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Variation of the fe/mn ratio of ferromanganese crusts from the Central North Pacific: implication for paleoclimate changes

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Progress in Natural Science:Materials International 2005年第6期15卷 530-537页

作者： LU Zunli1**, LING Hongfei1*, ZHOU feng1, JIANG Shaoyong1, CHEN Xiaoming1 and ZHOU Huaiyang2(1. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China 2. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China)Department of Earth & Environmental Science, University of Rochester State Key Laboratory for Mineral Deposits Research Department of Earth Sciences Nanjing University Nanjing 210093 China Department of Earth & Environmental Science University of Rochester Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China

Contents of fe, mn and other elements in four ferromanganese crusts recovered from the central North Pacific are analyzed at high depth-resolution by electron microprobe for reconstructing factors controlling their deposition. Manganese (IV) in hydrogenetic ferromanganese crusts is mainly supplied as colloidal precipitates from the Oxygen Minimum Zone (OMZ), which concentrates high amounts of dissolved mn (II). The iron is derived from carbonate dissolution and silicate particles of eolian dust. An increase in paleoproductivity during cooler climate would potentially lead to a decrease in mn deposition due to enlargement of the OMZ which has a “temporary storage” effect for mn. On the other hand, not affected by the OMZ, the iron entering the fe-mn crust would likely increase with the eolian dust input and surface productivity at glacial stages. As a result, the increasing fe/mn ratio should indicate a cooling climate. This is supported by the following observations. In the profile of the past 1 Ma, the variations of fe/mn ratio coincide with benthic oxygen isotope fluctuation in glacial-interglacial cycles. Three episodes with high fe/mn ratios, approximately at 2.6, 1.8 and 0.8 Ma, are detected within the past 3 Ma and coincide with major climate transitions and cooling events. The secular evolution pattern of fe/mn ratio in the Cenozoic is similar to the Pb isotope evolution which is mainly controlled by eolian dust and related to climate. The fe/mn evolution pattern is also broadly consistent with the global deep-sea oxygen isotope records. Therefore, fe/mn ratio recorded in the ferromanganese crusts may be a new proxy for climate change.

关键词： fe/mn ratio ferromanganese crust central North Pacific paleoclimate.

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