Varve sediments of the Lake Meerfelder Maar

Lake Meerfelder Maar (Germany) provides a varved record from the Last Glacial/Interglacial transition back to ca 1500 years BP. This study shows results for the Holocene sequence from new cores collected in 2009 based on varve counting, microfacies and micro-XRF analyses. The main goal of combining those analyses is to provide a new approach for interpreting long-term palaeolimnological proxy data and testing the climate-proxy stationarity throughout the current interglacial period. Varve counting provides a new independent Holocene chronology (MFM2012) with an estimated counting error of 1-0.5% and supported by 14C dating. Varve structure and thickness and geochemical composition of the varves give information about the main environmental processes that affect the lake and its catchment as well as the possible climate variability behind. Varves are couplets of i) a spring/summer laminae composed of monospecific diatom blooms and ii) an autumn/winter sub-layer made of minerogenic material and re-worked sediments. Thickness of the varves and sub-layers reflect lake variability and allow seasons to be distinguished as well as seasonal proxies. Changes in the winter minerogenic influx into the lake are reflected by Ti intensities and the Si/Ti ratio as a indicator for diatom concentration, which can be used as a proxy for water circulation during the early spring. Long-term variability of geochemical composition shows a reduction of the detrital material input (Ti) at 5,000 varve yrs BP and a visible sensitivity to water mixing (Si/Ti) during the Late Holocene. Variations of Ti intensities during the early and mid-Holocene do not show a clear relationship with climate. In contrast, higher values of the Si/Ti ratio together with thicker varves have been interpreted as wind-stress phases, which coincide with centennial variability of European cold/wet episodes during the Late Holocene. Our findings show that a long-term change in the lake and/or variability of the climate system can influence proxy sensitivity of a lacustrine record.

德国米尔费尔德马尔湖（Lake Meerfelder Maar）保存了一套可追溯至末次冰期/间冰期过渡时段、年代可达约1500年BP的纹泥（varved）沉积记录。本研究基于2009年采集的新岩芯，通过纹泥计数、微相（microfacies）分析以及微区X射线荧光光谱（micro-XRF）分析，获取了全新世序列的研究结果。整合上述分析手段的核心目标，是为解译长期古湖沼学（palaeolimnological）代用资料提供全新研究范式，并检验当前间冰期时段内气候-代用指标的平稳性。纹泥计数建立了一套独立的全新世年代序列（MFM2012），估算计数误差为1%~0.5%，并经碳十四（14C）测年验证。纹泥的结构、厚度及其地球化学组成，可反映影响湖泊及其流域的主要环境过程，以及背后潜在的气候变率。纹泥由两类韵律层构成：一是由单种硅藻水华组成的春/夏季层，二是由陆源矿物物质与再沉积沉积物构成的秋/冬季亚层。纹泥及其亚层的厚度可反映湖泊环境变化，不仅能够区分季节信号，还可提取季节性代用指标。冬季输入湖泊的陆源矿物通量变化，可通过钛（Ti）元素强度以及指示硅藻浓度的硅钛（Si/Ti）比值进行反映，该指标可作为早春时段水循环的代用指标。地球化学组成的长期变化显示，在5000纹泥年BP时，碎屑物质输入（以Ti元素为代表）出现降低，且在全新世晚期对水体混合作用（以Si/Ti比值为代表）表现出显著敏感性。全新世早中期的Ti元素强度变化与气候并无明确关联。与之相反，Si/Ti比值偏高且纹泥厚度更大的时段，被解释为风应力增强阶段，这与全新世晚期欧洲百年尺度冷湿事件的变率高度吻合。本研究结果表明，湖泊系统的长期变化及/或气候系统的变率，会对湖相记录的代用指标敏感性产生显著影响。