Rock varnish is a dark coating composed mostly of clays and manganese and iron oxides1 that accretes on rock surfaces in all terrestrial environments2. Perry and Adams3 first observed micron-scale manganese-rich and manganese-poor (relatively iron-rich) layers in varnish and proposed that these may be related to unspecified environmental changes. Electron microprobe analyses reported here suggest that Mn:Fe ratios vary systematically with depth in varnishes on K/Ar-dated volcanic rocks from the Coso and Cima volcanic fields, eastern California, and on talus flatirons in the Negev Desert, Israel. These microchemical laminations probably reflect past fluctuations in the level of aeolian alkalinity, and possibly climatic change. This new indicator of terrestrial environmental change may be of importance to research on geomorphology, archaeology, palaeoclimatology, and Quaternary studies in arid environments.
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