Risa Madoff

Date of Award

January 2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

First Advisor

Jaakko Putkonen

Abstract

Regolith transport within the uppermost 30-50 cm of Earth's surface is responsive to shifts in climate. Rates of surface change affect accumulation rates of sediment and nutrients, thereby affecting ecology, surface stability, and the ability to date landform surfaces. However, little is known about the quantitative effects past climates have had on the long term evolution of the landscape. A space-for-time substitution method was used to study this knowledge gap. Known degradation rates from widely varying climates were used to generate a transfer function where published paleo-temperature records from the Death Valley sediment core (DV93-1) were applied to derive paleo-degradation rates at Mono Basin moraine, CA. Together with a published glacial chronology for Mono Basin, CA for the approximately last 100 kyrs, the method related past climates to varying rates of landform degradation.

In conventional applications, landscape evolution models use a diffusion equation, based on a transport law, to describe a linear relation between degradation and slope. In the transport equation, q = k (dz/dx), a topographic diffusivity coefficient, k, expresses all the combined effects other than slope on landform degradation. However, as it has been used, k does not express the diverse responses of surface processes to shifts in climate. This study used numerical modeling to vary the coefficient of topographic diffusivity in accordance with documented variations in climate to model the cross-section profile of an 85 kyr old Mono Basin moraine.

Results from hillslope profile modeling with various scenarios - an optimized constant k, time-varying k, and a current-time k - were compared. In comparison with the time-varying value, an optimized constant k, overestimated the surface elevation by up to 10% during the first 60 kyrs and underestimated it by up to approximately the same amount during a 5 thousand year time span 20 to 15 kyrs ago. Application of the current k value, assumed to reflect current interglacial climate, underestimated surface elevation by up to 32%. The results provide a first step in relating past climate shifts to variable erosion rates and surface processes through time, particularly with respect to the uppermost 1-2 meters of regolith mantling landforms.

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