Geochemical insights into the relationship of rock varnish and adjacent mineral dust fractions

Abstract

Rock varnishes are μm-thin, dark, manganese(Mn)-rich crusts that accrete in the order of few μm/ka on weathering-resistant lithologies. Although these crusts can form in all climates, they are best known in arid to semi-arid settings. Aeolian dust is understood as a major contributor to the distinct trace metal and REE enrichments in rock varnish. However, the exact proportions of abiotic and biotic formation mechanisms that may explain the oxidation-reactions of Mn2+ to Mn4+, present as Mn oxyhydroxides in the varnish, are still a matter of ongoing debate.We present here the first systematic study of trace element enrichment processes between the uppermost layer of the varnish sequence and their adjacent <50 μm and >50 μm dust fractions across a selection of 41 major and trace elements. This approach is used to investigate samples from three fully arid deserts: the An Nafud in Saudi Arabia, the Negev in Israel, and the Mojave in the USA, which are compared to the significantly different environment of the semi-arid Knersvlakte in South Africa. A new in situ trace element analysis protocol was developed to perform femto- and nanosecond sector-field LA-ICP-MS microanalyses at high-spatial resolution, which allows us to measure the most recent varnish layers for their comparison with recent dust. In agreement with previous studies, all varnishes are enriched in Mn, Pb, Ce, Co, Ba, Zn, Ni, and the Rare-Earth Elements (REE). Here we demonstrate that fine (<50 μm) dust is characterized by similar trace element trends as the varnishes, at overall lower mass fractions. Dust >50 μm has low trace element mass fractions, and enrichment patterns plotting distinctly away from varnish and fine dust. Based on these geochemical patterns, our results indicate a general enrichment mechanism from fine dust to varnish. Previous studies suggested dust to play an integral role in providing the trace elements that are incorporated into the varnish by pH-Eh fluctuations in short-term rain and fog events. We amalgamate and refine previous growth models by providing direct evidence that leaching of about 10% of the Mn and other trace elements from clay minerals in rain or fog droplets at pH ~5 and subsequent scavenging on varnish surfaces at pH ~8 leads integrated over time to the distinct enrichment patterns of the varnish, while initial Mn oxyhydroxide formation is suggested to follow pathways of metal oxide mediated photo-catalysis.For the semi-arid occurrence in the Knersvlakte we present a distinct growth model as both environment, varnish, and dust composition differ significantly from the arid settings. Here, thick, metallic-looking varnish occurs mainly on the rim of quartz pebbles, lacks microlamination, and likely has upscaled growth processes. Among other aspects, we suggest a more complex interplay between photo-catalysis, nocturnal condensation events on quartz pebbles with subsequent water trapping at the rock-soil-atmosphere interface, due to a salic top-soil layer, to mainly account for these differences.