Anna Ladenberger

The concept of compositional data analysis in practice - Total major element concentrations in agricultural and grazing land soils of Europe

Applied geochemistry and environmental sciences invariably deal with compositional data. Classically, the original or log-transformed absolute element concentrations are studied. However, compositional data do not vary independently, and a concentration based approach to data analysis can lead to faulty conclusions. For this reason a better statistical approach was introduced in the 1980s, exclusively based on relative information. Because the difference between the two methods should be most pronounced in large-scale, and therefore highly variable, datasets, here a new dataset of agricultural soils, covering all of Europe (5.6 million km(2)) at an average sampling density of 1 site/2500 km(2), is used to demonstrate and compare both approaches. Absolute element concentrations are certainly of interest in a variety of applications and can be provided in tabulations or concentration maps. Maps for the opened data (ratios to other elements) provide more specific additional information. For compositional data XY plots for raw or log-transformed data should only be used with care in an exploratory data analysis (EDA) sense, to detect unusual data behaviour, candidate subgroups of samples, or to compare pre-defined groups of samples. Correlation analysis and the Euclidean distance are not mathematically meaningful concepts for this data type. Element relationships have to be investigated via a stability measure of the (log-)ratios of elements. Logratios are also the key ingredient for an appropriate multivariate analysis of compositional data.

The Baltoscandian margin detrital zircon signatures of the central Scandes

Abstract In central parts of the Scandinavian Caledonides, detrital zircon signatures provide evidence of the change in character of the Baltoscandian crystalline basement, from the characteristic Late Palaeoproterozoic granites of the Transscandinavian Igneous Belt (TIB, c. 1650–1850 Ma) in the foreland Autochthon to the typical, mainly Mesoproterozoic-age profile (c. 950–1700 Ma) of the Sveconorwegian Orogen of southwestern Scandinavia in the hinterland. Late Ediacaran to Early Cambrian shallow-marine Vemdal quartzites of the Jämtlandian Nappes (Lower Allochthon) provide strong bimodal signatures with TIB (1700–1800 Ma) and Sveconorwegian, sensu stricto (900–1150 Ma) ages dominant. Mid-Ordovician turbidites (Norråker Formation) of the Lower Allochthon in Sweden, sourced from the west, have unimodal signatures dominated by Sveconorwegian ages with peaks at 1000–1100 Ma, but with subordinate components of older Mesoproterozoic zircons (1200–1650 Ma). Latest Ordovician shallow-marine quartzites also yield bimodal signatures, but are more dispersed than in the Vemdal quartzites. In the greenschist facies lower parts of the Middle Allochthon, the Fuda (Offerdal Nappe) and Särv Nappe signatures are either unimodal or bimodal (950–1100 and/or 1700–1850 Ma), with variable dominance of the younger or older group, and subordinate other Mesoproterozoic components. In the overlying, amphibolite to eclogite facies lower part of the Seve Nappe Complex, where the metasediments are dominated by feldspathic quartzites, calcsilicate-rich psammites and marbles, most units have bimodal signatures similar to the Särv Nappes, but more dispersed; one has a unimodal signature very similar to the Ordovician turbidites of the Jämtlandian Nappes. In the overlying Upper Allochthon, Lower Köli (Baltica-proximal, Virisen Terrane), Late Ordovician quartzites provide unimodal signatures dominated by Sveconorwegian ages (sensu stricto). Further north in the Scandes, previously published zircon signatures in quartzites of the Lower Allochthon are similar to the Vemdal quartzites in Jämtland. Data from the Kalak Nappes at 70°N are in no way exotic to the Sveconorwegian Baltoscandian margin. They do show a Timanian influence (ages of c. 560–610 Ma), as would be expected from the palinspastic reconstructions of the nappes. Thus the detrital zircon signatures reported here and published elsewhere provide supporting evidence for a continuation northwards of the Sveconorwegian Orogen in the Neoproterozoic, from type areas in the south, along the Baltoscandian margin of Baltica into the high Arctic. Supplementary material: LA-ICP-MS U–Pb analyses are available at http://www.geolsoc.org.uk/SUP18699.

Tectonometamorphic evolution of the Åreskutan Nappe–Caledonian history revealed by SIMS U–Pb zircon geochronology

Abstract Secondary ionization mass spectrometry (SIMS) U–Pb dating of zircons from the Åreskutan Nappe in the central part of the Seve Nappe Complex of western central Jämtland provides new constraints on the timing of granulite–amphibolite-facies metamorphism and tectonic stacking of the nappe during the Caledonian orogeny. Peak-temperature metamorphism in garnet migmatites is constrained to c. 442±4 Ma, very similar to the ages of leucogranites at 442±3 and 441±4 Ma. Within a migmatitic amphibolite, felsic segregations crystallized at 436±2 Ma. Pegmatites, cross-cutting the dominant Caledonian foliation in the Nappe, yield 428±4 and 430±3 Ma ages. The detrital zircon cores in the migmatites and leucogranites provide evidence of Late Palaeoproterozoic, Mesoproterozoic to Early Neoproterozoic source terranes for the metasedimentary rocks. The formation of the ductile and hot Seve migmatites, with their inverted metamorphism and thinning towards the hinterland, can be explained by an extrusion model in which the allochthon stayed ductile for a period of at least 10 million years during cooling from peak-temperature metamorphism early in the Silurian. In our model, Baltica–Laurentia collision occurred in the Late Ordovician–earliest Silurian, with emplacement of the nappes far on to the Baltoscandian platform during the Silurian and early Devonian, Scandian Orogeny lasting until c. 390 Ma.

Torellian (c. 640 Ma) metamorphic overprint of Tonian (c. 950 Ma) basement in the Caledonides of southwestern Svalbard

Ion microprobe dating in Wedel Jarlsberg Land, southwestern Spitsbergen, provides new evidence of early Neoproterozoic ( c . 950 Ma) meta-igneous rocks, the Berzeliuseggene Igneous Suite, and late Neoproterozoic ( c . 640 Ma) amphibolite-facies metamorphism. The older ages are similar to those obtained previously in northwestern Spitsbergen and Nordaustlandet where they are related to the Tonian age Nordaustlandet Orogeny. The younger ages complement those obtained recently from elsewhere in Wedel Jarlsberg Land of Torellian deformation and metamorphism at 640 Ma. The Berzeliuseggene Igneous Suite occurs in gently N-dipping, top-to-the-S-directed thrust sheets on the eastern and western sides of Antoniabreen where it is tectonically intercalated with younger Neoproterozoic sedimentary formations, suggesting that it provided a lower Tonian basement on which upper Tonian to Cryogenian sediments (Deilegga Group) were deposited. They were deformed together during the Torellian Orogeny, prior to deposition of Ediacaran successions (Sofiebogen Group) and overlying Cambro-Ordovician shelf carbonates, and subsequent Caledonian and Cenozoic deformation. The regional importance of the late Neoproterozoic Torellian Orogeny in Svalbards Southwestern Province and its correlation in time with the Timanian Orogeny in the northern Urals as well as tectonostratigraphic similarities between the Timanides and Pearya (northwestern Ellesmere Island) favour connection of these terranes prior to the opening of the Iapetus Ocean and Caledonian Orogeny.

Pressure–temperature evolution of a kyanite–garnet pelitic gneiss from Åreskutan: evidence of ultra-high-pressure metamorphism of the Seve Nappe Complex, west-central Jämtland, Swedish Caledonides

Abstract New evidence is presented for ultra-high-pressure metamorphism of kyanite–garnet pelitic gneiss in the Åreskutan Nappe of the Seve Nappe Complex, in the central part of the Scandinavian Caledonides. Modelled phase equilibria for a peak pressure assemblage garnet+phengite+kyanite+quartz (coesite) in the NCKFMMnASH system record pressure and temperature conditions of c. 26–32 kbar at 700–720 °C, possibly up to ultra-high-pressure conditions. Subsequent decompression, simultaneous with an increase of temperature to c. 800–820 °C, led to partial melting largely owing to the dehydration and breakdown of phengite. Based on existing isotope age data, we conclude that the Middle Seve Nappe in central Jämtland experienced deep subduction in the late(st) Ordovician, prior to decompression and partial melting of the pelitic protoliths during Early Silurian extrusion, giving way in the Mid to Late Silurian to thrusting on to the Baltoscandian platform. Nappe emplacement probably continued into and through the Early Devonian.

Low density geochemical mapping and mineral exploration: application of the mineral system concept

The mineral exploration industry is used to very high sample densities (100s to 1000s of samples/km2) for geochemical exploration in order to define drill targets. Lately, geoscience organizations in many countries have been geochemically mapping increasingly larger areas at progressively lower sampling densities (1 site/100 to 1 site/18 000 km2). A single ore body is too small a target and cannot be expected to be discovered at such low sample densities; indeed numerous deposits could be hidden within a 100 × 100 km grid cell. However, mineral systems, which include all geological ingredients and processes necessary for the generation of mineral deposits, form much larger targets that could be identified even at such low sampling densities. Examples from some European low density geochemical surveys where patterns emerged that may have implications for mineral exploration are shown and discussed. It is concluded that low density geochemical mapping holds great promise in the early stages of mineral exploration programmes in guiding subsequent effort into the more fertile regions. Interpretation of these maps, however, may need a different approach than that used in classical, high density mapping exercises, where only ‘high values’ of certain metals are the traditional target.

U-Th signatures of agricultural soil at the European continental scale (GEMAS): Distribution, weathering patterns and processes controlling their concentrations

Agricultural soil (Ap-horizon, 0-20cm) samples were collected in Europe (33 countries, 5.6millionkm2) as part of the GEMAS (GEochemical Mapping of Agricultural and grazing land Soil) soil-mapping project. The GEMAS survey area includes diverse groups of soil parent materials with varying geological history, a wide range of climate zones, and landscapes. The soil data have been used to provide a general view of U and Th mobility at the continental scale, using aqua regia and MMI® extractions. The U-Th distribution pattern is closely related to the compositional variation of the geological bedrock on which the soil is developed and human impact on the environment has not concealed these genuine geochemical features. Results from both extraction methods (aqua regia and MMI®) used in this study support this general picture. Ternary plots of several soil parameters have been used to evaluate chemical weathering trends. In the aqua regia extraction, some relative Th enrichment-U loss is related to the influence of alkaline and schist bedrocks, due to weathering processes. Whereas U enrichment-Th loss characterizes soils developed on alkaline and mafic bedrock end-members on one hand and calcareous rock, with a concomitant Sc depletion (used as proxy for mafic lithologies), on the other hand. This reflects weathering processes sensu latu, and their role in U retention in related soils. Contrary to that, the large U enrichment relative to Th in the MMI® extraction and the absence of end-member parent material influence explaining the enrichment indicates that lithology is not the cause of such enrichment. Comparison of U and Th to the soil geological parent material evidenced i) higher capability of U to be weathered in soils and higher resistance of Th to weathering processes and its enrichment in soils; and, ii) the MMI® extraction results show a greater affinity of U than Th for the bearing phases like clays and organic matter. The comparison of geological units with U anomalies in agricultural soil at the country scale (France) enables better understanding of U sources in the surficial environment and can be a useful tool in risk assessments.

GEMAS: CNS concentrations and C/N ratios in European agricultural soil

A reliable overview of measured concentrations of TC, TN and TS, TOC/TN ratios, and their regional distribution patterns in agricultural soil at the continental scale and based on measured data has been missing - despite much previous work on local and the European scales. Detection and mapping of natural (ambient) background element concentrations and variability in Europe was the focus of this work. While total C and S data had been presented in the GEMAS atlas already, this work delivers more precise (lower limit of determination) and fully quantitative data, and for the first time high-quality TN data. Samples were collected from the uppermost 20cm of ploughed soil (Ap horizon) at 2108 sites with an even sampling density of one site per 2500km2 for one individual land-use class (agricultural) across Europe (33 countries). Laboratory-independent quality control from sampling to analysis guaranteed very good data reliability and accuracy. Total carbon concentrations ranged from 0.37 to 46.3wt% (median: 2.20wt%) and TOC from 0.40 to 46.0wt% (median: 1.80wt%). Total nitrogen ranged from 0.018 to 2.64wt% (median: 0.169wt%) and TS from 0.008 to 9.74wt% (median: 0.034wt%), all with large variations in most countries. The TOC/TN ratios ranged from 1.8 to 252 (median: 10.1), with the largest variation in Spain and the smallest in some eastern European countries. Distinct and repetitive patterns emerge at the European scale, reflecting mostly geogenic and longer-term climatic influence responsible for the spatial distribution of TC, TN and TS. Different processes become visible at the continental scale when examining TC, TN and TS concentrations in agricultural soil Europe-wide. This facilitates large-scale land-use management and allows specific areas (subregional to local) to be identified that may require more detailed research.