Analysis of ferromanganese deposits from the Northwestern Pacific seamounts

Fine-scale dating is crucial to understanding the growth and environments of formation of marine manganese deposits. The paleomagnetic method of dating of manganese nodules and crusts has been attempted but with no success so far. We measured remanent magnetization (RM) on up to 75-mm-thick hydrogenetic crusts at intervals of 2.5 mm after mineralogical and chemical examination, careful separation, and alternative field demagnetization. Stable sharp RM and well-correlated polarity-change patterns between four samples made it possible to identify the Pliocene-Quaternary magnetic chrons in them. These data show that the three crusts have grown continuously at rates of 14-17 mm/m.y. since the Early Pliocene. This is three or four times faster than those for Central Pacific seamount hydrogenetic crusts. Estimates of growth rate based on determination of Co-flux are within this range. Magnetostratigraphy therefore appears to be a powerful method for dating hydrogenetic manganese crusts when supported by conventional dating methods. These results encourage finer-scale investigations which promise more detailed paleoceanographic reconstruction.

高精度定年对于理解海底锰矿床的形成过程与形成环境至关重要。此前学界曾尝试利用古地磁方法对锰结核与锰结壳进行定年，但迄今尚未取得成功。本研究在完成矿物学与化学分析、精细分选及交变场退磁处理后，以2.5 mm的间隔对厚度可达75 mm的水成锰结壳样品开展了剩余磁化强度（remanent magnetization, RM）测试。四个样品之间稳定清晰的剩余磁化强度信号与高度一致的极性转换模式，使得我们能够识别其中蕴含的上新世-第四纪磁极性时。这些数据表明，自上新世早期以来，这三块锰结壳的连续生长速率为14~17毫米/百万年。该生长速率是中太平洋海山的水成锰结壳生长速率的3~4倍。基于钴通量测定得到的生长速率估算值也处于这一区间内。因此，在传统定年方法的佐证下，磁地层学似乎可成为水成锰结壳定年的有效手段。本研究结果为后续开展更高精度的研究提供了支撑，有望实现更精细的古海洋学重建。