NSF Org:	EAR Division Of Earth Sciences
Recipient:	THE TRUSTEES OF COLUMBIA UNIVERSITY IN THE CITY OF NEW YORK
Initial Amendment Date:	June 7, 2013
Latest Amendment Date:	June 30, 2014
Award Number:	1315254
Award Instrument:	Continuing Grant
Program Manager:	Eva Zanzerkia EAR  Division Of Earth Sciences GEO  Directorate for Geosciences
Start Date:	June 15, 2013
End Date:	May 31, 2016 (Estimated)
Total Intended Award Amount:	$234,587.00
Total Awarded Amount to Date:	$234,587.00
Funds Obligated to Date:	FY 2013 = $119,078.00 FY 2014 = $115,509.00
History of Investigator:	James Davis (Principal Investigator) jdavis@ldeo.columbia.edu Benjamin Holtzman (Co-Principal Investigator) Meredith Nettles (Co-Principal Investigator)
Recipient Sponsored Research Office:	Columbia University 615 W 131ST ST NEW YORK NY  US  10027-7922 (212)854-6851
Sponsor Congressional District:	13
Primary Place of Performance:	Columbia University Lamont-Doherty Earth Observatory 61 Route 9W Palisades NY  US  10964-1707
Primary Place of Performance Congressional District:	17
Unique Entity Identifier (UEI):	F4N1QNPB95M4
Parent UEI:
NSF Program(s):	Geophysics
Primary Program Source:	01001314DB NSF RESEARCH & RELATED ACTIVIT 01001415DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s):
Program Element Code(s):	157400
Award Agency Code:	4900
Fund Agency Code:	4900
Assistance Listing Number(s):	47.050

ABSTRACT

Mantle viscosity plays a fundamental role in the long-term evolution of Earth, controlling cooling of its interior, convective flow in its mantle (which drives plate tectonics), and the stability of its rotation axis. Knowledge of mantle viscosity is also crucial for modeling the long-term changes in Earth?s shape known as Glacial Isostatic Adjustment, or postglacial rebound. These changes are associated with Earth?s glacial cycles, and accurate estimates of mantle viscosity appear to be key to resolving a number of ongoing debates in climate research. Our knowledge of the viscosity of the mantle, though, is highly uncertain, since Earth?s interior cannot be directly observed, but only probed indirectly through observations at its surface. Two of our most useful tools for this are seismology, which provides high-resolution information regarding important material boundaries in the interior of the Earth, and geodesy, which provides observations relating to postglacial rebound, such as crustal deformation, variations in sea-level and gravity, and changes in Earth?s spin axis.

The project will combine seismic models and geodetic observations to estimate parameters of an Earth model that includes composition, temperature, degree of melt, and other thermodynamic and compositional parameters. By using seismological models and geodetic observations simultaneously, the project can potentially take advantage of the best resolving power and sensitivities of both observational techniques to determine a model, unified in the sense of fitting both types of observations. The primary question to be addressed in this project is how feasible (in terms of capability for constraining multiple parameters of the Earth model) such an approach is. Along with state-of-the-art seismic and geodetic information, the project will employ recently developed methodology for calculating rheological properties across the broad spectrum of time scales relevant to geophysics, from seismic wave frequencies to strain rates associated with plate tectonics. This study will lead to preliminary three-dimensional models for mantle rheological parameters based on seismic models, and to consistent estimates of deformation, gravity, and sea level change associated with postglacial rebound; these will lead to an improved understanding of mantle convection and the impact of long-term climate change on the solid Earth.

PROJECT OUTCOMES REPORT

Disclaimer

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.

The major goal of the project was to study the feasibility of implementing a model for the rheology of Earth's mantle that reflects, to the best of our ability, the physics and thermodynamics of the materials that are believed to make up the mantle.  These would lead to time-dependent viscosities, which could have important implications for the prediction of Glacial Isostatic Adjustment (GIA)---also known as postglacial rebound---as well as for the interpretation of geodetic data that measure Earth's motions due to GIA.  The main hurdle was to develop a theory for this time depdendence that extrapolated from short time scales (as experienced in the laboratiry or by seismic waves) and to implement this theory into code that calculated the viscoelastic response of Earth to loading by nacient glaciers.

In incorporating the time-depdnent rhology in the lithosphere and upper asthenosphere into models of GIA response, we have needed to adapt two pre-existing methods: (1) incorporation of time-dependent deformation into the models of the GIA response, and (2) interpolation of frequency-dependent mechanical behavior of Earth's mantle and lithosphere into simplified, linear mechanical models that can be solved for the GIA response.

We have run into a problem in achieving the union of these two approaches and are investigating several approaches.  This involves both the approach used to calculate the GIA response, and the interpolation to GIA time-scales. Scientifically, this issue is illuminating but it nevertheless is a hinderance to achieving the goals of the proposed research.  In solving for GIS deformation, the approach generally used is to solve a series of equations to find its zero-crossings for a range of frequencies.  The solution is approximated using numerical schemes that assume a linearity of behavior such that the zeros can be interpolated accurately.  However, the non-linearity of the proposed rehology is too great for this appraoch to work without additional modifications to the code.

Last Modified: 06/13/2016
Modified by: James L Davis

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