum using a new WLD lidar survey. In the subtidal zone, δ¹³C profiles are similar between cores, and OC loadings (OC/SSA) are typical of riverine sediments, with trends in OC concentration primarily controlled by mineral surface area. In the intertidal and supratidal zones, decreasing δ¹³C, increasing OC/SSA, and increasing Δ¹⁴C values are consistent with the deposition of OC from in-situ vegetation. We used a δ¹³C binary mass-balance mixing model with plant and recent mineral deposit endmembers to find that half of the OC in the intertidal and supratidal zones is autochthonous. We found a linear relationship between δ¹³C-δ¹³C₀ and ln(C/C₀) for these zones, which is more consistent with a time-dependent mass-balance mixing model influenced by the Suess effect than isotopic fractionation from microbial decomposition. These findings reveal a shift from mineral-surface-area-driven OC trends in subtidal deposits to vegetation-driven OC accumulation trends in young soils as deltaic islands emerge and mature.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-19188

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