Deglacial grounding-line retreat in the Ross Embayment, Antarctica, controlled by ocean and atmosphere forcing

Reconstructing past grounding-line evolution can help inform future sea level projections by constraining marine ice sheet sensitivities to changes in climate. The Ross Embayment, the largest sector of Antarctica, experienced substantial grounding-line retreat since the Last Glacial Maximum. However, different interpretations for the timing and spatial pattern of deglacial grounding-line retreat in this region persist, suggesting either very high or low sensitivity to external forcings. Complicating matters is the sparse paleoclimate record, which is limited spatially and temporally. In this thesis, I address these issues by analysing the output of two transient climate simulations in relation to Antarctic ice core and marine sediment records, and performing and analysing the largest ensemble to date of regional ice sheet model simulations of the last deglaciation in the Ross Sea. The climate models and paleoclimate proxy records exhibit key differences in the timing, magnitude and duration of millennial-scale climate change events through the deglacial period. Using this diverse set of deglacial climate trajectories as ocean and atmosphere forcings, the ice sheet model ensemble produces a wide range of ice sheet responses, supporting the view that external forcings are the main drivers of past grounding-line retreat in the region. The simulations demonstrate that atmospheric conditions early in the deglacial period can enhance or diminish ice sheet sensitivity to rising ocean temperatures, thereby controlling the initial timing and spatial pattern of grounding-line retreat. Through the Holocene, grounding-line position is more sensitive to sub-shelf melt rates as the ocean cavity below the ice shelf expands. Model parameters that control the physical properties of the bed, deformation of the continental shelf, and rheological properties of the ice strongly influence the sensitivity of ice sheets to external forcing. Basin-wide differences in these forcings, driven by oceanic and atmospheric circulation, and spatial heterogeneity of bed properties likely contribute to the asynchronous pattern of retreat in the eastern and western parts of the embayment, as indicated by marine and terrestrial proxy records.

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Deglacial climate and ice sheet evolution of the Ross Embayment, Antarctica

10.26686/wgtn.17142116.v1 ◽

2021 ◽

Author(s):

◽

Daniel P. Lowry

Keyword(s):

Spatial Pattern ◽

Climate Models ◽

Ice Sheet ◽

Model Parameters ◽

Atmospheric Conditions ◽

Last Deglaciation ◽

Proxy Records ◽

External Forcings ◽

Wide Range ◽

Grounding Line

Reconstructing past grounding-line evolution can help inform future sea level projections by constraining marine ice sheet sensitivities to changes in climate. The Ross Embayment, the largest sector of Antarctica, experienced substantial grounding-line retreat since the Last Glacial Maximum. However, different interpretations for the timing and spatial pattern of deglacial grounding-line retreat in this region persist, suggesting either very high or low sensitivity to external forcings. Complicating matters is the sparse paleoclimate record, which is limited spatially and temporally. In this thesis, I address these issues by analysing the output of two transient climate simulations in relation to Antarctic ice core and marine sediment records, and performing and analysing the largest ensemble to date of regional ice sheet model simulations of the last deglaciation in the Ross Sea. The climate models and paleoclimate proxy records exhibit key differences in the timing, magnitude and duration of millennial-scale climate change events through the deglacial period. Using this diverse set of deglacial climate trajectories as ocean and atmosphere forcings, the ice sheet model ensemble produces a wide range of ice sheet responses, supporting the view that external forcings are the main drivers of past grounding-line retreat in the region. The simulations demonstrate that atmospheric conditions early in the deglacial period can enhance or diminish ice sheet sensitivity to rising ocean temperatures, thereby controlling the initial timing and spatial pattern of grounding-line retreat. Through the Holocene, grounding-line position is more sensitive to sub-shelf melt rates as the ocean cavity below the ice shelf expands. Model parameters that control the physical properties of the bed, deformation of the continental shelf, and rheological properties of the ice strongly influence the sensitivity of ice sheets to external forcing. Basin-wide differences in these forcings, driven by oceanic and atmospheric circulation, and spatial heterogeneity of bed properties likely contribute to the asynchronous pattern of retreat in the eastern and western parts of the embayment, as indicated by marine and terrestrial proxy records.

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Description of a hybrid ice sheet-shelf model, and application to Antarctica

Geoscientific Model Development ◽

10.5194/gmd-5-1273-2012 ◽

2012 ◽

Vol 5 (5) ◽

pp. 1273-1295 ◽

Cited By ~ 136

Author(s):

D. Pollard ◽

R. M. DeConto

Keyword(s):

Ice Sheet ◽

Last Deglaciation ◽

Ice Flow ◽

Ice Shelves ◽

Continental Scale ◽

Hybrid Combination ◽

Grounding Line ◽

Basal Sliding ◽

The Last Deglaciation

Abstract. The formulation of a 3-D ice sheet-shelf model is described. The model is designed for long-term continental-scale applications, and has been used mostly in paleoclimatic studies. It uses a hybrid combination of the scaled shallow ice and shallow shelf approximations for ice flow. Floating ice shelves and grounding-line migration are included, with parameterized ice fluxes at grounding lines that allows relatively coarse resolutions to be used. All significant components and parameterizations of the model are described in some detail. Basic results for modern Antarctica are compared with observations, and simulations over the last 5 million years are compared with previously published results. The sensitivity of ice volumes during the last deglaciation to basal sliding coefficients is discussed.

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Holocene thinning of Darwin and Hatherton glaciers, Antarctica, and implications for grounding-line retreat in the Ross Sea

The Cryosphere ◽

10.5194/tc-15-3329-2021 ◽

2021 ◽

Vol 15 (7) ◽

pp. 3329-3354

Author(s):

Trevor R. Hillebrand ◽

John O. Stone ◽

Michelle Koutnik ◽

Courtney King ◽

Howard Conway ◽

...

Keyword(s):

Catchment Area ◽

Ross Sea ◽

Three Dimensional ◽

Ice Sheet ◽

Early Holocene ◽

Last Deglaciation ◽

Grounding Line ◽

The Last Deglaciation ◽

Transantarctic Mountains ◽

Exposure Ages

Abstract. Chronologies of glacier deposits in the Transantarctic Mountains provide important constraints on grounding-line retreat during the last deglaciation in the Ross Sea. However, between Beardmore Glacier and Ross Island – a distance of some 600 km – the existing chronologies are generally sparse and far from the modern grounding line, leaving the past dynamics of this vast region largely unconstrained. We present exposure ages of glacial deposits at three locations alongside the Darwin–Hatherton Glacier System – including within 10 km of the modern grounding line – that record several hundred meters of Late Pleistocene to Early Holocene thickening relative to present. As the ice sheet grounding line in the Ross Sea retreated, Hatherton Glacier thinned steadily from about 9 until about 3 ka. Our data are equivocal about the maximum thickness and Mid-Holocene to Early Holocene history at the mouth of Darwin Glacier, allowing for two conflicting deglaciation scenarios: (1) ∼500 m of thinning from 9 to 3 ka, similar to Hatherton Glacier, or (2) ∼950 m of thinning, with a rapid pulse of ∼600 m thinning at around 5 ka. We test these two scenarios using a 1.5-dimensional flowband model, forced by ice thickness changes at the mouth of Darwin Glacier and evaluated by fit to the chronology of deposits at Hatherton Glacier. The constraints from Hatherton Glacier are consistent with the interpretation that the mouth of Darwin Glacier thinned steadily by ∼500 m from 9 to 3 ka. Rapid pulses of thinning at the mouth of Darwin Glacier are ruled out by the data at Hatherton Glacier. This contrasts with some of the available records from the mouths of other outlet glaciers in the Transantarctic Mountains, many of which thinned by hundreds of meters over roughly a 1000-year period in the Early Holocene. The deglaciation histories of Darwin and Hatherton glaciers are best matched by a steady decrease in catchment area through the Holocene, suggesting that Byrd and/or Mulock glaciers may have captured roughly half of the catchment area of Darwin and Hatherton glaciers during the last deglaciation. An ensemble of three-dimensional ice sheet model simulations suggest that Darwin and Hatherton glaciers are strongly buttressed by convergent flow with ice from neighboring Byrd and Mulock glaciers, and by lateral drag past Minna Bluff, which could have led to a pattern of retreat distinct from other glaciers throughout the Transantarctic Mountains.

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Antarctic ice dynamics amplified by Northern Hemisphere sea level forcing

10.5194/egusphere-egu21-9256 ◽

2021 ◽

Author(s):

Natalya Gomez ◽

Michael Weber ◽

Peter Clark ◽

Jerry Mitrovica ◽

Holly Han

Keyword(s):

Mass Loss ◽

Sea Level ◽

Northern Hemisphere ◽

Ice Sheet ◽

Last Deglaciation ◽

Grounding Line ◽

The Last Deglaciation ◽

Global Sea Level ◽

The Last Glacial Maximum ◽

The Last Glacial

A longstanding hypothesis for near-synchronous evolution of global ice sheets over ice-age cycles invokes an interhemispheric sea-level forcing whereby sea-level rise due to ice loss in the Northern Hemisphere in response to insolation and greenhouse gas forcing causes grounding-line retreat of marine-based sectors of the Antarctic Ice Sheet (AIS). Recent studies have shown that the AIS experienced substantial millennial-scale variability during and after the last deglaciation, with several times of recorded increased iceberg flux and grounding line retreat coinciding, within uncertainty, with well documented global sea-level rise events, providing further evidence of this sea-level forcing. However, the sea level changes associated with ice sheet mass loss are strongly nonuniform due to gravitational, deformational and Earth rotational effects, suggesting that the response of the AIS to Northern Hemisphere sea-level forcing is more complicated than previously modelled.We adopt an ice-sheet model coupled to a global sea-level model to show that a large or rapid Northern Hemisphere sea-level forcing enhances grounding-line advance and associated mass gain of the AIS during glaciation, and grounding-line retreat and AIS mass loss during deglaciation. Relative to models without these interactions, including the Northern Hemisphere sea-level forcing leads to a larger AIS volume during the Last Glacial Maximum (about 26,000 to 20,000 years ago), subsequent earlier grounding-line retreat and millennial-scale AIS variability throughout the last deglaciation. These findings are consistent with geologic reconstructions of the extent of the AIS during the Last Glacial Maximum and subsequent ice-sheet retreat, and with relative sea-level change in Antarctica.&#160;

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Climate and ice sheet evolutions from the last glacial maximum to the pre-industrial period with an ice sheet – climate coupled model

10.5194/cp-2021-39 ◽

2021 ◽

Author(s):

Aurélien Quiquet ◽

Didier M. Roche ◽

Christophe Dumas ◽

Nathaëlle Bouttes ◽

Fanny Lhardy

Keyword(s):

North American ◽

Large Scale ◽

Climate Model ◽

Global Climate ◽

Ice Sheet ◽

Freshwater Flux ◽

Oceanic Circulation ◽

Last Deglaciation ◽

Grounding Line ◽

The Last Deglaciation

Abstract. The last deglaciation offers an unique opportunity to understand the climate – ice sheet interactions in a global warming context. In this paper, to tackle this question, we use an Earth system model of intermediate complexity coupled to an ice sheet model covering the Northern Hemisphere to simulate the last deglaciation and the Holocene (26–0 ka BP). We use a synchronous coupling every year between the ice sheet and the rest of the climate system and we ensure a closed water cycle considering the release of freshwater flux to the ocean due to ice sheet melting. Our reference experiment displays a gradual warming in response to the forcings, with no abrupt changes. In this case, while the amplitude of the freshwater flux to the ocean induced by ice sheet retreat is realistic, it is sufficient to shut down the Atlantic meridional overturning from which the model does not recover within the time period simulated. However, with reduced freshwater flux we are nonetheless able to obtain different oceanic circulation evolutions, including some abrupt transitions between shut-down and active circulation states in the course of the deglaciation. The fast oceanic circulation recoveries lead to abrupt warming phases in Greenland. Our simulated ice sheet geometry evolution is in overall good agreement with available global reconstructions, even though the abrupt sea level rise at 14.6 kaBP is underestimated, possibly because the climate model underestimates the millenial- scale temperature variability. In the course of the deglaciation, large-scale grounding line instabilities are simulated both for the Eurasian and North American ice sheets. The first instability occurs in the Barents-Kara seas for the Eurasian ice sheet at 14.5 kaBP. A second grounding line instability occurs circa 12 kaBP in the proglacial lake that formed at the southern margin of the North American ice sheet. With additional asynchronously coupled experiments, we assess the sensitivity of our results to different ice sheet model choices related to surface and sub-shelf mass balance, ice deformation and grounding line representation. While the ice sheet evolutions differ within this ensemble, the global climate trajectory is only weakly affected by these choices. In our experiments, only the abrupt shifts in the oceanic circulation due to freshwater fluxes are able to produce some millenial-scale variability since no self-generating abrupt transitions are simulated without these fluxes.

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Evolution of mean ocean temperature in Marine Isotope Stages 5-4

10.5194/cp-2021-8 ◽

2021 ◽

Author(s):

Sarah Shackleton ◽

James A. Menking ◽

Edward Brook ◽

Christo Buizert ◽

Michael N. Dyonisius ◽

...

Keyword(s):

Noble Gas ◽

Ice Sheet ◽

Ocean Temperature ◽

Atmospheric Conditions ◽

Last Deglaciation ◽

Glacial Cycle ◽

The Last Deglaciation ◽

Gas Measurements ◽

Mis 5 ◽

The Last Glacial

Abstract. Deglaciations are characterized by relatively fast and near-synchronous changes in ice sheet volume, ocean temperature, and atmospheric greenhouse gas concentrations, but glacial inceptions occur more gradually. Understanding the evolution of ice sheet, ocean, and atmospheric conditions from interglacial to glacial maximum provides important insight into the interplay of these components of our climate system. Using noble gas measurements in ancient ice samples, we reconstruct mean ocean temperature (MOT) from 74 to 59.5 ka BP, covering the Marine Isotope Stage (MIS) 5-4 boundary, MIS 4, and part of the MIS 4-3 transition. Comparing this MOT reconstruction to previously published MOT reconstructions from the last glacial cycle, we find that the majority of interglacial-glacial ocean cooling occurred across MIS 5, and MOT reached full glacial levels by MIS 4 (−2.7 ± 0.3 °C relative to the Holocene). Comparing MOT to contemporaneous records of CO2 and benthic 𝛿18O, we find that ocean cooling and the solubility pump can explain most of the CO2 drawdown and increase in 𝛿18O across MIS 5. The timing of ocean warming and cooling in our record indicates that millennial scale climate variability plays a crucial role in setting mean ocean temperature during this interval, as seen during other periods, such as the last deglaciation.

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Climate and ice sheet evolutions from the last glacial maximum to the pre-industrial period with an ice-sheet–climate coupled model

Climate of the Past ◽

10.5194/cp-17-2179-2021 ◽

2021 ◽

Vol 17 (5) ◽

pp. 2179-2199

Author(s):

Aurélien Quiquet ◽

Didier M. Roche ◽

Christophe Dumas ◽

Nathaëlle Bouttes ◽

Fanny Lhardy

Keyword(s):

Ice Sheet ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Freshwater Flux ◽

Oceanic Circulation ◽

Last Deglaciation ◽

Grounding Line ◽

Meridional Overturning ◽

Reference Experiment ◽

The Last Deglaciation

Abstract. The last deglaciation offers an unique opportunity to understand the climate–ice-sheet interactions in a global warming context. In this paper, to tackle this question, we use an Earth system model of intermediate complexity coupled to an ice sheet model covering the Northern Hemisphere to simulate the last deglaciation and the Holocene (26–0 ka). We use a synchronous coupling every year between the ice sheet and the rest of the climate system and we ensure a closed water cycle considering the release of freshwater flux to the ocean due to ice sheet melting. Our reference experiment displays a gradual warming in response to the forcings, with no abrupt changes. In this case, while the amplitude of the freshwater flux to the ocean induced by ice sheet retreat is realistic, it is sufficient to shut down the Atlantic meridional overturning circulation from which the model does not recover within the time period simulated. However, with reduced freshwater flux we are nonetheless able to obtain different oceanic circulation evolutions, including some abrupt transitions between shut-down and active circulation states in the course of the deglaciation. The inclusion of a parameterisation for the sinking of brines around Antarctica also produces an abrupt recovery of the Atlantic meridional overturning circulation, absent in the reference experiment. The fast oceanic circulation recoveries lead to abrupt warming phases in Greenland. Our simulated ice sheet geometry evolution is in overall good agreement with available global reconstructions, even though the abrupt sea level rise at 14.6 ka is underestimated, possibly because the climate model underestimates the millennial-scale temperature variability. In the course of the deglaciation, large-scale grounding line instabilities are simulated both for the Eurasian and North American ice sheets. The first instability occurs in the Barents–Kara seas for the Eurasian ice sheet at 14.5 ka. A second grounding line instability occurs ca. 12 ka in the proglacial lake that formed at the southern margin of the North American ice sheet. With additional asynchronously coupled experiments, we assess the sensitivity of our results to different ice sheet model choices related to surface and sub-shelf mass balance, ice deformation and grounding line representation. While the ice sheet evolutions differ within this ensemble, the global climate trajectory is only weakly affected by these choices. In our experiments, only the abrupt shifts in the oceanic circulation due to freshwater fluxes are able to produce some millennial-scale variability since no self-generating abrupt transitions are simulated without these fluxes.

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Description of a hybrid ice sheet-shelf model, and application to Antarctica

Geoscientific Model Development Discussions ◽

10.5194/gmdd-5-1077-2012 ◽

2012 ◽

Vol 5 (2) ◽

pp. 1077-1134 ◽

Cited By ~ 13

Author(s):

D. Pollard ◽

R. M. DeConto

Keyword(s):

Ice Sheet ◽

Last Deglaciation ◽

Ice Flow ◽

Ice Shelves ◽

Continental Scale ◽

Grounding Line ◽

Basal Sliding ◽

Ice Retreat ◽

The Last Deglaciation

Abstract. The formulation of a 3-D ice sheet-shelf model is described. The model is designed for long-term continental-scale applications, and has been used mostly in paleoclimatic studies. It uses a hybrid combination of the scaled Shallow Ice and Shallow Shelf Approximations for ice flow. Floating ice shelves and grounding-line migration are included, with parameterized ice fluxes at grounding lines that allows relatively coarse resolutions to be used. All significant components and parameterizations of the model are described in some detail. Basic results for modern Antarctica are compared with observations, and simulations over the last 5 million yr are shown to be similar to previously published results using an earlier model version. The sensitivity of ice retreat during the last deglaciation to basal sliding coefficients is discussed.

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Transient climate simulations of the deglaciation 21–9 thousand years before present; PMIP4 Core experiment design and boundary conditions

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-9045-2015 ◽

2015 ◽

Vol 8 (10) ◽

pp. 9045-9102 ◽

Cited By ~ 2

Author(s):

R. F. Ivanovic ◽

L. J. Gregoire ◽

M. Kageyama ◽

D. M. Roche ◽

P. J. Valdes ◽

...

Keyword(s):

Working Group ◽

Ad Hoc ◽

Climate Models ◽

Ice Sheet ◽

Before Present ◽

Last Deglaciation ◽

Transient Simulations ◽

The Last Deglaciation ◽

Set Up ◽

Core Simulation

Abstract. The last deglaciation, which marked the transition between the last glacial and present interglacial periods, was punctuated by a series of rapid (centennial and decadal) climate changes. Numerical climate models are useful for investigating mechanisms that underpin the events, especially now that some of the complex models can be run for multiple millennia. We have set up a Paleoclimate Modelling Intercomparison Project (PMIP) working group to coordinate efforts to run transient simulations of the last deglaciation, and to facilitate the dissemination of expertise between modellers and those engaged with reconstructing the climate of the last 21 thousand years. Here, we present the design of a coordinated Core simulation over the period 21–9 thousand years before present (ka) with time varying orbital forcing, greenhouse gases, ice sheets, and other geographical changes. A choice of two ice sheet reconstructions is given, but no ice sheet or iceberg meltwater should be prescribed in the Core simulation. Additional focussed simulations will also be coordinated on an ad-hoc basis by the working group, for example to investigate the effect of ice sheet and iceberg meltwater, and the uncertainty in other forcings. Some of these focussed simulations will focus on shorter durations around specific events to allow the more computationally expensive models to take part.

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