MASS FLUX AND GRAIN SIZE VARIATIONS OF EOLIAN DUSTS AS A NEW PROXY TOOL FOR TRACKING OF THE INTERTROPICAL CONVERGENCE ZONE IN THE NORTHEAST PACIFIC

Latitudinal position of the Intertropical Convergence Zone (ITCZ), where the southeast and northeast trade winds converge, has shifted through geologic past. Its latitudinal position is recorded in deep-sea cores as a form of changing source regimes from the Asia to Central/South America or vice versa because the ITCZ is acting as an effective hemispheric dust barrier. Because of it, source indicators such as mineralogical, geochemical, and isotopic compositions of eolian dusts were investigated to understand the paleo-position of the ITCZ at a certain geologic time. However, careful examination of existing data reveals a significant difference in mass flux and grain size distribution of eolian dust across the ITCZ. It implies that these two variables might be used as a proxy tool for the paleo-position of the ITCZ. In order to evaluate this hypothesis, a 328 cm-long piston core from the northeast equatorial Pacific at 16°12'N, 125°59'W was investigated for eolian mass flux and grain size distribution with its mineralogical, elemental, and isotopic compositions. The eolian mass flux of the lower section (below 250 cm, 7.6-15.5Ma) was very uniform at 5.2 ± 0.1 mg/cm²/10³yr, while that of the upper section above 250cm (to 7.6Ma) was 12.0 ± 1.0 mg/cm²/10³yr. It is over 2 times higher than that of the lower section. The median grain sizes of eolian dusts were also distinctive across the 250 cm boundary; 8.4~8.0f in the lower section and 8.8~8.6f in the upper section. The determined eolian mass fluxes and grain sizes are well compared in magnitude to those of central Pacific sediments in the upper section and equatorial/south Pacific sediments in the lower section. The analytical results of other source indicators of eolian dust are also consistent with those of mass flux and grain size and point out the Central/South American source for the lower section (smectite-rich, e_Nd = -5.1~-6.8, ⁸⁷Sr/⁸⁶Sr = 0.7069~0.7078, ²⁰⁸Pb/²⁰⁴Pb = 38.68~38.71) and the Asian source for the upper section (illite-rich, e_Nd = -8.4~-10.3, ⁸⁷Sr/⁸⁶Sr = 0.7134~0.7214, ²⁰⁸Pb/²⁰⁴Pb = 38.69~38.93). All these results suggest that the study site was located beneath the ITCZ at ~7.6 Ma and also that the distinct differences in eolian mass flux and grain size observed across the ITCZ can be used to track the paleo-latitude of the ITCZ in deep-sea cores.