Spatial patterns in accretion on barrier-island salt marshes

On minerogenic barrier-island salt marshes, sedimentation is spatially heterogeneous. Although the main forcing factors for sedimentation are known, much less is known about the characteristic sizes of this spatial patterning. Such patterning gives information on the spatial component of salt-marsh formation and on the uncertainty in measured accretion rates. We used variograms (geostatistics) to study the size of spatial patterns in the thickness of salt-marsh deposits, based on a database of over 10,000 soil cores. These were taken at various spatial scales ranging from metres to kilometres, along a chronosequence representing 10–150 years of salt-marsh formation at three barrier islands in the Wadden Sea (south-eastern North Sea).The general complexity of salt-marsh accretion was reflected in the observed patterns of the thickness of the marsh deposits. The patterns were nested and ranged in horizontal size from 3 m on sites with micro-topography, to 900 m at the scale of the entire marsh. Their structure and size changed with salt-marsh age and there was no characteristic pattern size. Although the pre-marsh topography is an important large-scale control, during salt-marsh development independent spatial patterns are superimposed.When scaling up data on salt-marsh sedimentation, the presence of spatial patterning adds uncertainty to the prediction. The consequence of the complexity of these patterns is that the spatial uncertainty is a (not necessarily linear) function of the area under consideration, which can only be quantified if it is explicitly measured. Our findings therefore pose a cautionary note to studies of salt-marsh resilience to sea-level rise: reliable estimates can only be derived if they are based on measurements that take into account the entire salt marsh.

► Sedimentation on minerogenic barrier-island salt marshes is spatially heterogeneous.
► We used over 10,000 cores to describe these patterns on various spatial scales.
► Spatial patterns in salt-marsh deposits are nested and change in size through time.
► Elevation sets general trend, but independent accretion patterns develop with time.
► Only accretion measurements with sufficient spatial coverage are reliable.