近百年来长江水下三角洲高分辨率洪水沉积记录及其控制机理

夏季洪涝灾害对长江流域社会经济造成了严重的影响，急需开展多时空尺度流域洪水发生规律和控制机理的研究，但因洪水器测数据年限短和缺少有效的古洪水沉积记录研究方法而难以实现。选取长江水下三角洲YEC1701柱状样顶部100 cm进行高分辨率XRF岩心连续扫描（XRFCS），粒度、有机碳、N元素、δ13C和210Pb定年分析，并与流域洪水器测或历史记录等进行对比研究，以期建立高效的古洪水沉积记录代用指标。结果表明，长江水下三角洲沉积物中Zr/Rb峰值通常对应粒度粗组分高值和较高的C/N值、偏负的δ13C值，所在层位的沉积年龄与流域洪水事件发生年份有很好的对应关系。由此判断，Zr/Rb值可作为判别长江古洪水沉积的重要代用指标。长江流域于1930—2017年间共发生22次洪水事件，利用XRFCS获得的10 mm和2 mm间隔Zr/Rb值可分别识别出其中的11次和18次，识别率为50%和80%。由此推荐XRFCS样品测试间隔要小于年沉积速率一半以内，以提高古洪水事件的识别率。多源洪水和降雨数据分析表明，长江洪水年际至千年尺度发生规律主要受厄尔尼诺-南方涛动和东亚夏季风、南亚夏季风活动的共同影响。但早期长江洪水资料分辨率较低，长江水下三角洲有较连续的洪水沉积记录，运用XRFCS毫米级分辨率的Zr/Rb值可重建全新世高分辨率的长江古洪水发生历史，更精细地分析气候变化对洪水发生规律的控制机理，为预测全球变化背景下长江洪水演变趋势提供依据。

High resolution flood records in the Yangtze subaqueous delta during the past century and control mechanism

Summer flood hazards have a strong influence on the social economy of the Yangtze River Basin, so it is in dire need of investigating multiple spatiotemporal variations in floods and control mechanisms, but this is handled by short time extent of instrumental flood data and lack of effective research methods for palaeoflood deposition records.A sediment core (YEC1701)collected from the Yangtze subaqueous delta was studied in detail with its top 100 cm through using high-resolution XRF core scanner (XRFCS), and measurements of grain size, organic carbon and nitrogen, stable carbon isotope (δ13C), and210Pb compositions.The above study results were compared with the observing instruments or documental flood data to establish an effective proxy for paleo-flood depositions by comparison with instrumental or documental flood data.The results show that the peak value of Zr/Rb in the subaqueous delta sediments of the Yangtze River usually corresponds to the high value of coarse grain composition, the high value of C/N, and the negative value of δ13C.The sedimentary age of the strata in the delta corresponds well to the year of flood events in the basin.Therefore, the Zr/Rb ratio can be used as an important proxy index for the identification of palaeo-flood sediments in the Yangtze River.Totally, 22 flood events occurred in the period 1930-2017 in the Yangtze River basin, 11 and 18 of which were identified by the XRFCS Zr/Rb data in terms of 10 mm and 2 mm measurement intervals with effective recognition rates of 50% and 80% respectively.It is therefore recommended to perform XRFCS measurement with a smaller interval than half of sedimentation rates for better recognition rates of flood events.Multiple source data of river floods and precipitations were analyzed to show that river floods in Yangtze River basin are majorly influenced by ENSO (El Niño-Southern Oscillation), EASM (East Asian summer monsoon) and SASM (South Asian summer monsoon) over different time scales from multiple years to millennium.However, the time resolution for earlier flood records is very low, and it can be greatly improved by employing XRFCS mm-scaled Zr/Rb ratio of continuous flood depositions in the Yangtze subaqueous delta.This will also improve our understanding of controlling mechanisms of flood events and then better prediction of flood variation in response to global climate change.