Near-infrared spectroscopy for estimating soil burn severity

Mestrado em Estudos AmbientaisForest fires are a natural phenomenon and occurred long before human kind was around, serving important ecosystem functions. In the past decades, however, some parts of the world have seen marked increases in the frequency and spatial extent of wildfires. This includes Portugal, where forest fires have, on average, affected 100.000 ha of rural lands per year since the mid1970s. In general, the direct and indirect effects of fires depend strongly on the temperatures to which vegetation and soil are exposed. In the case of wildfires – as opposed to prescribed burning or experimental fires - these temperatures can hardly ever be measured. Therefore, wildfire impacts are commonly assessed using proxies based on the consumption of the vegetation and the colour of the ashes deposited on the soil surface. These so-called burn severity indices typically provide qualitative estimates, distinguishing between low, medium and high severity. Recently, however, near- infrared (NIR) reflectance spectroscopy was successfully applied to estimate the maximum temperatures reached (MTR) by soils heated under laboratory conditions. The present study wanted to explore the potential of NIR for estimating MTR in soils burnt by wildfires. To this end, the work addressed two main topics: (i) spatial variability in the relationships between soil heating temperatures in a muffle and the corresponding NIR-based MTR estimates, both between and within study sites; (ii) the importance of this spatial variability in estimating MTRs of wildfire-burnt soil samples. A number of NIR-based models was constructed and used to predict the known MTR of laboratory-heated soil samples. One of the two long-unburnt study sites revealed marked variability over short distances, whereas the other did not. The models based on larger sample numbers, however, provided robust MTR predictions, even when these models involved samples from the two study sites. This probably reflected the sites comparable parent materials, soils and land cover (eucalypt plantations in schist soils). The best achieved models were used to estimate MTR by soil samples from a wildfire occurred in the central-north of Portugal, in the year 2010.. According to the index proposed in this work and the maximum temperatures reached estimations, the soil burn severity of the studied sites was moderate to high in surface samples, and low to moderate in the sub-surface samples

Upscaling calcite dissolution rates in a tight reservoir sandstone

Calcite is a highly abundant mineral in the Earth’s crust and occurs as a cement phase in numerous siliciclastic sediments, where it often represents the most reactive component when a fluid percolates through the rock. Hence, the objective of this study is to derive calcite dissolution rates on different scales in a reservoir sandstone using mineral surface experiments combined with vertical scanning interferometry (VSI) and two types of core plug experiments. The 3D geometry of the calcite cement phase inside the rock cores was characterized by X-ray micro-computed tomography (µXCT) and was used to attempt dissolution rate upscaling from the mineral surface to the core scale. Initially (without upscaling), our comparison of the far-from-equilibrium dissolution rates at the mineral surface (µm-mm-scale, low fluid residence time) and the surface normalized dissolution rates obtained from the core experiments (cm-scale, high fluid residence time) revealed differences of 0.5–2 orders of magnitude. The µXCT geometric surface area connected to the open pore space (GSA$_{Cc,open}$) considers the fluid accessibility of the heterogeneously distributed calcite cement that can largely vary between individual samples, but greatly affects the effective dissolution rates. Using this parameter to upscale the rates from the µm- to the cm-scale, the deviation of the upscaled total dissolution rates from the measured total dissolution rates was less than one order of magnitude for all investigated rock cores. Thus, GSA$_{Cc,open}$ showed to be reasonably suitable for upscaling the mineral surface rates to the core scale

A Statistical Approach for Analysis of Dissolution Rates Including Surface Morphology

Understanding mineral dissolution is relevant for natural and industrial processes that involve the interaction of crystalline solids and fluids. The dissolution of slow dissolving minerals is typically surface controlled as opposed to diffusion/transport controlled. At these conditions, the dissolution rate is no longer constant in time or space, an outcome observed in rate maps and correspondent rate spectra. The contribution and statistical prevalence of different dissolution mechanisms is not known. Aiming to contribute to close this gap, we present a statistical analysis of the variability of calcite dissolution rates at the nano- to micrometer scale. A calcite-cemented sandstone was used to perform flow experiments. Dissolution of the calcite-filled rock pores was measured using vertical scanning interferometry. The resultant types of surface morphologies influenced the outcome of dissolution. We provide a statistical description of these morphologies and show their temporal evolution as an alternative to the lack of rate spatial variability in rate constants. Crystal size impacts dissolution rates most probably due to the contribution of the crystal edges. We propose a new methodology to analyze the highest rates (tales of rate spectra) that represent the formation of deeper etch pits. These results have application to the parametrization and upscaling of geochemical kinetic models, the characterization of industrial solid materials and the fundamental understanding of crystal dissolution

Enhanced weathering potentials—the role of in situ CO2 and grain size distribution

International audienceThe application of rock powder on agricultural land to ameliorate soils and remove carbon dioxide (CO 2 ) from the air by chemical weathering is still subject to many uncertainties. To elucidate the effects of grain size distribution and soil partial pressure of carbon dioxide ( p CO 2 ) levels on CO 2 uptake rates, two simple column experiments were designed and filled nearly daily with an amount of water that simulates humid tropical conditions, which prevail in areas known for being hotspots of weathering. Multiple materials (dunite, basanite, agricultural oxisol, a combination of the latter two, and loess) were compared under ambient and 100% CO 2 atmosphere. In a second series, single material columns (dunite) were filled with three different grain size distributions. Total alkalinity, pH, major ions, and dissolved silica were determined in the outflow water of the columns for about 300 days. Under ambient atmospheric conditions, the CO 2 consumption was the lowest in the oxisol column, with 100 t CO 2 km −2 year −1 , while dunite and basanite showed similar consumption rates (around 220 t CO 2 km −2 year −1 ). The values are comparable to high literature values for ultramafic lithologies. Interestingly, the mixture of basanite and oxisol has a much higher consumption rate (around 430 t CO 2 km −2 year −1 ) than the basanite alone. The weathering fluxes under saturated CO 2 conditions are about four times higher in all columns, except the dunite column, where fluxes are increased by a factor of more than eleven. Grain size distribution differences also play a role, with the highest grain surface area normalized weathering rates observed in the columns with coarser grains, which at first seems counterintuitive. Our findings point to some important issues to be considered in future experiments and a potential rollout of EW as a carbon dioxide removal method. Only in theory do small grain sizes of the spread-material yield higher CO 2 drawdown potentials than coarser material. The hydrologic conditions, which determine the residence times in the pore space, i.e., the time available for weathering reactions, can be more important than small grain size. Saturated-CO 2 column results provide an upper limit for weathering rates under elevated CO 2

The pseudomorphic replacement of marble by apatite: The role of fluid composition

© 2016 Elsevier B.V. The replacement of a natural carbonate rock (Carrara marble) by apatite was used as a model to study the role of fluid chemistry in replacement reactions, focusing on the mineralogy, chemical composition, and porosity of the replacement product. Carrara marble was reacted with diammonium phosphate solutions ((NH4)2HPO4), in the presence and absence of four salt solutions (NH4Cl, NaCl, NH4F, and NaF) at different ionic strengths, at 200 °C and autogenous pressure. The replacement products were analyzed using powder X-ray diffraction, Scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and Raman spectroscopy. The reaction in all samples resulted in pseudomorphic replacements and shared the characteristics of an interface-coupled dissolution-precipitation mechanism. Increasing the ionic strength of the phosphate fluid increased the replacement rates. With a fixed concentration of phosphate, replacement rates were reduced with the addition of NH4Cl and NaCl and increased significantly with the addition of NaF and NH4F. The addition of different salts resulted in specific porosity structures resulting from the formation of different phosphate phases. Chloride-containing fluids showed a higher degree of fluid percolation through grain boundaries. This study illustrates the significant impact that small differences in solvent composition can have in the progress of replacement reactions, the nature of the products and the resultant porosity

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones. © 2003 Elsevier Science Ltd. All rights reserved