FORECASTING THE FATE OF LOUISIANA’S COASTAL WETLANDS USING PAST AND PRESENT MARSH RECORDS

The future of coastal wetlands is a critical issue given the wide range of goods and services they provide. Louisiana hosts about 40% of the coastal wetlands in the US, yet an area roughly the size of the state of Delaware has been lost in the past century. Numerous research efforts are underway to better understand and predict coastal marsh dynamics under conditions of accelerated sea-level rise. Here we present two entirely different yet complementary datasets of marsh sustainability that provide strikingly different results. Measurements of present-day marsh surface-elevation change at 240 sites collected through the Coastwide Reference Monitoring System shows that on average, marshes in the Mississippi Delta gain elevation at rates of 6.6 ± 7.9 mm/yr with respect to a datum at ~25 to 45 m depth below the surface. In contrast, the neighboring Chenier Plain exhibits rates of -0.5 ± 7.6 mm/yr. This suggest that marshes in the Mississippi Delta may be somewhat resilient to the higher rates of relative sea-level (RSL) rise expected in the future. However, a study of Holocene paleo-marsh evolution as inferred from several hundred boreholes and tied to known rates of RSL rise shows that marshes drown once rates exceed 3 to 4 mm/yr. These seemingly contradictory findings can be reconciled due to the fact that paleo-marsh drowning typically took multiple decades or centuries to occur, i.e., much longer than the 5-10 yr long records of present-day marsh change. Thus, the apparent present-day resilience of Mississippi Delta marshes is of a temporary nature and ultimate drowning appears inevitable.