| Ferromanganese crust is shell-like precipitates growing on hard substrate in the marine environment. Hydrogenous ferromanganese crusts are considered to incorporate elements from ambient seawater during their growth on seamounts. Besides, after the crusts’formation, the major elements (like Fe and Mn) and some trace elements (like REE) seldom migrate. Accordingly, the distribution of elements such as Fe and Mn can reflect the chemical characteristics of the ambient seawater during the formation of the crusts, while the compositions of isotopes such as Nd, Pb, and Hf can trace the ocean circulation and source provenances. Therefore, investigation on the elements and isotopes of the crust profiles can reveal and reconstruct the evolution history about the paleocirculation, weathering inputs into the ocean, and the paleoclimate changes.In order to further understand the ocean circulation pattern of the central North Pacific and the relationship between the crusts’elements and the climate changes, crust MP3D07and CXD55have been analyzed for high-time resolution element contents by electron probe, and the other two crusts located near Mariana arc but at different water depths (MKD13:1530m, MDD53:2700m) have been analyzed for both the element contents and Nd isotope compositions. Co contents in the crusts are used to estimate the ages of these crusts.The high-time resolution electron probe results show that the Fe/Mn ratios of both crusts in this study reveal rhythmic variations, which coincide with benthic oxygen isotope fluctuation in glacial-interglacial cycles since～1Ma and the Fe/Mn ratios in glacial stages are higher than those in interglacial stages. Such distinct glacial and interglacial signal extracted from the ferromanganese crusts indicates that the Fe-Mn contents of crusts are related to global climate change. During the glacial stages, together with the influence of Subarctic water mass, the increased eolian input which brought large amount of iron and other nutritive elements led to higher primary productivity in Pacific surface water, and thus increased iron flux to the deep water by the release of biological particles, which were responsible for the increase of the Fe/Mn ratio of ferromanganese crusts.Based on the Nd isotopic records of the two crusts (MKD13and MDD53), the evolution of western Pacific deep water circulation since Miocene is explored. From the early to late Miocene, Nd isotopic compositions of MDD53remained stable, and they were also characterized by the least radiogenic signatures (sNd:-4.0to-5.0) compared with crusts of similar water depths in the Miocene North Pacific. Afterward, an abrupt increase in its ENd value occurred in the Pliocene. In contrast, Nd isotopes of MKD13became more radiogenic with time in the Miocene and were almost invariable thereafter. The continual increase in8Nd of shallower crust MKD13is interpreted as reflecting progressive closure of Indonesian Seaway in the Miocene, while the deep western boundary current sourced from the Southern Pacific may have dominated Nd isotopes of deeper crust MDD53during the same time interval. The lack of Nd isotopic variation of MKD13in the Pliocene indicates that there was no change in Nd sources in shallower waters and the final restriction between the Indian and Pacific might have only occurred since then. Therefore the observed large shift to more radiogenic Nd isotopes of MDD53in the Pliocene should have not been resulted from changes in vertical input from shallower to deeper water. Instead, a decreased ventilation of deep southern component current along the studied water depth range (-2700m) might have continued in the Pliocene. |
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