30 000 years of productivity and salinity variations in the late Quaternary Cariaco Basin revealed by dinoflagellate cysts

Dinoflagellate cysts and other palynomorphs were studied from ODP Hole 1002C in the Cariaco Basin over the past 30 000 years. The assemblage shifts between a dominance of heterotrophic dinoflagellate cysts (mainly Brigantedinium spp., Lejeunecysta spp., Selenopemphix nephroides and Stelladinium reidii ) and autotrophic dinoflagellate cysts (mainly Spiniferites ramosus , Lingulodinium machaerophorum and Operculodinium centrocarpum ). These assemblage shifts are associated with stronger upwelling during stadials and stronger river influx during interstadials. Increases in productivity caused by enhanced upwelling are coupled to improved preservation and vice versa. More stratified water is indicated by higher abundances of L. machaerophorum and succeeds Heinrich events. The average process length of L. machaerophorum can be used to track changes in salinity, since this shows a similar pattern as the δ¹⁸O_SW (paired Mg/Ca −δ¹⁸O) reconstruction. During the last glacial, conditions were more saline than during the current interglacial. On a millennial scale, changes in salinity are opposite to open ocean salinities and the hydrological proxies, which can be explained by a modulation of the signal by stratification, isolation of the Basin or advection of freshwater masses. These results highlight both generalities and particularities of the palaeoecological record of this tropical semi-enclosed basin.     

Ordovician low- to intermediate-Mg calcite marine cements from Sweden: marine alteration and implications for oxygen isotopes in Ordovician seawater

Petrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO₃, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low- to intermediate-Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO₃ content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light-brown turbid fibrous calcite has low MgCO₃ (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ¹³C and δ¹⁸O values of all fibrous calcite form a tight field (δ¹³C=1·7 to 3·1‰ PDB, δ¹⁸O= ? 2·6 to ? 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ¹⁸O values are larger than adjacent meteoric or burial cements, which have δ¹⁸O δ ? 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First-generation equant and bladed calcite cements, which pre-date fibrous calcite, are interpreted as unaltered, low-Mg calcite marine cements based on δ¹³C and δ¹⁸O data (δ¹³C = 2·3 to 2·7‰ PDB, δ¹⁸O= ? 2·8 to ? 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first-generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ¹⁸O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ¹⁸O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed.     

The Geographer's Craft: Teaching GIS in the Web

The Geographer's Craft is a two-semester course introducing GIS and geographic research methods using active-learning, problem-solving techniques. All of the materials for the course were developed in hypertext form in the World-Wide Web (WWW). The organization of this electronic textbook and laboratory manual is explained and related to principles for using the WWW to enrich GIS education. Teaching strategies are discussed as are lessons learnt in developing hypertext courseware. The value of enlisting student participation in the development process is stressed. Note is made of the ways in which Web-based materials may lead to the emergence of distance of education programmes in GIS. The project homepage is http://www.utexas.edu/depts/grg/gcraft/contents.html     

The Late Devensian and Flandrian history of the Teith valley, Scotland

Two episodes of glaciation are identified in the Teith valley, central Scotland. During the earlier episode, the valley was wholly occupied by an ice sheet, but during the later episode, correlated with the Loch Lomond Readvance, ice only occupied the upper part of the valley. The deglaciation which followed each episode was marked in particular by sequences of kame and outwash terraces. A terrace related to the second episode grades into a large fan buried beneath a sequence of marine deposits in the nearby Forth valley. The latest of these, the carse clays, are related to Flandrian terraces in the Teith valley.     

Evidence for the Repeated Folding of Isotherms during Regional Metamorphism

Petrologic and structural investigations in a polydeformed terranein southwestern New Hampshire show that more than one stageof noncoaxial folding has significantly affected the metamorphichistory of this region. Detailed examinations of both isograd-isothermpatterns and mineral reaction histories within pelitic rockssuggest that pre-existing, nearly horizontal isotherms werefolded during an early stage of folding about north-south axes.After folding, thermal relaxation resulted in cooling in theanticlinal portions of the early folds and heating in the synclinalregions. This process of isotherm folding and re-equilibrationwas repeated during a later stage of east-west folding, resultingin complex isograd patterns and mineral reaction histories. Detailed mapping of isograd and isotherms in the pelitic rocksof this region shows a complex patten. High grade and high temperatureassemblages occur along early and late synclinal axes, withthe highest grade assemblages occurring at the intersectionof the two synclinal axes. Conversely, the lowest grade assemblagesare found at the intersection of the early and late anticlinalfolds. Four different types of fold intersections result from the twostages of noncoaxial folding. Pelitic rocks at each of thesefour different fold intersections show different and contrastingmineral reaction histories. At the intersections of early andlate synclines the rocks show evidence for continuous heating,while continuous cooling trends are seen at intersections ofearly and late anticlines. Complex reaction histories are observedat the intersections of early synclines and late anticlinesand early anticlines and late synclines, which show heating-coolingand cooling-heating trends, respectively. These results show that over small areas of a metamorphic terranedifferent samples may show widely different P-T paths. Therefore,without careful structural analysis and thorough sampling, tectonicinterpretations based solely on paths determined from a fewwidely scattered samples may lead to erroneous conclusions.     

The Grayback Pluton: Magmatism in a Jurassic Back-Arc Environment, Klamath Mountains, Oregon

The Jurassic Grayback pluton was emplaced in a back-arc settingbehind a contemporaneous oceanic arc. Th\alphae main stage ofthe pluton consists of an early, reversely zoned tonalite togabbro that was intruded by synplutonic noritic and gabbroicmagmas. Late-stage activity was characterized by intrusion oftonalitic and granitic dikes, many of which contain mafic enclavesand hybrid zones. Most mafic rocks in the pluton are calc-alkaline,with characteristic magnesian clinopyroxene, calcic cores inplagioclase, and elemental abundances similar to H₂O-rich arcbasalts. However, some mafic rocks contain relatively Fe-richclinopyroxene, lack calcic cores in plagioclase, and are compositionallysimilar to evolved high-alumina tholeiite. Compositional variation in the main stage can be modeled inpart by fractional crystallization and crusted assimilationduring which parental calc-alkaline basalt evolved to graniticcompositions. Cumulates related to this process are representedby modally variable melagabbro and pyroxenite. Mixing of basalticand tonalitic magmas accounts for the compositions of most main-stageintermediate rocks, but mixing of basaltic and granitic magmaswas uncommon until late in the pluton's history. Oxygen, Sr and Nd isotopic data indicate that virtually allmain-stage magmas in the pluton contain a crustal component.Isotopic and trace element data further suggest that late-stagetonalitic dikes represent melts derived from older, metavolcanicarc crust Deep crustal contamination of main-stage rocks tookplace below the level of emplacement, probably in a magma-richzone where basalts ponded and mixed with crustal melts. The Grayback pluton illustrates the diversity of Jurassic back-arcmagmatism in the Klamath province and demonstrates that ancientmagmatism with arc-like features need not be situated in anarc setting. KEY WORDS: Grayback Pluton; Klamath Mountains; Oregon; back arc; crustal contamination ^*Corresponding author     

The Quantitative Mineralogical Composition and Variation of the Lacorne, La Motte, and Preissac Granitic Complex, Quebec, Canada

Variation of quartz percentage, colour index, potash feldspar/plagioclaseratio, feldspar percentage, and specific gravity within theLacorne, La Motte, and Preissac granitic complex is studiedby means of trend surface analyses based on 198 specimens. Markedpatterns of variation emerge; the more clear-cut patterns relateto massifs with mafic- and potash feldspar-rich centres, andquartzose peripheries. The trend components suggest that theLacorne massif is a composite body, and re-evaluation of thefield evidence supports this prediction. These results are usedto demonstrate a quantitative technique and its applicationto geographically distributed data of granitic complexes; aplea is made for the collection of areally distributed quantitativedata of all types.