Abstract

This study shows that mineral surface coatings in upper soil horizons can preserve products of past abiotic and biotic soil processes and are thus able to record environmental changes on the nanoscale. Contrary to studies involving paleosols, the soil memory of coatings can only be deciphered with high-resolution transmission electron microscopy (HRTEM) on samples prepared using focused ion beam technology. To demonstrate this method, mineral grains were collected in the shallow soils of the Greater Sudbury area, Ontario, Canada, where large fluctuations in the soil pH have occurred during emissions of vast amounts of sulfuric acid and particulate matter (a result of decades of smelter activity in the Sudbury mining district) and subsequent reclamation efforts using lime and phosphates. Observations with HRTEM indicate that limited diffusion of elements in silicified Fe-hydroxide coatings has been the key to the preservation of a rarely observed mineralogy and chemistry. The most intriguing features include petrified cocci in proximity to a structural analogue of green rust (formed prior to the peak of smelter emissions), pockets of jarosite encapsulated by amorphous silica (peak of the smelter emissions), chains of biogenic magnetite, and Fe-(hydr)oxides with high amounts of adsorbed phosphate species (during and after reclamation). These observations indicate that chemical processes in confined pore and interface spaces are commonly in disequilibrium with the surrounding soil, and that Fe-silica–bearing coatings can be excellent tracking tools for the evolution of the chemistry of soils and sediments.

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