bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deformation histories and structural assemblages of glacier ice in a non-steady flow regime
Authors: Lawson Wendy J. , Sharp Martin J. , Hambrey Michael J. 
Published: April 2008

Abstract
Deformation histories of ice exposed at various locations on the centreline in the ablation area of surge-type Variegated Glacier were estimated using a technique based on evaluation of strain-rate fields derived from velocity gradients. The analysis indicates that ice exposed in the upper part of the ablation area a few years after the 1982–83 surge had experienced a relatively simple deformation history that included one surge-related high magnitude deformation event. Ice exposed in the terminal lobe at the same time had experienced six surge phases during its approximately 100 year-long residence time. The histories of accumulation of cumulative strain are complex, and indicate that measures of cumulative strain can mask the effects of large but transient strain events. They also demonstrate that substantial cumulative strain can be ‘undone’ during subsequent deformation to leave a cumulative strain signal that is unrepresentative of earlier cumulative strain states.
Structural relationships in general do not reflect the complexity of the deformation histories experienced by the ice. A preliminary analysis of the relationships between the deformation histories and structural assemblages suggests, in particular, that brittle structures are reactivated several times during the history of the ice.
The stress distribution related to the boudinage of a visco-elastic material: examples from a polar outlet glacier
Authors: Marmo Brett A. , Wilson Christopher J. L. 
Published: April 2008

Abstract
Quantitative stress measurements related to the development of mesoscale structures in rock are difficult, if not impossible. A method for determining the stress distribution and history during the development of mesoscale boudinage structures in ice is introduced here. Boudinage structures in fracture trace ice have been observed in the outlets glaciers of the Framnes Mountains, east Antarctica. Fracture traces are preserved when creasses fill with surface melt water, which freezes to form coarse-grained columnar ice. A 4.0 km flow-parallel traverse across the Central Ice Stream of the Framnes Mountains is presented to illustrate the boudinage of fracture traces with a mean width of 0.30 m. Field-based measurement of the geometric evolution of boudinage structures has been combined with surface flow rate measurements to quantitatively determine the strain rate at which the boudinage structures formed. The strain rate measurements provide boundary constraints on several two-dimensional finite difference models that have been used to analyse the stress distribution related to the formation of boudinage structures in a visco-elastic solid. The results reveal the development of pressure-shadows during the boundinage of layered rocks, and demonstrate the degree of refraction of stress across rheological boundaries, which have important ramifications for the analysis of planar and linear fabrics in rocks.
The potential contribution of high-resolution glacier flow modelling to structural glaciology
Authors: Hubbard Alun , Hubbard Bryn 
Published: April 2008

Abstract

The three-dimensional stress and strain fields derived through high-resolution flow modelling of Haut Glacier d’Arolla, Switzerland are used to predict the generation, passage and surface expression of a variety of structural forms at the glacier. Flow vectors and strain ellipses are computed and illustrated in plan-form and long section. The model is used to predict the formation and orientation of surface crevasses, and, once healed, the downglacier evolution of their traces. Similarly, the evolution of primary stratification where it crops out at the glacier surface is predicted. The resulting stratification pattern compares well with that revealed in aerial photographs of the glacier. Finally, the three-dimensional strain field is used to track the (accumulation area) burial, (englacial) transport and (ablation area) exposure of ice deposited within a pre-defined elevation range. This
deposition-transport-exposure
tracking allows the location of ice that was initially deposited at any defined location on the glacier to be identified. Such information is of significance in interpreting, for example, the distribution of ash and isotopic horizons within a glacier. We conclude that high-resolution three-dimensional flow modelling has the potential to provide a powerful tool for investigating the genesis and evolution of valley glacier structures.
Continuity comes first: recent progress in understanding subglacial deformation
Authors: Alley R. B. 
Published: April 2008

Abstract
Subglacial till deformation is glaciologically and geologically important, but is difficult to model owing to numerous uncertainties related to till generation and the flow law for till deformation. I review recent results with the hope of at least focusing the uncertainties. Fine-grained subglacial tills often are sufficiently soft that continuity issues are more important than the ‘flow law’ of the till in affecting glacier behavior. Non-steady forcing, coarse clasts and an irregular ice-till contact favour till deformation to significant depths and thus rapid subglacial transport of till. Over long times, site history is important in till continuity and thus in glacier behaviour.
Evidence against pervasive bed deformation during the surge of an Icelandic glacier
Authors: Fuller Sarah , Murray Tavi 
Published: April 2008

Abstract
Hagafellsjökull Vestari, Iceland, surged in 1980 and advanced about 700 m. Subsequent retreat of the ice front has exposed a suite of streamlined bedforms and former subglacial sediments. Investigations were undertaken on the recently deglaciated forefield to explore the degree of ice-bed coupling during the surge. The morphometry of flutes at this site suggest that the ice was relatively well coupled to its bed. Widespread till wedges show that ploughing was a dominant process. These observations suggest that pervasive deformation of the sedimentary bed at high water pressures in surge was a strong possibility. However, evidence from the micromorphology of drumlin sediments shows laterally continuous clay bands preserved between till layers within a drumlin. Thus pervasive deformation of the whole sediment thickness did not occur. There is evidence only of a thin layer of deformation (<16 cm). We propose that at low effective pressures there was a reduction in the strength of the ice-bed interface rather than a reduction in sediment strength. Motion was concentrated at the sole with localized and shallow deformation where clasts ploughed through the substrate.
Laboratory investigations of the strength, static hydraulic conductivity and dynamic hydraulic conductivity of glacial sediments
Authors: Hubbard Bryn , Maltman Alex 
Published: April 2008

Abstract

We report the first data on how effective pressure and deformation influence the hydraulic conductivity of glacial sediments. Fifty eight static hydraulic conductivity tests, 28 triaxial deformation tests and 25 dynamic hydraulic conductivity tests (in which hydraulic conductivity is measured simultaneously with deformation) have been undertaken on seven samples of glacial sediments recovered from the margins of Haut Glacier d’Arolla, Switzerland, and from Traeth y Mwnt, mid Wales. Testing reveals that hydraulic conductivity is inversely related to effective pressure, particularly at effective pressures below
c.
100 kPa. Over the full range of effective pressures used (50–900 kPa), this relationship is best described by a negative power law above a base hydraulic conductivity value, termed
K
0
. The value of
K
0
varies between samples by over three orders of magnitude, from 10
−8
m s
−1
to 10
−11
m s
−1
. These values vary directly, but weakly, with the square of the effective grain-size of the samples tested.

Dynamic testing revealed a commonly repeated pattern of sample failure: axial stress approached a maximum value after which the sample deformed in a ductile manner from its initially cylindrical shape to a barrel shape. Most commonly, sample deformation and failure were accompanied by a decrease in hydraulic conductivity, although increases were also recorded. Dynamic testing also resulted in strongly linear relationships between effective pressure and the yield stress at failure. Such relationships are broadly consistent with a Mohr-Coulomb type model, revealing significant inter-sample variability in frictional resistance, and a cohesive term that is statistically indistinguishable from zero.
Moraine-mound formation by englacial thrusting: the Younger Dryas moraines of Cwm Idwal, North Wales
Authors: Graham David J. , Midgley Nicholas G. 
Published: April 2008

Abstract

The Younger Dryas (
c.
11-10 ka
bp
) moraine-mound complex (‘hummocky moraine’) in the historically important site of Cwm Idwal, North Wales, has previously been interpreted using periglacial, subglacial, ice-marginal and englacial models. In this paper the morphology and sedimentology of these landforms is described and the competing hypotheses tested against this evidence. It is demonstrated that an englacial thrusting model, developed for polythermal glaciers in Svalbard, best fits the available evidence. Thrusting probably resulted from longitudinal compression against a reverse bedrock slope, although a frozen snout, downglacier of sliding ice, may also have been a trigger. It is suggested that the role of ice-deformation, especially thrusting, in landform development has been underestimated, and that the englacial thrusting model may find application in the interpretation of other sites in the palaeo-landform record.
Molecular evidence on the evolution of the Bivalvia
Authors: Campbell D. C. 
Published: April 2008

Abstract
Despite widespread agreement on the monophyly of several major taxa of bivalves, others remain uncertain and the relationships among them are debated. The present study compares new and published morphological phylogenies with new analyses based on 18S gene sequences.
All but one family and all superfamilies in the Bivalvia were monophyletic in all the analyses. Several higher taxa, including most subclasses and orders, were also resolved as monophyletic. Only Myoida shows strong evidence for polyphyly, with at least two origins from Veneroida. Autobranchia was supported as monophyletic in the parsimony analyses. Within Pteriomorphia, Ostreoida is the sister taxon of Pterioida, if not derived from within it, rather than closest to Pectinoida. The numerous points of agreement with morphology based analyses suggests that both types of evidence are converging on a common phylogeny; however, differences remain to be resolved by further study.
A new look at early bivalve phylogeny
Authors: Cope John C. W. 
Published: April 2008

Abstract

In the 30 years since publication of the bivalve
Treatise
, (Moore, R. C. (ed.) 1969.
Treatise on Invertebrate Paleontology. Part N. Mollusca 6, Bivalvia
, Geological Society of America and University of Kansas) important new faunas have been described from the early and mid Cambrian and from the early and mid Ordovician. These contain significant new forms, including some long-ranging intermediate groups, that indicate the relationships between the principal bivalve clades, but lack of fossils from the late Cambrian and earliest Ordovician is a major hindrance. The principal phase of bivalve diversification followed on from the evolution of the filibranch gill in the latest Cambrian or earliest Ordovician. The fundamental division of the class is into two subclasses, Protobranchia and Autolamelli branchiata; links between the two can be demonstrated in the early Ordovician. Major divisions of each subclass are recognized as superorders.

Within the Protobranchia, the Nuculoida developed specialist food-gathering palps and an enlarged foot. Diverging early from the protobranch stock were other bivalves that lived symbiotically with sulphur-oxidizing chemoautotrophic bacteria; this allowed colonization of anaerobic substrates and produced two distinct stocks: the deeply infaunal anteriorly elongate Solemyoida and the shallower infaunal Nucinelloida.
The Autolamellibranchiata, initially identified by strongly asymmetrical hinges, diversified in three directions, each characterized by distinctive hinges. The Trigonioida were characterized by ligamental nymphs and frequently denticulate teeth, and rapidly regained greater symmetry; the Anomalodesmata also developed a strong ligamental insertion within nymphs and largely lost their dentition, whilst the Heteroconchia, principally with a shell including a complex crossed-lamellar structure, had various combinations of cardinal and lateral teeth. Heteroconch diversifications were mainly in the Mesozoic and Cenozoic, but one Ordovician group, the Glyptarcoidea, is a good ancestor for the Pteriomorphia.
The following new taxa are proposed: Cardiolarioidea superfam. nov., Eritropidae fam. nov. and Catamarcaidae fam. nov.
A preliminary phylogeny for rudist bivalves: sifting clades from grades
Authors: Skelton Peter W. , Smith Andrew B. 
Published: April 2008

Abstract

The taxonomic history of the late Jurassic-Cretaceous rudist bivalves (Superfamily Hippuritoidea) is reviewed and a new phylogeny based on cladistic analysis of 32 skeletal characters is proposed. Informative characters include: relative thickness and structures of the outer (calcitic) shell layer; valve asymmetry and attachment to the substrate by either the left or right valve; form of the ligament; and, in the aragonitic inner shell, the dentition and shelly supports (myophores) for the adductor muscles, as well as accessory cavities and blind-ending (pallial) canals. The myocardinal complex (teeth and myophores) is especially important for discriminating clades previously lumped in paraphyletic or even polyphyletic taxa (e.g. Caprinidae,
sensu lato
, in the bivalve
Treatise
(Moore, R. C. (ed.) 1969.
Treatise on Invertebrate Paleontology, Part N. Mollusca 6, Bivalvia
, Geological Society of America and University of Kansas).

As outgroup, a megalodontid bivalve was used, as this shares three derived traits (massive dentition, modified parivincular ligament and posterior myophores) in common with rudists. The clade of all rudists is united by possession of an outer shell layer of fibrillar prismatic calcite (albeit greatly reduced in a few taxa). Two subclades are distinguished according to the attachment of the shell — either by the left valve (all ‘diceratids’, except
Diceras
and
Valletia
, plus the monophyletic requieniids), or by the right valve (
Diceras, Valletia
and all other rudists). Monophyly of some previously established families is substantiated. These include Caprinidae,
sensu stricto
(Skelton, P. W. 1978.
Philosophical Transactions of the Royal Society of London, Series B
,
284
, 305–318), Radiolitidae (with
Agriopleura
as sister taxon) and Hippuritidae (with
Tepeyacia
emerging as a possible sister taxon), while others are resolved into distinct clades. There is some evidence for the Polyconitidae Mac Gillavry (1937), though with negligible bootstrap support.
On becoming cemented: evolutionary relationships among the genera in the freshwater bivalve family Etheriidae (Bivalvia: Unionoida)
Authors: Bogan Arthur E. , Hoeh Walter R. 
Published: April 2008

AbstractA robust phylogeny for the Unionoida is emerging and presumed relationships of some major clades are being questioned. The Etheriidae or freshwater oysters has been a distinct family for over 160 years and currently contains three cemented genera:Acostaea(Columbia, South America),Pseudomulleria(India) andEtheria(Africa and Madagascar). Starobogatov (1970,Nauka, 1–372), Mansur and da Silva (1990,Amazoniana,11(2), 147–166) and Bonetto (1997,Biociências,5, 113–142) present conflicting testable hypotheses regarding the evolution of these taxa. Using cytochrome c oxidase subunit I DNA sequences the evolutionary relationships of these three genera has been examined, by comparing them to representatives of 30 other unionoid taxa from around the world. These analyses placeAcostaeaandEtheriawithin the Mycetopodidae whilePseudomulleriafalls within the Unionidae. A monophyletic Etheriidae, composed of cemented freshwater bivalves, is not supported by the present analyses. Furthermore, the analyses indicate that cementation in the Unionida has evolved at least twice.
Relevance of sperm ultrastructure to the classification of giant clams (Mollusca, Cardioidea, Cardiidae, Tridacninae)
Authors: Keys Jennifer L. , Healy John M. 
Published: April 2008

Abstract

Examination of sperm ultrastructure in six out of eight of the living species of giant clams (‘Tridacnidae’) supports reduction in status of the Tridacnidae to a subfamily of the Cardiidae (Tridacninae), as suggested by recent cladistic analyses based on shell, anatomical and molecular characters. Tridacninae spermatozoa are all of the aquasperm type, featuring, in anterior-posterior sequence: a conical acrosomal vesicle, an oblong to rod-shaped nucleus, a short midpiece region (proximal and distal centrioles surrounded by four round mitochondria); and a flagellum (9 + 2 pattern axoneme). Substantial differences occur between species with respect to the shape and length of the nucleus, and in the spatial reationship between the acrosomal complex and the nuclear apex. Although the two extant genera can be distinguished on sperm features —
Tridacna
, nuclear peg associated with acrosome, centriolar connective absent;
Hippopus
, nuclear peg absent, connective present — no defining feature of the Tridacninae can be detected. Within
Tridacna
, the species
T. (Chametrachea) maxima
and
T. (C.) crocea
are distinguished from other species of the genus by a much finer nuclear peg and a considerably smaller acrosome. In contrast, and against expectation,
T. (C.) squamosa
shows acrosomal and nuclear dimensions very close to those obtained for
T. (Tridacna) gigas
. Recently proposed phylogenies and classificatory changes for ‘tridacnids’ are discussed in the light of available sperm data.
Functional anatomy, chemosymbiosis and evolution of the Lucinidae
Authors: Taylor John D. , Glover Emily A. 
Published: April 2008

AbstractAll Lucinidae species studied so far possess sulphide-oxidizing, chemosymbiotic bacteria housed in bacteriocytes of gill filaments. The ecology, functional anatomy and evolution of the Lucinidae must be considered in relation to this symbiosis. The ctenidia have been extensively studied but other anatomical structures peculiar to lucinids have received much less attention. Reviewed are the morphological diversity of living lucinids, highlighting features of their anatomy including ctenidia, pallial apertures, anterior adductor mucsle, pallial blood vessel and mantle gills. The latter are much more complex than previously understood and are here redescribed. They comprise folded structures located near the anterior adductor muscle inCodakia, PhacoidesandLucina, and on the septum ofAnodontia. These are interpreted as secondary respiratory surfaces, their location enabling the separation of the anterior inflow of oxygenated water from sulphide-containing water. The latter is released from the sediment by the probing activities of the highly extensible foot and is pumped over the gill through the pedal gape and perhaps also via the exhalant tube. The shell features ofIlioniafrom the Silurian Period suggests that the lucinid chemosymbiosis is an ancient association.
Carboniferous praecardioid bivalves from the exceptional Buckhorn Asphalt biota of south-central Oklahoma, USA
Authors: Yancey T. E. , Heaney M. J. 
Published: April 2008

Abstract

Unusual Pennsylvanian bivalves recovered from the Buckhorn Asphalt Quarry of south-central Oklahoma are members of the Order Praecardioida, Family Lunulacardiidae. This is the youngest occurrence of praecardioids in North America. These praecardioids have an ontogeny characterized by a veneriform juvenile shell, changing abruptly to an elongate adult shell with sharp carinae, accompanied by strong rotation of the juvenile shell and change in ligament. These shells provide new evidence of dentition and ligament for lunulacardiids and the first documentation of praecardioid shell microstructure. The Buckhorn taxa possess stout hinge teeth, indicating that lunulacardiids are not edentulous. Preserved ligament in multiple growth stages indicates that beaks are prosogyrous and the flattened truncated portion of the shell is posterior. A revised diagnosis of the Lunulacardiidae is presented. New taxa:
Buckhornia carteri
n. gen., n. sp.
The nature and origin of taxonomic diversity gradients in marine bivalves
Authors: Crame J. A. 
Published: April 2008

Abstract
Bivalves have been fundamental in developing the understanding of large-scale biodiversity patterns in the marine realm. A new study based on 29 regional bivalve faunas indicates that both latitudinal and longitudinal gradients are not so regular in form as was once imagined. The northern hemisphere latitudinal gradient has a marked step, or inflection, at 20–30°N, and in the southern hemisphere Australia forms a distinct diversity hotspot. A longitudinal gradient runs between a large tropical high diversity focus in the southern China-Indonesia-Australia region and a somewhat smaller one in the Panamic-Caribbean region.
It is unlikely that the marked asymmetry in both latitudinal and longitudinal gradients can be explained entirely by either the geometric patterns of species ranges in relation to geographical boundaries (the mid-domain effect) or the operation of contemporary, or equilibrium, factors. The role of history must be considered too, and in some cases this could involve a timescale of at least 60 Ma. For example, Australia has moved progressively northwards throughout the Cenozoic era and this seems to have led to the development of a unique assemblage of both tropical and temperate taxa around the continent. When Australia eventually collided with southeast Asia, some 15 Ma ago, three previously distinct tropical marine faunas were brought into close juxtaposition.
However, something else seems to have been involved in order to generate a marked pantropical Neogene diversification event in bivalves and other marine invertebrate taxa. It is striking how this event coincided with a prolonged interval of global cooling and the suggestion has been made that glacioeustatic cycles may have promoted the formation of a tropical species diversity pump. In concert with regular and profound sea-level changes, new taxa were created in marginal tropical regions and then concentrated in certain core regions. These same glacioeustatic cycles may also have served to accentuate the latitudinal limits of many tropical taxa.
As the knowledge of large-scale taxonomic diversity gradients in the marine realm has increased, so too has the realization that they are at least the partial products of major historical events.
Analysing the latitudinal diversity gradient in marine bivalves
Authors: Jablonski David , Roy Kaustuv , Valentine James W. 
Published: April 2008

Abstract

Marine bivalves of the eastern Pacific continental shelf show a strong diversity gradient from the Arctic Ocean to the tropics. This gradient is underlain by strong diversity trends in both infaunal and epifaunal bivalves, contrary to Thorson’s influential hypothesis (1952,
Verhandlungen der Deutschen Zoologischen Gesellschaft
,
1951
, 267–327), and is significantly correlated with mean sea-surface temperature; either raw data or a residuals analysis yields
p
< 0.0001. Patterns differ according to trophic group and phylogeny; however, suspension feeders conform to the general bivalve diversity gradient, as do facultative deposit feeders such as tellinids, while deposit feeding protobranchs do not. Infaunal and epifaunal diversity gradients have different slopes so that their ratio changes with latitude; data on Jurassic and Cretaceous bivalves suggests that this ratio has varied in slope and intercept over geological time.