MISSISSIPPI DELTA MARSH TIPPING POINTS LINKED TO A HOLOCENE RELATIVE SEA-LEVEL RECORD: IMPLICATIONS FOR FUTURE COASTAL MARSH SUSTAINABILITY

Coastal wetlands provide Louisiana with abundant ecosystem services, including protection from coastal hazards. Sea level is projected to rise by as much as 1 m in the coming century due to anthropogenic climate change. Short-term (< 10 years) coastal marsh sustainability studies suggest rates of marsh surface elevation gain of ~1 cm/yr, which would enable marshes to keep pace with projected rates of sea-level rise. However, it remains unknown what the sustainability of these ecosystems is in the longer term. Here we investigate the multi-decadal to century scale marsh response to varying rates of relative sea-level rise over the past 8500 years through a detailed stratigraphic analysis in association with 72 sea-level index points from the Mississippi Delta. Our preliminary results identify two marsh sustainability tipping points. At sea-level rise rates > 8 mm/yr, marshes drown almost instantaneously (i.e., within a few decades) and are entirely unsustainable. However, even at rates as low as ~3 mm/yr, marshes eventually drown and are converted into shallow lagoons over century timescales. We suggest that the long-term compaction effect on marsh response to sea-level rise may not be captured in short-term marsh sustainability studies, resulting in too optimistic assessments of marsh resiliency.