Accumulating sediments won’t save tidal marshes; they subside in response to accelerated sea level rise

While elevation gain due to gradual sediment accumulation can provide tidal marshes a measure of resiliency to rising sea levels, if sediments accumulate too quickly, they can subside and instead cause marshes to sink, researchers studying data from four continents report. The findings reveal new constraints on the ability of marshes to weather accelerating sea level rise (SLR). And while they reconcile previous discrepancies between contemporary observations and Holocene records, they also suggest that many marshes worldwide will be unable to keep up with rising seas under current climate change projections. Tidal marshes are among the most vulnerable of Earth’s ecosystems; because they only occur within tightly defined elevational ranges relative to mean sea level, they are under an increasing threat from accelerating sea level rise. While tidal marshes can respond to sea level rise by slowly creeping inland where they can, research has shown that some marshes are gaining elevation through the vertical accumulation of sediments and organic matter – a process that seemingly makes them resilient to rising seas. However, assessments of this resilience have revealed conflicting results, which vary across locations and between contemporary instrument observations and Holocene geological data. The root cause of these discrepancies isn’t fully understood but is important to grasp, for predicting tidal marshes’ fate in the face of future sea level rise. By comparing marsh elevation adjustment data from 97 sites across four continents, Neil Saintilan discovered a relationship between sediment accumulation and marsh subsidence, which helps to explain the observed variable response to sea level rise. Saintilan et al. found that, although marshes can gradually accrete more sediment to gain elevation and keep up with slow relative SLR, they can reach a point where the accretion rate becomes too high, resulting in sediment compaction and marsh subsidence. According to the authors, sediment subsidence increases nonlinearly with sediment accumulation such that at higher rates of SLR, marshes begin to sink instead of gain elevation. Thus, marsh elevation gain is constrained in relation to sea level rise, and the mechanisms that can promote marsh resilience at low rates of SLR may result in their failure under the accelerated rates projected for the future.

For reporters interested in trends, a June 2022 study in Science Advances suggested that landward migration of wetlands may not compensate for losses in sea level rise.