S.J. Kemp

Precise dating of low-temperature deformation: Strain-fringe analysis by 40Ar-39Ar laser microprobe

Pyritized graptolites from the Welsh Basin (United Kingdom) slate belt acted as rigid bodies during cleavage formation, and epizonal white micas formed within the resulting. strain shadows, orthogonal to the principal stress orientation. Although the quantities of mica are small, they are a pure synkinematic mineral and have been dated by Ar-40-Ar-39 infrared laser microprobe as a means to dating cleavage. Four samples of strain-fringe mica from different hand samples yielded ages ranging from 394.4 +/- 3.1 to 397.8 +/- 1.8 Ma (2sigma), with a mean age of 396.1 +/- 1.4 Ma (2sigma). By focusing on minerals that are unequivocally synkinematic, this technique provides a novel solution to the problems of isotopically dating slaty cleavage. Previous studies have predominantly relied on dating whole-rock slate samples or separated illite grains by Ar-40-Ar-39 techniques; problems encountered included (1) separating the effects of isotopic contamination by detrital phases, (2) Ar-39 loss during the irradiation of illite mineral separates, and (3) thermally induced Ar-40 loss in nature from fine-grained minerals. By circumventing these problems, this new method provides the first unequivocal and high-precision age data for Acadian deformation in the well-characterized Welsh Basin slate belt. With such precision, the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent-continent collision.

A Petrographic Investigation of two Sequential Extraction Techniques Applied to Anaerobic Canal Bed Mud

The application of sequential extraction procedures to determine metal speciation in sediments is fraught with uncertainty regarding what is actually dissolving or re-precipitating at each stage. In order to choose an appropriate scheme for the investigation of contaminated anaerobic mud two different sequential extraction procedures (Kersten and Förstner, 1986; Quevauviller, 1998) were investigated using a Cryogenic SEM (CryoSEM) technique coupled with energy dispersive X-ray analysis (EDXA). This enabled assessment of the degree of reagent selectivity and any re-precipitation associated with the respective methods. Analysis of the non-leached sediment revealed the most abundant authigenic minerals in order of decreasing abundance to be Fe2+-phosphate vivianite (Fe3(PO4)2·8H2O), mixed Fe, Zn, Cu sulphides, pyrite and calcite. After each stage of the sequential extraction the sediment residue was examined using CryoSEM. After extraction of the exchangeable fraction no obvious evidence of mineral dissolution was observed. Calcite was not completely dissolved during the carbonate extraction stage of either procedure. Vivianite began to dissolve in the carbonate extraction stage of both procedures and was completely dissolved by oxide extraction stage of both procedures. The sediment leached by acidified ammonium oxalate, contained abundant Fe oxalate crystals, suggesting that a large proportion of the Fe released from the vivianite has been re-precipitated. The Fe oxalate was then dissolved with the subsequent sulphide fraction. The technique used to extract the sulphide and organic fraction is the same in both schemes and no sulphide or metal rich organic matter was found in either residue.

Palaeocene–Eocene paratropical floral change in North America: responses to climate change and plant immigration

The Initial Eocene Thermal Maximum (IETM) at c. 55 Ma represents a period of rapid global warming that lasted less than 200 ka. The response of vegetation to such an event, and particularly warm-adapted highly diverse vegetation types, is poorly understood. Using pollen floral, clay mineral and stable carbon isotope analyses of sediments from the upper Tuscahoma Formation on the eastern US Gulf Coast (eastern Mississippi and western Alabama), we document paratropical floral changes across the Palaeocene–Eocene boundary from the Wahalak #2 and lower Harrell cores. Data indicate strong changes in the abundance of kaolinite that correlate with changes in relative abundance of native pollen taxa. There is no evidence for a transient, extra-tropical flora on the US Gulf Coast that may characterize the IETM. Immigration and extinction are not associated with this event. Instead, Early Eocene plant immigration occurs in pulses and therefore is not associated directly with climate change during the IETM. The two cores share the same regional species pool but compositional differences are stronger between cores than they are either through changes in environment, increased soil erosion or chemical weathering, or through the introduction of non-native plants. Our data suggest that vegetation change across the Palaeocene–Eocene boundary is not a single event but rather a sequence of cascading events.

Potential and pitfalls in establishing the provenance of earth-related samples in forensic investigations

ABSTRACT: Earth scientists are often asked to establish or constrain the likely provenance of very small quantities of earth‐related material as part of a forensic investigation. We tested the independent and collective interpretations of four experts with differing analytical skills in the prediction of sample provenance for three samples from different environmental settings. The methods used were X‐ray diffraction, scanning electron microscopy, the assessment of pollen assemblages, and structural characterization of organic matter at the molecular level. Independent interpretations were less accurate than those where multiple techniques were combined. Collective interpretation was very effective in the assessment of provenance for two of the three sites where the mineralogy and plant communities were distinctive. At the other site, although the mineralogical analysis correctly identified the Triassic mudstone soil parent material, Carboniferous spores from domestic coal were initially interpreted as deriving directly from bedrock. Such an interpretation could be a common pitfall owing to anthropogenic redistribution of material such as coal.

Clay mineral reaction progress : the maturity and burial history of the Lias group of England and Wales

Abstract The clay mineral assemblages and microtextures of a suite of mudrocks from the Lias Group of England and Wales indicate important regional differences in burial history. Samples from the northern Cleveland Basin are characterized by illite-smectite (I-S, 90% illite) and little carbonate whilst samples from the southern Worcester and Wessex basins contain less mature discrete smectite and are often calcite- and dolomite-rich. Lias Group rocks have been buried to 4 km in the Cleveland Basin but to <2 km in the Worcester and Wessex basins. Burial in the Cleveland Basin is deeper than previously estimated and does not need a local heating event. Illite- smectite (80% illite) detected in samples from the East Midlands Shelf suggests burial to 3 km, again deeper than previous estimates for this region.

The morphologies and compositions of depleted uranium particles from an environmental case-study

Abstract Uraniferous particles from contaminated environmental samples were analysed by scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDXA) and microfocus extended X-ray absorption fine structure (μEXAFS) spectroscopy. The particles of interest are uranium oxides, which were released into the environment by the combustion of scrap depleted uranium (DU) metal at a factory in Colonie, New York, USA. Most of the identified particles appear to have primary, ‘as emitted’ morphologies; some have evidence of minor dissolution, including corrosion pitting. Polycrystalline and often hollow microscopic spheres were identified, which are similar to particles produced by DU munitions impacting armoured targets. They are attributed to the autothermic oxidation of melt droplets. The compositions of the analysed spheres are dominated by UO2+x with variable amounts of U3O8, two of the least soluble and least bioaccessible phases of U. These particles, collected from dusts and soils, have survived more than 25 y in the terrestrial environment. This study further supports the case for using Colonie as an analogue for battlefield DU contamination.

Potentially harmful elements (PHEs) in scalp hair, soil and metallurgical wastes in Mitrovica, Kosovo: the role of oral bioaccessibility and mineralogy in human PHE exposure.

Internationally publicized impacts upon human health associated with potentially harmful element (PHE) exposure have been reported amongst internally displaced populations (IDPs) in Mitrovica, Kosovo, following the Kosovan War. Particular concern has surrounded the exposure to Pb indicated by the presence of highly elevated concentrations of Pb in blood and hair samples. This study utilizes a physiologically-based in-vitro extraction method to assess the bioaccessibility of PHEs in surface soils and metallurgical waste in Mitrovica and assesses the potential daily intake of soil-bound PHEs. Maximum As (210mgkg(-1)), Cd (38mgkg(-1)), Cu (410mgkg(-1)), Pb (18790mgkg(-1)) and Zn (8500mgkg(-1)) concentrations in surface soils (0-10cm) are elevated above guideline values. Samples with high PHE concentrations (e.g. As >1000mgkg(-1); Pb >1500mgkg(-1)) exhibit a wide range of bioaccessibilities (5.40 - 92.20% in the gastric (G) phase and 10.00 - 55.80% in the gastric-intestinal (G-I) phase). Samples associated with lower bioaccessibilities typically contain a number of XRD-identifiable primary and secondary mineral phases, particularly As- and Pb-bearing arsenian pyrite, beudantite, galena and cerrusite. Quantification of the potential human exposure risk associated with the ingestion of soil-associated PHEs indicates that on average, 0.01μg Cd kg(-1) BW d(-1), 0.16μg Cu kg(-1) BW d(--1), 0.12μg As kg(-1) BW d(-1), 7.81μg Pb kg(-1) BW d(-1), and 2.68μg Zn kg(-1) BW d(-1) could be bioaccessible following ingestion of PHE-rich soils in the Mitrovica region, with Pb, and to a lesser extent As, indicating the likely possibility of local populations exceeding the recommended tolerable daily intake. Lead present within surface soils of the area could indeed have contributed to the human Pb burden due to the high bioaccessibility of Pb present within these soils (13.40 - 92.20% in the gastric phase). Data for Pb levels in scalp hair (≤120μgg(-1)) and blood (≥650μgdL(-1); WHO, 2004) for children that have lived within IDP camps in Mitrovica indicate significant Pb uptake has indeed taken place. The highly bioaccessible nature of soil-associated PHEs in this study highlights the need for appropriate environmental management approaches that limit the exposure of local populations to these contaminated soils.

Geochemistry and petrography of phosphorus in urban canal bed sediment

Abstract Urban canal sediment provides an opportunity to investigate the processes controlling P geochemistry in a system dominated by anthropogenic input. To achieve this a combined study was undertaken of pore water geochemistry and petrology of the top 25 cm canal bed sediment from two urban canals in the English West Midland conurbation. Processes in the sediment are dominated by the geochemistry of Fe, P and organic matter. Results indicate that sediment re-suspension by boat traffic has a major influence on pore water chemistry. In addition to the sediment water interface an additional interface at 6–7 cm depth was identified and corresponds to the maximum depth of sediment re-suspension by boat traffic. Phosphorus in the sediment occurs predominantly as vivianite. Low PO4 concentrations occur close to the sediment water interface in summer due to frequent re-suspension and Fe oxidation. High PO4 concentrations occur close to the sediment water interface in winter when boat traffic is infrequent. The greatest potential to exchange phosphate between sediment and water column will therefore occur during infrequent boat traffic regimes.

40Ar–39Ar isotope constraints on the age of deformation in Charnwood Forest, UK

40 Ar– 39 Ar dating by a combination of spot analysis and step-heating has been carried out on mica fabrics from slaty cleavage and a shear zone in the Neoproterozoic to Cambrian rocks of Charnwood Forest, Leicestershire. The results indicate that crust adjacent to the eastern margin of the Midlands Microcraton was affected by localized epizonal metamorphism and penetrative deformation between 425 and 416 Ma (Silurian to earliest Devonian), somewhat earlier than the Acadian Phase of the Caledonian orogeny recorded elsewhere in southern Britain. The Charnwood cleavage geometry suggests that the deformation arose within a dextral transpressional regime along the eastern margin of the Midlands Microcraton. This tectonism may have overlapped with pre-Acadian sinistral transtensional movements documented for the Welsh Basin and Lake District areas, to the west and north of the Midlands Microcraton.

Polyphase low-grade metamorphism of the Ingleton Group, northern England, UK: a case study of metamorphic inversion in a mudrock succession

A series of boreholes in Horton Quarry, northwest Yorkshire (Horton-in-Ribblesdale Inlier) penetrated mudstones and slates belonging to the Austwick Formation (Windermere Supergroup) overlying laminated mudstones of the Ingleton Group. Illite (IC) and chlorite (ChC) crystallinity measurements indicate a metamorphic inversion between the two groups of mudrocks. The Windermere Supergroup mudrocks are mostly in the high anchizone or epizone, whereas the Ingletonian samples are lower grade in terms of IC, and are mostly deep diagenetic zone or low anchizone. Hence younger strata at higher grades rest on older strata at lower grades, creating a metamorphic inversion. Ingletonian slates exposed at Pecca Falls on the River Twiss show epizonal and anchizonal IC values, and greywacke samples from Ingleton Quarry contain pumpellyite. This suggests that grade in the Ingletonian may increase to the NW from the Horton to Ingleton inliers. K-white mica b cell dimensions show further differences between the Ingleton Group and the Windermere Supergroup. The Ingletonian samples are characterized by low b cell values (8.989–9.035, mean 9.007 A), whereas the Windermere Supergroup has higher values in the range 9.022–9.034, mean 9.027 A. The Windermere Supergroup values are similar to those recorded from the Windermere Supergroup of the southern Lake District, and Lower Palaeozoic rocks from the Scottish Southern Uplands, and are consistent with metamorphism in a low heat flow, convergent geotectonic setting. The Ingletonian b cell values suggest metamorphism in a higher heat flow setting, most likely an extensional basin. The metamorphic inversion at Horton and differences in K-white mica b cell dimensions suggest that the Ingleton Group and Windermere Supergroup strata evolved in different geotectonic settings and record two separate metamorphic events. The discovery of the metamorphic inversion at Horton provides further evidence in favour of an Ordovician rather than Neoproterozoic depositional age for the controversial Ingleton Group.