Occurence and thickness of flood layers in varved sediments of Lake Ammersee

Microfacies analyses and X-ray fluorescence scanning (µ-XRF) at sub-mm resolution were conducted on the varved Mid- to Late Holocene interval of two sediment profiles from pre-alpine Lake Ammersee (southern Germany). The coring sites are located in a proximal (AS10prox) and distal (AS10dist) position towards the main tributary River Ammer, in 1.8 km distance from each other. To shed light on sediment distribution within the lake, particular emphasis was (1) the detection of intercalated detrital layers and their micro-sedimentological features, and (2) intra-basin correlation of these event deposits. Detrital layers were dated by microscopic varve counting, verified by accelerator mass spectrometry 14C dating of terrestrial plant macrofossils. Since ~5500 varve years (vyr) BP, in total 1573 detrital layers were detected in either one or both of the investigated sediment profiles. Based on their microfacies, geochemistry, and proximal-distal deposition pattern, detrital layers were interpreted as River Ammer flood deposits. Earlier studies on flood layer seasonality have proven that flood layer deposition occurs predominantly during spring and summer, the flood season at Lake Ammersee. Most prominent features of the record are the onset of regular flood layer deposition at ~5500 vyr BP in AS10prox and ~ 2800 vyr BP in AS10dist as well as three major increases in mean flood layer thickness at ~5500, 2800, and 400 vyr BP. Integrating information from both sediment profiles allowed to interpret these changes in terms of shifts towards higher mean flood intensity. Proposed triggering mechanisms are gradual reduction in Northern Hemisphere orbital summer forcing and superimposed centennial-scale solar activity minima. Likely responses to this forcing are enhanced equator-to-pole temperature gradients and changes in synoptic-scale atmospheric circulation. The consequences for the Ammersee region are more intense cyclones leading to extremer rainfall and flood events in spring and summer.本研究对德国南部阿尔卑斯山前阿姆湖（Lake Ammersee）的两处沉积岩芯剖面的纹层状中全新世至晚全新世地层，开展了亚毫米级分辨率的微相分析与X射线荧光扫描（µ-XRF）。两处岩芯采样点分别位于朝向主要支流阿姆河（River Ammer）的近端（AS10prox）与远端（AS10dist）区域，两者间距1.8千米。 为阐明湖泊内部的沉积物分布特征，本研究重点开展两项工作：一是识别夹层状碎屑岩层及其微沉积学特征，二是对该类事件沉积开展盆内对比。碎屑岩层的年代通过显微纹层计数确定，并通过陆生植物大化石的加速器质谱（accelerator mass spectrometry, AMS）14C测年进行验证。 自约5500个纹层年（varve years, vyr）BP以来，在两处研究沉积岩芯剖面中，共识别出1573套碎屑岩层（单剖面或双剖面均有分布）。根据其微相、地球化学特征以及近端-远端沉积分布模式，这些碎屑岩层被判定为阿姆河洪水沉积。此前针对洪水层季节分布的研究表明，阿姆湖的洪水沉积主要形成于春季与夏季——即该湖的洪水频发期。 该记录最显著的特征包括：AS10prox剖面于约5500 vyr BP、AS10dist剖面于约2800 vyr BP分别开始出现规律性洪水层沉积；同时在约5500、2800及400 vyr BP三个时间节点，洪水层平均厚度出现三次显著增长。结合两处沉积岩芯剖面的信息，可以将上述变化解释为洪水平均强度持续升高的过程。 研究提出的触发机制包括：北半球夏季轨道辐射强迫的逐渐减弱，以及叠加其上的百年尺度太阳活动极小期。该强迫机制可能引发的响应包括：赤道-极地温度梯度加剧，以及天气尺度大气环流发生改变。这对阿姆湖区域造成的影响是：更强的气旋活动导致春季与夏季出现更为极端的降雨与洪水事件。