bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluid-assisted large strains in a crustal-scale décollement (Hercynian Belt of South Brittany, France)
Authors: Le Hebel F. , Gapais D. , Fourcade S. , Capdevila R.
Published: April 2008

Crustal deformations often occur along gently-dipping shear zones, where the scale of fluid transfers and the coupling between deformations and fluid availability are important questions. We present a structural and geochemical study of felsic rocks deformed at about 350–400°C within a crustal-scale décollement zone from the Hercynian Belt of South Brittany. At regional scale, deformations resulted in large distributed strains accumulated by dissolution-crystallization of quartz and feldspars. Mass transfers produced veins growing parallel to the foliation. Veins were major precipitation zones, while adjacent rocks were the main zones of dissolution. Deformation patterns, vein mineralogy, and oxygen isotope data show that both the amount of fluids and the scale of fluid flow were limited, and that the dominant transfer mechanism was diffusion. Results suggest that the deformation zone has acted as a trap for early fluids rather than as a syn-kinematic fluid channel.
Dominance of microstructural processes and their effect on microstructural development: insights from numerical modelling of dynamic recrystallization
Authors: Piazolo S. , Bons P. D. , Jessell M. W. , Evans L. , Passchier C. W.
Published: April 2008

The influence of the dominance of different processes on the microstructural development of a quartzite was investigated using the numerical model ‘ELLE’. Dynamic recrystallization of a polycrystalline aggregate was simulated by the concurrent operation of viscous deformation, lattice rotation, subgrain formation, rotational recrystallization, nucleation of new grains from strongly strained grains and recovery. The different observed microstructural characteristics depend on the relative rates at which grain boundary migration, subgrain formation, recrystallization by rotation and nucleation affect the microstructure. Observed sizes of recrystallized grains are significantly influenced by these different relative rates of processes. These rates are determined by parameters that mainly depend on temperature, fluid absence or presence, shear stress and strain rate. Therefore, the specific conditions at which deformation took place have to be taken into account if recrystallized grain sizes are used for palaeopiezometry. Comparison and combination of our results with experimental data and observations in natural examples provide the possibility of interpreting microstructures quantitatively in terms of temperature and shear strain rate.
Strain localization by fracturing and reaction weakening — a mechanism for initiating exhumation of subcontinental mantle beneath rifted margins
Authors: Handy M. R. , Stünitz H.
Published: April 2008

Rift-related strain localization in spinel lherzolite from an exhumed passive continental margin in the Southern Alps involved two stages. (1) Critical fracturing coincided with heterogeneous nucleation of plagioclase, olivine, and hornblende aggregates to form discrete, ultrafine-grained (0.5–0.6 µm) shear zones oriented at high angles to the pre-existing foliation in the host rock. The syntectonic replacement of spinal lherzolite by lower pressure, plagioclase-hornblende lherzolite documents extensional exhumation under high temperature (700–900 °C) conditions accompanied by limited fluid infiltration. Deformation involved a combination of dislocation creep (ol) and diffusion-accommodated viscous granular flow (plag, ol, hbl aggregates). (2) Hydrous deformation at lower temperatures (200–400 °C) involved the formation of serpentine-chlorite mylonite and cataclasite along discrete, anastomozing shear zones oriented at low angles to the pre-existing foliation. Both stages involved drastic weakening, particularly once the shear zones coalesced subparallel to the extensional shearing plane. The top of the lithospheric mantle was initially strong, but is inferred to have become weaker than both the underlying mantle and the overlying mafic lower crust. The interconnection of such strong-then-weak delamination zones to form trans-lithospheric extensional shear zones accelerated rifting and led to the exhumation of subcontinental mantle during the late stages of continental breakup.
GPR stratigraphy used to infer transgressive deposition of spits and a barrier, Lake Bonneville, Stockton, Utah, USA
Authors: Smith Derald G. , Simpson Chris J. , Jol Harry M. , Meyers Richard A. , Currey Donald R.
Published: April 2008

Ground penetrating radar (GPR) stratigraphic profiles of the classic cross-valley barrier and associated spits of Late Pleistocene Lake Bonneville, near Stockton, Utah, are used to infer transgressive depositional style and internal sedimentary structures. From onlapping patterns of radar reflections, which mimic subsurface stratigraphy, we reconstruct the following depositional sequence and style: (1) at the north end of the Rush Valley, the barrier formed by vertical accretion while keeping pace with hydro-isostatic-forced basin subsidence and/or slow lake-level rise; (2) a reorientation of the longshore transport pathway, induced by continued basin subsidence and/or a lake-level rise, produced a spit that prograded 2.5 km southwestward into Rush Valley. The NW-dipping radar reflections from the spit onlap SE-dipping reflections from the back-barrier, indicating that this spit was deposited after the barrier; (3) a final rise in lake level and/or basin subsidence again reoriented longshore transport and deposited the smaller upper spit. Radar reflections from the upper spit onlap the proximal eastern margin of the Stockton spit. This upper spit is the final landform deposited during the Bonneville highstand. The depositional sequence inferred from radar stratigraphy agrees with the transgressive hypothesis formulated in 1890 by G. K. Gilbert.
Structure of a Pleistocene push moraine revealed by GPR: the eastern Veluwe Ridge, The Netherlands
Authors: Bakker Marcel A. J. , Van Der Meer Jaap J. M.
Published: April 2008

The classic idea on the architecture of the push moraines of The Netherlands is one of imbricated thrusts. This study shows a more complete model of the structural style within a push moraine. We have mapped three glaciotectonic styles using 50 MHz ground penetrating radar (GPR).

The moraines are composed of coarse-grained unconsolidated sediments of fluvial and glaciofluvial origin. Maximum penetration depth is about 45 m. In a 12 km cross-section over the eastern Veluwe Ridge we have encountered the following glaciotectonic styles. Style I is found near the former ice margin and consists of imbricated thrust sheets, with thrust planes dipping towards the former glacier. Style II, further away from the glacier, is a combination of folds and thrust structures. The folds have a wavelength of up to 200 m and an amplitude of 40 m. Style III is almost entirely composed of large-scale folds with wavelengths of 400 m maximum. Toward the most distal parts of the ridges the folds flatten out.

The study shows that GPR is capable of imaging large-scale glaciotectonic structures, resulting in a factual model of the eastern Veluwe push-moraine architecture and better insight into postglacial denudation amounts and push-moraine genesis.
The recognition of barrage and paludal tufa systems by GPR: case studies in the geometry and correlation of Quaternary freshwater carbonates
Authors: Pedley Martyn , Hill Ian
Published: April 2008

Tufas provide virtually the only sedimentary and proxy-environmental records within karstic terrains. However, they are difficult to access. Shallow geophysical prospecting techniques, such as resistivity and shallow seismic reflection, fail to define the often complex internal bedform details in tufa deposits and many deposits appear too well lithified to auger-sample. Nevertheless, the application of ground penetrating radar (GPR) permits the recognition of up to five distinct types of radar reflectors that can be directly related to distinct lithologies commonly seen in tufa cores: (1) well-lithified phytoherms produce sharp, sinuous and often complexly truncated bright signals; (2) soft lime muds produce subhorizontal, laterally continuous lower contrast (dull) laminar bedform signals; (3) organic-rich deposits (sapropels and peats) produce poorly focused dull responses, often with internal ‘noise’; (4) the tops of bladed and coarse-grained deposits, such as flint gravel, give a strong bright signal; and (5) the associated presence of clay-grade lime silts and muds within the top of gravel beds produces the same top-bed signal as 4, but internal details of the deposit are masked and a remarkably homogeneous dull signal response is typical throughout the lower parts of the deposit.

From these GPR responses it is possible to make meaningful three-dimensional comparisons of the internal geometries of Holocene tufa deposits. Problematic tufa deposits in the valleys of the Derbyshire Wye and the Hampshire Test, UK, are presented to illustrate the universal value of GPR surveying for fresh-water carbonate recognition and for providing key information on valley-bottom resurgence locations.
Amplitude analysis of repetitive GPR reflections on a Lake Bonneville delta, Utah
Authors: Kruse Sarah E. , Jol Harry M.
Published: April 2008

Several recent theoretical studies have documented the sensitivities of the amplitude and waveform of a ground penetrating radar (GPR) reflection to the contrast in electromagnetic properties across the reflecting contact. Here we show that, in a setting with repetitive layering, it is possible to place constraints on conductivity and variations in permittivity within layers and across layer boundaries. The data set consists of 50 MHz, 100 MHz and 200 MHz profiles that image subparallel dipping bedding planes in a gravelly deltaic forest facies on a Lake Bonneville delta, Utah, USA. Strongly reflecting horizons with 1–2 m spacings bound packages with finer internal layering. From finite-difference time-domain (FDTD) simulations of radarwave propagation through such strata, constraints are placed on the variations in permittivity across larger-scale and finer-scale layering. Modelling the relative amplitudes of reflections demonstrates that the finer-scale permittivity contrasts are ∼0.4–0.8 times that of the 1–2 m layering. Amplitude v. offset (AVO) analysis yields an upper bound of ∼3.5 for the contrast in permittivity at the larger-scale layer boundaries. Overall signal attenuation requires the average conductivity to be in the range of 0.7–0.8 mS/m.
Introduction
Authors: Jones Stuart J. , Frostick Lynne E.
Published: April 2008

The stratigraphic record stored in sedimentary basins had traditionally been interpreted in terms of tectonic subsidence, climate change and sea-level rise or fall. During the last decade, however, the fundamental control of sediment flux in creating sedimentary sequences, and thus the need to understand variations in sediment flux in order to interpret the sedimentary record, has been recognized. Sediment flux is a first-order control on the pattern and distribution of sedimentary facies in depositional basins. Consequently, basin fills reflect the sediment flux across their margins and the hinterland conditions driving the sediment flux provide evidence of dynamic geomorphic processes. In context, the drainage systems and sedimentary basin can be regarded as a ‘production line’ with the sedimentary record giving valuable insight into long-term landscape evolution and geomorphological processes illuminating the evolution of sedimentary basins. This special publication contains a set of papers on sediment supply to basins, with a focus on clastic sediments. It presents a mix of hinterland and sedimentary basin studies with a gradation from orogenic belts to the deep marine. The papers present a current perspective on controls and constraints on sediment supply, a model and empirically based driven understanding of sediment flux and the interaction of geomorphology, landscape evolution and sedimentary geology to provide a more complete picture of the Earth system.
Quantification of river-capture-induced base-level changes and landscape development, Sorbas Basin, SE Spain
Authors: Stokes M. , Mather A. E. , Harvey A. M.
Published: April 2008

The Aguas/Feos river system of the Sorbas Basin, SE Spain was captured by an aggressive subsequent stream
c.
100 ka. The consequence of the capture event was twofold: (1) basin-scale drainage reorganization via beheading of the southward flowing Aguas/Feos system and re-routing the drainage eastwards into the Vera Basin; and (2) the creation of a new, lower base level and associated upstream propagation of a wave of incision.

The sequence of pre- and post-capture events are well established from previous studies of the Quaternary terrace record. Using these studies, this paper makes the first attempt to quantify the impact of river capture in terms of spatial and temporal variations in rates of incision, sediment flux and surface lowering. This was carried out through construction of 43 valley cross-sections from the ‘captured’ (Upper Aguas), ‘beheaded’ (Feos) and ‘capturing’ streams (Lower Aguas) within the central-southern parts of the Sorbas Basin. Dated pre- and post-capture terrace and corresponding strath levels were plotted on to the valley cross-sections enabling incision amounts, rates and valley cross-sectional areas to be calculated. Sediment fluxes were calculated using a mean valley section method. Surface lowering calculations were made through reconstruction of the top basin-fill surface and subtraction from the modern contour values.

The lowering of base level has resulted in a dramatic increase in incision upstream of the capture site by a factor of 4 to 20. This in turn has been associated with significant pre- and post-capture changes in valley shape. The increased incision resulted in dramatic post-capture increases in valley erosion upstream and downstream of the capture site by a factor of 2 to 9 which can be related to changes in associated stream power as a function of increased gradient and discharge. In excess of 60% of the landscape change can be accounted for by valley-constrained erosion as opposed to overall surface lowering.
Tracer pebble entrainment and deposition loci: influence of flow character and implications for riffle-pool maintenance
Authors: Milan D. J. , Heritage G. L. , Large A. R. G.
Published: April 2008

This paper considers the influence of flow character upon scour and deposition loci of tracer clasts in a gravel-bed river, and discusses implications for riffle-pool maintenance. Overall, bars were found to be the dominant depositional zones where over 54% of the tracer clasts accumulated during a 13-month period, followed by riffles (31%) and, finally, pools (<15%). Variability in the location of scour and deposition zones were apparent and could be broadly linked to four flow categories: (i) low-magnitude, high-frequency flows below 29% bankfull appeared responsible for intra-unit re-distribution of sediment particles; (ii) medium-magnitude and -frequency flows (up to 70% bankfull) appeared capable of inter-unit transfer, with pool scour and immediate deposition on riffle heads downstream, and some movement from riffles to bar edges and heads; (iii) high-magnitude, low-frequency flows (70–90% bankfull) appeared capable of riffle-riffle transport, with routing around bar edges; and (iv) very high-magnitude, very low-frequency flows (bankfull and over) capable of bar to bar transport and clast transport distances exceeding the length of a single riffle-pool unit. Tracers originating from riffles do not appear to be fed into pools on the outside of meander bends, instead they appear to be routed over shallower bar surfaces. High competence and low sediment supply explains the coarse nature of the pools (
D
50
= 110 mm) in comparison to the riffles (
D
50
= 85 mm). An improved understanding of the sediment transport mechanisms operating during different flood types is needed to better predict morphological response to changes in hydrological regime and sediment supply.
A fractionation model for sediment delivery
Authors: Tipper John C.
Published: April 2008

The delivery of sediment into basins can readily be studied using process-independent models. The simplest such model is the discrete fractionation model in two dimensions, without erosion. It represents the path down which sediment is delivered as a simple set of discrete steps, and assumes that the sediment is delivered down those steps in a series of discrete events. This model has two parameters — the feed volume and the fractionation coefficient. The feed volume is the volume of sediment fed to the first step in the delivery path, and the fractionation coefficient is the proportion of the sediment reaching a step that is then moved on. Several variants of this model are described here, each involving restrictions on one or both of these parameters. The model has obvious and important applications, but these will be difficult to realize until the problem of parameter estimation has been solved. Fortunately, it seems possible to solve this problem by removing some of the model’s restrictions; these were built into it initially for analytical reasons. Removing these restrictions also makes the model applicable to a wider range of sedimentation systems.
The history of sediment flux to Atchafalaya Bay, Louisiana
Authors: McManus John
Published: April 2008

The Atchafalaya became a free-flowing river system in the mid-nineteenth century. It progressively increased its diversion of waters from the Mississippi until, by the mid-twentieth century, it threatened to capture the total flow of the main river. The installation of control structures has, temporarily at least, prevented total diversion, limiting the flows to 30% of the Mississippi; water discharge and sediment transport into the Atchafalaya have led to deposition throughout the basin. A major system of lakes and bayous has become largely filled with deltaic sands and layered silts, so that the waters currently drain almost directly into the Atchafalaya Bay. The natural outlet and one artificially created outlet each carry waters from the system. As a result of major flooding in 1973, sediment was flushed from the Atchafalaya Basin and began the subaerial emergence of deltas, which had been slowly building below the bay water surface since the 1950s. The two deltas have provided, parallel growth one largely in natural form, and the other substantially modified by the dredging needed to maintain a navigable routeway to Morgan City and the interior of the basin. The sub-aerial deltas initially grew rapidly, a process apparently slowing, but the sediment transported to the depositional areas has not diminished, rather the area of deposition within the receptor basin is increasing, so that apparent growth inevitably slows. By 1994 the two deltas occupied an area of more than 153 km
2
above the −0.6 m measuring datum, created in approximately 30 years. In each case the sediments show upward passage from fine clays to laminated silts and fine sands, within a succession rarely more that 3 m in thickness, as defined by the original 1890 bathymetry of the Bay. Within the Bay itself resuspension of deposited fine sediment due to wave activity and water set-up and set-down in response to variations in the directions of coastal winds creates plumes of material that travel through the basin and may leave to form suspended sediment plumes on the inner parts of the continental shelf. A total of 20–25 such plumes annually drift initially east of the Bay outlet but later move west towards the chenier coasts of western Louisiana and Texas.
Late Quaternary turbidite input into the east Mediterranean basin: new radiocarbon constraints on climate and sea-level control
Authors: Reeder Michael S. , Stow Dorrik A. V. , Rothwell R. Guy
Published: April 2008

The Late Pleistocene-Holocene (0–30 ka
bp
) allochthonous sedimentation in the Herodotus Basin of the eastern Mediterranean has been controlled, in part, by a combination of regional climatic change and eustatic sea-level fluctuation. A new series of radiocarbon dates, made on planktonic foraminifers and pteropod shells taken from the pelagic and hemipelagic intervals between individual turbidite units, has given bracketing dates for each major turbidity current event that deposited sand and mud on the Herodotus Basin plain. Two partly independent cycles are evident. Climate-induced cycles have lead to an alternation of periods of turbidites sourced from the Nile delta-fan system with those from the North African shelf and Anatolian rise. These correlate with pluvial and inter-pluvial climatic periods recognized in the Nile hinterland. Sea-level cycles have tended to focus turbidite emplacement, from whatever source, at periods of sea-level fall within the latest Wisconsin and sea-level rise from the Wisconsin-Holocene period. In addition to the Herodotus Basin Megaturbidite (HBM) described previously, six other beds with volumes in excess of 25 km
3
and wide lateral extent across the basin can be termed megaturbidites. There is no simple sea-level or climate control on the timing of these events, so we must conclude that triggering and emplacement of megaturbidites is independent and variable.
From graphical display to dynamic model: mathematical geology in the Earth sciences in the nineteenth and twentieth centuries
Authors: Howarth Richard J.
Published: April 2008

Graphical displays were used early in geophysics and crystallography, mineralogy, petrology and structural geology by the early 1800s, but nineteenth-century geology obstinately remained mainly descriptive. Charles Lyell’s quantitative classification of the Tertiary Sub-Era in 1828 was a notable exception. Nevertheless, by 1920 the quantitative approach had become established. W. C. Krumbein, who introduced the computer into geology in 1958, encouraged use of probabilistic sampling and process-response models. Early work focused on databases, statistical data analysis and display. By the 1970s, stochastic simulation, deterministic modelling and spatial ‘geostatistics’ (pioneered by Matheron and his co-workers), were of growing importance. The introduction of the personal computer and the graphical user interface in the 1980s brought well-proven quantitative methods out of the research environment onto the workbench and into the field. Since the mid-1980s, the analysis, display and modelling of behaviour in three dimensions, underpinned by spatial statistics, computational fluid-flow, visualization technology, etc., has proved of economic benefit to mining, petroleum geology and hydrogeology. Other, computationally intensive, methods likely to be of importance in the Earth sciences are the application of ‘robust’ statistical methods, increasing use of Bayesian methods in uncertainty (risk) estimation (as a result of a renewed interest in statistical intervals and forecasting), and computational mineralogy.
Norman Levi Bowen (1887–1956) and igneous rock diversity
Authors: Young Davis A.
Published: April 2008

By the beginning of the twentieth century, differentiation had emerged as the leading theory to explain the chemical and mineralogical diversity of igneous rocks. Soret diffusion, liquid immiscibility, compositional stratification of magma by gravity (Gouy-Chaperon effect), volatile transport, crystal settling and other processes had been advocated as mechanisms of differentiation, but no consensus was achieved regarding a dominant mechanism.

During a career spent primarily at the Geophysical Laboratory, Washington DC, Norman Levi Bowen (1887–1956) initiated a new approach to petrology. On the basis of experimental studies of rock-forming silicates and on physicochemical principles, Bowen argued against the importance of Soret diffusion, liquid immiscibility, volatile transport and assimilation as major causes of diversity. He formulated a comprehensive theory of differentiation that emphasized the role of separation of crystals from liquid. He reasoned that rocks of the subalkaline igneous rock series, including granite, have been derived from parental basalt by crystal separation, e.g. settling, filter-pressing or armouring of crystals.

Apart from its scientific merits, Bowen’s achievement rested on personal, institutional and technical factors that included his determination to dedicate virtually his entire career to solution of the problem of igneous rock diversity; his affiliation with the Geophysical Laboratory; the prior development of the quenching method and the calibration of the temperature scale to very high temperatures; and the influence of Arthur L. Day.
Metamorphism today: new science, old problems
Authors: Touret Jacques L. R. , Nijland Timo G.
Published: April 2008

A concise history of the discipline of metamorphic petrology is presented, from the eighteenth-century concepts of Werner and Hutton to the end of the twentieth century.
Plate tectonics, terranes and continental geology
Authors: Le Grand Homer E.
Published: April 2008

The ‘modern revolution’ in the Earth sciences is associated with the emergence of plate tectonics in the late 1960s. The assumption that the crust of the Earth was composed of a small number of rigid, non-deformable, mobile plates enabled a quantitative, kinematic description of current geological processes and reconstructions of past plate interactions. The simple model of plate theory
c.
1970, for example its depiction of a subduction zone, has since undergone considerable refinement. However, some geologists, especially those concerned with questions of continental tectonics, contend that plate theory in its current form is of limited value in addressing questions of continental tectonics, and prefer to employ the concept of allochthonous terranes in characterizing, describing and interpreting regional geology. These geologists may understandably take the view that plate tectonics is a kinematic grand generalization but thus far not particularly useful in making sense of the rocks at the local level.
From terrestrial magnetism to geomagnetism: disciplinary transformation in the twentieth century
Authors: Good Gregory A.
Published: April 2008

In 1900, researchers interested in Earth’s magnetism generally proclaimed all facets of magnetic phenomena to be within their purview. Most researchers in this field referred to themselves as ‘magneticians’ first and physicists or geologists second. After World War II, specialization increased. A number of distinct research areas appeared over several decades: the geodynamo theory and the study of the core-mantle boundary; palaeomagnetism and its growing connection to geology; the production of induced fields in Earth’s crust; and, among others, the electromagnetic phenomena of the upper atmosphere and near space. The former unity dissolved and the field fragmented. One result of fragmentation has been a loss of memory and a consequent misinterpretation of an important part of the history of geoscience. This paper relates the challenges of recovering a history obscured by later events.
Diversification and biogeography of bivalves during the Ordovician Period
Authors: Cope John C. W.
Published: April 2008

Bivalves have a wide distribution in the Lower and to a lesser extent Middle Cambrian rocks, but they have not yet been certainly identified in the Upper Cambrian. Recent discoveries have significantly increased our knowledge of Lower Ordovician bivalve faunas and their explosive radiation from the Early Ordovician apparently coincides with the evolution of the feeding gill. Early Ordovician faunas were confined to the siliciclastic facies of Gondwanan shelf seas; most genera were clearly latitudinally constrained, but others apparently migrated over wide latitudes. By the Mid-Ordovician, bivalves had begun to escape the confines of Gondwana and marked latitudinal differences in the composition of the faunas became apparent, with pteriomorphians showing clear preference for low latitudes, whilst heteroconchs clearly preferred median to high latitudes; surprisingly, nuculoids were both most diverse in terms of species and most abundant as a percentage of individuals within the bivalve populations at low latitudes. It was in the Late Ordovician that bivalves colonized the low-latitude carbonate platforms of Laurentia and Baltica, leading to a second major diversification particularly within the pteriomorphian bivalves, which developed semi-infaunal and epifaunal habits; they became the dominant low-latitude bivalve group. The latest-Ordovician eustatic regression which exposed the low-latitude carbonate platforms resulted in a major reduction in the epifaunal and semi-infaunal bivalves involving extinction of many genera, including the only Ordovician boring bivalves.
A brief history of the Indus River
Authors: Clift Peter D.
Published: April 2008

The Indus River system is one of the largest rivers on the Asian continent, but unlike the Ganges-Brahmaputra system, the drainage of the Indus is dominated by the western Tibetan Plateau, Karakoram and tectonic units of the Indus Suture Zone, rather than the High Himalaya. The location of the river system relative to the Indus Suture Zone explains the deep exhumation north of that line in the Karakoram, compared with the modest erosion seen further east in Tibet. The modern Indus cuts Paleogene fluvial sedimentary rocks of the Indus Group located along the Indus Suture Zone in Ladakh, northern India. After the final marine incursion within the Indus Group in the early Eocene (<54.6 Ma), palaeo-current indicators changed from a north-south flow to an axial, westward pattern, synchronous with a marked change in sediment provenance involving erosion of South Tibet. The Indus probably was initiated by early Tibetan uplift following the India-Asia collision. The river has remained stationary in the suture since Early Eocene time, cutting down through its earlier deposits as they were deformed by northward folding and thrusting associated with the Zanskar backthrust at
c.
20 Ma. The Indus appears to have been located close to its present position within the foreland basin since at least Mid-Miocene time (
c.
18 Ma), and to have migrated only
c.
100 km east since Early Eocene time. In the Arabian Sea Paleogene fan sedimentation was significant since at least Mid-Eocene time (
c.
45 Ma). Sediment flux to the mid fan and shelf increased during Mid-Miocene time (after 16 Ma) and can be correlated with uplift of the Murray Ridge preventing sediment flow into the Gulf of Oman, tectonic uplift and erosion in the Karakoram and western Lhasa Block, and an enhanced monsoon triggered by that same uplift. Sedimentation rates fell during Late Miocene to Recent time. The Indus represents 18% of the total Neogene sediment in the basins that surround Asia, much more than all the basins of Indochina and East Asia combined (
c.
11%). Unlike the rivers of East Asia, which have strongly interacted as a result of eastward propagating deformation in that area, the Indus has remained uninterrupted and represents the oldest known river in the Himalayan region.
Monsoon-driven export fluxes and early diagenesis of particulate nitrogen and its δ
15
N across the Somalia margin
Authors: Brummer G. J. A. , Kloosterhuis H. T. , Helder W.
Published: April 2008

Settling nitrogen fluxes intercepted by sediment traps on the mid-slope and in the deep basin off Somalia show a consistent annual range of 3.4 ± 0.2‰ in their stable isotope composition. Seasonal minima in δ
15
N of 3.7‰ are associated with the moderate N fluxes derived from coastally upwelled water, which is rapidly carried offshore along eddy margins passing over the mooring sites during the SW monsoon (June–September). Coastal upwelling, offshore transport and deep wind mixing cease at the end of the SW monsoon, leading to enhanced utilization of the up to 20 μM of NO
3
−
in the photic layer, maxima in the N export flux, and an increasing δ
15
N by Rayleigh distillation. Yet as stratification develops, nutrient exhaustion follows and export production collapses as the δ
15
N increases to over 7‰. Cyanobacterial N
2
fixation probably diminishes the δ
15
N by 0.4–1.6‰ during the autumn intermonsoon (November–December) when settling N fluxes are lowest. Nutrient utilization remains high during the NE monsoon (January–March), when nutrient entrainment by deep wind mixing results in enhanced N export with maxima in δ
15
N of up to 7.4‰. Annual N fluxes have virtually the same δ
15
N of 6.0‰ in all traps despite considerable differences in both N flux and δ
15
N between the traps during the year and at different depths. In comparison with the annual δ
15
N of 6.0‰ arriving on the sea floor, core-top sediments are enriched by +0.6‰ on the upper slope (at 487 m) increasing to +2.9‰ in the deep basin (at 4040 m), whereas the N sediment burial efficiency declines from about 17% to 3%. It appears that the extent of oxic decomposition at the sediment-water interface is the most likely cause of such isotope enrichment. Similar positive gradients in δ
15
N with bottom depth have been reported from other continental margin transects and are generally attributed to increased nutrient utilization in the photic ocean with distance offshore. As for Somalia, nitrogen isotope fractionation as a result of oxic decomposition on the bottom rather than nutrient utilization at the ocean surface may account for the observed increase of sedimentary δ
15
N down continental margins in general.
Formation of varve-like laminae off Pakistan: decoding 5 years of sedimentation
Authors: Lückge A. , Reinhardt L. , Andruleit H. , Doose-Rolinski H. , von Rad U. , Schulz H. , Treppke U.
Published: April 2008

We studied Holocene sediments from the northeastern Arabian Sea near Pakistan, which were obtained from the same location in 1993 and 1998, to determine the composition and origin of laminated sediments for this 5 year time interval. Methods included geochemical, sedimentological and palaeontological analyses. We then compared our results with meteorological records, and satellite and sediment trap data. We suggest that short-term (few days) heavy rainfall periods in the hinterland and at the coast lead finally to flood events causing the deposition of light-coloured layers as event deposits on the continental slope. These layers are characterized by low percentages of biogenic compounds (i.e. organic matter, coccoliths and diatoms) and interpreted to have been deposited mainly during the winter season, when heavy rainfall can be expected. The thickness of the light layers seems to be related to the intensity of precipitation during a single flood event. In the 1997–1998 El Niño year, which was characterized by the strongest anomalies for the last 20 years, the thickest layer was deposited. The dark layers accumulate over the remaining larger part of the year and are characterized by an elevated input of biogenic material (marine organic matter, skeletal opal, foraminifera and coccoliths).
Discovery of the Toba Ash (
c.
70 ka) in a high-resolution core recovering millennial monsoonal variability off Pakistan
Authors: Von Rad Ulrich , Burgath Klaus-Peter , Pervaz Muhammad , Schulz Hartmut
Published: April 2008

A discrete Toba Ash layer in the northeastern Arabian Sea was detected near the base of a 20.2 m long piston core (289KL) recovered from the oxygen minimum zone off the Indus delta. In addition to the Toba Ash, we discovered two highly disseminated, vitreous, rhyolitic ‘ash layers’ in two annually laminated box cores: a ‘Younger Ash’ (about
ad
1885–1900), and an ‘Older Ash’ (about
ad
1815–1830). The glass shards were probably derived from eruptions of Indonesian volcanoes, although it was not possible to correlate these two ashes with well-known historical eruptions. We discuss source, transport and deposition of distal ash-fall layers in the Arabian Sea, which are derived from violent ultra-Plinian eruptions on the Indonesian volcanic archipelago, as well as their use for palaeoclimatic correlation. Core 289KL has a complete, high-resolution stratigraphic record of the past 75 ka with 21 interstadials (IS) or Dansgaard-Oeschger (D-O) cycles and equivalents of Heinrich events H1–H6. The high-frequency record of this core shows rapid climate oscillations with periods around 1.5 ka and can be tuned precisely to the δ
18
O record of a Bay of Bengal core and to the GISP-2 ice core from Greenland. The Toba event (70 ± 4 ka
bp
), which is well documented in the Arabian Sea and Bay of Bengal records at the end of IS-20, as well as in the Greenland ice, is an excellent stratigraphic marker horizon to validate this correlation. The apparent synchronous appearance of the various D-O oscillations and Heinrich events, which has been documented for many northern hemisphere localities, can be explained only by fairly rapid atmospheric circulation changes. Changes in the intensity of the Indian summer monsoon are tightly coupled with suborbital climate oscillations in the northern hemisphere via atmospheric moisture and heat circulation.
The seasonal and vertical distribution of living planktic foraminifera in the NW Arabian Sea
Authors: Peeters Frank J. C. , Brummer Geert-Jan A.
Published: April 2008

The NW Arabian Sea is characterized by a strong seasonal contrast in surface water hydrography. During the SW monsoon of 1992, we encountered strong coastal upwelling characterized by low sea surface temperatures (SST), high nutrient concentrations, a shallow thermocline and a near-surface chlorophyll maximum. By contrast, the hydrography during the NE monsoon of 1993 was characterized by a relatively warm nutrient-depleted surface mixed layer and a deep chlorophyll maximum. We show that the faunal composition, depth habitat and abundance of living planktic foraminifera respond to the hydrographic changes controlled by the seasonally reversing monsoon system. Total shell concentrations (> 125 μm) ranged from 4 to 332 individuals (ind.) m
−3
during upwelling and from 3 to 85 ind. m
−3
during the non-upwelling season. During upwelling, the fauna was dominated by
Globigerina bulloides
. During non-upwelling the fauna was characterized by relatively high concentrations of tropical symbiont-bearing species such as
Globigerinoides ruber, Globigerinoides sacculifer
and
Globigerinella siphonifera
, whereas concentrations of
Globigerina bulloides
were an order of magnitude lower. Factor analysis on 15 species yields an upwelling assemblage (UA), a tropical assemblage (TA) and a subsurface assemblage (SA). A fourth factor represents the distribution of the species
Globigerina falconensis
, which is mainly found in subsurface waters during the non-upwelling period (NE monsoon). A model is presented to calculate the base of the productive zone from the vertical shell concentration profile of a given species. The model is validated by comparing the range in calcification temperatures of
G. bulloides
, derived from its δ
18
O, with the
in situ
sea-water temperature range of the productive zone as predicted from the model. It appears that shell growth (calcite precipitation) is restricted to the productive zone as defined by this method. The average calcification temperature of
G. bulloides
corresponds to the point of maximum change in the shell concentration profile (i.e. the inflection point). For most shallow-dwelling species, the inflection point is found at or below the depth of the chlorophyll maximum, although above the main thermocline. This study indicates that the depth habitat and abundance of different species varies seasonally. Consequently, the abundance and stable isotope composition of specimens in the fossil record reflects a mixture of specimens that were produced at various depths during the different seasons.
Planktic foraminifera, particle flux and oceanic productivity off Pakistan, NE Arabian Sea: modern analogues and application to the palaeoclimatic record
Authors: Schulz Hartmut , von Rad Ulrich , Ittekkot Venugopalan
Published: April 2008

We use the flux of bulk sediment (CaCO
3
, biogenic opal, organic carbon, lithogenic material), and of planktic foraminifera (PF) and other shell-bearing plankton from sediment trap EPT-2 off Pakistan to (1) constrain the seasonal pattern of regional productivity and (2) search for indications of the NE monsoon winter situation that may serve as a modern analogue to better reveal the seasonal climatic signals preserved in the sedimentary record of the Arabian Sea. Our trap data show a clear seasonality of fluxes that can also be traced in the composition of non-bioturbated (varved) summer and winter sediment laminae preserved within the oxygen minimum zone. In EPT-2, the flux of PF is low during summer, but during winter and late spring it is higher, as at trap station WAST, in the upwelling area of the western Arabian Sea.
Globigerina bulloides
, a PF species linked to summer upwelling and high productivity, is of minor importance off Pakistan. In contrast,
Globigerina falconensis
dominates in flux and relative abundance, and is indicative of winter mixing, when NE monsoonal winds cool the highly saline surface waters and break up stratification. An enhanced horizontal flux of suspended sediments stirred up on the shelf and upper slope is clearly shown by the peak in occurrence of small benthic foraminifera during winter. Altogether, our data suggest that the particle flux in the northeastern Arabian Sea is determined by local sediment resuspension and winter productivity rather than by summer monsoonal upwelling, representing a ‘non-upwelling’ environment, in contrast to the ‘summer upwelling’ regime off Oman, Somalia and southern India. We used this evidence to reconstruct the seasonal intensity of both monsoons for the past 25 ka: the SW and NE monsoon both were weak during the last glacial period. The NE monsoon peaked during the cool phases of the glacial to interglacial climatic transition (i.e. during the Younger Dryas (YD) and Heinrich Event H1). The SW monsoon was reinforced after the YD. Both monsoons were enhanced during early Holocene time, when summer insolation and hence atmospheric forcing was at a maximum.
Scotland’s denudational history: an integrated view of erosion and sedimentation at an uplifted passive margin
Authors: Hall Adrian , Bishop Paul
Published: April 2008

Denudational history is commonly reconstructed from basin sediments derived from the denuded source area, and less frequently from the source area itself. Northern Britain is an important source area for the surrounding sedimentary basins and this paper reviews the erosional history of Scotland from Devonian time to the present using evidence both from onshore geology and geomorphology and from patterns of sedimentation in surrounding basins. Cover rocks were extensive in Scotland during late Palaeozoic time but the persistence of sediment source areas within the upland areas of Scotland makes it unlikely that basement highs were ever completely buried, and depths of post-Devonian erosion of basement have been correspondingly modest (< 1–2 km). During Mesozoic time, Scotland experienced several major erosional cycles, beginning with uplift, reactivation of relief and stripping of cover rocks, followed by progressive reduction of relief through etchplanation and culminating in extensive marine transgressions in Late Triassic, Late Jurassic and Late Cretaceous time. Mid-Paleocene pulses of coarse sediment to the Moray Firth Basin coincided with major uplift. This uplift was associated with major differential tectonics within the Highlands, with warping and faulting along the margins of the Minch and the inner Moray Firth Basins. Tectonic activity was renewed on a lesser scale in late Oligocene time and continued into Late Neogene time. Differential weathering and erosion under the warm to temperature humid climates of Neogene time created the major elements of the preglacial relief, with formation of valleys, basins, scarps and inselbergs, features often closely adjusted to lithostructural controls and, in some cases, with precursors that can be traced back to Devonian time. The history that can be ‘read’ from the onshore region complements the source area history interpreted from sedimentary basins derived from these areas.
Late Neogene development of the UK Atlantic margin
Authors: Stoker M. S.
Published: April 2008

The late Neogene (Pliocene-Holocene) interval witnessed a significant change in sedimentation style across the UK Atlantic margin that culminated in its present morphological expression. The onset of change is marked by the creation of a regional, angular, erosional unconformity that can be traced from the Hebrides and West Shetland margins into the adjacent deep-water basins of the Rockall Trough and Faeroe-Shetland Channel. In the Rockall Trough, the unconformity is a submarine erosion surface that is dated as early Pliocene in age, betweenc.3.85 and 4.5 Ma. On the Hebrides and West Shetland margins, the dating of the unconformity is slightly less well constrained and spans the latest Miocene(?)-early Pliocene (c.5.5–3.8 Ma) interval. The formation of this unconformity may have resulted from the seaward tilting and subsidence of the shelf margin, which may have further modified the oceanographic circulation pattern in the adjacent basins. The sedimentary response to this event was relatively quick in the deep-water basins, which preserve a record of early Pliocene (post 3.85 Ma) to Holocene sediment-drift accumulation, albeit with a shift in the focus of sedimentation relative to the underlying strata. In contrast, major prograding wedges, which have contributed extensively to the construction of the Hebrides and West Shetland margins and to a lesser extent the Rockall Bank, date essentially from late Pliocene time and largely correlate with the influx of ice-rafted material to the margin. However, indications for a restricted lower Pliocene component to the shelf-margin succession suggest that this apparent delay or lag in sedimentation, relative to the basins, may be a natural response to the rate of denudation of the adjacent landmasses. The regional observations off NW Britain support the concept of Neogene tectonic uplift.
Ridge-plume interaction in the North Atlantic and its influence on continental breakup and seafloor spreading
Authors: Smallwood J. R. , White R. S.
Published: April 2008

Development of the rifted continental margins and subsequent seafloor spreading in the North Atlantic was dominated by interaction between the Iceland mantle plume and the continental and oceanic rifts. There is evidence that at the time of breakup a thin sheet of particularly hot asthenospheric mantle propagated beneath the lithosphere across a 2500 km diameter region. This event caused transient uplift, massive volcanism and intrusive magmatism, and a rapid transition from continental stretching to seafloor spreading. Subsequently, the initial plume instability developed to an axisymmetric shape, with thec.100 km diameter central core of the Iceland plume generating 30–40 km thick crust along the Greenland-Iceland-Faroes Ridge. The surrounding 2000 km diameter region received the lateral outflow from the plume, causing regional elevation and the generation of thicker and shallower than normal oceanic crust. We document both long-term (10–20 Ma) and short-term (3–5 Ma) fluctuations in the temperature and/or flow rate of the mantle plume by their prominent effects on the oceanic crust formed south of Iceland. Lateral ridge jumps in the locus of rifting are frequent above the regions of hottest asthenospheric mantle, occurring in both the early history of seafloor spreading, when the mantle was particularly hot, and throughout the generation of the Greenland-Iceland-Faroes Ridge.