
NSF Org: |
OPP Office of Polar Programs (OPP) |
Recipient: |
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Initial Amendment Date: | August 7, 2019 |
Latest Amendment Date: | August 7, 2019 |
Award Number: | 1851094 |
Award Instrument: | Standard Grant |
Program Manager: |
Kelly Brunt
kbrunt@nsf.gov (703)292-0000 OPP Office of Polar Programs (OPP) GEO Directorate for Geosciences |
Start Date: | August 15, 2019 |
End Date: | July 31, 2024 (Estimated) |
Total Intended Award Amount: | $469,370.00 |
Total Awarded Amount to Date: | $469,370.00 |
Funds Obligated to Date: |
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History of Investigator: |
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Recipient Sponsored Research Office: |
7 LEBANON ST HANOVER NH US 03755-2170 (603)646-3007 |
Sponsor Congressional District: |
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Primary Place of Performance: |
14 Engineering Drive Hanover NH US 03755-4401 |
Primary Place of
Performance Congressional District: |
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Unique Entity Identifier (UEI): |
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Parent UEI: |
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NSF Program(s): |
ANT Glaciology, Antarctic Science and Technolo |
Primary Program Source: |
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Program Reference Code(s): |
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Program Element Code(s): |
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Award Agency Code: | 4900 |
Fund Agency Code: | 4900 |
Assistance Listing Number(s): | 47.078 |
ABSTRACT
The ice of the polar ice sheets is among the purest substances on Earth, yet the small amount of impurities --such as acids-- are important to how the ice flows and what can be learned from ice cores about past climate. The goal of this project is to understand the role of such acids on the deformation of polycrystalline ice by comparing the deformation behavior of pure and sulfuric acid-doped samples. Sulfuric acid was chosen both because of its importance for interpreting past climate and because it can lead to water veins in ice at low temperatures. This work will focus on the location, movement, and impact of acids in polycrystalline ice that are more complex than in single crystals of ice. By deforming samples and performing microstructural characterization, the role of acids on deformation rate, grain evolution, and the movement of the acids themselves, will be assessed. The work will lead to the education of a Ph.D. student at Dartmouth College, introduce undergraduate students to research at both the University of Washington and Dartmouth College.
Despite the ubiquitous use of the constitutive relation for ice commonly referred to as "Glen's Flow Law", significant uncertainty exists particularly with regard to the role of impurities and the development of oriented fabrics. The aim of this project is to improve the constitutive relationship for ice by performing deformation tests and microstructural characterization of pure and sulfuric acid-doped ice. The project will focus on sulfuric acid's impact on ice viscosity, fabric evolution, and diffusivity. Sulfuric acid can have both direct and indirect effects on the mechanical properties of polycrystalline ice. The direct effects change the dislocation velocity and/or density, and the indirect effects change the grain size and fabric. The complexity and interaction of these effects means that it is not possible to understand the effects of sulfuric acid by simply examining ice core specimens. In this project, the team will deform four types of ice: lab-grown ice samples doped with similar-to-natural concentrations of sulfuric acid, lab-grown high-purity ice, layered doped and pure ice, and natural ice from Antarctic ice cores. Deformation will be performed in both uniaxial compression and simple shear. The addition of simple shear tests is critical for relating the laboratory-observed deformation behavior to the behavior of polar ice sheets where the shear strain dominates ice motion in basal ice. After deformation to strains from 5 percent up to 25 percent, the microstructural development will be assessed with methods including a variety of scanning electron microscope techniques, Raman microscopy, synchrotron-based Nano-X-ray fluorescence, and ion chromatography. These analysis techniques will allow the determination of 1) the segregation and movement of impurities, 2) the rate of grain-boundary migration, 3) the number of recrystallized grains; and 4) the full orientation of the ice crystals. The results will enable both microstructural modeling of the effects of sulfuric acid and numerical modeling of diffusion in ice cores. The net result will be a better understanding of ice deformation that improves ice-core interpretation and ice-sheet modeling.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
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This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Ice sheets contain soluble impurities which not only serve as chemical proxies of past climate but also significantly influence ice deformation. The major objectives of this project were to uncover the direct and indirect effects of sulfuric acid on ice viscosity and fabric evolution. Sulfuric acid is particularly significant because it provides the strongest evidence for the diffusion of chemical signals and has been shown to decrease the strength of single and polycrystals of ice. To achieve this, a series of constant load (creep) uniaxial compression tests and constant strain rate shear tests were conducted on sets of sulfuric acid-doped and high-purity polycrystalline ice samples. The mechanical and microstructural behavior of the ice were recorded as functions of temperature, strain, strain rate and impurity location, i.e. within the grains, at the grain boundaries, or both.
The ice samples were prepared by freezing thin plates of ice, breaking them up, sieving them to a 1-2mm diameter range, and then loading them into a mold. The mold is then chilled as de-aerated water is flowed through the mold from the bottom up. The mechanical behavior was then measured using a linear variable differential transducer to record strain and time. Additionally, the microstructural evolution of the ice was studied using a combination of an optical microscope (OM) and a scanning electron microscope (SEM).
Compression results showed that sulfuric acid-doped ice exhibits creep rates 1.5 to 3 times faster than high-purity polycrystalline ice At -12oC, sulfuric acid within the grains has a higher softening effect, while at -3oC, sulfuric acid both within the grains and at the grain boundaries exhibits a higher softening effect. Besides this, the softening effects of the sulfuric acid were greater at -12oC. The shear tests on the other hand demonstrated that sulfuric acid has a slight strengthening effect on polycrystalline ice. Optical microscopy analysis of the post-mortem samples showed that sulfuric acid doping resulted in larger grain sizes in the compressed samples and smaller grain sizes in the shear-tested samples. Fabric analysis using both electron backscatter diffraction in a SEM and the Universal stage-equipped OM demonstrated that sulfuric acid may also induce faster fabric development, hence anisotropy. Energy dispersive X-ray spectroscopy in the SEM showed that during deformation, sulfuric acid migrates to the grain boundary, forming a liquid-layer. However, at higher temperatures, there is a possibility of the sulfuric acid being distributed across both the grains and the grain boundaries after deformation due to an increase in sulfuric acid solubility within the grains.
We also explored the microstructural evolution and recrystallization processes during the creep of firn (multi-year snow). To achieve this, we subjected samples of firn from Summit, Greenland to constant load and temperature conditions. The results indicate that recrystallization in firn starts in secondary creep by strain-induced boundary migration and nucleation and growth of new grains. These results have implications for improved firn densification modeling.
The results of this project will aid in developing more accurate ice flow models, which will result in improved interpretation of ice core records through better-informed constraints on ice deformation and vertical thinning.
This project resulted in the education of a PhD student, as well as the training of a post-doctoral fellow and three undergraduate students. The results of this project were disseminated to diverse audiences at various scientific conferences including the 2023 Physics and Chemistry of Ice Conference, the 2022 and 2023 American Geophysical Union meetings, the 2022 and 2024 Northeast Glaciology meetings, and the 2023 and 2024 Ice Core Open Science meetings.
Last Modified: 08/22/2024
Modified by: Ian Baker
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