Paleosols, or fossil soils, document past atmospheric composition, climate, and terrestrial landscapes and are key to understanding the composition and evolution of the Earths atmosphere and the environments in which life developed and evolved. Micromorphological, geochemical and isotopic studies of two Precambrian weathering profiles (Steep Rock, Canada and Hokkalampi, Finland) place constraints on the age of soil formation, document pedogenic processes and post-pedogenic alteration and metamorphism, and yield information about the terrestrial environment at a critical time in Earth history. The textures and geochemistry of the Early Proterozoic Hokkalampi paleosol and the Archean Steep Rock paleosol are consistent with in situ, subaerial weathering under varying soil redox conditions. The upper portion of both profiles have lost greater than 40% of their iron relative to the parent material, similar to Phanerozoic vertisols, while retaining Fe3+/Fe2+ ratios greater than 1. Redistribution of phosphorus in the Steep Rock profile and thorium in some Hokkalampi profiles suggests that organic ligands could have enhanced the mobility of redox-sensitive elements. In addition, copper mobility in the Steep Rock profile could indicate the presence of oxygen in the soil. Loss of base cations in both paleosols is similar to modern ultisols or oxisols, which form in warm, moist environments. Whole rock Rb-Sr isochrons indicate that post-pedogenic potassium metasomatism affected both weathering profiles, and the apparent ages are consistent with the timing of regional greenschist metamorphic events. Whole rock Sm-Nd isotope data demonstrate that rare earth element (REE) systematics were largely unaffected by metamorphism, and that the observed mobility and fractionation of REE could represent a record of Precambrian weathering processes. In the Hokkalampi profiles, Sm-Nd isotope data suggest that pedogenesis took place 2.35±0.19 Ga; this also places a minimum age for Huronian glaciation in the area. The Sm-Nd age of 3.02±0.09 Ga for the Steep Rock paleosol suggests that pedogenesis occurred very soon (less than 60 Ma) after emplacement and erosional uplift of the parent granitoid. This study demonstrates that Rb-Sr and Sm-Nd analysis combined with geochemical, field, and microscopic examination of Precambrian paleosols can help constrain the nature and timing of ancient Earth-surface processes.
| Identifer | oai:union.ndltd.org:PITT/oai:PITTETD:etd-08092007-085938 |
| Date | 27 September 2007 |
| Creators | Stafford, Sherry L. |
| Contributors | Hiroshi Ohmoto, Charles E. Jones, Edward G. Lidiak, Brian Stewart, Rosemary C. Capo |
| Publisher | University of Pittsburgh |
| Source Sets | University of Pittsburgh |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.library.pitt.edu/ETD/available/etd-08092007-085938/ |
| Rights | restricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Pittsburgh or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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