# Characterisation of Coastal Sediment Properties from Spectral Reflectance Data
## 4 Discussion  
The presence of a wave-dominated shoreface and wide supratidal zone with migrating transverse dunes (Figures 1 and 4) is typical of the south- and southeast-facing South African coast, in which the dunes have a net eastward migration rate of some 3-12 m yr\({}^{-1}\)[48, 49, 39]. Sediment grain size analysis of field samples from Oyster Bay shows that overall they are fairly texturally uniform but with some significant differences in properties between different beach-dune sub-environments (Table 2) and a wide range of carbonate contents (Table 1). The values obtained for grain size properties are similar to other beach-dune systems in the region, e.g., [50, 51]. The relatively limited textural and compositional differences mean that it is sometimes difficult to distinguish between such coastal samples, especially in wave-dominated shoreface environments and wind-affected supratidal environments, where sediments are relatively well sorted e.g., [2, 4, 11, 52]. This is certainly the case with beach-dune sediments along the South African coast, e.g., [39, 45, 49, 51, 53]. However, detailed statistical analysis shows that different landforms and beach-dune settings at Oyster Bay have different sediment properties (Table 2). This is particularly the case with grain size endmembers (fine and coarse sand) and CaCO\({}_{3}\) values that reflect the presence of broken marine shells (not land snail shells) and thus a shoreface source. It is also notable that there are similar overall spectral signatures of all samples (Figure 5), irrespective of their depositional environment (Table 2), which means that the depositional environment of any one sediment sample cannot be resolved by their spectral signature alone. One potential reason for this is that all of the field sediment samples examined here are quartz-dominated (data not shown) and thus, variations in mineralogy cannot be considered as a significant control on their spectral signatures, unlike in previous studies, e.g., [29, 47, 54]. Although the spectral signature of interstitial water is a dominant feature in other previous studies of coastal sediments e.g., [25, 26, 32], we deliberately excluded this by drying the samples prior to analysis. This enabled the spectral data of the Oyster Bay samples to be a better representation of grain size and CaCO\({}_{3}\) properties (Figure 6), which is the primary aim of this study. Spectral characteristics of sand systems (beaches, dunes and deserts) have been examined in several studies, e.g., [54, 55], and these highlight the potential application of spectral analysis techniques to inform on, in particular, mineralogy and depositional environments [29, 32, 34, 35, 36, 47]. Similar to this study, the VNIR part of the spectrum has been previously identified as the most useful in terms of sediment discrimination [55]. There are fewer studies, however, that have looked at grain size data and CaCO\({}_{3}\) content. In detail, the spectral reflectance of these samples, however, revealed some fundamental differences between CaCO\({}_{3}\) content, fine sand proportion and _span_ (Figure 6). These properties also show statistically significant correlations with certain wavelengths (Table 4). However, despite the evidence for some differences in spectral reflectance at different wavelengths for samples with different values of CaCO\({}_{3}\) and fine sand (Figure 10), this does not mean that spectral reflectance can be used to predict values of these sediment properties in unknown samples. This is because measured reflectance values at any wavelength are the net result of all grains within the entire sample and not one single component such as shell fragments. In addition, detridal sediment samples of different provenance or found in different depositional environments could have a range of lithologies, water, organic content or other materials, such as microplastics, that may affect spectral reflectance, e.g., [56].