## 4 Discussion
This was proven both by punctual EDX analysis, which showed that the crumbled parts of the surface (loose material) have low Mn content compared to the compact surface of the varnish, and SEM-EDX elemental mapping, in which it is clearly visible that the dust deposited in the depressions of the crust is mainly composed of Al, K and Fe, while Mn is concentrated in the actual varnish layer. Enrichment of Mn in the varnish layer by a factor of \(\sim\)100 [19] compared to its primary source of material, i.e., windborne dust, is one of the main points of discussion in the literature, especially related to whether or not Mn enrichment is due to abiotic [19, 70, 71] or biogenic factors [26, 27, 46]. Raman analysis allowed us to partially characterize the minerals present in the black varnish. Identification of iron oxides was quite straightforward, and hematite and magnetite were detected, consistent with the results of other Raman studies on rock varnish [72, 73, 74]. The identification of Mn oxides was more challenging, as also previously reported by [PERSON] et al. [72], who were not able to identify Mn oxides by Raman spectroscopy on the rock varnish analyzed. Nevertheless, our results seem to indicate that birmesite-like Mn oxides might be present, which are addressed as the most common Mn compounds in desert varnish [19]. Interestingly, the Raman spectra of the orange layer of sample EZ4 and of the reddish layer emerging from the black crust of the other samples showed similar spectral features, indicating the presence of iron oxides, a possible presence of Mn oxides, and, at higher wavenumber, the peaks around 1320 and 1600 cm\({}^{-1}\) suggested the presence of carbon. Although it is believed that Raman spectroscopy cannot distinguish between biogenic and abiotic sources of carbon, [PERSON] et al. [72] address the possibility that the graphitic carbon found on the desert varnish in their study might be the result of \"highly thermally process carbonaceous residues from living organisms\". Raman spectroscopy also pointed out the presence of carotenoid pigments in the samples, which are used by microorganism as protection against radiation, therefore indicative of microbial activity. A wide variety of bacteria detected in this study are known to synthesize carotenoids, for example the bacterioruberin from _Rubrobacter_ spp., which might be the source of the carotenoid pigment we detected on the reddish layer beneath the black crust. We could also prove the presence of carotenoids directly on cyanobacteria aggregates, in which the most common pigment is \(\beta\)-carotene. The Raman measurements to detect biopigments were preliminary, and these results could be implemented in future studies. The combination of microscope observations and SEM-EDX analysis on the petrographic thin sections showed very interesting results. Optical microscopy unveiled the presence of weathered areas beneath the black crust, and SEM-EDX elemental maps revealed that in these areas only the Si component of the bedrock is present while the Ca and Mg matrix is missing. The same elemental distribution was observed in sample EZ4, in which the orange layer was mainly composed of Si, also highlighting the presence of silicified fossils widely present in the bedrock and the lack of the Ca matrix. These observations brought us to the conclusion that the orange layer, rather than a different kind of superficial crust, might be a weathered area of the bedrock similar to the ones observed in the other samples, but in this case, the superficial black varnish is completely missing. The typical elements normally detected in it, such as Al, K and Fe, are distributed in the weathered area, including also a low proportion of Mn. These observations might suggest that elements present in the black crust had been mobilized by microorganisms, contributing to the slow degradation of the black varnish.