Upstream flow effects revealed in the EastGRIP ice-core using a Monte Carlo inversion of a 2D ice-flow model

The North East Greenland ice-stream (NEGIS) is the largest active ice-stream on the Greenland ice-sheet and is a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice-core is drilled in the upstream part of the NEGIS, termed the East Greenland ice-core project (EastGRIP). Upstream flow effects introduce non-climatic bias in ice-cores and are particularly strong at EastGRIP due to high ice-flow velocities and the location in an ice-stream on the eastern flank of the Greenland ice-sheet. Understanding and ultimately correcting for such effects requires information on the source area and the local atmospheric conditions at the time of ice deposition. We use a two-dimensional Dansgaard-Johnsen model to simulate ice-flow along three approximated flow-lines between the summit of the ice-sheet and EastGRIP. Model parameters are determined using a Monte Carlo inversion by minimizing the misfit between modeled isochrones and isochrones observed in radio-echo-sounding images. We calculate backward-in-time particle trajectories to determine the source area of ice found in the EastGRIP core today and present estimates of surface elevation and past accumulation-rates at the deposition site. The thinning function and accumulated strain obtained from the modeled velocity field provide useful information on the deformation history in the EastGRIP ice. Our results indicate that increased accumulation in the upstream area is predominantly responsible for the constant annual layer thickness observed in the upper part of the ice column at EastGRIP. Inverted model parameters suggest that the imprint of basal melting and sliding is present in large parts along the flow profiles and that most internal ice deformation happens close to the bedrock. The results of this study can act as a basis for applying upstream corrections to a variety of ice-core measurements, and the model parameters can be useful constraints for more sophisticated modeling approaches in the future.&#160;

Download Full-text

Upstream flow effects revealed in the EastGRIP ice core using a Monte Carlo inversion of a two-dimensional ice-flow model

10.5194/tc-2021-63 ◽

2021 ◽

Author(s):

Tamara Annina Gerber ◽

Christine Schøtt Hvidberg ◽

Sune Olander Rasmussen ◽

Steven Franke ◽

Giulia Sinnl ◽

...

Keyword(s):

Monte Carlo ◽

Ice Core ◽

Ice Sheet ◽

Source Area ◽

Model Parameters ◽

Two Dimensional ◽

Ice Flow ◽

Ice Stream ◽

Flow Effects ◽

Upstream Flow

Abstract. The Northeast Greenland Ice Stream (NEGIS) is the largest active ice stream on the Greenland Ice Sheet (GrIS) and a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice core is drilled in the upstream part of the NEGIS, termed the East Greenland Ice-Core Project (EastGRIP). Upstream flow effects introduce non-climatic bias in ice cores and are particularly strong at EastGRIP due to high ice-flow velocities and the location inside an ice stream on the eastern flank of the GrIS. Understanding and ultimately correcting for such effects requires information on the source area and the local atmospheric conditions at the time of ice deposition. We use a two-dimensional Dansgaard–Johnsen model to simulate ice flow along three approximated flow lines between the summit of the ice sheet (GRIP) and EastGRIP. Model parameters are determined using a Monte Carlo inversion by minimizing the misfit between modelled isochrones and isochrones observed in radio-echo-sounding (RES) images. We calculate backward-in-time particle trajectories to determine the source area of ice found in the EastGRIP ice core and present estimates of surface elevation and past accumulation rates at the deposition site. Our results indicate that increased accumulation in the upstream area is predominantly responsible for the constant annual layer thicknesses observed in the upper part of the ice column at EastGRIP. Inverted model parameters suggest that the imprint of basal melting and sliding is present in large segments along the flow profiles and that most internal ice deformation happens in the lower half of the ice column. The results of this study act as a basis for applying upstream corrections to a variety of ice-core measurements, and the model parameters are useful constraints for more sophisticated modelling approaches in the future.

Download Full-text

Upstream flow effects revealed in the EastGRIP ice core using Monte Carlo inversion of a two-dimensional ice-flow model

The Cryosphere ◽

10.5194/tc-15-3655-2021 ◽

2021 ◽

Vol 15 (8) ◽

pp. 3655-3679

Author(s):

Tamara Annina Gerber ◽

Christine Schøtt Hvidberg ◽

Sune Olander Rasmussen ◽

Steven Franke ◽

Giulia Sinnl ◽

...

Keyword(s):

Monte Carlo ◽

Ice Core ◽

Ice Sheet ◽

Ice Cores ◽

Model Parameters ◽

Two Dimensional ◽

Flow Lines ◽

Ice Flow ◽

Ice Stream ◽

Upstream Flow

Abstract. The Northeast Greenland Ice Stream (NEGIS) is the largest active ice stream on the Greenland Ice Sheet (GrIS) and a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice core is drilled in the upstream part of the NEGIS, termed the East Greenland Ice-core Project (EastGRIP). Upstream flow can introduce climatic bias into ice cores through the advection of ice deposited under different conditions further upstream. This is particularly true for EastGRIP due to its location inside an ice stream on the eastern flank of the GrIS. Understanding and ultimately correcting for such effects requires information on the atmospheric conditions at the time and location of snow deposition. We use a two-dimensional Dansgaard–Johnsen model to simulate ice flow along three approximated flow lines between the summit of the ice sheet (GRIP) and EastGRIP. Isochrones are traced in radio-echo-sounding images along these flow lines and dated with the GRIP and EastGRIP ice-core chronologies. The observed depth–age relationship constrains the Monte Carlo method which is used to determine unknown model parameters. We calculate backward-in-time particle trajectories to determine the source location of ice found in the EastGRIP ice core and present estimates of surface elevation and past accumulation rates at the deposition site. Our results indicate that increased snow accumulation with increasing upstream distance is predominantly responsible for the constant annual layer thicknesses observed in the upper part of the ice column at EastGRIP, and the inverted model parameters suggest that basal melting and sliding are important factors determining ice flow in the NEGIS. The results of this study form a basis for applying upstream corrections to a variety of ice-core measurements, and the inverted model parameters are useful constraints for more sophisticated modelling approaches in the future.

Download Full-text

Tracer transport in an isochronal ice-sheet model

Journal of Glaciology ◽

10.1017/jog.2016.111 ◽

2016 ◽

Vol 63 (237) ◽

pp. 22-38 ◽

Cited By ~ 3

Author(s):

ANDREAS BORN

Keyword(s):

Ice Core ◽

Ice Sheet ◽

Vertical Motion ◽

Greenland Ice Sheet ◽

Ice Cores ◽

Model Parameters ◽

Tracer Transport ◽

Geochemical Tracers ◽

Tuning Method ◽

Polar Ice Sheets

ABSTRACTThe full history of ice sheet and climate interactions is recorded in the vertical profiles of geochemical tracers in polar ice sheets. Numerical simulations of these archives promise great advances both in the interpretation of these reconstructions and the validation of the models themselves. However, fundamental mathematical shortcomings of existing models subject tracers to spurious diffusion, thwarting straightforward solutions. Here, I propose a new vertical discretization for ice-sheet models that eliminates numerical diffusion entirely. Vertical motion through the model mesh is avoided by mimicking the real-world flow of ice as a thinning of underlying layers. A new layer is added to the surface at equidistant time intervals, isochronally, thus identifying each layer uniquely by its time of deposition and age. This new approach is implemented for a two-dimensional section through the summit of the Greenland ice sheet. The ability to directly compare simulations of vertical ice cores with reconstructed data is used to find optimal model parameters from a large ensemble of simulations. It is shown that because this tuning method uses information from all times included in the ice core, it constrains ice-sheet sensitivity more robustly than a realistic reproduction of the modern ice-sheet surface.

Download Full-text

Bed topography and subglacial landforms in the onset region of the Northeast Greenland Ice Stream

Annals of Glaciology ◽

10.1017/aog.2020.12 ◽

2020 ◽

Vol 61 (81) ◽

pp. 143-153 ◽

Cited By ~ 3

Author(s):

Steven Franke ◽

Daniela Jansen ◽

Tobias Binder ◽

Nils Dörr ◽

Veit Helm ◽

...

Keyword(s):

Boundary Conditions ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ultra Wideband ◽

Ice Flow ◽

Detailed Model ◽

Dynamic Component ◽

Bed Topography ◽

Ice Stream ◽

Topography Model

AbstractThe Northeast Greenland Ice Stream (NEGIS) is an important dynamic component for the total mass balance of the Greenland ice sheet, as it reaches up to the central divide and drains 12% of the ice sheet. The geometric boundary conditions and in particular the nature of the subglacial bed of the NEGIS are essential to understand its ice flow dynamics. We present a record of more than 8000 km of radar survey lines of multi-channel, ultra-wideband radio echo sounding data covering an area of 24 000 km2, centered on the drill site for the East Greenland Ice-core Project (EGRIP), in the upper part of the NEGIS catchment. Our data yield a new detailed model of ice-thickness distribution and basal topography in the region. The enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity, potentially over several glacial–interglacial cycles. Off-nadir reflections from the ice–bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms. Our new bed topography model will help to improve the basal boundary conditions of NEGIS prescribed for ice flow models and thus foster an improved understanding of the ice-dynamic setting.

Download Full-text

Ice flow between the Greenland Ice Core Project and Greenland Ice Sheet Project 2 boreholes in central Greenland

Journal of Geophysical Research Atmospheres ◽

10.1029/97jc00268 ◽

1997 ◽

Vol 102 (C12) ◽

pp. 26851-26859 ◽

Cited By ~ 19

Author(s):

C. Schøtt Hvidberg ◽

D. Dahl-Jensen ◽

E. D. Waddington

Keyword(s):

Ice Core ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ice Flow ◽

Core Project

Download Full-text

East Greenland ice core dust record reveals timing of Greenland ice sheet advance and retreat

Nature Communications ◽

10.1038/s41467-019-12546-2 ◽

2019 ◽

Vol 10 (1) ◽

Cited By ~ 10

Author(s):

Marius Folden Simonsen ◽

Giovanni Baccolo ◽

Thomas Blunier ◽

Alejandra Borunda ◽

Barbara Delmonte ◽

...

Keyword(s):

Sea Level ◽

Ice Core ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Interglacial Period ◽

Last Deglaciation ◽

Glacial Cycle ◽

East Greenland ◽

Ice Cap ◽

Climate Forcings

Abstract Accurate estimates of the past extent of the Greenland ice sheet provide critical constraints for ice sheet models used to determine Greenland’s response to climate forcing and contribution to global sea level. Here we use a continuous ice core dust record from the Renland ice cap on the east coast of Greenland to constrain the timing of changes to the ice sheet margin and relative sea level over the last glacial cycle. During the Holocene and the previous interglacial period (Eemian) the dust record was dominated by coarse particles consistent with rock samples from central East Greenland. From the coarse particle concentration record we infer the East Greenland ice sheet margin advanced from 113.4 ± 0.4 to 111.0 ± 0.4 ka BP during the glacial onset and retreated from 12.1 ± 0.1 to 9.0 ± 0.1 ka BP during the last deglaciation. These findings constrain the possible response of the Greenland ice sheet to climate forcings.

Download Full-text

A multi-approach skill-score procedure to optimize continental-scale ice-sheet models

10.5194/egusphere-egu2020-18968 ◽

2020 ◽

Author(s):

Ilaria Tabone ◽

Alexander Robinson ◽

Jorge Alvarez-Solas ◽

Javier Blasco ◽

Daniel Moreno ◽

...

Keyword(s):

Large Scale ◽

Thermal State ◽

Spatial Scales ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Skill Score ◽

Model Parameters ◽

Ice Flow ◽

Flow Equations ◽

Continental Scale

Simulations of large-scale ice sheet models are crucial to understand the long-term evolution of an ice sheet and its response to climate forcings. However, solving the ice-flow equations and processes proper of the ice sheet at large spatial scales requires reducing the model computational complexity to a certain degree. To do so, coarse-resolution models represent several physical processes and ice characteristics through model parameterisations. Ice-sheet boundary conditions (e.g. basal sliding, surface ablation, grounded and marine basal melting) as well as unconstrained ice-flow properties (e.g. ice-flow enhancement factor) are some examples. However, choosing the best parameter values to well represent such processes is a demanding exercise. Statistical methods, from simple to advanced techniques involving Bayesian approaches, have been taken into account to evaluate the model performance. Here we optimise the performance of a new state-of-the-art hybrid ice-sheet-shelf model by applying a skill-score method based on a multi-misfits approach. A large ensemble of paleo-to-present transient simulations of the Greenland ice sheet (GrIS) is produced through the Latin Hypercube Sampling technique. Results are then evaluated against a variety of information, comprising the present-day state of the ice sheet (e.g. ice thickness, ice velocity, basal thermal state) as well as available paleo reconstructions (e.g. glacial maximum extent, past elevation at the ice core sites). Results are then assembled to generate a single skill-score value based on a gaussian approach. The procedure is applied to various model parameters to evaluate the best choice of values associated with their parameterisations.&#160;

Download Full-text

Heat flux and the North East Greenland Ice Stream

10.5194/egusphere-egu21-13450 ◽

2021 ◽

Author(s):

Paul D. Bons ◽

Tamara de Riese ◽

Steven Franke ◽

Maria-Gema Llorens ◽

Till Sachau ◽

...

Keyword(s):

Heat Flux ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Heat Fluxes ◽

High Heat Flux ◽

Geothermal Heat ◽

Radiogenic Heat ◽

East Greenland ◽

North East ◽

Ice Stream

The prominent North East Greenland Ice Stream (NEGIS) is an exceptionally large ice stream in the Greenland Ice sheet. It is over 500 km long, originates almost at the central ice divide, and contributes significantly to overall ice drainage from the Greenland Ice sheet. Surface velocities in the inland part of the ice stream are several times higher inside NEGIS than in the adjacent ice sheet. Modelling NEGIS is still a challenge as it remains unclear what actually causes and controls the ice stream.An elevated geothermal heat flux is one of the factors that are being considered to trigger or drive the fast flow inside NEGIS. Unfortunately, the geothermal heat flux below NEGIS and its upstream area is poorly constrained and estimates vary from close to the global average for continental crust (ca. 60 mW/m2) to values as high as almost 1000 mW/m2. The latter would cause about 10 cm/yr of melting at the base of the ice sheet.We present a brief survey of global geothermal heat flux data, especially from known hotspots, such as Iceland and Yellowstone. Heat fluxes in these areas that are known to be among the hottest on Earth rarely, if ever, exceed 300 mW/m2. A plume hotspot or its trail can therefore not cause heat fluxes at the high end of the suggested range. Other potential factors, such as hydrothermal fluid flow and radiogenic heat, also cannot raise the heat flux significantly. We conclude that the heat flux at NEGIS is very unlikely to exceed 100-150 mW/m2, and future modelling studies on NEGIS should thus be mindful of implementing realistic geothermal heat flux values. If NEGIS is not the result of an exceptionally high heat flux, we are left with the exciting challenge to find the true trigger of this fascinating structure.

Download Full-text

Exceptionally high heat flux needed to sustain the Northeast Greenland Ice Stream

The Cryosphere ◽

10.5194/tc-14-841-2020 ◽

2020 ◽

Vol 14 (3) ◽

pp. 841-854 ◽

Cited By ~ 1

Author(s):

Silje Smith-Johnsen ◽

Basile de Fleurian ◽

Nicole Schlegel ◽

Helene Seroussi ◽

Kerim Nisancioglu

Keyword(s):

Heat Flux ◽

Ice Core ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

High Heat ◽

High Heat Flux ◽

Good Representation ◽

Geothermal Heat ◽

Ice Dynamics ◽

Ice Stream

Abstract. The Northeast Greenland Ice Stream (NEGIS) currently drains more than 10 % of the Greenland Ice Sheet area and has recently undergone significant dynamic changes. It is therefore critical to accurately represent this feature when assessing the future contribution of Greenland to sea level rise. At present, NEGIS is reproduced in ice sheet models by inferring basal conditions using observed surface velocities. This approach helps estimate conditions at the base of the ice sheet but cannot be used to estimate the evolution of basal drag in time, so it is not a good representation of the evolution of the ice sheet in future climate warming scenarios. NEGIS is suggested to be initiated by a geothermal heat flux anomaly close to the ice divide, left behind by the movement of Greenland over the Icelandic plume. However, the heat flux underneath the ice sheet is largely unknown, except for a few direct measurements from deep ice core drill sites. Using the Ice Sheet System Model (ISSM), with ice dynamics coupled to a subglacial hydrology model, we investigate the possibility of initiating NEGIS by inserting heat flux anomalies with various locations and intensities. In our model experiment, a minimum heat flux value of 970 mW m−2 located close to the East Greenland Ice-core Project (EGRIP) is required locally to reproduce the observed NEGIS velocities, giving basal melt rates consistent with previous estimates. The value cannot be attributed to geothermal heat flux alone and we suggest hydrothermal circulation as a potential explanation for the high local heat flux. By including high heat flux and the effect of water on sliding, we successfully reproduce the main characteristics of NEGIS in an ice sheet model without using data assimilation.

Download Full-text