## 3 Results
### Micro-Raman Spectroscopy  
Raman spectra acquired on the black varnish were quite complex. Depending on the measurement spot, the spectra displayed different features confirming the heterogeneity of the varnish composition. In many cases, the overall spectral profile matches well the spectral features of the pigment burnt Sienna, which is a mixture of iron oxides, quartz and aluminum oxides. An example is reported in Figure 9a--spectrum A, in which, more specifically, the bands at 224, 291, 408 and 1322 cm\({}^{-1}\) can be assigned to hematite (\(\alpha\)-Fe\({}_{3}\)O\({}_{4}\)), while the band around 670 cm\({}^{-1}\) is diagnostic for magnetite (Fe\({}_{2}\)O\({}_{3}\)) [40]. In the region between 500 and 700 cm\({}^{-1}\), the peaks of the spectra acquired on the black varnish are not well resolved and a broad band with shoulders is observed; therefore, clear assignment of signals is challenging. In this spectral region, the characteristic peaks of Mn oxides are found, so their presence could not be clearly identified. Nevertheless the peaks visible at around 400 cm\({}^{-1}\), at 502 cm\({}^{-1}\) and at 574 cm\({}^{-1}\) (Figure 9a--spectrum B) might be assigned to birnessite or birnessite-like minerals [41], a manganese oxide often reported in the literature as part of the desert varnish [19]. Quartz (sharp band at 465 cm\({}^{-1}\)) was often identified in the spectra of the black varnish, and in some areas anatase (TiO\({}_{2}\)) (band at 143 cm\({}^{-1}\)) and pseudobrookite (Fe\({}_{2}\)TiO\({}_{5}\)) were also detected. The spectra obtained on the orange layer emerging from the black varnish (Figure 9a--spectrum C) showed a spectral profile similar to the one of the orange crust of sample EZ4 (Figure 9a--spectrum D), showing characteristic features of iron oxides (bands around 220, 290 and 400 cm\({}^{-1}\)) and quartz (bands at 207 and 465 cm\({}^{-1}\)) at lower wavenumbers. In the range from 1000 to 1800 cm\({}^{-1}\), EZ4 presented a complex spectrum, with overlapping broad bands that did not allow clear interpretation. The peaks in this range might be related to organic pigments such as carotenoids combined with amorphous carbon (peaks at 1320 and 1600 cm\({}^{-1}\)). The presence of carotenoids was clearly confirmed in a reddish area beneath the black varnish (transversal section of the sample). Indeed, the Raman spectrum of the area analyzed (Figure 9b) showed three sharp bands at 1000 cm\({}^{-1}\), 1151 cm\({}^{-1}\) and 1505 cm\({}^{-1}\) typical of carotenoids, which are characterized by three main skeletal features: the stretching of C=C in the region around 1520 cm\({}^{-1}\), the stretching of C-C around 1150 cm\({}^{-1}\) and the bending of C=CH around 1000 cm\({}^{-1}\)[42]. Figure 8: SEM image in backscattered electron mode and EDX elemental mapping (Si, Ca, Mg and Al) of sample EZ4. On one sample, Raman spectra were acquired in a green area visibly colonized by cyanobacterial aggregates. The spectral features of carotenoids were again identified but with a shift in the wavenumbers (1002 cm\({}^{-1}\), 1152 cm\({}^{-1}\) and 1510 cm\({}^{-1}\)) compared to the spectrum obtained on the reddish area (Figure 9b). The shift might indicate the presence of a different type of carotenoid pigment in the green area. According to [PERSON] et al. [42], the position of the stretching band of the C=C group is diagnostic for the discrimination of different carotenoids, and they assigned the band at 1506 cm\({}^{-1}\) to the carotenoid pigment bacterium and the C=C signal at higher wavenumbers (1510 cm\({}^{-1}\)) to \(\beta\)-carotene. The positions of the two bands correspond very well to the one shown in the Raman spectra of carotenoids obtained in this study: 1505 cm\({}^{-1}\) in the case of the orange spot and 1510 cm\({}^{-1}\) in the case of the green spot. Despite this, attribution of the two spectra to one carotenoid pigment rather than the other cannot be certain solely by Raman spectroscopy.