High-resolution X-ray fluorescence profiling of hybrid event beds: Implications for sediment gravity flow behaviour and deposit structure

Hybrid event beds form when turbidity currents that transport or locally acquire significant quantities of mud decelerate. The mud dampens turbulence driving flow transformations, allowing both mud and sand to settle into dense, near-bed fluid layers and debris flows. Quantifying details of the mud distribution vertically in what are often complex tiered deposits is critical to reconstructing flow processes and explaining the diverse bed types left by mud-bearing gravity flows. High-resolution X-ray fluorescence core scanning provides continuous vertical compositional profiles that can help to constrain mud distribution at sub-millimetre scale, offering a significant improvement over discrete sampling. The approach is applied here to cores acquired from the Pennsylvanian Ross Sandstone Formation, western Ireland, where a range of hybrid event beds have been identified. Raw X-ray fluorescence counts are calibrated against element concentrations and mineral abundances determined on coincident core plugs, with element and element log-ratios used as proxies to track vertical changes in abundances of quartz, illite (including mica), chlorite and calcite cement. New insights include ‘stepped’ (to higher values) as opposed to ‘saw-tooth’ vertical changes in mud content and the presence of compositional banding that would otherwise be overlooked. Hybrid event beds in basin floor sheets that arrived ahead of the prograding fan system have significantly cleaner sandy components than those in mid-fan lobes. The latter may imply that the heads of the currents emerging from mid-fan channels entrained significant mud immediately before they collapsed. Many of the H3 debrites are bipartite with a sandier H3a division attributed to re-entrainment and mixing of a trailing debris or fluid mud flow (H3b) with sand left by the forward part of the flow. Hybrid event bed structure may thus partly reflect substrate interaction and mixing during deposition, and the texture of the bed divisions may not simply mirror those in the suspensions from which they formed.     

Analcime: A characteristic authigenic phase of andean alluvium,northern chile

Analcime (NaAlSi₂O₆.H₂O) forms a characteristic authigenic phase in Andean alluvium of northern Chile. Three stratigraphic units ranging from the lower Cretaceous to the Miocene were studied in order to investigate the presence of analcime in diagenetic assemblages of different ages. The units were deposited in forearc, foreland/back-arc, and intra-arc basinal settings. Characteristics of other sedimentary analcime occurrences were examined in order to define conditions favourable for analcime precipitation. Zeolite zonation and evidence for the original presence of volcanic glass, common to many sedimentary analcime occurrences, are absent in the studied units. Analcime precipitation requires a high Na⁺/H⁺ ratio, relatively low Si⁴⁺/Al³⁺ activity and low H₂O activity. Diagenetic examination of the three stratigraphic units revealed early quartz and feldspar overgrowths, mechanical clay infiltration, and dissolution of labile rock and mineral fragments, particularly intermediate plagioclase (An_30–80). Conditions for these processes are favoured by oxidizing groundwater at near neutral pH. Later diagenesis involved the precipitation of analcime, non-ferroan calcite, quartz, and authigenic clays as the main cementing phases. These formed due to the concentration of oxidizing groundwater and resulted from its continued reaction with detrital feldspars and rock fragments of andesitic and dacitic composition. Analcime precipitation in Andean alluvium resulted from the evolution of saline, alkaline brines under an arid, evaporative climate in closed hydrographic basins with circulating groundwater systems.     

Bioaccessibility and mobilisation of copper and zinc in estuarine sediment contaminated by antifouling paint particles

Clean estuarine sediment amended with antifouling paint particles has been digested in biologically relevant reagents in order to evaluate the bioaccessibilities of Cu and Zn to deposit feeders in coastal environments where boat maintenance is important. Concentrations of Cu and Zn in the estuarine sediment of about 20 and 70 μg g⁻¹, respectively, increased to about 930 and 330 μg g⁻¹, respectively, on addition of 1.3% of a composite of fractionated paint particles collected from a boat repair facility. Seawater containing the vertebrate bile salt, sodium taurocholate, representative of surfactants in the digestive environment of deposit feeders, mobilised significantly greater quantities of metal (up to about 2 μg g⁻¹ of both Cu and Zn) than seawater alone, presumably through complexation and exchange reactions. Seawater solutions of the protein, bovine serum albumin (BSA), a surrogate for proteinaceous material and amino acids encountered in the digestive tract, mobilised even greater quantities of metal (up to about 80 and 40 μg g⁻¹ of Cu and Zn, respectively) via strong complexation, although addition of taurocholate reduced this capacity through interactions between the two reagents. Overall, and through feeding, burrowing and bioirrigation, infaunal invertebrates are predicted to greatly accelerate the rate of mobilisation and local dispersal of metals in sediment contaminated by antifouling paint particles.     

Geothermal activity at the archaeological site of Aghia Kyriaki and its significance to Roman industrial mineral exploitation on Melos,Greece

The geothermal setting of the archaeological site at Aghia Kyriaki, Southeast Melos (or Milos) was investigated in order to help clarify the possible role of the site in mineral exploitation on Melos in Roman times. There are active sulfurous fumaroles in the area and these were also potential sources of sulfur and alum‐group minerals in Roman times. However, geothermal activity has been ongoing in Southeast Melos for hundreds of thousands of years, and extensive hydrothermal alteration of basement rocks to the northeast of the site has produced “white rocks” containing additional potential industrial minerals such as kaolin and alunite. The archaeological remains occur within, but mainly near the surface, of a deeply gullied sequence of late Quaternary alluvial sediments, which consist mainly of metamorphic detritus but are rich in sulfates; the remains contain pottery sherds through the entire sequence of about 40 m. They were deposited on an earlier gullied topography of felsic tuffs overlying the metamorphic basement. Pervasive and veinlike intense reddish alteration of these sediments is probably mainly due to superheated fluid escaping from depth. Field observations demonstrate that this took place after the main phase of building but was likely to be ongoing during occupation of the site. While industrial minerals and geothermal energy would therefore have been available in the Roman period, any relationship of the site to mineral exploitation will have to be determined by archaeological excavation. © 2003 Wiley Periodicals, Inc.     

Are there any Type 2 QSOs? The case of AXJ0341.4–4453

The X-ray source AXJ0341.4–4453 was described by Boyle et al. as a Type 2 active galactic nucleus (AGN) at z =0.672 based on the absence of broad emission lines in the observed wavelength range 4000–7000 Å. We obtained a new spectrum of AXJ0341.4–4453 extending to 9600 Å which reveals broad Balmer lines and other characteristics of Seyfert 1 galaxies. The FWHM of broad H β is at least 1600 km s⁻¹, while [O  iii ] λ 5007 has FWHM=730 km s⁻¹. The flux ratio [O  iii ] λ 5007/H β =1. Thus, AXJ0341.4–4453 is by definition a narrow-line Seyfert 1 galaxy, or perhaps a moderately reddened Seyfert 1 galaxy, but it is not a Type 2 QSO. Although examples of the latter have long been sought, particularly in connection with the problem of the X-ray background, there is still virtually no evidence for the existence of any Type 2 QSO among X-ray selected samples.     

Chemical versus Enzymatic Digestion of Contaminated Estuarine Sediment: Relative Importance of Iron and Manganese Oxides in Controlling Trace Metal Bioavailability

Chemical and enzymatic reagents have been employed to determine available concentrations of Fe, Mn, Cu and Zn in contaminated estuarine sediment. Gastric and intestinal enzymes (pepsin, pH 2, and trypsin, pH 7·6, respectively) removed significantly more metal than was water-soluble or exchangeable (by seawater or ammonium acetate), while gastro-intestinal fluid of the demersal teleost, Pleuronectes platessa L. (plaice), employed to operationally define a bioavailable fraction of contaminants, generally solubilized more metal than the model enzymes. Manganese was considerably more available than Fe under these conditions and it is suggested that the principal mechanism of contaminant release is via surface complexation and reductive solubilization of Mn oxides, a process which is enhanced under conditions of low pH. Of the chemical reagents tested, acetic acid best represents the fraction of Mn (as well as Cu and Zn) which is available under gastro-intestinal conditions, suggesting that the reducing tendency of acetate is similar to that of the ligands encountered in the natural digestive environment. Although the precise enzymatic and non-enzymatic composition of plaice gastro-intestinal fluid may be different to that encountered in more representative, filter-feeding or burrowing organisms, a general implication of this study is that contaminants associated with Mn oxides are significantly more bioavailable than those associated with Fe oxides, and that contaminant bioavailability may be largely dictated by the oxidic composition of contaminated sediment.     

Chromium(VI) distributions in the Arctic and the Atlantic Oceans and a reassessment of the oceanic Cr cycle

Chromium(VI) concentrations ranging between 3.0 and 6.1 nmol l⁻¹ and 3.1 and 7.3 nmol l⁻¹ were found in the Arctic and Atlantic Oceans, respectively. The vertical profiles show modest depletion of chromium(VI) in surface waters, but poor overall correlations between Cr(VI) and nutrient profiles. Given that Cr(VI) is the dominant oxidation state of chromium in open-ocean waters, these data are combined with literature data to reassess the distribution of Cr in oceanic waters. It is concluded that while Cr shows some characteristics of both “recycled” and “accumulated” vertical profiles, it does not fall clearly within either group.     

Assessment and integration of conventional,RTK-GPS and image-derived beach survey methods for daily to decadal coastal monitoring

Coastal monitoring across a broad range of time-scales was recognized in the latest report by the Intergovernmental Panel on Climate Change as key to better understanding the likely impacts of climate change at the coast. A unique and historic coastal monitoring program undertaken at the Collaroy–Narrabeen embayment in south-eastern Australia comprises: 1) 30 years of monthly conventional (Emery method) surveys of five cross-shore profile lines; 2) three years of monthly three-dimensional surveys of the entire embayment using RTK-GPS mounted to an all-terrain vehicle (ATV); and 3) four years of hourly shoreline measurements using coastal imaging technology (ARGUS). This study evaluates the strengths and limitations of conventional, RTK-GPS and image-derived surveys for coastal monitoring at daily to decadal coastal time-scales. High-accuracy RTK-GPS was used to first assess the accuracy of the conventional and image-derived survey methods. The magnitude of daily to decadal coastal variability was then characterized by calculating the temporal semivariogram of the integrated survey dataset. With both measurement errors and the degree of beach variability quantified, the corresponding signal-to-noise ratios (SNR) of each survey method at different time-scales were determined. The value of the simple and cost-effective Emery method was verified by this analysis, with measurement error significantly smaller than the degree of overall beach variability (SNR = 8.4). The accuracy, speed and efficiency of ATV-mounted RTK-GPS meanwhile make it suitable for three-dimensional beach surveys. Image-derived surveys were found to be an effective means of remotely measuring the considerable degree of beach variability identified at time-scales of less than one month. These measurements however become indistinguishable from survey noise (i.e. SNR ≤ 1) when considering typical weekly (or smaller) variations at large distances from the cameras.     

Simultaneous plasma and magnetic-field measurements of probable tangential discontinuities in the solar wind

Changes in magnetic-field and plasma data from Pioneer 6 across selected discontinuities are found to agree with those predicted for tangential discontinuities. Substantial velocity shears are found across the discontinuities, the largest observed being about 80 km/sec. From the observation of velocity shears and of the asymmetry in the orientation of tangential discontinuities, an East-West asymmetry in the solar-wind speed is inferred: Fast streams should tend to come from the West and slow streams from the East with respect to the average direction. This effect is discussed in relation to the rotation of the sun.