SOUTHWARD SHIFTING OF THE INTER-TROPICAL CONVERGENCE ZONE IN THE WESTERN PACIFIC DURING THE LATE TERTIARY: EVIDENCE FOUND IN FERROMANGANESE CRUSTS FROM SEAMOUNTS NEAR THE MARSHALL ISLANDS

Hydrogenous ferromanganese crusts (Fe-Mn crusts) were dredged from four seamounts (OSM7, OSM2, Lomilik, and Lemkein from NW to SE) near the Marshall Islands in the western Pacific, which are aligned parallel to direction of the movement of the Pacific plate. The Fe-Mn crusts of this study show distinct four layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediments. Detrital (Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The property of layer 2 indicated that this layer had grown under the condition of high biogenic and detrital flux. Such a condition can be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet. Considering the present location and paleotracking of seamounts, layer 2 had formed during these seamounts were beneath the ITCZ, whereas layer 1 started to grow after the seamounts moved out of the ITCZ due to northwestward movement of the Pacific plate. Crusts from higher latitude result in thicker layer 1 as a result of longer growth period. Ages of the boundary between layers 1 and 2 increase northward, and were estimated at 15.3, 10.8, 7.3, and 3.3 Ma for OSM7, OSM2, Lomilik, and Lemkein, respectively. Assuming a constant velocity of the Pacific plate of 0.3°/Myr, the latitudes of the seamounts at the time of initiation of the layer 1 growth were calculated to be 12.4°N at 15.3 Ma, 10.7°N at 10.8 Ma, 9.5°N at 7.3 Ma, and 8°N at 3.3 Ma. This result suggests that the position of ITCZ had shifted southward constantly as a result of asymmetric cooling of northern and southern hemisphere during the Late Tertiary.