Geochemistry of Fe-Mn crusts at seamounts in the Northwest Pacific

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.

本数据集的研究对象为采自西太平洋4座海山（OSM7、OSM2、Lomilik与Lemkein）的水成铁锰结壳（Hydrogenetic ferromanganese crusts），这些海山沿北西-南东向展布，与太平洋板块（Pacific Plate）的运动方向一致。该结壳发育4层结构清晰的单元，各单元具有独特的结构与地球化学特征。最顶层的第1层相对富集锰（Mn）、钴（Co）、镍（Ni）与钼（Mo）；而下伏的第2层则相对富集铝（Al）、钛（Ti）、钾（K）、铷（Rb）以及铜（Cu）、锌（Zn）与过剩钡（excess Ba）。第2层的结构与地球化学特征表明，其形成于高生物源与碎屑物通量的沉积环境中，这类环境多见于赤道太平洋区域，即热带辐合带（Intertropical Convergence Zone, ITCZ）与赤道高生产力区之间。结合海山的回溯路径来看，第2层应形成于各海山位于赤道太平洋下方的时期。与之相对，第1层的形成时间应为海山从热带辐合带（ITCZ）向西北方向移动之后。这一解释与4块结壳的第1层厚度变化规律相符：第1层厚度向西北方向逐渐增厚。各结壳的层1-层2界面年代可作为热带辐合带（ITCZ）北缘的潜在代用指标，其年代同样向西北方向逐渐变老，其中OSM7、OSM2、Lomilik与Lemkein对应的界面年代分别为17 Ma、11 Ma、8 Ma与5 Ma。假设太平洋板块运动速率为0.3°/百万年（Myr），则界面形成时期，这4座海山分别位于北纬12°、11°、9°与8°的位置。该结果表明，西太平洋热带辐合带（ITCZ）的北缘自中中新世（middle Miocene）以来已向南迁移，这一结论与东太平洋的相关研究结果一致。