Multiple dating approach (14C, 230Th/U and 36Cl) of tsunami-transported reef-top boulders on Bonaire (Leeward Antilles) – Current achievements and challenges
Multiple dating approach (14C, 230Th/U and 36Cl) of tsunami-transported reef-top boulders on Bonaire (Leeward Antilles) – Current achievements and challenges
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对博内尔岛（背风安的列斯群岛）海啸搬运的礁顶巨石进行多重测年方法（14C、230Th/U 和 36Cl）â 当前的成就和挑战
DOI：
10.1016/j.margeo.2017.03.007
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发表时间：
2017
期刊：
影响因子：
2.9
通讯作者：
Rixhon
中科院分区：
文献类型：
--
作者：
Rixhon
确定潮上粗碎屑沉积物的搬运/沉积时间是困难的，这限制了它们在推断高能波事件的频率-量级模式方面的价值。在博内尔岛（加勒比海背风安的列斯群岛），这些沉积物形成了突出的地貌，据推测，至少对于最大的碎屑来说，它们是由一次或几次全新世海啸搬运的。尽管有大型的14c和电子自旋共振（ESR）年龄数据集可用于主要的珊瑚碎石脊和壁垒，但这些数据是否反映了主要事件的时间，以及这些数据集如何因珊瑚碎片的重新加工而产生偏差，仍然存在争议。为了克服目前对单一巨石错位定年的挑战，采用了三种不同的定年方法并进行了比较：(i)对附着在巨石上的钻孔双壳类进行14C定年；（ii）巨砾下沉积后、次生方解石流岩和陆生微生物岩的230Th/U定年；（iii）透过测量珊瑚中的36cl浓度，确定翻覆巨石的地表暴露年代。迄今为止，方法（ii）和（iii）从未应用于沿海巨石矿床。三个14c的年龄估计超过40 ka，即很可能超出了该方法的适用性，这归因于沉积后成岩作用，这使人们对该方法在当地环境中的实用性产生了怀疑。230 /U年龄明显趋同，均指向晚全新世（1.0-1.6 ka），是该运输事件的最小年龄。微生物岩样品的年龄为1.23±0.23 ka，两种流岩样品按地层顺序排列：较老（碳酸盐沉淀开始）和较年轻的流岩层的年龄分别为1.59±0.03 ka和1.23±0.03 ka。从翻倒的巨石顶部采集的四个珊瑚样本也得到了类似的36cl浓度测量值。然而，计算出的年龄受到很大的不确定性的影响，主要是由于天然氯浓度高。在对平台出现以来的遗传成分和化学剥蚀（引起额外的不确定性）进行校正后，计算出的36cl年龄在4块巨石中有3块的年龄在2.5±1.3和3.0±1.4 ka之间，而第4块的年龄为6.1±1.8 ka，可能与较高的遗传有关。230 /U和36cl的年龄估计与博内尔西北部沉积物岩心的异质壳层调查得出的< 3.3 ka的海啸年龄一致。虽然对沉积后方解石流岩的230th /U定年似乎是最可靠和/或最准确的方法，但这些结果表明，在碳酸盐岩-礁环境中，应用方法对海上巨石的错位定年的潜力和目前的局限性。
Dating the transport/deposition time of supratidal coarse-clast deposits is difficult, limiting their value for inferring frequency-magnitude patterns of high-energy wave events. On Bonaire (Leeward Antilles, Caribbean), these deposits form prominent landforms, and transport by one or several Holocene tsunamis is assumed at least for the largest clasts. Although a large dataset of14C and electron spin resonance (ESR) ages is available for major coral rubble ridges and ramparts, it is still debated whether these data reflect the timing of major events, and how these datasets are biased by the reworking of coral fragments. As an attempt to overcome the current challenges for dating the dislocation of singular boulders, three distinct dating methods are implemented and compared: (i)14C dating of boring bivalves attached to the boulders; (ii)230Th/U dating of post-depositional, secondary calcite flowstone and subaerial microbialites at the underside of the boulders; and (iii) surface exposure dating of overturned boulders via36Cl concentration measurements in corals. Approaches (ii) and (iii) have never been applied to coastal boulder deposits so far. The three14C age estimates are older than 40 ka, i.e. most probably beyond the applicability of the method, which is attributed to post-depositional diagenetic processes, shedding doubt on the usefulness of this method in the local context. The remarkably convergent230Th/U ages, all pointing to the Late Holocene period (1.0–1.6 ka), are minimum ages for the transport event(s). The microbialite sample yields an age of 1.23 ± 0.23 ka and both flowstone samples are in stratigraphic order: the older (onset of carbonate precipitation) and younger flowstone layers yield ages of 1.59 ± 0.03 and 1.23 ± 0.03 ka, respectively. Four coral samples collected from the topside of overturned boulders yielded similar36Cl concentration measurements. However, the computed ages are affected by large uncertainties, mostly due to the high natural chlorine concentration. After correction for the inherited component and chemical denudation since platform emergence (inducing additional uncertainty), the calculated36Cl ages cluster between 2.5 ± 1.3 and 3.0 ± 1.4 ka for three of four boulders whilst the fourth one yields an age of 6.1 ± 1.8 ka, probably related to a higher inheritance. These230Th/U and36Cl age estimates are coherent with a suggested tsunami age of < 3.3 ka obtained from the investigation of allochthonous shell horizons in sediment cores of northwestern Bonaire. Whilst230Th/U dating of post-depositional calcite flowstone appears to be the most robust and/or accurate approach, these results illustrate the potential and current limitations of the applied methods for dating the dislocation of supralittoral boulders in carbonate-reef settings.
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DOI：
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发表时间：
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期刊：
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