scholarly journals Inversion of IceBridge gravity data for continental shelf bathymetry beneath the Larsen Ice Shelf, Antarctica

2012 ◽  
Vol 58 (209) ◽  
pp. 540-552 ◽  
Author(s):  
James R. Cochran ◽  
Robin E. Bell
Keyword(s):  
Continental Shelf ◽  
Ocean Circulation ◽  
Gravity Data ◽  
Radar Data ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Free Air ◽  
Free Air Gravity ◽  
Larsen Ice Shelf ◽  
Possible Cause

AbstractA possible cause for accelerated thinning and break-up of floating marine ice shelves is warming of the water in the cavity below the ice shelf. Accurate bathymetry beneath large ice shelves is crucial for developing models of the ocean circulation in the sub-ice cavities. A grid of free-air gravity data over the floating Larsen C ice shelf collected during the IceBridge 2009 Antarctic campaign was utilized to develop the first bathymetry model of the underlying continental shelf. Independent control on the continental shelf geologic structures from marine surveys was used to constrain the inversion. Depths on the continental shelf beneath the ice shelf estimated from the inversion generally range from about 350 to 650 m, but vary from <300 to >1000 m. Localized overdeepenings, 20-30 km long and 900-1000 m deep, are located in inlets just seaward of the grounding line. Submarine valleys extending seaward from the overdeepenings coalesce into two broad troughs that extend to the seaward limit of the ice shelf and appear to extend to the edge of the continental shelf. The troughs are generally at a depth of 550-700 m although the southernmost mapped trough deepens to over 1000 m near the edge of the ice shelf just south of 68° S. The combination of the newly determined bathymetry with published ice-draft determinations based on laser altimetry and radar data defines the geometry of the water-filled cavity. These newly imaged troughs provide a conduit for water to traverse the continental shelf and interact with the overlying Larsen C ice shelf and the grounding lines of the outlet glaciers.

scholarly journals Seabed topography beneath Larsen C Ice Shelf from seismic soundings

2013 ◽  
Vol 7 (4) ◽  
pp. 4177-4206
Author(s):  
A. M. Brisbourne ◽  
A. M. Smith ◽  
E. C. King ◽  
K. W. Nicholls ◽  
P. R. Holland ◽  
...  
Keyword(s):  
Velocity Profile ◽  
Ocean Circulation ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Gravity Inversion ◽  
Oceanic Circulation ◽  
Ice Shelf ◽  
Free Air ◽  
Cavity Thickness ◽  
Free Air Gravity

Abstract. Seismic reflection soundings of ice thickness and seabed depth were acquired on the Larsen C Ice Shelf in order to test a sub-shelf bathymetry model derived from the inversion of IceBridge gravity data. A series of lines were collected, from the Churchill Peninsula in the north to the Joerg Peninsula in the south, and also towards the ice front. Sites were selected using the bathymetry model derived from the inversion of free-air gravity data to indicate key regions where sub-shelf oceanic circulation may be affected by ice draft and sub-shelf cavity thickness. The seismic velocity profile in the upper 100 m of firn and ice was derived from shallow refraction surveys at a number of locations. Measured temperatures within the ice column and at the ice base were used to define the velocity profile through the remainder of the ice column. Seismic velocities in the water column were derived from previous in situ measurements. Uncertainties in ice and water cavity thickness are in general <10 m. Compared with the seismic measurements, the root-mean-square error in the gravimetrically derived bathymetry at the seismic sites is 162 m. The seismic profiles prove the non-existence of several bathymetric features that are indicated in the gravity inversion model, significantly modifying the expected oceanic circulation beneath the ice shelf. Similar features have previously been shown to be highly significant in affecting basal melt rates predicted by ocean models. The discrepancies between the gravity inversion results and the seismic bathymetry are attributed to the assumption of uniform geology inherent in the gravity inversion process and also the sparsity of IceBridge flight lines. Results indicate that care must be taken when using bathymetry models derived by the inversion of free-air gravity anomalies. The bathymetry results presented here will be used to improve existing sub-shelf ocean circulation models.

scholarly journals Seabed topography beneath Larsen C Ice Shelf from seismic soundings

2014 ◽  
Vol 8 (1) ◽  
pp. 1-13 ◽  
Author(s):  
A. M. Brisbourne ◽  
A. M. Smith ◽  
E. C. King ◽  
K. W. Nicholls ◽  
P. R. Holland ◽  
...  
Keyword(s):  
Velocity Profile ◽  
Ocean Circulation ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Gravity Inversion ◽  
Seismic Velocities ◽  
Oceanic Circulation ◽  
Ice Shelf ◽  
Free Air ◽  
Free Air Gravity

Abstract. Seismic reflection soundings of ice thickness and seabed depth were acquired on the Larsen C Ice Shelf in order to test a sub-ice shelf bathymetry model derived from the inversion of IceBridge gravity data. A series of lines was collected, from the Churchill Peninsula in the north to the Joerg Peninsula in the south, and also towards the ice front. Sites were selected using the bathymetry model derived from the inversion of free-air gravity data to indicate key regions where sub-ice shelf oceanic circulation may be affected by ice draft and seabed depth. The seismic velocity profile in the upper 100 m of firn and ice was derived from shallow refraction surveys at a number of locations. Measured temperatures within the ice column and at the ice base were used to define the velocity profile through the remainder of the ice column. Seismic velocities in the water column were derived from previous in situ measurements. Uncertainties in ice and water cavity thickness are in general < 10 m. Compared with the seismic measurements, the root-mean-square error in the gravimetrically derived bathymetry at the seismic sites is 162 m. The seismic profiles prove the non-existence of several bathymetric features that are indicated in the gravity inversion model, significantly modifying the expected oceanic circulation beneath the ice shelf. Similar features have previously been shown to be highly significant in affecting basal melt rates predicted by ocean models. The discrepancies between the gravity inversion results and the seismic bathymetry are attributed to the assumption of uniform geology inherent in the gravity inversion process and also the sparsity of IceBridge flight lines. Results indicate that care must be taken when using bathymetry models derived by the inversion of free-air gravity anomalies. The bathymetry results presented here will be used to improve existing sub-ice shelf ocean circulation models.

Constraining ice shelf basalt melting rates from isochrone data

2021 ◽  
Author(s):  
Vjeran Visnjevic ◽  
Reinhard Drews ◽  
Clemens Schannwell ◽  
Inka Koch
Keyword(s):  
Ocean Circulation ◽  
Radar Data ◽  
Ice Shelf ◽  
Ice Dynamics ◽  
Ice Flow ◽  
Ice Shelves ◽  
Inverse Approach ◽  
History Of ◽  
Forward And Inverse Modeling ◽  
Basal Melting

&lt;p&gt;Ice shelves buttress ice flow from the continent towards the ocean, and their disintegration results in increased ice discharge.&amp;#160; Ice-shelf evolution and integrity is influenced by surface accumulation, basal melting, and ice dynamics. We find signals of all of these processes imprinted in the ice-shelf stratigraphy that can be mapped using isochrones imaged with radar.&lt;/p&gt;&lt;p&gt;Our aim is to develop an inverse approach to infer ice shelf basal melt rates using radar isochrones as observational constraints. Here, we investigate the influence of basalt melt rates on the shape of isochrones using combined insights from both forward and inverse modeling. We use the 3D full Stokes model Elmer/Ice in our forward simulations, aiming to reproduce isochrone patterns observed in our data. Moreover we develop an inverse approach based on the shallow shelf approximating, aiming to constrain basal melt rates using isochronal radar data and surface velocities. Insights obtained from our simulations can also guide the collection of new radar data (e.g., profile lines along vs. across-flow) in a way that ambiguities in interpreting the ice-shelf stratigraphy can be minimized. Eventually, combining these approaches will enable us to better constrain the magnitude and history of basal melting, which will give valuable input for ocean circulation and sea level rise projections.&lt;/p&gt;

scholarly journals Long melt seasons on ice shelves of the Antarctic Peninsula: an analysis using satellite-based microwave emission measurements

2002 ◽  
Vol 34 ◽  
pp. 127-133 ◽  
Author(s):  
Mark A. Fahnestock ◽  
Waleed Abdalati ◽  
Christopher A. Shuman
Keyword(s):  
Time Series ◽  
Antarctic Peninsula ◽  
Threshold Value ◽  
Microwave Emission ◽  
Ice Shelf ◽  
Ice Shelves ◽  
Gradient Ratio ◽  
The Antarctic ◽  
Larsen Ice Shelf ◽  
Possible Cause

AbstractWe have examined the record of melt-season duration on the Antarctic Peninsula using two techniques for detecting the presence of a melt signal in microwave-emission time series covering the period 1978–2000. We have obtained similar estimates of melt-season length using the cross-polarized gradient ratio (XPGR) technique and calibrations previously applied in Greenland and a technique which detects the jump in emission caused by melt without using a sensor- and frequency-dependent threshold value. The close correspondence between results from the two techniques on peninsula ice shelves suggests that the XPGR analysis can be used over the length of the time series. The results show that the long melt seasons of 1992/93 and several later years were exceptional occurrences on the northern parts of the Larsen Ice Shelf. These melt seasons were followed by disintegration events, supporting a possible cause-and-effect relationship.

scholarly journals An improved bathymetry compilation for the Bellingshausen Sea, Antarctica, to inform ice-sheet and ocean models

2010 ◽  
Vol 4 (4) ◽  
pp. 2079-2101 ◽  
Author(s):  
A. G. C. Graham ◽  
F. O. Nitsche ◽  
R. D. Larter
Keyword(s):  
Ocean Circulation ◽  
Ice Sheet ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Shelves ◽  
Sea Floor ◽  
Bellingshausen Sea ◽  
Warm Deep Water ◽  
Basal Melting

Abstract. The southern Bellingshausen Sea (SBS) is a rapidly-changing part of West Antarctica, where oceanic and atmospheric warming has led to the recent basal melting and break-up of the Wilkins ice shelf, the dynamic thinning of fringing glaciers, and sea-ice reduction. Accurate sea-floor morphology is vital for understanding the continued effects of each process upon changes within Antarctica's ice sheets. Here we present a new bathymetric grid for the SBS compiled from shipborne echo-sounder, spot-sounding and sub-ice measurements. The 1-km grid is the most detailed compilation for the SBS to-date, revealing large cross-shelf troughs, shallow banks, and deep inner-shelf basins that continue inland of coastal ice shelves. The troughs now serve as pathways which allow warm deep water to access the ice fronts in the SBS. Our dataset highlights areas still lacking bathymetric constraint, as well as regions for further investigation, including the likely routes of palaeo-ice streams. The new compilation is a major improvement upon previous grids and will be a key dataset for incorporating into simulations of ocean circulation, ice-sheet change and history. It will also serve forecasts of ice stability and future sea-level contributions from ice loss in West Antarctica, required for the next IPCC assessment report in 2013.

Sources and transport of glacial meltwater in the Bellingshausen Sea, Antarctica

2021 ◽  
Author(s):  
Peter Sheehan ◽  
Karen Heywood ◽  
Andrew Thompson ◽  
Mar Flexas
Keyword(s):  
Ocean Circulation ◽  
World Ocean ◽  
Cyclonic Circulation ◽  
Shelf Break ◽  
Multiple Sources ◽  
Ice Shelf ◽  
Transport Pathways ◽  
Ice Shelves ◽  
Bellingshausen Sea ◽  
The Impact

&lt;p&gt;Quantifying meltwater content and describing transport pathways is important for understanding the impact of a warming, melting Antarctica on ocean circulation. Meltwater fluxes can affect density-driven, on-shelf flows around the continent, and the formation of the dense water masses that ventilate abyssal regions of the world ocean. We present observations collected from two ocean gliders that were deployed in the Bellingshausen Sea for a period of 10 weeks between January and March of 2020.&lt;span&gt;&amp;#160; &lt;/span&gt;Using multiple high-resolution sections, we quantify both the distribution of meltwater concentrations and lateral meltwater fluxes within the Belgica Trough in the Bellingshausen Sea. We observe a cyclonic circulation in the trough, in agreement with previous studies. A meltwater flux of 0.46 mSv is observed flowing northwards in the&lt;span&gt;&amp;#160; &lt;/span&gt;western limb of the cyclonic circulation. A newly identified meltwater re-circulation (0.88 mSv) is observed flowing back towards the ice front (i.e. southwards) with the eastern limb of the cyclonic circulation. In addition, 1.16 mSv of meltwater is observed flowing northeastward, parallel to the shelf break, with the northern limb of the cyclonic circulation. Peak meltwater is concentrated into two layers associated with different density surfaces: one approximately 150 m deep (27.4 kg m&lt;sup&gt;-3&lt;/sup&gt;) and one approximately 200 m deep (27.6 kg m&lt;sup&gt;-3&lt;/sup&gt;}). The deeper of these layers is characterised by an elevated optical backscatter, which indicates a more turbid water mass. The shallower layer is less turbid, and is more prominent closer to the shelf break and in the eastern part of the Belgica Trough. We hypothesise that the deeper, turbid meltwater layer originates locally from the Venables Ice Shelf, whereas the shallower, less turbid meltwater layer, comprises meltwater from ice shelves in the eastern Bellingshausen Sea. The broad distribution of meltwater from multiple sources suggests the potential for remote interactions and feedbacks between the various ice shelves that abut the Bellingshausen Sea.&lt;/p&gt;

Sea floor topography modeling by cumulating different types of gravity information

2020 ◽  
Author(s):  
Lucia Seoane ◽  
Benjamin Beirens ◽  
Guillaume Ramillien
Keyword(s):  
New England ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Geoid Height ◽  
Extended Kalman Filtering ◽  
Sea Floor ◽  
Single Beam ◽  
Free Air ◽  
Free Air Gravity ◽  
Great Meteor Seamount

&lt;p&gt;We propose to cumulate complementary gravity data, i.e. geoid height and (radial) free-air gravity anomalies, to evaluate the 3-D shape of the sea floor more precisely. For this purpose, an Extended Kalman Filtering (EKF) scheme has been developed to construct the topographic solution by injecting gravity information progressively. The main advantage of this sequential cumulation of data is the reduction of the dimensions of the inverse problem. Non linear Newtonian operators have been re-evaluated from their original forms and elastic compensation of the topography is also taken into account. The efficiency of the method is proved by inversion of simulated gravity observations to converge to a stable topographic solution with an accuracy of only a few meters. Real geoid and gravity data are also inverted to estimate bathymetry around the New England and Great Meteor seamount chains. Error analysis consists of comparing our topographic solutions to accurate single beam ship tracks for validation.&lt;/p&gt;

The unification of gravity data for Ireland-Northern Ireland

2020 ◽  
Vol 39 (2) ◽  
pp. 135-143
Author(s):  
Sajjad Sajjadi ◽  
Zdeněk Martinec ◽  
Patrick Prendergast ◽  
Jan Hagedoorn ◽  
Libor Šachl ◽  
...  
Keyword(s):  
Northern Ireland ◽  
Tide Gauge ◽  
Gravity Data ◽  
Base Station ◽  
Base Stations ◽  
Data Format ◽  
Free Air ◽  
Vertical Datum ◽  
Systematic Biases ◽  
Free Air Gravity

The systematic biases and errors associated with gravity data in Ireland and Northern Ireland and the conversion of gravity to a consistent and unified system are analyzed. The gravity data in Ireland and Northern Ireland are given in different coordinate systems (Irish Grid and Irish Transverse Mercator), different gravity base stations (Dunsink and Cambridge), and different vertical datums (Malin Head and Belfast tide gauge). The conversion of the gravity data to a consistent system, which refers to unified coordinates, base station, and vertical datum, is essential in geophysics and geodesy, especially in geoid determination. A new standardized and unified data format is computed and proposed for the supply of gravity data for Ireland and Northern Ireland to minimize the potential of misinterpreting the data. As part of this study, simple Bouguer and free-air gravity anomaly maps are produced for Ireland and Northern Ireland to give an example of how to integrate the data.

The Origin of Lunar Mascon Basins

Science ◽  
2013 ◽  
Vol 340 (6140) ◽  
pp. 1552-1555 ◽  
Author(s):  
H. J. Melosh ◽  
Andrew M. Freed ◽  
Brandon C. Johnson ◽  
David M. Blair ◽  
Jeffrey C. Andrews-Hanna ◽  
...  
Keyword(s):  
Gravity Data ◽  
Gravity Anomalies ◽  
Element Model ◽  
Isostatic Adjustment ◽  
Melt Pool ◽  
Free Air ◽  
Free Air Gravity ◽  
Bull's Eye ◽  
Gravity Recovery ◽  
The Impact

High-resolution gravity data from the Gravity Recovery and Interior Laboratory spacecraft have clarified the origin of lunar mass concentrations (mascons). Free-air gravity anomalies over lunar impact basins display bull’s-eye patterns consisting of a central positive (mascon) anomaly, a surrounding negative collar, and a positive outer annulus. We show that this pattern results from impact basin excavation and collapse followed by isostatic adjustment and cooling and contraction of a voluminous melt pool. We used a hydrocode to simulate the impact and a self-consistent finite-element model to simulate the subsequent viscoelastic relaxation and cooling. The primary parameters controlling the modeled gravity signatures of mascon basins are the impactor energy, the lunar thermal gradient at the time of impact, the crustal thickness, and the extent of volcanic fill.

Seismic investigation of the Larsen Ice Shelf, Antarctica: in search of the Larsen Basin

1995 ◽  
Vol 7 (2) ◽  
pp. 181-190 ◽  
Author(s):  
E.P. Jarvis ◽  
E.C. King
Keyword(s):  
Gravity Data ◽  
Hot Water ◽  
Seismic Velocities ◽  
Ice Shelf ◽  
Sedimentary Sequences ◽  
Reflection Data ◽  
Sample Interval ◽  
James Ross Island ◽  
Small Team ◽  
Larsen Ice Shelf

Seismic reflection surveys were carried out over the Larsen Ice Shelf to examine the extent of the observed sedimentary sequences of the Larsen Basin as suggested by aeromagnetic and gravity data. The surveys were carried out with a small team of six, working from Skidoo motor toboggans and Nansen sledges. Charges of up to 8 kg were fired in hot-water drilled holes up to 9 m deep and 6 sec records made by a 48 channel TI DFS V system with a 4 ms sample interval. By towing a 2.4 km cable behind a Skidoo it was possible to obtain 2.4 km of 24 fold data per day. The reflection data were supplemented by shallow refraction surveys using a 12 channel Nimbus seismograph and by a 12 km expanding spread experiment. The refraction data gave velocities of 1305 ± 20 m s−1 for surface snow and 3154 m s−1 for the top 100 m of shelf ice. The 24 km of reflection data showed high seismic velocities with weak shallow reflectors, characteristics which are quite different from the known basin fill on James Ross Island. It is concluded that the surveys were done outside the basin and that the depth to basement estimates made from the aeromagnetic data do not provide a reliable guide to the extent of the basin.

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