Zinc isotope variations in deep-sea carbonates from the eastern equatorial Pacific over the last 175 ka

The carbonate fraction of sediment core ODP 849, leg 138, located in the eastern equatorial Pacific, mostly consisting of coccoliths, was separated and analyzed for its Zn isotopic composition. The overall variation in Zn isotopic composition, as determined by multiple-collector, magnetic-sector, inductively coupled plasma mass spectrometry, was found to be on the order of 1 parts per thousand (expressed in delta(66)Zn, where delta(x)Zn = [(Zn-x/Zn-64)(sample)/(Zn-x/ Zn-64)(standard) - 1] x 10(3) and x = 66, 67 or 68) over the last 175 ka. The analytical precision was 0.04 parts per thousand and the overall reproducibility was usually better than 0.07 parts per thousand. The Zn isotopic composition signal exhibits several marked peaks and a high-frequency variability. A periodogram of the delta(66)Zn signal showed two periodicities of 35.2 and 21.2 ka. We suggest that the latter is caused by the precession of the Earth's axis of rotation. The periodogram exhibits a minimum at 41.1 ka, thus showing that the Zn isotopic composition is independent of the obliquity in the eastern equatorial Pacific. The range of 666 Zn values observed for the carbonate fraction of ODP 849 overlaps with the range observed for Fe-Mn nodules in the world's oceans, which suggests that seawater/carbonate Zn isotope fractionation is weak. We therefore assume that most of the Zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean. The CIDP 849 delta(66)Zn record seems to follow the changes in the insolation cycles. Changes in the late summer/fall equatorial insolation modulate the intensity of the equatorial upwelling, hence the mixing between deep and surface waters. We propose that during decreased summer/fall equatorial insolation, when a steep thermocline can develop (El Ni (n) over tildeo-like conditions), the surface waters cannot be replenished by deep waters and become depleted in the lighter Zn isotopes by biological activity, thus resulting in the progressive increase of the 666 Zn values of the carbonate shells presumably in equilibrium with surface seawater. (C) 2003 Elsevier Science B.V. All rights reserved.