Trace element partitioning in the Morasko meteorite from Poznan, Poland

The nickel–iron meteorite of Morasko shows isolated inclusions of troilite in the bulk mass of a Fe,Ni-alloy. During a segregation of the FeS phase chalcophile trace elements were collected from the melt. The solidification of the Fe,Ni-phase occurred probably later, incorporating thereby mainly siderophile trace elements. To prove this general assumption selected trace elements were determined and reveal in the two phases of the present meteorite a characteristic distribution pattern. The meteorite of Morasko is in close conformity to the iron meteorite group IAB with rounded dark FeS inclusions.     

Seasonal Variability of Adsorption and Exchange Equilibria in Soil Waters

Chemical analyses for major ions have been conducted on waters,collected on an approximately weekly basis over the period April, 1993 toNovember, 1996, that drain three small experimental ecosystems(sandboxes) at Hubbard Brook, New Hampshire. One sandbox is planted withpine trees, another with grass, and the third is left bare (actually itis covered sporadically by bryophytes and lichens). Results show linearcorrelations, independent of discharge, between the concentrations ofdissolved Na⁺ and K⁺ on the one hand andCa⁺⁺ and Mg⁺⁺ on the other for all threesandboxes. No correlations between singly charged and doubly chargedcations were found. These correlations are interpreted to represent cationexchange equilibria between soil waters and clay minerals plus soil organicmatter. The correlation slope, representing the exchange constant, for Na vsK is different for the pine-covered sandbox than for the other two whereasfor Ca vs Mg the correlation is independent of the presence or absence oftrees. We interpret this as representing a shift of cation exchangeequilibria in the pine sandbox by the activities of growing trees.Concentrations of Na, K, Ca, Mg, and H₄SiO₄from the barren and grass-lined sandboxes were found to vary seasonally witha marked sinusoidal pattern which was independent of the discharge from eachsandbox. (The discernment of a similar pattern in the tree lined sandbox wasdifficult due to a lack of discharge over much of the year.) Concentrationmaxima occurred in August and minima in February, and there is a closeparallelism with soil temperature. We interpret this as representingtemperature induced variations in cation exchange equilibria and silicaadsorption. Independence from highly varying water discharge, e.g.,. thataccompanying severe rainstorms, indicates rapidly re-attained equilibrium.Variations in the concentrations of cations are likely due to exchange withunmeasured cations, probably H⁺ or dissolved Al species, as aresult of possible seasonal changes in internal acid production and externalinput of acid rain to the sandboxes. Internal production may represent aresponse to seasonal changes in respiration rate as it responds toseasonally varying temperature. Added to this is the effect of temperatureon exchange equilibrium. Seasonal variations in dissolved silica are mostlikely due to the dependence of adsorption/desorption equilibria ontemperature. The temperature dependence of a number of silica-consumingreactions are consistent with the measured values.     

Sedimentary pyrite formation: An update

Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H₂S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals.Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High $\text{C}\text{S}$ ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low $\text{C}\text{S}$ ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.     

More evidence that the Chicxulub impact predates the K/T mass extinction

Abstract Yaxcopoil‐1 (Yax‐1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the Cretaceous‐Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous.     

Attribution of uranium ore concentrates using elemental and anionic data

The capability to correctly identify the geological or geographical source of unknown uranium ore concentrates (UOCs) has obvious nuclear security benefits. This paper reports on a scoping study where the trace elemental and anionic compositions of 24 UOC samples, sourced mainly from mines in Australia and Canada, were examined for their ability to allow attribution of the sample to a particular geological U deposit type or to a particular geographical source. Results of statistical analysis using canonical analysis of principal coordinates (CAP) showed that samples originating from certain U ore deposit types, especially phosphorite and quartz-pebble conglomerate, contained a distinct impurity composition. Samples grouped according to their geographical region of origin appeared to contain distinctive impurities in certain cases (Elliot Lake and Bancroft, Ontario). The key impurities responsible for differentiating groups of samples from a particular geological deposit type were identified and the use of certain impurities as signatures of processing history is discussed. The methodology described in this scoping study provides a promising approach for more comprehensive databases.     

Dissolved organic carbon from the traditional jute processing technique and its potential influence on arsenic enrichment in the Bengal Delta

Dissolved organic C (DOC) plays an important role in the mobilization of As from sediments. In West Bengal, the widely used technique for obtaining jute fiber involves retting of the jute plant in ponds (hereafter such ponds are termed jute decomposing ponds) for several weeks, which produces significant amounts of DOC in the ponds. These ponds thus act as point sources of DOC and supply huge quantities of organic C to the Bengal Delta sediments. This study has been carried out to investigate the role of such DOC in enriching the groundwater with As in the Bengal Delta. Data clearly show that due to the effect of DOC, As is mobilized from the upper 2.6 m of the sediment profile, and is fixed between 2.6 and 6.1 m, while the lower part (6.1–9 m) largely remains unaffected. The reducing conditions mainly developed due to the decay of the percolating DOC seem to help the mobilization and transportation of As and other redox sensitive elements (Fe, Mn), as well as elements (Cu, Zn) attached to oxy-hydroxides of those redox-sensitive elements. Experiments also indicate that if the DOC production at the surface continues for a longer period of time, the zone of As fixation (2.6–6.1 m) may get shifted further downwards and ultimately intercept the water table resulting in As enrichment of groundwater.     

Mechanism of feldspar weathering—I. Experimental studies

It has been widely accepted that a chemically altered, protective surface layer regulates the dissolution, and hence the weathering, of plagioclase feldspars under Earth's surface conditions. In this study, we examine this hypothesis in detail with the aid of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Using these techniques, we have been unable to find any direct evidence suggesting the presence of a chemically altered coating on feldspar surfaces which have been weathered in the lab. Instead, our results suggest that the mechanism controlling feldspar weathering is a surface controlled reaction.Based on SEM observations and measurements of rate of release of silica, we postulate that the process of feldspar dissolution proceeds in two stages in the lab. Initially, one observes the dissolution of ultrafine (? 1 μm diameter) particles which are produced during grinding of the sample, and which adhere tenaciously to the surfaces of larger grains. This is the stage which results in the non-linear rates of dissolution which are commonly observed in the lab (parabolic kinetics). Secondly, the dissolution occurs at sites of excess surface energy such as at dislocations or similar crystal defects. This process yields linear rates of dissolution, and it dominates the weathering of feldspars in the field.     

Mechanism of pyroxene and amphibole weathering—I. Experimental studies of iron-free minerals

The short term (2–40 days) dissolution of enstatite, diopside, and tremolite in aqueous solution at low temperatures (20–60°C) and pH 1–6 has been studied in the laboratory by means of chemical analyses of reacting solutions for Ca²⁺, Mg²⁺, and Si(OH)₄ and by the use of X-ray photoelectron spectroscopy (XPS) for detecting changes in surface chemistry of the minerals. All three minerals were found to release silica at a constant rate (linear kinetics) providing that ultrafine particles, produced by grinding, were removed initially by HF treatment. All three also underwent incongruent dissolution with preferential release of Ca and/or Mg relative to Si from their outermost surfaces. The preferential release of Ca, but not Mg for diopside at pH 6 was found by both XPS and solution chemistry verifying the theoretical prediction of greater mobility of cations located in M₂ structural sites. Loss mainly from M₂ sites also explains the degree of preferential loss of Mg from enstatite at pH 6; similar structural arguments apply to the loss of Ca and Mg from the surface of tremolite. In the case of diopside and tremolite initial incongruency was followed by essentially congruent cation-plus-silica dissolution indicating rapid formation of a constant-thickness, cation-depleted surface layer. Cation depletion at elevated temperature and low pH (~ 1) for enstatite and diopside was much greater than at low temperature and neutral pH, and continued reaction resulted in the formation of a surface precipitate of pure silica as indicated by solubility calculations, XPS analyses, and scanning electron microscopy.From XPS results at pH 6, model calculations indicate a cation-depleted altered surface layer of only a few atoms thickness in all three minerals. Also, lack of shifts in XPS peak energies for Si, Ca, and Mg, along with undersaturation of solutions with respect to all known Mg and Ca silicate minerals, suggest that cation depletion results from the substitution of hydrogen ion for Ca²⁺ and/or Mg²⁺ in a modified silicate structure and not from the precipitation of a new, radically different surface phase. These results, combined with findings of high activation energies for dissolution, a non-linear dependence on a_H⁺ for silica release from enstatite and diopside, and the occurrence of etch pitting, all point to surface chemical reaction and not bulk diffusion (either in solution or through altered surface layers) as the rate controlling mechanism of iron-free pyroxene and amphibole dissolution at earth surface temperatures.     

Oceanic events and biotic effects of the Cenomanian-Turonian anoxic event, Tarfaya Basin, Morocco

Profound biotic changes accompanied the late Cenomanian δ¹³C excursion and OAE2 in planktic foraminifera in the Tarfaya Basin of Morocco. Planktic foraminifera experienced a severe turnover, though no mass extinction, beginning with the rapid δ¹³C excursion and accelerating with the influx of oxic bottom waters during the first peak and trough of the excursion. Species extinctions equaled the number of evolving species, though only the disaster opportunists Guembelitria and Hedbergella thrived along with a low oxygen tolerant benthic assemblage. The succeeding δ¹³C plateau and organic-rich black shale deposition marks the anoxic event and maximum biotic stress accompanied by a prolonged drop in diversity to just two species, the dominant (80–90%) low oxygen tolerant Heterohelix moremani and surface dweller Hedbergella planispira. After the anoxic event other species returned, but remained rare and sporadically present well into the lower Turonian, whereas Heterohelix moremani remained the single dominant species. The OAE2 biotic turnover suggests that the stress to calcareous plankton was related to changes in the watermass stratification, intensity of upwelling, nutrient flux and oxic levels in the water column driven by changes in climate and oceanic circulation. Results presented here demonstrate a 4-stage pattern of biotic response to the onset, duration, and recovery of OAE2 that is observed widely across the Tethys and its bordering epicontinental seas.     

Magnetic susceptibility as a possible correlation tool in Quaternary alluvial stratigraphy

Correlation of the fluvial magnetic susceptibility (MS) record of borehole Devavanya‐1 in the Körös Basin (eastern Hungary) with Chinese aeolian MS records (Jingbian, Lingtai) and the marine δ¹⁸O record from the Equatorial Pacific (V28‐239) is established here based on cross‐correlations and singular spectral analysis. A basin‐scale well‐to‐well correlation based on magnetic susceptibility records was also performed involving unpublished cores. To refine the age model, a Monte Carlo simulation was conducted using the Chinese Jingbian section as a tuning target. Spectral analysis of the tuned record revealed c. 400, c. 100 and c. 41 ka cycles over the 2.5 million years of the Quaternary fluvial succession. To ensure a complementary palaeoclimate proxy, the full width at half maximum of smectites was measured as a facies‐independent indicator of weathering intensity. This investigation was carried out on a subset of samples involved in MS measurements representing a c. 400 ka time interval across the top of the Olduvai subchron. A phase‐shift between MS and weathering intensity recorded in the clay mineralogy indicates different response times of the considered proxies. The fluvial MS record is determined by the climatic control on delivery and preservation of magnetic minerals, mainly of magnetite. Under cold‐and‐dry climate these minerals were released owing to frost shattering in the adjacent hinterlands and were transported to alluvial plains in the early postglacial periods thanks to the increasing discharge of rivers. With further warming the weathering‐sensitive magnetic minerals soon disappeared from the soils of the catchment area and thus from the fluvial load. As a result, in fluvial successions early postglacial warmings are expressed by the occurrences of MS maxima (magnetic episodes), while the palaeotemperature maximum and the subsequent cooling remain concealed within the tract of low MS values. The early postglacial magnetic episodes may serve as ideal stratigraphical markers in regional and global correlations.