Clay swelling mechanism in clay-bearing sandstones

Swelling clays in stone can generate damaging stresses during a wetting or a drying cycle, which lead to deterioration of building stones such as Portland Brownstone. There are two primary types of swelling identified for clays: short-range, ordered intracrystalline swelling, and long-range, continuous osmotic swelling. Identification of the swelling mode is important for understanding and ultimately preventing swelling damage. Through comparison of XRD and swelling experiments with cationic pretreatments and organic solvents, we demonstrate that intracrystalline swelling is the primary mode of swelling present in three different stones, including Portland Brownstone. The results highlight the importance of the counterbalancing cation to the swelling process, and a method for characterizing the intracrystalline swelling in sandstones is developed. Finally, the implications of long-term swelling behavior for stones are discussed.     

A Numerical Case Study of the Implications of Secondary Circulations to the Interpretation of Eddy-Covariance Measurements Over Small Lakes

The CASES-99 experimental data are used to analyze turbulence behaviour under a range of stable conditions using an adaptive method based on Hilbert spectral analysis. The characteristic scales of intrinsic mode functions vary between different stratifications. The second-order Hilbert marginal spectra display clear separation between fine-scale turbulence and large-scale motions. After removing the large-scale motions, the statistical characteristics of the reconstructed signals confirm the distinction of different stratifications in the fine-scale range. The correlation coefficient analyses reveal that the Hilbert spectral analysis method separates turbulence from large-scale motions in the stable boundary layer.     

Nutrient pulses, plankton blooms, and seasonal hypoxia in western Long Island Sound

Development of seasonal hypoxia was studied weekly in the western narrows of Long Island Sound (WLIS) during the summers of 1992 and 1993 by measuring hydrographic properties, biological oxygen demand (BOD), biomass, production, and mortality of phytoplankton and bacterioplankton in the water column. Dissolved oxygen in bottom waters was low and variable during stratified periods (19–51% saturation), oscillating in and out of hypoxic conditions (defined as <3 mg O₂ l⁻¹ or 94 μM O₂). Hypoxia was more prevalent in 1993 than in 1992, corresponding to greater water column stratification in 1993. Microbial BOD in bottom waters appeared to be fueled by delivery of autochthonous carbon from phytoplankton blooms rather than allochthonous carbon input. Phytoplankton production responded to elevated NH₄ ⁺ concentrations, especially when the mixed layer was shallow. NH₄ ⁺ concentrations generally varied as a function of the preceding week's rainfall (r²=0.765). Bacterial production did not covary with phytoplankton production, yet was closely correlated with particulate organic carbon, which was chlorophyll-rich. Results indicate that the timing and severity of hypoxia development are strongly coupled to allochthonous input of NH₄ ⁺ after heavy precipitation. Observations illustrate for the first time that bottom waters in this system oscillate in and out of hypoxia on an almost weekly basis rather than sustain them over the entire stratified period. The frequency of these oscillations depends upon variations in nutrients, planktonic production and export, and bottom water ventilation.     

Transient, longitudinal, sedimentary furrows in the York River subestuary, Chesapeake Bay: Furrow evolution and effects on seabed mixing and sediment transport

Sedimentary furrows in fine-grained sediments have been observed in a variety of settings ranging from the deep ocean and deep lake bottoms to shallow estuaries and are commonly described as persistent, long-term features of the seabed. A series of 12 sidescan sonar surveys over the course of three years reveal that transient, longitudinal sedimentary furrows regularly form and then occasionally dissipate within the middle portion of the York River. Varying furrow morphologies were observed depending on current conditions, ranging from large regularly space (0.7–7 m) linear furrows during low current conditions to large patches of meandering furrows as the mean current increases or no bed forms during the higher current conditions. Based on²¹⁰Pb and¹³⁷Cs profiles of kasten cores, differences in physical mixing depths of ∼25 cm between cores collected <2 m apart indicate a high degree of small-scale spatial heterogeneity within the seabed. By documenting the position of kasten cores using a digital sidescan sonar system, we showed that a core taken within a furrow had a mixing depth 15 cm shallower than an adjacent core taken between furrows. A time-series of mixing depths over the 35 mo of the study reveals that, along with the ∼25 cm scale differences in mixing depths due to the formation and destruction of furrows, there is a longer temporal signal of mixing producing 100-cm-scale changes in mixing depths on the annual to interannual time frame. Although the formation and destruction of the furrows appear to be a significant process contributing to decimeter-scale seabed mixing, there is a longer-term unknown process which is controlling the meter-scale seabed mixing.     

Production of Reactive Oxygen Species in the Rhizosphere of a <Emphasis Type="Italic">Spartina-</Emphasis>Dominated Salt Marsh Systems

This paper reports the presence of a metastable mixture of Fe(II), O₂, superoxide and hydrogen peroxide in sediment pore water in organic carbon-rich sediments in Spartina alterniflora-dominated salt marsh systems. Field measurements at two different estuarine sites in South Carolina (one heavily urbanized and a protected research reserve) showed a broad region of reactive oxygen species (ROS) production more than 15 cm below the sediment surface within and immediately adjacent to the rhizospheres of S. alterniflora. Dissolved Fe(II) was positively correlated with hydrogen peroxide indicating a possible abiotic pathway to ROS production (r ² = 0.94). The null hypothesis was tested that Fe(II) inventories were maintained by protective ligands and thus unreactive with respect to O₂ consumption and ROS production. The addition of an Fe-binding ligand, DTPA, resulted in rapid decline of ROS in pore water, indicating that Fe(II) was labile. The half-life of superoxide under the measured solution conditions was calculated and found to be less than a second. The combination of high lability and persistent ROS was interpreted to indicate a high rate of Fe(II) and O₂ supply to the pore water. The ²²⁴Ra/²²⁸Th disequilibrium was measured to determine the potential for advective mass transfer of dissolved oxygen via pore water exchange. The estimated pore water exchange of 54 L m^?2 day^?1 was significant but could not support the measured production of ROS alone, the direct exchange of O₂ from the S. alterniflora root system may have contributed significantly to ROS production in the sediments.     

Pivotal principles for digital earth development in the twenty-first century

ABSTRACT

Reductionist thinking will no longer suffice to address contemporary, complex challenges that defy sectoral, national, or disciplinary boundaries. Furthermore, lessons learned from the past cannot be confidently used to predict outcomes or guide future actions. The authors propose that the confluence of a number of technology and social disruptors presents a pivotal moment in history to enable real time, accelerated, and integrated action that can adequately support a ‘future earth’ through transformational solutions. Building on more than a decade of dialogues hosted by the International Society for Digital Earth (ISDE), and evolving a briefing note presented to delegates of Pivotal 2015, the paper presents an emergent context for collectively addressing spatial information, sustainable development, and good governance through three guiding principles for enabling prosperous living in the twenty-first century. These are: (1) open data, (2) real-world context, and (3) informed visualization for decision support. The paper synthesizes an interdisciplinary dialogue to create a credible and positive future vision of collaborative and transparent action for the betterment of humanity and planet. It is intended that these Pivotal Principles can be used as an elegant framework for action toward the Digital Earth vision, across local, regional, and international communities and organizations.     

Toward a mechanistic understanding of larval dispersal: insights from genomic fingerprinting of the deep-sea hydrothermal vent tubeworm Riftia pachyptila

Larval dispersal is critical for the maintenance of species populations in patchy and ephemeral hydrothermal vent habitats. On fast‐spreading ridges, such as the East Pacific Rise, rates of habitat turnover are comparable to estimated lifespans of many of the inhabiting species. Traditionally, dispersal questions have been addressed with two very different approaches, larval studies and population genetics. Population genetic studies of vent‐endemic species have been informative for determining whether patterns of dispersal are suggestive of stepping stone or island models and estimating rates of gene flow (effective migrants per generation) over broad geographic ranges. However, these studies leave fundamental questions unanswered about the specific mechanisms by which larvae disperse and species maintain their populations and biogeographic ranges. With the goal of examining genetic structure and elucidating alternative larval dispersal mechanisms, we employed a genomic DNA fingerprinting technique, amplified fragment length polymorphisms (AFLPs). To assess the potential utility of AFLPs, and genetic structure of the hydrothermal vent tubeworm Riftia pachyptila, genomic ‘fingerprints’ were recovered from 29 individuals from five vent fields spanning a distance of up to c. 5000 km along the East Pacific Rise. In contrast to previous population genetic studies that found little to no genetic structure using allozymes and mitochondrial DNA, genetic analyses of 630 polymorphic AFLP loci identified distinct subclades within R. pachyptila populations. Significant levels of differentiation were observed among populations from all vent regions as well as within each region. Discrete assemblages of tubeworms separated by as little as c. 400 m within a given vent region were genetically distinguishable and cohorts (based on size‐frequency distribution) within an aggregation were found to be most closely related. These results suggest that mechanisms of larval dispersal act to retain cohort fidelity in R. pachyptila.