Ultrafine-scale magnetostratigraphy of marine ferromanganese crust
Ultrafine-scale magnetostratigraphy of marine ferromanganese crust
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DOI：
10.1130/g31610.1
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发表时间：
2011-03
期刊：
影响因子：
5.8
通讯作者：
H. Oda;A. Usui;I. Miyagi;M. Joshima;B. Weiss;Chris Shantz;L. Fong;K. McBride;Rene Harder;F. Baudenbacher
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文献类型：
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作者：
H. Oda;A. Usui;I. Miyagi;M. Joshima;B. Weiss;Chris Shantz;L. Fong;K. McBride;Rene Harder;F. Baudenbacher
水成铁锰结壳是海底的铁锰氧化物化学沉淀物，经过几千万年的生长。它们的化学、矿物学和结构变化的长期记录是深海环境变化的档案。然而，环境重建需要可靠的高分辨率年龄测定。早期使用放射化学和稳定同位素的年代学方法提供了铁锰结壳的年龄模型，但在毫米尺度上有局限性。例如，通常被认为是最可靠的技术的10 Be/ 9 Be计时法的可靠性取决于10 Be的产生和保存是恒定的假设，并且需要对10 Be半衰期的准确了解。为了克服这些限制，我们采用了另一种计时技术，磁性地层学，从西北太平洋的50毫米厚的水成铁锰结壳（D96-M4）。亚毫米级的磁条起源于平行于层理方向的大致相反磁化区域，使用称为扫描SQUID（超导量子干涉装置）显微镜的高分辨率磁力测量技术在地壳的薄切片上清晰地识别出来。通过将磁条的边界与已知的地磁反转相关联，我们确定了平均增长率为5.1 ± 0.2 mm/m.y.，与10 Be/ 9 Be法（6.0 ± 0.2mm/m.y.）计算值相差16%以内。这是迄今为止对地质样本进行的最精细的磁性地层学研究。使用SQUID显微镜的超细尺度磁性地层学是估算水成铁锰结壳年龄和生长速率的一种强有力的新年代学工具。它提供了时间上的限制与天文校准的磁性地层时间尺度（1-40千年）承诺的准确性。
Hydrogenetic 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 10 Be/ 9 Be chronometry, commonly considered the most reliable technique, depends on the assumption that the production and preservation of 10 Be are constant, and requires accurate knowledge of the 10 Be 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 10 Be/ 9 Be 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.).