Paleoecological and climatic changes of the Upper Lerma Basin, Central Mexico during the Holocene

The record of Almoloya Lake in the Upper Lerma basin starts with the deposition of the late Pleistocene Upper Toluca Pumice layer. The data from this interval indicate a period of climatic instability that lasted until 8500 cal yr B.P., when temperature conditions stabilized, although moisture fluctuations continued until 8000 cal yr B.P. Between 8500 and 5000 cal yr B.P. a temperate climate is indicated by dominance of Pinus. From 5000 to 3000 cal yr B.P. Quercus forest expanded, suggesting a warm temperate climate: a first indication of drier environmental conditions is an increase in grassland between 4200 and 3500 cal yr B.P. During the Late Holocene (3300 to 500 cal yr B.P.) the increase of Pinus and grassland indicates temperate dry conditions, with a considerable increase of Pinus between 1100 and 950 cal yr B.P. At the end of this period, humidity increased. The main tendency during the Holocene was a change from humid to dry conditions. During the Early Holocene, Almoloya Lake was larger and deeper; the changing humidity regime resulted in a fragmented marshland, with the presence of aquatic and subaquatic vegetation types.     

Holocene vegetation and climatic variations in Central India: A study based on multiproxy evidences

Palynology, texture, mineralogy, geochemistry, and magnetic susceptibility analysis of a 2 m deep sediment core from Padauna Swamp, southeastern Madhya Pradesh infers that between 8600 and 7500 cal yr BP a warm and relatively less-humid climate prevailed with open tree-savannahs dominated by grasses followed by sedges, Artemisia and members of Chenopodiaceae/Amaranthaceae with scanty trees viz., Schrebera, Aegle marmelos and Sterculia urens. This is well supported by lower organic to carbonate carbon ratio, coarser texture having relatively low CIA and magnetic susceptibility values and presence of some primary minerals. Between 7500 and 6250 cal yr BP the tree-savannahs were succeeded by open mixed deciduous forests with the invasion of a few more trees viz., Madhuca indica, Holoptelea, Emblica officinalis, Mitragyna parvifolia and members of Anacardiaceae in response to onset of a warm and humid climate. A considerable rise in organic carbon generated from the degradation of plentiful biomass along with increase in clay content with signs of kaolinite and increase in immobile over mobile elements with slightly higher CIA and magnetic susceptibility values also suggest climatic amelioration. The presence of ruderal plants such as Artemisia, Cannabis sativa and Cheno/Am further infers initiation of human activities in the region. Between 6250 and 2800 cal yr BP, the mixed deciduous forests became more diverse and dense, subduing grasses and other herbaceous elements. Sporadic incursion of Shorea robusta (Sal) in forest floristic was recorded around 5000 cal yr BP. The overall change in the vegetation mosaic reflects that a warm and more-humid climate prevailed in the region, probably on account of invigoration of southwest monsoon. This observation is further corroborated by other proxy data showing a spurt in organic/inorganic carbon ratio, increase in clay content with matured mineralogy, significantly higher CIA and magnetic susceptibility values. Since 2800 cal yr BP onwards, the modern Sal dominated deciduous forests were established indicating continuation of warm and more-humid climate including timely arrival of SW monsoon coinciding with the shedding of Sal seeds as they are viable for a very short period.     

Holocene paleoenvironmental changes inferred from diatom assemblages in sediments of Kusawa Lake, Yukon Territory, Canada

The southwest Yukon Territory, Canada, is an important region for recovering sensitive records of Holocene paleoclimatic change. More information is needed, however, to constrain the timing of the major Holocene climatic transitions, and to understand associated impacts on different ecosystems. For example, paleolimnological studies have focused on small lakes and ponds, but the history of large lakes has received little study. We analyzed diatom assemblages, species richness, valve concentrations, and biogenic silica, in the sediments of Kusawa Lake (60°16.5'N; 136°10.9'W; 671 m a.s.l.) to reconstruct the responses of this large (surface area = 142 km²), deep (Z_max = 135 m) freshwater ecosystem to Holocene climatic transitions. Diatoms colonized the lake soon after ice retreat, around 11,000 cal yr BP; assemblages throughout the record were dominated by planktonic types. Diatom concentrations and biogenic silica were high during the Holocene Thermal Maximum between 10,700 and 7300 cal yr BP, then began to decrease in response to cooling associated with orbitally driven reductions in insolation. Diatom assemblages shifted towards taxa with lower surface water temperature optima after 8300 cal yr BP, perhaps in response to abrupt and progressive cooling. Our study confirms that diatom assemblages in large lakes are sensitive to regional-scale paleoclimatic changes.     

Biogeochemistry and paleoclimate variability during the Holocene: a record from Mansar Lake,Lesser Himalaya

Investigation of biogeochemistry and amino acids on a 30-m-long core from Mansar Lake has thrown light on palaeoclimate variability during the Holocene period. The C/N ratio between 6 and 10 with some deviations and δ13C mostly between −20 and −22‰ in the shallow core, as well as a C/N ratio greater than 13 reaching from 19 to 20 and correspondingly lower δ13C of –28‰ in the deeper core suggest an aquatic source of carbon in the former and a cellulose-rich land plant source in the latter. This is supported by the abrupt increase in organic carbon content in the deeper core compared to the shallow core, which indicates a hot and wet climatic regime during the early Holocene and dry and cold during the late Holocene period. The amino acid data AA-C/C% and AA-N/N% are higher in shallow sediments compared to deeper sediments, indicating an aquatic plant source in the shallow core and greater supply of land plant sources in the deeper core. The lower percentage of the non-protein amino acids β-alanine (ALA) and γ-amino-butyric acid in the shallow core compared to the deeper core indicates different sources of organic matter in the lake basin. The higher amino acid ratio Asp/β-Ala (16.99 av.) and Glu/γ-Aba (18.18 av.) in the shallow core and lower ratios (10.32 and 12.41 av.) in the deeper core, and Asp/Glu (1.52 av.) and β-Ala/γ-Aba (1.61 av.) ratios in the former, which are potential indicators of the nature of the organic matter, are higher in the shallow core relative to the deeper core (1.33 and 1.23 av.), indicating relative biodegradation of organic matter in deeper sediments. It has been observed that the organic matter associated with the dry season is relatively less biodegraded, as evidenced from their higher ratios, and is more biodegraded in the wet season as their ratios are lower in the river sediments. In the absence of a bacterial contribution of organic matter from the soil source in this lake, since Mansar Lake is a non-drainage type, it is envisaged that the climatic variation may be responsible for biodegradation in the deeper core sediments. Therefore, the C/N ratio and δ13C values supported by amino acid data, the latter being significant in revealing primary productivity and a terrestrial source of organic matter, suggest a hot and wet climatic regime during the early Holocene (ca. 7580 bp) and a dry and cold in the late Holocene period (ca. 4050 bp).     

Palynological evidence for climatic and oceanic variability off NW Africa during the late Holocene

Pollen and organic-walled dinoflagellate cyst assemblages from core GeoB 9503-5 retrieved from the mud-belt ( 50 m water depth) off the Senegal River mouth have been analyzed to reconstruct short-term palaeoceanographic and palaeoenvironmental changes in subtropical NW Africa during the time interval from ca. 4200 to 1200 cal yr BP. Our study emphasizes significant coeval changes in continental and oceanic environments in and off Senegal and shows that initial dry conditions were followed by a strong and rapid increase in humidity between ca. 2900 and 2500 cal yr BP. After ca. 2500 cal yr BP, the environment slowly became drier again as indicated by slight increases in Sahelian savannah and desert elements in the pollen record. Around ca. 2200 cal yr BP, this relatively dry period ended with periodic pulses of high terrigenous contributions and strong fluctuations in fern spore and river plume dinoflagellate cyst percentages as well as in the fluxes of pollen, dinoflagellate cysts, fresh-water algae and plant cuticles, suggesting “episodic flash flood” events of the Senegal River. The driest phase developed after about 2100 cal yr BP.     

新疆阿尔泰全新世双湖沉积物正构烷烃分布及其环境意义

文章对新疆阿尔泰双湖沉积物进行了古气候、古环境变化研究。双湖（48°52'N,87°02'E）位于阿尔泰山中部,是山谷岩石滑塌阻塞形成的堰塞湖,湖长约1200m,宽度约300m,最大水深9m。项目使用重力钻在双湖中心钻取了172cm无扰动岩芯,根据核素测年（¹³⁷Cs,²¹⁰Pb,AMS¹⁴C）建立了9.1cal. ka B.P.以来的时间标尺,以正构烷烃分布特征（ACL_23~33、nC₂₇/nC₃₁、P_aq）为指标,重建了该区的古植被、古气候变化。沉积物中正构烷烃以高碳数为主,中长链正构烷烃存在显著的奇碳优势。链长指标（ACL_23~33）、nC₂₇/nC₃₁指标、P_aq之间具有很强的相关性关系,表明指标可能受到相似的气候驱动因素影响,反映了区域植被和湖泊水生生物的演替历史：9.1~8.8cal. ka B.P.期间水生植被输入较少,陆生植被中草本植物相对占比较高;8.8~3.3cal. ka B.P.期间大型水生植物大量繁殖,汇水区木本植物增多;3.3cal. ka B.P.至今湖泊周围的木本植物减少,草本植物相对占比增多,水生植物输入减少。双湖沉积物ACL_23~33指标自9.1cal. ka B.P.以来呈逐渐增加的趋势,可能表明温度逐渐增加,这一结果与前人在该区利用泥炭br-GDGT重建的温度序列具有相似的变化趋势,但在百-千年尺度上,温度变化不一致,可能与指标的敏感性和定年精度有关。

     

新疆四棵树地区西山窑组孢粉组合及古气候意

四棵树凹陷位于准噶尔盆地南缘西段乌苏市境内,蕴含丰富的油气和煤炭资源。由于匮乏化石证据,制约四棵树凹陷侏罗纪地层的地质时代的确定。对西山窑组沉积环境和孢粉分析研究,恢复重建了其古环境,探讨了本区中西山窑时期的植被演替及环境演变。西山窑组岩性以泥质细砂岩、浅灰绿色泥岩、粉砂岩、泥岩为主。通过对孢粉化石分析研究,建立了Cyathidites minor—Osmundacidites —Cycadopites—Disacciatrileti（COCD）孢粉组合。组合中蕨类植物孢子与裸子植物花粉含量大体相当,蕨类植物孢子以紫萁科的Osmundacidites为主,桫椤科孢子也有一定含量,其他常见分子包括Deltoidospora、Neoraistrickia等,裸子植物以松柏类两气囊花粉含量为主,均为我国侏罗纪常见属种。通过对西山窑组孢粉植物群的分析,认为西山窑组孢粉组合时代属于中侏罗世早期,该孢粉植物群总体上反映了温暖湿润的热带—亚热带气候特点,植被类型以针阔混交林为主。     

Palynological Evidence for Climatic Change and Human Activity during the Holocene on Minorca (Balearic Islands)

Four pollen diagrams from Minorca (Balearic Islands) have been correlated with other previously studied sequences from Majorca and Minorca to define a Holocene landscape sequence for the region from 8000 yr B.P. to the present. The lower part of the pollen diagrams reflects a climatic phase with more rain and less-marked seasonality than today. Significant quantities ofCorylus, Buxus,and mesophilous taxa are found. In the middle part, between 5000 and 4000 yr B.P., a strong change is recorded in composition and structure of the vegetational landscape, with vegetation appearing that was adapted to Mediterranean conditions. This episode coincided with the first human colonization of the island and also with a widespread climatic change in the western Mediterranean region. The change in taxa was complex and some sclerophyllous taxa suchOleaplayed an important role in the transformation of the landscape physiognomy from the mid-Holocene until the present. Although human activities have removed much of the Mediterranean vegetation on the Balearic Islands, it seems clear that the changes have been brought about, in part, by increasing dryness.     

晚新生代以来天山北缘沉积记录及其稳定同位素变化的环境意义

晚新生代以来,天山北缘的沉积记录与天山的隆升密切相关.研究表明:始-渐新世安集海河组主要沉积发育水平层理的绿色、灰绿色湖泊相泥岩;晚渐新世沙湾组中上部岩石组合特征为紫红色曲流河相砂岩、粉砂岩和砾岩夹层;早中新世塔西河组下部主要为灰绿色、褐红色砂、泥岩互层,上部主要为土黄色砂岩和粉砂岩夹层;晚中新世-早上新世独山子组为辫状河相灰色砾岩和砂砾岩沉积;晚上新世-早更新世西域组主要为厚层砾岩堆积.土壤碳酸盐中稳定同位素数据表明:安集海河组δ13C平均值为-3.9‰,δ18O平均值为-5.0‰;沙湾组中上部δ13C平均值为-8.1‰,δ18O平均值为-11.0‰;塔西河组δ13C平均值为-5.9‰;δ18O平均值为-7.7‰;独山子组δ13C平均值为-8.5‰,δ18O平均值为-10.3‰;西域组δ13C平均值为-6.0‰,δ18O平均值为-10.4‰.同位素结果和沉积记录共同表明,始-渐新世为湿润温暖气候;晚渐新世为干旱寒冷气候;早中新世早期为半湿润、半温暖气候,晚期为干旱、寒冷气候;晚中新世.早上新世为干旱寒冷气候;晚上新世-下更新世为干旱、半温暖气候.结合古地磁数据和全球气候变化,天山北缘晚新生代以来发生五次主要气候变化事件,即晚渐新世(沙湾组时期中上部)气候转换,约24Ma(沙湾组时期顶部)气候突变,早中新世内部(约24～19Ma)气候转换事件,晚中新世(约7Ma)C4植被迅速扩张和约3.17Ma以来气候的剧烈震荡.     

广东雷州半岛晚渐新世—早更新世孢粉共存因子分析及古气候变化重建

广东省雷州半岛新生代钻孔ZKA01揭露的地层序列自下向上为渐新统涠洲组、中新统下洋组、角尾组、灯楼角组、上新统望楼港组、下更新统湛江组和中更新统北海组,涠洲组—望楼港组为滨浅海沉积,湛江组和北海组为陆相河湖相沉积。本文在ZKA01钻孔地层中自下向上88个层位中获取的29311粒孢粉化石的81个属中,选取了常见的种子植物花粉种属42个,通过共存因子分析法,定量重建了研究区晚渐新世—早更新世的古气候参数,划分出晚渐新世—早中新世(25~17 Ma)、中中新世(17~13.5 Ma)、晚中新世—上新世初期(13.5~4 Ma)和上新世—早更新世(4~1.5 Ma)4个气候演化阶段。孢粉共存因子定量法重建的研究区晚渐新世—早更新世4个阶段的古气候变化过程能较好地与全球气候变化的趋势相匹配,晚渐新世—早中新世温度降低的时间拐点大致可与Mi1a气候变冷事件相吻合。中中新世可以与中中新世气候适宜期(MMCO)相对应,表现为炎热潮湿的气候特征。中中新世晚期的气温是下降的,在一定程度上响应了Mi3中中新世气候变冷事件。     

Stratigraphy and paleoenvironmental changes in the Yangtze Delta during the Late Quaternary

The Yangtze Delta area may be subdivided into paleo-valley and paleointerfluves of the last glaciation. The postglacial transgressive sedimentary cycle (PTSC) on the front zones of the paleointerfluves is composed of marsh-nearshore and barrier-lagoon, shallow marine and nearshore-tidal flat units, with a basement in stiff clay. The PTSC in back zones of the paleointerfluves consists of lacustrine-marsh deposits. The PTSC in the incised valley contains river channel, floodplain-estuary, estuarine-shallow marine and deltaic units, with an erosional surface at its bottom. The stiff clay and the erosional surface constitute the PTSC lower boundary.The stiff clay, consisting of a paleosol, experienced deposition alternating with pedogenesis during the falling of sea-level during δ¹⁸O stage 3; ongoing pedogenesis in the sea-level lowstand of δ¹⁸O stage 2; and early diagenesis after paleointerfluve inundation by the sea-level rise of δ¹⁸O stage 1. The climate during the paleosol formation was temperate with more than 500-mm a⁻¹ rainfall and frequently fluctuating groundwater.The Yangtze River incised its course during falling sea level of stage 3, and a huge incised valley was formed during the sea-level lowstand (stage 2). The filling of the incised valley took place during the postglacial sea-level rise, and delta formation occurred at a late stage of PTSC development.     

上海东部地区全新世孢粉组合及古植被和古气候

通过对上海东部地区南汇鹤鸣Hm孔、东海Dh1井、高桥G2孔3口钻井全新世地层的孢粉研究,划分出5个孢粉组合带和3个亚带,恢复了本地区古植被演替和古气候波动的5个阶段：第1阶段为针阔叶混交林-草地,反映气候温凉略湿（前北方期）;第2阶段为含常绿阔叶树的针阔叶混交林,反映气候温和略干（北方期）;第3阶段为以常绿栎类、栲属、杨梅等为主的常绿阔叶林,反映气候热暖潮湿（大西洋期）;第4阶段是以栎、松、禾本科为主的针阔叶混交林,反映气候温暖略干（亚北方期）;第5阶段是以落叶栎类、常绿栎类、松为主的落叶阔叶、常绿阔叶、针叶混交林-草地,反映气候温暖湿润（亚大西洋期）。这为本地区的地层年代划分和对比提供了可靠的证据,为全新世古植被、古气候及古环境的重建提供了重要的孢粉学资料。     

川东北早全新世降水变化及其意义

岩溶洞穴沉积是研究古气候环境变化的重要地质材料[1-2]。可以高精度定年和高分辨率釆样的石笋为研究一些短时间尺度气候事件如厄尔尼诺-南方涛动(ENSO)提供了重要信息,如Frappier等[3]发现中美洲伯利兹一支石笋碳同位素很好地记录了1971-2000年南方涛动指数(S0I)变化.     

Climate variability in the Aral Sea basin (Central Asia) during the late Holocene based on vegetation changes

High-resolution pollen analyses ( 50 yr) from sediment cores retrieved at Chernyshov Bay in the NW Large Aral Sea record shifts in vegetational development from subdesertic to steppe vegetation in the Aral Sea basin during the late Holocene. Using pollen data to quantify climatic parameters, we reconstruct and date for the first time significant changes in moisture conditions in Central Asia during the past 2000 yr. Cold and arid conditions prevailed between ca. AD 0 and 400, AD 900 and 1150, and AD 1500 and 1650 with the extension of xeric vegetation dominated by steppe elements. These intervals are characterized by low winter and summer mean temperatures and low mean annual precipitation (P_mm < 250 mm/yr). Conversely, the most suitable climate conditions occurred between ca. AD 400 and 900, and AD 1150 and 1450, when steppe vegetation was enriched by plants requiring moister conditions (P_mm  250–500 mm/yr) and some trees developed. Our results are fairly consistent with other late Holocene records from the eastern Mediterranean region and the Middle East, showing that regional rainfall in Central Asia is predominantly controlled by the eastern Mediterranean cyclonic system when the North Atlantic Oscillation (NAO) is in a negative phase.     

秦岭东段卢氏盆地始新世沉积序列与气候环境变迁

始新世是新生代重要的暖期,其气候环境的特征与演化备受学界关注。位于秦岭东段的卢氏盆地保存了>1000 m厚的连续的始新世河湖相沉积物,是揭示古气候和环境变化规律的宝贵沉积记录。在前人工作基础上,本研究利用沉积学、古生物学、碎屑年代学等方法对卢氏盆地始新世地层的沉积特征、年代和环境进行了分析。结果表明：1）碎屑磷灰石裂变径迹与碎屑锆石U-Pb年龄指示沉积序列年代晚于约60 Ma,与前人将张家村组-卢氏组化石组合对比到中始新世的结论基本吻合。2）始新统张家村组沉积相演化表现为冲积扇→河流→洪泛平原或滨湖,反映盆地拉张、初期成湖的过程;卢氏组为滨浅湖→深湖/半深湖→滨浅湖或洪泛平原→浅湖/半深湖,反映湖泊开始稳定出现后的扩张、收缩和复活过程;大峪组为河流→洪泛平原→冲积扇,反映湖泊淤满消亡后河流与冲洪积物相继充填的历史。3）沉积相、化石组合、盐类矿物种类揭示中始新世卢氏盆地古气候为半湿润半干旱,地表景观呈现径流活跃、动物种类多样、树木茂盛、发育淡水至微咸水湖的特征,并可能具有一定的季节性。卢氏盆地沉积序列提供了始新世暖期中国中部气候环境变化的重要证据。

     

准噶尔盆地西南缘四棵树地区西山窑组孢粉组合及古气候意义

四棵树凹陷位于准噶尔盆地南缘西段乌苏市境内,蕴含丰富的油气和煤炭资源.由于匮乏化石证据,制约四棵树凹陷侏罗纪地层的地质时代的确定.对西山窑组沉积环境和孢粉分析研究,恢复重建了其古环境,探讨了本区中西山窑时期的植被演替及环境演变.西山窑组岩性以泥质细砂岩、浅灰绿色泥岩、粉砂岩、泥岩为主.通过对孢粉化石分析研究,建立了Cy...     

Mineralogy and palynology of the Mesopotamian plain sediments, Central Iraq

This work was done on the representative section (230 cm) in Dora site which located in Baghdad in the central of the Mesopotamian Plain. Eleven soil samples were examined mineralogically and palynologically. Grain size analyses showed that silt is the higher fraction (61%), clay is the least (9%), whereas sand forms 30%. Translucent and opaque heavy minerals contributed 57% from the sand fraction, whereas the light minerals formed just 43%. The type of heavy minerals and its maturity defined four provenances—they are granitic, basic igneous and metamorphic rocks—that belong to the high lands of south Turkey and north of Iraq. The fourth provenance is the sedimentary rocks which are located north of Mesopotamian Plain around the Tigris River and their tributaries. Dinoflagellate within thin bed of marine origin clearly indicated the transgression of the Arabian Sea northward through Tigris and Euphrates valleys or high marine waves reached to Baghdad before 10,000 years P.B. approximately. This means that the marine sediments were also an additional source of the Mesopotamian Plain. Three specific climatic features are identified via palynological interpretation; a temperate-wet climate extended from 10,000 to 5,000 years P.B. followed by dry period extended from 5,000 to 3,500 years P.B. and the last climatic feature is characterized by oscillation of wet to dry periods which is dominant since 3,500 till now.     

Late Pleistocene to early Holocene lake level and paleoclimate insights from Stansbury Island, Bonneville basin, Utah

This paper reports on recent multiproxy research conducted to determine the chronology of lake-level fluctuations recorded in sediments from a natural exposure at a classic Bonneville basin site. Grain size, carbonate percentage, magnetic susceptibility, amount of charcoal, and diatom community composition data were collected from the 16 lacustrine units that compose the 122 cm stratigraphic column in Stansbury Gulch. Trends observed in the measured proxies reveal several significant changes in lake level, and thereby effective moisture, over the approximately 14,500 yr time span represented by the sediments. Results (1) verify the effectiveness of the multiproxy approach in Bonneville basin studies, which has been underutilized in this region, (2) reaffirm the double nature of Lake Bonneville's Stansbury oscillation, (3) suggest a previously undocumented post-Gilbert highstand of Great Salt Lake, and (4) identify possible teleconnections between climate events in the Bonneville basin and events in the North Atlantic at about 20,500 and 7500 ¹⁴C yr BP.     

Evaluation of Sand Dunes Stabilization Techniques in Baiji District, Iraq

INTRODUCTIONAsandduneisanyaccumulationofsand grainsshapedintoamoundorridgebythewindunderthein fluenceofgravity .Sanddunesarecomparabletootherformsthatappearwhenafluidmovesoveraloosebed .Dunesarefoundwhereverloosesandiswindblown(Bagnold ,196 0 ) .MostofIraqianlandsareaffectedbywinderosion ,theformationandmovementofsanddunes ,particularlyinthemiddleandsouthofIraq .Thetotalareacoveredbysanddunesisapproximately 2mil lionshectares ,andthreatensmanyothermillionshectares (DougramejiandKaul,197…     

Formation of Ulan Buh desert and its environmental changes during the Holocene

Extensive investigations and studies on topography, sedimentary and chronology show new evidence for the formation and evolution of the Ulan Buh desert during early Holocene. Evidence on clay-sand strata and plant roots under interdune lowlands, lake shorelines covered by plenty of Corbicula largillierti and large amounts of dry salt lakes in the central region of the desert prove that many megalakes existed in the hinterland of Ulan Buh desert. Several OSL samples collected from Aeolian sands overlying lacustrine sediments in profiles around Jilantai Salt Lake and interdune lowlands in the southern Ulan Buh desert suggest that the desert began around 7 ka B.P.. The formation of Ulan Buh desert may have resulted from the shrinking of Jilantai megalakes and sands blown from exposed loose sediments. __________ Translated from Journal of Desert Research, 2007, 27(6): 927–931 [译自: 中国沙漠]