Organic Matter Accretion, Shallow Subsidence, and River Delta Sustainability

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 similar 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 similar to 0.35-1.14 m of compaction but leave a net elevation gain of similar to 0.01-1.75 m, depending on the sediment delivery rate and stiffness of underlying strata. Plain Language Summary River deltas constitute some of the most valuable but also most vulnerable environments on the planet. Their elevation right above sea level is controlled by a delicate balance between sediment deposition and compaction. If sediment delivery is reduced due to dam construction in the hinterland, for example, sedimentation rates decrease. Continued sediment compaction may prevent deltas from keeping up with sea-level rise and increase the risk of drowning. In this study, we investigate the magnitude of compaction that occurs in deltaic wetlands. We find that highly organic sediments are particularly susceptible to compaction, especially when they are buried by sand or mud. As a result, deltaic wetlands cannot rely on organic matter alone to keep pace with sea-level rise. These findings are relevant to delta restoration efforts where sediment is reintroduced to previously isolated wetlands to combat wetland loss. For restoration to be successful, elevation gained from new sediment deposition must outpace elevation lost to compaction. The portion of the Mississippi Delta where this restoration strategy has been planned indicates potentially favorable conditions for land building, provided that sediment loads are high, and the underlying material is strong and mineral rich. Key Points Coastal sedimentary strata under effective stresses of >10 kPa, particularly organic-rich sediments, are likely substantially compacted A large portion of the compaction in such organic-rich strata occurs in the top 1-3 m and the first 100-500 years after deposition Though deposition in deltaic wetlands may drive compaction, river diversions in the Mississippi Delta will likely yield net elevation gain