Lacustrine sediments provide geochemical evidence of environmental change during the last millennium in southeastern Brazil

A 172 cm-long sediment core was collected from a small pristine lake situated within a centripetal drainage basin in a tropical karst environment (Ribeira River valley, southeastern Brazil) in order to investigate the paleoenvironmental record provided by the lacustrine geochemistry. Sediments derived from erosion of the surrounding cambisoils contain quartz, kaolinite, mica, chlorite and goethite. Accelerator mass spectroscopy (AMS) ¹⁴C dating provided the geochronological framework. Three major sedimentary units were identified based on the structure and color of the sediments: Unit III from 170 to140 cm (1030±60–730±60 yr BP), Unit II from 140 to 90 cm (730±60–360±60 yr BP) and Unit I from 90 to 0 cm (360±60–0 yr BP). Results of major and trace element concentrations were analysed through multivariate statistical techniques. Factor analysis provided three factors accounting for 72.4% of the total variance. F1 and F2 have high positive loadings from K, Ba, Cs, Rb, Sr, Sc, Th, light rare earth element (LREE), Fe, Cr, Ti, Zr, Hf and Ta, and high negative loadings from Mg, Co, Cu, Zn, Br and loss on ignition (LOI). F3, with positive loadings from V and non-metals As and Sb, accounts for a low percentage (9.7%) of the total variance, being therefore of little interpretative use. The profile distribution of F1 scores reveals negative values in Units I and III, and positive values in Unit II, meaning that K, Ba, Cs, Rb, Sr, Sc, Th, LREE, Fe, Cr, Ti, Zr, Hf and Ta are relatively more concentrated in Unit II, and Mg, Co, Cu, Zn and Br are relatively more abundant in Units I and III. The observed fluctuations in the geochemical composition of the sediments are consistent with slight variations of the erosion intensity in the catchment area as a possible response to variations of climatic conditions during the last millennium.     

Late Quaternary vegetation,climate and fire dynamics inferred from the El Tiro record in the southeastern Ecuadorian Andes

In order to study the stability and dynamics of mountain rainforest and paramo ecosystems, including the biodiversity of these ecosystems, the Holocene and late Pleistocene climate and fire variability, and human impact in the southeastern Ecuadorian Andes, we present a high‐resolution pollen record from El Tiro Pass (2810 m elevation), Podocarpus National Park. Palaeoenvironmental changes, investigated by pollen, spores and charcoal analysis, inferred from a 127 cm long core spanning the last ca. 21 000 cal. yr BP, indicate that grass‐paramo was the main vegetation type at the El Tiro Pass during the late Pleistocene period. The grass‐paramo was rich in Poaceae, Plantago rigida and Plantago australis, reflecting cold and moist climatic conditions. During the early Holocene, from 11 200 to 8900 cal. yr BP, subparamo and upper mountain rainforest vegetation expanded slightly, indicating a slow warming of climatic conditions during this period. From 8900 to 3300 cal. yr BP an upper mountain rainforest developed at the study site, indicated by an increase in Hedyosmun, Podocarpaceae, Myrsine and Ilex. This suggests a warmer climate than the present day at this elevation. The modern subparamo vegetation became established since 3300 cal. yr BP at El Tiro Pass. Fires, probably anthropogenic origin, were very rare during the late Pleistocene but became frequent after 8000 cal. yr BP. Copyright © 2007 John Wiley & Sons, Ltd.     

浙江瓶窑BHQ孔全新统孢粉组合特征与气候变化

利用浙江省瓶窑北湖BHQ孔长约14m的岩心,以4个AMS¹⁴C数据为年代依据,对49块样品进行孢粉分析。分析结果发现,样品中孢粉浓度差异较大,植物种类繁多,总计103个科/属。通过对孢粉组合特征的分析,结合岩性特征,划分出3个孢粉带和9个亚带,重建了研究区早—中全新世植被演替和气候冷暖波动的环境序列。19～8.9m沉积时段,对应于早全新世气候回暖期,研究区植被主要是以落叶栎、松、阿丁枫和枫杨为主,混有少量常绿栎和榆等的针叶—落叶阔叶混交林,常绿乔木花粉含量较低,说明研究区经历晚更新世晚期新仙女木事件后进入全新世,气候迅速转暖;8.9～6.7m沉积时段,对应于全新世最适宜期,为研究区水热条件配置最佳阶段,主要发育由落叶栎、常绿栎、松、阿丁枫、枫杨和榆等组成的常绿—落叶阔叶混交林,常绿乔木的种类和含量要远远高于前一阶段,指示气候变化的最适宜阶段;6.7～4.775m沉积时段,对应于中全新世气候波动期,孢粉组合发生明显波动,研究区植被主要以落叶栎、枫香、松、枫杨等落叶阔叶乔木为主,常绿栎花粉的含量急剧萎缩,表明研究区植被类型演替为落叶阔叶林,反映经历了较为剧烈的气候恶化事件,而本段高含量的稻属花粉(≥35μm)可能指示该时段研究区存在一定规模的古人类活动。     

浙江田螺山遗址区孢粉学记录的中全新世植被、环境和人类活动初步研究

浙江宁绍平原是研究河姆渡文化与自然环境发展关系的热点区域。选取田螺山遗址附近的TLS1402钻孔上部16.16 m的沉积物进行AMS ¹⁴C测定、孢粉和微炭屑分析,旨在研究宁绍平原中全新世植被历史、环境变化和人类活动的变化规律。结果表明：1）约8460~7700 a B.P.,植被类型以落叶阔叶林为主,含有常绿成分和针叶成分,揭示了气候温凉偏干的特点;7700~6300 a B.P.期间,乔灌木整体有所下降,波动显著,植被类型为亚热带常绿落叶阔叶混交林,气候暖湿,在7600~7500 a B.P.、7300~7200 a B.P.和6700~6300 a B.P.,粒径＞38 μm的禾本科明显增加,可能显示了人类的3个农业发展阶段。另外,微炭屑增加对应栽培作物花粉的增加,暗示植被变化可能与人类活动有关;6300 a B.P.至今,木本花粉含量总体大幅度下降,草本花粉含量升高,气候稍温凉偏干,植被类型仍以亚热带常绿落叶阔叶混交林为主,但植被变化可能受人类干扰。2）淡水藻类和沟鞭藻类含量表明,TLS1402钻孔所在地8460~7700 a B.P.可能为浅海湾环境,但也受到淡水注入影响。7700~7200 a B.P.为过渡时期;7200 a B.P.后环境发生较大变化,海平面上升速率大幅度降低,陆地逐渐生长,7000~6900 a B.P.河姆渡文化逐步开始发育,水稻农业发展,但之后受到海水影响,在7200~6700 a B.P.、6600~6500 a B.P.都出现农业发展低谷,分析认为和海水入侵有关。

     

Millennial-scale shifts in microtidal ecosystems during the Holocene: dynamics and drivers of change from the Po Plain coastal record (NE Italy)

Framed into a robust stratigraphic context, multivariate analyses on the Holocene palaeobiological record (pollen, benthic foraminifers, ostracods) of the Po coastal plain (NE Italy) allowed the investigation of microtidal ecosystems variability and driving parameters along a 35-km-long land–sea transect. Millennial-scale ecosystem shifts are documented by coeval changes in the meiofauna, reflecting variations in organic matter–water depth (shallow-marine environments) and degree of confinement-salinity (back-barrier settings). In-phase shifts of vegetation communities track unsteady water-table levels and river dynamics in freshwater palustrine areas. Five environmental–ecological stages followed one another crossing four tipping points that mark changes in relative sea level (RSL), climate and/or fluvial regime. At the culmination of Mediterranean RSL rise, after the 8200 event, remarkable growth of peatlands took place in the Po estuary, while low accumulation rates typified the shelf. At the transgressive–regressive turnaround (~7000 cal a bp ), the estuary turned into a delta plain with tidally influenced interdistributary embayments. River flow regime oscillations after the Climate Optimum (post-5000 cal a bp ) favoured isolation of the bays and the development of brackish wetlands surrounded by wooded peatlands. The youngest threshold (~800 cal a bp ), which led to the establishment of the modern delta, reflects a major avulsion of the Po River.     

Post-glacial landscape response to climate variability in the southeastern San Juan Mountains of Colorado, USA

Geomorphic mapping in the upper Conejos River Valley of the San Juan Mountains has shown that three distinct periods of aggradation have occurred since the end of the last glacial maximum (LGM). The first occurred during the Pleistocene–Holocene transition (~ 12.5–9.5 ka) and is interpreted as paraglacial landscape response to deglaciation after the LGM. Evidence of the second period of aggradation is limited but indicates a small pulse of sedimentation at ~ 5.5 ka. A third, more broadly identifiable period of sedimentation occurred in the late Holocene (~ 2.2–1 ka). The latest two periods of aggradation are concurrent with increases in the frequency of climate change in the region suggesting that Holocene alpine and sub-alpine landscapes respond more to rapid changes in climate than to large singular climatic swings. Soil development and radiocarbon dating indicate that hillslopes were stable during the Holocene even while aggradation was occurring in valley bottoms. Thus, we can conclude that erosion does not occur equally throughout the landscape but is focused upslope of headwater streams, along tributary channels, or on ridge tops. This is in contrast to some models which assume equal erosion in headwater basins.     

Holocene dryness and human occupation in Brazil during the “Archaic Gap”

An overview of the archaeological data produced over the last decades for Brazil, coupled with a background of recent paleoenvironmental studies, suggests that during the mid-Holocene vast areas of Central Brazil ceased to be occupied by human groups. Independent data from dated human skeletons, rock-shelter stratigraphy, and chronology of open-air sites converge to support the inference that these areas were depopulated or altogether abandoned. Paleoenvironmental data suggest that dryness events constitute the major cause behind the observed trends. This phenomenon expands the already perceived notion that climatic stresses had a major role in the shaping of human settlement patterns in marginal environments, such as deserts and high-altitude settings.     

Pollen‐ and diatom based environmental history since the Last Glacial Maximum from the Andean core Fúquene‐7, Colombia

The late Pleistocene–Holocene ecological and limnological history of Lake Fúquene (2580 m a.s.l.), in the Colombian Andes, is reconstructed on the basis of diatom, pollen and sediment analyses of the upper 7 m of the core Fúquene‐7. Time control is provided by 11 accelerator mass spectrometry (AMS) ¹⁴C dates ranging from 19 670 ± 240 to 6040 ± 60 yr BP. In this paper we present the evolution of the lake and its surroundings. Glacial times were cold and dry, lake‐levels were low and the area was surrounded by paramo and subparamo vegetation. Late‐glacial conditions were warm and humid. The El Abra Stadial, a Younger Dryas equivalent, is reflected by a gap in the sedimentary record, a consequence of the cessation of deposition owing to a drop in lake‐level. The early Holocene was warm and humid; at this time the lake reached its maximum extension and was surrounded by Andean forest. The onset of the drier climate prevailing today took place in the middle Holocene, a process that is reflected earlier in the diatom and sediment records than in the pollen records. In the late Holocene human activity reduced the forest and transformed the landscape. Climate patterns from the Late‐glacial and throughout the Holocene, as represented in our record, are similar to other records from Colombia and northern South America (the Caribbean, Venezuela and Panama) and suggest that the changes in lake‐level were the result of precipitation variations driven by latitudinal shifts of the Intertropical Convergence Zone. Copyright © 2003 John Wiley & Sons, Ltd.     

华北平原小冰期以来干湿变化与人类活动特征

气候变化尤其干湿变化对人类活动具有重要影响,高分辨率地重建过去的干湿变化是研究区域干湿变化的基础,同时也为未来气候变化提供依据。目前针对华北地区小冰期高分辨率的气候变化研究还较少,特别是干湿变化研究更少。本研究在3个AMS-¹⁴C测年的基础上,通过99个样品的孢粉、炭屑、粒度等指标高分辨率地重建了该地区小冰期以来的气候干湿变化特征及与人类活动强度的关系。结果表明：小冰期之前（约1340 A.D.）乔木花粉含量多在25%以上,指示流域植被以森林草地景观为主,沉积物颗粒较粗,沉积环境不稳定,气候较为湿润。小冰期期间（1340~1920 A.D.）乔木花粉多降低至15%以下,喜干的藜科、荨麻属和葎草属花粉含量明显增加,指示气候整体变干,但不同阶段变干程度又存在明显差异：小冰期早期（1340~1580 A.D.）孢粉组合中喜干的藜科花粉含量逐渐增加,湿生植物花粉含量减少,PCA axis 1得分值多在0~1之间,显示气候偏干。该时期与人类活动有关的炭屑浓度、农作物（禾本科≥35 μm）花粉和磷（P）含量均最低,说明小冰期早期研究区人类活动强度相对较弱;小冰期中期（1580~1800 A.D.）喜干的藜科、荨麻属和葎草属花粉含量达到最高值,PCA axis 1得分值较小冰期早期更正偏,指示气候更为干旱。炭屑浓度、伴人植物、磷（P）含量和农作物（禾本科≥35 μm）花粉均达最高值,指示人类活动强度明显增强;小冰期晚期（1800~1920 A.D.）湿生的禾本科和水生植物狐尾藻属等花粉含量达到最高值,PCA axis 1得分值负偏,指示气候变湿润。该阶段农作物（禾本科≥35 μm）花粉、炭屑浓度和磷（P）含量较上一时期均略有降低,表明人类活动强度较小冰期中期略有降低。小冰期以后（1920年至今）喜湿的禾本科和水生植物狐尾藻属花粉含量减少,与小冰期晚期相比,气候向着干旱化方向发展

     

中纬度亚洲现代间冰期气候变化的“西风模式”讨论

中纬度亚洲地区存在主要受季风环流影响的东南部湿润地区(简称季风区)和主要受西风环流控制的内陆干旱区(包括青藏高原北部高寒干旱区,简称西风区)。根据对近年来新发表的气候变化记录证据梳理总结,发现西风区在中—晚全新世气候湿润,与亚洲季风在早—中全新世强盛的格局显著不同。过去千年的西风区中世纪暖期干旱,而小冰期相对湿润,与此相对,万象洞石笋氧同位素记录则显示季风降水在中世纪暖期时整体处于高值,在小冰期处于低值段。在近百年,尤其是近50a,西北干旱区湿度增加,而季风影响范围内的西北东部和华北等地变得更干。不仅如此,在分属西风和季风影响区的青藏高原北部和南部,年代际—百年尺度上降水变化也表现出反相位关系。据此我们提出,亚洲中部西风带控制区在现代间冰期从数千年到年代际的各个时间尺度上均存在不同于季风区的湿度(降水)变化模式,称之为现代间冰期气候变化的西风模式。     

A record of Holocene climate change from lake geochemical analyses in southeastern Arabia

Lacustrine sediments from southeastern Arabia reveal variations in lake level corresponding to changes in the strength and duration of Indian Ocean Monsoon (IOM) summer rainfall and winter cyclonic rainfall. The late glacial/Holocene transition of the region was characterised by the development of mega-linear dunes. These dunes became stabilised and vegetated during the early Holocene and interdunal lakes formed in response to the incursion of the IOM at approximately 8500 cal yr BP with the development of C3 dominated savanna grasslands. The IOM weakened ca. 6000 cal yr BP with the onset of regional aridity, aeolian sedimentation and dune reactivation and accretion. Despite this reduction in precipitation, the lake was maintained by winter dominated rainfall. There was a shift to drier adapted C4 grasslands across the dune field. Lake sediment geochemical analyses record precipitation minima at 8200, 5000 and 4200 cal yr BP that coincide with Bond events in the North Atlantic. A number of these events correspond with changes in cultural periods, suggesting that climate was a key mechanism affecting human occupation and exploitation of this region.     

Vegetation changes and human impact inferred from an oxbow lake in southwestern Amazonia,Brazil since the 19th century

Pollen and X-ray fluorescence spectrometry (XRF) analyses from a 272 cm-long sediment core of Lago Amapá, an oxbow lake in western Amazonia, reveal the first palaeoecological investigation of late Holocene sediments in Acre state, Brazil. Radiocarbon dating of older sediments failed due to re-deposition of organic material but a historical map suggests that lacustrine deposition started at 1900 AD. We detected two periods of changes in sediment and vegetation, dominated by pioneer taxa especially Cecropia. The first period around 1900 AD is documenting an initial oxbow lake, with regular fluvial input (high Ti) and low accumulation of organic matter (low inc/coh ratio). During that period Andean pollen taxa originating from Peruvian Andean headwaters were deposited. A fully lacustrine phase started about 1950 AD and is characterized by prolonged periods of stagnant water (low Fe/Mn ratio). The increase of pioneer taxa, sedimentation rates and a reduction of most of the XRF element counts point to a period during which Lago Amapá was a more isolated lake which was flooded only during exceptional severe flood events and is catching mainly anthropogenic disturbances. The extensive human influence during this period was assumed by 1) the high occurrence of pioneer taxa and the absence of charcoal which could indicate changes in vegetation possibly as a result of logging, 2) the Ca and Ti/K ratio which reflect changes to a local sediment source, and 3) comparison of Landsat images from the last 30 years which shows broad changes in vegetation cover and land transformation in the peripheral areas of the oxbow lake.     

1961-2015年我国鲁东南地区降雪的气候特征分析

利用鲁东南地区18个代表站1961-2015年的逐日降水量、逐日天气现象、积雪深度资料,对近55 a来降雪的气候特征进行了统计分析。结果表明：鲁东南地区年均降雪日数、强降雪日数、降雪量、强降雪量及年均雪深、年最大积雪深度的空间分布总体上山区多于平原和沿海,区域差异明显。21世纪00年代以前为多雪时期,以后为少雪时期。近55 a的年均降雪日数、强降雪日数、降雪量、强降雪量及年均雪深、年最大积雪深度皆呈减少趋势,降雪由多转少的转折年份均在1993年,年均雪深、年最大积雪深度的减少分别出现在1987年、1986年。鲁东南地区降雪主要集中在1-2月份,3月份强降雪量最大,平均雪深、最大积雪深度的最大月份分别出现在11月份、3月份。降雪时段为10月23日-次年4月28日,降雪的初终日西北部山区皆为最早。降雪日数、强降雪日数、降雪量、强降雪量、雪深均存在3 a的周期,最大积雪深度存在4～5 a的周期。     

A multi‐proxy study of Holocene lake development,lake settlement and vegetation history in central Ireland

Stratigraphical investigations, geomorphological mapping, and diatom, plant macrofossil and pollen analyses were undertaken in and around two lakes in central Ireland to establish correlations between changes in lake conditions and catchment vegetation throughout the Holocene. Similar investigations of an adjacent mire reveal early Holocene changes in lake level and area. The palaeoecological data show high correlations related to variations in lake depth and area, catchment vegetation type, organic inputs and trophic status. Catchment‐scale deforestation is gradual and occurs through the Bronze and the Iron Ages, and the construction of a crannog in the early Medieval period (seventh century AD ) appears to be associated with a widespread increase in deforestation and mixed agriculture in the catchment. Both pollen and plant macrofossils suggest that one of the crannogs was used for crop storage in addition to domestic and any other activities. In the early to middle Holocene similarities in the proxy‐data appear to be climatically driven through changing lake levels and areal extent whereas the later Holocene record is clearly dominated by anthropogenic changes within the catchment and the construction of crannogs in the lakes. The advantages of combining multi‐proxy indicators of lake hydroecology with the vegetation record are illustrated. Copyright © 2005 John Wiley & Sons, Ltd.     

Climatic changes and alluvial-sedimentation settings in southeastern Transbaikalia in the Middle-Late Holocene (by the example of the Ilya floodplain)

Climatic changes in southeastern Transbaikalia in the Middle and Late Holocene and their influence on alluvial sedimentation environments are reconstructed from the results of study of the Ilya floodplain sediments (Alkhanai National Park). At the beginning of the Subboreal period, the regional climate became more arid, which led to a significant increase in steppe species communities in the landscapes. Intense climate aridization also took place at the beginning and at the end of the Subatlantic period of the Holocene. The alluvial-sedimentation rate increased during the weakening of aridization and decreased during cooling and the intensification of aridization. The obtained regional data are compared with data on the adjacent areas and the global climatic changes in the Northern Hemisphere.     

A high‐resolution record of mire development and climatic change spanning the Late‐glacial–Holocene boundary at Church Moss,Davenham (Cheshire,England)

Excavations of deposits filling a closed basin within glacial drift at Church Moss, Davenham, near Northwich (Cheshire, England) revealed a sequence of Late‐glacial and Early Holocene sediments. Analyses of pollen and plant and invertebrate macrofossils were undertaken, together with loss‐on‐ignition analyses and a programme of AMS radiocarbon dating, to provide a record of changing biostratigraphy and climatic and ecological regimes. The infilling of features identified as frost‐cracks in the till flooring the basin gave remains that reflected conditions of extreme cold towards the end of the Devensian. The pollen record from a 3.5 m sequence of peat towards the deepest part of the basin, supported by radiocarbon dates, shows that organic deposition was initiated during the Late‐glacial Interstadial and continued into the early part of the Holocene. There was some evidence for a cool episode during the interstadial, with amelioration prior to the rapid onset of the tundra conditions of the Loch Lomond Stadial. Following the stadial, amelioration was rapid. There was evidence from both central and marginal sequences for a mosaic of fen dominated by sedges and often also mosses, with short‐lived small pools through much of the succession. Change to terrestrial conditions proceeded intermittently, probably as a result of subsidence caused by solution of underlying salt‐bearing strata. Copyright © 2000 John Wiley & Sons, Ltd.     

京津冀地区城市人居环境气候舒适性评价

京津冀地区城市聚集,目前在京津冀协同发展规划下,随着都市圈产业结构调整,各城市面临大量人口流动迁移,人居环境气候舒适性将影响人口重布。以京津冀地区13个主要城市作为研究对象,根据1951-2013年的气候资料,采用温湿指数THI和风效指数K,综合评价各城市气候舒适度,并加以对比分析。结果表明：近63 a来,本区13个城市人居气候环境舒适性较高;区内各城市气候舒适性分布特点表现为由东北向西南,舒适性呈现降低趋势,河北省内秦皇岛、唐山、廊坊、邯郸等多个城市气候舒适性均高于北京天津两大城市。以全年气候舒适程度由高到低排序：秦皇岛>唐山>廊坊、邯郸、北京 > 承德 > 张家口 > 石家庄、邢台、保定、衡水 > 沧州、天津。这将成为引导人口分流的有利因素,驱动人口高度密集区进行有序合理疏散,缓解北京天津的大城市病问题。     

Grain‐size Age Model in Reconstructing Orbital‐ and Suborbital‐scale Climate Changes on the Northeastern Tibetan Plateau Since the Late Glacial

Reconstructions of past climatic changes on the northeastern Tibetan Plateau(NETP) can provide insights into the Asian summer monsoon(ASM) variability. However, the regional climate changes on both orbital and suborbital time scales since the late Glacial remain controversial. Here we present the results of high-resolution geochemical and grain-size analyses of a sediment core from Genggahai Lake, a small, shallow lake in this area. Changes of the accumulation rate of the core sediments show a s...