238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration?
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Abstract
A detailed TIMS study of (234Uexc/238U), (230Th/232Th), and Th/U ratios have been performed on the outermost margin of ten hydrogenous FeMn crusts from the equatorial Pacific Ocean and west-central Indian Ocean.Th/U concentration ratios generally decrease from the crust's surface down to 0.5-1 mm depth and growth rates estimated by uranium and thorium isotope ratios are significantly different in FeMn crusts from the Peru Basin and the west-central Indian Ocean. FeMn crusts from the same geographical area define a single trend in plots of Ln(234Uexc/238U) vs. Ln(230Th/232Th) and Th/U ratios vs. age of the analysed fractions. Results suggest that (l) hydrogenous FeMn crusts remain closed-systems after formation, and consequently (2) the discrepancy observed between the 230Th and 234U chronometers in FeMn crusts, and the variations of the Th/U ratios through the margin of FeMn crusts, are not due to redistribution of uranium and thorium isotopes after oxyhydroxide precipitation, but rather to temporal variations of both Th/U and initial thorium activity ratios recorded by the FeMn layers. Implications of these observations for determination of FeMn crust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in FeMn crusts might be related to changes in particle input to seawater and/or changes in ocean circulation during the last 150 ka.
Study Area
Publication type | Article |
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Publication Subtype | Journal Article |
Title | 238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration? |
Series title | Geochimica et Cosmochimica Acta |
DOI | 10.1016/S0016-7037(97)00187-7 |
Volume | 61 |
Issue | 17 |
Year Published | 1997 |
Language | English |
Publisher | Elsevier |
Description | 14 p. |
First page | 3619 |
Last page | 3632 |
Other Geospatial | Indian Ocean, Pacific Ocean, Peru Basin |
Google Analytic Metrics | Metrics page |