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Refining the Quaternary Geomagnetic Instability Time Scale (GITS):Lava flow recordings of the Blake and Post-Blake excursions

机译：完善第四纪地磁不稳定时标（GITS）：布雷克和布雷克后期游览的熔岩流记录

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摘要

The Blake excursion was among the first recognized with directional and intensity behavior known mainly from marine sediment and Chinese loess. Age estimates for the directional shifts in sediments are poorly constrained to about 118-100 ka, i.e., at the marine isotope stage (MIS) 5e/5d boundary. Moreover, sediments at Lac du Bouchet maar, France and along the Portuguese margin reveal what may be a "post-Blake" excursion at about 105-95 ka. The excursional directions are associated with a prominent paleointensity minimum between about 125 and 95 ka in global stacked records. Lava flow recordings of the Blake excursion(s) have, however, been questionable because precise ages required for correlation with these sediment records are lacking. To establish new, independent records of the Blake excursion, and link these into a larger Quaternary GITS, we have undertaken ~(40)Ar/~(39)Ar incremental heating and unspiked K-Ar experiments on groundmass from the transitionally magnetized Inzolfato flow on Lipari Island. We also obtained ~(40)Ar/~(39)Ar incremental heating results for a lava flow on Amsterdam Island originally thought to record the Mono Lake excursion and from the transitionally magnetized El Calderon basalt flow, New Mexico that was K-Ar dated by Champion et al. (1988) at 128±66 ka. Unspiked K-Ar ages of four samples from the Inzolfato flow are 102.5±4.7, 101.3±3.3, 97.1±2.6, and 96.8±3.1 ka and thus indistinguishable from one another. ~(40)Ar/~(39)Ar results are more complex, with three samples yielding discordant age spectra. Based on incremental heating data obtained in both the UW-Madison and Gif-sur-Yvette ~(40)Ar/~(39)Ar laboratories, a fourth sample yields six concordant age plateaus and a weighted mean age of 105.2±1.4 ka that we take as the best estimate of time since the flow erupted. Five ~(40)Ar/~(39)Ar incremental heating experiments on the Amsterdam Island lava yield a plateau age of 120±12 ka, whereas ages from two sites in the Calderon flow are 112±23 and 101±14 ka, together giving a weighted mean of 104±12 ka. The age of 120±12 ka from Amsterdam Island, though imprecise, correlates with the Blake excursion. In contrast, the 104e105 ka age obtained from both Lipari and New Mexico indicates that these lavas record a younger period of dynamo instability, most probably associated with the post-Blake excursion. These radioisotopic ages are consistent with the astronomical ages of two paleointensity minima in the PISO-1500 global stack. Our findings indicate that the Blake and post-Blake excursions are both global features of past geodynamo behavior and support the hypothesis that Brunhes chron excursions are temporally clustered into two groups of at least a half-dozen each spanning over 220 to 30 ka and 720 to 520 ka.
机译：布雷克偏移是最早发现的定向和强度行为之一，主要从海洋沉积物和中国黄土中得知。沉积物方向偏移的年龄估算几乎没有约束到约118-100 ka，即在海洋同位素阶段（MIS）5e / 5d边界。此外，法国Lac du Bouchet maar和葡萄牙边缘的沉积物揭示了大约105-95 ka的“布雷克后”漂移。在全球堆叠记录中，偏移方向与介于约125和95 ka之间的突出的古强度最小值相关。然而，布雷克漂移的熔岩流记录一直是有问题的，因为缺乏与这些沉积物记录相关的精确年龄。为了建立布雷克偏移的新的独立记录，并将其链接到更大的第四纪GITS中，我们对过渡磁化的Inzolfato流中的地面质量进行了〜（40）Ar /〜（39）Ar增量加热和非掺入K-Ar实验在利帕里岛上。我们还获得了阿姆斯特丹岛上熔岩流的〜（40）Ar /〜（39）Ar增量加热结果，最初认为该记录是记录莫诺湖的偏移以及新墨西哥州的过渡磁化El Calderon玄武岩流（K-Ar日期）由Champion等人撰写。 （1988）在128±66 ka。来自Inzolfato流的四个样品的未加标K-Ar年龄为102.5±4.7、101.3±3.3、97.1±2.6和96.8±3.1 ka，因此彼此之间没有区别。 〜（40）Ar /〜（39）Ar的结果更为复杂，其中三个样本产生了不一致的年龄谱。根据UW-Madison和Gif-sur-Yvette〜（40）Ar /〜（39）Ar实验室获得的增量加热数据，第四个样本产生了六个一致的年龄平稳期，加权平均年龄为105.2±1.4 ka自流爆发以来，我们将其作为对时间的最佳估计。在阿姆斯特丹岛熔岩上进行的五个〜（40）Ar /〜（39）Ar增量加热实验得出的高原年龄为120±12 ka，而Calderon流中两个位置的年龄合计为112±23和101±14 ka。加权平均值为104±12 ka。来自阿姆斯特丹岛的120±12 ka年龄虽然不精确，但与布莱克游览有关。相反，从利帕里（Lipari）和新墨西哥州获得的104e105 ka年龄表明，这些熔岩记录了更年轻的发电机不稳定时期，这很可能与布雷克之后的漂移有关。这些放射性同位素年龄与PISO-1500全球堆栈中两个古强度最小值的天文年龄一致。我们的发现表明，布雷克和布雷克之后的偏移都是过去的地震动行为的全球特征，并支持这样的假说，即布鲁日时间偏移在时间上分为两组，每组至少六个，跨越220至30 ka和720至520卡。

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