River-bed sediments display two universal downstream trends: fining, in which particle size decreases; and rounding, where pebble shapes evolve toward ellipsoids. Rounding is known to result from transport-induced abrasion; however many researchers argue that the contribution of abrasion to downstream fining is negligible. This presents a paradox: downstream shape change indicates substantial abrasion, while size change apparently rules it out. Here we use laboratory experiments and numerical modeling to show quantitatively that pebble abrasion is a curvature-driven flow problem. As a consequence, abrasion occurs in two well-separated phases: first, pebble edges rapidly round without any change in axis dimensions until the shape becomes entirely convex; and second, axis dimensions are then slowly reduced while the particle remains convex. Explicit study of pebble shape evolution helps resolve the shape-size paradox by reconciling discrepancies between laboratory and field studies, and enhances our ability to decipher the transport history of a river rock.

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A novel approach of studying dust, black crust and host limestones by using various techniques is presented in this paper. Samples were collected from limestone monuments located in rural, urban and industrial areas in Germany and Hungary. The paper focuses on differences in the mineralogical composition, major and trace element distribution of materials and their total polyaromatic hydrocarbons (PAH) concentration having different exposure to air pollutants. High concentration of gypsum was found in laminar and dendritic black crusts and even in the dust of both urban and rural areas, despite the low concentration of SO 2 in rural atmospheric environments. The black crusts show a typical microfabric with distinct layers of various gypsum-calcite mineral associations, silt-sized mineral fragments and black particle content. In dust, newly formed gypsum crystals were found along with mineral fragments, and siliceous and, less frequently, carbonaceous fly ash particles. High concentration of lead was found both in the samples of urban and rural areas of Germany and Hungary. Lead mostly accumulates in dust and also in the black crust. Highest lead concentrations were found not on the surface of the black crust, but close to the crust/limestone boundary according to LA-ICPMS analyses. It indicates that despite the ban of leaded petrol, lead is still present in the dust and the in the soiled zones of the built environment. Similarly to lead, no significant difference in the total PAH content of dust and crust samples were found in the two countries. Dusts are equally enriched in PAH in both countries, whereas black crusts contain fewer amounts. Most of the aqueous extracts of the samples were saturated with respect to sulphate, which also indicate the presence of gypsum, whilst in some German samples unsaturated conditions were detected. In summary, by using a combination of these analytical techniques, it was possible to distinguish rural and urban samples and to outline the role of dust in bringing pollutants to the stone surface. It is suggested that the analysed dust and stone samples indicate not only present, but also the past, pollution fluxes and as a consequence stones similar to sediments or surface waters can be used as environment indicators.

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