G42A-01
Calibrating a Glaciological Model of the Greenland Ice Sheet From the Last Glacial Maximum to Present-day Using Field Observations of Relative sea Level and ice Extent
We constrain a three-dimensional thermomechnical model of Greenland Ice Sheet evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using primarily observations of relative sea level as well as data on past ice extent. Our new model (Huy2) fits the majority of the observations and is characterised by a number of key features: (i) The ice sheet had a LGM excess volume (relative to present) of 4.1 m ice- equivalent sea-level which increased to a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin (there was a readvance during the Younger Dryas) and the final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in the northeast. As a result of this retreat, the modelled ice sheet reaches a minimum volume between 5 - 4 ka BP which corresponds to a rise of 0.17 m ice-equivalent sea-level since this time. Our results suggest that remaining discrepancies between the model and observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or a diachronous ice margin retreat.
G42A-02
Deepest Mantle Viscosity and Earth Rotation: A Further Refinement of the ICE-5G(VM2) Model of the Glacial Isostatic Adjustment Process
Recent analyses of the time-dependent gravity field delivered by the GRACE program have clearly demonstrated the accuracy of the ICE-5G(VM2) model of the global process of glacial isostatic adjustment insofar as the observed features of the signal associated with North American deglaciation are concerned. Since the model was published prior to the availability of GRACE measurements, this has been very satisfying. There are nevertheless minor misfits that continue to exist between the predictions of the model and other related observations. One of these concerns the ability of the model to fit observations of certain anomalies associated with the planet's rotational state, namely the non-tidal acceleration of planetary rotation and the ongoing wander of the pole towards the centroid of the ancient Laurentide ice sheet. I employ the fact that the viscosity of the mantle from the surface to ~1400 km depth is "pinned" by both relative sea level and GRACE data to investigate the modifications to the viscosity structure at greater depth that are required to eliminate the remaining misfits to the rotational constraints. These analyses demonstrate that both misfits are simultaneously removed by the introduction of a low viscosity region coincident with the D" boundary- layer adjacent to the core-mantle interface. This is a highly satisfactory result as such a low viscosity region is expected on a priori grounds. The result has implications concerning the accuracy of previously published models for the viscosity of the deepest mantle and these will be discussed.
G42A-03 INVITED
Constraints on Accumulation and Postglacial Rebound from GRACE and InSAR
We use monthly measurements of time variable gravity from the GRACE satellite gravity mission to determine spatial variation in ice mass (IM) trend and seasonality for over a six year period starting in Apr 2002. We compare spatial pattern and amplitude of the IM changes from GRACE with the one from the mass budget methods combining InSAR and regional climate modeling output. The two estimates are completely independent and characterized by different error sources. While largest source of error for the GRACE IM estimates is the uncertainties in the glacial isostatic adjustment (GIA) signal. For the mass budget method a large source of uncertainty can be traced to the surface mass balance components from regional climate modeling output. The comparison allows improved constrains on GIA and surface mass balance model output. We find that a thinner ice sheet at the Last Glacial Maximum may be required in the East Antarctic Ice sheet as well as in the Ross Sea region. In the Bellingshausen Sea area comparisons indicate that both a larger signal from the long-term accumulation and a larger GIA signal are likely. The adjusted forward model explanation for the Bellinghausen Sea time-dependent gravity is attributed to a combination of lower viscosity structure and geologically more recent ice mass loss.
G42A-04 INVITED
Glacial Isostatic Adjustment over Antarctica from combined GRACE and ICESat satellite data
The determination of present-day changes in the Antarctic mass balance through satellite gravity measurements is severely hampered by insufficient knowledge of ongoing glacial isostatic adjustment (GIA) due to the historic deglaciation in the Late-Pleistocene. Previous studies have speculated that it might be possible to distinguish between signals of ongoing GIA from past ice mass changes through the combination of satellite gravimetry and satellite altimetry. In this study, it is shown that by combining four years of gravity and altimetry data from the GRACE and ICESat satellite missions, the GIA contribution can indeed be isolated. The inferred GIA signal over the complete Antarctic continent supports Late-Pleistocene ice models derived from glacio-geologic studies, with important differences over the two main ice-shelves. The contribution of GIA mass change remains limited to less than 100 Gt/yr, which is considerably smaller than previously thought.
G42A-05
Assimilating geodetic data into GIA estimates over North America
Recently the amount of geodetic data over North America that can be used to constrain the ongoing impact of glacial isostatic adjustment (GIA) has greatly increased. In this presentation, we explore the possibility of using this data to improve the predictions of crustal motion and gravity trends due to GIA without referring the changes back to specific variations in the earth model parameters or the spatial and temporal history of the ice sheet. The overarching goal is to produce GIA estimates with realistic uncertainties that could be used in geodetic analysis to better constrain contemporary changes in, for example, continental water storage, ice sheet variations and sea level change. The assimilation procedure utilizes the covariance between different data types derived from forward model predictions that are differentiated by viscosity structure and lithospheric thickness. We illustrate the skill of the technique in recovering various forward model predictions when sampled in a similar manner to the real data sets in the presence of noise and unknown reference frame rate parameters. These tests also include GIA predictions generated using ice sheet and earth models not included in the covariance calculation. Finally, we discuss the results of applying the technique to GPS rate estimates from North America as well as GRACE data.
G42A-06 INVITED
Modelling the Glacial Isostatic Adjustment of Greenland on Millennial to Decadal Timescales
Near-field observations of sea-level change can provide useful information on ice-Earth interaction over a range of timescales. The majority of previous studies have considered data that record sea-level changes over millennial (1000 yr) timescales to constrain models of glacial isostatic adjustment (GIA) during the most recent deglaciation and through the Holocene. In this paper, we present results from a project aimed to improve our knowledge of Greenland ice sheet (GrIS) evolution and GIA over the past few centuries. We employ a model of GIA to interpret sea-level observations from Greenland reconstructed from proxy methods that document changes over the past 12 kyr (approx.) at millennial scale resolution and the past 500 years at up to decadal scale resolution. We focus on observations from the south west sector of the GrIS that show a steady millennial scale sea-level rise of a few mm/yr during the late Holocene that is interrupted by an abrupt acceleration between 1500 and 1600 AD and followed by sea levels that are stable to within a few decimetres from this time to present. Predictions based on two different ice models indicate that the observed sea-level signal results from a combination of processes operating on different timescales. We conclude that the model constraints provided by the new high resolution sea-level observations fill a data gap (between traditional millennial-scale geological data and satellite era measurements) that is critical to accurately interpret recent geodetic observations of ice mass changes and solid Earth motion.
G42A-07 INVITED
Geodetic observations to estimate ice mass changes and GIA in Antarctica and Greenland
The GRACE (Gravity Recovery and Climate Experiment) satellite mission launched in 2002 is able to observe mass changes in the Earth system through their gravitational effect. Hence, ice mass changes of the Antarctic and Greenland ice sheets may be inferred from GRACE data. However, the respective results published from different analyses differ considerably. A thorough understanding of the various error mechanisms involved in the GRACE data analysis and in the geophysical reductions is necessary to ensure reliable results with realistic uncertainty assessments as well as to advance the methods of analysis. The GIA signal is a crucial quantity for the separation of ice mass changes and the solid earth response. Therefore, complementary observations are needed for this purpose. We discuss the role of GPS observations in this context providing also estimates of the related accuracies. Finally, we present our results on Antarctic and Greenland ice mass changes obtained with an adapted methodology from the Release 04 monthly GRACE solutions by GeoForschungsZentrum Potsdam for the time interval from 08/2002 to 01/2008. We consider mass changes of the entire ice sheets but also of their individual large drainage basins. For Antarctica, for example, we detect ice mass loss which is clearly dominated by changes in the Amundsen Sea Sector and Northwest Marie Byrd Land (West Antarctica) while East Antarctica appears to be near balance.
G42A-08
Geodetic estimates of the vertical crustal velocity field in Antarctica
We present the initial results of the West Antarctic GPS Network (WAGN), which was initiated in the 2002/03 Antarctic field season. Specifically we present vertical velocity estimates for all WAGN stations with a total occupational time span of 3 years or more. Most of these station have been observed for at least four years, and many for 5 years or more. We present solutions at 12 stations in West Antarctica, as well as for 4 stations in the upper Antarctic Peninsula, and 7 CGPS stations in East Antarctica. We realized our reference frame by minimizing the vertical velocities of 202 CGPS stations located outside of the Antarctic continent, and by minimizing the horizontal velocities of 11 stations within Antarctica. The vertical velocities of the East Antarctic stations are all small, in agreement with most models for GIA. The observed pattern of vertical motion within West Antarctica and the Peninsula does not correspond well with any GIA model known to us. We shall discuss the implications for ice mass balance studies based on GRACE observations.