Date of Award

Spring 2009

Project Type

Thesis

Program or Major

Earth Sciences: Geochemical Systems

Degree Name

Master of Science

First Advisor

Joseph M Licciardi

Abstract

Developing quantitative records that uniquely resolve past temperatures is important for interpreting regional paleoclimate changes. Here I analyze the extent of amino acid racemization in fossil ostracodes, and use a previously published expression to calculate effective diagenetic temperatures (EDTs) in the Summer Lake basin of south-central Oregon for time intervals bracketed between 208 ka, 180 ka, 74 ka, and present day. Geochronological control is provided by a published age-depth model for the Ana River section in the Summer Lake basin. Reverse-phase high performance liquid chromatography (HPLC) was used to measure the enantiomeric composition of aspartic and glutamic acids in Candona and Limnocythere ostracode valves collected from 33 sedimentary horizons within a ∼15-m section of lake sediments exposed along the Ana River. An average of 9 replicate subsamples were analyzed for each sample, and the relative difference in the rate of racemization in the two genera was quantified by analyzing multiple pairs from the same stratigraphic horizons. Between 208-180 ka (MIS 7-6), the EDT was 7.0 +/- 2.6°C, which overlaps with the current mean annual temperature near the site (9.5°C). The EDT for the time interval between 180-74 ka (MIS 6-4) was -3.2 +/- 1.3°C and from 74 ka to present the EDT was 2.7 +/- 1.2°C. EDT estimates spanning the MIS 6-4 are unexpectedly low given that this interval encompasses the last interglaciation, and suggests that either the Ana River section age-depth model is inaccurate, or other factors in addition to temperature influenced the rate of racemization in the ostracode valves analyzed.

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