238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration?

A detailed TIMS study of (²³⁴U_exc/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(²³⁴U_exc/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.