Does Load-Induced Shallow Subsidence Inhibit Delta Growth?

The ability of deltas to persist by building new land is critical to maintaining these vital ecologic environments that are often home to major economic and population centers. However, the deposition of land-building sediment triggers load-induced shallow subsidence which may undermine the effectiveness of natural and engineered emergent landforms. Here, we present a new method to quantify shallow subsidence in a 6,000–8,000 km² relict bayhead delta of the Mississippi Delta using the mouth bar to overbank stratigraphic boundary that formed near sea level, temporally constrained by optically stimulated luminescence dating. Vertical displacement rates at this boundary, averaged over 750–1,500 years, are on the order of a few mm/yr. Total subsidence scales to ∼50% of the thickness of overlying deposits, significantly greater than the 28%–35% loss estimated for inland localities underlain by peat, indicating that bay muds in the study area are more compaction-prone than terrestrial organic-rich deposits. Modeling shows a modest reduction of ∼13% in deltaic land-area gain under a realistic compaction scenario for 1,000 years of simulated delta progradation, compared to a no-compaction scenario. Our findings indicate that load-driven compaction does not majorly hinder land-area gain and may in fact promote long-term growth at engineered sediment diversions through channel maintenance driven by compaction, thereby adding further support to this restoration strategy.