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Abstract

This work presents a detailed 87Sr/86Sr isotope curve for the 7.5–9.8 Ma time interval obtained by analyzing isotope compositions of the Orbulina universa planktonic foraminifer species from the Mediterranean Gibliscemi section (southern Sicily). The available astronomical tuning of the section provided the opportunity to assess a direct control of the Milankovitch climate cyclicity on the seawater Sr isotope changes. Results of spectral analysis suggest a linear forcing of the 400 ky eccentricity component on the 87Sr/86Sr ratios of the Mediterranean seawater. The recorded amplitude of Sr isotope 400 ky cycles ranges between ± 5.5 × 10-5 and ± 6 × 10-5 around the long-term trend for the Tortonian at global scale. Such a ∆87Sr is of the same order of magnitude of that measured by Capo and DePaolo (1990) and Dia et al. (1991) for the Pleistocene 100 ky glacial-interglacial cycles and about two times larger than that reported for site 758 by Clemens et al. (1993) for the same periodic oscillations. Mass-balance calculations applied to our dataset suggest that periodic changes of about 100–150%in the riverine inputs can account for the amplitude oscillations of 87Sr/86Sr ratios recorded in the Mediterranean during the Tortonian,thus emphasizing the high potential of this basin as good recorder of climate-induced seawater Sr isotope changes.

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