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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Magnetotelluric Analysis for Greenland and Postglacial Isostatic Evolution (MAGPIE)

Alternative title: Magnetotellurisk Analyse for Grønland og Postglasial Isostatisk Evolusjon (MAGPIE)

Awarded: NOK 8.7 mill.

Project Manager:

Project Number:

288449

Application Type:

Project Period:

2019 - 2023

Location:

Partner countries:

Melting of the Greenland Ice Sheet has accelerated during the past decade due to climate warming. This melting is now considered a major contributor to global sea level rise, and a serious threat to future coastlines. Thus, it is vital that we accurately monitor the patterns and volumes of melting. Ice loss is monitored by measuring gravity and ground height, but ground motion beneath the ice affects these measurements. Beneath Greenland, the solid Earth is moving as the weight of the melting ice is lifted off, in a process called glacial isostatic adjustment (GIA). The rate of GIA uplift depends critically on how easily mantle rocks flow - their viscosity. Viscosity is poorly constrained beneath Greenland and is likely to be complex. This uncertainty in viscosity leads to uncertainties in ice loss measurements. This project seeks to develop new constraints on rock viscosity beneath Greenland by collecting geophysical data on the ice sheet. During the summer of 2019, the MAGPIE team travelled to EastGRIP station in center of the Greenland Ice Sheet and collected magnetotelluric (MT) data across the ice within 100 km of the station. Data collection was not possible during the summers of 2020 and 2021, but in the summer of 2022 we returned to collect MT data out of Raven and Summit stations, which are positioned south of EastGRIP station. These two campaigns (2019 and 2022) provided excellent MT coverage across the interior of Greenland. The MT data provides information about Earth's electrical conductivity at depth. At shallow depths, this provides a constraint on subglacial melt, which is important for glacial dynamics. At deeper depths, the MT data is sensitive to the temperature and water content of mantle rocks. Because these factors also control mantle viscosity, we can use the MT data to map viscosity variations beneath Greenland. A PhD student has developed a method that combines MT data with other geophysical constraints to constrain mantle viscosity. We have applied this method to Scandinavia using existing seismic and MT constraints. Our predictions of mantle viscosity beneath Scandinavia compare well to independent constraints from GIA modelling and geological observations of past uplift, which confirms the usefulness of our new method. A postdoctoral researcher has developed models that explore the interaction between the hot Iceland plume and the cold rocks beneath Greenland. These models show that this interaction likely removed some of the cold rocks beneath Greenland, introducing more heat beneath Greenland. Rocks that were melted by this heat may have carried even more heat toward the surface. These processes likely introduced extra heat to the base of the ice sheet in some parts of Greenland, and may help to explain volcanism patterns around the edges of Greenland. This knowledge of how a mantle plume affects the rocks beneath Greenland helps us to understand the MT data that we collected on the ice sheet. Another PhD student has developed a new numerical modelling technique for GIA that can accommodate large viscosity variations. This code has been tested and benchmarked, and is now available for scientists studying GIA problems worldwide. We have already used it to show that a low viscosity region beneath a melting ice sheet significantly accelerates and amplifies ground uplift across a broad region. This finding is important for understanding land uplift patterns around Greenland. In particular, we have developed new GIA models that include a low-viscosity plume track across Greenland associated with the Iceland Plume. We find that recent melting of the ice sheet causes much faster land uplift along the low-viscosity plume track compared to other parts of Greenland. In particular, our calculations predict rapid uplift in southeast Greenland due to recent and rapid deglaciation of the Kangerlussuaq and Helheim Glaciers, which lie over the youngest parts of the plume track. Indeed, geodetic observations of ground uplift in this region are faster than anywhere else in Greenland, confirming our calculations. Even faster uplift should result from accelerated melting in the future. Such changes in the ground surface will affect future patterns of ice melting, and sea level worldwide. The MAGPIE project has contributed greatly to our understanding of how heterogeneities in mantle structure can be detected, and how they affect patterns of heat flow and ground uplift in deglaciating regions. We have applied this knowledge to southeast Greenland and discovered the causes of rapid ground uplift there. The MAGPIE project leaves behind a new numerical code for studying GIA problems, a new method for estimating mantle viscosity from geophysical observations, and invaluable MT data from the Greenland Ice Sheet. The MAGPIE discoveries and legacy will be useful for future scientists studying complex interactions between mantle structure, ice sheet dynamics, and climate change.

The computational aspects of the MAGPIE project resulted in the training of two PhD students (Maaike Weerdesteijn and Florence Ramirez) and provided postdoctoral opportunities for two PhD graduates (Björn Heyn and Florence Ramirez). These individuals gained critical expertise in numerical modelling, research project organization, and research skills such as oral presentation and scientific writing. The fieldwork components of the MAGPIE project also provided opportunities to gain polar field experience for two PhD students (Silje Smith-Johansen at UiB and Maaike Weerdesteijn at UiO) and for PI Clint Conrad. For Kate Selway, who organized and directed the fieldwork on the Greenland Ice Sheet, the project offered an opportunity to develop leadership and logistical skills for scientific research in polar regions. The scientific output of MAGPIE has resulted in several impacts that are important for society. First, the heat from the Iceland Plume is still emerging, which changes our understanding of the thermal structure at the base of parts of the Greenland Ice Sheet. Future glacial models should take in to account this excess heat. This finding will impact our understanding of glacial flow dynamics of the Greenland Ice Sheet, with consequences for predictions of future ice melting and associated sea level change. Second, the trace of the Iceland Plume should be associated with a path of low-viscosity mantle beneath Greenland. We have found that deglaciation above this path causes rapid uplift because of its thinner elastic lithosphere and lower upper mantle viscosities. This is important for two reasons. First, this uplift is used to measure the amount of ice melting – so rapid uplift will affect our estimates of melting rates. Second, the uplift itself may change the rate of future ice melting by lifting the remaining ice to higher elevations and by shifting the location where ice meets ocean (the grounding line). Understanding this process improves our ability to predict the consequences of future climate change on the ice sheet. This will in turn improve predictions of future ice melting and sea level rise, which is important for coastal regions worldwide. Third, we have developed a method for constraining large variations in mantle viscosity beneath the ice sheet, using seismic and magnetotellurics data. This capability to infer viscosity variations using geophysical measurements will improve our ability to understand geodynamic processes polar regions, where land is uplifting due to unloading of ice. It may also be important in non-polar regions because changes in lake levels and groundwater hydrology also impart loads on Earth’s surface. As future climate change affects hydrology worldwide, it may be important to understand viscosity variations outside of polar regions in order to understand and predict the Earth’s response. The MAGPIE method for constraining viscosity variations will provide a framework for developing this understanding.

Rising sea levels due to melting of the Greenland ice sheet threaten to drastically impact global environments. It is therefore vital to measure ice sheet melting, but this has proved challenging because measurements of the ice sheet’s mass and elevation, which both decrease as the ice melts, are also sensitive to movements of the Earth's ground surface. Greenland, and most of the Arctic, is still deforming in a viscous response to deglaciation since the last ice age. How much does this glacial isostatic adjustment (GIA) affect ice loss calculations? At present we do not know: both GIA and mantle viscosity are poorly constrained for Greenland. Furthermore, Greenland’s complex geologic history, with a recent passage over the Iceland Plume, probably created large lateral viscosity variations beneath Greenland that complicate the GIA response. In fact, the GIA correction has been called 'the largest source of uncertainty in [Greenland's] ice mass estimate' (Velicogna, 2009). In this project we aim to improve GIA models for Greenland. First, we will collect the first ever magnetotelluric data on the Greenland ice cap. We will use the resulting electrical conductivity models, together with other geological and geophysical data, to constrain mantle temperatures and compositions beneath Greenland. From these we will infer lateral viscosity variations and test for a hot, low-viscosity channel beneath the Iceland Plume track. Second, we will build a new set of GIA models for Greenland by repurposing a mantle flow code with adaptive mesh refinement to solve the GIA problem with 3D viscosity variations. This open-source code will be available to the geophysical community for solving similar problems, which are also vitally important in Antarctica. Using this code, we will generate the first GIA models for Greenland that include constrained 3D viscosity variations. These predictions of GIA uplift will enable us to greatly improve estimates for modern-day ice loss in Greenland.

Publications from Cristin

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FRINATEK-Fri prosj.st. mat.,naturv.,tek