Monitoring of the nano-structure response of natural clay under mechanical perturbation using small angle X-ray scattering and digital image correlation

Acta Geotechnica - This paper introduces a new experimental method to monitor the evolving intra-particle, nanometre-scale response during hydro-mechanical tests on undisturbed wet clay samples...     

Palaeogeographical changes in response to glacial–interglacial cycles,as recorded in Middle and Late Pleistocene seismic stratigraphy,southern North Sea

Offshore stratigraphic records from the North Sea contain information to reconstruct palaeo-ice-sheet extent and understand sedimentary processes and landscape response to Pleistocene glacial–interglacial cycles. We document three major Middle to Late Pleistocene stratigraphic packages over a 401-km² area (Norfolk Vanguard/Boreas Offshore Wind Farm), offshore East Anglia, UK, through the integration of 2D seismic, borehole and cone penetration test data. The lowermost unit is predominantly fluviatile [Yarmouth Roads Formation, Marine Isotope Stage (MIS) 19–13], including three northward-draining valleys. The middle unit (Swarte Bank Formation) records the southernmost extent of tunnel valley-fills in this area of the North Sea, providing evidence for subglacial conditions most likely during the Anglian stage (MIS 12) glaciation. The Yarmouth Roads and Swarte Bank deposits are truncated and overlain by low-energy estuarine silts and clays (Brown Bank Formation; MIS 5d–4). Smaller scale features, including dune-scale bedforms, and abrupt changes in cone penetration test parameters, provide evidence for episodic changes in relative sea level within MIS 5. The landscape evolution recorded in deposits of ~MIS 19–5 are strongly related to glacial–interglacial cycles, although a distinctive aspect of this low-relief ice-marginal setting are opposing sediment transport directions under contrasting sedimentary process regimes.     

Leaching characteristics of a high-calcium fly ash as a function of pH: a potential source of selenium toxicity

Using a modified extraction procedure, the effect of pH on the leaching of selected elements from Ca-rich (Type C) power plant fly ash was studied. Continuous additions of acetic acid were used to maintain pH values of fly ash slurries at 4.0, 6.0 and 8.0 for 24 h and an additional set was leached at its natural pH (average 11.8) value. Analyses for Se, As, Ca, Cd, Cr, Fe, Na and Pb showed that the highest concentrations occur in the leachate at pH 4.0 and decline with increasing pH. Concentrations of Cr and Fe increased slightly between neutral and high pH. Arsenic, Cd, Cr, Pb and Se concentrations exceeded the Environmental Protection Agency's toxicity criteria at pH 4.0. Selenium was above its toxicity level at pH values near 7 but the other elements were below their respective toxicity levels near neutral pH. Because recent studies show adverse effects of Se on aquatic life at far lower concentrations than the current Environmental Protection Agency's standard, high-Ca, power plant fly ashes represent a potentially hazardous pollutant to surface and subsurface waters.     

A chironomid-based reconstruction of late glacial summer temperatures in the southern Carpathians (Romania)

Late glacial and early Holocene summer temperatures were reconstructed based on fossil chironomid assemblages at Lake Brazi (Retezat Mountains) with a joint Norwegian–Swiss transfer function, providing an important addition to the late glacial quantitative climate reconstructions from Europe. The pattern of the late glacial temperature changes in Lake Brazi show both similarities and some differences from the NGRIP δ¹⁸O record and other European chironomid-based reconstructions. Our reconstruction indicates that at Lake Brazi (1740 m a.s.l.) summer air temperature increased by ~ 2.8°C at the Oldest Dryas/Bølling transition (GS-2/GI-1) and reached 8.1–8.7°C during the late glacial interstade. The onset of the Younger Dryas (GS-1) was characterized by a weak (< 1°C) decrease in chironomid-inferred temperatures. Similarly, at the GS-1/Holocene transition no major changes in summer temperature were recorded. In the early Holocene, summer temperature increased in two steps and reached ~ 12.0–13.3°C during the Preboreal. Two short-term cold events were detected during the early Holocene between 11,480–11,390 and 10,350–10,190 cal yr BP. The first cooling coincides with the Preboreal oscillation and shows a weak (0.7°C) temperature decrease, while the second is characterized by 1°C cooling. Both cold events coincide with cooling events in the Greenland ice core records and other European temperature reconstructions.     

Direct physical evidence of dolomite recrystallization

The possibility of recrystallization is a long‐standing barrier to deciphering the genetic origin of dolomites. There is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event. Results from 65 new high‐temperature dolomite synthesis experiments (1 m , 1·0 Mg/Ca ratio solutions at 218°C) demonstrate dolomite recrystallization affecting stoichiometry, cation ordering and nanometre‐scale surface texture. The data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization, which occur by various dissolution–precipitation reactions. During the first phase (induction period), no dolomite forms despite favourable conditions. The second phase (replacement period) occurs when Ca‐rich dolomite products, with a low degree of cation ordering, rapidly replace calcite reactants. During the replacement period, dolomite stoichiometry and the degree of cation ordering remain constant, and all dolomite crystal surfaces are covered by nanometre‐scale growth mounds. The third phase (primary recrystallization period), which occurs in the experiments between 97% and 100% dolomite, is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering. The end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat, laterally extensive layers. The fourth phase of the reaction (secondary recrystallization period) occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time. Inferences of recrystallization, in natural dolomite, based on cation order or stoichiometry of dolomite, usually depend on assumptions about the precursor dolomite subjected to recrystallization. If it is assumed that the experimental evidence presented here is applicable to natural, low‐temperature dolomites, then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization.     

Elemental and mineralogical changes in soils due to bioturbation along an earthworm invasion chronosequence in Northern Minnesota

Minnesota forested soils have evolved without the presence of earthworms since the last glacial retreat. When exotic earthworms arrive, enhanced soil bioturbation often results in dramatic morphological and chemical changes in soils with negative implications for the forests’ sustainability. However, the impacts of earthworm invasion on geochemical processes in soils are not well understood. This study attempts to quantify the role of earthworm invasion in mineral chemical weathering and nutrient dynamics along an earthworm invasion chronosequence in a sugar maple forest in Northern Minnesota. Depth and rates of soil mixing can be tracked with atmospherically derived short lived radioisotopes ²¹⁰Pb and ¹³⁷Cs. Their radioactivities increase in the lower A horizon at the expense of the peak activities near the soil surface, which indicate that soil mixing rate and its depth reach have been enhanced by earthworms. Enhanced soil mixing by earthworms is consistent with the ways that the vertical profiles of elemental and mineralogical compositions were affected by earthworm invasion. Biologically cycled Ca and P have peak concentrations near the soil surface prior to earthworm invasion. However, these peak abundances significantly declined in the earthworm invaded soils presumably due to enhanced soil mixing. It is clear that enhanced soil mixing due to earthworms also profoundly altered the vertical distribution of most mineral species within A horizons. Though the mechanisms are not clear yet, earthworm invasion appears to have contributed to net losses of clay mineral species and opal from the A horizons. As much as earthworms vertically relocated minerals and elements, they also intensify the contacts between organic matter and cations as shown in the increased amount of Ca and Fe in organically complexed and in exchangeable pools. With future studies on soil mixing rates and elemental leaching, this study will quantitatively and mechanically address the role of earthworms in geochemical evolution of soils and forests’ nutrient dynamics.     

Expression of the Early Toarcian negative carbon-isotope excursion in separated carbonate microfractions (Jurassic,Paris Basin)

The causes of the pronounced negative excursion in carbon-isotope values that was recorded during the Early Toarcian Oceanic Anoxic Event (T-OAE) are still under debate, particularly with regard to the local versus global pattern of the excursion, and the extent to which recorded signals are under a diagenetic control. In this study we employ a novel microseparation technique in order to investigate the isotopic and mineralogical characteristics of different size fractions of the carbonate content from a Toarcian section recovered from the Sancerre–Couy borehole, southern Paris Basin. Beyond the recognition of a ? 6‰ δ¹³C excursion in the bulk carbonate content, our data also demonstrate that biogenic particles (such as coccoliths) and inorganic grains precipitated as early diagenetic phases (including dolomite) both record the excursion with the same magnitude. Although several black shales occur through the Paris Basin Toarcian section, it is only that associated with the onset of the OAE that coincides with a large negative carbon-isotope excursion. Taken together these observations indicate that during this event, the entire water column was characterized by homogeneous carbon-isotope values; such a pattern is incompatible with the idea that the negative excursion was generated simply through the upwelling of bottom waters enriched in re-mineralized organic carbon (cf. “the Küspert model”), since this would have required a strong vertical gradient in the water column. Additionally, the Paris Basin data show that the decrease in carbonate δ¹³C values during the OAE occurred in several discrete steps (each of some ? 2‰), as has previously been found for organic carbon substrates in other European sections. The stepped nature of the isotopic profile, which is part of a stratigraphic signature previously ascribed to Milankovitch forcing, is compatible with regular pulsed input of light carbon into the whole atmosphere–ocean system from a climatically sensitive source such as gas hydrate, or from thermal methanogenesis of organic-rich sediments in the Karoo–Ferrar large igneous province. Contrasts in the amplitude of the negative carbon-isotope excursion on a regional scale remain an important unexplained aspect of the Toarcian record.     

Thermal effects in the ultrarelativistic two-stream instability

The dispersion relation for longitudinal waves in a one-dimensional ultrarelativistic plasma is calculated. Analytical and numerical results for the growth rate and frequency of the two-stream instability are presented as a function of the energy spread in the denser stream when the dilute stream is cold. The case of energy spreads in both beams is investigated numerically: it is found that relatively small energy spreads in both streams can lead to suppression of the instability.