TRACE AND RARE EARTH ELEMENTAL COMPOSITIONS OF THE OPERATIONALLY DEFINED EOLIAN DUST FRACTION OF CORE SEDIMENTS FROM THE NORTHEAST EQUATORIAL PACIFIC AND THEIR IMPLICATIONS ON PALEOLOCATION OF THE INTERTROPICAL CONVERGENCE ZONE

The Intertropical Convergence Zone (ITCZ), where the southeast and northeast trade winds converge, is the effective climatological barrier that separates the southern and northern hemispheres in dust budget. A few studies suggested that it was located farther north than its present position at 5¨¬ to 10¨¬N during Cenozoic, but its position through geologic time is still in debate. We propose its paleolocation at around 15¨¬N during Middle Miocene based on geochemical and mineral compositions of core sediments recovered from 16¨¬N and 125¨¬W of the northeast equatorial Pacific. The operationally defined eolian dust component (ODED) of the lower part sediments that extend from 250cm (9Ma) to 328 cm (14 Ma), the bottom of the core, is characterized by 1) the dominance of smectite and plagioclase over quartz and illite, and 2) REE-deletion and strong positive Eu anomaly when normalized to average shale composition. These geochemical characteristics are common in the sediments containing significant fraction of volcanic components and are generally found in surface sediments of the equatorial Pacific in the south of the ITCZ. In contrast, the ODED component of the upper section (<9Ma) is dominated by quartz and illite, and shows more shale-like REE and trace element compositions. These characteristics are commonly observed in surface sediments in the north of the ITCZ. If we assume that there have been no significant changes in the composition of eolian dust transported by the northeast and southeast trade winds during the last 14 Ma, these results suggest that the ITCZ was located near the study area at around 9 Ma. This interpretation was derived mainly based on geochemical characteristics of surface sediments distributed in the equatorial Pacific. In order to reach the solid conclusion, however, further isotopic and geochemical characterizations are required for the equatorial surface sediments in the south of the ITCZ as well as the studied sediments. We exclude the volcanic arcs of the Pacific Rim as a source for the volcanic material in the lower section because the Pacific Rim is known volcanically quiet prior to 6.3 Ma.