PRESENT-DAY RATES AND PATTERNS OF SHALLOW SUBSIDENCE IN COASTAL LOUISIANA, USA

Louisiana’s coastal wetlands constitute an important ecological and economic resource that is in peril due to rapid relative sea-level rise. While eustatic sea-level rise affects all coastlines, subsidence is a major contributor to sea-level rise related risks in coastal Louisiana. Currently, the spatial pattern and present-day rates of land-surface subsidence in this region are poorly constrained and predictions for the future therefore carry large uncertainties. The Coastwide Reference Monitoring System (CRMS), a major effort initiated by the US Geological Survey to monitor Louisiana’s coastal wetlands, offers unique new opportunities to study shallow subsidence on sub-decadal timescales. All of the ~400 CRMS sites include a rod-surface elevation table combined with marker horizons, resulting in detailed records of surface-elevation change and vertical accretion rates. Here we use these data to determine the rates and patterns of shallow (<45 m) subsidence throughout coastal Louisiana over the past 5-7 years, primarily due to sediment compaction. For the 226 sites with at least 5 years of observation, the range of shallow subsidence rates varies by several tens of mm. A statistical analysis of the relationship between shallow subsidence and other wetland characteristics (e.g., organic matter content, salinity) will help to elucidate the spatial patterns of modern subsidence, with the objective to improve subsidence predictions.