Holocene hydrological changes in south-western Mediterranean as recorded by lake-level fluctuations at Lago Preola,a coastal lake in southern Sicily,Italy

This paper presents a high-resolution lake-level record for the Holocene at Lago Preola (Sicily, southern Italy) based on a specific sedimentological approach, with a chronology derived from AMS radiocarbon dates. It gives evidence of three major successive palaeohydrological periods, with (1) a pronounced dryness during the early Holocene until ca 10300 cal BP, (2) a highstand from ca 10300 to 4500 cal BP, and (3) a marked lowstand from 4500 cal BP to present. Large amplitude lake-level fluctuations characterise two transition phases at ca 10300–9000 and 6400–4500 cal BP. Period 2 was interrupted between 8300 and 7000 cal BP by a dry phase that was punctuated to ca 7300 cal BP by the deposition of a tephra from neighbouring Pantelleria Island. Comparisons of the Preola record with other palaeohydrological records along north–south and west–east transects in the Mediterranean show contrasting patterns of hydrological changes: north (south) of around 40°N latitude, the records highlight a mid-Holocene period characterised by lake-level minima (maxima). Humid mid-Holocene conditions over the Mediterranean south of 40°N were probably linked to a strong weakening of the Hadley cell circulation and of monsoon winds. We suggest that the maximum of humidity in the Mediterranean during the mid-Holocene was characterised by humid winters to the north of 40°N and humid summers to the south. On a multi-centennial scale, the high-resolution palaeohydrological reconstructions in the central Mediterranean area reveal a strong climate reversal around 4500–4000 cal BP, with contrasting changes in the hydrological cycle. In addition to seasonal and inter-hemispherical changes related to orbital forcing, this major oscillation might be related to non-linear responses of the climatic system to the gradual decrease in summer insolation at northern latitudes. Another major climate oscillation around 7500–7000 cal BP may have resulted from the combined effects of (1) a strong rate of change in insolation, and (2) variations in solar activity. Finally, comparisons of the Preola lake-level record with Sicilian pollen records suggest a strong influence of moisture availability on vegetation development in Sicily. Very dry early Holocene conditions probably prevented the expansion of coastal evergreen forests, while decreasing moisture availability since the onset of the late Holocene may have exacerbated effects of intensive land-use.     

Holocene palaeohydrological changes in the northern Mediterranean borderlands as reflected by the lake-level record of Lake Ledro,northeastern Italy

A lake-level record of Lake Ledro (northern Italy) spans the entire Holocene with a chronology derived from 51 radiocarbon dates. It is based on a specific sedimentological approach that combines data from five sediment profiles sampled in distinct locations in the littoral zone. On a millennial scale, the lake-level record shows two successive periods from 11,700 to 4500 cal yr BP and from 4500 cal yr BP to the present, characterized by lower and higher average lake levels, respectively. In addition to key seasonal and inter-hemispherical changes in insolation, the major hydrological change around 4500 cal yr BP may be related to a non-linear response of the climate system to orbitally-driven gradual decrease in insolation. The Ledro record questions the notion of an accentuated summer rain regime in the northern Mediterranean borderlands during the boreal insolation maximum. Moreover, the Ledro record highlights that the Holocene was punctuated by successive centennial-scale highstands. Correlations with the Preboreal oscillation and the 8.2 ka event, and comparison with the atmospheric ¹⁴C residual record, suggest that short-lived lake-level fluctuations developed at Ledro in response to (1) final steps of the deglaciation in the North Atlantic area and (2) variations in solar activity.     

Early-Holocene climatic oscillations recorded by lake-level fluctuations in west-central Europe and in central Italy

On the basis of lake-level data, the period of the Preboreal oscillation (PBO) at 11 300–11 150 cal yr BP as defined by the GRIP oxygen-isotope record appears to correspond to wetter climatic conditions in west-central Europe and to a marked drying in north-central Italy. Additional short-lived phases of higher lake level have been identified before the PBO at ca 11 450–11 400 and 11 350 cal yr BP in west-central Europe, and at ca 11 500 cal yr BP in Italy. Such multiple climatic oscillations around the PBO have been observed in various records in northwestern Europe. On the basis of various proxies, a map of changes in moisture over western Europe during the PBO indicates a mid-latitude zone between 58 and 43°N characterised by wetter conditions, while a drier climate developed in southern and northern Europe. This wet middle zone shows a larger extension and suggests a more meandering, weaker Atlantic Westerly Jet during the PBO than during the 8.2 kyr event. A comparison of lake-level records in west-central Europe with (1) outbursts from North American and north-European proglacial lakes, and (2) variations in solar activity as reflected by ¹⁴C and the ¹⁰Be records supports the hypothesis that the PBO was a response to successive meltwater pulses at 11 300, 11 250, 11 200 and 11 170 cal yr BP and to a sudden decrease in solar activity at 11 250 cal yr BP. This study points to the necessity of developing integrated multi-proxy approaches to construct more robust regional event stratigraphies, and of better documenting palaeohydrological changes in the Mediterranean area.     

Climatic oscillations in central Italy during the Last Glacial–Holocene transition: the record from Lake Accesa

This paper presents an event stratigraphy based on data documenting the history of vegetation cover, lake‐level changes and fire frequency, as well as volcanic eruptions, over the Last Glacial–early Holocene transition from a terrestrial sediment sequence recovered at Lake Accesa in Tuscany (north‐central Italy). On the basis of an age–depth model inferred from 13 radiocarbon dates and six tephra horizons, the Oldest Dryas–Bølling warming event was dated to ca. 14 560 cal. yr BP and the Younger Dryas event to ca. 12 700–11 650 cal. yr BP. Four sub‐millennial scale cooling phases were recognised from pollen data at ca. 14 300–14 200, 13 900–13 700, 13 400–13 100 and 11 350–11 150 cal. yr BP. The last three may be Mediterranean equivalents to the Older Dryas (GI‐1d), Intra‐Allerød (GI‐1b) and Preboreal Oscillation (PBO) cooling events defined from the GRIP ice‐core and indicate strong climatic linkages between the North Atlantic and Mediterranean areas during the last Termination. The first may correspond to Intra‐Bølling cold oscillations registered by various palaeoclimatic records in the North Atlantic region. The lake‐level record shows that the sub‐millennial scale climatic oscillations which punctuated the last deglaciation were associated in central Italy with different successive patterns of hydrological changes from the Bølling warming to the 8.2 ka cold reversal. Copyright © 2006 John Wiley & Sons, Ltd.     

Reconstruction and regional correlation of Holocene lake-level fluctuations in Lake Bysjön, South Sweden

Holocene lake-level fluctuations in Lake Bysjön are reconstructed from recorded changes in the sediment limit and the content of reworked minerogenic matter in the sediment. These recorded changes are related to past lake-level fluctuations by correlation to fluctuations convincingly demonstrated in earlier studies. A correlation of regionally significant fluctuations in South Sweden is presented, and the climatic interpretation is discussed. A distinct lowering in lake level culminated at about 9,500-9,200 B.P., recording a major period of drier climate in the earlier part of the Holocene. After a succeeding period of increased humidity, recorded by rising and relatively higher lake level, another major period of increased dryness began at about 6,800-6,500 B.P. In contrast to the older period, the climate was not uniformly drier, but anumber of demonstrated lake-level fluctuations suggest a fluctuating climate. From the reconstruction in Bysjön, dryness culminated at about 4,900-4,600 B.P., and the major period lasted until about 2,900-2,600 B.P. After a succeeding rise in lake level, another distinct lowering is recorded some time between 1,800 B.P. and 1,200 B.P.     

苏北平原沉积孢粉组合记录的全新世气候突变

通过中国东部苏北平原厚3.4 m的PM4剖面孢粉分析结果,重建了该地区全新世以来古植被更替过程,查明了5.20~3.00 cal.ka B.P.中国东部沿海平原百年尺度的海平面变化历史,并探讨了沿海平原植被对海平面变化的响应。孢粉结果表明,全新世以来该地区的植被与气候经历了以下5个阶段:1)11.90~8.80 cal.ka B.P.,亚热带常绿、落叶阔叶混交林繁盛,气候温暖湿润;2)8.80~7.40 cal.ka B.P.,发育了常绿、落叶阔叶混交林-草地,气候相对冷干;3)7.40~5.20 cal.ka B.P.,植被为常绿、落叶阔叶混交林,温度降低,降水增加;4)5.20~1.08 cal.ka B.P.,植被退化为盐生草甸;5)1.08 cal.ka B.P.以来植被中湿生草本群落繁盛。根据藜科、蒿属、禾本科和莎草科花粉的含量变化及粒度、有孔虫特征重建了相对海平面的变化历史,5.20~4.55 cal.ka B.P.、4.25~4.05 cal.ka B.P.与4.55~4.25 cal.ka B.P.、4.05~3.00 cal.ka B.P.分别为海平面上升和下降的阶段。研究区4.25~4.05 cal.ka B.P.海平面上升可与大范围尺度的"4.2 ka B.P.气候事件"相对应,可能受到了当时东亚季风减弱、苏北平原强降水与大规模洪水事件的共同驱动。     

Geophysical evidence for Holocene lake-level change in southern California (Dry Lake)

Bird, B. W., Kirby, M. E., Howat, I. M. & Tulaczyk, S. 2009: Geophysical evidence for Holocene lake-level change in southern California (Dry Lake). Boreas , 10.1111/j.1502-3885.2009.00114.x. ISSN 0300-9483.
Ground penetrating radar (GPR) data are used in combination with previously published sediment cores to develop a Holocene history of basin sedimentation in a small, alpine lake in southern California (Dry Lake). The GPR data identify three depositional sequences spanning the past 9000 calendar years before present (cal. yr BP). Sequence I represents the first phase of an early Holocene highstand. A regression between <8320 and >8120 cal. yr BP separates Sequence I from Sequence II, perhaps associated with the 8200 cal. yr BP cold event. Sequence II represents the second phase of the early-to-mid Holocene highstand. Sequence IIIa represents a permanent shift to predominantly low lake stands beginning ∼5550 cal. yr BP. This mid-Holocene shift was accompanied by a dramatic decrease in sedimentation rate as well as a contraction of the basin's area of sedimentation. By ∼1860 cal. yr BP (Sequence IIIb), the lake was restricted to the modern, central basin. Taken together, the GPR and core data indicate a wet early Holocene followed by a long-term Holocene drying trend. The similarity in ages of the early Holocene highstand across the greater southern California region suggests a common external forcing – perhaps modulation of early Holocene storm activity by insolation. However, regional lake level records are less congruous following the initial early Holocene highstand, which may indicate a change in the spatial domain of climate forcing(s) throughout the Holocene in western North America.     

Younger Dryas and early Holocene lake-level fluctuations in the Jura mountains, France

Lake-level fluctuations in the Jura mountains (France) during the Younger Dryas and the early Holocene are reconstructed using sedimentological analyses. Major transgressive phases culminated just before the Laacher See tephra deposition, at the beginning of the Younger Dryas, between 9000 and 8000 BP and between 7000 and 6000 BP. The Younger Dryas appears to be characterized by increasing dryness. Other major lowering phases occurred during the middle Allerød and during the Preboreal. A transgressive event developed between c . 9700 and 9500 BP. These palaeohydrological changes can be related to climatic oscillations reconstructed from pollen and isotopic records in Swiss lakes, from glacier movements and timberline variations in the Alps, and from isotopic records in the Greenland ice sheet.     

内蒙古中部达尔罕茂明安联合旗腾格尔诺尔湖泊 沉积物记录的全新世晚期环境演变

通过对内蒙古中部腾格尔诺尔湖泊沉积、粒度组成、孢粉组合等特征的综合分析,重建了该地区全新世晚期1700aBP以来的环境演变过程。结果表明,该地区1700aBP以来的气候环境变化可以分为4个阶段:1700～1170aBP,气候温暖较湿润;1170～840aBP,为温干气候期;840～200aBP,气候转为温暖湿润,间有短期的冷干波动;200aBP以来气候以温干为主,晚期气候有向凉干转变的趋势。     

鲁西孟家屯岩组中发现红柱石和锌尖晶石

侯增谦、高永丰、黄卫于２００１年发表的《西藏高原雅鲁藏布江北岸蛇绿岩带的发现及其地质意义》一文中，把所发现的蛇绿岩带描述为“谢通门蛇绿岩带东起大竹卡区，西至谢通门以西，长数百千米，宽３～５km，由一系列自北而南逆冲推覆的构造岩片构成，其中，基性玄武熔岩岩片、巨厚辉长岩岩片、超镁铁质辉石岩－纯橄岩岩片，依次由雅江近岸至冈底斯弧内部呈叠瓦状排布。变质橄榄岩出露零星，少许孤立的岩体出露于雅江近岸，发育强烈碳酸盐化和蛇纹石化；大量散布的岩群出露于堆积岩岩片的北侧，多在冈底斯弧花岗岩基和弧火山－沉积岩系中呈…     

内蒙古中部达尔罕茂明安联合旗腾格尔诺尔湖泊沉积物记录的全新世晚期环境演变

通过对内蒙古中部腾格尔诺尔湖泊沉积、粒度组成、孢粉组合等特征的综合分析,重建了该地区全新世晚期1700aBP以来的环境演变过程。结果表明,该地区1700aBP以来的气候环境变化可以分为4个阶段:1700～1170aBP,气候温暖较湿润;1170～840aBP,为温干气候期;840～200aBP,气候转为温暖湿润,间有短期的冷干波动;200aBP以来气候以温干为主,晚期气候有向凉干转变的趋势。     

青藏高原东北缘冬给错纳湖全新世湖面波动

青藏高原处于东亚季风、印度季风和西风环流交互作用区.末次冰消期以来,太阳辐射对该地区的古气候环境产生了重要影响,湖泊随着季风系统的变化发生了明显的水位升降,对湖岸阶地的形成起到了直接作用.本项研究重建了青藏高原东北缘冬给错纳湖湖岸阶地记录的湖面波动历史,试图了解青藏高原季风系统演变过程.通过湖泊北岸265 cm厚湖岸阶地沉积物粒度、碳酸盐、矿物、元素和介形虫古环境指标,结合OSL年代模式,分析表明在约10. 2 ka B. P.之前水体较浅;约10. 2~9. 0 ka B. P.湖面开始上升,气候凉湿;9. 0~8. 5 ka B. P.为印度季风强盛期,湖面明显上升,降雨量增高、温度上升;8. 5~7. 9 ka B. P.湖面降低与气候变冷有关;7. 9~7. 0 ka B. P.印度季风开始减弱,气温、降雨下降,但有效湿度较大,湖面降低;7. 0~6. 1 ka B. P.湖面上升可能与低蒸发作用有关,印度季风仍然影响该地区;6. 1~5. 2 ka B. P.,印度季风衰退,气候逐渐变冷、降雨量减小、水体变浅;5. 2~4. 6 ka B. P.气候冷干,有效湿度减小,湖面进一步下降;4. 6 ka B. P.至今气候干冷,东亚季风衰退,湖面下降,期间也可能受西风环流影响而有短暂的降雨增加时期.     

大兴安岭阿尔山天池沉积物中正构烷烃记录揭示的全新世古气候变化

我国东北地区地处东亚季风北缘,在高、低纬过程的共同影响下,区域气候和环境变化较为复杂。文章选取大兴安岭中段的阿尔山天池长2.5m的全新世以来的湖泊沉积岩芯（ACL17C）作为研究对象,对源于植物叶蜡的正构烷烃及相关代用指标进行分析,结果表明阿尔山天池自10.0cal. ka B.P.以来,正构烷烃浓度所代表的湖泊及流域的生产力逐渐增强,并在5.0~3.0cal. ka B.P.期间达到峰值,结合其他指标推测该阶段湖泊水位较高,有机物输入稳定,沉积速率较慢,气候温暖湿润,可能是全新世气候适宜期;2.3~1.5cal. ka B.P.期间,正构烷烃浓度下降,沉积物颗粒变粗,指示湖泊水位突降,藻类含量上升,植被状况较差,可能发生了区域性的气候变干事件;1.5cal. ka B.P.之后,相关指标显示植被状况转好,湖泊水位上升。通过对比大、小兴安岭及附近区域的植被及气候演化结果,表明该区域全新世气候适宜期可能发生在中、晚全新世,与其他地区记录的早、中全新世适宜期存在显著差异。

     

Gypsum–anhydrite transformations in Messinian evaporites of central Tuscany (Italy)

The Messinian succession of Tuscany (central Italy) contains three evaporitic units. Among the several exposed evaporitic lithofacies, only selenitic gypsum precipitated directly from evaporating brines. All the other facies, nodular microcrystalline gypsum, gypsarenites and gypsum laminites, despite their macroscopic differences, display the same petrographic textures, indicating that they are the product of dehydration of gypsum to give anhydrite which has been successively rehydrated to secondary gypsum. These secondary facies show an entire array of textures ranging from cloudy ameboid (xenotopic) with anhydrite relics, to idiotopic without anhydrite relics, that are here interpreted as a sequence of progressive stages of rehydration. The presence of completely hydrated petrofacies at the core of nodules which display a less hydrated rim suggests that these rocks have undergone at least two cycles of a dehydration–rehydration process. This interpretation is supported by the presence of satin spar veins that are replaced by microcrystalline gypsum. Satin spar itself is considered to be a by-product of anhydrite hydration. The first dehydration–rehydration event affected the entire gypsum deposit, producing a completely hydrated (idiotopic) facies together with satin spar veins; the second affected only veins, fractures and the rims of nodules, turning the first generation of satin spar and idiotopic gypsum into cloudy ameboid gypsum. Sedimentary structures typical of sabkha environments indicate for the youngest formation that the first dehydration and rehydration process occurred syndepositionally. The preservation of primary gypsum facies only at sites with condensed sections, indicate for the oldest two formations that the first dehydration event occurred upon burial. This event has been estimated to have occurred in the earliest Pliocene. After the Early Pliocene, dehydration was favored even at shallow depths, due to an increased heat flow related with the emplacement of local crustal magmatic bodies. Rehydration possibly occurred when these formations were uplifted and exposed to ground and/or meteoric water. The Volterra Basin has undergone alternating subsidence and uplift events, that can account for two dehydration–rehydration processes at least, also driven by alternating circulation, in the tectonic fractures, of fresh and salty water, the latter derived from dissolution of Messinian halite.     

Pollen-inferred Late-Glacial and Holocene climate in southern Balkans (Lake Maliq)

High-temporal resolution analysis of pollen records from Lake Maliq (Albania) provides quantitative estimates of monthly temperature and precipitation changes since the last deglaciation. The climate parameters were estimated using the best modern analogue technique with an updated modern pollen-climate database composed of 2748 surface samples. The record shows two main cooling phases in the Maliq area (the Oldest and Younger Dryas) and a cooling event around 8200 years, which suggests that the forcing factors driving climate variations in the North Atlantic area since the Last Glacial period also extended their influence into the Mediterranean area. The Oldest and Younger Dryas are also characterized by an arid climate and a change in the seasonality of precipitation: the summer precipitation tends to be greater during the cooling phases than during the temperate periods. The Holocene climate is relatively stable and the values of each parameter reach their modern levels, except for an arid event between 8300 and 8100 cal BP.     

Hydrological changes during the Roman Climatic Optimum in northern Tuscany (Central Italy) as evidenced by speleothem records and archaeological data

Study of the climate in the Mediterranean basin during different historical periods has taken on a particular importance, particularly regarding its role (together with other factors) in the evolution of human settlement patterns. Although the Roman age is traditionally considered a period with a favourable climate, recent studies have revealed considerable complexity in terms of regional climate variations. In this paper, we compare the hydrological change from speleothem proxy records with flood reconstructions from archaeological sites for Northern Tuscany (central Italy). We identify a period of oscillating climatic conditions culminating in a multidecadal dry event during the 1st century bc , followed by a century of increased precipitation at the beginning of the Roman Empire and subsequently a return to drier conditions in the 2nd century ad. The period of rainfall increase documented by the speleothems agrees with both the archaeological flood record as well as historical flood data available for the Tiber River, ca. 300 km to the south. These data also suggest a return to wetter conditions following the 3nd and 4rd centuries ad.     

Holocene climate changes in the mid-high-latitude-monsoon margin reflected by the pollen record from Hulun Lake, northeastern Inner Mongolia

Pollen-assemblage data from a sediment core from Hulun Lake in northeastern Inner Mongolia describe the changes in the vegetation and climate of the East Asian monsoon margin during the Holocene. Dry steppe dominated the lake basin from ca. 11,000 to 8000 cal yr BP, suggesting a warm and dry climate. Grasses and birch forests expanded 8000 to 6400 cal yr BP, implying a remarkable increase in the monsoon precipitation. From 6400 to 4400 cal yr BP, the climate became cooler and drier. Chenopodiaceae dominated the interval from 4400 to 3350 cal yr BP, marking extremely dry condition. Artemisia recovered 3350-2050 cal yr BP, denoting an amelioration of climatic conditions. Both temperature and precipitation decreased 2050 to 1000 cal yr BP as indicated by decreased Artemisia and the development of pine forests. During the last 1000 yr, human activities might have had a significant influence on the environment of the lake region. We suggest that the East Asian summer monsoon did not become intensified until 8000 cal yr BP due to the existence of remnant ice sheets in the Northern Hemisphere. Changes in the monsoon precipitation on millennial to centennial scales would be related to ocean-atmosphere interactions in the tropical Pacific.     

云南华坪全新世大暖期百年尺度的季风气候的石笋记录

通过对云南华坪葫芦洞FL4石笋进行高精度的ICP—MS—230Th/U测年和高分辨率的碳、氧同位素分析,建立了该地区6 060-4 185 a BP间高分辨率的西南季风气候变化时间序列,进而揭示了该时段发生的3次季风减弱事件。 这3次百年尺度（持续时间为90～240a）的干旱寒冷事件,分别发生在6 060－5 950 a BP、5 380－5 140 a BP、4 810-4 620 a BP,呈台阶状演变;而石笋的碳同位素记录揭示了2次强降水事件,分别发生在5 503－5 443 a BP和4 210-4 185 a BP,持续时间分别为25a和60a。石笋碳、氧同位素记录的西南季风减弱以及强降水事件明显受太阳辐射强度的控制。分辨率为3~10 a的碳、氧同位素记录表明,在百年尺度的西南季风气候变化上,叠加了一系列十年尺度的气候突变事件,呈锯齿状的高频波动。这些短时间尺度的季风气候波动事件与树轮14C 残差、冰芯记录极为相似,反映低纬度地区石笋记录的季风气候与高纬度及北极地区的气候具有极好的可比性,可能主要是受中低纬度太阳辐射强度以及北半球大气环流的影响,太阳辐射强度的变化是控制印度季风的快速推进或退出(萎缩)以及百年尺度上的气候波动的主要动因。      

Geochemistry of regional aquifer systems hosted by carbonate-evaporite formations in Umbria and southern Tuscany (central Italy)

In central Italy Mesozoic carbonates represent the principal reservoir of freshwater of the region. The hydrogeological setting is linked to the geological evolution of the Apennine chain and is generally characterised by a lower aquifer and one or more shallower aquifers separated by thin aquicludes. In these systems, groundwater composition is the result of a complex array of regional and local geochemical processes. The main geochemical processes are the dissolution of calcite, the influx of deeply derived CO₂ related to a regional process of mantle degassing, dedolomitization and mixing with deep saline fluids. The occurrence of saline fluids, characterised by a Na–Cl(HCO₃) composition, is related to the presence of a deep regional aquifer at the base of Mesozoic carbonates. The extremely high pCO₂ values computed for the saline waters suggest that the deep aquifer is also a structural trap for the mantle derived CO₂ during its ascent towards the surface. In central Italy, geological and geophysical data highlight the presence of two different crustal sectors: the eastern sector, where the geometry of the Apennine thrust belt is still preserved, and the western sector, where the compressive structures are dislocated by important extensional deformations. In the western sector, the normal faults disrupting the compressive structures allow the mixing of the deep Na–Cl(HCO₃) fluids with the shallow groundwater causing a salinity increase and the natural deterioration of groundwater quality.