Luminescence dating of beach ridges for characterizing multi-decadal to centennial deltaic shoreline changes during Late Holocene, Mekong River delta
Luminescence dating of beach ridges for characterizing multi-decadal to centennial deltaic shoreline changes during Late Holocene, Mekong River delta
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DOI：
10.1016/j.margeo.2012.08.004
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发表时间：
2012-10
期刊：
影响因子：
2.9
通讯作者：
T. Tamura;Y. Saito;M. Bateman;V. L. Nguyen;T. Ta;D. Matsumoto
中科院分区：
文献类型：
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作者：
T. Tamura;Y. Saito;M. Bateman;V. L. Nguyen;T. Ta;D. Matsumoto
对过去十年到百年规模的三角洲海岸线变化的实证理解对于理解未来几十年和几个世纪海岸的命运至关重要。我们测试了石英光激发光 (OSL) 海滩山脊测年法在过去 3500 年来限制湄公河三角洲海岸线变化的有效性。对湄公河三角洲中部茶荣三角洲平原的 47 个 OSL 年龄进行了分析。 Tra Vinh 海滩的山脊向后弯曲并分叉，显示出海岸线变化的层次结构，其中包括不连续的变化 c．由于三角洲前坝/岛屿的出现和拉长，以及随后的沙嘴下流堆积而导致向海5公里。沙嘴堆积因海岸线方向的变化而中断。每一次不连续的移动都会形成一组海滩山脊，称为山脊。人们发现海滩山脊沉积物具有优异的发光特性，导致c的年龄不确定性较低。 5%。 OSL年代学与近几十年来海岸线的变化以及海滩山脊下的潮滩沉积物的放射性碳年龄非常吻合。年表清楚地说明了 3500 年（相对于公元 2010 年）以来的海岸退化。存在两种主要类型的海滩山脊：反曲山脊和树干山脊。无论是顺流还是向海，弯曲山脊的 OSL 年龄始终较年轻，记录了十年到百年规模的海岸线迁移，尤其是在过去 1500 年中。相比之下，干脊的系统年龄较少，因为干脊是在沉积物可能被波浪改造的地方形成的。上升主干山脊的年龄特征是每个山脊组的年龄都相对较年轻，这代表了上升海岸的广泛侵蚀和再沉积。海岸线方向的重大变化发生在 400-500 年左右，表明小冰河时代开始时东北冬季风加强。茶荣三角洲平原的进积速率可以用两种方式描述：不连续移动的频率和三角洲平原面积的增长率。两者都表明海岸在过去一千年中定期扩张。据推断，过去几十年来，由于河坝建设和河沙疏浚，海岸线的供沙量减少，可能会影响现代和未来海岸线的行为，这可以与本研究中重建的受人类影响较小的、百年到十年规模的过去海岸线变化进行比较。
Empirical understanding of decadal- to centennial-scale deltaic shoreline changes in the past is essential for understanding the fate of coasts in the coming decades and centuries. We tested the effectiveness of quartz optically-stimulated luminescence (OSL) dating of beach ridges to constrain shoreline changes of the Mekong River delta over the last 3500years. Forty-seven OSL ages have been analyzed from the Tra Vinh delta plain, central Mekong River delta. The Tra Vinh beach ridges are recurved and branching, showing the hierarchy of shoreline changes, which include discontinuous shifts c. 5km seawards caused by the emergence and elongation of a delta-front bar/island, and subsequent downdrift accretion of spits. The spit accretion is interrupted by changes of shoreline orientation. Each of the discontinuous shifts resulted in a cluster of beach ridges, which is referred to as ridgeset. The beach ridge sediments were found to have excellent luminescence properties resulting in low age uncertainties of c. 5%. The OSL chronology agrees well with shoreline changes over recent decades and with radiocarbon ages of tidal flat sediment underlying the beach ridges. The chronology clearly illustrates the coastal progradation from 3500years (relative to AD 2010) onwards. Two main types of beach ridge are present: recurved and trunk ridges. OSL ages of recurved ridges are consistently younger both downdrift and seawards, documenting decadal- to centennial-scale shoreline migration especially over the last 1500years. Trunk ridges in contrast have less systematic ages because a trunk ridge is formed where the sediment is likely to have been reworked by waves. Ages of updrift trunk ridges characteristically show relatively young ages in each ridgeset, representing the extensive erosion and resedimentation of the updrift coast. Major changes in shoreline orientation occurred around 400–500years, suggesting strengthening of the northeasterly winter monsoon at the beginning of the Little Ice Age. The rate of progradation of the Tra Vinh delta plain is described in two ways: frequency of the discontinuous shift, and growth rate of the delta plain area. Both suggest the coast has expanded regularly over the last 1000years. A decrease in sand supply to the coast in the last few decades due to river dam construction and fluvial sand dredging is inferred, possibly affecting the behavior of the modern and future shorelines, which can be compared with the less human-influenced, centennial- to decadal-scale past shoreline changes reconstructed in this study.