An ecophysiological explanation for manganese enrichment in rock varnish

Desert varnish is a dark rock coating that forms in arid environments worldwide. It is highly and selectively enriched in manganese, the mechanism for which has been a long-standing geological mystery. We collected varnish samples from diverse sites across the western United States, examined them in petrographic thin section using microscale chemical imaging techniques, and investigated the associated microbial communities using 16S amplicon and shotgun metagenomic DNA sequencing. Our analyses described a material governed by sunlight, water, and manganese redox cycling that hosts an unusually aerobic microbial ecosystem characterized by a remarkable abundance of photosynthetic Cyanobacteria in the genus Chroococcidiopsis as the major autotrophic constituent. We then showed that diverse Cyanobacteria, including the relevant Chroococcidiopsis taxon, accumulate extraordinary amounts of intracellular manganese—over two orders of magnitude higher manganese content than other cells. The speciation of this manganese determined by advanced paramagnetic resonance techniques suggested that the Cyanobacteria use it as a catalytic antioxidant—a valuable adaptation for coping with the substantial oxidative stress present in this environment. Taken together, these results indicated that the manganese enrichment in varnish is related to its specific uptake and use by likely founding members of varnish microbial communities.

The “Kalgutinsky” Style in the Rock Art of Central Asia

On the basis of petroglyphic sites Kalgutinsky Rudnik (Kalgutinsky mine) on the Ukok Plateau, Baga-Oigur and Tsagaan-Salaa in northwestern Mongolia, a distinct “Kalgutinsky” style of rock art of the Russian and Mongolian Altai is described. The distance between these sites is about 20 km. This group is marked by very specifi c stylistic features, common technological properties, a narrowly defi ned motif, featuring only animals, and a very intense desert varnish. All these features and the proximity of the sites suggest that they should be regarded as a special group, which we term the “Kalgutinsky” style and date to the Upper Paleolithic on the basis of several criteria. Images of mammoths at Baga-Oigur and Tsagaan-Salaa are similar to those known in the classic Upper Paleolithic cave art of Western Europe. An entire set of stylistic features typical of the “Kalgutinsky” canon is seen also in the representations of mammoths, and this manner is consonant with that of European Upper Paleolithic rock art. Our fi ndings suggest that a peculiar “Kalgutinsky” style existed and, moreover, that it represented a separate Central Asian locus of Upper Paleolithic rock art.

Growth of desert varnish on petroglyphs from Jubbah and Shuwaymis, Ha’il region, Saudi Arabia

Petroglyphs, engraved throughout the Holocene into rock varnish coatings on sandstone, were investigated in the Ha’il region of northwestern Saudi Arabia, at Jabal Yatib, Jubbah, and Shuwaymis. The rock art has been created by removing the black varnish coating and thereby exposing the light sandstone underneath. With time, the varnish, a natural manganese (Mn)-rich coating, grows back. To study the rate of regrowth, we made 234 measurements by portable X-ray fluorescence (pXRF) on intact varnish and engraved petroglyphs. Since many petroglyphs can be assigned to a specific time period, a relationship between their ages and the Mn surface densities (DMn) of the regrown material could be derived. This relationship was improved by normalizing the DMn in the petroglyphs with the DMn of adjacent intact varnish. In turn, we used this relationship to assign a chronologic context to petroglyphs of unknown ages. Following the removal of the varnish by the artist and prior to the beginning of Mn oxyhydroxide regrowth, a thin Fe-rich film forms on the underlying rock. This initial Fe oxyhydroxide deposit may act as catalyst for subsequent fast Mn oxidation. After a few decades of relatively rapid growth, the regrowth of the Mn-rich varnish slows down to about 0.017 µg cm–2 a–1 Mn, corresponding to about 0.012% a–1 Mn of the intact varnish density, or about 1.2 nm a–1, presumably due to a change of the catalytic process. Our results suggest that petroglyphs were engraved almost continuously since the pre-Neolithic period, and that rock varnish growth seems to proceed roughly linear, without detectable influences of the regional Holocene climatic changes.