58 research outputs found
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Processes and patterns of glacier-influenced sedimentation and recent tidewater glacier dynamics in Darbel Bay, western Antarctic Peninsula
AbstractBathymetric data of unprecedented resolution are used to provide insights into former ice dynamics and glacial processes in a western Antarctic Peninsula embayment. An assemblage of submarine glacial landforms, which includes subglacially produced streamlined features and ice-marginal ridges, reveals the former pattern of ice flow and retreat. A group of more than 250 small (< 1–3 m high, 10–20 m wide) and relatively evenly spaced recessional moraines was identified beyond the margin of Philippa Glacier. The small recessional moraines are interpreted to have been produced during short-lived, possibly annual re-advances of a grounded ice margin during overall retreat. This is the first time that these features have been shown to be part of the assemblage of landforms produced by tidewater glaciers on the Antarctic Peninsula. Glacier-terminus changes during the last four decades, mapped from LANDSAT satellite images, were analysed to determine whether the moraines were produced during recent still-stands or re-advances of Philippa Glacier and to further investigate the short-term (annual to decadal) variability in ice-marginal position in tidewater glacier systems. The asynchronous response of individual tidewater glaciers in Darbel Bay is interpreted to be controlled mainly by local topography rather than by glacier catchment-area size.C.L. Batchelor was in receipt of a Junior Research Fellowship from Newnham College, Cambridge during this work. K.A Hogan and R.D Larter are supported by the Polar Science for Planet Earth programme funded by the Natural Environment Research Council, U.K
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Past water flow beneath Pine Island and Thwaites glaciers, West Antarctica
Abstract. Outburst floods from subglacial lakes beneath the Antarctic Ice Sheet
modulate ice-flow velocities over periods of months to years. Although
subglacial lake drainage events have been observed from
satellite-altimetric data, little is known about their role in the
long-term evolution of ice-sheet basal hydrology. Here, we
systematically map and model past water flow through an extensive area
containing over 1000 subglacial channels and 19 former lake basins exposed
on over 19 000 km2 of seafloor by the retreat of Pine Island and
Thwaites glaciers, West Antarctica. At 507 m wide and 43 m deep on average,
the channels offshore of present-day Pine Island and Thwaites glaciers
are approximately twice as deep, 3 times as wide, and cover an area over
400 times larger than the terrestrial meltwater channels comprising the
Labyrinth in the Antarctic Dry Valleys. The channels incised into bedrock
offshore of contemporary Pine Island and Thwaites glaciers would have been
capable of accommodating discharges of up to 8.8×106 m3 s−1. We suggest that the channels were formed by episodic discharges
from subglacial lakes trapped during ice-sheet advance and retreat over
multiple glacial periods. Our results document the widespread influence of
episodic subglacial drainage events during past glacial periods, in
particular beneath large ice streams similar to those that continue to
dominate contemporary ice-sheet discharge.
UK Natural Environment Research Council’s iSTAR programme (grant nos. NE/J005703/1, NE/J005746/1, and NE/J005770/1).
James D. Kirkham: Debenham Scholarship from the Scott Polar Research Institute, University of Cambridge, and a UK Natural Environment Research Council Ph.D. studentship awarded through the Cambridge Earth System Science Doctoral Training Partnership (grant no. NE/L002507/1
Revealing the former bed of Thwaites Glacier using sea-floor bathymetry: Implications for warm-water routing and bed controls on ice flow and buttressing
Abstract. The geometry of the sea floor immediately beyond
Antarctica's marine-terminating glaciers is a fundamental control on
warm-water routing, but it also describes former topographic pinning points
that have been important for ice-shelf buttressing. Unfortunately, this
information is often lacking due to the inaccessibility of these areas for
survey, leading to modelled or interpolated bathymetries being used as
boundary conditions in numerical modelling simulations. At Thwaites Glacier
(TG) this critical data gap was addressed in 2019 during the first cruise of
the International Thwaites Glacier Collaboration (ITGC) project. We present more than 2000 km2 of new multibeam
echo-sounder (MBES) data acquired in exceptional sea-ice conditions
immediately offshore TG, and we update existing bathymetric compilations.
The cross-sectional areas of sea-floor troughs are under-predicted by up to
40 % or are not resolved at all where MBES data are missing, suggesting that
calculations of trough capacity, and thus oceanic heat flux, may be
significantly underestimated. Spatial variations in the morphology of
topographic highs, known to be former pinning points for the floating ice
shelf of TG, indicate differences in bed composition that are supported by
landform evidence. We discuss links to ice dynamics for an overriding ice
mass including a potential positive feedback mechanism where erosion of
soft erodible highs may lead to ice-shelf ungrounding even with little
or no ice thinning. Analyses of bed roughnesses and basal drag contributions
show that the sea-floor bathymetry in front of TG is an analogue for extant
bed areas. Ice flow over the sea-floor troughs and ridges would have been
affected by similarly high basal drag to that acting at the grounding zone
today. We conclude that more can certainly be gleaned from these 3D
bathymetric datasets regarding the likely spatial variability of bed
roughness and bed composition types underneath TG. This work also addresses
the requirements of recent numerical ice-sheet and ocean modelling studies
that have recognised the need for accurate and high-resolution bathymetry to
determine warm-water routing to the grounding zone and, ultimately, for
predicting glacier retreat behaviour.
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The periodic topography of ice stream beds: Insights from the Fourier spectra of mega-scale glacial lineations
Ice stream bed topography contains key evidence for the ways ice streams interact with, and are potentially controlled by, their beds. Here we present the first application of two-dimensional Fourier analysis to 22 marine and terrestrial topographies from 5 regions in Antarctica and Canada, with and without mega-scale glacial lineations (MSGLs). We find that the topography of MSGL-rich ice stream sedimentary beds is characterized by multiple, periodic wavelengths between 300 and 1200 m and amplitudes from decimeters to a few meters. This periodic topography is consistent with the idea that instability is a key element to the formation of MSGL bedforms. Dominant wavelengths vary among locations and, on one paleo ice stream bed, increase along the direction of ice flow by 1.7 ± 0.52% km−1. We suggest that these changes are likely to reflect pattern evolution via downstream wavelength coarsening, even under potentially steady ice stream geometry and flow conditions. The amplitude of MSGLs is smaller than that of other fluvial and glacial topographies but within the same order of magnitude. However, MSGLs are a striking component of ice stream beds because the topographic amplitude of features not aligned with ice flow is reduced by an order of magnitude relative to those oriented with the flow direction. This study represents the first attempt to automatically derive the spectral signatures of MSGLs. It highlights the plausibility of identifying these landform assemblages using automated techniques and provides a benchmark for numerical models of ice stream flow and subglacial landscape evolution.The research was funded by the NE/J004766/1 UK NERC New Investigator and a SAGES 728 PECRE grants awarded to MS
Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks.
Marine ice-cliff instability (MICI) processes could accelerate future retreat of the Antarctic Ice Sheet if ice shelves that buttress grounding lines more than 800 metres below sea level are lost. The present-day grounding zones of the Pine Island and Thwaites glaciers in West Antarctica need to retreat only short distances before they reach extensive retrograde slopes. When grounding zones of glaciers retreat onto such slopes, theoretical considerations and modelling results indicate that the retreat becomes unstable (marine ice-sheet instability) and thus accelerates. It is thought that MICI is triggered when this retreat produces ice cliffs above the water line with heights approaching about 90 metres. However, observational evidence confirming the action of MICI has not previously been reported. Here we present observational evidence that rapid deglacial ice-sheet retreat into Pine Island Bay proceeded in a similar manner to that simulated in a recent modelling study, driven by MICI. Iceberg-keel plough marks on the sea-floor provide geological evidence of past and present iceberg morphology, keel depth and drift direction. From the planform shape and cross-sectional morphologies of iceberg-keel plough marks, we find that iceberg calving during the most recent deglaciation was not characterized by small numbers of large, tabular icebergs as is observed today, which would produce wide, flat-based plough marks or toothcomb-like multi-keeled plough marks. Instead, it was characterized by large numbers of smaller icebergs with V-shaped keels. Geological evidence of the form and water-depth distribution of the plough marks indicates calving-margin thicknesses equivalent to the threshold that is predicted to trigger ice-cliff structural collapse as a result of MICI. We infer rapid and sustained ice-sheet retreat driven by MICI, commencing around 12,300 years ago and terminating before about 11,200 years ago, which produced large numbers of icebergs smaller than the typical tabular icebergs produced today. Our findings demonstrate the effective operation of MICI in the past, and highlight its potential contribution to accelerated future retreat of the Antarctic Ice Sheet
Technology-dependency among patients discharged from a children's hospital: a retrospective cohort study
BACKGROUND: Advances in medical technology may be increasing the population of children who are technology-dependent (TD). We assessed the proportion of children discharged from a children's hospital who are judged to be TD, and determined the most common devices and number of prescription medications at the time of discharge. METHODS: Chart review of 100 randomly selected patients from all services discharged from a children's hospital during the year 2000. Data were reviewed independently by 4 investigators who classified the cases as TD if the failure or withdrawal of the technology would likely have adverse health consequences sufficient to require hospitalization. Only those cases where 3 or 4 raters agreed were classified as TD. RESULTS: Among the 100 randomly sampled patients, the median age was 7 years (range: 1 day to 24 years old), 52% were male, 86% primarily spoke English, and 54% were privately insured. The median length of stay was 3 days (range: 1 to 103 days). No diagnosis accounted for more than 5% of cases. 41% were deemed to be technology dependent, with 20% dependent upon devices, 32% dependent upon medications, and 11% dependent upon both devices and medications. Devices at the time of discharge included gastrostomy and jejeunostomy tubes (10%), central venous catheters (7%), and tracheotomies (1%). The median number of prescription medications was 2 (range: 0–13), with 12% of cases having 5 or more medications. Home care services were planned for 7% of cases. CONCLUSION: Technology-dependency is common among children discharged from a children's hospital
Magnetism, FeS colloids, and Origins of Life
A number of features of living systems: reversible interactions and weak
bonds underlying motor-dynamics; gel-sol transitions; cellular connected
fractal organization; asymmetry in interactions and organization; quantum
coherent phenomena; to name some, can have a natural accounting via
interactions, which we therefore seek to incorporate by expanding the horizons
of `chemistry-only' approaches to the origins of life. It is suggested that the
magnetic 'face' of the minerals from the inorganic world, recognized to have
played a pivotal role in initiating Life, may throw light on some of these
issues. A magnetic environment in the form of rocks in the Hadean Ocean could
have enabled the accretion and therefore an ordered confinement of
super-paramagnetic colloids within a structured phase. A moderate H-field can
help magnetic nano-particles to not only overcome thermal fluctuations but also
harness them. Such controlled dynamics brings in the possibility of accessing
quantum effects, which together with frustrations in magnetic ordering and
hysteresis (a natural mechanism for a primitive memory) could throw light on
the birth of biological information which, as Abel argues, requires a
combination of order and complexity. This scenario gains strength from
observations of scale-free framboidal forms of the greigite mineral, with a
magnetic basis of assembly. And greigite's metabolic potential plays a key role
in the mound scenario of Russell and coworkers-an expansion of which is
suggested for including magnetism.Comment: 42 pages, 5 figures, to be published in A.R. Memorial volume, Ed
Krishnaswami Alladi, Springer 201
Ice sheet–free West Antarctica during peak early Oligocene glaciation
One of Earth’s most fundamental climate shifts – the greenhouse-icehouse transition 34 Ma ago – initiated Antarctic ice-sheet build-up, influencing global climate until today. However, the extent of the ice sheet during the Early Oligocene Glacial Maximum (~33.7–33.2 Ma) that immediately followed this transition, a critical knowledge gap for assessing feedbacks between permanently glaciated areas and early Cenozoic global climate reorganization, is uncertain. Here, we present shallow-marine drilling data constraining earliest Oligocene environmental conditions on West Antarctica’s Pacific margin – a key region for understanding Antarctic ice sheet-evolution. These data indicate a cool-temperate environment, with mild ocean and air temperatures preventing West Antarctic Ice Sheet formation. Climate-ice sheet modeling corroborates a highly asymmetric Antarctic ice sheet, thereby revealing its differential regional response to past and future climatic change
On the Origin and Trigger of the Notothenioid Adaptive Radiation
Adaptive radiation is usually triggered by ecological opportunity, arising
through (i) the colonization of a new habitat by its
progenitor; (ii) the extinction of competitors; or
(iii) the emergence of an evolutionary key innovation in
the ancestral lineage. Support for the key innovation hypothesis is scarce,
however, even in textbook examples of adaptive radiation. Antifreeze
glycoproteins (AFGPs) have been proposed as putative key innovation for the
adaptive radiation of notothenioid fishes in the ice-cold waters of Antarctica.
A crucial prerequisite for this assumption is the concurrence of the
notothenioid radiation with the onset of Antarctic sea ice conditions. Here, we
use a fossil-calibrated multi-marker phylogeny of nothothenioid and related
acanthomorph fishes to date AFGP emergence and the notothenioid radiation. All
time-constraints are cross-validated to assess their reliability resulting in
six powerful calibration points. We find that the notothenioid radiation began
near the Oligocene-Miocene transition, which coincides with the increasing
presence of Antarctic sea ice. Divergence dates of notothenioids are thus
consistent with the key innovation hypothesis of AFGP. Early notothenioid
divergences are furthermore congruent with vicariant speciation and the breakup
of Gondwana
Neuron-glial Interactions
Although lagging behind classical computational neuroscience, theoretical and computational approaches are beginning to emerge to characterize different aspects of neuron-glial interactions. This chapter aims to provide essential knowledge on neuron-glial interactions in the mammalian brain, leveraging on computational studies that focus on structure (anatomy) and function (physiology) of such interactions in the healthy brain. Although our understanding of the need of neuron-glial interactions in the brain is still at its infancy, being mostly based on predictions that await for experimental validation, simple general modeling arguments borrowed from control theory are introduced to support the importance of including such interactions in traditional neuron-based modeling paradigms.Junior Leader Fellowship Program by “la Caixa” Banking Foundation (LCF/BQ/LI18/11630006
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