A synopsis of climatic and vegetational change in Southeast Asia

Tropical rain forest in Southeast Asia has developed within an extensive archipelago during the past 65 million years or more. During the Miocene (beginning 25 million years BP), rain forest extended much further north (to southern China and Japan); since that time it has contracted. During the Pleistocene (beginning 2.0 million years BP), development of continental glaciers at high latitudes was associated in Southeast Asia with lowered sea level, cooler temperatures, and modified rainfall patterns. Fossil pollen records demonstrate that Southeast Asian vegetation during the last glacial maximum (ca. 18 000 BP) differed substantially from that of today, with an increase in the extent of montane vegetation and savannah and a decline in rain forest. These data show that the distribution and extent of rain forest in Southeast Asia has historically been quite sensitive to climatic change.     

Kinetic analysis of cation exchange in birnessite using time-resolved synchrotron X-ray diffraction

In this study, we applied time-resolved synchrotron X-ray diffraction (TRXRD) to develop kinetic models that test a proposed two-stage reaction pathway for cation exchange in birnessite. These represent the first rate equations calculated for cation exchange in layered manganates. Our previous work has shown that the substitution of K, Cs, and Ba for interlayer Na in synthetic triclinic birnessite induces measurable changes in unit-cell parameters. New kinetic modeling of this crystallographic data supports our previously postulated two-stage reaction pathway for cation exchange, and we can correlate the kinetic steps with changes in crystal structure. In addition, the initial rates of cation exchange, R (ų min⁻¹), were determined from changes in unit-cell volume to follow these rate laws: R = 1.75[]^0.56, R = 41.1[]^1.10, R = 1.15[]^0.50. Thus, the exchange rates for Na in triclinic birnessite decreased in the order: Cs ? K > Ba. These results are likely a function of hydration energy differences of the cations and the preference of the solution phase for the more readily hydrated cation.     

Vibrational spectroscopy of beta-eucryptite (LiAlSiO<Subscript>4</Subscript>): optical phonons and phase transition(s)

The structural behavior of -eucryptite (LiAlSiO₄) has been investigated using infrared (IR) spectroscopy over a temperature range of 20 to 900 K and FT-Raman spectroscopy at room temperature. IR reflectance measurements show that -eucryptite possesses high reflectivity in the far-IR region, as is consistent with its reported superionic conductivity along the c-axis. On heating, the Li-related IR bands near 246 and 300 cm^–1 (with A₂ symmetry) broadened and weakened dramatically, presumably as a result of Li⁺ positional disordering along the structural channels parallel to c. The disordering process appears to induce a framework distortion, as is evidenced by the broadening of some vibrations of Si(Al)–O with increasing temperature. A change in slope in the temperature dependence of the phonon frequency near 300 cm^–1 and the linewidth of the 760 cm^–1 band at 715 K indicates that Li becomes completely disordered above this temperature. In addition, the temperature dependence of the linewidth for the 760 cm^–1 band exhibits an additional change in slope at 780 K, implying the existence of an intermediate state within this temperature range. The detailed structure of this intermediate phase, however, needs further study. Our IR data provide no indication of structural changes between room temperature and 20 K.     

Rapid geoacoustic characterization using a surface ship of opportunity

The problem of rapid classification of the sea-floor sediment is addressed using horizontal line array (HLA) acoustic data from a passing surface ship. The data are beamformed to improve signal-to-noise ratio. The rapid geoacoustic characterization (RGC) algorithm involves extracting acoustic observables from the data (normalized striation slope, time spread, and transmission-loss slope). A simple single homogenous sediment layer over an acoustic half-space model is used to compute forward estimates of the acoustic observables. An exhaustive search over the two-parameter model is performed. The two parameters searched over are the sediment compressional speed (Cp), which is a polynomial function of the mean grain size (/spl phi/), and sediment thickness (H). This approach provides a real-time technique for classifying the sediment in a way that successfully reproduces the basic physics of propagation.     

Structural modulations and phase transitions in β-eucryptite: an in-situ TEM study

Beta-eucryptite as grown by the flux method has been investigated by in-situ cold- and hot-stage transmission electron microscopy (TEM). Using electron diffraction and dark-field TEM imaging, we found no evidence for the merohedral twinning that accompanies structural collapse at the β-to-α-quartz transition, suggesting a true hexagonal symmetry for the aluminosilicate framework of β-eucryptite. Selected-area electron diffraction (SAED) patterns exhibited a variety of incommensurate structures along the three a axes with an average modulation period of about 6.5 a. These modulated structures arise from superperiodic stacking parallel to (100) of two structural units with different Li configurations. In-situ heating experiments show that the modulated structures undergo a reversible transition to the disordered modification at elevated temperatures. This transition arises from positional order-disorder of Li ions along the main structural channels. In addition, it appears that at constant low temperature, electron beam irradiation alone can induce a sequence of transitions similar to those that are thermally induced from the ordered modification to modulated states. Received: 12 November 1998 / Revised and accepted: 31 December 1998     

Characterization of manganese oxide precipitates from Appalachian coal mine drainage treatment systems

The removal of Mn(II) from coal mine drainage (CMD) by chemical addition/active treatment can significantly increase treatment costs. Passive treatment for Mn removal involves promotion of biological oxidative precipitation of manganese oxides (MnO_x). Manganese(II) removal was studied in three passive treatment systems in western Pennsylvania that differed based on their influent Mn(II) concentrations (20–150 mg/L), system construction (±inoculation with patented Mn(II)-oxidizing bacteria), and bed materials (limestone vs. sandstone). Manganese(II) removal occurred at pH values as low as 5.0 and temperatures as low as 2 °C, but was enhanced at circumneutral pH and warmer temperatures. Trace metals such as Zn, Ni and Co were removed effectively, in most cases preferentially, into the MnO_x precipitates. Based on synchrotron radiation X-ray diffraction and Mn K-edge extended X-ray absorption fine structure spectroscopy, the predominant Mn oxides at all sites were poorly crystalline hexagonal birnessite, triclinic birnessite and todorokite. The surface morphology of the MnO_x precipitates from all sites was coarse and “sponge-like” composed of nm-sized lathes and thin sheets. Based on scanning electron microscopy (SEM), MnO_x precipitates were found in close proximity to both prokaryotic and eukaryotic organisms. The greatest removal efficiency of Mn(II) occurred at the one site with a higher pH in the bed and a higher influent total organic C (TOC) concentration (provided by an upstream wetland). Biological oxidation of Mn(II) driven by heterotrophic activity was most likely the predominant Mn removal mechanism in these systems. Influent water chemistry and Mn(II) oxidation kinetics affected the relative distribution of MnO_x mineral assemblages in CMD treatment systems.     

Thermal expansion and structural transformations of stuffed derivatives of quartz along the LiAlSiO4–SiO2 join: a variable-temperature powder synchrotron XRD study

 The structural behavior of stuffed derivatives of quartz within the Li_{1− x} Al_{1− x} Si_{1+ x} O₄ system (0 ≤ x ≤ 1) has been studied in the temperature range 20 to 873 K using high-resolution powder synchrotron X-ray diffraction (XRD). Rietveld analysis reveals three distinct regimes whose boundaries are defined by an Al/Si order-disorder transition at x=∼0.3 and a β–α displacive transformation at x=∼0.65. Compounds that are topologically identical to β-quartz (0 ≤ x < ∼0.65) expand within the (0 0 1) plane and contract along c with increasing temperature; however, this thermal anisotropy is significantly higher for structures within the regime 0 ≤ x < ∼0.3 than for those with compositions ∼0.3 ≤ x < ∼0.65. We attribute this disparity to a tetrahedral tilting mechanism that occurs only in the ordered structures (0 ≤ x < ∼0.3). The phases with ∼0.65 ≤ x ≤ 1 adopt the α-quartz structure at room temperature, and they display positive thermal expansion along both a and c from 20 K to their α–β transition temperatures. This behavior arises mainly from a rotation of rigid Si(Al)-tetrahedra about the <100> axes. Landau analysis provides quantitative evidence that the charge-coupled substitution of Li+Al for Si in quartz dampens the α–β transition. With increasing Li+Al content, the low-temperature modifications exhibit a marked decrease in spontaneous strain; this behavior reflects a weakening of the first-order character of the transition. In addition, we observe a linear decrease in the α–β critical temperature from 846 K to near 0 K as the Li+Al content increases from x=0 to x=∼0.5. Received: 26 June 2000 / Accepted: 1 December 2000     

Molecular orbital modeling of aqueous organosilicon complexes: implications for silica biomineralization

Researchers recently have proposed that hypercoordinate Si-organic complexes can form in biologically relevant fluids, and they have reported the first evidence for a transient organosilicon complex generated within the life cycle of an organism (Kinrade et al., 2001b, 2002). These interpretations are based upon peak assignments of ²⁹Si NMR spectra that invoke Si-polyol (e.g., Si-sorbitol) complexes with Si in five- and six-fold coordination states. However, ab initio analyses of the proposed organosilicon structures do not reproduce the experimentally observed ²⁹Si NMR chemical shifts (Sahai and Tossell 2001, 2002 and this work). In place of the originally proposed structures, we have modeled one of the observed δ²⁹Si values with a 5-fold Si-disorbitol complex involving 5-membered ring configurations (i.e., Si-O-C-C-O). The calculated δ²⁹Si value of this new structure closely matches the observed δ²⁹Si peaks in the −100 to −102 ppm range. Likewise, ²⁹Si NMR peaks near −144 ppm were well fit by a model complex in which a 6-fold Si was complexed to three sorbitol molecules in 5-membered ring configurations. The ability to reproduce the observed NMR peaks using molecular orbital calculations provides support for the controversial role of hypercoordinate organosilicon species in the uptake and transport of silica by biological systems. The existence of such complexes in turn may explain other puzzles in Si biogeochemistry, such as the persistence of dissolved silica in concentrated biological fluids, the biofractionation of Si isotopes, and fractionation of Ge from Si.