A late quaternary record of eolian silt deposition in a maar lake, St. Michael Island, western Alaska

Recent stratigraphic studies in central Alaska have yielded the unexpected finding that there is little evidence for full-glacial (late Wisconsin) loess deposition. Because the loess record of western Alaska is poorly exposed and not well known, we analyzed a core from Zagoskin Lake, a maar lake on St. Michael Island, to determine if a full-glacial eolian record could be found in that region. Particle size and geochemical data indicate that the mineral fraction of the lake sediments is not derived from the local basalt and is probably eolian. Silt deposition took place from at least the latter part of the mid-Wisconsin interstadial period through the Holocene, based on radiocarbon dating. Based on the locations of likely loess sources, eolian silt in western Alaska was probably deflated by northeasterly winds from glaciofluvial sediments. If last-glacial winds that deposited loess were indeed from the northeast, this reconstruction is in conflict with a model-derived reconstruction of paleowinds in Alaska. Mass accumulation rates in Zagoskin Lake were higher during the Pleistocene than during the Holocene. In addition, more eolian sediment is recorded in the lake sediments than as loess on the adjacent landscape. The thinner loess record on land may be due to the sparse, herb tundra vegetation that dominated the landscape in full-glacial time. Herb tundra would have been an inefficient loess trap compared to forest or even shrub tundra due to its low roughness height. The lack of abundant, full-glacial, eolian silt deposition in the loess stratigraphic record of central Alaska may be due, therefore, to a mimimal ability of the landscape to trap loess, rather than a lack of available eolian sediment.     

Comparative studies of the kinetic parameters of various algaenans and kerogens via open-system pyrolyses

Kinetic parameters were determined for the first time, via open-system pyrolyses, on algaenans (highly resistant biomacromolecules that are selectively preserved during kerogen formation) isolated from extant microalgae. Parallel studies were also carried out on 10 kerogens exhibiting, with one exception, a low level of maturity. These kerogens included samples chiefly derived from the selective preservation of the above algaenans and samples mainly, or almost exclusively, derived from the “natural vulcanization” pathway. Important differences in activation energy (E_a) distributions were observed between the four algaenans investigated and correlated with their chemical structures. The kerogens predominantly derived from algaenan-selective preservation (Pula alginite, NE 70 and BJ 248 Torbanites, Rundle Oil Shale) also exhibited pronounced differences in E_a distributions. These distributions provided: (i) information on the diversity of the source materials; and (ii) reflected the occurrence of important differences in chemical structures and thermal behaviour between three of the tested kerogens, even though they are all classified as low maturity type I. The Kimmeridge Clay samples and the Lorca Oil Shale showed broad E_a distributions shifted to low energies when compared with the above algaenans and kerogens. Such shifts reflect an important (or even almost exclusive for some of these kerogens) contribution of materials originating from sulphur incorporation into various lipids during early diagenesis. Finally, the kinetic data derived for the nine low maturity fossil samples were extrapolated to a very low, geological heating rate of 3°C Ma⁻¹ and the generation rate curves and cumulative yield curves thus obtained were compared.     

Partial melting of metagreywackes, Part II. Compositions of minerals and melts

A series of experiments on the fluid-absent melting of a quartz-rich aluminous metagreywacke has been carried out. In this paper, we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe. This analytical work includes biotite, plagioclase, orthopyroxene, garnet, cordierite, hercynite, staurolite, gedrite, oxide, and glass, over the range 100–1000 MPa, 780–1025 °C. Biotites are Na- and Mg-rich, with Ti contents increasing with temperature. The compositions of plagioclase range from An₁₇ to An₃₅, with a significant orthoclase component, and are always different from the starting minerals. At high temperature, plagioclase crystals correspond to ternary feldspars with Or contents in the range 11–20 mol%. Garnets are almandine pyrope grossular spessartine solid solutions, with a regular and significant increase of the grossular content with pressure. All glasses are silicic (SiO₂ = 67.6–74.4 wt%), peraluminous, and leucocratic (FeO + MgO = 0.9–2.9 wt%), with a bulk composition close to that of peraluminous leucogranites, even for degrees of melting as high as 60 vol.%. With increasing pressure, SiO₂ contents decrease while K₂O increases. At any pressure, the melt compositions are more potassic than the water-saturated granitic minima. The H₂O contents estimated by mass balance are in the range 2.5–5.6 wt%. These values are higher than those predicted by thermodynamic models. Modal compositions were estimated by mass balance calculations and by image processing of the SEM photographs. The positions of the 20 to 70% isotects (curves of equal proportion of melt) have been located in the pressure-temperature space between 100 MPa and 1000 MPa. With increasing pressure, the isotects shift toward lower temperature between 100 and 200 MPa, then bend back toward higher temperature. The melting interval increases with pressure; the difference in temperature between the 20% and the 70% isotects is 40 °C at 100 MPa, and 150 °C at 800 MPa. The position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process. A comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 MPa. The difference in fertility between pelites and greywackes decreases with decreasing pressure. A review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust (greywackes, pelites, or orthogneisses) produces only peraluminous leucogranites. More mafic granitic compositions such as the various types of calk-alkaline rocks, or mafic S-type rocks, have never been obtained during partial melting experiments. Thus, only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments. Other types of granites involve other components or processes, such as restite unmixing from the source region, and/or interaction with mafic mantle-derived materials. Received: 11 July 1995 / Accepted: 27 February 1997     

Harmonic Analysis of Sedimentary Cyclic Sequences in Kansas, Midcontinent, USA

Several stratigraphic sequences in the Upper Carboniferous (Pennsylvanian) in Kansas (Midcontinent, USA) were analyzed quantitatively for periodic repetitions. The sequences were coded by lithologic type into strings of datasets. The strings then were analyzed by an adaptation of a one-dimensional Fourier transform analysis and examined for evidence of periodicity. The method was tested using different states in coding to determine the robustness of the method and data. The most persistent response is in multiples of 8–10 ft (2.5–3.0 m) and probably is dependent on the depositional thickness of the original lithologic units. Other cyclicities occurred in multiples of the basic frequency of 8–10 with persistent ones at 22 and 30 feet (6.5–9.0 m) and large ones at 80 and 160 feet (25–50 m). These levels of thickness relate well to the basic cyclothem and megacyclothem as measured on outcrop. We propose that this approach is a suitable one for analyzing cyclic events in the stratigraphic record.     

A vibrational spectroscopic study of the oxidation of pyrite by molecular oxygen

In situ flow-through attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy has been used to investigate the formation, and step-wise oxidation, of sulfoxyanions at the pyrite surface during oxidation by molecular oxygen. The surface was studied under two different pH regimes (pH 2.5 and pH 5.6) and under two light conditions (dark and ultraviolet light). It was experimentally observed that multiple sulfoxyanions were present at the pyrite surface during oxidation by molecular oxygen, spectroscopically illustrating the concept of sulfur step-wise oxidation. The results from photochemical experiments were complex and suggest a dependence on both pH and surface speciation.     

Are isolated stable rigid clasts in shear zones equivalent to voids?

Particles in shear enclose important information about a rock's past and can potentially be used to decipher the kinematic history and mechanical behavior of a certain outcrop or region. Isolated rigid clasts in shear zones often exhibit systematic inclinations with respect to the shear-plane at small angles, tending towards the instantaneous stretching direction of the shear zone. This shape preferred orientation cannot be easily explained by any of the analytical theories used in geology. It was recently recognized that a weak mantle surrounding the clast or a slipping clast–matrix interface might be responsible for the development of the observed inclinations. Physical considerations lead us to conjecture that such mantled, rigid clasts can be effectively treated as voids that are not allowed to change their shape. The resulting equivalent void conjecture agrees well with numerical and field data and has the following important geological implications. (i) Clasts in shear zones can have stable positions in simple shear without the requirement of an additional pure shear component. (ii) The stable orientation can be approached either syn- or antithetically; hence, the clast can rotate against the applied shear sense. (iii) The strain needed to develop a strong shape preferred orientation is small (γ≈1) and therefore evaluations based on other theories may overestimate strain by orders of magnitude. (iv) The reconstruction of far-field shear flow conditions and kinematic vorticity analysis must be modified to incorporate these new findings.     

Le Viséen supérieur d'Azarhare (Maroc central) : environnements de dépôt,datation et évolution diagénétique

The unexplored area of Azarhare in central Morocco is studied thanks to three sections composed of five lithological facies Main biostratigraphical and sedimentological results concern (1) the identification of Late Visean biozones, with important presence of problematic algae Ungdarella, (2) the regional extension of deposit sequences SD5, SD6 and SD7 previously defined, (3) and an analysis of the diagenetic kaolinite. To cite this article: A. Karim et al., C. R. Geoscience 337 (2005).     

Correlated trace element “vital effects” in tropical corals: A new geochemical tool for probing biomineralization

Micron-scale variations in the trace-element (TE) composition of tropical coral skeletons were measured using laser-ablation ICP-MS (LA-ICP-MS) as part of an investigation into the chemical processes underlying paleoenvrionmental proxy reconstructions. Fluctuations in B, Mg, Sr, Ba and U were measured at high spatial resolution in two Porites corals from the Great Barrier Reef (Australia), and the fine-scale fluctuations (< ∼1.0 mm) were compared with seasonal TE cycles in a third coral. Fine-scale TE variations were found to have a large amplitude over distances corresponding to less than 1 month growth. Variations were quasi-periodic and appeared to have characteristic wavelengths on weekly (6-7 d) and monthly (28 d) scales, although periodicity was not continuous and variations could not be matched either within or between individual corallites. Fine-scale variations between Mg, Sr and U were significantly correlated with each other (Sr and U are positively correlated, but negatively correlated with Mg). This 3D correlation “vector” has the same slope as the seasonal-scale Mg, Sr and U correlations, suggesting that the same chemical/biologic biomineralization process mediates trace element variations at both timescales. Importantly, the fine-scale variations are too large to be caused directly by daily to monthly fluctuations in sea-surface temperature. This means that seasonal variations in these elements cannot reflect purely inorganic temperature-dependent coprecipitation. Models of physicochemical calcification were developed to test whether changes in calcification rate could explain the trace-element correlations. The calculations show that increases in calcification rate will result in correlated decreases in all TE/Ca ratios. The models reproduce the Sr partition coefficient, trace-element correlation slopes, and amplitude of fine-scale variations for an average calcifying pH of 8.5, varying by ±0.2 pH units. The models, however, predict U partition coefficients which are too low, and cannot reproduce the negative correlation between Mg and the other trace elements, which may be caused by crystallographic factors.