X-ray CT characterisation of dune sands to inform geomorphological models

Coastal sand dunes are a key feature of coastlines worldwide, performing a number of functions in the landscape. They act as important coastal defences, reduce beach erosion, and provide habitats for a number of endangered species. Dune morphology and location are controlled by deposition and erosion by aeolian (wind) processes, and these processes are of great interest to geomorphologists, ecologists, and land managers.

In dune sand transport models, sand grain density is assumed to be a constant 2.65 g cm^-3, the density of quartz, as this is the most common sand-forming mineral in most beaches. However, many beach sands also include a proportion of denser minerals such as ilmenite (~4.7 g cm^-3) or magnetite (~5.17 g cm^-3). Although such minerals generally form a relatively small proportion of the overall composition, they may become concentrated at the surface, leading to local regions with a substantially higher proportion of heavier grains. The effect of this on wind threshold speeds required to erode dune surfaces is unknown.

We sampled transects of dune surfaces with higher concentrations of heavy minerals at the Ainsdale National Nature Reserve in Formby, Northwest England. Samples were scanned using a Nikon MCT X-ray CT at a resolution of ~5.7 microns voxel size. Density distributions of grains at the surface of the samples were compared with deeper levels within the sample, with a control sample and with other samples in the transects. Several transect samples were found to have denser grains at and near the surface compared with the overall distribution and with the control, as well as dense layers within the sample perhaps representing inactive buried dune surfaces. Characterisation of the mineralogy of the sand and correlation with the CT scans will allow us to quantify the density variation at the surface in different locations and provide more realistic input parameters for sand transport modelling.