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Ejecta blankets around impact craters are rarely preserved on Earth. Although impact craters are ubiquitous on solid bodies throughout the solar system, on Earth they are rapidly effaced and few records exist of the processes occurring during emplacement of ejecta. The Stac Fada Member of the Precambrian Stoer Group of Scotland has previously been described as volcanic in origin. However, shocked quartz and biotite provide evidence for high-pressure shock metamorphism, whilst chromium isotope values and elevated abundances of the platinum group metals and siderophile elements, indicate addition of meteoritic material. Thus the unit is reinterpreted as of impact origin. The ejecta blanket reaches >20 m in thickness, and contains abundant dark green, vesicular, devitrified glass fragments. Field observations suggest that the deposit was emplaced as a single fluidized flow, formed by an impact into water-saturated sedimentary strata. The continental geological setting and presence of groundwater, makes this deposit an analogue for Martian fluidized ejecta blankets.

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Several hundred Middle Palaeolithic (MP) archaeological sites have now been identified in the Arabian Peninsula. However, the study of lithic raw material properties and related procurement behaviours is still in its infancy. Here we describe raw material procurement and early stage lithic reduction at MP sites in the Jubbah palaeolake basin, in the Nefud Desert, northern Saudi Arabia. We describe the sites identified during our surveys, and we use petrographic studies to demonstrate that MP assemblages were mostly produced from differing forms of ferruginous quartzite. These raw materials do not substantially vary in composition, although they are not identical in terms of factors such as grain size and the proportion of iron oxide. We then describe the lithic technology at these sites, with a particular focus on the largest assemblage identified, Jebel Katefeh-12 (JKF-12), which provides detailed information on lithic reduction at a quartzite source. Analyses from this site are then considered together with data from other MP sites in the Jubbah basin, where similar raw material was used. The results indicate that factors such as initial clast size/shape and reduction intensity play important roles in influencing aspects of morphological and technological variability. Our results suggest that incursions of MP populations into northern Arabia were probably temporally limited, as might be expected in a marginal and generally arid region. MP raw material procurement sites provide a highly visible signal of these ephemeral incursions, providing information on the ways that human populations adapted to the challenging conditions of the Saharo-Arabian arid belt.

Microbes interact with metals and minerals in the environment altering their physical and chemical states, whilst in turn metals and minerals impact on microbial growth, activity and survival. The interactions between bacteria and dissolved chromium in the presence of iron minerals, and how these impact on Cr isotope variations, were investigated. Cr(VI) reduction experiments were conducted with two bacteria, Pseudomonas fluorescens LB 300 and Shewanella oneidensis MR-1, in the presence of two iron oxide minerals, goethite and hematite. Both minerals were found to inhibit the rates of Cr(VI) reduction by Pseudomonas, but accelerated those by Shewanella. The Cr isotopic fractionation factors generated by Shewanella were independent of the presence of the minerals (ε = -2.3 ‰). For Pseudomonas, the ε value was the same in both the presence and absence of goethite (-3.3 ‰); although, it was much higher (ε = -4.4 ‰) in the presence of hematite. The presence of aqueous Fe(III) in solution had no detectable impact on either bacterial Cr reduction rates nor isotopic fractionation factors. While the presence of aqueous Fe(II) induced rapid abiotic reduction of Cr, it had little impact on the bacterial Cr reduction rates and the corresponding isotope fractionation factors. The different effects that the presence of Fe minerals had on the Cr fractionation factors for reduction by the different bacterial species may be attributed to the way each bacteria attached to the minerals and their different reduction pathways. SEM images confirmed that Pseudomonas cells were much more tightly packed on the mineral surfaces than were Shewanella. The images also confirmed that Shewanella oneidensis MR-1 produced nanowires. The results suggest that the dominant Cr(VI) reduction pathway for Pseudomonas fluorescens LB 300 may have been through membrane-bound enzymes, whilst for Shewanella oneidensis MR-1 it was probably via extracellular electron transfer. Since different minerals impact differentially on bacterial Cr(VI) reduction and isotope fractionation, variations of mineralogies and the associated changes of bacterial communities should be taken into consideration if using Cr isotopes as proxies in the environment.