Southward shift of the Intertropical Convergence Zone in the western Pacific during the late Tertiary: Evidence from ferromanganese crusts on seamounts west of the Marshall Islands

Abstract

Hydrogenetic ferromanganese crusts were dredged from four seamounts in the western Pacific, OSM7, OSM2, Lomilik, and Lemkein, aligned in a NW‐SE direction parallel to Pacific Plate movement. The crusts consist of four well‐defined layers with distinct textural and geochemical properties. The topmost layer 1 is relatively enriched in Mn, Co, Ni, and Mo compared to the underlying layer 2, which is relatively enriched in Al, Ti, K, and Rb and Cu, Zn, and excess Ba. Textural and geochemical properties of layer 2 suggest growth conditions under high biogenic and detrital flux. Such conditions are met in the equatorial Pacific (i.e., between the Intertropical Convergence Zone (ITCZ) and equatorial high‐productivity zone). Layer 2 likely formed when each seamount was beneath the equatorial Pacific along its back track path. On the other hand, layer 1 probably started to grow after seamounts moved northwest from the ITCZ. This interpretation is consistent with the thickness of layer 1 across the four crusts, which increases to the northwest. Ages of the layer 1–layer 2 boundary in each crust, a potential proxy for northern margin of the ITCZ, also increase to the northwest at 17, 11, 8, and 5 Ma for OSM7, OSM2, Lomilik, and Lemkein, respectively. Assuming Pacific Plate motion of 0.3°/Myr, the seamounts were located at 12°N, 11°N, 9°N, and 8°N at the time of boundary formation. This result suggests that the north edge of the ITCZ has shifted south since the middle Miocene in the western Pacific, which agrees with information from the eastern Pacific.

Shifts in the Intertropical Convergence Zone, Himalayan exhumation, and late Cenozoic climate

“…1C), allowing for the differences in its reconstructed position (Lyle et al, 2002;Kim et al, 2006). This is consistent with the distribution of Early Miocene biogeographic provinces that indicates that this was a period during which warm water masses extended to unusually high latitudes in the Northern Hemisphere.…”

“…Other estimates have been made for the ITCZ paleolatitude. Kim et al (2006) analyzed the changes in composition of ferromanganese crusts from seamounts in the western Pacifi c (Fig. 1C).…”

Cenozoic sedimentary records of climate-tectonic coupling in the Western Himalaya

“…This model was largely based on the position of the smectite-illite transition that marks the boundary between wind-blown clay derived from Asia and that from Central and South America in the central Pacific Ocean (Lyle et al 2002). In the western Pacific, a less dramatic migration southward was noted on the basis of the composition of manganese crusts that are dependent on biologic and detrital fluxes (Kim et al 2006). As the monsoon weakened, this band of intense rainfall would have migrated south and became positioned over the region where the Inner Lesser Himalaya are now exposed.…”

Geology and geochemistry of ferromanganese nodules in the Japanese Exclusive Economic Zone around Minamitorishima Island

“…These Pacific seamounts crusts generally consist of a lower (older) phosphatized layer and an upper (younger) non-phosphatized layer (e.g., McMurtry et al, 1994;Koschinsky et al, 1997;Hein et al, 1993Hein et al, , 1999. Recent detailed descriptions (Pan et al, 2005;Kim et al, 2005Kim et al, , 2006, however, have shown that each of these layers is itself divided into two distinct layers for a total of four layers. The younger two layers of Fe-Mn crust, which formed after phosphatization by carbonate fluorapatite, are respectively characterized by mottled and porous or vuggy (inner; Layer 2), and massive (outer; Layer 1) texture (Pan et al, 2005;Kim et al, 2005Kim et al, , 2006.…”