Paleo sea-level indicators and proxies from Greenland in the GAPSLIP database and comparison with modelled sea level from the PaleoMIST ice-sheet reconstruction

One of the most common ways to assess ice-sheet reconstructions of the past is to evaluate how they impact changes in sea level through glacial isostatic adjustment. PaleoMIST 1.0, a preliminary reconstruction of topography and ice sheets during the past 80 000 years, was created without a rigorous comparison with past sea-level indicators and proxies in Greenland. The basal shear stress values for the Greenland ice sheet were deduced from the present day ice-sheet configuration, which were used for the entire 80 000 years without modification. The margin chronology was based on previous reconstructions and interpolation between them. As a result, it was not known if the Greenland component was representative of its ice-sheet history. In this study, I compile sea–level proxy data into the Global Archive of Paleo Sea Level Indicators and Proxies (GAPSLIP) database and use them to evaluate the PaleoMIST 1.0 reconstruction. The Last Glacial Maximum (c. 20 000 years before present) contribution to sea level in PaleoMIST 1.0 is about 3.5 m, intermediate of other reconstructions of the Greenland ice sheet. The results of the data-model comparison show that PaleoMIST requires a larger pre-Holocene ice volume than it currently has to match the sea-level highstands observed around Greenland, especially in southern Greenland. Some of this mismatch is likely because of the crude 2500 year time step used in the margin reconstruction and the limited Last Glacial Maximum extent. Much of the mismatch can also be mitigated if different Earth model structures, particularly a thinner lithosphere, are assumed. Additional ice in Greenland would contribute to increasing the 3–5 m mismatch between the modelled far-field sea level at the Last Glacial Maximum and proxies in PaleoMIST 1.0

Role of sediments in controlling the dynamics of paleo-ice sheets

The motion of glacial ice is predominantly controlled by basal conditions, which include a variety of parameters such as ice rheology, temperature, water content, the presence of sediments, and topography. Soft sediment deformation has long been hypothesized to be a dominant control on the size and dynamics of temperate ice sheets such as the Laurentide Ice Sheet. The transition from hard-bedded regions (areas that lack significant sediment cover) to soft sediment areas put a limit on the maximum volume of these ice sheets. When the ice sheet margin reached soft sediment cover, it may have caused the ice sheet to surge, with global-scale climatic impacts. Current generation ice sheet models only have limited control on how sediments modify the behavior of an ice sheet. We present a model of sediment deformation that can take into account the thickness, lithology and hydrology at the base of the ice sheet using the Parallel Ice Sheet Model (PISM). We assess how changes in sediment properties affect the advance and retreat of the ice sheet, including standstills in the margin when the ice sheet becomes restricted to the hard-bedded interior areas. We apply this model to the Wisconsin Glaciation (~85-11 kyrs ago) of the Laurentide ice sheet. We show how the distribution of sediments affect its growth and retreat. We specifically focus on how the soft bedded Hudson Bay impeded the growth of the ice sheet, up to the lead up to the Last Glacial Maximum. We also investigate the relationship between Dansgaard–Oeschger and Heinrich events and the basal dynamics of the ice sheets

Last interglacial sea level along the Patagonian coast

As part of the World Atlas of Last Interglacial Shorelines (WALIS), we critically review existing chronologically constrained last interglacial sea level indicators along the coast of Patagonia in Argentina, South America. Since Charles Darwin's voyages on the Beagle in the 1830s, the staircase-like shoreline platforms along the Patagonian coast have been used as evidence that it has been subject to uplift. Paleo-sea level indicators, ranging between about 7 and 24 m have been attributed to the Last Interglacial on the basis of electron spin resonance and U/Th dating on mollusc shells, and shows remarkable continuity along the entire coast. If the age of these indicators are accurate, then it would indicate that there is a significant uplift rate during the past 120,000 years, since these values exceed the generally accepted peak globally averaged sea level for the last interglacial. However, the Patagonian coast is close enough to the Antarctic and Patagonian ice sheets that there could be a signal from glacial-isostatic adjustment (GIA). This might be an alternative explanation for these higher than average paleo-sea level values. We test the magnitude of the effect of GIA on the coast of Patagonia using a simple ice reconstruction spanning the past two glacial cycles, using a variety of Earth models

Communicating the relevance of paleo research in the current societal environment

It is not an easy task for paleoscientists to communicate the relevance of their research to policy makers and funders. However, an increase in catastrophic environmental calamities related to climate change (e.g. landslide, droughts, flooding) demands a response both in terms of policy-making and future governmental decisions. Often, climate change in the recent past was linked to major shifts in human behavior, which masks the relative contribution of humans and nature. For example, the 4.2 ka BP aridification event was so severe that it may have triggered the collapse of several large civilizations (the Old Kingdom in Egypt and the Akkadian Empire in Mesopotamia; Gibbons 1993). Compilations of long-term records of past variability can help reduce the uncertainties on past, present and future climate changes, and thus support informed societal decisions. Therefore, policymakers should (and some may argue, must) consider the long-term perspective provided by paleoscience research

The impact of spatially varying ice sheet basal conditions on sliding at glacial time scales

Spatially variable basal conditions are thought to govern how ice sheets behave at glacial time scales (>1000 years) and responsible for changes in dynamics between the core and peripheral regions of the Laurentide and Fennoscandian ice sheets. Basal motion is accomplished via the deformation of unconsolidated sediments, or via sliding of the ice over an undeformable bed. We present an ice sheet sliding module for the Parallel Ice Sheet Model (PISM) that takes into account changes in sediment cover and incorporates surface meltwater. This model routes meltwater, produced at the surface and base of the ice sheet, toward the margin of the ice sheet. Basal sliding is accomplished through the deformation of water saturated sediments, or sliding at the ice-bed interface. In areas with continuous, water saturated sediments, sliding is almost always accomplished through sediment deformation. In areas with incomplete cover, sliding has a stronger dependence on the supply of water. We find that the addition of surface meltwater to the base is a more important factor for ice sheet evolution than the style of sliding. In a glacial cycle simulation, our model causes a more rapid buildup of the Laurentide Ice Sheet

The sensitivity of Northern Hemisphere ice sheets to atmospheric forcing during the last glacial cycle using PMIP3 models

The evolution of Northern Hemisphere ice sheets through the last glacial cycle is simulated with the glacial index method by using the climate forcing from one General Circulation Model, COSMOS. By comparing the simulated results to geological reconstructions, we first show that the modelled climate is capable of capturing the main features of the ice-sheet evolution. However, large deviations exist, likely due to the absence of nonlinear interactions between ice sheet and other climate components. The model uncertainties of the climate forcing are examined using the output from nine climate models from the Paleoclimate Modelling Intercomparison Project Phase III. The results show a large variability in simulated ice sheets between the different models. We find that the ice-sheet extent pattern resembles summer surface air temperature pattern at the Last Glacial Maximum, confirming the dominant role of surface ablation process for high-latitude Northern Hemisphere ice sheets. This study shows the importance of the upper boundary condition for ice-sheet modelling, and implies that careful constraints on climate output is essential for simulating realistic glacial Northern Hemisphere ice sheets

Early-career paleoscientists meet in the mountains of Aragon

Three days prior to the 5th PAGES Open Science Meeting (OSM), 80 ambitious early-career scientists (PhD students and postdoctoral researchers) met in the restored village of Morillo de Tou, Spain. The remote setting in the Pyrenees, the old style buildings constructed of turbidites, and the clear and sunny weather made this place an inspiring location to discuss past climate, environment and human interactions. Despite some grumblings about cold coffee served in small cups, the conference was a high-energy affair that promoted connections

Last interglacial sea-level proxies in the glaciated Northern Hemisphere

Because global sea level during the last interglacial (LIG; 130–115 ka) was higher than today, the LIG is a useful approximate analogue for improving predictions of future sea-level rise. Here, we synthesize sea-level proxies for the LIG in the glaciated Northern Hemisphere for inclusion in the World Atlas of Last Interglacial Shorelines (WALIS) database. We describe 82 sites from Russia, northern Europe, Greenland and North America from a variety of settings, including boreholes, riverbank exposures and along coastal cliffs. Marine sediments at these sites were constrained to the LIG using a variety of radiometric methods (radiocarbon, uranium–thorium, potassium–argon), non-radiometric methods (amino acid dating, luminescence methods, electron spin resonance, tephrochronology) as well as various stratigraphic and palaeo-environmental approaches. In general, the sites reported in this paper do not offer constraint on the global LIG highstand, but rather evidence of glacial isostatic adjustment (GIA)-influenced sea-level positions following the Marine Isotope Stage 6 glaciation (MIS 6; 191–130 ka). Most of the proxies suggest that sea level was much higher during the LIG than at the present time. Moreover, many of the sites show evidence of regression due to sea-level fall (owing to glacial isostatic uplift), and some also show fluctuations that may reflect regrowth of continental ice or increased influence of the global sea-level signal. In addition to documenting LIG sea-level sites in a large swath of the Northern Hemisphere, this compilation is highly relevant for reconstructing the size of MIS 6 ice sheets through GIA modelling. The database is available at https://doi.org/10.5281/zenodo.5602212 (Dalton et al., 2021).publishedVersio

Investigating the role of subglacial geology on ice sheet dynamics

Basal boundary conditions, such as basal geology, the presence of unconsolidated sediments, and hydrology, play a dominant role in the dynamics of ice sheets. One problem when studying existing ice sheets in Greenland and Antarctica is the lack of direct observations of these basal conditions. Studying paleo-ice sheet behaviour is advantageous in this respect, because these conditions are preserved in glacial landforms and sediments. By studying past ice sheet behaviour, we can provide analogues for modern behaviour. We investigate the role of basal geological conditions and hydrology on ice sheet dynamics using the ice sheet model PISM. We specifically focus on the North American ice sheet complex. We present datasets for this region that can be used in ice sheet models to investigate basal conditions including information on bedrock geology, the distribution of unconsolidated sediments, and the dominant grain size of the sediments. We use these datasets to investigate how they impact ice sheet dynamics with simulations over the last glacial cycle. We include a simple hydrology model that includes meltwater derived from the surface, and property changes depending on the basal geology from our datasets. Our results show that the behaviour of the ice sheet changes when there is a transition from regions with sparse sediment cover to complete sediment cover. One impact is that because ice can flow faster in regions like Hudson Bay, it can cause the Laurentide Ice Sheet to stabilize into a single continent-wide ice sheet faster than in a situation where sediments are not present. We also investigate the role of changes in basal geological conditions have in causing unstable behaviour in ice sheets

GRANTSISM: An Excel™ ice sheet model for use in introductory Earth science courses

GRANTISM (GReenland and ANTarctic Ice Sheet Model) is an educational Excel™ model introduced by Pattyn (2006). Here, GRANTISM is amended to simulate the Svalbard-Barents-Sea Ice Sheet during the Last Glacial Maximum, an analogue for the contemporary West Antarctic Ice Sheet. A new name, “GRANTSISM,” is suggested; the added S represents Svalbard. GRANTSISM introduces students of bachelor's or master's programs in Earth sciences (first or second cycle program in the Bologna system for higher education), but with little or no background in numerical modeling, to basic ice sheet modeling. GRANTSISM provides hands-on learning experiences related to ice sheet dynamics in response to climate forcing, and fosters understanding of processes and feedbacks. GRANTSISM was successfully used in noncompulsory courses in which students have been able to reproduce paleo-ice sheet evolution scenarios discussed here as examples. Students progressed further by designing, developing, and analyzing their own modeling scenarios. Here, we describe GRANTSISM and report on how learning activities with GRANTSISM were assessed by students who had no prior experience in ice sheet modeling. The response rate for a noncompulsory survey of the learning activity was less than 40%. A subsequent control experiment with a compulsory survey, however, showed the same patterns of answers, so the student response is considered representative. First, GRANTSISM is concluded to be a highly attractive tool to introduce learners with an interest in ice sheet behavior to ice sheet modeling. Second, it triggers an interest for more in-depth learning experiences related to numerical ice sheet modeling