VIEWS OF THE AAG: RESULTS OF THE LONG RANGE PLANNING MEMBERSHIP SURVEY

In this issue THE PROFESSIONAL GEOGRAPHER presents the first two reports in a series on the Association's long-range planning process, which was described by Past President Marcus in May. David Morgan outlines here the results of a survey of members' attitudes on a number of important questions.     

Contrasting interactions of sodium and potassium with H<Subscript>2</Subscript>O in haplogranitic liquids and glasses at 200 MPa from hydration–diffusion experiments

This study examines hydration–diffusion in the metaluminous haplogranite system at 200 MPa H₂O and 800–300°C. At 800°C hydration is accompanied by melting and uphill diffusion of sodium from anhydrous glass toward the region of hydration and melting, whereas potassium diffuses away from the hydration front and into anhydrous glass. Silicon and aluminum are simply diluted upon hydration. There is no change in molecular Al/(Na + K) throughout the entire hydration-diffusion aureole and, therefore, (1) there is no loss of alkalis to the vapor, and (2) K migrates to replace Na in order to maintain local charge balance required by ^IVAl. Alkali diffusion occurs over a viscosity contrast from 10^4.1 Pa s in hydrous liquid to 10^11.8–10^13.5 Pa s in anhydrous glass. From these results, we interpret that: (1) Na is structurally or energetically favored over K as a charge-balancing cation for ^IVAl in hydrous granitic liquids, whereas the opposite behavior has been observed for anhydrous melts, and (2) the diffusion of alkalis through silicate melts is largely independent of viscosity. Results from 600°C are similar to those at 800°C, but hydration at 300°C involves a loss of Na and concomitant increase in molar Al/(Na + K) in the hydration zone due to hydrogen-alkali exchange between fluid and glass. Hydration behavior at 400°C is transitional between those at 300°C and 600°C, suggesting that the change in hydration mechanism occurs near the glass transition.     

Marine regions and regionalism in South-east Asia

If problems of ocean management are to be solved the seas must be divided up into appropriate regions. Regional boundaries can be based on factors of physical geography or on economic, political or management principles. South-east Asia, with complicated land-sea boundaries, provides a number of examples of different types of marine regions, which can be used as case study examples of problems and solutions in regionalizing the oceans. Eight regional types and locations are described and discussed, and it is concluded that regional arrangements for management or the solution of problems are at present limited to three: the Malacca-Singapore Straits, Andaman Sea and Celebes Sea.     

Stable carbon isotope ratios in lake and swamp sediments as a proxy for prehistoric forest clearance and crop cultivation in the Neotropics

Close correspondence between stable carbon isotope ratios ( ¹³ C), pollen, and charcoal profiles in sediment cores from Laguna Zoncho and Machita swamp, Costa Rica, shows that prehistoric forest clearance and crop cultivation can be detected in the stable carbon isotope ratios of total organic carbon ( ¹3C _TOC ). Analyses of δ 13C _TOC complement evidence from pollen, charcoal, and phytoliths and provide a proxy that is sensitive to the intensity and/or proximity to core sites of prehistoric forest clearance and agriculture in watersheds. Stable carbon isotope analyses are particularly useful in situations in which other evidence of forest clearance and agriculture is limited.     

Experiments on the kinetics of partial melting of a leucogranite at 200 MPa H<Subscript>2</Subscript>O and 690–800°C: compositional variability of melts during the onset of H<Subscript>2</Subscript>O-saturated crustal anatexis

We have experimentally investigated the kinetics of melting of an aplitic leucogranite (quartz+sodic plagioclase of ≈Ab₉₀+K-feldspar+traces of biotite) at 690, 740, and 800°C, all at 200 MPa H₂O. Leucogranite cylinders, 3.5 mm in diameter and 7 mm in length, were run in the presence of excess H₂O using cold-seal pressure vessels for 11–2,925 h. At 690 and 740°C and any experimental time, and 800°C and short run times, silicate glass (melt at run conditions) occurs as interconnected films along most of the mineral boundaries and in fractures, with the predominant volume occurring along quartz/feldspars boundaries and quartz/plagioclase/K-feldspar triple junctions. Glass film thickness is roughly constant throughout a given experimental charge and increases with experimental temperature and run duration. The results indicate that H₂O-saturated partial melting of a quartzo-feldspathic protolith will produce an interconnected melt phase even at very low degrees (<5 vol%) of partial melting. Crystal grain boundaries are therefore completely occluded with melt films even at the lowest degrees of partial melting, resulting in a change in the mechanism of mass transport through the rock from advection of aqueous vapor to diffusion through silicate melt. At 690 and 740°C the compositions of glasses are homogeneous and (at both temperatures) close to, but not on, the H₂O-saturated 200 MPa haplogranite eutectic; glass compositions do not change with run duration. At 800°C glasses are heterogeneous and plot away from the minimum, although their molar ratios ASI (=mol Al₂O₃/CaO+Na₂O+K₂O) and Al/Na are constant throughout the entire charge at any experimental time. Glass compositions within individual 800°C experiments form linear trends in (wt%) normative quartz–albite–orthoclase space. The linear trends are oriented perpendicular to the 200 MPa H₂O haplogranite cotectic line, reflecting nearly constant albite/orthoclase ratio versus variable quartz/feldspar ratio, and have endpoints between the 800°C isotherms on the quartz and feldspar liquidus surfaces. With increasing experimental duration the trends migrate from the potassic side of the minimum toward the bulk rock composition located on the sodic side, due to more rapid (and complete) dissolution of K-feldspar relative to plagioclase. The results indicate that partial melting at or slightly above the solidus (690–740°C) is interface reaction-controlled, and produces disequilibrium melts of near-minimum composition that persist metastably for up to at least 3 months. Relict feldspars show no change in composition or texture, and equilibration between melt and feldspars might take from a few to tens of millions of years. Partial melting at temperatures well above the solidus (800°C) produces heterogeneous, disequilibrium liquids whose compositions are determined by the diffusive transport properties of the melt and local equilibrium with neighboring mineral phases. Feldspars recrystallize and change composition rapidly. Partial melting and equilibration between liquids and feldspars might take from a few to tens of years (H₂O-saturated conditions) at these temperatures well above the solidus.     

New evidence for extreme and persistent terminal medieval drought in California’s Sierra Nevada

The level of Cliff Lake, a small, subalpine, moraine-dammed lake in California’s south central Sierra Nevada, was approximately 5 m lower than present for 50 years or more approximately 600 years ago, this determined by radiocarbon dating of wood recovered from a submerged tree stump found in the lake. This finding corresponds to commensurate data from throughout much of western North America, suggesting the duration and magnitude of terminal medieval megadrought was similar throughout the region. Ultimately this datum helps give credence to the perspective that though late Holocene climate in California was indeed variable, the effects of terminal Medieval megadrought was similar across both time and broad geographic expanse.     

Sinton crater, Mars: Evidence for impact into a plateau icefield and melting to produce valley networks at the Hesperian-Amazonian boundary

The majority of martian valley networks are found on Noachian-aged terrain and are attributed to be the result of a ‘warm and wet’ climate that prevailed early in Mars' history. Younger valleys have been identified, though these are largely interpreted to be the result of localized conditions associated with the melting of ice from endogenic heat sources. Sinton crater, a 60 km diameter impact basin in the Deuteronilus Mensae region of the dichotomy boundary, is characterized by small anastomosing valley networks that are located radial to the crater rim. Large scale deposits, interpreted to be the remains of debris covered glaciers, have been identified in the area surrounding Sinton, and our observations have revealed the occurrence of an ice rich fill deposit within the crater itself. We have conducted a detailed investigated into the Sinton valley networks with all the available remote data sets and have dated their formation to the Amazonian/Hesperian boundary. The spatial and temporal association between Sinton crater and the valley networks suggest that the impact was responsible for their formation. We find that the energy provided by an asteroid impact into surficial deposits of snow/ice is sufficient to generate the required volumes of melt water needed for the valley formation. We therefore interpret these valleys to represent a distinct class of martian valley networks. This example demonstrates the potential for impacts to cause the onset of fluvial erosion on Mars. Our results also suggest that periods of glacial activity occurred throughout the Amazonian and into the Hesperian in association with variations in spin orbital parameters.     

Optimizing point-source parameters for scanning satellite surveys

We describe a method for deriving the position and flux of point and compact sources observed by a scanning survey mission. Results from data simulated to test our method are presented, which demonstrate that at least a 10-fold improvement is achievable over that of extracting the image parameters, position and flux, from the equivalent data in the form of pixel maps. Our method achieves this improvement by analysing the original scan data and performing a combined, iterative solution for the image parameters. This approach allows for a full and detailed account of the point-spread function (PSF), or beam profile, of the instrument. Additionally, the positional information from different frequency channels may be combined to provide the flux-detection accuracy at each frequency for the same sky position. Ultimately, a final check and correction of the geometric calibration of the instrument may also be included. The Planck mission was used as the basis for our simulations, but our method will be beneficial for most scanning satellite missions, especially those with non-circularly symmetric PSFs.     

Continental shelf nurseries and recruitment variability in American plaice and yellowtail flounder on the Grand Bank: insights into stock resiliency

In 1994, a directed fishing moratorium was declared for Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea) stocks because both stocks showed severe declines in abundance from heavy exploitation during the mid 1980s and early 1990s. Four years later, the fishery for yellowtail re-opened while the plaice stock has shown little recovery and the moratorium is still in effect. To assess the possible causes of the differences in recovery between species, we examined the spatial structure and environmental characteristics of the continental shelf nursery habitats of plaice and yellowtail, and their relationship to recruitment variability and overall population size. Depth plays a major influential role determining the spatial pattern and the abundance of juveniles of both species and in the case of plaice the spatial structure of the adult population also determines the amount of nursery area utilised by juveniles. Recruitment variability was higher in plaice than in yellowtail. We found year class synchrony in both species indicating that common environmental conditions and/or biological processes are affecting recruitment in a similar manner. Density-dependent regulation appears to be more severe in yellowtail and this should contribute to a more stable population when compared to plaice. These results are discussed in terms of resiliency of both stocks to over-exploitation.     

A GIS data model for public gardens

An enterprise GIS data model was developed for use in public gardens, which includes botanical gardens, arboreta, and zoos. The data model defines the representation of geographical phenomena, the attributes of each feature, and the relationships between them in order to support information management and analysis within public gardens. The data model was developed using a three‐stage process consisting of conceptual, logical, and physical design, each guided by stakeholder workshops and software testing. The data model includes the attributes of features necessary to create collection maps and perform collection analysis, and is designed to link to existing plant and animal records management systems (BG‐BASE and ZIMS) through a common key. The Public Garden Data Model consists of three modules (Base Map, Facilities and Infrastructure, and Basic Plant Records), unified into one comprehensive spatial data model for public gardens, and contains 59 feature classes and tables connected by 49 relationship classes.