南海北部夏季沿岸上升流近百年的强度变化

用全谱直读ICP-AES技术测定了海南岛东部（琼东）沿岸上升流区域滨珊瑚1906—1996年的Sr／Ca比值。结果显示珊瑚Sr／Ca比值重建的琼东海域的夏季海表温度偏低,强烈地受到东亚夏季风引起沿岸冷上升流的影响。结合西沙海域的滨珊瑚海表温度记录,首次重建了1906～1993年琼东沿岸风生上升流的强度指数变化序列。结果表明琼东上升流于1906～1993期间整体呈加强趋势,并具显著的年代际波动特征,同全球趋暖密切相关。此外,重建的上升流强度指数序列还揭示了大尺度环流——厄尔尼诺一南方涛动对琼东上升流强度变化的制约。     

Pollen evidence for variations in the southern margin of the westerly winds in SW Patagonia over the last 12,600 years

We report pollen and charcoal records from Vega Ñandú ( 51°0′S, 72°45′W), a small mire located near the modern forest-steppe ecotone in Torres del Paine National Park, southern Chile. The record shows an open landscape dominated by low shrubs and herbs between 12,600 and 10,800 cal yr BP, under cold and relatively humid conditions. Nothofagus experienced frequent, large-amplitude oscillations between 10,800 and 6800 cal yr BP, indicating recurrent transitions between shrubland/parkland environments, under warm and highly variable moisture conditions. A sustained increase in Nothofagus started at 6800 cal yr BP, punctuated by step-wise increases at 5100 and 2400 cal yr BP, implying further increases in precipitation. We interpret these results as indicative of variations in the amount of precipitation of westerly origin, with prominent increases at 6800, 5100, and 2400 cal yr BP. These pulses led to peak precipitation regimes during the last two millennia in this part of SW Patagonia. Our data suggest variations in the position and/or strength of the southern margin of the westerlies, most likely linked to variations in the extent and/or persistence of sea ice and sea-surface temperature anomalies in the Southern Ocean. Over the last two centuries the record shows a forest decline and expansion of Rumex acetosella, an exotic species indicative of European disturbance.     

Hydrologic variation during the last 170,000 years in the southern hemisphere tropics of South America

Despite the hypothesized importance of the tropics in the global climate system, few tropical paleoclimatic records extend to periods earlier than the last glacial maximum (LGM), about 20,000 years before present. We present a well-dated 170,000-year time series of hydrologic variation from the southern hemisphere tropics of South America that extends from modern times through most of the penultimate glacial period. Alternating mud and salt units in a core from Salar de Uyuni, Bolivia reflect alternations between wet and dry periods. The most striking feature of the sequence is that the duration of paleolakes increased in the late Quaternary. This change may reflect increased precipitation, geomorphic or tectonic processes that affected basin hydrology, or some combination of both. The dominance of salt between 170,000 and 140,000 yr ago indicates that much of the penultimate glacial period was dry, in contrast to wet conditions in the LGM. Our analyses also suggest that the relative influence of insolation forcing on regional moisture budgets may have been stronger during the past 50,000 years than in earlier times.     

Climatic change during the last 4000 years in the southern Tarim Basin,Xinjiang, northwest China

In this study, a ca. 4000 cal. yr ancient lacustrine (or wetland) sediment record at the southern margin of Tarim Basin is used to reconstruct the history of climate change. Six radiocarbon dates on organic matter were obtained. δ¹⁸O and δ¹³C of carbonate, pollen and sediment particle size were analysed for climate proxies. The proxies indicate that a drier climate prevailed in the area before ca. 1010 BC and during period 1010 BC–AD 500 climate then changed rapidly and continuously from dry to moist, but after about AD 500 climate generally shows dry condition. Several centennial‐scale climatic events were revealed, with the wettest spell during AD 450–550, and a relatively wetter interval between AD 930–1030. Pollen results show that regional climate may influence human agricultural activities. Spectral analysis of mean grain size (MGS) proxy reveals statistically pronounced cyclic signals, such as ca. 200 yr, ca. 120 yr, ca. 90 yr, ca. 45 yr and ca. 33 or 30 yr, which may be associated with solar activities, implying that solar variability plays an important role in the decadal‐ and centennial‐scale climate variations in the study area. Copyright © 2007 John Wiley & Sons, Ltd.     

Paleoceanography of the Santa Barbara Basin during the last 8000 years

The varved sediments of the Santa Barbara Basin off southern California, offer a unique opportunity to study the changes in oceanographic conditions of this nearshore area during the last 8000 yr. Quantitative analysis of Radiolaria found in recent surface sediment samples from the eastern North Pacific allows the identification of four “assemblages” which can be related to the physical oceanography of the California Current. Two assemblages are associated with the southerly flowing California Current, one with the main stream of the current (California Current Assemblage) and the other with the offshore flow along northern California (Central Assemblage). The two other assemblages are associated with the subtropical region of the eastern North Pacific (Subtropical Assemblage) and one found mostly off the coast of Baja California (Baja Assemblage). Analysis of the Radiolaria found in the varved sediments of a core from the Santa Barbara Basin give an 8000-yr continuous record of these four assemblages. The California and Baja Assemblages show only minor fluctuations in their importance in the sediments of the Santa Barbara Basin. The California Assemblage, however, shows a steady increase during this time period. Prior to 5400 yr B.P. the Radiolaria were predominately subtropical in character, whereas after 5400 yr B.P. the Central Assemblage becomes more important. Since 5400 yr B.P. most of the changes in the radiolarian fauna consist of fluctuations in the importance of these two assemblages. Past sea-surface temperatures for the month of February were calculated using the transfer function technique of J. Imbrie and N. G. Kipp (1971, In “The Late Cenozore Glacial Ages” L. K. Turekian, Ed.), (Chap. 5, Yale Univ. Press, New Haven, Conn.). The time series of paleotemperature estimates show major changes in the average February temperature of Santa Barbara Basin waters. The range of estimated temperatures (12°C) exceeds that of the historical observations of February temperatures in the Santa Barbara Basin but does not exceed the observed range for the California Current region. The intervals from 800 to 1800 yr B.P. 3600 to 3800 yr B.P. and 5400 to the end of the record appear to have been generally warmer than today. Comparison of the Holocene record of alpine glacial advances with the radiolarian assemblage and paleotemperature time series shows that the initiations of advances was coincident with a decrease in sea-surface temperatures and an increase in the importance of the Central Assemblage in the Santa Barbara Basin. The terminations of these advances were not marked by any consistent characteristic in the Santa Barbara Basin time series.     

Relative inactivity during the last 140,000 years of a portion of the La Paz fault,southern Baja California Sur,Mexico

Uranium-series dating of corals overlying the undeformed Punta Coyote gravels indicates that the underlying La Paz fault zone has been relatively inactive in this part of the Baja California peninsula during the last 140,000 years, and possibly for a significantly longer period. However, Holocene seismic activities along extensions of the fault zone north of Cabo San Lucas suggest potential seismic hazards for the city of La Paz (population 200,000), which lies about 6 km from the fault.     

The California current system during the last 136,000 years: response of the North Pacific High to precessional forcing

Alkenone sea surface temperature (SST) records were generated from the Ocean Drilling Program's (ODP) Sites 1014 and 1016 to examine the response of the California Current System to global climate change during the last 136 ka. The temperature differences between these sites (ΔSST_NEP=SST_ODP1014–SST_ODP1016) reflected the intensity of the California Current and varied between 0.4 and 6.1 °C. A high ΔSST_NEP (weaker California Current) was found for late marine isotope stage (MIS) 2 and early MIS 5e, while a low ΔSST_NEP (stronger California Current) was detected for mid-MIS 5e and MIS 1. Spectral analysis indicated that this variation pattern dominated 23- (precession) and 30-ka periods. Comparison of the ΔSST_NEP and SST based on data from core MD01-2421 at the Japan margin revealed anti-phase variation; the high ΔSST_NEP (weakening of the California Current) corresponded to the low SST at the Japan margin (the southward displacement of the NW Pacific subarctic boundary), and vice versa. This variation was synchronous with a model prediction of the tropical El Niño-Southern Oscillation behavior. These findings suggest that the intensity of the North Pacific High varied in response to precessional forcing, and also that the response has been linked with the changes of tropical ocean–atmosphere interactions.     

Estimates of the position of sea level between 140,000 and 75,000 years ago

The absolute elevations of sea level 103,000 and 82,000 years ago have been estimated as ?15 and ?13 m, respectively, from the present elevations of emergent reefs on Barbados (Broecker et al., 1968; Matthews, 1973; Bloom et al., 1974). The “Barbados model” requires two assumptions: (1) that sea level was +6 m 124,000 years ago, and (2) that rates of uplift on short individual traverses have been uniform during the last 125,000 years.A test of the derived values on Barbados itself does not yield uniform rates of uplift between 124,000 and 82,000 years ago. Less reliably dated strand line features on less uplifted coasts suggest that sea level 124,000 years ago differed from sea levels 103,000 and 82,000 years ago by smaller amounts than those suggested by the “Barbados model.” Such smaller differences yield more uniform rates of uplift between 124,000 and 82,000 years ago, on New Guinea as well as on Barbados, than do the larger. The “Barbados model” is not sufficiently precise to yield close estimates of past elevations of sea level. Better values will eventually be derived from low uplift coasts, when stratigraphic and radiometric data from them have achieved the credibility of data from moderate to high uplift coasts.     

Hydrological response of a dryland ephemeral river to southern African climatic variability during the last millennium

A long-term flood record from the Buffels River, the largest ephemeral river of NW South Africa (9250 km²), was reconstructed based on interpretation of palaeoflood, documentary and instrumental rainfall data. Palaeoflood data were obtained at three study reaches, with preserved sedimentary evidence indicating at least 25 large floods during the last 700 yr. Geochronological control for the palaeoflood record was provided by radiocarbon and optically stimulated luminescence (OSL) dating. Annual resolution was obtained since the 19th century using the overlapping documentary and instrumental records. Large floods coincided in the past within three main hydroclimatic settings: (1) periods of regular large flood occurrence (1 large flood/~30 yr) under wetter and cooler prevailing climatic conditions (AD 1600–1800), (2) decreasing occurrence of large floods (1 large flood/~100 yr) during warmer conditions (e.g., AD 1425–1600 and after 1925), and (3) periods of high frequency of large floods (~ 4–5 large floods in 20–30 yr) coinciding with wetter conditions of decadal duration, namely at AD 1390–1425, 1800–1825 and 1915–1925. These decadal-scale periods of the highest flood frequency seem to correspond in time with changes in atmospheric circulation patterns, as inferred when comparing their onset and distribution with temperature proxies in southern Africa.     

Eratosthenes Seamount: collisional processes in the easternmost Mediterranean in relation to the Plio-Quaternary uplift of southern Cyprus

Important new light is shed on subduction and incipient collisional processes in the Easternmost Mediterranean, notably the Plio-Quaternary uplift of the Troodos ophiolite, from new geophysical information collected during a cruise of the R.V. Gelendzhik in July 1993, as part of the Training-through-Research' programme. The data collected over the Eratosthenes Seamount formed part of the site-survey work for Leg 160 of the Ocean Drilling Program scheduled for spring 1995. The main results are that the Eratosthenes Seamount is in the process of actively subsiding, breaking-up and being thrust, beneath both Cyprus to the north and the Levantine Basin to the south. Northwards thrusting appears to post-date the Messinian, when evaporites accumulated around the lower flanks of a pre-existing seamount feature. Comparison with the geology of southern Cyprus and offshore areas suggests a causative link between northward underthrusting of the Eratosthenes Seamount and late Pliocene-mid Quaternary uplift of southern Cyprus, focused on the centre of the Troodos ophiolite.     

青藏高原40年来降水量时空变化趋势

利用青藏高原及其周边的97个气象台站1961-2000年的逐月降水量资料,以Mann-Kendall趋势检验方法结合主成分分析,分析了高原降水量40年来的时间变化趋势和空间演变特征,并探讨了其变化趋势的区域分异因素和可能的趋势突变时间。结果发现,高原站点40年年降水量大部分表现为增大趋势,只有青海东南部和南疆及西藏部分站点出现减小趋势,这一时间变化趋势的空间分布则大致表现为高原中东部和南北的反向变化,同时高原站点冷季降水量增大趋势明显;以冷、暖季降水量的第三载荷向量场分异进行的分区在10年年代际和逐年变化中都有明显体现,暖季区域分异因素的主要相关区域降水量变化趋势与相应载荷向量的时间变化相关显著,在1989年出现可信的突变时间点;冷季相关分异区域的降水量变化趋势在1987年出现明显增大趋势,同样与载荷向量的时间变化相关显著,但突变时间点检测不明显。     

Model for paleoceanographic reconstructions of the California Current during the last 8000 years

Two independent data sets are used to develop a model for reconstructing sea-surface temperature and dynamic height anomaly distributions for the California Current during the last 8000 years. The first data set, all hydrographic data available for the California Current region, was used to determine the statistical relationships between the historical record of sea-surface conditions in the Santa Barbara Basin and all one-degree-square grid points of the California Current area. Given these relationships and the second data set, an 8000-year record of sea-surface temperatures and dynamic height anomalies from the Santa Barbara Basin, past sea-surface conditions throughout the California Current can be estimated for times before historical observations. The 8000-year record of sea-surface conditions was estimated by analysis of the radiolarian fauna found in a varved sediment core from the Santa Barbara Basin (Pisias, N. G., 1978, Quaternary Research 10, 366–384). The reconstructions of sea-surface temperature and dynamic height anomalies indicate that at times of cold sea-surface conditions in the Santa Barbara Basin, the flow of the California Current was much stronger than it is today or was during the times of the warmest sea-surface conditions during the last 8000 years. The atmospheric circulation during the winter of 1950, the period of the coldest recorded sea-surface temperatures in the Santa Barbara Basin based on the historical data set, contained a strong northerly component in the winds which is consistent with the inferred increase in the California Current at times of cold sea-surface temperatures. Times of warm sea-surface temperatures in the Santa Barbara Basin are characterized by decreased southward flow of the California Current and a marked increase in northward flow into the Santa Barbara Basin itself. In the historical record, times of warm sea-surface temperatures are often associated with high precipitation in southern California. The atmospheric circulation during the winter of 1968–1969 is characterized by strong eastward flow over southern California and a northward transport of warm humid air from the tropics into the region of southern California producing the high rainfall observed. The persistence of this atmospheric circulation could produce the more zonal flow predicted for the California Current during times of warmer average conditions in the Santa Barbara Basin.     

Fluctuation in the level of pluvial Lake Lahontan during the last 40,000 years

Samples of algal tufa, gastropods and calcite-cemented sand were collected from the Walker and Pyramid Lake areas of the Lahontan Basin, Nevada. X-ray diffraction petrographic and radiocarbon analyses show that massive forms of tufa such as the dendritic variety contain secondary carbon-bearing material and therefore yield unreliable radiocarbon dates. Dense coating of tufa (lithoid), however, gave radiocarbon ages in agreement with dates on coexisting aragonite gastropods. Radiocarbon data from the study were combined with previously dated noncarbonate materials [Born, S. M. (1972). “Lake Quaternary History, Deltaic Sedimentation, and Mudlump Formation at Pyramid Lake, Nevada”, Center for Water Resources, Desert Research Inst., Reno, Nevada] to give an internally consistent record of lake level fluctuations for the past 40,000 years. The main features of the Lahontan chronology are (1) extreme high stands (1330 m above sea level) 13,500 to 11,000 and 25,000 to 22,000 B.P., (2) a moderate high stand (1260 m above sea level) 20,000 to 15,000 B.P., (3) a low stand of unknown elevation 40,000 to 25,000 B.P., (4) an extremely low stand 9000 to 5000 B.P., and (5) an overall increase in the size of Walker and Pyramid Lakes during the past 5000 years, until the late 19th century. Pore fluid data indicate that Walker Lake desiccated sometime during the period 9050 to 6400 B.P. Salts deposited as a result of this dessication are still undergoing dissolution causing a flux of chloride, carbon, and other solute species from the sediments to the overlying lake water. Pore fluid data obtained from Pyramid Lake sediments do not indicate the presence of a concentrated brine at depth. This suggests that Pyramid Lake did not dry completely during this period although it may have been severely reduced in size. There has been considerable disagreement regarding the occurrence of extreme arid conditions (altithermal period) since 10,000 B.P. [Mehringer, P. J. (1977). “Models and Great Basin Prehistory”. Desert Research Inst. Pub, Reno, Nevada]. The data of this study suggest that such a climatic regime did occur in the western Great Basin during the period 9000 to 5000 B.P.     

Constraints on ice-sheet thickness over Tibet during the last 40 000 years

The Late Quaternary glaciation of Tibet has received considerable attention in the last few decades due to its influence on the regional climate, especially the Asian summer monsoon. Recently, however, it has been argued that the Tibetan ice sheet also might have played an important role in initiating global-scale palaeoclimatic changes. Controversy, however, exists on the nature of Late Quaternary ice cover over Tibet due largely to the subjectivity in the interpretation of the sparse and complex geomorphological evidence. We have examined this problem in the light of δ ¹⁸O data (a temperature proxy) of ice cores-from the Dunde ice cap on the northern flank of Tibet. Considering only the gross features in the Dunde ice-core isotopic data, we have interpreted a temperature decrease of 4°–6°C and consequent lowering of equilibrium line altitude (ELA) in the range 700–850 m during the last glacial stage (LGS). This could have caused depression of the snow line below the mean altitutde of the Tibetan plateau, resulting in an areally extensive but marginally thick ice cover. However, if one also considers the possibility that precipitation on the Tibetan plateau during LGS may have been significantly lower than at present, the ELA depression would be much less than that estimated by considering the temperature effect alone.     

Forest–savanna–morichal dynamics in relation to fire and human occupation in the southern Gran Sabana (SE Venezuela) during the last millennia

The southern Gran Sabana (SE Venezuela) holds a particular type of neotropical savanna characterized by the local occurrence of morichales (Mauritia palm swamps), in a climate apparently more suitable for rain forests. We present a paleoecological analysis of the last millennia of Lake Chonita (4°39′N–61°0′W, 884 m elevation), based on biological and physico-chemical proxies. Savannas dominated the region during the last millennia, but a significant vegetation replacement occurred in recent times. The site was covered by a treeless savanna with nearby rainforests from 3640 to 2180 cal yr BP. Water levels were higher than today until about 2800 cal yr BP. Forests retreated since about 2180 cal yr BP onwards, likely influenced by a higher fire incidence that facilitated a dramatic expansion of morichales. The simultaneous appearance of charcoal particles and Mauritia pollen around 2000 cal yr BP supports the potential pyrophilous nature of this palm and the importance of fire for its recent expansion. The whole picture suggests human settlements similar to today – in which fire is an essential element – since around 2000 yr ago. Therefore, present-day southern Gran Sabana landscapes seem to have been the result of the synergy between biogeographical, climatic and anthropogenic factors, mostly fire.     

Vegetation and climate of the southern Chilean lake district during and since the last interglaciation

The palynology of stratigraphic sections from road-cut and gravel-pit exposures and from a fen and sphagnum bogs in the southern part of the Chilean lake district (40° 53′ S, 72°37′ W-41°24′ S, 72°53′ W) is the basis for interpreting vegetation and climate during the last interglaciation and glaciation (named Llanquihue Glaciation) and during the post-glacial. To help interpretation, modern pollen rain was studied in relation to vegetation and altitude along a transect on the west slope of the Andes, and average January (summer) temperatures were interpreted. The upper limit of closed Andean forest, where wind is a determinant, appears to be close to the 12°C January isotherm; parkland in southern Chile does not exceed the January isotherm of 9°C.Grassland and later southern beech forest are evident during the interglaciation that is dated at more than 39,900 radiocarbon yr. Climate of the grassland was relatively dry; during the forest phase, it was wet, cool, and approximately the same as at present. During Llanquihue Glaciation, average January temperature is estimated to have been about 8°C colder than today at 19,450 BP, some 5° colder shortly before 36,300 BP, and around 4° colder at 10,000 BP. Antarctic-alpine tundra or parkland, under colder, drier climate, is mostly in evidence in the vicinity of the study sites before about 12,000 BP. During the postglacial, forest communities occupied the lake district, and temperatures there were probably 1–2°C above (by 6500 BP) and as much as 2° below (4500-0 BP) the present-day average of about 16°.This pattern of climatic changes finds accord, in general terms, in other parts of the Southern Hemisphere where palynological, chronological, and glacial geological studies are reported. Postulated as a cause of these changes are shifts in the intensity of air mass circulation in antarctic latitudes.     

Continuous record of environmental changes in Chukotka during the last 350 thousand years

The comprehensive study of the upper 1283 cm of sediment from Lake El’gygytgyn, which formed nearly 4 Ma ago following a meteorite impact in northern Chukotka, yielded the first continuous record of the extreme changes in the Beringian climate and vegetation from the middle Middle Pleistocene to recent time (equivalent of marine isotope stages of 1–7 and the upper part of isotope stage 8). During this period, the climate was warmer than at present between 8600 and 10 7000 ¹⁴C years and during the Late Pleistocene (isotope substage 5e, 116–128 ka ago). In 2003, the German-Russian-USA expedition continued studying sediments of Lake El’gygytgyn to obtain new evidence of the change in the vegetation cover in the Middle Pleistocene and the first information on the Middle Pleistocene interglacial (isotope stage 9; 297–347 ka ago). Pollen spectra characterizing the Middle Pleistocene interglacial are similar to spectra of the early stage of the Early Pleistocene interglacial and the climatic optimum in the Pleistocene to Holocene transitional period. The climatic history of Lake El’gygytgyn is basic for stratigraphic interpretations and correlations in the eastern sector of the Arctic. These data also expand our understanding of climatic changes that are studied within the framework of the “Pole-Equator-Pole Paleoclimate,” “Past Global Changes,” and other international projects.