Modulation of the Zinc Isotope Variations in Deep-sea Carbonates from the Eastern Equatorial Pacific by Precession

Zinc participates in multiple biological processes, notably as cofactor in enzymatic photosynthetic reactions. It has been argued that fractionation between light and heavy isotopes of Zn in sinking particles is stronger after biological blooms (Maréchal et al., 2000). Considering that calcareous nannofossils, principally coccolithophoridae, deposit CaCO3 with O and C at or near equilibrium with surface seawater (Margolis et al., 1975), we have assumed a similar behavior for Zn. Using multiple-collector ICP-MS which allows high precision (40 ppm) measurements of isotopic ratios, we have investigated Zn isotope variation in the carbonate fraction, dominated by coccoliths and foraminifer fragments, of a deep-sea core (ODP 849, leg 138) located in the eastern equatorial Pacific. Results show δ ⁶⁶Zn values ranging from 0.13 to 0.64 ‰ over the last 175 kyr, with several marked peaks. The range of variation is a factor of thirteen larger than the overall analytical precision (0.04 ‰ ). The average δ ⁶⁶Zn value for ODP 849 (0.45 ‰ ) markedly departs from that of terrigenous material (0.24 ‰ ). Temperature induced Zn isotopic fractionation is probably insignificant since the SST variations are small (3-5° C) and the δ ⁶⁶Zn pattern lacks a glacial/interglacial signal. A periodogram of the δ ⁶⁶Zn record reveals a dominant peak at 21.3 kyr, most likely indicative of forcing by precession. The strong zonal Zn isotopic variations observed in ferromanganese nodules have been ascribed (Maréchal et al., 2000) to the amplitude of seasonal variations in biological productivity. The lighter Zn isotopes in high latitude surface waters become depleted by biological processes as a result of the development of a seasonal thermocline that prevents the Zn replenishment of the euphotic zone by deep water. On this basis, we hypothesize that the δ ⁶⁶Zn variability in the carbonate fraction of deep-sea sediment reflects the steepness of the thermocline. In tropical regions, trade winds intensity is controlled by the precession cycle (McIntyre & Molfino, 1996). Eckman pumping by southeast trade winds (STW) induces equatorial upwelling. Lower austral summer and higher austral winter insolation and infrequent El Nino like events weaken the STW, allowing the development of a deep thermocline correlatively with the decrease in the upwelling rate. Consequently, deep water does not fully replenish surface waters, which become gradually depleted in the lighter Zn isotope by biological activities. This phenomenon is reflected by high δ ⁶⁶Zn values in the deep-sea carbonates. In contrast, low δ ⁶⁶Zn values in carbonate sediments correspond to periods of strong STW, and, possibly, frequent El Nino events.