Climate drying and associated forest decline in the lowlands of northern Guatemala during the late Holocene     

Andreas D. Mueller  Gerald A. Islebe  Dustin A. Grzesik  Flavio S. Anselmetti  Mark Brenner  Jason H. Curtis  David A. Hodell  Kathryn A. Venz 《Quaternary Research》2009,71(2):133-141

Palynological studies document forest disappearance during the late Holocene in the tropical Maya lowlands of northern Guatemala. The question remains as to whether this vegetation change was driven exclusively by anthropogenic deforestation, as previously suggested, or whether it was partly attributable to climate changes. We report multiple palaeoclimate and palaeoenvironment proxies (pollen, geochemical, sedimentological) from sediment cores collected in Lake Petén Itzá, northern Guatemala. Our data indicate that the earliest phase of late Holocene tropical forest reduction in this area started at ∼ 4500 cal yr BP, simultaneous with the onset of a circum-Caribbean drying trend that lasted for ∼ 1500 yr. This forest decline preceded the appearance of anthropogenically associated Zea mays pollen. We conclude that vegetation changes in Petén during the period from ∼ 4500 to ∼ 3000 cal yr BP were largely a consequence of dry climate conditions. Furthermore, palaeoclimate data from low latitudes in North Africa point to teleconnective linkages of this drying trend on both sides of the Atlantic Ocean.  相似文献   

Environmental history of southern Patagonia unravelled by the seismic stratigraphy of Laguna Potrok Aike     

FLAVIO S. ANSELMETTI  DANIEL ARIZTEGUI†  MARC DE BATIST‡  A. CATALINA GEBHARDT§  TORSTEN HABERZETTL¶  FRANK NIESSEN§  CHRISTIAN OHLENDORF  BERND ZOLITSCHKA 《Sedimentology》2009,56(4):873-892

Laguna Potrok Aike, located in southernmost Patagonia (Argentina, 52°S) is a 100 m deep hydrologically closed lake that probably provides the only continental southern Patagonian archive covering a long and continuous interval of several glacial to interglacial cycles. In the context of the planned ‘International Continental Scientific Drilling Program’ initiative ‘Potrok Aike Maar Lake Sediment Archive Drilling Project’, several seismic site surveys that characterize in detail the sedimentary subsurface of the lake have been undertaken. Long sediment cores recovered the material to date and calibrate these seismic data. Laguna Potrok Aike is rimmed steeply, circular in shape with a diameter of ∼3·5 km and is surrounded by a series of subaerial palaeoshorelines, reflecting varying lake-level highstands and lowstands. Seismic data indicate a basinwide erosional unconformity that occurs consistently on the shoulder of the lake down to a depth of −33 m (below 2003 ad lake level), marking the lowest lake level during Late Glacial to Holocene times. Cores that penetrate this unconformity comprise Marine Isotope Stage 3-dated sediments (45 kyr bp ) ∼3·5 m below, and post-6800 cal yr bp transgressional sediments above the unconformity. This Middle Holocene transgression following an unprecedented lake-level lowstand marks the onset of a stepwise change in moisture, as shown by a series of up to 11 buried palaeoshorelines that were formed during lake-level stillstands at depths between −30 and −12 m. Two series of regressive shorelines between ∼5800 to 5400 and ∼4700 to 4000 cal yr bp interrupt the overall transgressional trend. In the basin, mound-like drift sediments occur after ∼6000 cal yr bp, documenting the onset of lake currents triggered by a latitudinal shift or an increase in wind intensity of the Southern Hemispheric Westerlies over Laguna Potrok Aike at that time. Furthermore, several well-defined lateral slides can be recognized. The majority of these slides occurred during the mid-Holocene lake-level lowering when the slopes became rapidly sediment-charged because of erosion from the exposed shoulder sediments. Around 7800 and 4900 cal yr bp , several slides went down simultaneously, probably triggered by seismic shaking.  相似文献   

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin,Llanbedr (Mochras Farm) borehole,Wales     

Jean‐Franois Deconinck  Stephen P. Hesselbo  Pierre Pellenard 《Sedimentology》2019,66(7):2769-2783

Early Jurassic climate is characterized by alternating cold and warm periods highlighted by studies based notably on oxygen isotopes measured on belemnite guards and other marine invertebrate shells. These climatic changes include changes in the hydrological cycle, and consequently weathering and runoff conditions. In order to clarify the erosion and weathering conditions during the Pliensbachian, this study determined the mineralogical composition of the clay fraction of 132 samples taken from the entire stage drilled in the Llanbedr (Mochras Farm) borehole (Cardigan Bay Basin). The clay mineral assemblages are composed of various proportions of chlorite, illite, illite/smectite mixed‐layers (R1 I–S), smectite and kaolinite, with possibly occasional traces of berthierine. The occurrence of abundant smectite indicates that the maximum burial temperature never exceeded 70°C. Consequently, clay minerals are considered mainly detrital, and their fluctuations likely reflect environmental changes. The variations in the proportions of smectite and kaolinite are opposite to each other. Kaolinite is particularly abundant at the base of the jamesoni Zone, in part coinciding with the δ¹³C negative excursion corresponding to the Sinemurian/Pliensbachian Boundary Event, and through the davoei Zone, whilst smectite is abundant in the upper part of jamesoni and base of ibex zones and through the subnodosus/gibbosus subzones of the margaritatus Zone. The kaolinite‐rich intervals reflect an intensification of hydrolysis and an acceleration of the hydrological cycle, while the smectite‐rich intervals indicate a more arid climate. The spinatum Zone is characterized by a distinct clay assemblage with abundant primary minerals, R1 I–S, kaolinite reworked from previously deposited sediments or from Palaeozoic rocks, and probably berthierine originating from contemporaneous ironstone‐generating environments of shallower waters. This mineralogical change by the end of the Pliensbachian likely reflects a transition from a dominant chemical weathering to a deeper physical erosion of the continent, probably related to a significant sea‐level fall consistent with a glacio‐eustatic origin.  相似文献   

河北坝上丰宁黄土剖面记录的230 ka BP以来的环境演化^*          下载免费PDF全文

毛欣  刘林敬  李长安  姜高磊  张玉芬  赵红梅  毕志伟 《古地理学报》2017,19(4):745-756

河北坝上地区位于东亚季风气候边缘区,对气候变化响应敏感。丰宁黄土剖面为研究该区域长时间尺度的环境演化提供了理想材料。通过丰宁黄土剖面地球化学元素和磁化率分析,结合光释光测年结果,恢复了该地区230,ka BP以来的环境演化历史; 通过野外观察,识别出了S₁古土壤之下发育的风化淋滤黄土层,探讨了S₁古土壤发育时期的风化淋滤特征及其所揭示的气候变化问题。结果表明: (1)230,ka BP以来的磁化率记录可与深海氧同位素3—8阶段进行细节上的对比,表明丰宁黄土堆积区对全球气候变化有着积极的响应;(2)S₁古土壤发育时期,由于受到强降水的影响,土壤中元素受到强烈淋溶从而向下迁移到L₂黄土中,造成L₂上部地球化学记录与磁化率的差异;(3)从S₁古土壤顶部到钙结核层之间土壤属于酸性淋溶土,可能相当于现今中国北亚热带的黄棕壤,其剖面由土壤层、风化淋滤黄土层和CaCO₃淀积层构成;(4)S₁古土壤发育时期的温度和降水量与现今江苏泗洪和六合地区相当,表明当时研究区为亚热带气候。  相似文献   

Glacier reconstruction and mass‐balance modelling as a geomorphic and palaeoclimatic tool     

Simon J. Carr  Sven Lukas  Stephanie C. Mills 《地球表面变化过程与地形》2010,35(9):1103-1115

The reconstruction of former mountain glaciers has long been used to examine the implications of rapid climate shifts, for example at the last glacial–interglacial transition, and for evaluating asynchronous behaviour of mountain glaciers compared with mid‐latitude ice sheets during the Late Quaternary. Glacier reconstruction has also been used as a source of palaeoclimatic information, based on the recognition of empirical relationships between glaciers and climate. This paper reviews the application and implications of a recently revised method of glacier reconstruction (Carr and Coleman, 2007 ), based around glaciological principles of mass‐balance. This study examines how this approach can be used to test geomorphological interpretations of former mountain glaciation and also to infer precipitation fields at sites of former glaciation. Sites of Younger Dryas niche and icefield glaciation in the British Isles demonstrate how this method can verify interpretations of marginal glaciation and begin to understand the different behaviour of outlet glaciers within the same environmental regime. Examination of a site of former niche glaciation in Southern Africa demonstrates how glacier reconstruction may be used to infer annual and seasonal precipitation values and strongly supports the idea that winter precipitation in Lesotho and SE South Africa was substantially greater than present‐day values during the last glacial cycle. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

洱海沉积物化学元素与古气候演化   总被引：48，自引：2，他引：46  

陈敬安  万国江 《地球化学》1999,28(6):562-570

测定了洱海沉积物中常量和微量元素的含量,利用因子分析方法,识别了控制沉积物化学组成的三个主要因子：流域侵蚀因子、早期成岩因子和湖 内生碳酸钙了。进一步 研究还发现,流域了和湖泊内生碳酸钙沉淀因子的因子得分曲线作为气候干湿和冷暖变化的代用曲线。从而为古气候重建提供了一条有效的新途径。结合沉积的柱＾２１０Ｐｂ和＾１３７Ｃ计年结果,初步评价了６００多年来洱海区域气候及环境演化历史,提示他该区暖干与冷湿相  相似文献   

末次盛冰期以来长江三角洲地区的沉积相和古地理   总被引：16，自引：6，他引：10       下载免费PDF全文

李从先  陈庆强  范代读  张家强  杨守业 《古地理学报》1999,1(4):12-25

末次盛冰期低海平面期间,长江三角洲地区可划分为2个古地理单元：古河谷和古河间地。下切河谷底部侵蚀面和古河间地顶面构成了冰后期海侵沉积旋回的底界面,它相当于层序地层学中的层序界面。位于河口湾-浅海相中的最大海侵面将冰后期海侵沉积旋回分为其下的海侵层序和其上的海退层序。随着δ¹⁸O ₃期的海平面下降,长江开始下切,至δ¹⁸O ₂期低海面时形成巨大的下切河谷。冰后期海平面上升引发的海侵造成了长江古河谷系的充填和河床、河漫滩－河口湾和部分河口湾－浅海相的形成,尔后的进积产生了部分河口湾－浅海相及三角洲相等。溯源堆积是产生下部河流沉积单元的主要过程,其中河漫滩沉积中出现的潮汐层理和少量小个体有孔虫说明了海洋因素的影响,河口湾－浅海相泥质沉积主要形成于最大海侵之时,三角洲的进积则产生了具有多期河口坝的三角洲。古河间地表面的硬粘土层经历了沉积和成壤作用交替、持续成壤作用和早期成岩作用,它们大致分别对应于δ¹⁸O ₃期、δ¹⁸O ₂期和δ¹⁸O ₁期,硬粘土层中留下了这3种作用的烙印。长江三角洲古河间地的古土壤母质属河漫滩相。持续成壤阶段河流基面和地下水位均较低,年降雨量约为500～800 mm,相当于现今的温带地区,干湿周期变化明显,地下水升降频繁。所有这些表明,当时并非干旱气候。  相似文献   

珊瑚的高精度热电离质谱定年和Sr/Ca温度计对古环境研究意义     

王兆荣  彭子成  倪守斌  马志邦 《海洋地质与第四纪地质》1999,19(4)

珊瑚等海底沉积物中留下了过去气候变化的高分辨率记录,利用高分辨中的取样方法和热电离质谱（TIMS）铀系法对其进行测年,可以提取很短时间间隔内的环境参数,进一步证实了海洋珊瑚骨骼的Sr／Ca与海水温度间有很好的相关关系,是恢复古环境的一个有效方法。  相似文献   

Palaeoclimatic record from a loess–soil profile in northeastern Bulgaria—I. Rock magnetic properties     

Diana Jordanova  & Nikolai Petersen 《Geophysical Journal International》1999,138(2):520-532

A detailed record of mineral magnetic properties of a loess–palaeosol profile comprising seven loess horizons, six interbedded palaeosols and recent soil at the top in NE Bulgaria is analysed. A strong contrast between the soil and loess susceptibilities as well as other concentration-dependent hysteresis parameters is present, similar to the well-documented magnetic characteristics of the Chinese loess ( Hus & Han 1992 ; Maher & Thompson 1992 ; Heller & Evans 1995 ; Hunt et al. 1995 ). The magnetic enhancement of the palaeosol units is caused by very fine-grained pedogenic magnetite with superparamagnetic behaviour. Thermomagnetic analyses on bulk material suggest magnetite and maghemite as the main ferrimagnetic carriers in both soil and loess horizons. Their relative proportions are shown to reflect different palaeoclimatic conditions. Chernozem soils, which include recent soil S₀ and first and second palaeosols S₁ and S₂ developed under steppe vegetation, show a high degree of low-temperature oxidation of the pedogenic magnetite to maghemite. This material is characterized by coercive force H _c showing even higher values than those of the parent loess material. The older palaeosols (S₄ to S₆ ) were formed during more humid climatic conditions and therefore probably developed as forest types. Rock magnetic data suggest the existence here of only partly oxidized magnetite grains. The behaviour of the thermomagnetic curves, characterized by a kink at 200 °C, may be due to either a release of internal stress (built up as a result of partial low-temperature oxidation) or interactions between two phases.  相似文献   

中国末次冰盛期以来湖泊水量变化及古气候变化机制解释   总被引：2，自引：0，他引：2  

薛滨  于革 《湖泊科学》2005,17(1):35-40

中国古湖泊数据库收录的42个湖泊,提供的湖泊水量每千年变化的空间信息,可以用来系统分析中国区域末次冰盛期以来大气环流变化的状况.研究结果表明:我国西部从末次冰盛期以来直至全新世中期均为较湿润的气候状况,推测冰期内的湿润条件主要与西风带的降水以及低温低蒸发密切相关,而全新世主要为夏季风降水增加所致;全新世晚期气候趋干明显.我国东部的大部分区域在冰盛期和晚冰期较为干旱;只是在全新世有效降水状况才有大幅度的改善,全新世中期夏季风降水的效应仍然相当显著,控制的范围可达整个中国西部,同时位于现代季风气候区的中国东部,有效降水的峰值区的变化似乎存在从北往南的穿时性,南方有效降水峰值出现在晚全新世.而西南季风区湿润状况的明显改善发生在晚冰期,比东南季风区发生的早,显然这与两个季风系统的相互消长有一定的关系.我国东北区的湿润状况改善的也较早,显示了独特的季风气候机制.  相似文献