越南岸外晚第四纪上升流与东亚夏季风变迁

通过对南海西部越南岸外１７９５４—３柱状样中浮游和底栖有孔虫的定量分析,结合碳酸钙、有机碳和稳定同位素数据,获得南海西部上升流区近２０万年来的古海洋学记录,进而讨论了晚第四纪东亚夏季风的变迁。结果表明,在间冰期,尤其是末次间冰期,越南岸外出现表层海水古温度低、温跃层浅、生产力高,同时底层海水富营养的现象,说明上升流的存在,且该上升流有从氧同位素５期(ＭＩＳ５期)向１期逐渐减弱的趋势。结合南海现代海流及生产力分布的研究,推断该上升流由东亚夏季风驱动,其强弱的变化说明,东亚夏季风在近２０万年来有间冰期增强、冰期减弱,且从ＭＩＳ５期向ＭＩＳ１期递减的特征。     

East Asian monsoon variations during Oxygen Isotope Stage 5: evidence from the northwestern margin of the Chinese loess plateau

We present the results of high-resolution multi-proxy climate studies of the S1 palaeosol, corresponding to oxygen isotope stage (OIS) 5, from the northwestern margin of the Chinese Loess Plateau area. Here, S1 is much thicker (ca. 6–8 m) than in the central Loess Plateau areas (ca. 2 m), where most previous studies have been conducted. Hence, much higher-resolution stratigraphic studies are possible, yielding more insight into the temporal variations of the East Asian monsoon during MIS 5. The frequency-dependent magnetic susceptibility, as well as the concentration of secondary carbonate, is used as an indicator of the summer monsoon intensity, and the median particle size as an indicator of the winter monsoon intensity. The results suggest that the northwestern margin of the Chinese Loess Plateau experienced the strongest summer monsoon intensity in sub-stage (OISS) 5e and the weakest in OISS 5a, among the three warmer periods during stage 5. The summer monsoon was weaker in OISS 5b than in OISS 5d. A dusty interval interrupted the second warmer period (5c) and a soil-forming event interrupted the first colder period (5d). The results also suggest that the directions of changes in the intensities of summer and winter monsoons may not always have been proportionately opposite. For example, the weakest summer monsoon occurred in OISS 5a during which the winter monsoon was not the strongest. We further conclude that the winter monsoon during the last interglacial was probably driven by global ice volume fluctuations, while the summer monsoon was primarily controlled by the northern hemisphere solar insolation and was probably modified by a feedback mechanism. That is, the climatic buffering effect of low-latitudinal oceans may have distorted the response of the summer monsoon to insolation variations. Finally, our results do not show the degree of climatic instability comparable to that recorded in the GRIP ice core for the last interglacial (OISS 5e), even though the study area is situated in a region which has been sensitive to climatic changes.     

Implications of a 24,000-Yr Palynological Record for a Younger Dryas Cooling and for Boreal Forest Development in Northeastern Siberia

A sediment core from Smorodinovoye Lake (SML), northeastern Siberia (area to the east of the Verkhoyansk Range) spanning the last 24,000 ¹⁴C yr indicates that vegetational and climatic changes in the upper Indigirka basin resemble those in eastern Siberia (Lena basin and westward). For example, maximum postglacial summer temperatures at SML probably occurred 6000–4000 ¹⁴C yr B.P., an age more in accordance with eastern than northeastern records. Larix arrived near the lake by 9600 ¹⁴C yr B.P., approximately when forests expanded in the east but ca. 1500 ¹⁴C yr later than forests were established in the neighboring upper Kolyma basin. Paleobotanical data further suggest that Larix possibly migrated southward from populations in the arctic lowlands of eastern Siberia and did not originate from interior refugia of the upper Kolyma basin. Although a Younger Dryas cooling has been noted in eastern Siberia, SML provides the first evidence from the northeast for a similar climatic reversal. Climatic variations seemingly have persisted between the Indigirka and Kolyma basins over at least the last 11,000 ¹⁴C yr, despite the proximity of the two drainages and the occurrence of major changes in boundary conditions (e.g., seasonal insolation, sea levels) that have influenced other regional climatic patterns.     

Paleoclimate of Southwestern China for the Past 50,000 yr Inferred from Lake Sediment Records

Long sediment cores (12.5 and 13.5 m) from two lakes in Yunnan Province were used to infer the paleoclimate of southwest China over the past 50,000 yr. During the Holocene and marine isotope stage (MIS 3), bio-induced carbonate precipitation and organic matter (OM) production was high, suggesting warm temperatures and high primary productivity. In contrast, sediment inorganic carbon (IC) and organic carbon (OC) concentrations were low in last glacial deposits from 38,000 to 12,000 cal yr B.P., indicating cool temperatures and low productivity. The 50,000-yr record has alternating peaks of carbonate and coarse-grain (>38 μm) quartz that reflect warm, moist interglacial or interstadial conditions alternating with cold, dry glacial or stadial conditions, respectively. Spectral analysis of the carbonate and quartz signals reveals power concentrated at periods of 7200 and 8900 cal yr, respectively, that may reflect a nonlinear climate response to precessional forcing at a time of reduced eccentricity modulation (McIntyre and Molfino, 1996). Oxygen isotope values of calcite from Yunnan lake cores indicate the summer monsoon was weak during the last glaciation from 50,000 to 12,000 cal yr B.P. The summer monsoon intensified between 12,000 and 8000 cal yr B.P., but weakened gradually in response to insolation forcing during the mid-to-late Holocene. Our results support the Overpeck et al. (1996) model that posits a weak summer monsoon during the last glaciation that responded nonlinearly to insolation forcing when its intensity was affected by Eurasian snow cover and ice-sheet extent. The summer monsoon intensified and responded linearly to seasonal insolation forcing in the Holocene when ice volume diminished.     

Reconstruction of millennial-scale variations in the East Asian summer monsoon over the past 300 ka based on the total carbon content of sediment from Lake Biwa,Japan

The total carbon content analysis for Takashima-oki core extracted from Lake Biwa with time resolutions of approximately 150 years was used to reconstruct millennial-scale climate change in the East Asian monsoon region over the past 300 ka. The total carbon content of the Takashima-oki core is an indirect proxy for the East Asian summer monsoon, because the total carbon content reflects primary productivity within the lake, which is controlled by precipitation in the catchment area. Using these data, we clarify temporal variations in the East Asian summer monsoon prior to MIS 5, which were previously unresolved. The observed variations show a ∼1,500-year cycle that is also recorded by paleoclimate indices in the North Atlantic region. We propose that the East Asian summer monsoon underwent abrupt millennium-scale changes during interglacial stages such as MIS 5 and 7.     

Fire and Vegetation History from the Coastal Rain Forest of the Western Oregon Coast Range

High-resolution charcoal and pollen analyses were used to reconstruct a 4600-yr-long history of fire and vegetation near Taylor Lake in the wettest forests of coastal Oregon. Today, fires in these forests are rare because the season of ignition does not coincide with months of dry fuels. From ca. 4600 to 2700 cal yr B.P. fire episodes occurred at intervals of 140±30 yr while forest vegetation was dominated by disturbance-adapted taxa such as Alnus rubra. From ca. 2700 cal yr B.P. to the present, fire episodes have become less common, occurring at intervals of 240±30 yr, and fire-sensitive forest taxa, such as Tsuga heterophylla and Picea sitchensis, have become more prominent. Fire occurrence during the mid-Holocene was similar to that of the more xeric forests in the eastern Coast Range and suggests that summer drought was widespread. After ca. 2700 cal yr B.P., a decrease in fire episode frequency suggests that cooler conditions and possibly increased summer fog allowed the establishment of present-day Picea sitchensis forests within the watershed. These results provide evidence that fire has been an important disturbance agent in the Coast Range of Oregon, and variations in fire frequency and climate have led to the establishment of present-day forests.     

南海北部MD12-3432站深海氧同位素MIS6期到MIS5期陆源沉积物元素比值反映的古气候变化

海洋沉积物中的某些主微量元素与沉积物源区有着密切的联系,它们之间的比值变化受到源区化学风化强度的影响,因此这些元素的比值变化可以反映出海洋沉积物源区古气候的变化。本文研究了南海北部陆坡MD12-3432站位深海氧同位素6期和5期(MIS6/5) 沉积物的主微量元素,发现其表现出良好的气候控制变化特征,K/Ti、Mg/Ti、Al/Ti、Fe/Ti、Co/Ti、Zn/Ti和V/Ti等比值在MIS 5期时较高,而在MIS6期时较低。南海北部的碎屑物质主要来自中国华南地区,沉积物中元素比值的变化表明间冰期时(MIS5)华南地区陆壳化学风化增强,说明该时期华南陆地气候环境温暖湿润,这可能是间冰期时东亚夏季风加强的结果;而冰期时南海北部沉积物源区化学风化减弱,则与此时东亚冬季风较强,华南地区气候干燥寒冷相关。同时,与表层海水生产力相关的Ba/Ti比值在间冰期较高而冰期较低,反映出南海北部在间冰期时表层生产力较高而冰期时相对较低,这可能是由于东亚夏季风增强带来更多降雨,陆地化学风化作用加强,大量营养成分随河流进入南海,导致南海北部表层海水生产力增加。     

东亚冬、夏古季风变化的相位差及热带太平洋在季风变化中的驱动作用

斋堂黄土剖面古气候记录表明：１)冬、夏季风在千年尺度上的变化存在相位差；２)冬、夏季风记录在变化趋势上具有互为消长的关系，而在变化幅度和频率方面则有明显的差异。这表明，冬、夏季风在千年尺度上的变化是分别由不同的因素和过程所控制。我们提出热带太平洋海气相互作用系统在东亚季风变化过程中起着直接的驱动作用。     

Late Quaternary changes in surface productivity and oxygen minimum zone (OMZ) in the northwestern Arabian Sea: Micropaleontologic and sedimentary record at ODP site 728A

Changes in the abundance of selected planktic foraminiferal species and some sedimentological parameters at ODP site 728A were examined to understand the fluctuations in the surface productivity and deep sea oxygenation in the NW Arabian Sea during last ∼540 kyr. The increased relative abundances of high fertility taxa, i.e., Globigerinita glutinata and Globigerina bulloides mainly during interglacial intervals indicate intense upwelling. Strong SW summer monsoon probably increased the upwelling in the western Arabian Sea during interglacial intervals and caused high surface productivities due to the lateral transport of eutrophic waters. Most of the glacial periods (i.e., MIS 2, 4, 6, 8 and 12) are characterized by higher relative abundances of Neogloboquadrina pachyderma and Neogloboquadrina dutertrei associated with Globigerinoides ruber. The more stratified condition and deep mixed layer due to increased NE winter monsoon are mainly responsible for the higher relative abundances of N. pachyderma during glacial periods. Some of the glacial intervals (i.e., MIS 6 and 8) are also characterized by pteropod spikes reflecting deepening of aragonite compensation depth (ACD) and relatively less intense oxygen minimum zone (OMZ) in this region due to deep sea mixing and thermocline ventilation, and relatively less intense surface productivity during winter monsoon. The interglacial periods are largely devoid of pteropod shells indicating more aragonite dissolution due to increased intensity of OMZ in the northwestern Arabian Sea.     

Late Pleistocene Glaciations in the Northwestern Sierra Nevada, California

Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 ¹⁰Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 ¹⁰Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.     

Decou pled Summer and Winter Monsoon in Northeast Tibet and Northwest Loess Plateau During the Last Interglaciation

DECOUPLED SUMMER AND WINTER MONSOON IN NORTHEAST TIBET AND NORTHWEST LOESS PLATEAU DURING THE LAST INTERGLACIATIONtheNationalKeyProjectforBasicResearchonTibetPlateau (G19980 4 80 0 )andtheNationalNaturalSci encesFoundationofChina (No.4 990     

Climate variability in the Salawusu River valley of the Ordos Plateau (Inner Mongolia,China) during Marine Isotope Stage 3

Fluctuations in climatic proxies of the Milanggouwan section in the Salawusu River valley of the Ordos Plateau (Inner Mongolia, China) during Marine Isotope stage 3 (MIS 3) coincide well with sedimentary cycles for palaeo‐mobile dune sands alternating with fluvial–lacustrine facies and palaeosols. We compared the palaeo‐mobile dune sands with modern mobile dune sands (products of a cold and dry climate dominated by the East Asian winter monsoon), whereas the fluvial–lacustrine facies and palaeosols were controlled by a wet–warm climate similar to that of the East Asian summer monsoon. The MIS 3 climate of the Salawusu River valley appears to have experienced at least nine wet–warm and ten cold–dry fluctuations, divided into five stages: MIS 3e (58 900–49500 yr BP), MIS 3d (49 500–40 700 yr BP), MIS 3c (40 700–36 900 yr BP), MIS 3b (36 900–27 000 yr BP) and MIS 3a (27 000–22 300 yr BP). The 19 cold–warm climatic fluctuations corresponded roughly to the GRIP and Guliyan records, and with fluctuations in the North Atlantic climate. Notable peaks in the spectral analysis occurred at 19 500 yr, 1020 yr, 640 yr and 500 yr. Our results show that the millennial–centennial climate was closely related to the relative strengths of East Asian monsoons, which are controlled by the North Atlantic thermohaline circulation, and which is also closely linked to the Sun's precession period. Copyright © 2008 John Wiley & Sons, Ltd.     

Comparison of the Climate during Marine Isotope Stage 9 and 11 Inferred from a Speleothem Isotope Record from Northern Norway

A stalagmite from northern Norway is dated with 12 thermal ionization mass spectrometry U-Th dates, and at least four separate growth periods are identified that correspond with marine isotope stages 9, 11, 13, and probably 15. The calcite is tested for isotopic equilibrium with the Hendy test. Oxygen isotope measurements on 231 subsamples on a vertical transect are used as a paleotemperature proxy. The detailed isotopic record from MIS 9 show apparent similarities to a Holocene record from the same cave, both in the climatic evolution and the overall temperatures: both show temperature oscillations changing from high-frequency, low-amplitude cycles in the beginning of the interglacial period to lower frequency, higher amplitude cycles in the later part of the interglacial period. The isotope record from MIS 11 shows a distinct isotopic event toward heavier values. The isotopic record together with the porous, humus-rich calcite are interpreted as indicating a warmer than present interglacial period with several episodes of heavy rainfall.     

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.     

浑善达克沙地南缘晚更新世黄土-古土壤序列的高分辨率磁性气候记录

浑善达克沙地处于季风边缘区,目前对该区域末次间冰期以来的古气候演化历史及其动力学机制尚不清楚。文章以浑善达克沙地南缘山间低洼地带北沟剖面总厚度4.72 m(未见底)的黄土-古土壤序列为研究对象,开展了光释光测年和精细的岩石磁学、环境磁学、地球化学和漫反射光谱分析。结果表明:1)该剖面记录了末次间冰期以来完整的风成堆积历史,并通过东亚夏季风指标(Rb/Sr及χARM/χ)与深海氧同位素曲线的对比建立了13万年来该黄土-古土壤序列精细的年龄模式;2)在黄土高原上常规的用来指示成壤强度和夏季风强度的磁参数并不完全适用于干旱-半干旱地区(400 mm等降水线附近)的风成黄土,而地球化学指标在指示季风边缘区的地球化学风化过程上表现出一定的合理性,北沟剖面复杂的磁信号可能是对末次冰期季风边缘区显著减弱的夏季风的阈值响应;3)在末次间冰期东亚夏季风可以延伸到内蒙古中东部,北沟剖面记录了浑善达克沙地南缘末次间冰期以来区域古季风的演化历史。     

East Asian monsoon instability at the stage 5a/4 transition

The physics involved in the abrupt climate changes of the late Quaternary have eluded paleoclimatologists for many years. More paleoclimatic records characteristic of different elements of the global climate system are needed for better understanding of the cause-feedback relationships in the system. The East Asian monsoon is an important part of the global climate system and the mechanical links between the East Asian monsoon and other climatic elements around the world may hold a key to our knowledge of abrupt climate changes in East Asia and probably over a larger part of the globe. Previous studies have detected millennial-scale winter monsoon oscillations during the last glaciation and probably also during the last interglaciation in loess sequences across China. However, less attention has been paid to the abrupt summer monsoon changes and the stage 5a/ 4 transition, an important period for the evolution of the East Asian monsoon when the global climate shifted towards the last glaciation. Here we report on two loess sections from eastern China which were dated using a thermoluminescence (TL) technique. The pedogenic and other sediment parameters suggest that the summer monsoon experienced a two-step abrupt retreat at the stage 5a/4 transition. The variations in the proxies for the winter monsoon are synchronized with the summer monsoon proxies during this brief interval, implying a direct and immediate link between high latitude and low latitude mechanisms. These changes may be correlated with similar climatic oscillations observed in the North Atlantic, Europe and Antarctica, raising the possibility that the forcing factors that induced these changes are global in extent.     

Holocene History of the Bering Sea Bowhead Whale (Balaena mysticetus) in Its Beaufort Sea Summer Grounds off Southwestern Victoria Island, Western Canadian Arctic

The fossil remains of 43 bowhead whales were mapped on the raised beaches of western Wollaston Peninsula, Victoria Island, Canadian Arctic, near the historic summer range limit of the Bering Sea stock in the Beaufort Sea. The elevations and radiocarbon ages of the remains demonstrate that the bowhead ranged commonly into the region following the submergence of Bering Strait at ca. 10,000 ¹⁴C yr B.P. until ca. 8500 ¹⁴C yr B.P. During the same interval, bowheads ranged widely from the Beaufort Sea to Baffin Bay. Subsequently, no whales reached Wollaston Peninsula until ca. 1500 ¹⁴C yr B.P. Late Holocene populations evidently were small, or occupations were brief, in comparison to those of the early Holocene. Although the late Holocene recurrence may relate to the expansion of pioneering Thule whalers eastward from Alaska, there are few Thule sites and limited evidence of Thule whaling in the area surveyed to support this suggestion.     

Climatic Regionalization and the Spatio-Temporal Occurrence of Extreme Single-Year Drought Events (1500–1998) in the Interior Pacific Northwest, USA

Tree-ring records from western juniper (Juniperus occidentalis var. occidentalis Hook.) growing throughout the interior Pacific Northwest identify extreme climatic pointer years (CPYs) (i.e., severe single-year droughts) from 1500–1998. Widespread and extreme CPYs were concentrated in the 16th and early part of the 17th centuries and did not occur again until the early 20th century. The 217-yr absence of extreme CPYs may have occurred during an extended period of low variance in the Pacific Decadal Oscillation. We mapped climatic boundaries for the interior Pacific Northwest based on the location of sites with similar precipitation variability indices. Three regions, the Northwest (based on chronologies from nine sites), the Southwest (four sites), and the East (five sites) were identified. Our results suggest that western juniper radial growth indices have substantial interannual variability within the northwestern range of the species (central Oregon), particularly when compared with western juniper growing in its eastern range (eastern Oregon, southeastern Idaho, and northern Nevada) and southwestern range (southern Oregon and northeast California). We suspect that the substantial differences in the variability of western juniper radial growth indices are linked to the influence of ENSO events on winter/spring precipitation amounts.     

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。     

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.