A 40,000-yr record of environmental change from Burial Lake in Northwest Alaska

Burial Lake in northwest Alaska records changes in water level and regional vegetation since ∼ 39,000 cal yr BP based on terrestrial macrofossil AMS radiocarbon dates. A sedimentary unconformity is dated between 34,800 and 23,200 cal yr BP. During all or some of this period there was a hiatus in deposition indicating a major drop in lake level and deflation of lacustrine sediments. MIS 3 vegetation was herb-shrub tundra; more xeric graminoid-herb tundra developed after 23,200 cal yr BP. The tundra gradually became more mesic after 17,000 cal yr BP. Expansions of Salix then Betula, at 15,000 and 14,000 cal yr BP, respectively, are coincident with a major rise in lake level marked by increasing fine-grained sediment and higher organic matter content. Several sites in the region display disrupted sedimentation and probable hiatuses during the last glacial maximum (LGM); together regional data indicate an arid interval prior to and during the LGM and continued low moisture levels until ∼ 15,000 cal yr BP. AMS ¹⁴C dates from Burial Lake are approximately synchronous with AMS ¹⁴C dates reported for the Betula expansion at nearby sites and sites across northern Alaska, but 1000-2000 yr younger than bulk-sediment dates.     

A 23,000-yr pollen record from Lake Euramoo, Wet Tropics of NE Queensland, Australia

A new extended pollen and charcoal record is presented from Lake Euramoo, Wet Tropics World Heritage rainforest of northeast Queensland, Australia. The 8.4-m sediment core taken from the center of Lake Euramoo incorporates a complete record of vegetation change and fire history spanning the period from 23,000 cal yr B.P. to present. The pollen record is divided into five significant zones; 23,000–16,800 cal yr B.P., dry sclerophyll woodland; 16,800–8600 cal yr B.P., wet sclerophyll woodland with marginal rainforest in protected pockets; 8600–5000 cal yr B.P., warm temperate rainforest; 5000–70 cal yr B.P., dry subtropical rainforest; 70 cal yr B.P.–AD 1999, degraded dry subtropical rainforest with increasing influence of invasive species and fire.The process of rainforest development appears to be at least partly controlled by orbital forcing (precession), though more local environmental variables and human activity are also significant factors. This new record provides the opportunity to explore the relationship between fire, drought and rainforest dynamics in a significant World Heritage rainforest region.     

A 12,000-Year Pollen Record from Lake Maliq, Albania

Pollen data from Lake Maliq, the first from Albania, contribute new information to the discussion of the vegetational, hydrological, and climatological history of the Balkans since 12,000 yr B.P. During late-glacial time, a perennial lake expanded at Maliq. It was surrounded by a complex vegetation association composed of steppe and mixed forest elements. The highly diverse forest flora suggest that late-glacial forest refugia were more developed here at middle altitude, rather than at higher altitude as previously suggested. The forest developed after 9800 yr B.P., while the water level remained high in the Korçë basin until 5000 yr B.P. Different environmental conditions, characterized by lower available moisture and warmer winters, progressively took place after this date. Human activity in the Korçë basin ca. 4500 yr B.P. was coeval with conditions characterized by an increase in winter temperatures and a decrease in summer moisture.     

Diatom, Pollen, and Chemical Evidence of Postglacial Climatic Change at Big Lake, South-Central British Columbia, Canada

Postglacial climatic conditions were inferred from cores taken from Big Lake in southern British Columbia. Low concentrations of nonarboreal pollen and pigments near the base of the core suggest that initial conditions were cool. Increases in both aquatic and terrestrial production suggest warmer and moister conditions until 8500 cal yr B.P. Hyposaline diatom assemblages, increases in nonarboreal pollen, and increased concentrations of pigments suggest the onset of arid conditions from 8500 to 7500 cal yr B.P. Slightly less arid conditions are inferred from 7500 until 6660 cal yr B.P. based on the diatoms, small increases and greater variability in biogenic silica and pigments, and higher percentages of arboreal pollen. At 6600 cal yr B.P., changes in diatoms, pigments, biogenic silica, and organic matter suggest that Big Lake became fresh, deep, and eutrophic until 3600 cal yr B.P., when water levels and nutrients decreased slightly. Our paleoclimatic inferences are similar to pollen-based studies until 6600 cal yr B.P. However, unlike these studies, our multiple lines of evidence from Big Lake imply large changes in effective moisture since 6000 cal yr B.P.     

A 2200-yr record of hydrologic variability from Foy Lake, Montana, USA, inferred from diatom and geochemical data

A 2200-yr long, high-resolution (∼5 yr) record of drought variability in northwest Montana is inferred from diatoms and δ¹⁸O values of bio-induced carbonate preserved in a varved lacustrine core from Foy Lake. A previously developed model of the diatom response to lake-level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data. High-frequency (decadal) fluctuations in the de-trended δ¹⁸O record mirror variations in wet/dry cycles inferred from Banff tree-rings, demonstrating the sensitivity of the oxygen-isotope values to changes in regional moisture balance. Low frequency (multi-centennial) isotopic changes may be associated with shifts in the seasonal distribution of precipitation. From 200 B.C. to A.D. 800, both diatom and isotope records indicate that climate was dry and lake level low, with poor diatom preservation and high organic carbon: nitrogen ratios. Subsequently, lake level rose slightly, although the climate was drier and more stable than modern conditions. At A.D. 1200, lake level increased to approximately 6 m below present elevation, after which the lake fluctuated between this elevation and full stage, with particularly cool and/or wetter conditions after 1700. The hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake's outlet. Spectral analysis of the δ¹⁸O data indicates that severe droughts occurred with multi-decadal (50 to 70 yr) frequency.     

吉林辉南二龙湾玛珥湖12.0 ka B.P.以来孢粉记录与气候变化

东北二龙湾玛珥湖孢粉记录显示,12.0 ka B.P.以来,该区植被和气候经历了以下变化：11.76～9.81 ka B.P. 发育针阔叶混交林,气候冷干。9.81～4.11 ka B.P.为落叶阔叶林植被景观,前期（9.81～8.86 ka B.P.）温干,为增温期;后期（8.86～4.11 ka B.P.）暖湿,为气候适宜期。4.11 ka B.P.至今,再次发育针阔叶混交林, 其中1.62 ka B.P.以前,气候温凉;之后湿度下降,气候凉干。研究表明:YD（11.56～10.21 ka B.P.）事件在二龙湾玛珥湖孢粉资料中有突出记录,4.0 ka B.P.左右开始的降温现象在东北东部地区广泛出现。     

新疆博斯腾湖表层沉积物的孢粉分析

通过对新疆博斯腾湖的表层沉积物孢粉的分析, 探讨了干旱区大型湖泊中表层沉积物孢粉与周围植被的关系以及孢粉在湖泊中的分布和传播规律. 研究发现, 博斯腾湖表层沉积物中孢粉主要是来源于周围荒漠植被中的藜科、蒿属、麻黄等植物以及湖泊四周浅水地带生长的香蒲等水生植物. 藜科、蒿属、麻黄等花粉的传播过程主要受大风作用的影响, 而且百分比含量在整个湖盆中的分布比较稳定, A/C值能够反映区域目前的干旱状况; 香蒲等水生植物花粉的百分比含量主要受其母体植物分布的影响. 云杉花粉在河口区域的相对高百分比含量表明其河流传播特征; 云杉花粉在湖泊中的低百分比含量与整个流域的植被及其分布状况以及区域的气候条件有关; 云杉花粉具有超代表性. 近河口湖泊表层样品孢粉组合受河流和近岸植被的影响较大, 但距离超过2 km以后湖泊沉积花粉百分比含量较为稳定. 孢粉的质量浓度分布显示, 孢粉在湖盆中的分布有比较强的趋中性. 博斯腾湖中部沉积物孢粉序列能够很好记录区域的植被变化历史和环境演化特征.     

Vegetation and Climate Inferred from Pollen Record in East Asian Region During MIS 5: A Review

The last interglacial, Marine Isotope Stage 5 (MIS 5, about 71~128 ka BP), is the closest to modern interglacial period, in which there are identifiable patterns that could give us clues into what will happen in the future. Pollen analysis, the study of fossil pollen and spores, is one of the key methods for the reconstruction of past vegetation and environment. Pollen data from 23 sites in eastern Asian region were reviewed to document regional patterns of vegetation and climate change during MIS 5 and to understand the large-scale controls on these changes. The regional patterns of vegetation during MIS 5 can be compared with the present-day vegetation. The climate, inferred from pollen record, was not stable during MIS 5. The substages (5a, 5b, 5c, 5d and 5e) of MIS 5 can be identi?ed by pollen assemblages in some areas, which describe the imprint of orbital-scale climate oscillations. East Asian monsoons have significantly contributed to the environment of modern eastern Asian monsoon region during MIS 5. However, the climate in Japan is also affected by the ocean currents and westerly circulation. The climate of the Tibetan Plateau is controlled by interactions of competing factors, including Indian monsoons, westerlies, and topography. In eastern Asian region, particularly in arid and semi-arid regions, the high-resolution pollen records need to be further studied for the better understanding of the climate change.     

Diatom Response to Global Warming in Douhu Lake,Southeast China

A large number of lacustrine sedimentary records indicate that global warming is the main factor leading to significant changes in diatom communities in lakes of the northern hemisphere.However,due to the intensification of human activities since 1850,some scholars have emphasized that the increasing lake trophic level may be the main reason for the changes in diatom communities.The debate is ongoing.In order to avoid falling into the complex relationship between diatom changes and the seasonal cycle that characterizes lakes in mid and high latitudes,we chose a lake located at a low latitude,where the relationship between diatoms and temperature is not indirect but direct.The diatom record spans the past ca.100 years and reveals that the abundance of Aulacoseira granulata increased from 1900 until 1985, replacing the previously dominant Aulacoseira ambigua.These changes are in agreement with the increasing trend in global temperature.Since 1985,the percentages of the small-celled Discostella stelligera and the benthic diatom Navicula heimansioides have increased,while Aulacoseira granulata has decreased.This latest shift is caused by further global warming.We conclude that warming is the main factor leading to changing diatom communities in Douhu Lake.     

东亚地区MIS 5时期孢粉记录的植被与气候研究进展

末次间冰期(深海氧同位素5阶段,MIS 5,71~128 ka BP)离现代间冰期最近,对其认识有助于预测未来环境,而化石孢粉是恢复古植被与古气候的重要指标。通过对选择出的东亚地区孢粉记录的具有年代数据和孢粉图谱的古植被和气候研究的综合分析,初步认为:①MIS 5的植被格局可与现代间冰期即全新世的植被格局对比。②各区孢粉记录了MIS 5气候波动,部分地区可划分出5a,5b,5c,5d和5e,存在轨道尺度变化,千年尺度事件较少发现。③东亚季风是影响现代东亚季风区环境的主要因素,但日本可能还受到西风环流和洋流的影响;西风、印度季风和地形是影响青藏高原环境的主要因素。④东亚地区,特别是干旱半干旱区,分辨率较高的孢粉记录的环境变化仍需更深入的研究。     

A 24,700-yr paleolimnological history from the Peruvian Andes

A new paleolimnological dataset from Lake Pacucha (13 °S, 3095 m elevation) in the Peruvian Andes provides evidence of changes in lake level over the past 24,700 yr. A late-glacial highstand in lake level gave way to an early-Holocene lowstand. This transition appears to have paralleled precessional changes that would have reduced insolation during the wet-season. The occurrence of benthic/salt-tolerant diatoms and CaCO₃ deposition suggest that the lake had lost much of its volume by c. 10,000 cal yr BP. Pronounced Holocene oscillations in lake level included a second phase of low lake level and heightened volatility lasting from c. 8300 to 5000 cal yr BP. While a polymictic lake formed at c. 5000 cal yr BP. These relatively wet conditions were interrupted by a series of drier events, the most pronounced of which occurred at c. 750 cal yr BP. Paleolimnological changes in the Holocene were more rapid than those of either the last glacial maximum or the deglacial period.     

Fluctuation in the level of pluvial Lake Lahontan during the last 40,000 years

Samples of algal tufa, gastropods and calcite-cemented sand were collected from the Walker and Pyramid Lake areas of the Lahontan Basin, Nevada. X-ray diffraction petrographic and radiocarbon analyses show that massive forms of tufa such as the dendritic variety contain secondary carbon-bearing material and therefore yield unreliable radiocarbon dates. Dense coating of tufa (lithoid), however, gave radiocarbon ages in agreement with dates on coexisting aragonite gastropods. Radiocarbon data from the study were combined with previously dated noncarbonate materials [Born, S. M. (1972). “Lake Quaternary History, Deltaic Sedimentation, and Mudlump Formation at Pyramid Lake, Nevada”, Center for Water Resources, Desert Research Inst., Reno, Nevada] to give an internally consistent record of lake level fluctuations for the past 40,000 years. The main features of the Lahontan chronology are (1) extreme high stands (1330 m above sea level) 13,500 to 11,000 and 25,000 to 22,000 B.P., (2) a moderate high stand (1260 m above sea level) 20,000 to 15,000 B.P., (3) a low stand of unknown elevation 40,000 to 25,000 B.P., (4) an extremely low stand 9000 to 5000 B.P., and (5) an overall increase in the size of Walker and Pyramid Lakes during the past 5000 years, until the late 19th century. Pore fluid data indicate that Walker Lake desiccated sometime during the period 9050 to 6400 B.P. Salts deposited as a result of this dessication are still undergoing dissolution causing a flux of chloride, carbon, and other solute species from the sediments to the overlying lake water. Pore fluid data obtained from Pyramid Lake sediments do not indicate the presence of a concentrated brine at depth. This suggests that Pyramid Lake did not dry completely during this period although it may have been severely reduced in size. There has been considerable disagreement regarding the occurrence of extreme arid conditions (altithermal period) since 10,000 B.P. [Mehringer, P. J. (1977). “Models and Great Basin Prehistory”. Desert Research Inst. Pub, Reno, Nevada]. The data of this study suggest that such a climatic regime did occur in the western Great Basin during the period 9000 to 5000 B.P.     

A 94-ka Pollen Record of Vegetation Change in Qaidam Basin,Northeastern Qinghai-Tibetan Plateau

正1 Introduction The northeastern Qinghai-Tibetan Plateau(NE QTP),located at a triple junction of influences of the Asian summer monsoon,westerly jet stream and Siberian high,is of considerable significance with regard to regional responses to global climate change.Qarhan Salt Lake is the largest playa located in the central eastern     

Metamorphism and Tectonics in Southern Madagascar: An Overview

The scope of this paper is to briefly summarize the general tectonic pattern and the metamorphic evolution of the continental crust from southern Madagascar. After a presentation of the main geophysical features of the Malagasy crust, the brittle and the ductile strain patterns are established at a crustal scale. The Pan-African metamorphic zonation is discussed and interpreted through a tectonic model related to a transpressive regime.     

Holocene Monsoon Changes Inferred from Lake Sediment Pollen and Carbonate Records, Northeastern Cambodia

Major Holocene monsoon changes in continental Southeast Asia are reconstructed from analysis of ¹⁴C-dated changes in pollen and organic/inorganic carbon in sediment cores taken from permanent, closed-basin, volcanic lakes in Ratanakiri Province, northeastern Cambodia. Analysis focuses on the nature and timing of monsoon changes, inferred from changes in vegetation and lake conditions. These data provide the first well-dated palynological record, covering most of the Holocene and continuous up to the present, from a terrestrial site in mainland Southeast Asia. The record from a 15-m core retrieved from Kara Lake, representing the last 9300 years, shows that the late Glacial conditions ended about 8500 ¹⁴C yr B.P., more than 1000 years later than sites in southwest China. Summer monsoon intensity increased over the period ca. 8400–5300 ¹⁴C yr B.P., similar to most other sites in the Asian monsoon region. A subsequent expansion of secondary forests at the expense of dense semievergreen forests suggest a drier climate leading to more frequent fire disturbance. After ca. 3500 ¹⁴C yr B.P. disturbance frequency may have increased further with increasing seasonality. From ca. 2500 ¹⁴C yr B.P. to the present, dense forest has recovered in a mosaic with annually burned dry forest, but climate may not be the main control on local vegetation dynamics in the late Holocene.     

青藏高原日土地区全新世中期以来介形类和孢粉组合变化及其古环境意义

本文综合分析了青藏高原日土地区RT-1剖面中介形类和孢粉种类组合的变化,结合三个光释光测年数据,探讨了该地区6510aBP以来古环境、古气候的演变。分析表明:日土地区在全新世中期曾有一个温暖湿润期(6170～5540aBP),之后气候环境频繁波动,逐渐趋向冷干,接近现代气候环境。43～30kaBP期间在鲁玛江冬错和班公错之间的古泛湖解体后,日土地区因为地势低洼,而在当时成为一个封闭的水体,并且一度与相邻的班公错相连。当时的水体一直持续到全新世,在经历了全新世暖湿期时短暂的湖涨期以后,逐渐萎缩,至约1570～1380aBP最终干涸消失,形成现今地貌。研究表明全新世大暖期在本地区也有反映,但是在高原深部,大暖期更为短暂,气候波动更为频繁。     

Reconstruction of Environmental Changes in Western Siberia: Evidence from the Diatom Complexes of Lake Sediments

Doklady Earth Sciences - The information content of lake sediments in Western Siberia for reconstruction of the latest environmental and climatic changes was evaluated. The possibility of reliable...     

Millennial-scale vegetation changes during the last 40,000 yr based on a pollen record from Lake Biwa, Japan

A high-resolution pollen record covering the last 40,000 yr (BIW95-4) from Lake Biwa, western Japan, shows regional vegetation responses to millennial-scale climate changes. From 40 to 30 ka, Cryptomeria japonica was dominant around the lake among pinaceous conifers and deciduous broad-leaved trees. During this period, fluctuations of C.japonica are correlated with Dansgaard-Oeschger (D-O) cycles recognized from the anhysteretic remanent magnetization (ARM) record. Increases in the abundance of this taxon may have been caused by wetter summer conditions influenced by the East Asian monsoon or increased snowfall on the Sea of Japan side of the Japanese archipelago. Between 29 and 14 ka, pinaceous conifer forests mainly composed of Pinus subgenus Haploxylon, Tsuga, and Picea trees developed. At approximately 23 ka, Picea trees increased in abundance as ARM values decreased. This expansion of Picea trees has been correlated with Heinrich event (HE) 2 in the North Atlantic. At about 14 ka, the distribution of broad-leaved forest (mainly composed of deciduous oaks) began to expand after D-O 1. Evidence of significant vegetation change related to the abrupt Younger Dryas cooling event has not been found.     

滇池盆地孢粉记录的29kaBP气候转型事件

位于低纬度高原上的滇池盆地的气候主要受西南季风及东亚季风的控制,盆地沉积物是气候变化的敏感记录.通过对该盆地北部钻孔MIS3晚期孢粉记录的研究,发现在29 kaBP该地区的气候明显发生了由暖干向凉湿的转型.29 kaBP之前,孢粉组合中,乔木以喜暖的壳斗科、山核桃、鹅耳枥等为主,草本植物中较为耐旱的菊科、石竹科、禾本科等较发育,气候暖干;29 kaBP之后,孢粉组合改变较大,松属花粉含量增加突出,喜湿的铁杉和云杉少量出现,草本植物中莎草科增加,蕨类增多,气候较之前偏凉湿.通过对比发现,该气候转型事件具有区域对比性.