June 2017 Issue Table of Contents
Coatings occur along interfaces between rocks and minerals and their environment. Coatings result from the wide variety of reactions and/or processes that occur at the interface between the lithosphere and the biosphere, hydrosphere, and atmosphere. Such coatings are biochemically, mineralogically and isotopically complex and have the potential to record changes in their immediate environment. The transition between a coating and its underlying host is abrupt and defined by a sharp interface at the nanoscale. Articles in this issue highlight new and exciting research in the field of coatings, focussing on coatings formed in deserts, soils, sediments, oceans, and on rocks from Mars.
Keywords: coatings, weathering rind, biofilm, ferromanganese crust, Mars, rock-inhabiting fungi
Coatings, in the sense of one substance covering another, define the surfaces of many rocks and minerals. They occur on rocks exposed to the atmosphere, biosphere, and hydrosphere and they cover mineral grains in soils, sediments, and anthropogenic features such as mine tailings and mine waste piles.
The occurrence of coatings on rocks and minerals was first described scientifically over two centuries ago by Alexander von Humboldt in his Personal Narrative of Travels to the Equinoctial Regions of the New Continent During the Years 1799–1804 (von Humboldt 1814), where von Humboldt correctly deduced that the dark covering on rocks in the Orinoco River (which flows in Venezuela and Columbia) was a Mn-rich accretion (Figs. 1A, 1B). Since then, our knowledge about coatings on rocks and minerals have increased steadily to the point where coatings on rocks and minerals are now studied to understand past and present geobiological, geomorphological, (astro-)biological, pedological, archaeological, and environmental processes (see articles in this issue).
Coatings form in various geological, bioclimatic, and anthropogenic environments. For example, black rock-coatings composed of Fe–Mn oxides and clay minerals (termed “rock varnish”) (Fig. 1) occur on different types of rock substrates in the arid environments of Antarctica, Arizona (USA) and Peru (Dorn 2013), whereas black Fe–Mn-oxide coatings also form on submarine rock surfaces (Koschinsky and Hein 2017 this issue) and can accumulate for far longer than any known terrestrial coating because erosion of the underlying rock material is much slower than in terrestrial settings. Terrestrial and submarine rock coatings represent mainly accretions derived from external constituents, but marine Fe–Mn crusts involve precipitation from sea water, while terrestrial rock coating formation varies greatly depending on the type of coating. Black coatings may also accrete on stone surfaces in air-polluted cities where the interaction between building stones and exposed rocks with sulfuric acid and/or nitric acid may result in the formation of coatings composed of amorphous silica, sulfates, and particulate matter (Fig. 2) (Smith and Prikryl 2007; Mantha et al. 2012).
Coatings on rocks and minerals are studied on the kilometer- (Fig. 3A), meter- (Fig. 3B), micrometer (Fig. 3C) and nanometer-scale (Fig. 3D). For example, the distribution of coatings on the surface of the Earth and on other terrestrial planets can be mapped using remote-sensing techniques (e.g. Malcolm et al. 2015). Details on their mineral, chemical, and isotope compositions can be analysed, for example, with bulk analytical tools such as X-ray diffraction, inductively coupled plasma mass spectrometry (ICP–MS) and conventional stable isotope techniques. Site-specific information on the nano- to micrometer scale can be gained through the combination of focused ion beam transmission- and transmission electron microscopy (e.g. Fig. 1C), scanning electron microscopy (Fig. 1B, 2B), synchrotron spectroscopy techniques, laser-ablation ICP–MS (Fig. 2C) and secondary ion mass spectroscopy.
In this issue of Elements, we present an overview of research on different types of coatings with respect to their formation on aerially exposed rocks, the role of bacteria and fungi during their formation, their ability to sequester contaminants and to preserve records of past and recent environmental processes, and on their potential role in understanding past biogeochemical processes on other terrestrial planets.
Coatings and weathering layers: formation, properties and environmental indicators
There is no clear definition for the term coating in the Earth sciences. Dorn et al. (2017 this issue) distinguish between weathering rinds and coatings. These authors define weathering rinds as a product of chemical weathering and coatings as a product material deposition from the surroundings, i.e. the constituents of a coating originate exclusively from the surroundings (Fig. 1A). Mantha et al. (2012) and Schindler and Singer (2017 this issue) consider coatings as heterogeneous surface layers that can contain constituents from the underlying rock or mineral and from the surroundings (Figs. 2A, 2B).
The distinction between the weathering rinds of rocks formed through chemical weathering and coatings formed by deposition from the exterior allows Dorn et al. (2017 this issue) to depict the dynamic transition between weathering processes occurring on the surface of the underlying rock and the infilling of newly formed pore spaces by rock-coating components. These authors show, however, that this case-hardening process eventually leads to the detachment of the rock coatings and, thus, to the loss of features such as rock art, the surfaces of stone monuments, and the facing stones of buildings.
Schindler and Singer (2017 this issue) emphasize that mineral surface coatings in soils are complex entities of nano-size abiotic- and biotic-formed mineral assemblages that contain constituents both from the underlying mineral and from the mineral’s surroundings. The authors show that, regardless of the origin of the coating constituents, nano-size pores within coatings allow sequestration of contaminants and the preservation of past and recent environmental processes in soils and sediments.
Gadd (2017 this issue) discusses the role of fungi and lichens as rock coatings and also as major geobiological agents altering Earth’s surface. The author focuses on the role of fungi during bioweathering, fungi’s ability to adapt to adverse environments, their organic carbon-based metabolism and their symbiotic relationships with other organisms, such as lichens. The author shows that the excretion of geo-active metabolites – such as organic, carbonic, and oxalic acids – is of fundamental importance in many environmental mineral transformation processes.
Whitley et al. (2017 this issue) present the use of microlaminations in rock varnish as both a tool to analyse paleoclimatic changes and also as a chronometric tool, a technique that allows the dating of commonly non-dateable stone artefacts and art, such as petroglyphs. Dating surface artefacts and petroglyphs provides valuable insight about the peopling of the Americas and the age and changing environmental adaptation of the prehistoric inhabitants of the hemisphere.
Koschinsky and Hein (2017 this issue) emphasize that iron–manganese oxyhydroxide coatings (Fe–Mn coatings) on submarine rock surfaces and on sediment represent vast archives of changing ocean conditions, including variations in climate, ocean currents, geological activity, erosion processes on land, and even anthropogenic impacts. These authors illustrate that slow-growing Fe–Mn coatings in deep oceans provide proxies of paleoceanographic changes over the time periods of hundreds of thousands to tens of millions of years. In contrast, fast-growing coatings in shallow waters on continental margins can record anthropogenic environmental impacts.
Marnocha (2017 this issue) explores the potential implications of rock coatings on Mars. Micro-environments in rock coatings can sometimes provide habitats for microbial life because such coatings contain sufficient inorganic nutrients as “microbe food” and can shield microbes from harmful radiation. This author suggests that rock coatings are able to preserve petrified biosignatures and that new technology in future Mars rover expeditions may provide valuable information about the potential occurrence of these signatures in rock coatings on Mars.
The reader can find below a glossary with the most important technical terms used in the articles of this issue of Elements.
Deciphering environmental records in coatings on rocks and minerals is a burgeoning field in the Earth sciences. The application of relatively new analytical tools and approaches in mineralogy (e.g. focussed ion beam/transmission electron microscopy) and (bio-)geochemistry (e.g. metal stable-isotope analyses) will assist in the deciphering of past and present environmental processes as recorded by soil and rock coatings. This information will help us to better understand the formation of coatings in the presence and absence of micro-organisms; the ability of coatings to sequester toxins in soils, rivers, lakes and oceans; and how coatings record past environmental changes. And all this may, one day, be done on the surfaces of other planets.
We thank the reviewers Phillip Larson and Athanasios Godelitsas for their constructive comments. MS thanks NSERC for a Discovery Grant.
Dorn RI (2013) Weathering and soils geomorphology: rock coatings. In: Pope GA (ed) Weathering and Soils Geomorphology. Treatise on Geomorphology, volume 4. Academic Press, San Diego. pp 70-97
Dorn RI, Mahaney W, Krinsley DH (2017) Case hardening: turning weathering rinds into protective shells. Elements 13: 165-169
Gadd GM (2017) Fungi, rocks, and minerals. Elements 13: 171-176
Koschinsky A, Hein JR (2017) Marine ferromanganese encrustations: archives of changing oceans. Elements 13: 177-182
Krinsley DH, DiGregorio B, Dorn RI, Razink J, Fisher R (2017) Mn-Fe-enhancing budding bacteria in century-old rock varnish, Erie Barge Canal, New York. Journal of Geology, 125: 317-336
Malcolm KJ, Leverington DW, Schindler M (2015) A Landsat-based study of black rock coatings proximal to base metal smelters, Sudbury, Ontario, Canada. International Journal of Remote Sensing 36: 3932-3960
Mantha NM, Schindler M, Murayama M, Hochella MF Jr (2012). Silica- and sulphate-bearing rock coatings in smelter areas: products of chemical weathering and atmospheric pollution I. Formation and mineralogical composition. Geochimica et Cosmochimica Acta 85: 254-274
Marnocha CL (2017) Rock coatings and the potential for life on Mars. Elements 13: 187-191
von Humboldt A (1814) Personal Narrative of Travels to the Equinoctial Regions of the New Continent During the Years 1799–1804 by Alexander von Humboldt and Aimé Bonpland. Longman, Hurst, Rees, Orme and Brown, London. 521 pp (trans. by H.M. Williams)
Schindler M, Singer D (2017) Mineral surface coatings: environmental records at the nanoscale. Elements 13: 159-164
Smith BJ, Prˇikryl R (2007) Diagnosis decay: the value of medical analogy in understanding the weathering of building stones. In:
Prikryl R, Smith BJ (eds) Building Stone Decay: From Diagnosis to Conservation. Geological Society, London, Special Publications 271, pp 1-8
Whitley DS, Calogero M, Santoro CS, Valenzuela D (2017) Climate change, rock coatings, and the archaeological record. Elements 13: 182-186