Longitudinal transport of turbidity currents—–a model study of Horgen events

Experiments were carried out in a 10 × 6 m basin to simulate turbidity currents generated by the Horgen Slumping Events of 1875. The conditions for kinematic similarity were satisfied and the experiments gave further insight into the mechanics of transport triggered by the Horgen slumps. The experimental turbidity currents laid down thick deposits on a subaqueous fan, and thin sheets of turbidite on the floor of the elongate basin through longitudinal transport, comparable with the simulated deposits in Lake Zurich. It is concluded that longitudinal transport is a general phenomenon of turbidity currents.     

Three-fold subdivision of the Allerød chronozone

A pollen diagram of a calcareous travertine bed near Kirf (West Germany) shows a tripartition of the Allerød chronozone. In northwestern and central-European diagrams this is not an uncommon phenom-enon. Arguments supporting a deduced climatic oscillation may also be derived from the average ice recession in southern Sweden, the Coleoptera assemblages in northwestern England, the isotope ¹⁸0/¹⁶O curve from the Gerzensee in Switzerland, and the Camp Century Ice Core in Greenland. The included sandy Laachcr See ash may suggest explosive volcanism again to be somehow related to inferred climatic changes.     

Rates and Timescales of Fractional Crystallization from 238U-230Th-226Ra Disequilibria in Trachyte Lavas from Longonot Volcano, Kenya

A suite of peralkaline trachytes from Longonot volcano, Kenya,which erupted during the last 6000 years, has been analysedfor major and trace elements, Pb and Nd isotopes, and U–Th–Radisequilibria. The lavas are divided into three stratigraphicgroups of trachytes (Lt²a, Lt²b and Lt³), and hybrid lavas,designated LMx¹ and LMx², which, respectively, pre-date andpost-date the Lt² lavas. Major and trace elements are consistent,with up to 37% within-group fractional crystallization of predominantlyalkali feldspar. The parental magma for the different trachytegroups had a more mafic composition—probably hawaiitic.Nd and Pb isotopes show minimal variation, both within and betweenmagma groups, and indicate that up to 10% comendite magma fromthe neighbouring Olkaria volcanic field may have intermixedwith the Longonot magma. (²³⁰Th/²³⁸U) disequilibria indicatethat limited U/Th fractionation occurred during the past 10kyr, whereas (²²⁶Ra/²³⁰Th) disequilibria reflect the effectof alkali feldspar fractionation >8 kyr ago in the Lt²a lavas,between 3 and 7 kyr ago in the Lt²b lavas and in the past 3kyr for the Lt³ lavas. (²²⁶Ra/²³⁰Th) disequilibria in the Lt²blavas are interpreted using a model that combines the equationsof radioactive decay and in-growth with Rayleigh crystallizationto give fractionation rates of about 0·2 x 10^–4/yearfor the evolution of hawaiite to trachyte, but more rapid ratesof up to 3 x 10^–4/year for fractionation within the trachytesequence. (²²⁶Ra/²³⁰Th) from two whole-rock–alkali feldsparpairs are interpreted to show the crystals formed at 5800 yearsBP (Lt²b) and 2800 years BP (Lt³), implying that phenocrystformation continued almost up to the time of eruption. The resultsstrongly indicate that fractionated magmas can be stored forperiods on the order of 1000–2500 years prior to eruption,whereas other magmas were erupted as fractionation was proceeding. KEY WORDS: trachyte; magma chambers; u-series; Kenya     

Surge mark formation and morphology

Surge marks have previously been described as hieroglyphs and more recently as erosional rills caused by the turbulent bank-swashing of storm waters upstream of obstructions. This paper presents evidence that surge marks have various forms directly related to the steepness of sediment slope. These features are due, at least in part, to deformation and not necessarily to erosion by surging water. The previous term dendritic surge mark may now only apply to one member of the surge mark formation.     

Mixed Magmas, Mush Chambers and Eruption Triggers: Evidence from Zoned Clinopyroxene Phenocrysts in Andesitic Scoria from the 1995 Eruptions of Ruapehu Volcano, New Zealand

Juvenile ejecta from the September and October 1995 eruptionsof Ruapehu volcano, New Zealand, indicate that mixing occurredbetween relatively higher- and lower-temperature (high-T andlow-T) andesitic magmas. Compositional zonations in clinopyroxenephenocrysts provide direct evidence for a pre-eruption crystal–meltmush chamber containing low-T magma, and elucidate the processesof magma mixing and eruption, following the injection of high-Tmagma. Many phenocrysts with Fe-rich cores derived from low-Tmagma have extremely reverse zoned mantles around slightly resorbedcores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70to     

Garnet-field Melting and Late-stage Refertilization in 'Residual' Abyssal Peridotites from the Central Indian Ridge

The role of residual garnet during melting beneath mid-oceanridges has been the subject of many recent investigations. Toaddress this issue from the perspective of melting residues,we obtained major and trace element mineral chemistry of residualabyssal peridotites from the Central Indian Ridge. Many clinopyroxeneshave ratios of middle to heavy rare earth elements (MREE/HREE)that are too low to be explained by melting in the stabilityfield of spinel peridotite alone. Several percent of meltingmust have occurred at higher pressures in the garnet peridotitestability field. Application of new trace element partitioningmodels, which predict that HREE are compatible in high-pressureclinopyroxene, cannot fully explain the fractionation of theMREE from the HREE. Further, many samples show textural andchemical evidence for refertilization, such as relative enrichmentsof highly incompatible trace elements with respect to moderatelyincompatible trace elements. Therefore, highly incompatibleelements, which are decoupled from major and moderately incompatibletrace elements, are useful to assess late-stage processes, suchas melt entrapment, melt–rock reaction and veining. Moderatelyincompatible trace elements are less affected by such late-stageprocesses and thus useful to infer the melting history of abyssalperidotites. KEY WORDS: abyssal peridotites; mantle melting; garnet     

Geochemical evolution of the Dras–Kohistan Arc during collision with Eurasia: Evidence from the Ladakh Himalaya, India

Abstract   The Dras 1 Volcanic Formation of the Ladakh Himalaya, India, represents the eastern, upper crustal equivalent of the lower crustal gabbros and mantle peridotites of the Kohistan Arc exposed in Pakistan. Together these form a Cretaceous intraoceanic arc now located within the Indus Suture zone between India and Eurasia. During the Late Cretaceous, the Dras–Kohistan Arc, which was located above a north-dipping subduction zone, collided with the south-facing active margin of Eurasia, resulting in a switch from oceanic to continental arc volcanism. In the present study we analyzed samples from the pre-collisional Dras 1 Volcanic Formation and the postcollisional Kardung Volcanic Formation for a suite of trace elements and Nd isotopes. The Kardung Volcanic Formation shows more pronounced light rare earth element enrichment, higher Th/La and lower ɛ_Nd values compared with the Dras 1 Volcanic Formation. These differences are consistent with an increase in the reworking of the continental crust by sediment subduction through the arc after collision. As little as 20% of the Nd in the Dras 1 Volcanic Formation might be provided by sources such as the Karakoram, while approximately 45% of the Nd in the Kardung Volcanic Formation is from this source. However, even before collision, the Dras–Kohistan Arc shows geochemical evidence for more continental sediment contamination than is seen in modern western Pacific arcs, implying its relative proximity to the Eurasian landmass. Comparison of the lava chemistry in the Dras–Kohistan Arc with that in the forearc turbidites suggests that these sediments are partially postcollisional, Jurutze Formation and not all pre-collisional Nindam Formation. Thus, the Dras–Eurasia collision can be dated as Turonian–Santonian (83.5–93.5 Ma), older than it was previously considered to be, but consistent with radiometric ages from Kohistan.     

THE SENSITIVITY OF TWO DISTRIBUTED NON-POINT SOURCE POLLUTION MODELS TO THE SPATIAL ARRANGEMENT OF THE LANDSCAPE

Distributed hydrological models are becoming increasingly complex with respect to spatial phenomena, and with the widespread availability of spatial data from GIS, this trend is likely to increase. In all such models the spatial arrangement of phenomena, such as soil properties and land-use categories is fundamental, and so the arrangement should have an influence on the model output. Testing for this influence we term spatial sensitivity analysis. Here, we report on the spatial sensitivity of two widely used models, AgNPS (agricultural non-point source pollution model) and ANSWERS (areal nonpoint source watershed environment response simulation). The input spatial data were subjected to spatially random mixing to varying degrees, such that the organized landscape became disorganized. The chemical discharge from AgNPS, and the sediment and water discharge from ANSWERS, are examined. In both cases most outputs exhibited little or no sensitivity to the spatial distribution of most input data. Only infiltration-related inputs produced large variations, but these changes were not in the sense that might have been predicted. Although the analytical methods used require further refinement, there must now be some doubt as to the validity of the models, and whether they repay their computational complexity. Furthermore, it is felt that spatial sensitivity analysis should become a fundamental part of the verification of all such models. © 1997 by John Wiley & Sons, Ltd.     

ASSESSMENT OF CLIMATE CHANGE AND FRESHWATER ECOSYSTEMS OF THE ROCKY MOUNTAINS,USA AND CANADA

The Rocky Mountains in the USA and Canada encompass the interior cordillera of western North America, from the southern Yukon to northern New Mexico. Annual weather patterns are cold in winter and mild in summer. Precipitation has high seasonal and interannual variation and may differ by an order of magnitude between geographically close locales, depending on slope, aspect and local climatic and orographic conditions. The region's hydrology is characterized by the accumulation of winter snow, spring snowmelt and autumnal baseflows. During the 2–3-month ‘spring runoff’ period, rivers frequently discharge > 70% of their annual water budget and have instantaneous discharges 10–100 times mean low flow. Complex weather patterns characterized by high spatial and temporal variability make predictions of future conditions tenuous. However, general patterns are identifiable; northern and western portions of the region are dominated by maritime weather patterns from the North Pacific, central areas and eastern slopes are dominated by continental air masses and southern portions receive seasonally variable atmospheric circulation from the Pacific and the Gulf of Mexico. Significant interannual variations occur in these general patterns, possibly related to ENSO (El Niño–Southern Oscillation) forcing. Changes in precipitation and temperature regimes or patterns have significant potential effects on the distribution and abundance of plants and animals. For example, elevation of the timber-line is principally a function of temperature. Palaeolimnological investigations have shown significant shifts in phyto- and zoo-plankton populations as alpine lakes shift between being above or below the timber-line. Likewise, streamside vegetation has a significant effect on stream ecosystem structure and function. Changes in stream temperature regimes result in significant changes in community composition as a consequence of bioenergetic factors. Stenothermic species could be extirpated as appropriate thermal criteria disappear. Warming temperatures may geographically isolate cold water stream fishes in increasingly confined headwaters. The heat budgets of large lakes may be affected resulting in a change of state between dimictic and warm monomictic character. Uncertainties associated with prediction are increased by the planting of fish in historically fishless, high mountain lakes and the introduction of non-native species of fishes and invertebrates into often previously simple food-webs of large valley bottom lakes and streams. Many of the streams and rivers suffer from the anthropogenic effects of abstraction and regulation. Likewise, many of the large lakes receive nutrient loads from a growing human population. We concluded that: (1) regional climate models are required to resolve adequately the complexities of the high gradient landscapes; (2) extensive wilderness preserves and national park lands, so prevalent in the Rocky Mountain Region, provide sensitive areas for differentiation of anthropogenic effects from climate effects; and (3) future research should encompass both short-term intensive studies and long-term monitoring studies developed within comprehensive experimental arrays of streams and lakes specifically designed to address the issue of anthropogenic versus climatic effects. © 1997 John Wiley & Sons, Ltd.     

Cosmogenic 10 Be ages on the Pomeranian Moraine, Poland

Rinterknecht, V. R., Marks, L., Piotrowski, J. A., Raisbeck, G. M., Yiou, F., Brook, E. J. & Clark, P. U. 2005 (May): Cosmogenic ¹⁰Be ages on the Pomeranian Moraine, Poland. Boreas , Vol. 34, pp. 186–191. Oslo. ISSN 0300–9483.
We measured the ¹⁰Be concentrations in boulders collected from the Pomeranian Moraine in Poland, providing the first direct dating of the southern margin of the Scandinavian Ice Sheet (SIS) in the Polish Lowland. The mean age of 8 ¹⁰Be ages of the Pomeranian Moraine in northwestern Poland is 14.30.8 ¹⁰Be ka, while in northeastern Poland the mean age of 19 ¹⁰Be ages of the moraine is 15.00.5 ¹⁰Be ka. Given the excellent agreement between the two age groups, we calculate a mean age of 14.80.4 ¹⁰Be ka for final deposition of the Pomeranian Moraine of northern Poland. The age of the Pomeranian Moraine suggests that the southern margin of the SIS was near its maximum extent in Poland at a younger time than previously inferred, and that retreat from the moraine at 14.80.4 ¹⁰Be ka probably occurred in response to the onset of the Bølling interstade.