Organic Matter Accretion, Shallow Subsidence, and River Delta Sustainability

Abstract

Abstract Globally, mineral sediment supply to deltaic wetlands has generally decreased so these wetlands increasingly rely on accretion of organic matter to keep pace with relative sea‐level rise (RSLR). Because organic‐rich sediments tend to be more compressible than mineral‐dominated sediments, deltaic wetland strata are vulnerable to compaction and drowning. Using an unprecedented data set of almost 3,000 discrete bulk density and organic‐matter measurements, we examine organic‐rich facies from coastal Louisiana to quantify the thickness lost to compaction and investigate whether sediments are able to maintain sufficient volume for the associated wetlands to keep pace with RSLR. We find that organic content as well as overburden thickness and density (which together determine effective stress) strongly control sediment compaction. Most compaction occurs in the top 1–3 m and within the first 100–500 years after deposition. In settings with thick peat beds, successions up to 14 m thick have been compacted by up to ∼50%. We apply geotechnical modeling to examine the balance between elevation gained from accretion and elevation lost to compaction due to renewed sediment deposition over a 100‐year timescale. Wetlands overlying mineral‐dominated lithologies may support the weight of deposition and allow net elevation gain. Model results show that reintroduction of sediment to a representative Mississippi Delta wetland site will likely cause another ∼0.35–1.14 m of compaction but leave a net elevation gain of ∼0.01–1.75 m, depending on the sediment delivery rate and stiffness of underlying strata.