ABSTRACT:

This paper researches the ferromanganese crust MHD79 from bottom to top 20 Os isotope data from the Central Pacific. The results showed that: in the Ferromanganese crust MHD79, each 187Os/188Os value low point corresponds to the Os content high point, and corresponds to a large-scale volcanic (magmatic) activity or meteorite impact event. In the early three stages of the ferromanganese crust MHD79, Os content gradually increased with the decreasing of 187Os/188Os, which demonstrates that Os content correlated with 187Os/188Os negatively during this period; in the stages IV and V, the Os contents still correlated negatively with the 187Os/188Os, but the 187Os/188Os value increased gradually; in the stage VI, the Os contents increased and correlated positively with the 187Os/188Os. The evolution of Os contents and 187Os/188Os in the ferromanganese crust MHD79 suggested that, mantle and extraterrestrial material were the main contributors to the Os in the ocean relative to terrestrial material from late Cretaceous to Paleocene; Thereafter, the terrestrial material input increased gradually; after Oligocene, the terrestrial material input increased rapidly and became the main contributor for the Os in the ocean.

INTRODUCTION

The ferromanganese oxides in marine sediments have recorded the seawater Os isotope composition when they were deposited. Therefore, sediments containing ferromanganese oxides, e.g. carbonates, metaliferous siliceous ooze, ocean clay, ferromanganese crust and ferromanganese nodules, may all be used in the research of seawater Os isotope composition evolution (Burton et al., 1999; Martin et al., 2001; Alves et al., 2002; McDaniel et al., 2004; Williams and Turekian, 2004; Dalai et al., 2005; Klemm et al., 2005; Fu et al., 2005; Paquay et al., 2008; Meng et al., 2008). Sharma et al. (1997) determined that the lower limit of Os residence time in the ocean can be 4.4 × 104a.

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