AN IMPROVED ALGORITHM FOR THE GENERALIZED RANK ANNIHILATION METHOD

An improved algorithm for the generalized rank annihilation method(GRAM)is presented.GRAM isa method for multicomponent calibration using two-dimensional instruments,such as GC-MS.In thispaper an orthonormal base is first computed and used to project the calibration and unknown sampleresponse matrices into a lower-dimensional subspace.The resulting generalized eigenproblem is thensolved using the QZ algorithm.The result of these improvements is that GRAM is computationally morestable,particularly in the case where the calibration sample contains chemical constituents not present inthe unknown sample and the unknown contains constituents not present in the calibration(the mostgeneral case).     

Mapping Submerged Aquatic Vegetation with GIS in the Caloosahatchee Estuary: Evaluation of Different Interpolation Methods

This article evaluates different spatial interpolation methods for mapping submerged aquatic vegetation (SAV) in the Caloosahatchee Estuary, Florida. Data used for interpolation were collected by the Submersed Aquatic Vegetation Early Warning System (SAVEWS). The system consists of hydro-acoustic equipment, which operates from a slow-moving boat and records bottom depth, seagrass height, and seagrass density. This information is coupled with geographic location coordinates from a Global Positioning System (GPS) and stored together in digital files, representing SAV status at points along transect lines. Adequate spatial interpolation is needed to present the SAV information, including density, height, and water depth, as spatially continuous data for mapping and for comparison between seasons and years. Interpolation methods examined in this study include ordinary kriging with five different semivariance models combined with a variable number of neighboring points, the inverse distance weighted (IDW) method with different parameters, and the triangulated irregular network (TIN) method with linear and quintic options. Interpolation results were compared with survey data at selected calibration transects to examine the suitability of different interpolation methods. Suitability was quantified by the determination coefficient (R2) and the root-mean-square error (RMSE) between interpolated and observed values. The most suitable interpolation method was identified as the one yielding the highest R2 value and/or the lowest RMSE value. For different geographic conditions, seasons, and SAV parameters, different interpolation methods were recommended. This study identified that kriging was more suitable than the IDW or TIN method for spatial interpolation of all SAV parameters measured. It also suggested that transect data with irregular spatial distribution patterns such as SAV parameters are sensitive to interpolation methods. An inappropriate interpolation method such as TIN can lead to erroneous spatial representation of the SAV status. With a functional geographic system and adequate computing power, the evaluation and selection of interpolation methods can be automated and quantitative, leading to a more efficient and accurate decision.     

Active synsedimentary tectonism on a mixed carbonate–siliciclastic continental margin: third‐order sequence stratigraphy of a ramp to basin transition,lower Sekwi Formation,Selwyn Basin,Northwest Territories,Canada

The lower part of the Early Cambrian Sekwi Formation in the Selwyn Basin of the Northwest Territories, Canada, is composed of two regional, unconformity‐bounded sequences, S0 and S1, which record the first widespread carbonate deposition during the initial Palaeozoic transgression onto the western margin of Laurentia. These Early Cambrian sequences are unique to the western North American Cordillera, representing the only record of primarily deep‐water deposition on a tectonically active, mixed carbonate–siliciclastic ramp during this period. More specifically, the geometry of the Sekwi ramp changed during deposition of S0 and S1, from a shallowly dipping homoclinal ramp during the S0 transgressive systems tract to a steeply dipping tectonically modified ramp during the early highstand systems tract of S0. The steeply dipping ramp profile of S0 was preserved into the early transgressive systems tract of S1. The Sekwi ramp returned to a gently sloping ramp during the late highstand systems tract of S1 and remained so throughout the remainder of Sekwi deposition. The evolving shape of the Sekwi ramp is attributed to syndepositional ‘down to the basin’ faulting during deposition of both S0 and S1 and is recorded by: (i) the westward thickening, irregular geometries of S0 and S1; (ii) geographical restriction of deep‐water facies (including sediment gravity flow deposits); (iii) the presence of large allochthonous blocks; and (iv) the clast composition of sediment gravity flow deposits. Sediment gravity flow deposits play an unusually important role in the sequence stratigraphic interpretation of the lower Sekwi Formation, as they delineate depositional packages, including the maximum flooding zone, the transitions between portions of systems tracts, and the inferred locations of syntectonic extensional faults. Syntectonic faults increased accommodation basinward of an extensive ooid‐shoal complex that developed along the Sekwi ramp crest, greatly influencing sequence geometry and initiating the downslope motion of sediment gravity flows. The syndepositional faulting probably was a continuation of extension that began during the latest Neoproterozoic rifting of western Laurentia. The composition of sediment gravity flow deposits track changing accommodation space on the lower Sekwi ramp and can be used to differentiate systems tracts that probably were related more to tectonism than eustasy.     

Dolomitization of Quaternary reef limestone, Aitutaki, Cook Islands

Six holes were drilled to depths of 30–69 m in the shallow lagoon of Aitutaki in the southern Cook Islands. One hole encountered pervasively dolomitized reef limestones at 36 m subbottom depth, which extended to the base of the drilled section at 69·3 m. This hole was drilled near the inner edge of the present barrier reef flat on the flank of a seismically defined subsurface ridge. Both the morphology and biofacies indicate that this ridge may represent an outer reef crest. Mineralogy, porosity and cementation change in concert downhole through three zones. Zone 1, 0–9 m, is composed of primary skeletal aragonite and calcite with minor void-filling aragonite and magnesian calcite cement of marine phreatic origin. Zone 2, 9–36 m, is composed of replacement calcite and calcite cement infilling intergranular, intragranular, mouldic and vuggy porosity. Stable isotopes (mean δ¹⁸O=—5·4‰ PDB for carbonate; δD =—50‰ SMOW for fluid inclusions) support the petrographic evidence indicating that sparry calcite cements formed in predominantly freshwater. Carbon isotope values of —4·0 to —11·0‰ for calcite indicate that organic matter and seawater were the sources of carbon. Zone 3, 36–69·3 m, is composed of replacement dolostone, consisting of protodolomite with, on average, 7 mol% excess CaCO₃ and broad and weak ordering X-ray reflections at 2·41 and 2·54 A. The fine-scale replacement of skeletal grains and freshwater void-filling cements by dolomite did not significantly reduce porosity. Stable isotopes (mean δ¹⁸O=+2·6‰₀ PDB for dolomite; maximum δD =—27‰ SMOW for fluid inclusions) and chemical composition indicate that the dolomite probably formed from seawater, although formation in the lower part of a mixed freshwater-seawater zone, with up to 40% freshwater contribution, cannot be completely ruled out. The carbon (δ¹³C=2·7‰) and magnesium were derived from seawater. Low-temperature hydrothermal iron hydroxides and associated transition metals occur in void space in several narrow stratigraphic intervals in the limestone section that was replaced by dolomite. The entire section of dolomite is also enriched in these transition metals. The metals dispersed throughout the dolostone section were introduced at the time of dolomitization by a different and later episode of hydrothermal circulation than the one(s) that produced the localized deposits near the base of the section. The primary reef framework is considered to have been deposited during several highstands of sea level. Following partial to local recrystallization of the limestone, a single episode of dolomitization occurred. Both tidal and thermal pumping drove large quantities of seawater through the porous rocks and perhaps maintained a wide mixing zone. However, the isotopic, geochemical and petrographic data do not clearly indicate the extent of seawater mixing.