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Pleistocene glaciations and their associated dramatic climatic conditions are suspected to have had a large impact on the groundwater flow system over the entire North American continent. Because of the myriad of complex flow-related processes involved during a glaciation period, numerical models have become powerful tools for examining groundwater flow system evolution in this context. In this study, a series of key processes pertaining to coupled groundwater flow and glaciation modeling, such as density- dependent (i.e., brine) flow, hydromechanical loading, subglacial infiltration, isostasy, and permafrost development, are included in the numerical model HydroGeoSphere to simulate groundwater flow over the Canadian landscape during the Wisconsinian glaciation (@120 ka to present). The primary objective is to demonstrate the immense impact of glacial advances and retreats during the Wisconsinian glaciation on the dynamical evolution of groundwater flow systems over the Canadian landscape, including surface-subsurface water exchanges (i.e., recharge and discharge fluxes) in both the subglacial and the periglacial environments. It is shown that much of the infiltration of subglacial meltwater occurs during ice sheet progression and that during ice sheet regression, groundwater mainly exfiltrates on the surface, in both the subglacial and periglacial environments. The average infiltration/exfiltration fluxes range between 0 and 12 mm/a. Using mixed, ice sheet thickness dependent boundary conditions for the subglacial environment, it was estimated that 15 to 70% of the meltwater infiltrated into the subsurface as recharge, with an average of 43%. Considering the volume of meltwater that was generated subsequent to the last glacial maximum, these recharge rates, which are related to the bedrock type and elastic properties, are historically significant and therefore played an immense role in the evolution of groundwater flow system evolution over the Canadian landmass over the last 120 ka. Finally, it is shown that the permafrost extent plays a key role in the distribution of surface-subsurface interaction because the presence of permafrost acts as a barrier for groundwater flow.
The original suggestion, by Rooth (1982) and refined by Broecker and al (1989), that the Younger Dryas was due to a reduction or shutdown in the large ocean heat conveyor (thermohaline circulation) triggered by a meltwater flood from eastward diversion of pro-glacial Lake Agassiz outflow, has been widely accepted for over a decade. This hypothesis fit inferred timing for a major reduction of southern outflow via the Mississippi river, as recorded in various paleoceanographic proxies from marine sedimentary cores from the Gulf of Mexico. Furthermore, paleoceanographic evidence for a reduction in the rate of the deep overturning circulation during the YD has become quite robust. However, over the last few years, a number of challenges have arisen. To assess our current understanding, I will address three key issues in turn.
The bed-thermal characteristics of a glacial systems model that has been calibrated against a large set of relative sea-level, geodetic, and strandline observations are examined for the previously glaciated sector of the North American continent. The model compares favourably against the present day extent of permafrost and against the observed temperature profiles from three deep boreholes when appropriate bed-thermal conductivities are employed. Estimates for the present day depth field of the lower permafrost boundary are presented. We find a significant disequilibrium in the lower permafrost boundary for most of the Arctic region, with present-day depth as much as 250 m shallower than the equilibrium value for present day climate forcing. This is largely due to the on-going response to the loss of ice cover from the glacial period.
The time evolution of the sub-glacial warm-based area fraction is also presented together with calibration derived confidence intervals. A peak warm-based fraction of $50\% \pm 6\%$ is obtained at last glacial maximum (LGM). The timing of the three largest ice volume maxima that were produced in response to the obliquity component of orbital forcing during the last glacial cycle matches that of the maxima for the warm-based area fraction with no significant phase delay. Warm-based conditions are required to enable ice streaming (fast flow) in the model. It is therefore hypothesized that the expansion of the area covered by warm-based ice played a critical role in producing a highly dynamic ice-sheet during both the most intense growth and recession phases.
We present a new deglacial meltwater drainage chronology for the North American ice sheet complex using a 3D glacial systems model calibrated against a large set of paleo-proxies. Results indicate that North America was responsible for a significant fraction of mwp1-a, with order 1.5 dSv or larger (100 year mean) peak discharges into both the Gulf of Mexico and the Eastern Atlantic and less than 1 dSv into the Arctic Ocean.
Our most significant result concerns discharge into the Arctic Ocean. The largest total discharge into the Arctic Ocean (ensemble mean values of 1.0 to 2.2 dSv) occurs during the onset of the Younger Dryas. The large majority of this discharge is locally sourced with reduction of the Keewatin ice dome being the largest contributor. Given that the only outlet from the Arctic Basin at this time was via Fram Strait into the Greenland-Iceland-Norwegian Seas, we hypothesize that this pulse was the trigger for the re-organization of thermohaline circulation that is thought to have been responsible for the Younger Dryas cold interval. In contradistinction with past inferences and subject to the imperfectly constrained ice-margin chronology, we also find that the Northwest outlet likely dominated much of the post -13 kyr drainage of Lake Agassiz.
The last deglaciation was abruptly interrupted by a millennial-scale reversion to glacial conditions. The cause of this Younger Dryas cold interval has been connected to a decrease in the rate of North Atlantic Deep Water formation and to a resultant weakening of the meridional overturning circulation due to surface freshening. In contrast, the earlier meltwater pulse 1-a event, of disputed provenance, produced no apparent reduction of the meridional overturning circulation. To identify the source of the freshwater forcing that weakened the meridional overturning circulation, we present the analysis of objectively-constrained drainage chronology ensembles for the North American ice-sheet derived from a calibrated Glacial Systems Model. We find that this ice-sheet sourced about half of meltwater pulse 1-a. During the onset of the Younger Dryas, we find that the largest combined meltwater/iceberg discharge was directed into the Arctic Ocean. Given that the only drainage outlet from the Arctic Ocean was via Fram Strait into the Greenland-Iceland-Norwegian Seas, where North Atlantic Deep Water is formed today, we hypothesize that it was this Arctic freshwater flux that triggered the Younger Dryas cold reversal.
Past reconstructions of the deglaciation history of the North American (NA) ice sheet complex have relied either on largely unconstrained and limited explorations of the phase space of solutions produced by glaciological models or upon geophysical inversions of relative sea level (RSL) data which suffer from incomplete geographic coverage of the glaciated regions, load history amplitude/timing ambiguities, and a lack of a priori glaciological self-consistency. As a first step in the development of a much more highly constrained deglaciation history, we present a synthesis of these two previously disjoint methodologies based on a large ensemble of glacial cycle simulations using a three dimensional thermo-mechanically coupled ice sheet model. Twenty glacial system model parameters, chosen so as to best cover the true deglacial phase space, were varied across the ensemble. Furthermore, a new high-resolution digitized ice margin chronology was imposed on the model in order to significantly limit the uncertainties associated with deglacial climate forcing. The model is simultaneously constrained by a large set of high quality RSL histories, a space geodetic observation of the present day rate of vertical motion of the crust from Yellowknife and a traverse of absolute gravity measurements from the west coast of Hudson Bay southward into Iowa.
The general form of the Last Glacial Maximum (LGM) ice topography that ensues when model results are subject to geophysical constraints is an ice sheet dominated by a large (3.3 to 4.3 km maximum ice thickness) Keewatin dome to the west of Hudson Bay connected to a major ice ridge running southeast to the Great Lakes, together with a Hudson Bay region that has relatively thin ice and an Arctic region heavily incised by open-water and/or ice-shelves. Geographically restricted fast flows due to sub-glacial till-deformation are shown to be critical to obtaining such a multi-domed late glacial Laurentide Ice Sheet structure, one that has been previously inferred on the basis of geomorphological data and that is required to fit the geophysical constraints. Our results further suggest that the NA contribution to LGM eustatic sea level drop is likely to be in the range of 60 to 75 m.
We examine the extent to which observations from the Greenland ice sheet combined with 3D dynamical ice sheet models and Semi-Lagrangian tracer methods can be used to constrain inferences of the Eemian evolution of the ice sheet, of the extent and frequency of summit migration during the 100 kyr ice age cycle, and of the deep geothermal flux of heat from the earth into the base of the ice sheet. Relative sea-level, present-day surface geometry, basal temperature, and age and temperature profiles from GRIP are imposed as constraints to tune ice sheet model and climate forcing parameters. Despite the paucity of observations, model-based inferences suggest a significant north-east gradient in geothermal heat flux. Our analyses also suggest that during the glacial cycle, the contemporaneous summit only occupied the present-day location during interglacial periods. Based on the development and use of a high resolution Semi-Lagrangian tracer analysis methodology for del 18 O we rule out isotropic flow disturbances due to summit migration as a possible source of the high Eemian variability of the GRIP del 18 O record. Finally, in contrast with results obtained in some recent attempts to infer the extent to which Greenland may have contributed to the anomalous high stand of Eemian sea-level, we find that conservative bounds for this contribution are 2 to 5.2 m, with a more likely range of 2.7 to 4.5 m.
Space-time reconstructions of the continental ice-sheets that existed at Last Glacial Maximum(LGM) have previously been produced using two entirely independent methodologies, respectively that based upon the use of theoretical models of ice-sheet accumulation and flow and that based upon the geophysical inversion of relative sea level (RSL) histories from previously ice-covered regions. The analyses described in this paper demonstrate the significant advantages that derive from the simultaneous application of both methods to the particular case of Greenland. We thereby show that the ICE-4G reconstruction of the glaciation history of this region from LGM to present, which was based upon the geophysical inversion of RSL data alone, was reasonably accurate in the peripheral regions where RSL data were available but inaccurate in the interior of the ice-sheet which was unconstrained by such information. We test the new model of Greenland glacial history determined by the simultaneous application of the constraints that derive from ice-sheet modelling and the geophysical inversion of RSL data by employing recently published geodetic inferences of mass-balance over the entire interior region of the ice sheet and of GPS measurements of vertical crustal motion. These observations, which were not employed to constrain the ice-sheet reconstruction, provide significant support for the new glacial history for Greenland that our analyses have led us to infer.
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