Dissolution Rates of Upper Mantle Minerals in an Alkali Basalt Melt at High Pressure: An Experimental Study and Implications for Ultramafic Xenolith Survival

The dissolution rates of the major upper mantle minerals olivine,orthopyroxene, clinopyroxene, spinel, and garnet have been determinedin an alkali basalt melt at superliquidus temperatures and 5,12, and 30 kb. At low pressure where olivine is the liquidusphase of the basalt, olivine has a slower dissolution rate thanclinopyroxene; however, at higher pressure where clinopyroxeneis the liquidus phase, clinopyroxene has a slower dissolutionrate than olivine. The relative rates of dissolution of olivineand clinopyroxene at each pressure are, therefore, governedby their relative stabilities in the melt and hence by the structureof the melt. As the degree of superheating above the liquidusincreases at each pressure, the dissolution rates of olivineand clinopyroxene converge, suggesting that the melt undergoestemperature-induced structural changes. Orthopyroxene has a dissolution rate similar to olivine at highpressure and similar to clinopyroxene at low pressure. Spinelhas the slowest dissolution rate at each pressure. Garnet dissolvesvery rapidly at 12 kb and at a comparable rate of olivine at30 kb. The dissolution rates determined in the experiments varyfrom 9.21 ? 10^–9cm s^–1 for spinel at 5 kbar and1250?C to 3.83 ? 10^–5cm s^–1 for garnet at 30 kband 1500?C. Textures produced during the dissolution experiments are relatedto mineral stability in the melt at each pressure and are independentof the degree of superheating. The mineral phases that are stableon or near the liquidus exhibit no reaction; whereas complexreaction textures and crystallization characterize dissolutionof minerals that are relatively unstable in the melt. Concentration profiles in the melt adjacent to the same crystalfor different experimental durations are identical, indicatingthat dissolution is time-independent and a steady-state process.However, cation diffusion coefficients calculated for single-componentoxides in the melt reveal that dissolution may not be completelycontrolled by diffusion of cations away from the crystal/meltinterface. The apparent diffusivities positively correlate withthe dissolution rate, which suggests that the stability of themineral is an important factor to consider when deriving diffusioncoefficients from these experiments. Other factors that maybe involved are multi-component effects and the nature of thediffusing species in the melt. A simple model has been constructed that predicts the survivalof ultramafic xenoliths in alkali basalt magmas as a functionof xenolith radius, magma ascent time and superheating. Theresults of the model suggest that the relative proportions ofperidotite and pyroxenite xenoliths brought to the surface inalkali basalts are generally representative of their proportionsas constituents of the upper mantle. Further experiments usingdifferent melt compositions are required to extend the model.     

A review of the origin and setting of tepees and their associated fabrics

Carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees. The types of tepees found in the geological record include: (1) Submarine tepees which form in shallow carbonate-saturated waters where fractured and bedded marine grainstones are bound by isopachous marine-phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements; (2) Peritidal and lacustrine tepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate. Peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine-vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material. Lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust. Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds. (3) Caliche tepees which are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid. In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrological setting.     

Podzol development, vegetation change and glacier variations at Haugabreen, southern Norway

¹⁴C dating and pollen analysis of the surface organic (LFH) horizons of several humo-ferric podzol profiles forming a soil catena close to the 'Little Ice Agc' outer moraine ridge of Haugabreen, southern Norway, are used to examine the timing and nature of podzol development at the low-/sub-alpine margin of the Jostedalsbreen area. Comparison with results from a palaeosol buried beneath the outer moraine shows that FH horizon development began as early as 5,265 ± 65 B.P., but that it was not synehronous across the profiles, the latest profile having a date of 3,590 ± 65 B.P. It is argued that surface organic horizons developed as a response to a deterioration of climate and possibly the recrudescence of the Myklebustbreen ice cap at c . 5,000 B.P., and that the dates for horizon initiation vary according to local topographic and soil-hydrologic conditions. It is still uncertain whether the hump-ferric podzols were preceded by brown earths or weakly podzolised sub-alpine podzolic soils, but at all sites where pollen evidence is available it appears that FH initiation took place beneath Betula woodland.     

A high-resolution diatom record of late-Quaternary sea-surface temperatures and oceanographic conditions from the eastern Norwegian Sea

Core MD95‐2011 was taken from the eastern Vøring Plateau, near the Norwegian coast. The section between 250 and 750 cm covers the time period from 13 000 to 2700 cal. yr BP (the Lateglacial and much of the Holocene). Samples at 5 cm intervals were analysed for fossil diatoms. A data‐set of 139 modern sea‐surface diatom samples was related to contemporary sea‐surface temperatures (SSTs) using two different numerical methods. The resulting transfer functions were used to reconstruct past sea‐surface temperatures from the fossil diatom assemblages. After the cold Younger Dryas with summer SSTs about 6°C, temperatures warmed rapidly to about 13°C. One of the fluctuations in the earliest Holocene can be related to the Pre‐Boreal Oscillation, but SSTs were generally unstable until about 9700 cal. yr BP. Evidence from diatom concentration and magnetic susceptibility suggests a change and stabilization of water currents associated with the final melting of the Scandinavian Ice Sheet at c. 8100 cal. yr BP. A period of maximum warmth between 9700 and 6700 cal. yr BP had SSTs 3–5°C warmer than at present. Temperatures cooled gradually until c. 3000 cal. yr BP, and then rose slightly around 2750 cal. yr BP. The varimax factors derived from the Imbrie & Kipp method for sea‐surface‐temperature reconstructions can be interpreted as water‐masses. They show a dominance of Arctic Waters and Sea Ice during the Younger Dryas. The North Atlantic current increased rapidly in strength during the early Holocene, resulting in warmer conditions than previously. Since about 7250 cal. yr BP, Norwegian Atlantic Water gradually replaced the North Atlantic Water, and this, in combination with decreasing summer insolation, led to a gradual cooling of the sea surface. Terrestrial systems in Norway and Iceland responded to this cooling and the increased supply of moisture by renewed glaciation. Periods of glacial advance can be correlated with cool oscillations in the SST reconstructions. By comparison with records of SSTs from other sites in the Norwegian Sea, spatial and temporal changes in patterns of ocean water‐masses are reconstructed, to reveal a complex system of feedbacks and influences on the climate of the North Atlantic and Norway.     

MODELLING RUNOFF GENERATION ON SMALL AGRICULTURAL CATCHMENTS: CAN REAL WORLD RUNOFF RESPONSES BE CAPTURED?

This paper focuses on the problem of quantifying real world catchment response using a distributed model and discusses the ability of the model to capture that response. The rainfall–runoff responses of seven small agricultural catchments in the eastern wheatbelt region of south-western Australia are examined. The variability in runoff generation and the factors that contribute to that variability (i.e. rainfall intensity, soil properties and topography) are investigated to determine if their influence can be captured in a mathematical model. The spatially distributed rainfall–runoff model used in this study is based on the TOPMODEL concepts of Beven and Kirkby (1979), and simulates runoff generation by both the infiltration excess and saturation excess mechanisms. Simulations with the model revealed the highly complex nature of catchment response to rainfall events. Runoff generation was highly heterogeneous in both space and time, with the runoff response being governed by the spatial variability of soil properties and topography, and by the temporal variation in rainfall intensity. Although the model proved capable of simulating catchment response for many events, the investigation has demonstrated that not all aspects of the variability associated with agricultural catchments (particularly the effects of land management) can be captured using this relatively simple model. © 1997 by John Wiley & Sons, Ltd     

Gibraltar woman and Neanderthal Man

Forbes' Quarry, on the Rock of Gibraltar, yielded a human skull in 1848, one of the earliest Neanderthal skeletal remains known to science. Fragments of a second Gibraltar skull, that of a child, were described from Devil's Tower rock shelter in 1928 and have recently been reconstructed and reinterpreted to emphasize the distinction of Homo neanderthalensis from H. sapiens . Neanderthal skeletal remains are confined to Europe, the Middle East and central Asia, their most recent occurrence arguably a refugium in southern Iberia. The race seemingly became extinct about 30 000 years Before Present, for reasons as yet unknown, but a programme of excavations in Gorham's and Vanguard caves on Gibraltar is in progress to elucidate palaeoenvironmental and behavioural changes as some of the last Neanderthals were succeeded stratigraphically by anatomically modern humans with an Upper Palaeolithic culture.     

Cyclicity in the nearshore marine to coastal, Lower Permian, Pebbley Beach Formation, southern Sydney Basin, Australia: a record of relative sea-level fluctuations at the close of the Late Palaeozoic Gondwanan ice age

The Lower Permian (Artinskian to Sakmarian) Pebbley Beach Formation (PBF) of the southernmost Sydney Basin in New South Wales, Australia, records sediment accumulation in shallow marine to coastal environments at the close of the Late Palaeozoic Gondwanan ice age. This paper presents a sequence stratigraphic re‐evaluation of the upper half of the unit based on the integration of sedimentology and ichnology. Ten facies are recognized, separated into two facies associations. Facies Association A (seven facies) comprises variably bioturbated siltstones and sandstones with marine body fossils, interpreted as recording sediment accumulation in open marine environments ranging from lower offshore to middle shoreface water depths. Evidence of deltaic influence is seen in several Association A facies. Facies Association B (three facies) comprises mainly heterolithic, interlaminated and thinly interbedded sandstone and siltstone with some thicker intervals of dark grey, organic‐rich mudstone, some units clearly filling incised channel forms. These facies are interpreted as the deposits of estuarine channels and basins. Throughout the upper half of the formation, erosion surfaces with several metres relief abruptly separate open marine facies of Association A (below) from estuarine facies of Association B (above). Vertical facies changes imply significant basinward shift of environment across these surfaces, and lowering of relative sea level in the order of 50 m. These surfaces can be traced over several kilometres along depositional strike, and are defined as sequence boundaries. On this basis, at least nine sequences have been recognized in the upper half of the formation, each of which is < 10 m thick, condensed, incomplete and top‐truncated. Sequences contain little if any record of the lowstand systems tract, a more substantial transgressive systems tract and a highstand systems tract that is erosionally truncated (or in some cases, missing). This distinctive stacking pattern (which suggests a dominance of retrogradation and progradation over aggradation) and the implied relative sea‐level drop across sequence boundaries of tens of metres are remarkably similar to some other studies of continental margin successions formed under the Neogene icehouse climatic regime. Accordingly, it is suggested that the stratigraphic architecture of the PBF was a result of an Icehouse climate regime characterized by repeated, high‐amplitude cycles of relative sea‐level change.     

Controls on late Quaternary incised‐valley dimension along passive margins evaluated using empirical data

Incised valleys are canyon‐like features that initially form near the highstand shoreline and evolve over geological time as rivers erode into coastal plains and continental shelves to maintain equilibrium‐gradient profiles in response to sea‐level fall. Most of these valleys flood during sea‐level rise to form estuaries. Incised‐valley morphology strongly controls the rate of creation of sediment accommodation, valley‐fill facies architecture and the preservation potential of coastal lithosomes on continental shelves, and affects coastal physical processes. Nonetheless, little is known about what dictates incised‐valley size and shape and whether these metrics can be used to explain principal formation processes. The main control on alluvial channel morphology over human time scales is discharge; this is based on numerous empirical studies and is well‐constrained because all variables are easily measured at this short time scale. Knowledge of long‐term river evolution over a complete glacio‐eustatic cycle, on the contrary, remains largely conceptual, experimental and based on individual systems because variables that are thought to drive morphological change are not easily quantified. In spite of this difficulty, existing models of incised‐valley formation at the coast suggest that valley evolution is driven largely by downstream forcing mechanisms, highlighting sea‐level and shelf gradient/morphology as the dominant controls on valley incision. Although valleys are cut by rivers, whose channels are a direct reflection of discharge, little empirical data exist in coastal areas to address the degree to which valley evolution is governed by upstream controls. The late Quaternary is the best time period to examine because it provides the most complete sedimentary record and many variables, including sea‐level, tectonics, substrate lithology and drainage network characteristics, are accurately constrained. Here, 38 late Quaternary valleys along the coast of two different passive continental margins are compared, which suggests that valley shape and size are governed primarily by upstream, intrinsic controls such as discharge. Valley width, depth and cross‐sectional area are found to be predictable at the highstand shoreline and are scaled with the size of their drainage basin, which has important implications for estimating sediment discharge to continental shelves and deep water environments during periods of low sea‐level.     

OVERVIEW OF THE PHYSICAL DYNAMICS OF THE CONTINENTAL MARGIN / Exposé sommaire de la dynamique physique de la marge continentale

Abstract

Recently acquired knowledge, both observational and conceptual, of the physical dynamics of the continental margin in mid-latitudes is reviewed. The major constituent processes on the continental margin are emphasized, and major unresolved questions are outlined. From this perspective, a few remarks on the interaction of river runoff and continental margin circulation are made.