Chronology of the Black Sea over the last 25,000 years

Deep-water sediments of the Black Sea deposited during Late Pleistocene and Holocene time are distinguished by three sedimentary units: (1) a microlaminated coccolith ooze mainly consisting of Emiliania huxleyi; (2) a sapropel; and (3) a banded lutite. The base of the first unit lies at 3,000 years B.P., that of the second at 7,000 years B.P., and that of the third at least at about 25,000 years B.P. Fossils and geochemical criteria are used to decipher the environmental events of this time period. Beginning with the base of the section dated at about 25,000 years B.P. we witness the final stage of metamorphosis from anoxic marine to oxic freshwater conditions. By the time this stage ended, about 22,000 years B.P., the Black Sea had become a truly freshwater habitat. The lake phase lasted about 12,000 to 13,000 years. Sedimentation rates were in the order of 1 m/10³ years, but began to decrease as sea level rose during the last 5,000 years of this phase (9,000–15,000 years B.P.). Starting at about 9,000 years B.P. and continuing to 7,000 years B.P., Mediterranean waters occasionally spilled over the Bosporus as a consequence of ice retreat and sea level rise. This marked the beginning of a gradual shift from freshwater to marine, and from well aerated to stagnant conditions. At about 7,000 years B.P. when deposition of unit 2 started, the H₂S zone was well established. Sedimentation rates dropped to 10 cm/10³ years. Environmental conditions similar to those of today finally became established around 3,000 years B.P., almost exactly the time when Jason and the Argonauts sailed through the Bosporus in search of the Golden Fleece.     

Dominant Northern Hemisphere climate control over millennial-scale glacial sea-level variability

Based on a radiocarbon and paleomagnetically dated sediment record from the northern Red Sea and the exceptional sensitivity of the regional changes in the oxygen isotope composition of sea water to the sea-level-dependent water exchange with the Indian Ocean, we provide a new global sea-level reconstruction spanning the last glacial period. The sea-level record has been extracted from the temperature-corrected benthic stable oxygen isotopes using coral-based sea-level data as constraints for the sea-level/oxygen isotope relationship. Although, the general features of this millennial-scale sea-level records have strong similarities to the rather symmetric and gradual Southern Hemisphere climate patterns, we observe, in constrast to previous findings, pronounced sea level rises of up to 25 m to generally correspond with Northern Hemisphere warmings as recorded in Greenland ice-core interstadial intervals whereas sea-level lowstands mostly occur during cold phases. Corroborated by CLIMBER-2 model results, the close connection of millennial-scale sea-level changes to Northern Hemisphere temperature variations indicates a primary climatic control on the mass balance of the major Northern Hemisphere ice sheets and does not require a considerable Antarctic contribution.     

巴颜喀拉山东端过去376年来夏季气温变化

巴颜喀拉山地处青藏高原东缘,是长江和黄河上游的分水岭,其生态环境与气候背景变化对下游地区的生态安全具有重要影响,但较短的器测记录限制了对该地区气候变化历史和机制的认识。本文基于巴颜喀拉山东端青海云杉（Picea crassifolia）486年树轮宽度年表,重建了该地区376年的夏季（6月、7月和8月）平均气温变化。结果显示,研究区经历了2个冷期（1724~1789年、1864~1994年）和3个暖期（1637~1723年、1790~1863年和1995~2012年）。经集合经验模态分解（Ensemble Empirical Mode Decomposition,简称EEMD）发现,重建温度序列的年代际和多年代际信号与太平洋年代际涛动（Pacific Decadal Oscillation,简称PDO）和北大西洋涛动（North Atlantic Oscillation,简称NAO）的变化有较好的一致性,表明PDO和NAO可能对研究区夏季温度变化具有重要影响,但具体影响机制需要气候模式来进一步揭示。     

Millennial-scale variability in Southern Hemisphere westerly wind activity over the last 5000 years in SW Patagonia

Here we report results from a high-resolution palynological record and stratigraphic/geochronologic data related to a Neoglacial event in Torres del Paine National Park, southern Chile (51°S, 71°W), to investigate climatic variations in Southwest Patagonia during the last 5000 years. The record reveals a stepwise expansion of Nothofagus-dominated woodlands and forests with discrete pulses at 4400, 2900, 1300, and 570 cal yr BP. Superimposed upon this trend we identify a relative opening of the woodlands between 4100–2900 and 2300–1300 cal yr BP. Closed-canopy forests dominated the landscape between 570–60 cal yr BP, followed by a rapid decline at the end of the 19th century that coincided with intense fire activity and the appearance of Rumex cf. acetocella, an exotic species introduced by European settlers. We interpret these changes as variations in the intensity and/or position of the southern margin of the westerly winds, which culminated with a net eastward shift of the forest–steppe ecotone during the Little Ice Age. We propose that millennial-scale changes in either the latitudinal position and/or the overall strength of the southern westerlies may be responsible for vegetation changes, fire occurrence, and the dynamic behavior of Patagonian glaciers during the last 5000 years. Because the modern maximum in near-surface wind velocities and precipitation is located between 48° and 50°S, we suggest that the core of the southern westerlies may have achieved this modern position ～570 years ago.     

Climate variability in the SE Alps of Italy over the past 17 000 years reconstructed from a stalagmite record

Stalagmite SV1 from Grotta Savi, located at the SE margin of the European Alps (Italy), is the first Alpine speleothem that continuously spans the past c . 17kyr. Extension rate and δ¹⁸O_c record for the Lateglacial probably reflect a combination of temperature and rainfall, with rainfall exerting the dominant effect. Low speleothem calcite δ¹⁸ O_c values were recorded from c . 14.5 and 12.35 kyr, during GI-1 (Bølling— Allerød) interstadial, which in our interpretation, was warm and wet. The GS-1 (Younger Dryas) was characterized by a shift to heavier δ¹⁸ O_c, coinciding with δ¹³C_c enrichment and extremely low extension rate (<8 μm/year). These characteristics indicate that GS-1 climate was cool and dry in the SE Alps. Calibration using historical data revealed that there is a positive δ¹⁸O_c/dT relationship. A 1°C rise in mean annual temperature should correspond to c . 2.85% increase of SV-1 δc¹⁸O_c. We reconstructed a slow and steady temperature rise of c . 0.5°C since 10 kyr BP, in broad agreement with reconstructions from pollen data for SE Europe. Stalagmite SV1 indicates that climate variability in the SE Alps has been influenced by the Mediterranean Sea for the past c . 17 kyr.     

Persistent sea surface temperature and declined sea surface salinity in the northwestern tropical Pacific over the past 7500 years

To understand Holocene climate evolutions in low-latitude region of the western Pacific, paired δ¹⁸O and Mg/Ca records of planktonic foraminifer Globigerinoides ruber (250–300 μm, sensu stricto, s.s.) from a marine core ORI715-21 (121.5°E, 22.7°N, water depth 760 m) underneath the Kuroshio Current (KC) off eastern Taiwan were analyzed. Over the past 7500 years, the geochemical proxy-inferred sea surface temperature (SST) hovered around 27–28 °C and seawater δ¹⁸O (δ¹⁸O_W) slowly decreased 0.2–0.4‰ for two KC sites at 22.7° and 25.3°N. Comparison with a published high-SST and high-salinity equatorial tropical Pacific record, MD98-2181 located at the Mindanao Current (MC) at 6.3°N, reveals an anomalous time interval at 3.5–1.5 kyr ago (before 1950 AD). SST gradient between the MC site and two KC site decrease from 1.5–2.0 °C to only 0–1 °C, and δ¹⁸O_W from 0.1–0.3‰ to 0‰ for this 2-kyr time window. The high SST and low gradient could result from a northward shift of the North Equatorial Current, which implies a weakened KC. The long-term descending δ¹⁸O_W and increasing precipitation in the entire low-latitude western Pacific and the gradually decreasing East Asian summer monsoonal rainfall during middle-to-late Holocene is likely caused by different land and ocean responses to solar insolation and/or enhanced moisture transportation from the Atlantic to Pacific associated with the southward movement of ITCZ.     

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.     

The last 25,000 years in the Eastern Plateau of Southern Brazil according to Alpes de São Francisco record

The palaeoenvironmental history has been studied based on palynology of a sedimentary profile from the Alpes de São Francisco bog (29°29′35′′S, 50°37′18′′W), São Francisco de Paula municipality, Rio Grande do Sul eastern Plateau, extreme Southern Brazil. The results indicate a regional cold and dry climate between 25,000 and 12,500 yr BP, interpreted from the grassland vegetation, forest taxa were present in refuges and the shallow local lake began to fill in. Climatic conditions became more aride after 16,000 yr BP, when grassland became rare. From 12,500 yr BP onwards, the climate began to change and at 11,000–9700 yr BP a warm and moist climate permitted the slight migration of pioneer arboreal taxa from refuges and locally a marsh formation. Between 9700 and 6500 yr BP a warm and dry climate resulted in reduction of grassland, confined the forest in refuges, dried out the marsh. The gradual increase of humidity between 6500 and 4000 yr BP allowed migration of forests from refuges and a bog developed. Between 4000 and 2000 yr BP Araucaria forest spread, indicating moister climate. The local bog expanded. From 2000 yr BP onwards, humid but warmer climate seems to result in a lower reproductive capacity of Araucaria forest taxa limiting its expansion. The bog reached the present-day in a decline condition. The results are compared to previous records from Southern Brazil highlands and some places from Argentina in order to better elucidate the climatic and vegetational history of these important South America areas during the late Quaternary.     

Synchronous variability changes in Alpine temperature and tree-ring data over the past two centuries

The understanding of extremes and their temporal distribution is useful in characterizing the behaviour of the climate system, and necessary for understanding their social and economic costs and risks. This task is analogous to the study of pointer years in dendrochronological investigations. Commonly used dendroclimatological methods, however, tend to result in an equalization of variance throughout the record by normalizing variability within moving windows. Here, we analyse a larger network of high-elevation temperature-sensitive tree sites from the European Alps processed to preserve the relative frequency and magnitude of extreme events. In so doing, temporal changes in year-to-year tree-ring width variability were found. These decadal length periods of increased or decreased likelihood of extremes coincide with variability measures from a long-instrumental summer temperature record representative of high-elevation conditions in the Alps. Intervention analysis, using an F-test to identify shifts in variance, on both the tree-ring and instrumental series, resulted in the identification of common transitional years. Based on a well-replicated network of sites reflecting common climatic variation, our study demonstrates that the annual growth rings of trees can be utilized to quantify past frequency and amplitude changes in extreme variability. Furthermore, the approach outlined is suited to address questions about the role of external forcing, ocean-atmosphere interactions, or synoptic scale changes in determining patterns of observed extremes prior to the instrumental period.     

Drivers of ecosystem and climate change in tropical West Africa over the past ∼540 000 years

A paucity of empirical non‐marine data means that uncertainty surrounds the impact of climate change on terrestrial ecosystems in tropical regions beyond the last glacial period. The sedimentary fill of the Bosumtwi impact crater (Ghana) provides the longest continuous Quaternary terrestrial archive of environmental change in West Africa, spanning the last ～1.08 million years. Here we explore the drivers of change in ecosystem and climate in tropical West Africa for the past ～540 000 years using pollen analysis and the nitrogen isotope composition of bulk organic matter preserved in sediments from Lake Bosumtwi. Variations in grass pollen abundance (0?99%) indicate transitions between grassland and forest. Coeval variations in the nitrogen isotopic composition of organic matter indicate that intervals of grassland expansion coincided with minimum lake levels and low regional moisture availability. The observed changes responded to orbitally paced global climate variations on both glacial–interglacial and shorter timescales. Importantly, the magnitude of ecosystem change revealed by our data exceeds that previously determined from marine records, demonstrating for the first time the high sensitivity of tropical lowland ecosystems to Quaternary climate change.

     

东南极冰盖Princess Elizabeth地区LGB69冰芯化学记录反映的南印度洋过去300 a大气环流变化

利用2002年中国南极考察期间在东南极冰盖Princess Elizabeth地区钻取的LGB69冰芯, 对其海盐离子与大气环流的关系进行了分析. 结果表明: 冰芯主要化学离子的经验正交函数(EOF)分解的第一特征向量(EOF1)可以作为描述海盐气溶胶的传输强度的代用指标, 可用于表征南印度洋准定常低压带海平面气压(SLP)的变化. 冰芯最近23 a(1979-2001年)的记录与SLP和低层风场的相关模态反映了海平面气压场准半年振荡(SAO)和南极大陆边缘下降风的季节特征. LGB69冰芯很好地记录了1708-2001年间南半球环状模(SAM)的变化特征, Na⁺时间序列呈现的3.5 a周期变化与SAM的周期变化具有很好地一致性. 在1970年SAM转为正位相后, 其与Na⁺的相关关系也由负相关转为正相关. LGB69冰芯的海盐离子可以作为重建过去近300 a海平面气压场和SAM变化的代用指标.     

Implications of climatic variability and climate change for water resources availability and management in West Africa

The paper examines: (a) trends in climatic variations and variability with particular emphasis on rainfall (b) the characteristics of climatic events, including floods and droughts, (c) seasonal variations in river flows, (d) mean annual trends in river flows and discharges, (e) local variations of extremes of rainfall and river discharges, (f) the effects of climatic variability and climate change on ground water variations, (g) the problems of acute shortage of freshwater, and (h) the prevalence of water stress whose characteristics would be worsened with the projected impacts of climate change. The results show that: (i) there are a lot of spatial and temporal variations in the characteristics of rainfall and the hydrological systems locally and regionally, although in general, there have been downward trends in rainfall and increases in water deficits and drought events, (ii) that flood events, which also have impacted adversely in many parts of the region, have also been witnessed. The paper then produces projections for future urban and rural water supplies in Nigeria, which is an epitome of West Africa and examines the two main categories of adaptation measures needed to improve water management, namely, those involving the water supply and water demand systems in the study region. Finally, the paper discusses the need to address a number of mechanisms for implementation of the various adaptation measures including: (a) building capacity and manpower, (b) promoting education and public awareness, (c) public participation and the involvement of stakeholders, (d) the establishment of both national and regional co-operation, and (e) the need for climatic and other environmental data collection and monitoring. This revised version was published online in July 2006 with corrections to the Cover Date.     

中亚地区过去2000年气候变化研究的数据挖掘

中亚地区是古丝绸之路的主要廊道,气候变率时空差异大,随着"一带一路"倡议的逐步推进,该地区过去2000年气候变化研究备受关注。文章参照世界数据中心古气候资料共享网收录文献的关键词,从Web of Science数据库检索该地区过去2000年气候变化研究文献;并结合中亚自然资源图集的政区、湖泊、山脉等地名信息,构建了面向该类文献的数据挖掘技术;据此提取了研究区（点）、时间、代用资料与研究结果等信息,分析了其间该地区冷暖与干湿变化研究的主要进展。主要结论有：1）中亚地区过去2000年气候变化的已有研究重点为局地温度、降水/干湿变化重建与冷暖、干湿、湖泊水位等的阶段性差异分析,其中尤以干湿相关研究居多;所用代用资料主要有树轮（占44.2%）、湖泊沉积物（占41.9%）、冰芯（占9.3%）等,其中湖泊沉积物、冰芯、石笋可覆盖整个2000年,而最长的树轮仅覆盖过去1100年,且多数集中在1700年之后;研究区集中分布于西天山及帕米尔高原、巴尔喀什湖、咸海及周边地区;研究时段虽覆盖整个2000年,但高分辨率重建工作主要集中于16世纪之后。2）中亚地区公元1~3世纪末、15世纪中期~19世纪中期气候相对寒冷,5世纪中期~9世纪末、12世纪中期~14世纪末和19世纪中期以后气候总体温暖,但在1230年前后和1680年前后也分别出现过显著转冷和回暖;20世纪后期的温暖程度可能与其前千年的最暖时段相当。西天山、帕米尔和咸海三地干湿的百年尺度阶段变化并不同步;其中在百年以上尺度,咸海及周边地区公元1~5世纪、10~12世纪中期相对偏干,5世纪~9世纪、12世纪后期~15世纪中期相对偏湿,此后缓慢转干持续至今;而在年代际尺度上,西天山和帕米尔山地降水自1980年起曾显著增加,但至2000年以后又显著转干。     

近30年黄河下游河道深泓线摆动特点

冲积河流深泓摆动具有复杂的时空变化特点,其摆动特点对于研究黄河下游河势变化及河床演变规律具有重要意义。以1986-2015年黄河下游91个淤积断面汛后实测地形资料为基础,计算下游断面及河段尺度的深泓摆动宽度及强度,分析近30年来黄河下游各河段的深泓摆动特点,并探讨游荡段深泓摆动强度的主要影响因素。结果表明:深泓摆动方向具有往复性,各断面深泓摆动宽度呈现上段大、中下段小的特点;小浪底水库运行后下游深泓摆动宽度大大降低,游荡段、过渡段和弯曲段年均深泓摆动宽度较水库运行前分别减小了47%、37%和41%;河床边界条件（滩槽高差、河床组成等）对游荡段深泓摆动强度有一定影响,但上游来水来沙条件是主要影响因素;建立了游荡段深泓摆动强度与该河段前4年平均水流冲刷强度的幂函数关系,并对经验公式进行了验证。     

Atmospheric dust variability from Arabia and China over the last 500,000 years

Atmospheric mineral dust aerosols affect Earth’s radiative balance and are an important climate forcing and feedback mechanism. Dust is argued to have played an important role in past natural climate changes through glacial cycles, yet temporal and spatial dust variability remain poorly constrained, with scientific understanding of uncertainties associated with radiative perturbations due to mineral dust classified as “very low”. To advance understanding of the dust cycle, we present a high-resolution dust record from the Red Sea, sourced principally from Arabia, with a precise chronology relative to global sea level/ice volume variability. Our record correlates well with a high-resolution Asian dust record from the Chinese Loess Plateau. Importing our age model from the Red Sea to the Chinese Loess Plateau provides a first detailed millennial-scale age model for the Chinese loess, which has been notoriously difficult to date at this resolution and provides a basis for inter-regional correlation of Chinese dust records. We observe a high baseline of dust emissions from Arabia and China, even through interglacials, with strong superimposed millennial-scale variability. Conversely, the distal EPICA Dome C Antarctic ice core record, which is widely used to calculate the radiative impact of dust variations, appears biased to sharply delineated glacial/interglacial contrasts. Calculations based on this Antarctic dust record will therefore overestimate the radiative contrast of atmospheric dust loadings on glacial/interglacial timescales. Additional differences between Arabian/Asian and circum-Saharan records reveal that climate models could be improved by avoiding ‘global mean’ dust considerations and instead including large-scale regions with different dust source variability.     

The area and absolute magnetic flux of sunspots over the past 400 years

A new series of yearly-mean relative sunspot numbers SN ₂ that has been extrapolated into the past (to 1610) is presented. The Kislovodsk series with the scale factor b = 1.0094 ± 0.0059 represents a reasonable continuation of the mean-monthly and mean-yearly total sunspot areas of the Greenwich series after 1976. The second maximum of the 24th solar-activity cycle was not anomalously low, and was no lower than 6 of the past 13 cycles. A series A ₂ of values for the total sunspot area in 1610–2015 has been constructed, and is complementary to new versions of the series of the relative number of sunspots SN ₂ and the number of sunspot groups GN ₂. When needed, this series can be reduced to yield a quantity having a clear physical meaning—the spot absolute magnetic flux Φ _Σ(t)[Mx] = 2.16 × 10¹⁹ A(t) [mvh]. The maximum sunspot area during the Maunder minimum is much higher in the new series compared to the previous version. This at least partially supports the validity of arguments that cast doubt on the anomalously low ampltude of the solar cycles during the Maunder minimum that has been assumed by many researchers earlier.     

Variation of slip and creep along the levant rift over the past 4500 years

Frequency—magnitude relations were established for a fault system embedded in a shear zone with dimensions 1000 km × 200 km that extends from the Gulf of Elat to the East-Anatolian fault. The resulting frequency—magnitude relations were found to be in good agreement with both the documented instrumental earthquake statistics for M_L 4.5 in the present century and microearthquake activity for M_L 1.5 recorded in Israel and some adjacent territories during 1976–1979. When these frequency—magnitude relations are extended to 2500 B.C., one can account for the maximal magnitude earthquakes that echo in the Bible and are manifested in archaeological excavations.

It is found that seismic slip-rates increase significantly from south toward north in such a way that in north-central Israel and Lebanon, seismic slip constitutes only 1/3 of the motion, the rest being attributed to visco-elastic processes. Further north, as one approaches the collision zone with the Turkish plate, aseismicity tends to disappear and the slip is purely seismic.