Inferring Histories of Accumulation Rate, Ice Thickness, and Ice Flow from Internal Layers in ...

314 Pages·2009·3.38 MB·English

Inferring Histories of Accumulation Rate,
Ice Thickness, and Ice Flow from Internal Layers
in Glaciers and Ice Sheets

Michelle R. Koutnik

A dissertation submitted in partial fulfillment of
the requirements for the degree of

Doctor of Philosophy

University of Washington

2009

Program authorized to offer degree: Earth and Space Sciences

University of Washington
Graduate School

This is to certify that I have examined this copy of a doctoral dissertation by

Michelle R. Koutnik

and have found that it is complete and satisfactory in all respects,
and that any and all revisions required by the final
examining committee have been made.

Chair of the Supervisory Committee:

________________________________________________
Edwin D. Waddington

Reading Committee:

________________________________________________
Edwin D. Waddington

________________________________________________
Howard B. Conway

_______________________________________________
Dale P. Winebrenner

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Abstract

Inferring Histories of Accumulation Rate, Ice Thickness, and Ice Flow
from Internal Layers in Glaciers and Ice Sheets

Michelle R. Koutnik

Chair of Supervisory Committee:

Professor Edwin D. Waddington
Earth and Space Sciences

Spatial and temporal variations in past accumulation, ice thickness, and ice
flow of polar ice sheets are weakly constrained on Earth, and are
fundamental unknowns on Mars. On Earth, the spatial and temporal
histories of accumulation and ice-sheet flow are necessary to recreate ice-
volume and sea-level histories, and are important to properly interpret ice-
core chemistry. On Mars, accumulation and ice-flow histories are necessary
to decipher the connection between climate and ice-mass formation,
evolution, and observable structure.
Internal layers in ice sheets on Earth and on Mars have been
observed with ice-penetrating radar. These layers preserve information
about how the ice sheet responded to past spatial and/or temporal changes
in accumulation rate and ice flow, and present-day internal-layer shapes
observed by radar are the most accessible remaining record of this past
information. Deeper layers contain information from further in the past,
making them highly valuable, but they are more difficult to decipher.

In this work, an inverse problem is solved to infer transients in
accumulation rate, ice-sheet thickness, and ice flow from the shapes of deep
internal layers. While some details of these histories can be recovered from
ice cores, ice cores represent conditions at only a single point. However, the
approach presented here is more robust in combination with ice-core data.
If internal layers are dated, for example by an intersecting ice core, then
radar-observed internal layers provide both spatial and temporal
information. Each layer represents a past surface of a particular age that
has been subsequently buried by accumulation and also modified by ice
flow.
In this work, the goal of solving this inverse problem is to find a set of
model parameters (e.g. accumulation-rate history) that have the minimum
variation required to explain the data (e.g. internal-layer shapes). The
process of internal-layer formation is described with a 2.5-D
thermomechanical ice-flow flowband model. Estimates of the data are
matched to measured values within their uncertainties, and to an expected
tolerance. We seek an accumulation pattern that is spatially smooth, and a
parameter set that is consistent with characteristic values of the
parameters. This dissertation presents this inverse approach, and discusses
applications to data from Antarctica and from Mars.