Nina Kirchner

Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica

The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.

Dynamic response of granular and porous materials under large and catastrophic deformations

I. Avalanches, Debris and Mud Flows.- Rapid Granular Avalanches.- Gravity-Driven Rapid Shear Flows of Dry Granular Masses in Topographies with Orthogonal and Non-Orthogonal Metrics.- A Lagrangian-Eulerian Finite-Volume Method for Simulating Free Surface Flows of Granular Avalanches.- Experimental Information on the Dynamics of Dry-Snow Avalanches.- Numerical Simulation of Dry-Snow Avalanche Flow over Natural Terrain.- Particle Image Velocimetry (PIV) for Granular Avalanches on Inclined Planes.- Existence of Avalanching Flows.- Group Theoretic Methods and Similarity Solutions of the Savage-Hutter Equations.- II. Porous and Granular Materials. Fundamentals and Dynamical Processes.- Dynamics of Hypoplastic Materials: Theory and Numerical Implementation.- Acoustic Waves in Porous Solid-Fluid Mixtures.- Dissipation Influence on Cooling of 2-Dimensional Hard Needle Systems.- Induced Anisotropy in Rapid Flows of Nonspherical Granular Materials.- III. Porous and Granular Materials. Subscale-and Micromechanical Effects.- Modelling Particle Size Segregation in Granular Mixtures.- Gravity-Induced Segregation of Cohesionless Granular Mixtures.

Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation

The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions >1-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (∼140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.

Massive remobilization of permafrost carbon during post-glacial warming

Recent hypotheses, based on atmospheric records and models, suggest that permafrost carbon (PF-C) accumulated during the last glaciation may have been an important source for the atmospheric CO2 rise during post-glacial warming. However, direct physical indications for such PF-C release have so far been absent. Here we use the Laptev Sea (Arctic Ocean) as an archive to investigate PF-C destabilization during the last glacial–interglacial period. Our results show evidence for massive supply of PF-C from Siberian soils as a result of severe active layer deepening in response to the warming. Thawing of PF-C must also have brought about an enhanced organic matter respiration and, thus, these findings suggest that PF-C may indeed have been an important source of CO2 across the extensive permafrost domain. The results challenge current paradigms on the post-glacial CO2 rise and, at the same time, serve as a harbinger for possible consequences of the present-day warming of PF-C soils.

Dynamically coupling the non-linear Stokes equations with the shallow ice approximation in glaciology

We propose and implement a new method, called the Ice Sheet Coupled Approximation Levels (ISCAL) method, for simulation of ice sheet flow in large domains during long time-intervals. The method couples the full Stokes (FS) equations with the Shallow Ice Approximation (SIA). The part of the domain where SIA is applied is determined automatically and dynamically based on estimates of the modeling error. For a three dimensional model problem, ISCAL computes the solution substantially faster with a low reduction in accuracy compared to a monolithic FS. Furthermore, ISCAL is shown to be able to detect rapid dynamic changes in the flow. Three different error estimations are applied and compared. Finally, ISCAL is applied to the Greenland Ice Sheet on a quasi-uniform grid, proving ISCAL to be a potential valuable tool for the ice sheet modeling community.

Possible iceberg-produced submarine terraces in Hambergbukta, Spitsbergen

Submarine landforms produced by drifting icebergs are common on the sedimentary beds of the polar seas. High-resolution multibeam-bathymetric images have revealed a variety of iceberg-related landforms, ranging from 1 to 2 m high corrugation ridges (e.g. Jakobsson et al. 2011; Graham et al. 2013) to large linear to curvilinear ploughmarks several tens of kilometres long and tens of metres deep, that are often distributed chaotically on polar continental shelves (e.g. Dowdeswell et al. 2010). Extensive lateral erosion of submarine ridge-top sediments in the central Arctic Ocean has also been attributed to the impact of deep-drafted icebergs that were once much more abundant in the Arctic than they are today (Jakobsson et al. 2008). Submarine terraces with remarkably flat surfaces were observed on the crest and along the distal slope of a large terminal-moraine ridge at the mouth of the Hambergbukta (Fig. 1a, c), a fjord in southeastern Spitsbergen (Fig. 1b). The terraces were recorded at several bathymetric levels from 29–66 m in water depth with their surface areas varying from c. 0.04 km2 to >2 km2 (Fig. 1a, d). Fig. 1. ( a ) Swath-bathymetric imagery …

Submarine medial moraines in Hambergbukta, southeastern Spitsbergen

Sediments are incorporated into glaciers mainly by basal erosion and freeze-on, and by debris-fall onto the ice surface along the slopes of adjacent mountainsides. Flow-parallel debris ridges formed at the confluence of glaciers are known as medial moraines (Benn & Evans 2010). Medial moraines have a number of different origins (Eyles & Rogerson 1978; Benn & Evans 2010), including ingestion of basal marginal debris in deep crevasses and redistribution of supra-, en- and subglacial debris in the shear zone between converging glaciers (e.g. Sharp 1988; Vere & Benn 1989; Hambrey & Glasser 2003). Medial moraines up to tens of kilometres long and tens of metres high can be preserved after glacier retreat, marking the former flow path of ice masses. Medial moraines have also been shown to exert significant control over surging glaciers by inhibiting and/or redirecting glacier flow. Locally persistent subglacial meltwater conduits located under the medial moraines can also inhibit glacier surges by draining subglacial meltwater efficiently and decreasing basal water pressure (Benn et al. 2009). Hambergbukta …

Arctic environmental change beyond instrumental records: introduction and overview

The Arctic is changing rapidly in response to recent climate warming. Understanding these changes and their causes, and predicting their future trajectory is a key challenge for science and one with significant socio-economic implications given the wider impact of rising sea levels due to polar ice sheet melt. While observations of recent change over the last few decades are of key importance, so too is an assessment of the longer term magnitude, rate and pattern of Arctic environmental change prior to the start of instrumental records. The collection of papers in this special issue stems from a meeting of the PAST Gateways (Palaeo-Arctic Spatial and Temporal Gateways) network in Kristineberg, Sweden in 2017. Past Gateways is an IASC (International Arctic Science Council) endorsed thematic network, the goal of which is to understand Arctic environmental change across decadal to millennial scales and in the period preceding instrumental records. The network brings together Arctic scientists in the fields of glacial and marine geology, palaeoceanography, palaeoecology, permafrost and numerical modelling. The suite of papers in this special issue are organised into two broad themes, both of which are major foci of PAST Gateways: (1) Arctic palaeoceanography and sea ice, and, (2) Growth and decay of Arctic glaciers and ice sheets.

Interplay of grounding-line dynamics and sub-shelf melting during retreat of the Bjørnøyrenna Ice Stream

The Barents Sea Ice Sheet was a marine-based ice sheet, i.e., it rested on the Barents Sea floor during the Last Glacial Maximum (21 ky BP). The Bjørnøyrenna Ice Stream was the largest ice stream draining the Barents Sea Ice Sheet and is regarded as an analogue for contemporary ice streams in West Antarctica. Here, the retreat of the Bjørnøyrenna Ice Stream is simulated by means of two numerical ice sheet models and results assessed against geological data. We investigate the sensitivity of the ice stream to changes in ocean temperature and the impact of grounding-line physics on ice stream retreat. Our results suggest that the role played by sub-shelf melting depends on how the grounding-line physics is represented in the models. When an analytic constraint on the ice flux across the grounding line is applied, the retreat of Bjørnøyrenna Ice Stream is primarily driven by internal ice dynamics rather than by oceanic forcing. This suggests that implementations of grounding-line physics need to be carefully assessed when evaluating and predicting the response of contemporary marine-based ice sheets and individual ice streams to ongoing and future ocean warming.

Dynamically coupling Full Stokes and Shallow Shelf Approximationfor marine ice sheet flow using Elmer/Ice (v8.3)

Ice flow forced by gravity is governed by the full Stokes (FS) equations, which are computationally expensive to solve due to the nonlinearity introduced by the rheology. Therefore, approximations to the FS equations are commonly used, especially when modeling a marine ice sheet (ice sheet, ice shelf, and/or ice stream) for 103 years or longer. The shallow ice approximation (SIA) and shallow shelf approximation (SSA) are commonly used but are accurate only for certain parts of an ice sheet. Here, we report a novel way of iteratively coupling FS and SSA that has been implemented in Elmer/Ice and applied to conceptual marine ice sheets. The FS–SSA coupling appears to be very accurate; the relative error in velocity compared to FS is below 0.5 % for diagnostic runs and below 5 % for prognostic runs. Results for grounding line dynamics obtained with the FS–SSA coupling are similar to those obtained from an FS model in an experiment with a periodical temperature forcing over 3000 years that induces grounding line advance and retreat. The rapid convergence of the FS–SSA coupling shows a large potential for reducing computation time, such that modeling a marine ice sheet for thousands of years should become feasible in the near future. Despite inefficient matrix assembly in the current implementation, computation time is reduced by 32 %, when the coupling is applied to a 3-D ice shelf.