The Bahama Islands contain many abandoned dissolution caves at elevations between two and seven metres above current sea level. The development of dissolution caves in tropical carbonate islands is dependent on the position and nature of the freshwater lens. Lens position is controlled by sea level, which in stable carbonate platforms like the Bahamas is a function of glacioeustatic sea level still stands. Caves in the Bahamas that are currently subaerial must have developed during past higher sea levels. During the Late Quaternary, sea levels higher than present have been relatively short-lived, and that limits the amount of time that a freshwater lens could be situated at the elevation required for the cave formation. The Bahama Islands are low-lying landforms where only aeolian ridges extend to elevations higher than six metres above current sea level. Past high sea level events greatly reduced the exposed land area of the Bahama Islands, thus also limiting both the catchment for and size of freshwater lenses. Caves must be younger than the rock in which they are developed; most subaerial Bahamian caves are found in limestones that are less than 150000 years old. Development of large dissolution caves under these limitations of time and lens size requires a powerful dissolutional mechanism. The mixing of discharging freshwater with tide-pulsed incoming marine water under the flanks of emergent dune ridges may have produced the conditions necessary. Bahamian caves formed by this process are phreatic chambers with complex interconnections and blind tubes. Their presence demonstrates that significant dissolution can occur rapidly as a result of the mixing of fresh and marine waters beneath small carbonate islands.

The porosityofyoung limestonesexperiencing meteoricdiagenesisin the vicinityof theirdeposition(eagenetic karst) is mainly a doubleporosityconsistingoftouching-vug channelsandpreferredpassagewayslacingthrougha matrixof interparticleporosity. Incontrast,the porosityoflimestonesexperiencingsubaerialerosionfollowingburialdiagenesisandupliftitelogenetic karst) ismainlya doubleporosityconsisting ofconduits within a network offractures. The stark contrast between these two kinds of karst is illustratedby their position on a graph showing thehydraulic characteristics ofanequivalentporousmediumconsistingofstraight,cylindricaltubes(II-Dspace,wheren isporosity,D isthediameter ofthe tubes, and log n is plotted against log D).Studiesofthe hydrologyof small carbonateislandsshow that large-scale,horizontalhydraulic conductivity(K) increasesby ordersof magnitude duringtheevolutionofeogenetickarst. Earlierpetrologicstudieshaveshownthereis littleifany changeinthetotalporosityofthe limestoneduring eogeneticdiagenesis. The limestone of eogenetickarst, therefore,trackshorizontallyinn-D space. Incontrast,the path from initialsedimentary material to telogenetickarst comprises a descent on the graph with reductionof /l during burial diagenesis,then a sideways shift with increasing D due to openingoffractures during upliftand exposure,and finallyan increaseinD and n during developmentofthe conduits alongthe fractures.Eogeneticcaves are mainly limited to boundariesbetween geologic unitsand hydrologic zones: streamcaves at the contact between carbonates and underlying impermeablerocks(andcollapse-origin cavesderivedtherefrom); verticalcavesalongplatform-margin fractures;epikarst;phreatic pockets(bananaholes)along the watertable;and flankmargin caves thatformas mixing chambers at thecoastalfreshwater-saltwater"interface". In contrast,thecavernsoftelogenetic karstarepartof a systemofinterconnected conduitsthat drain anentireregion. The eogeneticcavesof small carbonate islandsare, for the most part, not significantlyinvolved in the drainageofthe island.

No abstract

Background Aspergillus flavus infection and aflatoxin contamination of maize pose negative impacts in agriculture and health. Commercial maize hybrids are generally susceptible to this fungus. Significant levels of host plant resistance have been observed in certain maize inbred lines. This study was conducted to identify maize genes associated with host plant resistance or susceptibility to A. flavus infection and aflatoxin accumulation.ResultsGenome wide gene expression levels with or without A. flavus inoculation were compared in two resistant maize inbred lines (Mp313E and Mp04∶86) in contrast to two susceptible maize inbred lines (Va35 and B73) by microarray analysis. Principal component analysis (PCA) was used to find genes contributing to the larger variances associated with the resistant or susceptible maize inbred lines. The significance levels of gene expression were determined by using SAS and LIMMA programs. Fifty candidate genes were selected and further investigated by quantitative RT-PCR (qRT-PCR) in a time-course study on Mp313E and Va35. Sixteen of the candidate genes were found to be highly expressed in Mp313E and fifteen in Va35. Out of the 31 highly expressed genes, eight were mapped to seven previously identified quantitative trait locus (QTL) regions. A gene encoding glycine-rich RNA binding protein 2 was found to be associated with the host hypersensitivity and susceptibility in Va35. A nuclear pore complex protein YUP85-like gene was found to be involved in the host resistance in Mp313E.ConclusionMaize genes associated with host plant resistance or susceptibility were identified by a combination of microarray analysis, qRT-PCR analysis, and QTL mapping methods. Our findings suggest that multiple mechanisms are involved in maize host plant defense systems in response to Aspergillus flavus infection and aflatoxin accumulation. These findings will be important in identification of DNA markers for breeding maize lines resistant to aflatoxin accumulation.

No abstract

Blue holes are karst features that were initially described from Bahamian islands and banks, which have been documented for over 100 years. They are water-filled vertical openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries. Their deep blue color, for which theyare named, is the resultof theirgreat depth, and they may lead to cave systems belowsea level. Blue holes are polygenetic in origin, having formed: by drowning of cHssolutional sinkholes and shaftsdeveloped in the vadose zone; by phreatic dissolution along an ascending halocline; by progradational collapse upward from deep dissolution voids produced in the phreatic zone; or by fracture of the bank margin. Blue holes are the cumulative result of carbonate deposition and cHssolution cycles which have been controlled by Quaternary glacioeustatic fluctuations of sea-level.Blue holes have been widely studied during the past 30 years, and they have providedinformation regarding karst processes, global climate change, marine ecology, and carbonate geochemistry. The literature contains a wealth of references regarding blue holes that are at times misleading, and often confusing. To standaIdize use of the term bluehole,and to familiarize the scientific community with their nature, we herein define them as follows: "Blue holes are subsurface voids that are developed in carbonate banks and islands; are open to the earth's surface; contain tidally-influenced waters of fresh, marine, or mixedchemistry; extend below sea level for a majority of their depth; and may provide access to submerged cavepassages." Blue holes are foundin two settings: ocean holes open directly into the present marine environment and usually contain marinewater with tidal flow; inland blue holes are isolated by present topography from surface marine conditions, and open directly onto the land surface or into an isolated pond or lake, and contain tidally-influenced water of a variety of chemistries from fresh to marine.

No abstract

No abstract