bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fossils as geological clocks
Authors: Callomon John H. 
Published: April 2008

Abstract

To reconstruct the history of the Earth we need to know
what
happened and
when — events
and their
dates
— and we should like to know
how
it happened and
why — processes
and their
rates
. To date a historical event we need a timescale for reference — a
calendar
— and a means of placing events in this timescale — a
clock
. Direct access to the primary physical calendar, of time measured in years by means of elemental radiometry as clock, is possible in only a minority of geological problems. By far the richest historical source in the Phanerozoic Eon has been the stratigraphical analysis of sedimentary rocks by means of fossils, the approach pioneered by William Smith. The succession of fossil biotae found in the rocks is used to construct the calendar of
relative
time, the familiar geological calendar defining the standard chronostratigraphical timescale still in process of refinement today. Rocks are then dated through time correlations with this scale by means of their
guide
fossils (von Buch) as clocks. The power to measure the rates of geological processes then depends on the time
resolution
achievable by means of fossils, the time
intervals
between distinguishable events, the
finesse
of the calendar.

The present-day state of play is reviewed, both in the refinement of the geological calendar and the finesse that has been attained. Comparison of the geological calendar with our familiar human historical calendar reveals some illuminating parallels as well as some important differences. Illustrative examples are taken from the Jurassic Period (170 Ma BP) and its ammonites as clocks.
Time, life and the Earth
Authors: Manning Aubrey 
Published: April 2008

Abstract
Modern developments in the Earth sciences have revealed, for the first time, how our planet actually ‘works.’ For biologists, they have brought a fresh understanding of the interwoven histories of the physical and the living worlds. In turn, new biological discoveries show how early in Earth’s history life arose and how it was able to flourish even under the apparently hostile conditions of the young solar system. In particular the high ocean temperatures following meteor impacts would not have precluded the successful progress of so-called hyperthermophilic prokaryotic organisms.
The early problems which evolutionary biologists faced, when presented with estimates of the age of the Earth as a few tens of millions of years, have now been replaced by arguments concerning mechanisms underlying the variable rates and patterns of evolution.
Life and the Earth have reciprocally interacted over billions of years and repeatedly planetary processes have led to mass extinctions whose dramatic results can be seen in the fossil record even though the details of such events remain problematical. So far mass extinctions have resulted in increased opportunities for the survivors; the current human-induced extinction is unlikely to do so.
Dating the origin of modern humans
Authors: Stringer Christopher B. 
Published: April 2008

Abstract

One of the most serious past difficulties facing realistic tests of evolutionary models for modern human origins was the lack of widely applicable dating procedures that could reach beyond the practical limits of radiocarbon dating. Moreover, the amount of fossil material that had to be sacrificed to obtain a conventional radiocarbon date meant that human fossils could generally only be dated indirectly through supposedly associated materials. Because of these limitations, the transition from Neanderthals to modern humans in Europe was believed to have occurred about 35 000 radiocarbon years ago, but detailed reconstruction of the processes involved (for example, evolution or population replacement) was not practicable. In the Levant, the transition period from Neanderthals to modern humans was believed to lie only slightly beyond this 35 000-year watershed. Elsewhere, in Africa, east Asia and Australasia, the chronology for modern human origins was even more difficult to establish. However, over the last fifteen years, radiocarbon and non-radiocarbon physical dating techniques such as luminescence and electron spin resonance have been increasingly refined, leading to a revolution in our understanding of the timescale for human evolution, particularly for the last 200 000 years. Although each dating method has its own strengths and weaknesses, the picture that now emerges is one of a gradual evolution of Neanderthal morphology in Europe, in parallel with a similar evolution of modern humans in Africa. Modern humans also appear surprisingly early in the Levant (
c.
100 ka ago) and Australia (
c.
60 ka ago). However, many uncertainties still surround the process of establishment of our species globally.
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract

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

Abstract

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

Abstract
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.