Jan A. Piotrowski

The Last Deglaciation of the Southeastern Sector of the Scandinavian Ice Sheet

The Scandinavian Ice Sheet (SIS) was an important component of the global ice sheet system during the last glaciation, but the timing of its growth to or retreat from its maximum extent remains poorly known. We used 115 cosmogenic beryllium-10 ages and 70 radiocarbon ages to constrain the timing of three substantial ice-margin fluctuations of the SIS between 25,000 and 12,000 years before the present. The age of initial deglaciation indicates that the SIS may have contributed to an abrupt rise in global sea level. Subsequent ice-margin fluctuations identify opposite mass-balance responses to North Atlantic climate change, indicating differing ice-sheet sensitivities to mean climate state.

Subglacial hydrology in north-western germany during the last glaciation: groundwater flow, tunnel valleys and hydrological cycles

Abstract Numerical modelling of groundwater dynamics in an overpressured system of subglacial aquifers and aquitards under the marginal portion of the Scandinavian ice sheet in northwestern Germany has been coupled with calculations of basal meltwater production rates and with field evidence of meltwater erosional features. Largely fine-grained subglacial sediments with relatively low hydraulic conductivities had a capacity to drain only about 25% of all basal meltwater produced in the relevant subglacial catchment area. The rest of the meltwater was evacuated through tunnel valleys in spontaneous outburst events. Subglacial drainage cycles are proposed, in which each cycle begins with meltwater drainage through the substratum (groundwater flow), followed by water ponding at the ice/bed interface, and ends in catastrophic meltwater releases through tunnel valleys.

Were deforming subglacial beds beneath past ice sheets really widespread

Abstract The concept of widespread, large-strain deformation of subglacial unconsolidated, unfrozen, sediments during Pleistocene glaciations has inconsistencies in the geological record. Numerous properties of tills and related sediments are difficult to reconcile with pervasive strains, as predicted by the deforming bed theory. While accepting glacier-bed deformation as a geological process occurring under certain circumstances, we propose that it was much less widespread than believed by some. Instead, we believe that basal sliding and englacial transport are major ice movement and debris-transport mechanisms.

Tunnel-valley formation in northwest Germany—geology, mechanisms of formation and subglacial bed conditions for the Bornhöved tunnel valley

The Bornhoved tunnel valley consists of a relatively broad, elongated depression filled primarily with tills, and a narrow channel in the axial part filled with outwash, glaciolacustrine sediments and tills. The tunnel valley is at least 13 km long, 222 m deep (bottom at 191 m b.s.l.), and extends parallel to the axis of a peripheral sink between two Permian salt diapirs in the substratum. It is suggested that the initiation of the tunnel valley was controlled by a differential response of the bed materials to ice overriding on top of and between the salt structures. The Bornhoved tunnel valley is a polygenetic feature that resulted from different erosional processes throughout all three main glaciations in northern Germany. During the Elsterian Glaciation glaciotectonic squeezing occurred, to be later replaced by subglacial meltwater erosion. During the ice retreat and the Holsteinian Interglacial, the valley was filled with fine-grained glaciolacustrine and marine sediments, which were subsequently removed and redeposited further down-ice as a 200-m-thick glaciotectonic melange by the first Saalian advance. The second Saalian advance caused only minor changes and the tunnel valley was reactivated as a subglacial channel during the Weichselian Glaciation. It is suggested that at the time of the first Weichselian ice advance a large subglacial water reservoir developed in the area of the Baltic Sea basin and caused a rapid, surge-like ice movement. As the ice sheet advanced out of the Baltic Sea basin, drainage of the water reservoir was prevented by the ice toe overriding the permafrost on the Saalian highlands. During the ice retreat, frozen ground was left beyond the ice margin and subglacial meltwater catastrophically drained through the tunnel valley with a discharge roughly estimated at about 3.75 × 103 m3/s. It seems possible that drainage of the subglacial water collector through the Bornhoved tunnel valley and other similar channels south of the Baltic Sea led to increased ice-sheet basal friction, and was one reason for successively smaller extents of the subsequent Weichselian advances. Evidence against pervasively deforming bed materials in the vicinity of the Bornhoved tunnel valley in the Weichselian Glaciation indicates that the valley served as an efficient drainage path for porewater from the subglacial sediments.

Response of sediment to ice-sheet loading in northwestern Germany : effective stresses and glacier-bed stability

Laboratory tests on sediment over-ridden by the last ice sheet in northwestern Germany reveal very low ice-induced pre-consolidation and high palaeo-pore-water pressures. Sediment consolidation at the base of the glacier was largely controlled by hydraulic properties of the substratum. Generally low permeabilities of the bed caused sustained high pore-water pressure in over-ridden sediments close to the flotation point. This implies a serious possibility of hydraulic lifting of the ice sheet. It is believed that the reduced basal coupling limited the transformation of glacier shear stress on to the bed sediments, which is indicated by a lack of sedimentological evidence for widespread pervasive bed deformation. Ice motion was probably focused at the glacier sole by some combination of sliding and ploughing. However, isolated spots with deformation occur, so that the subglacial system in the study area can be characterized as a stable/deforming mosaic.

Subglacial groundwater flow during the last glaciation in northwestern Germany

Abstract Reconstruction of groundwater flow in overpressured soft sediments overridden by the last (Weichselian) ice sheet in northwestern Germany about 18,000 ka B.P, based on numerical modelling, reveals a complete reorganisation of groundwater dynamics as compared to the present system. Groundwater flow, controlled by potentiometric surface high up in the ice sheet and by a steep hydraulic gradient at the glacier periphery, was here up to 30 times faster than at present and a vertical flow component penetrated the entire thickness of Quaternary sediments reaching some 200 m in places.

Microstructures and microshears as proxy for strain in subglacial diamicts : Implications for basal till formation

Ring-shear experiments were used to study subglacial sediment deformation and the development of S-matrix microstructures and microshears to constrain glacially induced deformation in diamicts. The experiments approximated subglacial conditions, and the diamict was sheared and sampled incrementally to strains of 0, 7, 18, 33, 57, and 107. At strains between 7 and 18, a steady association of S-matrix microstructures developed, which did not evolve further despite continued shearing. The I L -index, a measure of microshear-orientation strength parallel to the shearing direction correlates log-linearly with strain, which indicates its potential use as strain proxy. The ring-shear data were compared with the characteristics of Quaternary basal tills from Denmark, Poland, and Svalbard. Based on I L -index values, we conclude that these tills only experienced strain of ∼10 1 , which is orders of magnitude lower than expected for deformation tills. This suggests that pervasive subglacial deformation and sediment advection in the mobile layer at the ice-bed interface may be less significant than previously assumed.

Geostatistical regionalization of glacial aquitard thickness in northwestern Germany, based on fuzzy kriging

In many instances hydrogeological parameters obtained by conventional methods for selected localities within an aquifer or an aquitard are not sufficient for adequate regionalization at the scale of the entire layer. Here, we demonstrate an application of the fuzzy kriging method in regionalization of hydrogeological data, in which the set of conventional, crisp values is supplemented by imprecise information subjectively estimated by an expert. It is believed that such an approach eventually may reflect the real-world conditions more closely than a traditional crisp-value approach, because the former does not impose exactness artificially on phenomena which are diffuse by their nature. Spatial interpolation was done for the thickness of one of the major aquitards (till and glaciolacustrine clay) in northwestern Germany. The dataset consists of 329 crisp values from boreholes supplemented by 172 imprecise values defined as fuzzy numbers. It is demonstrated that the reliability of regionalization was higher, compared to regionalization performed with the crisp dataset only. Fuzzy kriging was performed with FUZZEKS (Fuzzy Evaluation and Kriging System) developed at the Ecosystem Research Center at the University of Kiel.

Ice stream motion facilitated by a shallow-deforming and accreting bed

Ice streams drain large portions of ice sheets and play a fundamental role in governing their response to atmospheric and oceanic forcing, with implications for sea-level change. The mechanisms that generate ice stream flow remain elusive. Basal sliding and/or bed deformation have been hypothesized, but ice stream beds are largely inaccessible. Here we present a comprehensive, multi-scale study of the internal structure of mega-scale glacial lineations (MSGLs) formed at the bed of a palaeo ice stream. Analyses were undertaken at macro- and microscales, using multiple techniques including X-ray tomography, thin sections and ground penetrating radar (GPR) acquisitions. Results reveal homogeneity in stratigraphy, kinematics, granulometry and petrography. The consistency of the physical and geological properties demonstrates a continuously accreting, shallow-deforming, bed and invariant basal conditions. This implies that ice stream basal motion on soft sediment beds during MSGL formation is accommodated by plastic deformation, facilitated by continuous sediment supply and an inefficient drainage system.

Discrete element modeling of subglacial sediment deformation

[1] The Discrete Element Method (DEM) is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm-based glaciers. Complementary to analog experiments, the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain. The geometry of the heterogeneous stress network is visible in the form of force-carrying grain bridges and adjacent, volumetrically dominant, inactive zones. We demonstrate how the shear zone thickness and dilation depend on the level of normal (overburden) stress, and we show how high normal stress can mobilize material to great depths. The particle rotational axes tend to align with progressive shear strain, with rotations both along and reverse to the shear direction. The results from successive laboratory ring-shear experiments on simple granular materials are compared to results from similar numerical experiments. The simulated DEM material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress. These results demonstrate that the DEM is a viable alternative to continuum models for small-scale analysis of sediment deformation. It can be used to simulate the macromechanical behavior of simple granular sediments, and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain.