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

Abstract

A detailed TIMS study of ( 234U exc/ 238U), ( 230Th/ 232Th), and Th/U ratios have been performed on the outermost margin of ten hydrogenous Fe Mn 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 Fe Mn crusts from the Peru Basin and the west-central Indian Ocean. Fe Mn 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 Fe Mn crusts remain closed-systems after formation, and consequently (2) the discrepancy observed between the 230Th and 234U chronometers in Fe Mn 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 Fe Mn layers. Implications of these observations for determination of Fe Mn crust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in Fe Mn crusts might be related to changes in particle input to seawater and/or changes in ocean circulation during the last 150 ka.