Artifacts from manganese reduction in rock samples prepared by focused ion beam (FIB) slicing for X-ray microspectroscopy

The spatial distribution of transition metal valence states is of broad interest in the microanalysis of geological and environmental samples. An example is rock varnish, a natural manganese (Mn)-rich rock coating, whose genesis mechanism remains a subject of scientific debate. We conducted scanning transmission X-ray microscopy with near-edge X-ray absorption fine-structure spectroscopy (STXM-NEXAFS) measurements of the abundance and spatial distribution of different Mn oxidation states within the nano- to micrometer thick varnish crusts. Such microanalytical measurements of thin and hard rock crusts require sample preparation with minimal contamination risk. Focused ion beam (FIB) slicing was used to obtain ∼100–1000&thinsp;nm thin wedge-shaped slices of the samples for STXM, using standard parameters. However, while this preparation is suitable for investigating element distributions and structures in rock samples, we observed artifactual modifications of the Mn oxidation states at the surfaces of the FIB slices. Our results suggest that the preparation causes a reduction of Mn4+ to Mn2+. We draw attention to this issue, since FIB slicing, scanning electron microscopy (SEM) imaging, and other preparation and visualization techniques operating in the kilo-electron-volt range are well-established in geosciences, but researchers are often unaware of the potential for the reduction of Mn and possibly other elements in the samples.</p

Lengthening of maize maturity time is not a widespread climate change adaptation strategy in the US Midwest

Increasing temperatures in the US Midwest are projected to reduce maize yields because warmer temperatures hasten reproductive development and, as a result, shorten the grain fill period. However, there is widespread expectation that farmers will mitigate projected yield losses by planting longer season hybrids that lengthen the grain fill period. Here, we ask: (a) how current hybrid maturity length relates to thermal availability of the local climate, and (b) if farmers are shifting to longer season hybrids in response to a warming climate. To address these questions, we used county‐level Pioneer brand hybrid sales (Corteva Agriscience) across 17 years and 650 counties in 10 Midwest states (IA, IL, IN, MI, MN, MO, ND, OH, SD, and WI). Northern counties were shown to select hybrid maturities with growing degree day (GDD°C) requirements more closely related to the environmentally available GDD compared to central and southern counties. This measure, termed “thermal overlap,” ranged from complete 106% in northern counties to a mere 63% in southern counties. The relationship between thermal overlap and latitude was fit using split‐line regression and a breakpoint of 42.8°N was identified. Over the 17‐years, hybrid maturities shortened across the majority of the Midwest with only a minority of counties lengthening in select northern and southern areas. The annual change in maturity ranged from −5.4 to 4.1 GDD year−1 with a median of −0.9 GDD year−1. The shortening of hybrid maturity contrasts with widespread expectations of hybrid maturity aligning with magnitude of warming. Factors other than thermal availability appear to more strongly impact farmer decision‐making such as the benefit of shorter maturity hybrids on grain drying costs, direct delivery to ethanol biorefineries, field operability, labor constraints, and crop genetics availability. Prediction of hybrid choice under future climate scenarios must include climatic factors, physiological‐genetic attributes, socio‐economic, and operational constraints

Interdisciplinary studies on Rock Varnish

This work is a study of rock varnishes collected from different locations worldwide and from several environments. The focus was set on the microanalytical investigation of i) the geochemistry of different varnishes and adjacent collected mineral dust, ii) the structures of different varnishes, and iii) signs for a definite proof for a biogenic compound necessary for the formation of rock varnish. To this end, a combination of different cutting-edge analytical techniques was utilized to investigate the rock varnishes. Rock varnish is a black, micrometer thin, sedimentary crust on top of rock surfaces. It occurs on almost all lithologies and can be found for instance in deserts, in caves, on mountains and plateaus, in large cities, Antarctica, at waterfronts, and possibly even on Mars. Rock varnish consists of a mixture of dust grains as its main component, and a Mn-rich matrix material as cement component. A biogenic contribution to the genesis has been suggested. Rock varnish is a phenomenon that has defied more than 200 years of intermittent scientific examination. Methods and instruments used in this study were amongst others fs/ns LA-ICP-MS, fs LA-MC-ICP-MS, solution ICP-MS, portable XRF, stationary XRF, EPMA, NanoSIMS, FIB- and Microtome-slicing, STXM-NEXAFS, SEM, Raman spectroscopy, EDX, XRD, and TEM. More than 150 individual rock samples were investigated for their chemical composition by fs LA-ICP-MS, and on each sample about 10-30 measurements were conducted to obtain enough statistical accuracy since the crusts are very inhomogeneous. On the basis of the results, several samples were selected that were additionally investigated by other methods mentioned above, to receive further information about, e.g., nano- and microstructures, element distributions, in-situ oxidation state distributions, and isotope ratios. Mineral dust samples were collected and investigated by fs LA-ICP-MS and XRD, due to their role as potential element and material source for rock varnish. Additionally, STXM-NEXAFS measurements, Raman spectroscopy, extensive PCR, qPCR, and NGS studies, EPR measurements, light microscopic investigations, δ13C measurements, as well as fluorescence microscopy analyses on DAPI, DRAQ5, and OsO4 fixed thin sections were performed in the search for proof of a biogenic genesis of rock varnish. It was found that rock varnishes from disparate environments differ, and that the term rock varnish can thus be subdivided and categorized. It was possible to analyze the trace element composition of the microstructure of varnish without contamination from the host rock. Images of nano- and microstructures were obtained for a large set of varnishes and for varnishes from different environments. It was furthermore possible to determine and locate Mn oxidation states at the nanometer scale within single samples and by this show the reduction of the manganese matrix around cavities, a sign for previous redox reactions. The results indicate that the airborne dust is the major contributor to the genesis of varnishes found in deserts, vehicle emissions the main source of varnish on facades in urban areas, and water the source of crusts within river splash zones. No indicator for a possible biogenic origin was found in any varnish sample, even though a large variety of tests was performed. However, one would expect to find organisms capable of oxidizing Mn, at least in the most recently formed layers, or on the surface of rock varnish. Thus, an abiogenic genesis of rock varnish seems most plausible. In addition to the measurements, technical developments were made, such as the development of a new microanalytical reference material for Mn-rich rocks, a calibration for portable XRF measurements of thin Mn-rich coatings, and a measurement technique that allows the measurement of femtogram amounts of powdered material (such as dust) with high precision and accuracy. By providing these technical developments and a broad variety of information about different varnish types, as well as details about individual samples, significant cornerstones are now given on the basis of which future investigations can proceed with the aim to disclose the secret of the genesis of rock varnish

Rock varnish on petroglyphs from the Hima region, southwestern Saudi Arabia: Chemical composition, growth rates, and tentative ages

We investigated rock varnish formed on sandstone and petroglyphs in the Hima area, southwestern Saudi Arabia. To characterize the rock varnish, we made in-situ measurements by portable x-ray fluorescence (pXRF) and analyzed samples by femtosecond laser-ablation inductively coupled–plasma mass spectrometry (fs LA-ICP-MS). Detailed chemical analysis of the rock varnish samples and adjacent soil or aeolian dust yielded information about the varnish’s geochemical context and formation mechanism. Untypically low positive Ce anomalies in the rock varnish samples correlated with negative Ce anomalies in the dust, supporting the hypothesis that the dust is the source of the varnish material. To study the varnish development, we made use of the fact that engraving the petroglyphs exposes a fresh bare sandstone surface without varnish, on which varnish regrows subsequently. We determined by pXRF the areal density of manganese (Mn) and iron (Fe) that had been deposited as rock varnish since the creation of the rock art. The rates of Mn deposition in the newly formed varnish were then estimated by correlating the areal density of Mn in Ancient Arabian and Old Arabic inscriptions with their known age ranges. The observed deposition rates showed substantial variability resulting from differences in exposure conditions of the rock surface, but were in a range comparable with that of our previous measurements in northwestern Arabia. This variability could be reduced significantly by referencing the measurements to the intact varnish adjacent to the individual petroglyphs. This normalization provided a much clearer relationship between varnish deposition and age, and enabled tentative ages to be assigned to rock art motifs without previously known ages. These tentative ages spanned most of the Holocene period and were consistent with the culturally or ecologically derived ages of the animal and human figures depicted in the rock art and the styles of scripts used in different periods

A new technique to determine element amounts down to femtograms in dust using femtosecond laser ablation-inductively coupled plasma-mass spectrometry

Atmospheric dust has a substantial impact on climate, and climate in turn affects the dust flux and origin. Knowing its composition helps to determine dust sources and climate processes. We have therefore determined 46 major and trace elements in dust samples, using in-situ 200 nm-femtosecond laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Very small test portions of 4-7 mu g were enough to enable analyses down to detection limits of femtograms and ng g(-1), respectively. The data agree within 40 to 15% with the reference values for test portions of 1 to 100 mu g, respectively. The femtosecond laser results are compared to those obtained using a 213 nm nanosecond laser, which show the advantages of the femtosecond laser, such as the ability to use non-matrix-matched calibration, reliable measurements of volatile elements, and smaller quantities of material ablation with similar results for the repeatability. The technique is especially useful for measurements of small amounts of dust on filters, and for 2D-distribution maps of selected elements to identify minerals or contamination. To demonstrate the applicability, five dust samples from Niger and Chad, in the Sahara zone, were analyzed. The samples are characterized by enrichment in light REEs and a distinct Eu anomaly, with surface soils as their source. (C) 2014 Elsevier B.V. All rights reserved

Growth of desert varnish on petroglyphs from Jubbah and Shuwaymis, Ha'il region, Saudi Arabia

Petroglyphs, engraved throughout the Holocene into rock varnish coatings on sandstone, were investigated in the Ha’il region of northwestern Saudi Arabia, at Jabal Yatib, Jubbah, and Shuwaymis. The rock art has been created by removing the black varnish coating and thereby exposing the light sandstone underneath. With time, the varnish, a natural manganese (Mn)-rich coating, grows back. To study the rate of regrowth, we made 234 measurements by portable X-ray fluorescence (pXRF) on intact varnish and engraved petroglyphs. Since many petroglyphs can be assigned to a specific time period, a relationship between their ages and the Mn surface densities (DMn) of the regrown material could be derived. This relationship was improved by normalizing the DMn in the petroglyphs with the DMn of adjacent intact varnish. In turn, we used this relationship to assign a chronologic context to petroglyphs of unknown ages. Following the removal of the varnish by the artist and prior to the beginning of Mn oxyhydroxide regrowth, a thin Fe-rich film forms on the underlying rock. This initial Fe oxyhydroxide deposit may act as catalyst for subsequent fast Mn oxidation. After a few decades of relatively rapid growth, the regrowth of the Mn-rich varnish slows down to about 0.017 µg cm–2 a–1 Mn, corresponding to about 0.012% a–1 Mn of the intact varnish density, or about 1.2 nm a–1, presumably due to a change of the catalytic process. Our results suggest that petroglyphs were engraved almost continuously since the pre-Neolithic period, and that rock varnish growth seems to proceed roughly linear, without detectable influences of the regional Holocene climatic changes

Suitability of Mn- and Fe-Rich Reference Materials for Microanalytical Research

Manganese- and iron-rich materials are of major geoscientific and economic interest, many of which contain microscopic features that provide valuable information. To obtain accurate results, a homogeneous microanalytical reference material for calibration is needed. Several researchers have used the Mn-and Fe-rich RMs, JMn-1, NOD-A-1, NOD-P-1 and FeMn-1, for this purpose; therefore, they were tested in this study to determine their suitability for microanalysis. Their homogeneity was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with two different types of lasers (nano-and femtosecond), with spot and line scan analyses and with different operating parameters, such as spot size, pulse repetition rate and fluence. As the established manganese nodule RMs revealed inhomogeneities for picogram to microgram test portions, we also investigated the new synthetic Fe- and Mn-rich RM, FeMnOx-1. FeMnOx-1 was found to be homogeneous for large (empyt set 40 mu m: 2% RSD repeatability) and small (empyt set 8-10 mu m: 10% RSD repeatability) spot sizes. This homogeneity is in the range of the homogeneous NIST SRM 610 and GSE-1G reference glasses. Furthermore, FeMnOx-1 revealed a large-scale homogeneity within uncertainties of a few per cent, using test portions in the ng range, when measuring four individual mounts of this material