River stabilisation due to changing climate and vegetation during the late Quaternary in western Tasmania, Australia

The Stanley River in western Tasmania, Australia, contains sub-fossil rainforest logs within the channel and floodplain. Of the more than 85 radiocarbon dates obtained, all but 3 date from 17 ka to the present and permit an interpretation of fluvial and related environmental changes over this period. Particular attention is focused on the interactive relationship between the river and its riparian rainforest. Following the Last Glacial Maximum, the Stanley River was a laterally active gravel-load system reworking most of its valley floor in the upstream reaches. With ameliorating conditions at the end of the Pleistocene, climate became less seasonal and flow regimes less energetic. Huon pines already present in the catchment, re-asserted themselves in the form of dense tree cover along the river banks and floodplains with basal floodplain deposition shifting from gravels to coarse sands and granules. By about 3.5 ka, a further change in climate reduced stream discharges substantially. As a result the channel reduced in size, transported finer sediment, became laterally stable, and the floodplain accreted with overbank deposits of sand and silt. Huon pines falling into the channel formed obstructions of woody debris, some surviving for 2 ka. These have reduced stream power and boundary shear stress, further contributing to channel stability. Generational sequences of Huon pines on the river banks, some extending back 1–2 ka, are additional evidence of this stability. Since the Pleistocene, changing climate and the re-establishment of dense riparian rainforest appear to have stabilised the river channels and floodplains of western Tasmania.     

Fluctuation dissipation in a general circulation model

This paper considers the climate response to step function changes in the solar constant in two versions of a general circulation model with simplified geography. The NCAR CCM0 model is applied to an all-land planet with no topography (Terra Blanda). In one version there is moisture in the air (as well as self-generated clouds) as evaporated from an ideal surface at a fixed 80% of saturation. In the other version there is no moisture in the atmosphere. We examine the decay of natural anomalies in the large-scale temperature field in each model and compare the time dependence of the ensemble average with the average temporal behavior of the response to step function changes in the solar constant. The fluctuation-dissipation theorem of statistical mechanics makes specific predictions about the relationship between the two curves. We conduct the experiments for both versions of the model since the sensitivity is quite different for each. The theorem is found to hold reasonably well in each case.     

Contemporary and palaeo channel patterns and the late quaternary stratigraphy of Cooper Creek,Southwest Queensland,Australia

Braided and anastomosing channels make up two major coexistent networks in the mud-dominated fluvial system of Cooper Creek, Southwest Queensland. The floodplain is characterized by a system of mud braids operative when floods inundate the whole alluvial surface. Anastomosing channels are inset deeper into the floodplain, operate at modern flows, and transport a traction load of sand. Shallow stratigraphic data show that an underlying sand sheet is unrelated to surface channel patterns and was formed by a system of meandering streams. According to preliminary dates based on thermoluminescence, the change from a sand- to mud-dominated fluvial regime took place between 50 000 and 200 000 years B.P., and probably reflects increasing aridity.     

Seasonality of materials transport through a coastal freshwater marsh: Riverine versus tidal forcing

Transports of nitrate and suspended solids were measured six times from January 1984 until January 1985 in a small freshwater tidal bayou in south-central Louisiana. The bayou and adjacent marshes are influenced by Atchafalaya River discharges, tides, and coastal weather patterns. Large net ebb-directed water transports occurred in winter, spring, and summer, coincident with high river discharges, indicating riverine dominance. A very small net flood-directed water transport occurred in fall, indicating tidally dominated hydrology. Nitrate and suspended solids transports were net ebb-directed in all seasons, but were two orders of magnitude higher during high river flow. Exports changed as hydrology switched from river dominated to tidally dominated, and as concentrations of materials changed. Comparison of suspended solids and nitrate concentrations in the river and bayou shows that these materials were usually lower in the bayou, indicating retention by the marsh/aquatic system.     

The Pan-African continental margin in northeastern Africa: evidence from a geochronological study of granulites at Sabaloka, Sudan

Ion microprobe zircon ages, a Nd model age and RbSr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton. The granulites, which are of both sedimentary and igneous derivation, occur as remnants in migmatites. Detrital zircon ages range from ≈ 1000 to ≈ 2650 Ma and prove the existence of Archaean to late Proterozoic continental crust in the sedimentary source region. The Nd model age for one sedimentary granulite is between 1.26 (T_CHUR) and 1.70 (T_DM) Ga and provides a mean crustal residence age for the sedimentary precursor. Igneous zircons in enderbitic gneiss crystallized at 719 ± 81 Ma ago, an age that also corresponds to severe Pb loss in the detrital zircons and which probably reflects the granulite event at Sabaloka. The RbSr data indicate isotopic homogenization at about 700 Ma ago in the granulites and severe post-granulite disturbance at ≈ 570 Ma in the migmatites. We associate this disturbance with hydration, retrograde metamorphism and anatexis that produced undeformed granites ≈ 540 Ma ago. The ≈ 700 Ma granulite event at Sabaloka suggests that this part of the Sudan belongs to the Pan-African Mozambique belt while the ancient Nile craton lay farther west. The gneisses studied here may represent the infrastructure of the ancient African continental margin onto which the juvenile arc assemblage of the Arabian-Nubian shield was accreted during intense horizontal shortening and crustal interstacking of a major collision event.     

Geochemistry,geochronology, and petrogenesis of a Late Precambrian (∼ 590 Ma) composite dike from the North Eastern Desert of Egypt

Late Precambrian (575–600 Ma) igneous activity in the North Eastern Desert of Egypt produced large volumes of compositionally bimodal magmas. A single composite dike was studied to further examine petrogenetic relationships between andesitic and rhyolitic melts. The dike consists of 1.5–2 m of andesite on either side of a 5–6 m wide rhyolite core. Contact relations indicate that the andesite and rhyolite simultaneously existed as porphyritic liquids. Wholerock Rb-Sr dating indicates an age of 591 Ma, but with considerable scatter. Andesite and rhyolite had similar initial⁸⁷Sr/⁸⁶Sr of 0.7032±2, indicating derivation from a low Rb/Sr source, either the upper mantle, very young upper crust, or depleted lower crust. The composition of the andesites on either margin cannot be distinguished; these are very similar to andesites of the Dokhan Volcanics of similar age. Correspondingly, the rhyolite is compositionally similar to the epizonal Pink Granites. This dike and others like it represent hypabyssal feeders for the extrusive and shallow intrusive members of the North Eastern Desert bimodal suite.Major and trace element studies of the dike rocks show no evidence of mixing. With the exception of the alkaline earths, there is no evidence of diffusional transport across the rhyolite-andesite contacts. REE patterns indicate that petrogenesis of the andesite must have left a garnetiferous residue: either 10% melting of LREE-enriched garnet lherzolite or 25% melting of eclogite, followed by a small amount of shallow fractionation. Petrogenesis of the rhyolites remains enigmatic. Partitioning of trace elements and different plagioclase compositions in andesite and rhyolite argue against an origin by liquid immiscibility. Major and trace element models indicate that the rhyolite formed by fractional crystallization of the andesite or by anatexis of young amphibolite-facies crust.

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Zusammenfassung SpÄtprÄkambrische (575–600 Ma) magmatische AktivitÄt in der »North Eastern Desert« Ägyptens förderte gro\e Mengen bimodaler Magmen. Um die petrogenetischen Beziehungen zwischen andesitischen und rhyolitischen Schmelzen zu studieren, wurde ein einzelner, »bimodaler« Gang (composite dyke) untersucht.Der Gang besteht aus einem Rand von 1,5–2 m mÄchtigem Andesit auf beiden Seiten eines 5–6 m mÄchtigen Kernbereiches von Rhyolit. Die Kontaktbeziehungen zeigen, da\ Andesite und Rhyolite gleichzeitig als porphyritische Schmelzen existierten. »Whole-Rock« Rb-Sr Datierung ergab ein Alter von 591 Ma, jedoch mit einem erheblichen Fehler. Andesit und Rhyolith hatten Ähnliche initiale⁸⁷Sr/⁸⁶Sr VerhÄltnisse von 0.7032±2, die eine Herkunft aus Bereichen mit niedrigen Rb/Sr VerhÄltnissen erkennen lassen: Oberer Mantel, sehr junge obere Kruste, oder verarmte untere Kruste. Die Zusammensetzung der Andesite von den verschiedenen Seiten des Ganges ist nicht zu unterscheiden. Sie sind den Andesiten der Dokhan Vulkanite Ähnlich und etwa gleich alt. Entsprechend sind die Rhyolite den epizonalen rosa Graniten (»pink granite«) in der Zusammensetzung Ähnlich. Dieser Gang und andere gleich ihm stellen die Zufuhrspalten für die extrusiven und seichtintrusiven Magmatite der »North Eastern Desert« bimodalen Folge dar.Haupt- und Spurenelemente der Ganggesteine zeigen keine Anzeichen einer Mischung. Mit Ausnahme der Erdalkalien gibt es keinen Hinweis auf eine Diffusion der Elemente über den Rhyolit-Andesit Kontakt. Die Verteilung der Seltenen Erden deutet auf ein granatführendes Residuum bei der Petrogenese der Andesite: entweder10% Aufschmelzen von Granat-Lherzolit, angereichert an leichten Seltenen Erden, oder 25% Aufschmelzen von Eklogit, gefolgt von geringer Fraktionierung unter niedrigem Druck.Die Petrogenese der Rhyolite bleibt unklar. Die Verteilung der Spurenelemente und die unterschiedliche Plagioklas-Zusammensetzung in Andesit und Rhyolith sprechen gegen eine Entstehung aus unvermischbaren Schmelzen. Modelle basierend auf Haupt- und Spurenelementen deuten darauf hin, da\ der Rhyolit entweder durch fraktionierte Kristallisation des Andesits entstand oder durch Anatexis junger Kruste im Bereich der Amphibolit-Fazies.

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Résumé L'activité magmatique au Précambrien supérieur (575–600 Ma) dans le »North Eastern Desert« d'Egypte a engendré de grandes quantités de magmas bimodaux. De manière à préciser les relations entre fusions andésitiques et rhyolitiques, un dyke composite a été étudié.Ce dyke est constitué d'un coeur rhyolitique de 5 à 6 m, encadré de deux bordures andésitiques de 1,5 à 2 m. Les contacts indiquent que ces deux roches ont coexisté sous la forme de liquides porphyriques. Une datation Rb-Sr sur roche totale donne un âge de 591 Ma, mais avec une approximation importante. L'andésite et la rhyolite ont un mÊme rapport initial de 0,7032±2, indiquant une source à faible rapport Rb/Sr: manteau supérieur, croûte supérieure jeune ou croûte inférieure appauvrie. Les compositions des deux bordures andésitiques sont identiques et très semblables à celle des andésites de mÊme âge du complexe volcanique du Dokhan. De mÊme, la rhyolite a une composition semblable à celle des granites roses épizonaux. Ce dyke et d'autres du mÊme type représentent les voies d'alimentation des termes extrusifs et intrusifs superficiels de la série bimodale du »North Eastern Desert«.L'étude des éléments majeurs et en traces ne montre aucun mélange entre les roches du dyke. A l'exception des alcalino-terreux, il n'existe aucune indication de diffusion des éléments à travers le contact rhyolite — andésite. La distribution des terres rares montre que la genèse de l'andésite doit avoir laissé un résidu grenatifère: soit une lherzolite à grenat enrichie en terres rares légères et représentant moins de 10% de taux de fusion, soit une éclogite (jusqu'à 25% de taux de fusion) suivie d'un léger fractionnement à basse pression.La pétrogenèse de la rhyolite reste obscure. La répartition des éléments en traces et la différence de composition du plagioclase entre l'andésite et la rhyolite plaident contre une origine par immiscibilité. Les modèles tirés des éléments majeurs et en traces indiquent comme origine pour la rhyolite soit la cristallisation fractionnée de l'andésite, soit l'anatexie de matériaux crustaux jeunes dans les conditions du facies des amphibolites.

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