Abstract Iron minerals provide sedimentary repositories of chemical information pertaining to Earth’s redox and biogeochemical evolution, from before the Great Oxidation Event some 2.5 billion years ago, to more recent events occurring up to and into the Cenozoic Era. The most powerful chemical information recorded in iron minerals comes in the form of trace-element signatures, most notably their concentrations and stable isotope compositions. Here we provide an introduction to iron mineralogy and the processes responsible for the accumulation and preservation of trace-element signatures in iron minerals, focusing on the deposition of iron minerals in three key ancient sedimentary archives: banded iron formations, ferromanganese crusts and black shales. We introduce the theory and practical use of non-traditional trace-element stable-isotope systems in redox and biogeochemical research, focusing on the recent use of iron, molybdenum and chromium stable isotopes to shed light on the redox and biogeochemical information stored in iron-rich sediments. By analysing both trace-element concentrations and stable isotope compositions recorded in iron minerals, iron-rich sedimentary archives are providing a unique window into the past, where changes in trace-element signatures shed light on major transitions in Earth’s redox and biogeochemical evolution.

Abstract Together with the Playa Hermosa Formation (Fm.), the Las Ventanas and San Carlos formations constitute the Maldonado Group, which is better developed in the southeastern part of Uruguay and covers an area of c. 200 km 2 . The total thickness of both units (i.e. Las Ventanas and San Carlos formations) reaches c. 1500 m, and comprises mafic and acidic volcanic rocks, pyroclastic rocks, diamictite, sandstone, conglomerate and pelite. Structurally, the Maldonado Group is extensively deformed, although variably, throughout the region. Strike–slip faults, westward-verging detachment faults, and folds with axis sub-parallel to the strike–slip planes are common features. The presence of pumpellyite, prehnite, chlorite and epidote in mafic rocks indicates very low- to low-grade metamorphic conditions. The Las Ventanas Fm. is characterized by basal conglomerate, diamictite, sandstone and siltstone that pass upwards into fine-grained rhythmites (pelite), and is the thickest unit of the Maldonado Group ( c. 1250 m). The San Carlos Formation ( c. 250 m) comprises fine-grained conglomerate, sandstone and mudstone towards the top. Both units lie on an angular unconformity above Palaeo- and Neoproterozoic basement and are overlain unconformably by late Ediacaran–lowermost Cambrian units. Reliable palaeomagnetic data indicate that the Maldonado Group accumulated at high palaeolatitudes; however, the palaeogeographical evolution of the Río de la Plata Craton during the Neoproterozoic remains conjectural. Radiometric data from intrusive bodies and cross-cutting strike–slip faults place the minimum age of the group at c. 565 Ma, whereas basement volcanic rocks dated at 590 ± 2 Ma interbedded with meta-sandstones hosting detrital zircons c. 600 million years old provide the best constraint on the maximum age of deposition. Given the absence of carbonate rocks, no chemostratigraphic studies (e.g. C, O, Sr) are available. The Las Ventanas and San Carlos formations are largely interpreted as units within a thick glacially influenced fan-delta sedimentary system formed during the early Ediacaran in a strike–slip basin. Based on stratigraphic and sedimentological characteristics it has been suggested that this succession, containing glacially influenced diamictite and dropstones, records a glacial period that occurred sometime between c. 570 and 590 million years ago. Ongoing research is focused on establishing the precise age of deposition of the Maldonado Group and on reconstructing the tectonic evolution of the basin. Further palaeomagnetic studies will be especially useful for determining the palaeogeography of the Río de la Plata Craton during the Ediacaran and establishing its relationships with neighbouring strata hosting similar successions.

Abstract A wide variety of ancient and modern hot spring systems are characterized by authigenic silica precipitation and sinter formation. In these systems, the chemical disequilibrium of venting hydrothermal fluids leads to the nucleation and growth of amorphous silica masses and simultaneously the mineralization, and potential fossilization, of many different types of microorganisms. Source waters originating from deep, hot reservoirs, at equilibrium with quartz, commonly contain dissolved silica concentrations significantly higher than the solubility of amorphous silica at 100°C (approximately 400 ppm; Figure 1 ) ( Gunnarsson and Arnórsson, 2000 ). Upon the discharge of these fluids at the surface, decompressional degassing and boiling, rapid cooling to ambient temperatures, evaporation, and changes in solution pH all work together to cause the solution to rapidly exceed amorphous silica solubility ( Fournier, 1985 ). Under these conditions, silicic acid dissolved in the monomer form Si(OH)4 spontaneously polymerizes initially to oligomers (e.g., dimmers, trimers and tetramers), and then to polymeric species with spherical diameters of 1-5 nm, as the silanol groups (-Si-OH-) of each oligomer condense and dehydrate to produce the siloxane (-Si-O-Si-) cores of larger polymers. The polymers quickly grow in size such that a bimodal composition of monomers and particles of colloidal dimensions (>5 nm) are generated ( Crerar et al., 1981 ). Depending on the degree of supersaturation, these either remain in suspension, due to the external silanol groups exhibiting a residual negative surface charge due to a low zero point of charge (around pH 2), they coagulate via cation

Abstract: Precambrian banded iron-formations (BIPs) are chemical sediments of hydrothermal origin and consist of Fe-rich minerals with alternating layers of chert. Because microorganisms potentially played a role in their precipitation, the study of bacterial-mineral interactions at modern hydrothermal environments may provide small-scale analogues to those conditions under which they accumulated. Interestingly, microbial populations currently growing at hot springs and deep-sea vents are commonly encrusted in iron and silicate minerals. Iron biomineralization occurs either passively through interaction between the reactive sites of the cell and dissolved cationic iron from the hydrothermal fluid, or actively through chemolithotrophic iron-oxidation by bacteria such as Gallionella genera. Amorphous silica precipitates on individual bacteria through hydrogen bonding between hydroxy groups in the extracellular polymers and hydroxyl groups in dissolved silica, with some colonies becoming completely cemented together within a siliceous matrix up to several micrometers thick. Iron-silicates form due to reactions between dissolved silica and cell-bound iron. In these predominantly nonspecific processes, bacterial cells simply catalyze reactions that are rendered possible by the supersaturated conditions created by the sudden physical and chemical changes induced through venting. Diagenetic reactions, some of which are also catalyzed by microorganisms growing in the sediment, can further alter the mineralogy of these primary precipitates, leading to the formation of secondary magnetite and siderite. In this way, all of the main mineralogical components of BIFs can be associated with microbial activity.