Biological soil crusts (BSCs) are bio-sedimentary complexes that play critical ecological roles in arid landscapes; however, the interactions between component biota and sediments are poorly understood. A detailed micromorphological investigation of BSC development and crust microstructure in the Muddy Mountains Wilderness Area, Nevada, examined features in thin section using pétrographie microscopy, light microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy. The >1800 microscopic observations were linked to crust macroscale features and soil geomorphology. Complex bio-sedimentary structures of BSCs reflect a dynamic genetic history and diverse formative processes, including: (i) stabilization and authigenic mineral precipitation; (ii) wetting-drying and expansion-contraction; (iii) dust capture; (iv) microscale mass wasting; and (v) vesicular (Av) horizon formation. A new conceptual model for hot deserts illustrates how these processes co-develop with BSC succession, during countless wet-dry cycles, to huild up pinnacle microtopography while simultaneously forming Av horizons in the bio-rich and hio-poor zones. Complex surficial and internal hio-sedimentary structures, which vary as a function of crust morphology, trap surface water for uptake hy crust organisms, while dust influx provides a source of nutrients. These phenomena influence landscape-scale water dynamics and biogeochemical cycling, increasing the availability of soil resources during times of biotic stress. Biological soil crusts uniquely facilitate the accumulation, morphology, and ecosystem function of dust and should, therefore, be considered critical agents in arid pedogenesis and landscape development.Abbreviations: BSC, biological soil crust; EPS, extracellular polymeric secretions; XPL, cross-polarized light.
Large-magnitude retroarc shortening of Cretaceous age is well documented in the Sevier orogenic belt of the western United States, and has been associated with eastward Franciscan subduction that began in the Middle-Late Jurassic, but evidence for major Late Jurassic retroarc shortening has been lacking. Here we report new Lu-Hf garnet geochronology, pressuretemperature (P-T) paths, and 40 Ar/ 39 Ar thermochronology data that document tectonic burial of Late Jurassic age in a Barrovian metamorphic terrain, the Funeral Mountains metamorphic core complex, California, located in the hinterland of the Sevier orogenic belt. The P-T paths determined from growth-zoned garnets in upper greenschist facies pelitic schist show steep P-T trajectories consistent with metamorphism during thrust loading. The age of thrust loading is constrained by a fi ve-point Lu-Hf garnet isochron to be 158.2 ± 2.6 Ma (2σ). Partial exhumation, recorded in 146-153 Ma 40 Ar/ 39 Ar muscovite cooling ages, closely followed garnet growth. Late Jurassic Barrovian metamorphism has not been previously recognized in Cordilleran metamorphic core complexes, possibly due to being obscured by Late Cretaceous to Tertiary deformation, magmatism, and metamorphism. This study fi nds that the period of large-magnitude crustal shortening in the retroarc extended into the Late Jurassic, and may have closely followed the formation of the coherent orogenic system associated with east-dipping Franciscan subduction.
The most common test methods used to evaluate alkali-silica reaction (ASR) are the concrete prism test (CPT) and the accelerated mortar bar test (AMBT). However, these tests were not found to be entirely reliable in predicting the performance of concrete under field conditions, especially when supplementary cementitious materials (SCMs) are used. Recently, two new test methods, the miniature concrete prism test (MCPT) and the concrete cylinder test (CCT), have been proposed but still need to be benchmarked with results from outdoor exposed blocks. In this paper, the results from the MCPT, CCT, CPT and exposed blocks are compared and their ability to properly evaluate the expected behavior of these mixtures in service with regard to ASR is discussed. Here, the results of mixtures made with four reactive aggregates: Spratt, Placitas (coarse aggregates), Wright, and Jobe (fine aggregates) and SCMs (fly ashes Classes F or C, slag cement, or silica fume) at different levels of cement replacement or lithium nitrate are presented. For these mixtures, only the MCPT was capable of properly classifying the efficiency of the ASR preventive measures, as compared with the long-term results obtained from the exposed blocks.
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