Geomagnetic, cosmogenic and climatic changes across the last geomagnetic reversal from Equatorial Indian Ocean sediments
Geomagnetic, cosmogenic and climatic changes across the last geomagnetic reversal from Equatorial Indian Ocean sediments
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DOI：
10.1016/j.epsl.2014.03.053
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发表时间：
2014-07
影响因子：
5.3
通讯作者：
J. Valet;Franck Bassinot;A. Bouilloux;D. Bourlès;S. Nomade;V. Guillou;F. Lopes;N. Thouveny;F. Dewilde
中科院分区：
文献类型：
--
作者：
J. Valet;Franck Bassinot;A. Bouilloux;D. Bourlès;S. Nomade;V. Guillou;F. Lopes;N. Thouveny;F. Dewilde
高分辨率记录的铍（10 Be）生产和相对古强度已获得跨越Matuyama-Brunhes（M-B）逆转从赤道印度洋（马尔代夫地区）。磁性和地球化学分析都是从相同的离散样本进行的，以避免任何人为的深度偏移。自生的10 Be浓度进行了归一化相对于9 Be，以纠正潜在的环境影响，而相对古强度来自reversium磁化强度后，占沉积物内的磁浓度的变化。相对古强度和10 Be/9 Be记录都具有大的偏差，并在反转中期达到峰值。与大多数以前的研究相反，尽管相对较高的沉积速率（4.7厘米/ka），我们观察到的10 Be/9 Be峰，相对古强度的最低值和方向的开关之间的完美同步，这表明生物扰动和沉积后的过程并没有影响磁记录。这使得位于海洋同位素阶段19内的反转的地层位置没有任何歧义，正如来自同一岩心的碳酸盐δ 18 O记录所揭示的那样。磁测数据描绘了一个两阶段的过程，在快速极性转换之前的一个异常事件，而只有第二阶段是目前在10 Be记录，类似于其他低纬度记录从印度尼西亚地区。利用轨道调谐的年龄模型，我们得到过渡中期的年龄为772 ka±5 ka，而过渡事件发生在大约20 ka之前。我们认为，双峰分布出现的汇编年龄的MB逆转的结果，从这两个事件的继承。来自澳大拉西亚撞击的微玻璃陨石被发现在过渡区（790 ka±5 ka BP）以下0.6 m处，并证实这一大型事件发生在极性转换前12 ka。玻璃陨石丰度的分布被用来反卷积的10 Be/9 Be信号。结果证实，铍的变化集中在过渡期间，因此可能存在多极地磁场（或在地磁极附近），有利于宇宙射线的穿透，从而增加了10 Be的生产。10 Be的情况下，在前体表明，目前的网站和印度尼西亚的远离地磁极和纬度间的大气混合是有限的。因此，在前兆期间印度尼西亚站点上方的地磁极位置与此期间相应的倾斜偶极场不相容，并表明低阶谐波占主导地位。
High-resolution records of beryllium (10 Be) production and relative paleointensity have been obtained across the Matuyama–Brunhes (M–B) reversal from the equatorial Indian Ocean (Maldives area). Both magnetic and geochemical analyses were performed from the same discrete samples to avoid any artificial depth offset. The authigenic 10 Be concentrations were normalized with respect to 9 Be in order to correct for potential environmental effects, while the relative paleointensity was derived from the remanent magnetization intensity after accounting for changes in magnetic concentration within the sediment. The relative paleointensity and the 10 Be/9 Be records are both characterized by large deviations, which culminate in the middle of the reversal. In contrast to most previous studies, and despite relative high deposition rate (4.7 cm/ka), we observed a perfect synchronism between the 10 Be/9 Be peak, the lowest value of relative paleointensity and the switch in direction, which indicates that bioturbation and post-depositional processes did not affect the magnetic record. This leaves no ambiguity for the stratigraphic position of the reversal located within Marine Isotopic Stage 19 as revealed by the planktonic δ 18 O record from the same core. The magnetic data depict a two-phase process with a precursory event preceding the rapid polarity switch, while only the second phase is present in the 10 Be record, similarly to other low latitude records from the Indonesian area. Using an orbitally-tuned age model, we obtain an age of 772 ka±5 ka for the middle of the transition, while the precursory event occurred almost 20 ka before. We believe that the bimodal distribution emerging from the compilations of the ages of the MB reversal results from the succession of these two events. Microtektites from the Australasian impact were found at 0.6 m below the transition (790 ka±5 ka BP) and confirm that this large event occurred 12 ka prior to the polarity transition. The distribution of tektite abundance was used to deconvolve the 10 Be/9 Be signal. The results confirm that the beryllium changes are concentrated during the transitional period, thus likely in presence of a multipolar geomagnetic field (or in the vicinity of a geomagnetic pole) that favored the penetration of cosmic rays and consequently increased the 10 Be production. The absence of 10 Be during the precursor indicates that the present site and the Indonesian ones were far away from a geomagnetic pole and that interlatitudinal atmospheric mixing was limited. The geomagnetic pole positions above the Indonesian sites during the precursor would thus be incompatible with the corresponding inclined dipolar field during this period, and suggest the dominance of low-degree harmonics.