An experimental study of Fe-Mg partitioning between olivine and orthopyroxene at 1173, 1273 and 1423 K and 1.6 GPa

The partitioning of Mg and Fe²⁺ between coexisting olivines and orthopyroxenes in the system MgO-FeO-SiO₂ has been investigated experimentally at 1173, 1273, 1423 K and 1.6 GPa over the whole range of Mg/Fe ratios. The use of barium borosilicate as a flux to promote grain growth, and the identification by back-scattered electron imaging of resulting growth rims suitable for analysis by electron microprobe, results in coexisting olivine and orthopyroxenene compositions determined to a precision of±0.003 to 0.004 in molar Fe/(Mg+Fe). Quasi-reversal experiments were performed starting with Mg-rich olivine and Fe-rich orthopyroxene (low K_D) and vice versa (high K_D), which produced indistinguishable results. The distribution coefficient, K_D, depends on composition and on temperature, but near Fe/(Mg+Fe)=0.1 (i.e. mantle compositions) these effects cancel out, and K_D is insensitive to temperature. The results agree well with previous experimental investigations, and constrain the thermodynamic mixing properties of Mg-Fe olivine solid solutions to show small near-symmetric deviations from ideality, with between 2000 and 8000 J/mol. Multiple non-linear least squares regression of all data gave a best fit with (implying 5450 J/mol at 1 bar) and , but the two W _G parameters are so highly correlated with each other that our data are almost equally well fit with , as obtained by Wiser and Wood. This value implies , apparently independent of temperature. Our experimental results are not compatible with the assessment of olivine-orthopyroxene equilibria of Sack and Ghiorso.     

Brokering Trust in Citizen Science

Citizen science (CS) information requires systematic review that incorporates a range of interests and concerns. Yet, there has been little research on what might constitute reviewing best practice to ensure CS is trusted by contributors and users of the data. Insights from a survey of all 1134 reviewers who curate the global eBird Project highlight the knowledge-brokering work involved to ensure CS data are trusted by both citizens and science. Drawing on scholarship focused on key drivers of useable knowledge for natural resource decision-making, we consider CS reviewing best practice to ensure CS can be useful to the producers and users of this knowledge. We find that CS reviewers need to be motivated to provide appropriate feedback to improve CS data, commit to reviewing practice that is respected by citizens, and ensure the information published is credible and be reviewed by a supportive and accountable network of fellow reviewers.     

Simulating a century of soil erosion for agricultural catchment management

Agricultural land management requires strategies to reduce impacts on soil and water resources while maintaining food production. Models that capture the effects of agricultural and conservation practices on soil erosion and sediment delivery can help to address this challenge. Historic records of climatic variability and agricultural change over the last century also offer valuable information for establishing extended baselines against which to evaluate management scenarios. Here, we present an approach that combines centennial‐scale reconstructions of climate and agricultural land cover with modelling across four lake catchments in the UK where radiometric dating provides a record of lake sedimentation. We compare simulations using MMF‐TWI, a catchment‐scale model developed for humid agricultural landscapes that incorporates representation of seasonal variability in vegetation cover, soil water balance, runoff and sediment contributing areas. MMF‐TWI produced mean annual sediment exports within 9–20% of sediment core‐based records without calibration and using guide parameter values to represent vegetation cover. Simulations of land management scenarios compare upland afforestation and lowland field‐scale conservation measures to reconstructed historic baselines. Oak woodland versus conifer afforestation showed similar reductions in mean annual surface runoff (8–16%) compared to current moorland vegetation but a larger reduction in sediment exports (26–46 versus 4–30%). Riparian woodland buffers reduced upland sediment yields by 15–41%, depending on understorey cover levels, but had only minor effect on surface runoff. Planting of winter cover crops in the lowland arable catchment halved historic sediment exports. Permanent grass margins applied to sets of arable fields across 15% or more of the catchment led to further significant reduction in exports. Our findings show the potential for reducing sediment delivery at the catchment scale with land management interventions. We also demonstrate how MMF‐TWI can support hydrologically‐informed decision making to better target conservation measures in humid agricultural environments. Copyright © 2018 John Wiley & Sons, Ltd.     

Extending NMR data for permeability estimation in fine-grained sediments

We combine nuclear magnetic resonance (NMR) transverse relaxation time data and gamma ray data to estimate lithology-dependent permeability in silt- and clay-rich sediments. This approach extends the utility of the Schlumberger-Doll Research (SDR) permeability equation from reservoirs to aquicludes and seals, and thus improves the value and robustness of NMR data. Data from Keathley Canyon, northern Gulf of Mexico show that NMR data can be used to define permeability from 10⁻¹⁸ to 10⁻¹⁴ m² (0.001–10 millidarcies) as calibrated and tested by direct measurements on core samples. We performed uniaxial, constant rate-of-strain consolidation experiments on sediments from Keathley Canyon to determine core-scale permeability. Permeabilities from these experiments were compared to permeabilities calculated from logging-while-drilling data. A better fit between log-derived permeability and laboratory-measured permeability was obtained using the SDR equation with a variable coefficient A, rather than a constant A as is typically used. We show how A is a function of lithology and can be modeled from gamma ray data. The relationship between A and gamma ray values suggests that variations in A are caused by platy clay minerals and the effect they have on the pore system. Our results provide improved means for permeability estimation for application in basin flow modeling, hydrocarbon migration modeling, and well completion design.     

A global map of rainfed cropland areas (GMRCA) at the end of last millennium using remote sensing

The overarching goal of this study was to produce a global map of rainfed cropland areas (GMRCA) and calculate country-by-country rainfed area statistics using remote sensing data. A suite of spatial datasets, methods and protocols for mapping GMRCA were described. These consist of: (a) data fusion and composition of multi-resolution time-series mega-file data-cube (MFDC), (b) image segmentation based on precipitation, temperature, and elevation zones, (c) spectral correlation similarity (SCS), (d) protocols for class identification and labeling through uses of SCS R²-values, bi-spectral plots, space-time spiral curves (ST-SCs), rich source of field-plot data, and zoom-in-views of Google Earth (GE), and (e) techniques for resolving mixed classes by decision tree algorithms, and spatial modeling. The outcome was a 9-class GMRCA from which country-by-country rainfed area statistics were computed for the end of the last millennium. The global rainfed cropland area estimate from the GMRCA 9-class map was 1.13 billion hectares (Bha). The total global cropland areas (rainfed plus irrigated) was 1.53 Bha which was close to national statistics compiled by FAOSTAT (1.51 Bha). The accuracies and errors of GMRCA were assessed using field-plot and Google Earth data points. The accuracy varied between 92 and 98% with kappa value of about 0.76, errors of omission of 2–8%, and the errors of commission of 19–36%.     

First evidence for the Cenomanian–Turonian oceanic anoxic event (OAE2, ‘Bonarelli’ event) from the Ionian Zone,western continental Greece

Integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils), chemostratigraphic (bulk C and O isotopes) and compound-specific organic geochemical studies of a mid-Cretaceous pelagic carbonate—black shale succession of the Ionian Zone (western Greece), provide the first evidence for the Cenomanian–Turonian oceanic anoxic event (OAE2, ‘Bonarelli’ event) in mainland Greece. The event is manifested by the occurrence of a relatively thin (35 cm), yet exceptionally organic carbon-rich (44.5 wt% TOC), carbonate-free black shale, near the Cenomanian–Turonian boundary within the Vigla limestone formation (Berriasian–Turonian). Compared to the ‘Bonarelli’ black-shale interval from the type locality of OAE2 in Marche–Umbria, Italy, this black shale exhibits greatly reduced stratigraphic thickness, coupled with a considerable relative enrichment in TOC. Isotopically, enriched δ¹³C values for both bulk organic matter (−22.2‰) and specific organic compounds are up to 5‰ higher than those of underlying organic-rich strata of the Aptian-lower Albian Vigla Shale member, and thus compare very well with similar values of Cenomanian–Turonian black shale occurrences elsewhere. The relative predominance of bacterial hopanoids in the saturated, apolar lipid fraction of the OAE2 black shale of the Ionian Zone supports recent findings suggesting the abundance of N₂-fixing cyanobacteria in Cretaceous oceans during the Cenomanian–Turonian and early Aptian oceanic anoxic events.     

Characteristics of deep groundwater flow in a basin marginal setting at Sellafield,Northwest England: Cl and halide evidence

A number of chemical and physical processes inside and outside a sedimentary basin (e.g. evaporite dissolution and topographic drive, respectively) affect groundwater flow near the basin’s margin. Contrasting formations at the margin, typically basinal sedimentary rocks and basement, are host to the interplay between these processes so that groundwater flows and compositions change within a relatively small volume. To interpret how groundwater flow and geochemistry have evolved, interactions between these processes must be understood. Such interactions were investigated near the margin of the East Irish Sea Basin in NW England, by sampling deep groundwaters (to 1500 m below sea level) from Ordovician volcanic basement rocks and Carboniferous to Triassic sedimentary cover rocks. Variable Br/Cl ratios and Cl concentrations in deep saline waters and brines indicate mixing patterns. Variations in ³⁶Cl/Cl constrain the timing of mixing. Relatively low Br/Cl ratios (ca. 1 × 10⁻³ by mass) characterise brine from the western sedimentary cover and reflect halite dissolution further west. Saline water with relatively high Br/Cl ratios (ca. 2 × 10⁻³ by mass) of uncertain origin occupies the eastern basement. These two waters mix across the area. However, mixing alone cannot explain variable ³⁶Cl/Cl ratios, which partly reflect differing in situ³⁶Cl production rates in different rock formations. Most ³⁶Cl/Cl ratios in groundwater sampled from the eastern metavolcanic basement (mean = 25 × 10⁻¹⁵) and western sedimentary cover (mean = 10 × 10⁻¹⁵) are at or close to equilibrium with in situ³⁶Cl production. These variations in ³⁶Cl/Cl across the site possibly took >1.5 Ma to be attained, implying that deep groundwater flow responded only slowly to the Quaternary glaciation of the site. Interplay between varied processes in basin marginal settings does not necessarily imply flow instability.