Ultrafine-scale magnetostratigraphy of marine ferromanganese crust

http://geology.geoscienceworld.org/content/39/3/227.full.pdf+htmlHydrogenetic ferromanganese crusts are iron-manganese oxide chemical precipitates on the seafloor that grow over periods of tens of millions of years. Their secular records of chemical, mineralogical, and textural variations are archives of deep-sea environmental changes. However, environmental reconstruction requires reliable high-resolution age dating. Earlier chronological methods using radiochemical and stable isotopes provided age models for ferromanganese crusts, but have limitations on the millimeter scale. For example, the reliability of 10Be/9Be chronometry, commonly considered the most reliable technique, depends on the assumption that the production and preservation of 10Be are constant, and requires accurate knowledge of the 10Be half-life. To overcome these limitations, we applied an alternative chronometric technique, magnetostratigraphy, to a 50-mm-thick hydrogenetic ferromanganese crust (D96-m4) from the northwest Pacific. Submillimeter-scale magnetic stripes originating from approximately oppositely magnetized regions oriented parallel to bedding were clearly recognized on thin sections of the crust using a high-resolution magnetometry technique called scanning SQUID (superconducting quantum interference device) microscopy. By correlating the boundaries of the magnetic stripes with known geomagnetic reversals, we determined an average growth rate of 5.1 ± 0.2 mm/m.y., which is within 16% of that deduced from the 10Be/9Be method (6.0 ± 0.2 mm/m.y.). This is the finest-scale magnetostratigraphic study of a geologic sample to date. Ultrafine-scale magnetostratigraphy using SQUID microscopy is a powerful new chronological tool for estimating ages and growth rates for hydrogenetic ferromanganese crusts. It provides chronological constraints with the accuracy promised by the astronomically calibrated magnetostratigraphic time scale (1–40 k.y.).Japan Society for the Promotion of Science (Grant-in-Aid for Scientific Research (21654071))National Science Foundation (U.S.) (Collaboration in Mathematical Geosciences Program

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