Late Cenozoic magmatism of the Bolivian Altiplano

Small basalt to dacite volcanic centers are distributed sparsely over the Bolivian Altiplano, behind the Andean volcanic front. Most are Pliocene to Recent in age, and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts. True phenocrysts are rare in the more mafic samples. Compared with Recent volcanic rocks from Andean stratovolcanoes, the Bolivian centers overlap in major element trends. Incompatible element contents tend to be higher, particularly in the eastern Altiplano. The ranges of isotopic compositions reflect ubiquitous crustal contamination. Pb isotope compositions are dominated by Pb from isotopically heterogeneous basement, resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region. Rocks sampled from the Bolivian Altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region. Major element trends are largely controlled by the fractionating phase assemblage (olivine, clinopyroxene and amphibole). Trace element and isotope systematics, however, defy realistic attempts at modeling due to the geographic scatter of samples, the uniformity of compositions at a given center, and the heterogeneity of the contaminant. Nevertheless, there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone. In particular, LIFE/HFSE (exemplified by Ba/Nb), and Zr/Nb ratios decrease from the arc front eastward into the Altiplano. These variations are not easily reconciled with control by crustal contamination alone. A model is proposed in which the asthenospheric source is fluxed by high Ba/Nb slab-derived fluid to induce melting. Beneath the arc, high fluid flux increases the Ba/Nb ratio of the asthenosphre and leads to high degrees of partial melting (high Zr/Nb). To the east, lower or no fluid flux leads to low Ba/Nb and low degrees of partial melting (low Zr/Nb). Melting in the back arc region of the Altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter-flowing asthenosphere. There is no unequivocal evidence that requires a significant role for the lithospheric mantle.     

Late Pleistocene glaciation of the Kleiner Arbersee area in the Bavarian Forest,south Germany

During Pleistocene mountain glaciation of the Bavarian Forest, south Germany, the Wurmian Kleiner Arbersee glacier left behind glacial landforms and sediments which are described, classified and interpreted using a combination of geomorphological, sedimentological, pedological, surveying and absolute dating methods. The latest Kleiner Arbersee glacier with a maximum length of 2600 m, a minimum width of 800 m and a thickness of 115 m formed an elongated cirque, four lateral moraines, one divided end moraine, one recessional moraine, a proglacial lake and a basin in which lake Kleiner Arbersee was established after deglaciation. Beyond the glacial limit the landscape is denuded by periglacial slope deposits which are differentiated from the glacigenic sediments based upon clast fabrics, clast shapes and sediment consolidation. Within the glacial limit sandy–gravelly to silty–gravelly tills are widely distributed, whereas glaciolacustrine sediments are restricted to a small area north of the lake. Small variations in the sand and silt fraction of the tills are explained by melt-out processes. Quartz, mica and chlorite derived from gneiss bedrock are dominant in the clay mineral spectrum of tills, but also gibbsite as a product of pre-Pleistocene weathering is present giving evidence of glacially entrained saprolites. An IRSL-date of glaciolacustrine sediments (32.4±9.4 ka BP) confirms the Wurmian age for the glaciation and radiocarbon ages of the basal sediments (12.3±0.4 and 12.5±0.2 ka BP uncalibrated) in the lake Kleiner Arbersee prove that the basin was ice-free before the Younger Dryas.     

Late Pleistocene glacial chronology of the Taylor Valley,Antarctica, and the global climate

Carbonate-rich lacustrine and deltaic deposits, containing thin beds of finely laminated carbonates and thick beds of silt, crop out at several sites in the Taylor Valley and have been encountered in cores obtained by the Dry Valley Drilling Project (DVDP). Fragments of the more indurated carbonate beds have widespread occurrence as part of the desert “lag gravel” which covers much of the valley floor. Analysis of the carbonates suggests that they were deposited as algal limestones from waters derived from the East Antarctic Ice Sheet via the Taylor Glacier at times which correspond to the previous three global interglacial periods, as evidenced by the ice volumes deduced from oxygen-isotopic analysis of oceanic cores. The lacustrine carbonates have been found up to 30 km beyond the present terminus of the Taylor Glacier, and up to 100 m above the level of Lake Bonney, into which the Taylor Glacier at present discharges. It is concluded that the Taylor Glacier has advanced during each of the previous three interglaciations, and it is suggested that this has been caused by a thickening of the East Antarctic Ice Sheet during the interglaciations.     

Late Pleistocene and Holocene vegetation and climate on the northern Taymyr Peninsula, Arctic Russia

Pollen data from a Levinson-Lessing Lake sediment core (74°28'N, 98°38'E) and Cape Sabler, Taymyr Lake permafrost sequences (74°33'N, 100°32'E) reveal substantial environmental changes on the northern Taymyr Peninsula during the last c. 32 000 [Formula: See Text]C years. The continuous records confirm that a scarce steppe-like vegetation with Poaceae, Artemisia and Cyperaceae dominated c. 32 000-10 300 [Formula: See Text]C yr BP, while tundra-like vegetation with Oxyria, Ranunculaceae and Caryophyllaceae grew in wetter areas. The coldest interval occurred c. 18 000 yr BP. Lateglacial pollen data show several warming events followed by a climate deterioration c. 10 500 [Formula: See Text]C yr BP, which may correspond with the Younger Dryas. The Late Pleistocene/Holocene transition, c. 10 300-10 000 [Formula: See Text]C yr BP, is characterized by a change from the herb-dominated vegetation to shrubby tundra with Betula sect. Nanae and Salix. Alnus fruticosa arrived locally c. 9000-8500 [Formula: See Text]C yr BP and disappeared c. 4000-3500 [Formula: See Text]C yr BP. Communities of Betula sect. Nanae, broadly distributed at c. 10 000-3500 [Formula: See Text]C yr BP, almost disappeared when vegetation became similar to the modern herb tundra after 3500-3000 [Formula: See Text]C yr BP. Quantitative climate reconstructions show Last Glacial Maximum summer temperature about 4°C below the present and Preboreal (c. 10 000 [Formula: See Text]C yr BP) temperature 2-4°C above the present. Maximum summer temperature occurred between 10 000 and 5500 [Formula: See Text]C yr BP; later summers were similar to present or slightly warmer.     

黔南晚二叠世深水相地层及双壳类动物群

根据生态环境和沉积环境的差异 ,黔南晚二叠世生物群和地层可分出两种类型 :“浅海台地型”和“次深海裂陷槽型”,后者为深水型。简述了深水相地层的特征 ,据地层内所含化石确证了地层时代 ;据所含菊石、遗迹化石及围岩的沉积特点和类型 ,证实研究区晚二叠世时为深水环境 ;较系统地研究了黔南晚二叠世深水相双壳类动物群 ,它们的特征为薄壳、壳体较小、个体数量多、壳表以光滑和同心饰为主。化石中以 Claraia,H unanopecten最为发育 ,建立了 H u-nanopecten exilis-Claraia primitiva组合 ,并与国内同期的双壳类组合进行了比较。     

Clay record of climate change since the mid‐Pleistocene in Jiujiang,south China

To obtain a better understanding of climate change in south China in the Quaternary, a clay mineralogical study was undertaken on the red earth profile at Jiujiang, using X‐ray diffraction (XRD) and particle‐size distribution analysis methods. The XRD results showed that the clay minerals of the Jiujiang red earth were mainly mixed‐layer illite–smectite (I/S), illite, kaolinite and vermiculite, with trace amounts of mixed‐layer kaolinite–smectite (K/S). Changes in clay mineral composition displayed a trend of three‐stage evolution. The higher mixed‐layer I/S clays and kaolinite contents in the lower portion suggest extremely warm and humid climates over the period c. 700 to c. 350 ka ago. A gradual decrease in I/S clays and kaolinite reveals a gradual climate change from warm/humid to cool/dry during the period c. 350 to c. 130 ka ago. The higher illite and vermiculite contents indicate a relatively cool and dry climate during the period since c. 130 ka ago. The particle‐size distribution pattern of the upper section was similar to that of the Xiashu loess, while that of the middle to lower section was similar to those of fluvially reworked red earth. A rapid increase in the abundance of large grain‐size components at 2.6 m depth indicates an intensification of the winter monsoon and a cool and dry climate during the period, in good agreement with results from the clay mineral composition and homogeneous structure. The red earth sequences in south China could probably be used to test the response of tropical to subtropical regions to global climate changes.     

莱州湾南岸滨海平原晚更新世以来古环境演变的孢粉记录

依据莱州湾南岸A1钻孔孢粉分析，结合14C、热释光测年、沉积物粒度、岩性等资料，对A1钻孔自下而上划分为7个孢粉组合带，并概括它们的主要特征。利用孢粉组合，探讨了120 ka BP 以来莱州湾南岸咸水入侵区的植被发展以及气候的冷暖、干湿交替变化。在85～76 ka BP、50～24 ka BP和10～4 ka BP出现三次暖湿期，分别对应于“羊口海侵”、“广饶海侵”和“垦利海侵”。在三次暖湿期中发生海陆交互相滨岸沼泽或湖泊相沉积，植被类型为落叶阔叶林或针阔叶混交林滨岸草原。在76～50 ka BP和24～10 ka BP两次冷干期为陆相沉积环境，前者出现以针叶林为主的草原植被类型，后者出现干冷的针叶林干旱草原或荒漠草原植被类型，它们分别对应于早大理亚冰期和晚大理亚冰期。孢粉组合所反映的莱州湾南岸晚更新世以来的环境演化同全球性的气候事件及渤海沿岸环境变化具有很好的可比性。     

Late Pleistocene climate change,nutrient cycling,and the megafaunal extinctions in North America

This study proposes an ecological mechanism for the terminal Pleistocene population collapse and subsequent extinction of North American megafauna. Observations of modern ecosystems indicate that feedback mechanisms between plant nutrient content, nitrogen cycling, and herbivore–plant interactions can vary between a nutrient accelerating mode favoring increased herbivore biomass and a nutrient decelerating mode characterized by reduced herbivore biomass. These alternate modes are determined largely by plant nitrogen content. Plant nitrogen content is known to be influenced by atmospheric CO₂ concentrations, temperature, and precipitation. It is argued that Lateglacial climate change, particularly increases in atmospheric CO₂, shifted herbivore–ecosystem dynamics from a nutrient accelerating mode to a nutrient decelerating mode at the end of the Pleistocene, leading to reduced megafaunal population densities. An examination of Sporormiella records – a proxy for megaherbivore biomass – indicates that megafaunal populations collapsed first in the east and later in the west, possibly reflecting regional differences in precipitation or vegetation structure. The fortuitous intersection of the climatically driven nitrogen sink, followed by any one or combination of subsequent anthropogenic, environmental, or extra-terrestrial mechanisms could explain why extinctions took place at the end of the Pleistocene rather than during previous glacial–interglacial cycles.     

Late Pleistocene and Holocene cool‐water carbonates of the Western Mediterranean Sea

Post‐glacial, neritic cool‐water carbonates of the Western Mediterranean Sea were examined by means of hydroacoustic data, sediment surface sampling and vibrocoring to unravel geometries and to reconstruct sedimentary evolution in response to the last sea‐level rise. The analysed areas, located on the Alboran Ridge, in the Bay of Oran, and at the southern shelf of the island of Mallorca, are microtidal and bathed by oligotrophic to weakly mesotrophic waters. Seasonal water temperature varies between 13 °C and 27 °C. Echosounder profiles show that the Bay of Oran and the southern shelf of Mallorca are distally steepened ramps, while the Alboran Ridge forms a steep‐flanked rugged plateau around the Alboran Island. In the three areas, an up to 10 m thick post‐glacial sediment cover overlies an unconformity. In Oran and Mallorca, stacked lowstand wedges occur in water depths of 120 to 130 m. On the Alboran Ridge and in the Bay of Oran, highstand wedges occur at 35 to 40 m. Up to 5 m long cores of upper Pleistocene to Holocene successions were recovered in water depths between 40 and 81 m. Deposits contain more than 80% carbonate, with mixed carbonate‐volcaniclastics in the lower part of some cores in Alboran. The carbonates consist of up to 53% of aragonite and up to 83% of high magnesium calcite. Radiocarbon dating of bivalve shells, coralline algae and serpulid tubes indicates that deposits are as old as 12 400 cal yr bp . The carbonate factories in the three areas are dominated mostly by red algae, but some intervals in the cores are richer in bivalves. A facies rich in the gastropod Turritella, reflecting elevated surface productivity, is restricted to the Mallorca Shelf. Rhodoliths occur at the sediment surface in most areas at water depths shallower than 70 m; they form a 10 to 20 cm thick veneer overlying rhodolith‐poor bioclastic sediments which, nonetheless, contain abundant red algal debris. This rhodolith layer has been developing for the past 800 to 1000 years. Similar layers at different positions in the cores are interpreted as reflecting in situ growth of rhodoliths at times of reduced net sedimentation. Sedimentary successions in the cores record the post‐glacial sea‐level rise and the degree of sediment exposure to bottom currents. Deepening‐upward trends in the successions are either reflected by shallow to deep facies transitions or by a corresponding change of depth‐indicative red algae. There are only weak downcore variations of carbonate mineralogy, which indicate that no dissolution or high magnesium to low magnesium calcite neomorphism occurs in the shallow subsurface. These new data support the approach of using the Recent facies distribution for interpretation of past cool‐water, low‐energy, microtidal carbonate depositional systems. Hydroacoustic data show that previous Pleistocene transgressive and highstand inner ramp deposits and wedges were removed during sea‐level lowstands and accumulated downslope as stacked lowstand wedges; this suggests that, under conditions of high‐amplitude sea‐level fluctuations, the stratigraphic record of similar cool‐water carbonates may be biased.     

Late Pleistocene and Holocene vegetation and climate records from Lake Kotokel, central Baikal region

Climatically driven Late Pleistocene and Holocene vegetation changes were reconstructed based on pollen records from the sediments of Lake Kotokel and Cheremushka Bog, located on the eastern shore of Lake Baikal. The described paleoenvironmental record has higher resolution than records collected from Lake Baikal and unites individual events identified in prior studies of bottom and onshore cores. Remarkable shifts in landscapes and expansions of index plants are as follows. Forest tundra and/or forest steppe landscape with birch, spruce, Artemisia, and Poaceae prevailed at ca. 50–25 ¹⁴C kyr BP. Tundra and/or steppe vegetation dominated by Artemisia and Poaceae was typical for the Last Glacial Maximum. The expansion of shrub birch and willow occurred at ca. 15.5 ¹⁴C kyr BP. Two peaks of spruce expansion at ca. 47.5–42.4 ¹⁴C kyr BP (Karginian time) and at ca. 14.5–13 ka (Bølling-Allerød warm intervals) suggest that the condition were more humid than today. A slight increase in Artemisia at ca. 11–10.5 ¹⁴C kyr BP (13–12 ka) was indicative of the Younger Dryas event. An expansion of birch forests with fir at ca. 12–6.4 ka suggests higher humidity. The currently dominant Scots and Siberian pine forests with birch expanded since 6.4 ka.     

祁连山南坡木里地区晚更新世晚期的气候变迁

通过对祁连山南坡木里地区晚更新世地层剖面AMS¹⁴C年代的测定,以及对沉积物粒度、磁化率及SiO₂、Al₂O₃、Fe₂O₃等氧化物含量的研究,重建了研究区晚更新世晚期44.8～35ka B.P.的气候变迁。木里地区晚更新世晚期44.8～35ka B.P.的气候可以分为3个阶段:(1)44.8～40.5ka B.P.,气候温暖湿润,41.9ka B.P.出现的暖事件与D-O振荡中的IS12对应;(2)40.5～35.5ka B.P.,气候波动频繁且在波动中变冷,该阶段成功捕捉到3次D-O振荡中的暖事件和1次Heinrich冷事件,3次暖事件与D-O振荡中的IS9、IS10、IS11对应,时间分别为35.8ka B.P.、37.5ka B.P.、39.7ka B.P.,冷事件与D-O振荡中的H3对应,时间为35.5ka B.P.;(3)35.5～35ka B.P.,气候波动大,具变暖趋势。整个阶段的气候波动与深海氧同位素第3阶段后期气候变化特征较吻合,亦揭示了全球变化信号在研究区的响应特征。     

Late Pleistocene Environments of the Central Ukraine

The Vyazivok loess sequence from the Dnieper Plain, Ukraine, documents regional environmental changes during the late Pleistocene and Holocene. Pedological and palynological analyses and low-field magnetic susceptibility document changes from dense temperate forest during the last interglacial maximum to open, harsh, loess–steppe during the latest Pleistocene. The Vyazivok section overlies hillwash derived from a lower Pleistocene terrace and consists of two stratified soil complexes (Kaydaky and Pryluky; marine isotope stage [MIS] 5 equivalent) separated by a layer of eolian dust (Tyasmyn silt). The lower soils in both complexes formed within forest. These soils are overlain by the Uday (MIS4) and Bug (MIS2) loess units, which are separated by boreal soils of the Vytachiv (MIS3) complex. The coldest conditions within the record occurred in the youngest loess. Holocene soils cap the Bug loess. The Vyazivok section shows remarkable similarities with other classical loess sequences in western Europe, the Czech Republic, and Austria. The Kaydaky, Pryluky, and Vytachiv deposits, correlate with the PKIII, PKII, and PKI soil complexes, respectively, of the Czech Republic. The Tyasmyn and Prylyky silt layers correspond to marker horizons from central Europe.     

Extensive carbonate algal bioherms in upper Pleistocene saline lakes of the central Altiplano of Bolivia

During the upper Pleistocene the Central Altiplano of Bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations. Development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands, discontinuously covering the 300-km-long and 100-km-wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of Uyuni. Distribution, size and shape of the bioherms are diverse due to various factors, e.g. the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth. On larger palaeoterraces, the build-ups coalesced to form platform-like carbonate accumulations. Although the morphologies closely resemble those induced by cyanobacteria, they were predominantly constructed by other plant communities, probably dominated by filamentous green algae. Cyanobacterial communities flourished in association with these plants, but they did not contribute significantly to the architecture of the bioherms; they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build-ups. A prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects. The build-ups located between 3660 and 3680 m elevation display up to three major parts: (1) a massive inner core formed during an early Minchin highstand, before 40 ka; (2) a large peripheral envelope deposited at about 40 ka (late Minchin) and (3) a thinner outermost crust formed during a late glacial event. Lake level dropped during interlacustrine stages, sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system, i.e. the present-day salar of Uyuni.     

Evidence for Late Pleistocene and Holocene sea-level,neotectonic and climate control in the coastal zone of northwest Portugal

The coastal zone of northwest Portugal can be subdivided into two geomorphological sectors: Sector 1, between the Minho River and the town of Espinho, where the coastal segments consist of estuaries, sandy and shingle beaches with rocky outcrops, and Holocene dune systems (foredunes and some migrating dunes with blow-outs). The estuaries and the foredunes in particular are very degraded by human activities. Sector 2, between Espinho and the Mondego Cape, where coastal lagoons and Holocene dune systems (foredunes, parabolic and transverse dunes) occur. This study deals with the macroscale, i.e. 100–1000 years, forcing by sea-level changes and neotectonic activity on the one hand, and mesoscale, i.e. 1–100 years, forcing by climate fluctuations on the other hand, on these (palaeo-)environments. It is shown in particular that sea-level changes and neotectonic activity play a dominant role in the evolution of the coastal zone since the Late Pleistocene. Sediment starvation on the shoreface is postulated to be one of the major causes for coastal erosion since at least the 15th century. The mesoscale role of climate is difficult to assess at the present stage of knowledge, mainly because of overprinting by the macroscale evolution of the coast. However, data on estuarine saltmarsh evolution in sector 1 point towards discrete changes in storminess, while the development of Medieval dune systems in sectors 1 and 2 are attributed to the Little Ice Age or, alternatively, to human occupation of the dune areas.     

Drainage evolution in south and east England during the Pleistocene

Two major river systems operated in southern and eastern England throughout the Pleistocene: the river Thames and the Solent river. Both rivers are axial streams of comparable size draining major basinal structures comprising similar Tertiary and Mesozoic rocks. Although the modem Thames flows broadly W-E in the London Basin, upstream of Reading it flows from the north to drain the south Midlands. It was diverted to its present course through London by glaciation in the Anglian (Elsterian) before which it flowed across East Anglia into the southem North Sea. The Solent river no longer exists since most of its course was drowned by eustatic sea-level rise during the Flandrian Stage (Holocene). Previously, it flowed eastwards across SE Dorset and S Hampshire as an extension of the modem river Frome in the Hampshire Basin. During periods of low sea-level (cold stages) it was a tributary of the 'Channel River'. Fluvial aggradations provide evidence of the former courses of these substantial rivers and their tributaries. The facies and sedimentary structures indicate that the bulk of the deposits in both systems accumulated in braided river environments under periglacial climates. Fossiliferous sediments provide biostratigraphical frameworks. During temperate periods the rivers adopted singlethread courses. Evolution of both rivers reflect their responses to climatic change, local geological structure and long-term tectonic activity. Both rivers are undoubtedly of considerable antiquity, their records potentially extend from the Early Pleistocene or Late Pliocene, but they may have originated in the early Tertiary.     

晚更新世以来冲绳海槽天文驱动的气候变化: 来自CSHC-15钻孔的证据

东海冲绳海槽是一个年轻的弧后盆地,自中新世以来接受了来自长江、黄河以及陆架输送的物质,具有高沉积速率和富有机质等沉积特征,但是沉积记录的年代学约束较为薄弱,影响了对气候信号变化的解读。本研究以冲绳海槽中北部CSHC-15钻孔的60 m连续样品为对象,选取高分辨率的主微量元素含量作为古气候替代指标开展天文年代学分析,采用碳同位素测年数据和底栖有孔虫氧同位素记录作为年龄约束,建立该钻孔约200 ka以来的沉积记录变化。通过对沉积物时间域Al、K、Ti等主微量元素含量序列进行频谱分析,观察到显著的100 ka短偏心率、41 ka斜率、约23 ka岁差以及10 ka半岁差周期。由于该钻孔的底栖有孔虫种属存在差异,在没有放射性同位素年龄的约束下,氧同位素值与国际标准曲线LR04的对比结果效果不佳。因此,本研究利用La04天文轨道模型的岁差周期进行约束,进行精细的天文调谐,以获得可靠的地层年代框架;此外,结合贝叶斯统计的年龄模型分析方法,计算了该天文年代标尺的误差范围,从而进一步提高了对冲绳海槽晚更新世以来沉积记录年代的可靠性,其结果有助于深入探究气候变化和环境演化的影响。

     

Vegetation and climate change on the Bolivian Altiplano between 108,000 and 18,000 yr ago

A 90,000-yr record of environmental change before 18,000 cal yr B.P. has been constructed using pollen analyses from a sediment core obtained from Salar de Uyuni (3653 m above sea level) on the Bolivian Altiplano. The sequence consists of alternating mud and salt, which reflect shifts between wet and dry periods. Low abundances of aquatic species between 108,000 and 50,000 yr ago (such as Myriophyllum and Isoëtes) and marked fluctuations in Pediastrum suggest generally dry conditions dominated by saltpans. Between 50,000 yr ago and 36,000 cal yr B.P., lacustrine sediments become increasingly dominant. The transition to the formation of paleolake “Minchin” begins with marked rises in Isoëtes and Myriophyllum, suggesting a lake of moderate depth. Similarly, between 36,000 and 26,000 cal yr B.P., the transition to paleolake Tauca is also initiated by rises in Isoëtes and Myriophyllum; the sustained presence of Isoëtes indicates the development of flooded littoral communities associated with a lake maintained at a higher water level. Polylepis tarapacana-dominated communities were probably an important component of the Altiplano terrestrial vegetation during much of the Last Glacial Maximum (LGM) and previous wet phases.