| 摘要: |
| 长江三角洲地层的年代学研究对探讨晚第四纪以来我国东部沿海地区陆海交互作用历史以及海岸地貌的形成与演化具有重要价值。本研究选择长江三角洲YZ07和EGQD14两支钻孔开展系统光释光（optically stimulated luminescence，OSL）测年研究，总结前期已发表的50个年代数据并收集整理该区域相关钻孔的年代地层资料，探讨在可靠的年代框架约束下晚第四纪长江三角洲沉积地层演变特征，取得了以下几点认识：（1）在长江三角洲地区，常规OSL测年以较细石英颗粒OSL测年效果较为理想；对粗颗粒石英，若存在长石包裹体对石英OSL信号的影响，建议用脉冲释光测年（POSL）技术加以解决。（2）以YZ07孔为例，14C测年和OSL测年结果对比表明在三角洲海岸地区，14C测年方法建立的年代地层框架需要谨慎对待，建议在此区域开展年代地层学研究时可以尝试将两种测年方法结合使用，以确保年代地层框架的准确和可靠。（3）针对争议较大的氧同位素3阶段（MIS3）沉积环境问题，EGQD14孔给出了新的年代学证据，认为MIS3阶段（约30 — 50 ka）长江三角洲可能是陆地河流相沉积环境为主导，这也得到了区域相邻的几支钻孔沉积记录的佐证。（4）两支钻孔从年代学角度证实了前人关于末次盛冰期（Last Glacial Maximum，LGM）以来长江三角洲发育下切河谷的沉积学判识和地层学论断。此外，多钻孔的年代地层学特征也验证了全新世时段海平面变化控制下的长江三角洲及海岸演变的时空动态。 |
| 关键词: 晚第四纪 长江三角洲 陆-海交互沉积地层 石英OSL测年 |
| DOI：10.7515/JEE182046 |
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| 基金项目:国家自然科学基金项目（41501003）；江苏省自然科学青年基金项目（BK20181106）；中国科学院南京地理与湖泊研究所自主研究启动项目（NIGLAS2017QD11）； 中国科学院青年创新促进会人才项目（2015251） |
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| Optical dating of the land-sea interactive deposits of the Yangtze River delta since the Late Quaternary |
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GAO Lei, LONG Hao
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State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (NIGLAS), Nanjing 210008, China
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| Abstract: |
| Background, aim, and scope Chronology is vital for studying the Late Quaternary coastal geomorphology and the land-sea interaction. In this study, we aimed to analyze the chronological characteristics of sediments in the Yangtze River delta based on the dataset of collections from our previous two cores’ sedimentary records and from other cores’ records recently reported in the adjacent areas. We attempt to illustrate the spatiotemporal features of the Yangtze River delta sedimentary evolution since MIS3 in the scope of chronostratigraphic investigations. Materials and methods In current study, we synthesized several chronological sequences according to the dataset of 50 ages from our previous published two cores (YZ07 and EGQD14) in the Yangtze River delta together with a few ages from other cores in the adjacent areas. Each age determined by quartz OSL dating techniques in the dataset is re-examined through the critical items, including characteristics of OSL signals, the IR-OSL depletion ratios, as well as the dose rates. Results Combing our recent studies with the relevant archives from the adjacent cores, we re-analyzed its sedimentary environment changes and coastal dynamics since MIS3. It tended to show that a terrestrial-dominated environment happened at MIS3, a depositional hiatus occurred in LGM and variable patterns existed in the Holocene. Discussion Finer quartz grains, e.g., 4—11 μm or 63—100 μm, would be much better than that of 100—200 μm in routine OSL dating, as the later might suffer from feldspar inclusions and resulted in OSL ages underestimation. Fortunately, the problem could be resolved by using the pulsed OSL dating technique. Comparison of the chronologies between OSL and radiocarbon dating, it shows that 14C ages are severe inversion against with the stratigraphic sequence while OSL ages display a well correlation with the strata. Therefore, we suggest combining the two dating techniques for coastal-deltaic deposits. Conclusions New chronological evidence of core EGQD14 reveals that a fluvial dominated sedimentary environment happened during MIS3 (30—50 ka). Similar conclusions were set up on the records from the adjacent cores. A deposition hiatus occurred due to the decline of sea level and the formation of the incised valley during the LGM. In the Holocene, the incised-valley underwent stacking with the accommodation variation as sea level changes. In these processes, the Yangtze River delta initiated and gradually developed seaward. The accumulation rate is characterized with three periods, namely to be fast deposition in the early Holocene and in the late Holocene (~2 ka to 0 ka) but relative slow sedimentation during the mid-to-late Holocene. Recommendations and perspectives However, the chronological framework of the strata in the Yangtze River delta beyond MIS3 is still poor so far. In the future, we should pay more attention to the sedimentary stratigraphy of the last glaciation using more newfangled luminescence dating techniques (e.g., K-feldspar and/or polymineral post-IR IRSL dating). |
| Key words: Late Quaternary Yangtze River delta land-sea interaction quartz OSL dating |
