Holocene climate as reflected by a malacological sequence at Verrières, France

Though numerous analyses have been made of Holocene pollen sequences, they come from similar environmental contexts, mainly peat deposits. Land snails can provide good palaeoecological and palaeoclimatical data in different drier environmental settings. The Verrières deposits, located in the Seine Valley, southeast of Paris, provide rich and abundant malacofaunas. We compare the well–defined local biostratigraphy with other mollusc stratigraphies from Burgundy, the closet site to the studied region. Multivariate analysis of the malacofaunas indicates that temperature and moisture did not always vary in parallel during the Holocene. On the other hand, Verrières malacofaunas reflect the main Holocene changes, as observed in the classical pollen series, confirming the reliability of the local biostratigraphy. The younger Dryas in Verrières was cold and cry. This was followed by the Preboreal phase, which is not well preserved at Verrières, but shows cool and humid conditions. The Boreal and Subboreal both show a cold and moist event bounded by two temperature phases. The Atlantic is also divided into two temperate phases by a cool and moist event. The Subatlantic shows temperature oscillations with cool peaks, but moisture shows a continuous trend to dryness.     

Climate-driven changes in lake conditions during late MIS 3 and MIS 2: a high-resolution geochemical record from Les Echets, France

A new set of geochemical data obtained from a long sediment sequence from the Les Echets basin provides a new record of palaeoenvironmental change for western Europe. The sequence covers the late part of Marine Isotope Stage (MIS) 3 and MIS 2 ( c . 46.1–15.0 cal. kyr BP) and extends over 30 m of sediment, allowing for high sampling resolution. Distinct cyclic fluctuations in aquatic productivity proxies suggest a tempo of environmental change at Les Echets that resembles the one established for Dansgaard-Oeschger cycles and Heinrich events. Increases in organic carbon are paralleled by high hydrogenous contents and enriched δ¹³C_org and C_org/N_tot ratios, implying a mixture of aquatic and land-derived constituents. The stable isotope values are directly linked to primary aquatic productivity and the input of terrestrial organic matter, enriched during warm, productive, periods, and drastically depleted during cold periods, particularly Heinrich events. Phases of relatively high productivity correlate with Dansgaard-Oeschger interstadials 8–2, while Heinrich events 3 and 2 are also clearly delineated by distinct lithological horizons with very low organic contents, high dry densities and enhanced organic matter oxidation. A large hiatus occurring about the time of Heinrich event 4 suggests that the cold and dry climate reconstructed for this event in the North Atlantic region also greatly affected Les Echets and its catchment.     

北京地区闪电活动与气溶胶浓度的关系研究

基于2015—2017年北京闪电定位网(BLNET)总闪资料与35个自动空气质量监测站PM2.5数据,分析了北京地区(39.5°N—41.0°N,115.0°E—117.5°E)夏季(6—8月)闪电活动与PM2.5的时空分布特征,同时针对117次雷暴天气,探讨了气溶胶浓度变化对闪电活动的可能影响.结果表明:PM2.5浓度及总闪密度均呈现自西北向东南升高的空间分布特征.闪电峰值在污染背景下出现的时间(19∶00LT)晚于清洁背景下(15∶00LT)约4h,且总闪百分比(～20%)可达清洁背景下(～9%)的两倍.对雷暴前1～4h的PM2.5浓度与时间窗(12∶00—22∶00LT)内总闪数目的中位数进行相关分析,发现PM2.5浓度低于130μg·m-3时,PM2.5与总闪数存在明显正相关,此时气溶胶可能通过影响云微物理过程进而影响雷暴的对流发展,增强闪电活动;PM2.5大于150μg·m-3时,总闪数随PM2.5浓度的增加呈减少趋势,可能的原因是高气溶胶浓度下地面太阳辐射显著下降,对流活动受到抑制,导致闪电活动减少.当PM2.5浓度在130～150μg·m-3时,两者关系不明显.