Earth and Planetary Sciences ETDs

Publication Date

5-1-2010

Abstract

The "Green Sahara" pluvial phases that alternated with North African hyper-aridity during the Pleistocene are well recognized in tropical and Mediterranean marine records. However, comparatively few studies have investigated the terrestrial expression of these pluvials, in part because of the paucity of paleohydrologic archives in the desert. In this study, we show that the travertine record of Egypt's Western Desert constitutes a promising terrestrial proxy for North African paleohydrology. Integrating our reconnaissance sampling of travertine and modern groundwaters from five important oasis areas with data from previously published studies, we combine high-precision U/Th dating, geologic characterization, and stable isotope and 87Sr/86Sr geochemistry to contribute to a record of Egyptian pluvial periods for the last ~650 ka. We show that changing hydrologic head controlled voluminous travertine deposition and dictated its landscape position, and that major depositional episodes were largely synchronous across the Western Desert, suggesting a regional signal. We confirm previous findings that large volume deposition occurred across oasis areas at ~125 ka, as well as constraining major deposition from ~450-600 ka. We also show that at least some lacustrine deposits at Dahkla Oasis associated with paleolithic artifacts are 300-350 ka rather than ~130 ka. A comparison of travertine geochemistry with modern groundwater chemistry suggests that a consistent Nubian groundwater source has fed travertine deposition over the last half million years. Dakhla Oasis has an enriched 87Sr/86Sr signature in both modern groundwater (0.7170-0.7211) and travertine (~0.7098), reflecting water circulation through radiogenic basement rocks; higher 87Sr/86Sr in modern waters is interpreted to be due to deeper tapping of modern pumped waters relative to past artesian conditions. Travertine stable isotopic signatures from Dahkla also differ from the other oases and are interpreted to reflect its lacustrine rather than spring-mound depositional environment. These observations lead to a depositional model in which travertine accumulations around paleo-oasis springs reflect episodes of enhanced spring discharge deriving from high hydrologic head in the Nubian aquifer system. Increased head in the artesian system, in turn, is interpreted as a response to greater precipitation in southern groundwater recharge areas. Importantly, discharge from these travertine-depositing springs includes significant upward flux of deeply-derived carbonic fluids through faults in paleo-oasis areas. Thus, in this model, large-scale travertine accumulations serve as an archive of wet intervals in the Ethiopian-Sudan recharge region, which are then expressed in oasis springs following the short (<10 ka) lapse time it takes for transmission of high head pressure from the highlands to the oasis springs. This idea is supported by the fact that peak times of large volume travertine deposition are associated, roughly, with sapropels, indicating response to major regional pluvial episodes. However, our data do not show a coherent correlation to glacial cycles, suggesting that previous emphasis on travertine's association with glacial forcings should be scrutinized. In summary, our study reveals that travertine deposition in broadly synchronous regional episodes across the Western Desert is consistent with the pluvial events recognized in marine records. Subject to further testing, we interpret large volume travertine deposition in Egypt's Western Desert to be a pluvial indicator ultimately responding to orbital forcing.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Advisor

Crossey, Laura J.

First Committee Member (Chair)

Karlstrom, Karl E.

Second Committee Member

Asmerom, Yemane

Third Committee Member

Fawcett, Peter J.

Project Sponsors

This work is supported by National Science Foundation grant #1004276 from International Programs and Hydrologic Sciences to Drs. Crossey and Karlstrom; GJ received the University of New Mexico's EEE Black Fellowship, and a National Science Foundation Graduate Research Fellowship.

Language

English

Keywords

U/Th geochronology, 87Sr/86Sr, stable isotopes, travertine, pluvials, Egypt

Document Type

Thesis

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