Time-dependent degradation of biotic carbonates and the search for past life on Mars

Searching for traces of extinct and/or extant life on the surface of Mars is one of the major objectives for remote-sensing and in-situ exploration of the planet. In the present paper we study the infrared (IR) spectral modifications induced by thermal processing on differently preserved calcium carbonate fossils, in order to discriminate them from their abiotic counterparts.The main conclusion of this study is that the degree of alteration of the fossils, derived from IR spectral analysis, seems to be well correlated with the sample age, and that terrestrial fossils after a billion years are so altered that it becomes impossible to trace their biotic origin. Since it is reasonable to assume that the putative Martian fossils should be at least 3.5 billion years old, this would imply that our spectroscopic method could not be able to detect them, if their degradation rate were the same as that we have found in usual conditions for the terrestrial fossils. However, due to the different climate evolution of the two planets, there is the possibility of having two different degradation rates, much lower for Mars than for Earth, especially if the fossils are embedded in a protective layer, such as a clay deposit. In this case IR spectroscopy, coupled with thermal processing, can be a useful tool for discriminating between abiotic and biotic (fossil) carbonate samples collected on the Martian surface.     

Earthquake response of a multiblock nuclear reactor graphite core: Experimental model vs simulations

The complex dynamics of a quarter‐scale model of a graphite nuclear reactor core, representative of the second generation of British advanced gas‐cooled nuclear reactors, is investigated numerically and experimentally. Advanced gas‐cooled nuclear reactor cores are polygonal, multilayer, arrays of graphite bricks, with each brick allowed to rock by design relative to each other in accordance with the boundary conditions. A 35 000 DOF, nonlinear finite element model of the core created by Atkins Nuclear, was analysed on a high performance computing facility at the University of Bristol, and a corresponding 8 t physical model, equipped with 3200 data acquisition channels, was built and tested on the University of Bristol 6‐DOF shaking table. In this paper, the two models are subjected to a series of (1) synthetic earthquake and (2) idealised harmonic input motions. The experimental data are used to compare and verify the two models and explore the dynamics of the core. A kinematic model of the response is also developed based solely on geometric constraints. The results are presented in the form of response maps and graphs. Important conclusions are drawn as to the dynamics and earthquake response of such systems, which inform numerical model validation. It is found that contrary to the case of a small number of rocking blocks that exhibit highly complex response patterns, the behaviour of the model at hand is both smooth and repeatable. An analogy between the response of the core and that of dense granular matter exhibiting particle interlocking and dilatancy is highlighted.     

Post-rift volcanism and high heat-flow at the ocean-continent transition of the eastern Gulf of Aden

A high heat-flow (∼900 mW m⁻²) has been observed over a volcanic structure at the Ocean-Continent Transition in the Eastern Gulf of Aden (Oman margin). The anomaly is superposed to a progressive increase of heat-flow across the margin and can be interpreted either by (1) heat refraction, (2) fluid discharge or (3) cooling magma. The two first explanations cannot be ruled out definitely by modelling analysis, but require unlikely thermal conductivity or permeability values. The third one implies that the latest activity of the volcano was about 100 000 years old and therefore continued c. 18 Ma after the break-up of Africa and Arabia. This potential mechanism is consistent with other lines of evidence of post-rifting activity in the Gulf of Aden and could invalidate the conventional assumption that rifted-margins become passive after the break-up of continents.     

Chronology of mercury enrichment factors in reef corals from western Venezuela

Mining and deforestation in the early 20th century, the development of petrochemical industries during the 1950s, and the constant weathering of natural deposits of cinabrium (HgS) have made Golfo Triste, Venezuela, a region impacted by mercury (Hg). We studied the chronology of Hg in coral skeletons of Siderastrea siderea (1 colony, 1900-1996) and Montastraea faveolata (2 colonies, 1930-1999) from Parque Nacional San Esteban. Maximum values of Hg/Ca ratios and standard deviations of Hg enrichment factors occurred in the 1940s, 1960s, and 1980s, and matched maxima of decadal rainfall. Values from the 1950s and 1970s matched periods of abundant but constantly decreasing rainfall and hence were best explained by the combination of runoff and the sudden bioavailability of Hg in the region. This sudden availability likely was associated with activities of the chlorine-caustic soda and fertilizer plants of Morón petrochemical complex, industries that started producing large amounts of Hg in 1958.     

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.     

Inorganic ions and trace metals bulk deposition at an Atlantic Coastal European region

The inorganic chemical composition (major ions and trace metals) of bulk deposition samples collected monthly with bulk collectors at seven Atlantic Coastal European cities (Galicia, Northwest of Spain) during wet season (September 2011 to March 2012) has been assessed and compared. Trace metals (Al, As, Ba, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sr, V and Zn) were analysed in soluble fraction and non-soluble fraction (after acid extraction) of the bulk deposition by inductively coupled plasma-mass spectrometry. Major inorganic ions (Cl^?, NO₃ ^?, SO₄ ^2?, Na⁺, K⁺, Ca²⁺, Mg²⁺ and NH₄ ⁺) were analysed in the soluble fraction of the bulk deposition by capillary zone electrophoresis. Univariate analysis (ANOVA and Multiple Range Test) according to the location of each sampling site was performed. Results also suggest a great influence of cleaner Atlantic air masses. After partition coefficients and enrichment factor estimation, similar sources could be assigned for the ionic and metal composition of bulk deposition at seven urban sites.     

Autoradiographic detection of species-specific thermal stress effects on natural phytoplankton assemblages

Species-specific photosynthetic responses to stress were detected in mixed phytoplankton assemblages using autoradiography. In laboratory experiments subjecting a mixture of the diatom Phaeodactylum and the flagellate Pseudoisochrysis to control and transient thermal stress treatments of +5, +10, +15 and +20°C, both species responded negatively with Phaeodactylum appearing more sensitive. Whole community measures of stress (¹⁴C uptake and DCMU-fluorescence) showed alterations in the photosynthetic response of the two-species mixtures with increasing ΔT, but did not sort out species-specific differences. In two field trials at an estuarine power plant, after entrainment through the plant's cooling system, the photosynthetic rate of a diatom, Cyclotella sp., was reduced whereas the photosynthetic rate of a thecate dinoflagellate, Prorocentrum minimum, was enhanced. Field DCMU-fluorescence measurements of stress failed to detect the observed species-specific entrainment effects.     

A study on the dissolution rates of K-Cr(VI)-jarosites: kinetic analysis and implications

Background

The presence of natural and industrial jarosite type-compounds in the environment could have important implications in the mobility of potentially toxic elements such as lead, mercury, arsenic, chromium, among others. Understanding the dissolution reactions of jarosite-type compounds is notably important for an environmental assessment (for water and soil), since some of these elements could either return to the environment or work as temporary deposits of these species, thus would reduce their immediate environmental impact.

Results

This work reports the effects of temperature, pH, particle diameter and Cr(VI) content on the initial dissolution rates of K-Cr(VI)-jarosites (KFe₃[(SO₄)_2 ? X(CrO₄)_X](OH)₆). Temperature (T) was the variable with the strongest effect, followed by pH in acid/alkaline medium (H₃O⁺/OH^?). It was found that the substitution of CrO₄ ^2?in Y-site and the substitution of H₃O⁺ in M-site do not modify the dissolution rates. The model that describes the dissolution process is the unreacted core kinetic model, with the chemical reaction on the unreacted core surface. The dissolution in acid medium was congruent, while in alkaline media was incongruent. In both reaction media, there is a release of K⁺, SO₄ ^2? and CrO₄ ^2? from the KFe₃[(SO₄)_2 ? X(CrO₄)_X](OH)₆ structure, although the latter is rapidly absorbed by the solid residues of Fe(OH)₃ in alkaline medium dissolutions. The dissolution of KFe₃[(SO₄)_2 ? X(CrO₄)_X](OH)₆ exhibited good stability in a wide range of pH and T conditions corresponding to the calculated parameters of reaction order n, activation energy E _A and dissolution rate constants for each kinetic stages of induction and progressive conversion.

Conclusions

The kinetic analysis related to the reaction orders and calculated activation energies confirmed that extreme pH and T conditions are necessary to obtain considerably high dissolution rates. Extreme pH conditions (acidic or alkaline) cause the preferential release of K⁺, SO₄ ^2? and CrO₄ ^2? from the KFe₃[(SO₄)_2 ? X(CrO₄)_X](OH)₆ structure, although CrO₄ ^2? is quickly adsorbed by Fe(OH)₃ solid residues. The precipitation of phases such as KFe₃[(SO₄)_2 ? X(CrO₄)_X](OH)₆, and the absorption of Cr(VI) after dissolution can play an important role as retention mechanisms of Cr(VI) in nature.

     

The at-rest earth pressure coefficient prediction using simple elasto-plastic constitutive models

The paper deals with the theoretical prediction of the at-rest earth pressure coefficient for normally consolidated soils (k_0(NC)) using simple elasto-plastic constitutive models. In the first part of the work, the k_0(NC) expressions derived from critical state soil mechanics models are critically discussed. It is shown that, adopting typical values of the models parameters, the experimental k_0(NC) values are usually over-predicted. In the second part, a possible modification of the critical state model formulation, which allows to predict lower k_0(NC) values for a fixed angle of shearing resistance, is considered. Finally, a discussion of the results and some brief conclusions are proposed.