Thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions

The thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated. Members of these series were either coprecipitated or synthesized hydrothermally and were characterized by XRD, FTIR, electron microprobe analysis, ICP-MS, and thermal analysis. Partial alkali substitution and vacancies on the Fe/Al sites were observed in all cases, and the solids studied can be described by the general formula K_1-x-yNa_y(H₃O)_xFe_zAl_w(SO₄)₂(OH)_6-3(3-z-w)(H₂O)_3(3-z-w). A strong preferential incorporation of Fe over Al in the jarosite/alunite structure was observed. Heats of formation from the elements, ΔH°_f, were determined by high-temperature oxide melt solution calorimetry. The solid solutions deviate slightly from thermodynamic ideality by exhibiting positive enthalpies of mixing in the range 0 to +11 kJ/mol. The heats of formation of the end members of both solid solutions were derived. The values ΔH°_f = −3773.6 ± 9.4 kJ/mol, ΔH°_f = −4912.2 ± 24.2 kJ/mol, ΔH°_f = −3734.6 ± 9.7 kJ/mol and ΔH°_f = −4979.7 ± 7.5kJ/mol were found for K_0.85(H₃O)_0.15Fe_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, K_0.85(H₃O)_0.15Al_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, Na_0.7(H₃O)_0.3Fe_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9, and Na_0.7(H₃O)_0.3Al_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9 respectively. To our knowledge, this is the first experimentally-based report of ΔH°_f for such nonstoichiometric alunite and natroalunite samples. These thermodynamic data should prove helpful to study, under given conditions, the partitioning of Fe and Al between the solids and aqueous solution.     

The Thermodynamics of Ordered Perovskites on the CaTiO3-FeTiO3 Join

The enthalpies of formation from ilmenite, FeTiO₃, and perovskite, CaTiO₃, of two intermediate ordered perovskite phases, CaFeTi₂O₆ and CaFe₃Ti₄O₁₂, have been measured at 801°C using oxide melt solution calorimetry. These data, in combination with experiments at high pressure and temperature, indicate that below 1518±50°C CaFe₃Ti₄O₁₂ is stable at lower pressures (∼7 GPa at 1200°C) than CaFeTi₂O₆ (∼13 GPa at 1200°C). This relationship should be reversed, and CaFeTi₂O₆ should become stable at lower pressures than CaFe₃Ti₄O₁₂, at temperatures above 1518±50°C. These intermediate phases are of petrological interest because they form as a reaction between two minerals, ilmenite and perovskite, which are commonly associated in kimberlites, and because their pressure-temperature range of formation overlaps that of origin of kimberlites. Received: 10 November 1997 / Revised; accepted: 15 January 1998     

Calorimetric study of high-pressure phase transitions among the CdGeO3 polymorphs (pyroxenoid,garnet, ilmenite,and perovskite structures)

At high pressures, CdGeO₃ pyroxenoid transforms to garnet, then to ilmenite, and finally to perovskite. Enthalpies of transition among the four phases were measured by high temperature calorimetry. The entropies of transition and slopes of the boundaries were calculated using the measured enthalpies and free energies calculated from the phase equilibrium data. Pyroxenoid and garnet are very similar energetically. However garnet is a high pressure phase because of its lower entropy and smaller volume. The pyroxenoid-garnet transition has a small positiveP-T slope. Ilmenite is intermediate in enthalpy between garnet and perovskite, but is lower in entropy than both phases. Therefore the garnet-ilmenite transition has a positivedP/dT, while a negativedP/dT is calculated for the ilmenite-perovskite transition. The thermochemical data for the CdGeO₃ phases are generally consistent with the observed high pressure phase relations. The high entropy of perovskite relative to ilmenite, observed in several ABO₃ comounds including CdGeO₃, is related to the structural features of perovskite, in which relatively small divalent cations occupy the large sites of 8–12 fold coordination. The thermochemistry of the CdGeO₃ polymorphs shows several similarities to that of the CaGeO₃ system.     

Environments of the Russian Plain during the Middle Valdai Briansk Interstade (33,000–24,000 yr B.P.) Indicated by Fossil Mammals and Plants

Abundant mammal (45 localities) and plant (52 sections) fossil data collected from the Russian Plain deposits of the Middle Valdai Briansk Interstade indicate a period of noticeable warming. These materials were jointly analyzed. The data were organized in the database software PARADOX and then moved to the GIS program ARC/INFO. Mathematical methods as well as traditional research methods were used for the analysis. The reconstructed biogeographical provinces illustrate the specific environmental and climatic conditions of the Briansk Interstade. Analogues of modern natural zones did not exist at this time on the Russian Plain. The Briansk landscapes reflect the moderate-cool climate of this period, when the majority of northern subarctic plant and mammal species extended their ranges considerably to the south to the Central Russian Plain. At the same time, steppe species penetrated farther north and west. This combination shows that the continuous forest zone was destroyed during the Briansk Inter-stade. Five biogeographical provinces have been reconstructed from the north to the south for the Russian Plain during the Briansk warming on the basis of joint analysis of mammalian and botanical data.     

A conceptual model of the copper–porphyry ore formation based on joint analysis of deep 3D geophysical models: Sorskoe complex (Russia) case study

Joint analysis of deep three-dimensional models of the electrical resistivity, seismic velocity, and density of the complex hosting the Sorskoe Cu–Mo deposit (Russia) is carried out aimed at finding geophysical markers characterizing the areas of ore generation, transportation and deposition. The three-dimensional lithology model of the study area is built based on the empirical relationship between the silica content of the rocks and seismic velocities. It is in agreement with geological and geochemical studies provided in this area earlier and could be used as a basis for forecasting locations of the copper–molybdenum ore deposits at depth. A conceptual model of the copper–porphyry complex explaining the mechanisms of ore generation, transportation from the lower to the upper crust and deposition in the upper crust is suggested. In particular, it is supposed that post-magmatic supercritical gas–water ore-bearing fluids are upwelling through the plastic crust due to the sliding of the fluid films along the cleavage planes of the foliated rocks while at the depths of the brittle upper crust this mechanism could be changed by volumetric fluid transportation along the network of large pores and cracks.     

δ15N and δ13C in the Mondego estuary food web: Seasonal variation in producers and consumers

Assessments of temporal variation in stable carbon and nitrogen ratios were used to examine seasonal trends of the water column and benthic food webs in the Mondego estuary (Portugal). There was a marked seasonality in weather and water column conditions, including nutrient supply and chlorophyll concentrations. In spite of the pronounced environmental changes, we found little evidence of seasonal variation in δ¹³C and δ¹⁵N of producers and consumers in the Mondego estuary, with a few notable exceptions. Nitrogen isotope ratios in macrophytes (Zostera noltii, Ulva sp., Enteromorpha sp., and Gracilaria sp.), and in two grazers (Idotea chelipes and Lekanesphaera levii) increased during late summer, with the highest δ¹⁵N values being measured in July, during a period of elevated temperatures and drought, which may have favored high rates of denitrification and heavier δ¹⁵N values. The results suggest that stable-isotope values from macrophytes and selected grazers are useful as tracers of seasonal changes in nitrogen inputs into estuaries, and that those of consumers reflect other factors beyond seasonal variations in N and C sources.     

Sub-recent nitrogen-isotope trends in sediments from Skagerrak (North Sea) and Kattegat: Changes in N-budgets and N-sources?

We determined ¹⁵N/¹⁴N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand (δ¹⁵N = 5‰) and the imprint of river-borne nitrogen input into the SE North Sea (δ¹⁵N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ¹⁵N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ¹⁵N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea.     

Amplitude of ENSO cycles in the Santa Barbara Basin,off California,during the past 15 000 years

Varve thickness time series from ODP Site 893 in the Santa Barbara Basin (off California) were analysed to determine variation in the strength of El Niño/Southern Oscillation (ENSO) cycles during the past 15 000 yr. Mean varve thickness and variance changed over time, with thicker varves before ～8000 yr BP indicative of wetter than modern climates. A 100‐yr running standardisation was applied to correct for non‐stationarity. The contribution of ENSO‐scale variability was then estimated as the amplitude of 3–8 year bandpassed data. Results show multidecadal‐ to centennial‐scale modulation of the amplitude. On average, however, the amplitude of ENSO scale variability remained constant throughout the past 15 000 yr. We therefore conclude that, although the expression of ENSO cycles may have changed during the Holocene, there is no indication for a significant change in amplitude of interannual variability. Copyright © 2005 John Wiley & Sons, Ltd.     

Fluvial transport of suspended sediment and organic carbon during flood events in a large agricultural catchment in southwest France

Water draining from a large agricultural catchment of 1 110 km² in southwest France was sampled over an 18‐month period to determine the temporal variability in suspended sediment (SS) and dissolved (DOC) and particulate organic carbon (POC) transport during flood events, with quantification of fluxes and controlling factors, and to analyze the relationships between discharge and SS, DOC and POC. A total of 15 flood events were analyzed, providing extensive data on SS, POC and DOC during floods. There was high variability in SS, POC and DOC transport during different seasonal floods, with SS varying by event from 513 to 41 750 t; POC from 12 to 748 t and DOC from 9 to 218 t. Overall, 76 and 62% of total fluxes of POC and DOC occurred within 22% of the study period. POC and DOC export from the Save catchment amounted to 3090 t and 1240 t, equivalent to 1·8 t km^?2 y^?1 and 0·7 t km^?2 y^?1, respectively. Statistical analyses showed that total precipitation, flood discharge and total water yield were the major factors controlling SS, POC and DOC transport from the catchment. The relationships between SS, POC and DOC and discharge over temporal flood events resulted in different hysteresis patterns, which were used to deduce dissolved and particulate origins. In both clockwise and anticlockwise hysteresis, POC mainly followed the same patterns as discharge and SS. The DOC‐discharge relationship was mainly characterized by alternating clockwise and anticlockwise hysteresis due to dilution effects of water originating from different sources in the whole catchment. Copyright © 2011 John Wiley & Sons, Ltd.