共查询到20条相似文献，搜索用时 31 毫秒
1.
A coastal dunefield on Groote Eylandt, in the Gulf of Carpentaria, northern Australia, is stratigraphically described and dated using the coarse fraction thermoluminescence dating technique. Four phases of dune activity have been identified: (1) Modern active transverse and parabolic dunes. (2) A parabolic dunefield apparently stabilized less than 2000 yr BP. (3) A parabolic dunefield stablized between 6000 and 4800 yr BP. (4) A basal dunefield unit emplaced prior to 100,000 yr BP.The current dune systems are an expression of dune activation and stabilization events in the Holocene, but were formed from the deflation of an extensive pre-Holocene dunefield. The destruction of the pre-Holocene dunefield appears to have been caused by sea-level rise at the end of the Pleistocene and during the early Holocene. The ages derived from the dunefield accord well with chronostratigraphic investigations of coastal dune systems elsewhere in northern Australia and support theories of regional environmental change during the Holocene. 相似文献
2.
通过对巢湖湖泊沉积岩芯不同粒级炭屑浓度和磁化率等指标变化的研究,揭示了全新世以来的区域火环境变化及人类活动影响。结果表明：① 全新世早期,气候由寒冷干旱向温暖湿润过渡,但仍较为干旱,炭屑浓度也相对略高,火活动比较频繁,地方性和区域性火时有发生;② 全新世中期是最温暖湿润的适宜期,炭屑浓度出现全新世以来的低值,火活动微弱,但在文化兴盛时期炭屑浓度出现峰值,则归因于人类活动的影响;③ 全新世晚期,在气候趋于凉干的环境背景下,人类活动的增强更加剧了火活动的频率和强度,炭屑浓度大幅增加;④ 近200 a炭屑浓度的降低可能与湖区附近已没有足够生物量引起火灾有关。 相似文献
3.
Charcoal preserved in lake sediments is commonly used to reconstruct past trends in fire occurrence. However, interpretation of the charcoal record is often complicated, as changes in charcoal influx could represent natural shifts in fire regimes associated with changes in climate, changes in vegetation, or changes in patterns of anthropogenic burning. Here we examine sedimentary charcoal records from three lakes on the Caribbean islands of Hispaniola and Puerto Rico: Laguna Saladilla in the Dominican Republic, Lake Miragoane in Haiti, and Laguna Tortuguero in Puerto Rico. All records are based on microscopic charcoal fragments quantified from pollen slides and cover the last 7,000 or more years of the Holocene. We compare charcoal influx values to archeological and palynological evidence of human activity and explore the role of increasing winter insolation over the Holocene in driving increased charcoal deposition beginning ca. 6,000–5,000 cal yr BP. An increase in charcoal influx at Laguna Tortuguero at ca. 5,200 cal yr BP, previously interpreted as a signal of human settlement predating archeological evidence, may instead reflect insolation-driven shifts in winter drying that led to more frequent and possibly more intense natural fires. Decreased charcoal influx around 3,200 cal yr BP may signal human modification of the environment that altered fire frequency and/or intensity. Comparing the records from these three lowland Caribbean sites highlights possible intervals of synchronous, climate-driven burning as distinct from more localized anthropogenic burning. 相似文献
4.
华北平原晚冰期以来气候环境演变研究对该地区社会发展、灾害风险评估和科学应对未来全球增温背景下极端降水和洪涝事件具有重要意义。本文以华北平原中部白洋淀地区高阳剖面（BG-2019）为研究对象,通过高精度AMS14C、OSL定年技术和高分辨率孢粉组合、粒度组分分析,恢复和重建了白洋淀地区晚冰期以来（距今13710 a—今）区域植被演替和气候环境变化历史。结果显示：BG-2019剖面在距今10270~13710 a和距今4630~5400 a发育湖相沉积,距今3470~3700 a发育沼泽相沉积;距今7130~8000 a发育河流—入湖三角洲相沉积,距今3700~4630 a和距今3230~3470 a发育河流相沉积;距今8000~10270 a和距今5400~7130 a存在明显的沉积间断/地层缺失;表明采样剖面所在位置缺乏连续的湖相地层。晚冰期白洋淀为局地小湖沼;中全新世湖沼较发育、范围广,但也不是连续广袤的湖相沉积;晚全新世湖泊范围收缩。晚冰期和全新世白洋淀流域植被景观存在显著差异;晚冰期气候寒冷干燥,平原发育以蒿属、藜亚科、禾本科和菊科等为主的草地,周围山地森林覆盖度低;中全新世气候温暖湿润,平原大部仍发育以蒿属、藜亚科和禾本科为主的草地,湖区水蕨和水生植物繁盛,周围山地生长松属、栎属为主的针阔混交林,森林覆盖度增高;晚全新世气候温和偏干,平原仍是以蒿属、藜亚科和禾本科等为主的草地,西部山地生长以松属为主的针阔混交林,森林覆盖度较高。 相似文献
5.
Jennifer M. Galloway R. Timothy Patterson Christine T. Doherty Helen M. Roe 《Journal of Paleolimnology》2007,38(4):569-588
Pollen and diatoms preserved in the radiocarbon dated sediments of Two Frog Lake in the Seymour-Belize Inlet Complex of the
central mainland coast of British Columbia document postglacial climate change. Two Frog Lake was isolated from the sea prior
to 11,040 ± 50 yr BP (13,030 cal. yr BP) when the climate was cool and dry, and open Pinus contorta woodlands covered the landscape. These woodlands were replaced by a mixed conifer forest ca. 10,200 yr BP (ca. 12,300 cal.
yr BP) when the climate became moister. A relatively dry and warm early Holocene climate allowed Pseudotsuga menziesii to migrate northward to this site where it grew with Picea, Tsuga heterophylla and Alnus. The climate became cooler and moister at ca. 8,000 yr BP (ca. 9,200 cal. yr BP), approximately 500–1,000 years prior to
sites located south of Two Frog Lake and on the Queen Charlotte Islands, but contemporary with sites on the northern mainland
coast of British Columbia and south coastal Alaska. Climate heterogeneity in central coastal British Columbia appears to have
occurred on a synoptic scale, suggesting that atmospheric dynamics linked to a variable Aleutian Low pressure system may have
had an important influence on early Holocene climate change in the Seymour-Belize Inlet Complex. The transition to cooler
and moister conditions facilitated the expansion of Cupressaceae and the establishment of a modern-type coastal temperate
rainforest dominated by Cupressaceae and T. heterophylla. This was associated with progressive lake acidification. Diatom changes independent of vegetation change during the late
Holocene are correlative with the mid-Neoglacial period, when cooler temperatures altered diatom communities. 相似文献
6.
西辽河流域中晚全新世气候环境演变及其对农牧业演替的影响 总被引：1，自引：0，他引：1
气候变化是人类文明演化的重要驱动力之一,中国东北西辽河流域地处季风气候敏感带,早中全新世以来古文化演替频繁,具有研究气候变化与早期农业文明响应的重要价值。本文基于西辽河流域兴隆洼（XLW）剖面高精度年代学框架开展多种气候代用指标分析,重建了中晚全新世以来西辽河流域的气候环境演变历史。结果显示,距今5.0 ka以来研究区经历了冷干—暖湿—冷干的气候演变过程。5.0—3.7 cal. ka BP期间,磁化率、烧失量等指标的低值反映了气候由全新世大暖期向冷干环境的转变,可能与东亚夏季风的衰退有关。该时期红山文化衰退,逐渐被以渔猎和农业并重的小河沿文化所取代,东北地区遗址数量有所减少并出现明显南偏。3.7 cal. ka BP后,磁化率、烧失量等指标的高值指示了夏季风的增强,水热条件的改善促进了旱作农业的发展,为夏家店下层文化的发展提供了有利条件。这一阶段东北地区出现人口大爆发,遗址数量达到峰值。2.8 cal. ka BP后,磁化率与烧失量波动降低,表明夏季风波动频繁,气候逐渐转冷干,水热条件的衰退可能导致了夏家店上层文化时期旱作农业的退化与游牧业的发展,且遗址分布出现南偏。 相似文献
7.
H. Inda F. García-Rodríguez L. del Puerto V. Acevedo D. Metzeltin C. Castiñeira R. Bracco J. B. Adams 《Journal of Paleolimnology》2006,35(4):699-713
Paleolimnological data are presented on trophic development, climatic change and sea level variations in Rocha Lagoon, a 72 km2 coastal lagoon in southern Uruguay. Using a sediment core that extended from 7000 to about 3700 yr BP, analyses of organic
matter, carbonate, diatoms and chrysophyte cysts were used to track the early Holocene paleolimnological conditions of Rocha
Lagoon. Opal phytoliths were also counted and identified, both temperature and humidity indices were calculated, and Opal
Phytolith Association Zones (OPAZ) were identified by performing Principal Coordinates Analysis (PCO). Diatom Association
Zones (DAZ) corresponding to marine/brackish and brackish/freshwater episodes were closely related to changes in trophic state.
Those DAZ representing marine/brackish stages exhibited a lower trophic state than those DAZ dominated by brackish and freshwater
diatoms. This highlights that during the first Holocene marine transgression, Rocha Lagoon did not continuously exhibit marine/brackish
conditions as reported in previous papers. Instead, three brackish/freshwater episodes related to sea level variation and
changes in humidity were identified. The first episode, by ~6000 yr BP, was related to sea level change as no significant
changes in either temperature or humidity indices were observed. The second episode, between 5000 and 4400 yr BP, was related
to both a sea level decrease and an increase in humidity, as a transition from humid to very humid climate was inferred. Concomitant
decreases in salinity and increases in trophic state were also observed. The third episode, after ~4000 yr BP, was related
to the end of the first Holocene regressive phase when sea level was slightly below present levels. Further decreases in salinity
and increases in trophic state were detected. The paleoclimatic trends inferred in this study were in close agreement with
other regional studies on climatic change, as cool temperatures were inferred. However, major changes in humidity were also
detected. A humid to very humid climate was inferred for ~7000–4500 yr BP, but the occurrence of a semiarid/arid climate was
inferred for ~4500–3700 yr BP. Our data suggest that during transgressive and regressive events there might be higher frequency
and lower amplitude sea level oscillations that might lead to changes in salinity and trophic state of coastal aquatic systems.
Such oscillations could only be tracked by high resolution analyses of sedimentary records and could be best interpreted with
complementary data on paleoclimate. In addition, microfossils such as diatoms and opal phytoliths were shown to be very sensitive
to such paleoenvironmental changes. 相似文献
8.
We present a record of Holocene environmental change on the east coast of Korea, inferred using pollen, carbon-isotopic composition (??13C), total organic carbon, total sulfur, carbon/nitrogen ratios, particle size analysis and major element geochemistry in a sediment core from Soonpogae Lagoon. A multi-proxy paleoenvironmental approach had not previously been applied in Korea and allowed us to reconstruct climate and vegetation change, sea-level rise, lagoon development, and human impact on the east coast of Korea over the last 8,000?years. Evidence from Soonpogae Lagoon supports the following three conclusions: (1) As a drying trend prevailed on the east coast of Korea after ~5,900?cal?yr BP, chemical weathering weakened and herbaceous plants became more important than during the previous humid phase (Holocene Climate Optimum), (2) Sea-level rise on the east coast slowed dramatically about 6,800?cal?yr BP, resulting in low rates of sedimentation in Soonpogae Lagoon, and (3) Soonpogae Lagoon was almost completely isolated from the sea by sand barriers when human impact intensified ~2,100 BP. 相似文献
9.
Cyril Aubert Elodie Brisset Morteza Djamali Arash Sharifi Philippe Ponel Belinda Gambin Tayebeh Akbari Azirani Frédéric Guibal Hamid Lahijani Abdolmajid Naderi Beni Jacques-Louis de Beaulieu Ali Pourmand Valérie Andrieu-Ponel Alain Thiéry Emmanuel Gandouin 《Journal of Paleolimnology》2017,58(2):151-167
We reconstructed the paleohydrologic and climatic history of the Lake Neor region, NW Iran, from the end of the late glacial to the middle Holocene (15,500–7500 cal yr BP). Subfossil chironomid and pollen assemblages in a sediment core from a peatland located south of Lake Neor enabled identification of four main hydrologic phases. The period 15,500–12,700 cal yr BP was characterized by a relatively dry climate with an open landscape, suggested by the abundance of Irano-Turanian steppe plants (e.g. Amaranthaceae, Artemisia and Cousinia). Dominance of several shallow-water and semi-terrestrial chironomid taxa (e.g. Pseudosmittia, Smittia/Parasmittia and Paraphaenocladius/Parametriocnemus) during this period is indicative of lower water tables in the wetland. Between 12,700 and 11,300 cal yr BP, chironomid taxa indicate higher wetland water tables, as suggested by the presence of Zavrelia, Chironomus anthracinus/plumosus-type and Micropsectra, which are inhabitants of open-water, lacustrine areas. The open-steppe vegetation remained dominant in the watershed during this time. Increasing wetland moisture could be explained by: (1) cool summers that reduced the evaporation rate; and/or (2) a decrease in duration of the summer dry season. The period 11,300–8700 cal yr BP was characterized by lower wetland moisture, contemporaneous with a delay in the expansion of deciduous forest, suggesting persistent dry climate conditions throughout the beginning of the Holocene, which may have been related to the intensified seasonality of precipitation. Around 8700 cal yr BP, higher wetland water levels, inferred from chironomids, occurred simultaneously with the onset of regional deciduous forest expansion, probably caused by a shortening of the summer dry period. We concluded that chironomids are appropriate paleoecological proxies to investigate global and local hydrologic variability in the Middle East. 相似文献
10.
近年来,“全新世温度谜题”已经受到全球古气候学者的广泛关注,为了解决这一谜题,需要在全球不同区域进行更多的全新世温度重建。帕米尔高原位于亚洲内陆核心区域,目前有关帕米尔高原全新世气候变化的研究相对较少,且已有的研究主要集中于相对湿度(或降水)变化的研究,而涉及温度变化的成果则相对较少。论文首先研究了表土碳同位素与气候因子之间的相关关系,结果显示帕米尔高原的δ13Corg与温度正相关;进一步在7个AMS 14C测年数据的支持之下,基于175个泥炭δ13Cα-cellulose分析,重建了帕米尔高原过去约5000 a的温度变化历史。结果发现：帕米尔高原晚全新世以来整体呈现波动升温趋势,约5000~3600 cal a BP阶段处于缓慢降温期;约3600~200 cal a BP处于波动升温期;驱动机制分析显示,约3600 cal a BP之前温度下降主要是夏季太阳辐射下降导致的,约3600 cal a BP之后温度上升是由温室气体辐射强迫增强导致的。 相似文献
11.
Stefan Doberschütz Peter Frenzel Torsten Haberzettl Thomas Kasper Junbo Wang Liping Zhu Gerhard Daut Antje Schwalb Roland Mäusbacher 《Journal of Paleolimnology》2014,51(2):253-266
This study focuses on Holocene monsoon dynamics on the central Tibetan Plateau (TP) inferred using a sediment record from Lake Nam Co. A high-resolution (decadal) multi-proxy approach, using geochemical, micropaleontological, and sedimentological methods was applied. Fifteen AMS-14C ages were used to establish the chronology, assuming a reservoir effect of 1,420 years. Our results point to a first strong monsoonal pulse at Lake Nam Co ~11.3 cal ka BP, followed by colder and drier conditions until ~10.8 cal ka BP. A warm and humid climate from ~10.8 to ~9.5 cal ka BP is related to a strong summer monsoon on the central TP, triggering a lake-level highstand at ~9.5 cal ka BP. Declining minerogenic input after ~9.5 cal ka BP indicates less moisture availability until ~7.7 cal ka BP. Following stable conditions between ~7.7 and ~6.6 cal ka BP, a warm and wet climate is inferred for the time span from ~6.6 to ~4.8 cal ka BP. A change towards drier conditions after ~4.8 cal ka BP points to a weakening of the Indian Ocean Summer Monsoon on the central TP, further diminished after ~2.0 cal ka BP. A short-term wet spell occurred from ~1.5 to ~1.3 cal ka BP. By comparing the results derived from Lake Nam Co with several lacustrine records from the central, northern, and eastern TP, a similar but not synchronous pattern of monsoon-driven paleoenvironmental change is observed. Although the general trend of lake and catchment evolution on the TP during the Holocene is also reproduced in this record, pronounced spatial and temporal offsets with respect to distinct climate events were detected, suggesting periods of non-uniform moisture and temperature evolution. 相似文献
12.
中国北方沙漠地区全新世气候变化的周期性探讨 总被引：4，自引：0，他引：4
随着高精度全新世气候变化研究的深入,千、百年尺度上气候波动、周期性变化和气候事件不断被发现。北大西洋深海沉积物中发现的IRD事件、千年尺度上的约1500年周期性气候振荡已经被许多研究者共识,并在陆地湖泊、冰心乃至沙地和沙漠的沉积物中均有发现。百年尺度上的512年、400年、200年等周期性气候振荡还有待深入研究。研究表明,全新世以来,中国北方地区沙漠的扩大与缩小与全球气候的周期性变化具有密切的相关性,尤其是与千年尺度上的1500年的周期性震荡有关;数年-数十年尺度上的周期性气候振荡对我国沙尘暴的发生起着明显的控制作用。 相似文献
13.
13 ka BP 以来黄土高原西部的植被与环境演化 总被引：2，自引：0，他引：2
通过对黄土高原西部三个剖面的孢粉记录分析, 重建了该区13 ka BP 以来详细的植被 与气候演化序列。结果表明, 12.1 ka BP 以前, 研究区植被以干草原为主, 气候寒冷干燥。 12.1~9.8 ka BP 植被变化显著。期间出现两次显著的湿润期, 分别为12.1~11.4 ka BP、 11.2~11.0 ka BP, 可与博令暖期和阿勒罗得暖期对比; 两次持续时间和强度明显不同的干旱 期出现在11.4~11.2 ka BP 和11.0~9.8 ka BP, 可分别与中仙女木事件和新仙女木事件对比。 经过短暂的快速变湿后, 9.6~7.6 ka BP 研究区植被以疏林草原为主, 气候波动频繁但总体温 和偏干。7.6~4.0 ka BP 森林和森林草原植被出现, 气候温暖湿润。其中6.6~5.8 ka BP 温带落 叶阔叶林发育, 为研究区全新世最适宜期。自4.0 ka BP 以来研究区草原和荒漠草原交替出 现, 气候在总体干冷的环境下存在次一级的干湿波动。 相似文献
14.
Pollen records from the Chinese Loess Plateau revealed a detailed history of vegetation variation and associated climate changes during the last 13.0 ka BP. Before 12.1 ka BP, steppe or desert-steppe vegetation dominated landscape then was replaced by a coniferous forest under a generally wet climate (12.1–11.0 ka BP). The vegetation was deteriorated into steppe landscape and further into a desert-steppe landscape between 11.0 and 9.8 ka BP. After a brief episode of a cool and wet climate (9.8–9.6 ka BP), a relatively mild and dry condition prevailed during the early Holocene (9.6–7.6 ka BP). The most favourable climate of warm and humid period occurred during mid-Holocene (7.6–~4.0 ka BP) marked by forest-steppe landscape and vegetation alternatively changed between steppe and desert- steppe from ~4.0 to ~1.0 ka BP. 相似文献
15.
粒度和磁化率记录的毛乌素沙地东缘全新世气候变化 总被引：1，自引：2，他引：1
通过对毛乌素沙地东缘大柳塔剖面地层沉积物粒度及磁化率特征的分析，结合光释光测年结果，探讨了毛乌素沙地全新世气候变化。结果表明：（1）剖面沉积物粒度组成以中砂、粗砂为主，古土壤中黏粒组分含量高（3.32%），风成砂中几乎不含黏粒组分（0.01%），粒度参数的平均粒径Mz和分选系数σ也表现为古土壤高值、风成砂低值；（2）磁化率在全剖面呈有规律的变化，低频、高频磁化率在古土壤中呈高值，湖沼相、弱成壤次之，风成砂最小；低频磁化率χlf与黏粒（<2 μm）含量、平均粒径Mz呈显著正相关，垂向剖面上变化规律一致，指示了相似的气候环境意义；（3）毛乌素沙地在10.39 ka BP附近存在明显的3次冷干-暖湿气候波动；10.39~9.34 ka BP，气候冷干，风沙活动盛行；9.34~8.68 ka BP，冬季风衰退，夏季风增强，成壤作用强烈；8.68~8.29 ka BP，气候寒冷干燥；8.29~2.72 ka BP，气候整体温暖湿润，在6.55~3.80 ka BP达到鼎盛后转向冷干；2.72~1.34 ka BP，冬季风占主导，沙丘活化，沙漠扩张；1.34 ka BP至今，逐渐接近现代气候。毛乌素沙地的全新世气候变化是东亚冬、夏季风此消彼长作用下全球气候变化的区域响应。 相似文献
16.
花海古湖泊外源碎屑矿物含量揭示的河西走廊早、中全新世降水变化 总被引：1，自引：0，他引：1
花海地区位于河西走廊的西段,在全新世早期和中期亚洲季风增强时,该区域降水是否很好地响应亚洲季风的变化,以及季风的水汽输送是否会影响到该区域是仍需继续探讨的问题.本文通过对花海古湖泊沉积物样品的矿物测定,分析了花海古湖泊早、中全新世湖相沉积阶段中外源碎屑矿物含量的变化,并以此重建了该区域早全新世(10.47～8.87 cal ka BP)和中全新世(8.87～5.5 cal ka BP)的降水变化.结果显示,早全新世时期花海湖泊外源碎屑矿物含量高于中全新世时期,表明了该区域早全新世时期的降水高于中全新世时期,反映了该区域早、中全新世千年尺度的降水变化响应了印度季风的强弱变化,降水量受到了印度季风强度的影响.在全新世中期,由于西风环流的增强,花海地区水汽输送可能受到了印度季风和西风环流的双重影响.但全新世最湿润期出现在中全新世,与早全新世降水强度高于中全新世的结果并不一致.这种差异可能主要是由于中全新世时期较低的湖面蒸发所造成的. 相似文献
17.
毛乌素沙漠东南缘萨拉乌苏河流域米浪沟湾剖面MGS1层段含有11个由古沙丘砂与河湖相或古土壤构成的沉积旋回。通过对该层段粒度和CaCO3的分析,发现其河湖沼相和古土壤的平均粒径、分选系数、偏度和峰态的值明显高于古沙丘砂;CaCO3含量在古沙丘砂及其上覆的河湖相和古土壤中依次变化于0.07%~0.32%（平均值为0.16%）、0.05%~1.50%（0.27%）和0.06%~1.42%（0.64%）,并与平均粒径呈显著相关（相关系数0.8）;构成与沉积旋回波动韵律相同的11个峰谷交替的粒度和CaCO3旋回,指示了全新世气候冷、暖的变化过程和东亚冬夏季风千年尺度交替变化的气候旋回。这些旋回中的冬季风主导时期可与北大西洋8 200、5 000、4 000、2 700、1 400 a BP等寒冷事件进行对比,和中国若干地点的气候波动具有良好对应关系,可能表明全新世东亚季风在全球大气、海洋、陆地和全球冰量变化的共同作用下,表现为干冷冬季风和暖湿夏季风的相互交替的千年尺度的气候变化特征。 相似文献
18.
Hydrology of Dali Lake in central-eastern Inner Mongolia and Holocene East Asian monsoon variability 总被引：12，自引：1，他引：11
Jule Xiao Bin Si Dayou Zhai Shigeru Itoh Zaur Lomtatidze 《Journal of Paleolimnology》2008,40(1):519-528
Lacustrine records from the northern margin of the East Asian monsoon generate a conflicting picture of Holocene monsoonal precipitation change. To seek an integrated view of East Asian monsoon variability during the Holocene, an 8.5-m-long sediment core recovered in the depocenter of Dali Lake in central-eastern Inner Mongolia was analyzed at 1-cm intervals for total organic and inorganic carbon concentrations. The data indicate that Dali Lake reached its highest level during the early Holocene (11,500–7,600 cal yr BP). The middle Holocene (7,600–3,450 cal yr BP) was characterized by dramatic fluctuations in the lake level with three intervals of lower lake stands occurring 6,600–5,850, 5,100–4,850 and 4,450–3,750 cal yr BP, respectively. During the late Holocene (3,450 cal yr BP to present), the lake displayed a general shrinking trend with the lowest levels at three episodes of 3,150–2,650, 1,650–1,150 and 550–200 cal yr BP. We infer that the expansion of the lake during the early Holocene would have resulted from the input of the snow/ice melt, rather than the monsoonal precipitation, in response to the increase in summer solar radiation in the Northern Hemisphere. We also interpret the rise in the lake level since ca. 7,600 cal yr BP as closely related to increased monsoonal precipitation over the lake region resulting from increased temperature and size of the Western Pacific Warm Pool and a westward shifted and strengthened Kuroshio Current in the western Pacific. Moreover, high variability of the East Asian monsoon climate since 7,600 cal yr BP, marked by large fluctuations in the lake level, might have been directly associated with variations in the intensity and frequency of the El Niño-Southern Oscillation (ENSO) events. 相似文献
19.
Paleolimnological data are presented on trophic development, climatic change and sea level variations in Negra Lagoon, a 142 km2 coastal lagoon in southern Uruguay. Using a sediment core that extended to the early Holocene, analyses of organic matter, carbonate, diatoms and opal phytoliths allowed us to track changes in trophic state and paleosalinity levels, which were closely related to Holocene sea level variation, temperature and humidity. Diatom Association Zones (DAZ) corresponding to transgressive events were dominated by marine/brackish taxa and relatively low organic matter values, while those DAZ corresponding to regressive events showed increases in brackish/freshwater diatoms and organic matter. Opal Phytoliths Association Zones (OPAZ) also were identified, temperature and humidity indices were calculated, and climatic trends were inferred. During the middle Holocene (i.e., ~5000–4000 yr BP), cool–dry climatic conditions were established. After ~4000 yr BP, a transition from cool to warm–humid conditions was observed, but fully warm humid conditions established only after ~1700 yr BP. Such climatic amelioration led to increases in trophic state. 相似文献
20.
16 ka以来黄土高原东亚夏季风变化的环境磁学记录 总被引：1，自引：1，他引：0
末次冰消期以来的气候变化是第四纪古气候研究的重要内容。本文对黄土高原西部和平镇剖面16 ka以来的黄土沉积序列（厚度为3.5 m）进行了高分辨率环境磁学研究。结果表明：该黄土序列的磁性颗粒主要由细颗粒磁铁矿主导。其磁化率和频率磁化率具有相同的变化特征,与黄土高原中部和东部的黄土磁化率变化机制一致,即黄土高原西部和平镇黄土的磁化率和频率磁化率仍然受成土作用形成的细颗粒磁铁矿控制,因此其磁化率和频率磁化率仍然可以反映季风降雨的变化特征,并且其反映的16 ka以来季风降雨变化与南方石笋氧同位素记录的季风变化特征一致。通过详细的海陆综合对比,本文认为16 ka至11 ka间东亚季风降雨的变化受到北半球太阳辐射和北半球区域温度的共同影响,而11 ka之后的夏季风降雨减少主要受太阳辐射降低的影响。 相似文献