scholarly journals Ice Flow along the Byrd Station Strain Network, Antarctica

1979 ◽  
Vol 24 (90) ◽  
pp. 15-28 ◽  
Author(s):  
I. M. Whillans
Keyword(s):  
Strain Rate ◽  
Isotopic Ratio ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheet ◽  
Surface Strain ◽  
Rate Data ◽  
Progressive Development ◽  
Bed Form ◽  
Oxygen Isotopic

AbstractThe flow of the Antarctic ice sheet near Byrd Station is modeled using surface net accumulation-rate data, surface strain-rate data, and core-hole tilting results. The model empirically allows for the progressive development of ice fabric and for values of the vertical strain-rate nearer to zero at depth, and adjusts the strain-rates according to the effect of the climatic warming at the beginning of the Holocene.The validity of the model is supported by the agreements between calculated bed form and that measured by radar sounding, and between calculated and measured present-day ice-sheet thinning-rates. The ice was about 200 m thicker before thinning.The depth-age relationship for the Byrd Station ice core shows that the climatic change represented by the oxygen isotopic ratio of the ice began some 5000 years sooner than in north Greenland (Hammer and others 1978), but ended at about the same time.

scholarly journals Ice Flow along the Byrd Station Strain Network, Antarctica

1979 ◽  
Vol 24 (90) ◽  
pp. 15-28 ◽  
Author(s):  
I. M. Whillans
Keyword(s):  
Strain Rate ◽  
Isotopic Ratio ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheet ◽  
Surface Strain ◽  
Rate Data ◽  
Progressive Development ◽  
Bed Form ◽  
Oxygen Isotopic

AbstractThe flow of the Antarctic ice sheet near Byrd Station is modeled using surface net accumulation-rate data, surface strain-rate data, and core-hole tilting results. The model empirically allows for the progressive development of ice fabric and for values of the vertical strain-rate nearer to zero at depth, and adjusts the strain-rates according to the effect of the climatic warming at the beginning of the Holocene.The validity of the model is supported by the agreements between calculated bed form and that measured by radar sounding, and between calculated and measured present-day ice-sheet thinning-rates. The ice was about 200 m thicker before thinning.The depth-age relationship for the Byrd Station ice core shows that the climatic change represented by the oxygen isotopic ratio of the ice began some 5000 years sooner than in north Greenland (Hammer and others 1978), but ended at about the same time.

scholarly journals Mass Balance of the Greenland Ice Sheet at Dye 3

1985 ◽  
Vol 31 (108) ◽  
pp. 198-200 ◽  
Author(s):  
Niels Reeh ◽  
Niels S. Gundestrup
Keyword(s):  
Mass Balance ◽  
Accumulation Rate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Bottom Layer ◽  
Surface Strain ◽  
Ice Thickness ◽  
Strain Rates ◽  
Confidence Limits ◽  
Rate Of Increase

AbstractThe mass balance of the Greenland ice sheet at Dye 3 is estimated on the basis of observations of ice thickness, accumulation rate, surface velocities, and surface strain-rates. The calculations indicate a rate of increase of surface elevation of 3 cm/year, with 95% confidence limits of −3 cm/year and +9 cm/year. Previous estimates of the mass balance of the Greenland ice sheet by the same method reported large imbalances; these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth-averaged quantities. The most reliable observations indicate that the interior regions of the Greenland ice sheet are at present thickening at a rate of a few centimetres per year; a contributing cause for this may be the slow thinning of a bottom layer of relatively soft Wisconsin ice.

scholarly journals Modelling binge–purge oscillations of the Laurentide ice sheet using a plastic ice sheet

2008 ◽  
Vol 48 ◽  
pp. 177-182 ◽  
Author(s):  
H.C. Steen-Larsen ◽  
D. Dahl-Jensen
Keyword(s):  
Melting Point ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Climate Feedbacks ◽  
Heinrich Events ◽  
Basal Sliding ◽  
Pressure Melting ◽  
Temperature Records

AbstractA simple combined heat and ice-sheet model has been used to calculate temperatures at the base of the Laurentide ice sheet. We let the ice sheet surge when the basal temperature reaches the pressure-melting temperature. Driving the system with the observed accumulation and temperature records from the GRIP ice core, Greenland, produces surges corresponding to the observed Heinrich events. This suggests that the mechanism of basal sliding, initiated when the basal temperature reaches the melting point, can explain the surges of the Laurentide ice sheet. This study highlights the importance of the surface temperature and accumulation rate as a means of forcing the timing and strength of the Heinrich events, thus implying important ice-sheet climate feedbacks.

scholarly journals The topology of ice-sheet centres

1991 ◽  
Vol 37 (126) ◽  
pp. 220-227 ◽  
Author(s):  
J.F. Nye
Keyword(s):  
Strain Rate ◽  
Ice Sheet ◽  
Vertical Axis ◽  
Horizontal Component ◽  
Surface Strain ◽  
Principal Strain ◽  
Circular Symmetry ◽  
Two Component ◽  
Horizontal Strain ◽  
Structurally Stable

AbstractAn ice sheet will, in general, possess points where the horizontal component of velocity is zero, and some of these will be ice centres, occurring close to summits. The paper examines the possible flow patterns near such points. The corresponding horizontal strain-rate pattern is studied by considering an ice sheet which initially has perfect circular symmetry about a vertical axis. Before perturbation there is an isotropic point for the horizontal surface strain rate at the centre. It may be shown, on purely topological grounds and without any reference to the mechanism of flow, that, when the symmetry is broken, this point, being degenerate and structurally unstable, breaks up into two structurally stable components. The breakup always occurs in essentially the same way. Around the two component points the trajectories of principal strain-rate directions always have the lemon pattern. The contours of equal principal strain rate around them are usually hyperbolic; however, if the unperturbed flow pattern had a very pronounced spiral character, they would be elliptic. This behaviour is in contrast to that of the ice centre itself, which remains unsplit.

scholarly journals Air Content Of The Byrd Core And Past Changes In The West Antarctic Ice Sheet

1982 ◽  
Vol 3 ◽  
pp. 269-273 ◽  
Author(s):  
D. Raynaud ◽  
I. M. Whillans
Keyword(s):  
Stable Isotope ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheet ◽  
Ice Cores ◽  
Isotope Ratios ◽  
Ice Age ◽  
Gas Content ◽  
Stable Isotope Ratios ◽  
Antarctic Ice Sheet

Analyses of ice cores taken from the Antarctic ice sheet can provide information on the environmental conditions under which the ice was formed. New results from measurements of gas content and stable isotope ratios in the Byrd station ice core are discussed and interpreted in terms of past iceflow changes.165 selected ice samples from 32 different depth levels along the core were processed for total gas content V and stable isotope ratios. This large dataset is used to discuss the variability and significance of the values of V at different depths. The short term variations of V are mainly explained by heterogeneities of the pore volume when the firn pores close off.The general trends in the values of V with depth are then used to investigate the possibility of past changes in the ice sheet. They suggest near-steady flow during the past few tens of thousands of years and that a thickening of about 200 to 250 m occurred in this area of the ice sheet at the end of the last ice age. This thickening could be due to a change in the accumulation rate.

Surface strain accumulation and the seismic moment tensor

1997 ◽  
Vol 87 (5) ◽  
pp. 1345-1353
Author(s):  
J. C. Savage ◽  
R. W. Simpson
Keyword(s):  
Strain Rate ◽  
Accumulation Rate ◽  
Moment Tensor ◽  
Surface Strain ◽  
Seismogenic Zone ◽  
Strain Accumulation ◽  
Average Strain ◽  
Average Strain Rate ◽  
Double Couple ◽  
Scalar Moment

Abstract Although the scalar moment accumulation rate within the seismogenic zone beneath a given area is sometimes deduced from the observed average surface strain accumulation rate over that same area (e.g., Working Group on California Earthquake Probabilities, 1995), the correspondence between the two is very uncertain. The equivalence between surface strain accumulation and scalar moment accumulation is based on Kostrov's (1974) relation between the average strain rate over a volume and the moment-rate tensor for that volume. The average strain rate over the volume is replaced by the average strain rate measured at the free surface to deduce an approximate moment-rate tensor. Only in exceptional circumstances will that moment-rate tensor correspond to a double-couple mechanism, a mechanism that can be represented by a scalar moment accumulation rate. More generally, the moment tensor must be resolved into the superposition of two or more double-couple mechanisms, and that resolution can be done in many ways, each with its own scalar moment rate. Thus the resolution is not unique. This is demonstrated by deducing scalar moment accumulation rates for a GPS network that covers most of California south of San Francisco. It is shown that resolutions into different double-couple mechanisms lead to scalar moment accumulation rates differing by factors of ∼2. We suggest that the minimum scalar moment rate equivalent to principal surface strain rates ɛ1 and ɛ2 acting over the area A is M0(min) = 2μHA Max (¦ɛ1¦, ¦ɛ2¦, ¦ɛ1 + ɛ2¦), where μ is the rigidity and H the depth of seismogenic zone, and the function Max is equal to the largest of its arguments. Within the uncertainites of measurement, the scalar moment accumulation rate in southern California based on that approximation is in balance with the average historic seismic moment release rate so that no current earthquake deficit need be accumulating.

scholarly journals Topographic control of regional accumulation rate variability at South Pole and implications for ice-core interpretation

2004 ◽  
Vol 39 ◽  
pp. 214-218 ◽  
Author(s):  
Gordon S. Hamilton
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheet ◽  
Ice Cores ◽  
Snow Accumulation ◽  
Topographic Effects ◽  
South Pole ◽  
Accumulation Rates ◽  
Surface Slope ◽  
Ice Dynamics

AbstractSnow-accumulation rates are known to be sensitive to local changes in ice-sheet surface slope because of the effect of katabatic winds. These topographic effects can be preserved in ice cores that are collected at non-ice-divide locations. The trajectory of an ice-core site at South Pole is reconstructed using measurements of ice-sheet motion to show that snow was probably deposited at places of different surface slope during the past 1000 years. Recent accumulation rates, derived from shallow firn cores, vary along this trajectory according to surface topography, so that on a relatively steep flank mean annual accumulation is ∼18% smaller than on a nearby topographic depression. These modern accumulation rates are used to reinterpret the cause of accumulation rate variability with time in the long ice-core record as an ice-dynamics effect and not a climate-change signal. The results highlight the importance of conducting ancillary ice-dynamics measurements as part of ice-coring programs so that topographic effects can be deconvolved from potential climate signals.

scholarly journals Vostok (Antarctica) ice-core time-scale from datings of different origins

2004 ◽  
Vol 39 ◽  
pp. 283-292 ◽  
Author(s):  
Andrey N. Salamatin ◽  
Elena A. Tsyganova ◽  
Vladimir Ya. Lipenkov ◽  
Jean Robert Petit
Keyword(s):  
Time Scale ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheet ◽  
Monte Carlo Sampling ◽  
Age Dating ◽  
Ice Age ◽  
Sheet Flow ◽  
Glacier Ice ◽  
Different Origins

AbstractThree different approaches to ice-core age dating are employed to develop a depth–age relationship at Vostok, Antarctica: (1) correlating the ice-core isotope record to the geophysical metronome (Milankovich surface temperature cycles) inferred from the borehole temperature profile, (2) importing a known chronology from another (Devils Hole, Nevada, USA) paleoclimatic signal, and (3) direct ice-sheet flow modeling. Inverse Monte Carlo sampling is used to constrain the accumulation-rate reconstruction and ice-flow simulations in order to find the best-fit glaciological time-scale matched with the two other chronologies. The general uncertainty of the different age estimates varies from 2 to 6 kyr on average and reaches 6–15 kyr at maximum. Whatever the causes of this discrepancy might be, they are thought to be of different origins, and the age errors are assumed statistically independent. Thus, the average time-scale for the Vostok ice core down to 3350m depth is deduced consistent with all three dating procedures within the standard deviation limits of ±3.6 kyr, and its accuracy is estimated as 2.2 kyr on average. The constrained ice-sheet flow model allows, at least theoretically, extrapolation of the ice age–depth curve further to the boundary with the accreted lake ice where (at 3530m depth) the glacier-ice age may reach ∼2000 kyr.

scholarly journals How warm was Greenland during the last interglacial period?

2016 ◽  
Author(s):  
Amaelle Landais ◽  
Valérie Masson-Delmotte ◽  
Emilie Capron ◽  
Petra M. Langebroek ◽  
Pepijn Bakker ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Water Isotopes ◽  
Interglacial Period ◽  
Last Interglacial ◽  
Surface Warming ◽  
Last Interglacial Period ◽  
Industrial Level

Abstract. The last interglacial period (LIG, ~ 129–116 thousand years ago) provides the most recent case study for multi-millennial polar warming above pre-industrial level and a respective response of the Greenland and Antarctic ice sheets to this warming, as well as a test bed for climate and ice sheet models. Past changes in Greenland ice sheet thickness and surface temperature during this period were recently derived from the NEEM ice core records, North-West Greenland. The NEEM paradox has emerged from an estimated large local warming above pre-industrial level (7.5 ± 1.8 °C at the deposition site 126 ka ago without correction for any overall ice sheet altitude changes between the LIG and pre-industrial) based on water isotopes, together with limited local ice thinning, suggesting more resilience of the real Greenland ice sheet than shown in some ice sheet models. Here, we provide an independent assessment of the average LIG Greenland surface warming using ice core air isotopic composition (δ15N) and relationships between accumulation rate and temperature. The LIG surface temperature at the upstream NEEM deposition site without ice sheet altitude correction is estimated to be warmer by +7 to +11 °C (+8 °C being the most likely estimate according to constraints on past accumulation rate) compared to the pre-industrial period. This temperature estimate is consistent with the 7.5 ± 1.8 °C warming initially determined from NEEM water isotopes. Moreover, we show that under such warm temperatures, melting of snow probably led to a significant firn shrinking by ~ 15 m. Climate simulations performed with present day ice sheet topography lead to much smaller warming but larger amplitudes (up to 5 °C) can be obtained from changes in sea ice extent and ice sheet topography. Still, ice sheet simulations forced by 5 °C surface warming lead to large ice sheet decay that are not compatible with existing data. Our new, independent temperature constrain therefore reinforces the NEEM paradox.

scholarly journals Mass Balance of the Greenland Ice Sheet at Dye 3

1985 ◽  
Vol 31 (108) ◽  
pp. 198-200 ◽  
Author(s):  
Niels Reeh ◽  
Niels S. Gundestrup
Keyword(s):  
Mass Balance ◽  
Accumulation Rate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Bottom Layer ◽  
Surface Strain ◽  
Ice Thickness ◽  
Strain Rates ◽  
Confidence Limits ◽  
Rate Of Increase

AbstractThe mass balance of the Greenland ice sheet at Dye 3 is estimated on the basis of observations of ice thickness, accumulation rate, surface velocities, and surface strain-rates. The calculations indicate a rate of increase of surface elevation of 3 cm/year, with 95% confidence limits of −3 cm/year and +9 cm/year. Previous estimates of the mass balance of the Greenland ice sheet by the same method reported large imbalances; these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth-averaged quantities. The most reliable observations indicate that the interior regions of the Greenland ice sheet are at present thickening at a rate of a few centimetres per year; a contributing cause for this may be the slow thinning of a bottom layer of relatively soft Wisconsin ice.

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