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.     

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.     

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.     

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…     

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.     

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.     

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。      

Holocene lake-level fluctuations in Jura and the northern subalpine ranges, France: regional pattern and climatic implications

Holocene lake-level fluctuations in Jurassian and French subalpine lakes are reconstructed using sedimentological analysis, and a regional pattern of palaeohydrological changes is put forward. Major transgressive phases culminated at about 8500 BP, 6500 BP, 4800 BP, 3500–2300 BP and 450 BP. (1) The synchroncity of Holocene lake-level changes demonstrated for a large number of lakes, (2) close correlations attested between certain lakes and (3) the coincidence between rises in lake-levels in Jura and French subalpine ranges, and glacier advances in Swiss and Austrian Alps argue for a climatic control of these Holocene lake-level fluctuations.     

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.     

Tephra layers from Holocene lake sediments of the Sulmona Basin, central Italy: implications for volcanic activity in Peninsular Italy and tephrostratigraphy in the central Mediterranean area

We present a new tephrostratigraphic record from the Holocene lake sediments of the Sulmona basin, central Italy. The Holocene succession is represented by whitish calcareous mud that is divided into two units, SUL2 (ca 32 m thick) and SUL1 (ca 8 m thick), for a total thickness of ca 40 m. These units correspond to the youngest two out of six sedimentary cycles recognised in the Sulmona basin that are related to the lake sedimentation since the Middle Pleistocene. Height concordant U series age determinations and additional chronological data constrain the whole Holocene succession to between ca 8000 and 1000 yrs BP. This includes a sedimentary hiatus that separates the SUL2 and SUL1 units, which is roughly dated between <2800 and ca 2000 yrs BP. A total of 31 and 6 tephra layers were identified within the SUL2 and SUL1 units, respectively. However, only 28 tephra layers yielded fresh micro-pumices or glass shards suitable for chemical analyses using a microprobe wavelength dispersive spectrometer. Chronological and compositional constraints suggest that 27 ash layers probably derive from the Mt. Somma-Vesuvius Holocene volcanic activity, and one to the Ischia Island eruption of the Cannavale tephra (2920 ± 450 cal yrs BP). The 27 ash layers compatible with Mt. Somma-Vesuvius activity are clustered in three different time intervals: from ca 2000 to >1000; from 3600 to 3100; and from 7600 to 4700 yrs BP. The first, youngest cluster, comprises six layers and correlates with the intense explosive activity of Mt. Somma-Vesuvius that occurred after the prominent AD 79 Pompeii eruption, but only the near-Plinian event of AD 472 has been tentatively recognised. The intermediate cluster (3600–3100 yrs BP) starts with tephra that chemically and chronologically matches the products from the “Pomici di Avellino” eruption (ca 3800 ± 200 yrs BP). This is followed by eight further layers, where the glasses exhibit chemical features that are similar in composition to the products from the so-called “Protohistoric” or AP eruptions; however, only the distal equivalents of three AP events (AP3, AP4 and AP6) are tentatively designated. Finally, the early cluster (7600–4700 yrs BP) comprises 12 layers that contain evidence of a surprising, previously unrecognised, activity of the Mt. Somma-Vesuvius volcano during its supposed period of quiescence, between the major Plinian “Pomici di Mercato” (ca 9000 yrs BP) and “Pomici di Avellino” eruptions. Alternatively, since at present there is no evidence of a similar significant activity in the proximal area of this well-known volcano, a hitherto unknown origin of these tephras cannot be role out. The results of the present study provide new data that enrich our previous knowledge of the Holocene tephrostratigraphy and tephrochronology in central Italy, and a new model for the recent explosive activity of the Peninsular Italy volcanoes and the dispersal of the related pyroclastic deposits.     

The Upper Pleistocene to Holocene sediments on the Mediterranean island of Lampedusa (Italy)

The island of Lampedusa lies on the northern edge of the African continental shelf, but during some Quaternary marine lowstands it was joined to the African continent. The study and dating of the aeolian, alluvial, detrital sediments, calcareous crusts and speleothems have established that the climatic–environmental variations recorded on the island can be related chronologically to those known for northern Libya, Tunisia and the Italian peninsula. During the Last Glacial Maximum, phases of Saharan dust accumulation on Lampedusa occurred, and were coeval with dust accumulation in crater lakes and on high mountains in central‐southern Italy, and with phases of glacial advance in the Apennines and in the Alps. During the late Holocene, accumulation of Saharan dust on Lampedusa occurred but there was little accumulation of dust on the northern side of the Mediterranean Sea. With the new data from Lampedusa, it is possible to envisage two different scenarios of atmospheric circulation relating to the Last Glacial Maximum and to the late Holocene. During the Last Glacial Maximum, southerly atmospheric circulation brought rainfall to the southern slopes of the Alps and to the Apennines. During the late Holocene, a prevalent westerly atmospheric circulation became established in the northern Mediterranean. Copyright © 2004 John Wiley & Sons, Ltd.     

安徽巢湖湖泊沉积物孢粉-炭屑组合记录的全新世以来植被与气候演变

利用湖泊钻孔的孢粉及炭屑分析资料,探讨了安徽巢湖湖泊沉积物孢粉-炭屑组合记录的全新世以来植被与气候演变。孢粉-炭屑记录表明：本区9870-6040 cal.{a BP},植被是以壳斗科的落叶、常绿属种为主的落叶阔叶、常绿阔叶混交林,气候呈现温和略干的特点;6040-4860 cal.{a BP},植被是以落叶栎类、栗属、青冈属和栲/石栎属为主的落叶阔叶、常绿阔叶混交林,气候由前一阶段的温和略干变为温暖湿润,进入全新世最适宜气候期;4860-2170 cal.{a BP},植被是以落叶栎类占绝对优势的落叶阔叶、常绿阔叶混交林,气温逐渐降低、湿度下降,气候温和干燥,环境整体向着干旱趋势转化,2170 cal.{a BP}左右出现一次明显的干旱事件;2170-1040 cal.{a BP},森林退缩,落叶阔叶、常绿阔叶混交林迅速被破坏,演替成以禾本科为主的草地,气候处于转型时期,总体上温和湿润;1040-200 cal.{a BP},木本植物仅零星分布,可能存在针叶阔叶混交林植被,植被类型逐渐演替成以禾本科、蒿属和蓼属等为主的草丛,其中包括较多的农作物,气候温凉稍湿;200 cal.{a BP}以来,湖区周围零星分布有阔叶乔木,松林在远处可能有分布,植被仍是以禾本科、蒿属和蓼属等为主的草丛,气候相对温暖湿润,但也有若干变凉干的波动,湖泊富营养化水平逐渐增加,人类活动已经对环境变化产生了较大的影响。     

Hydrogeological study of the intensely exploited aquifer of the Santa Croce leather-producing district, Tuscany (central Italy)

A hydrogeological study was undertaken to define the groundwater circulation in the Santa Croce area (Tuscany, central Italy) where the existing multilayered aquifer has long been intensively exploited. Investigations carried out on about 150 wells revealed the existence of a deep piezometric depression (to 20?m below sea level) which drains groundwater from the entire surrounding area. Samples from about 70 water points, collected twice in 2007, were analysed for major elements and stable isotope composition. Three major groups of waters, which mix in the study area, have been distinguished: (1) waters of the Ca?CMg?CHCO₃ type mainly flowing along the Arno River Plain; (2) waters of the Na?CK?CCl/Ca?CMg?CHCO₃?CSO_4, type with SO₄ content up to 275?mg/L, inflowing from the Pisane Hills; (3) relatively high-salinity waters mainly of the Na?CHCO₃ type which, with Cl concentrations up to 750?mg/L, likely arise from a normal fault located at the foot of the northern hills. The characteristics of the different components are greatly affected by significant modifying processes such as cation exchange and sulfate reduction. The achieved conceptual model suggests the southern hills as the main recharging area of the aquifer system from which water circulation, characterized by pathways of different length and depth, develops.     

Thermal modelling of the Larderello geothermal field (Tuscany,Italy)

We present a 3-D thermal model of the Larderello geothermal field (Tuscany) to evaluate (1) the extent and contribution of the heat transfer mechanisms (conduction vs. convection) at the intermediate-upper crust levels, (2) the variability of the heat and mass fluxes entering from below and (3) the crucial role of the formation permeability. The model, composed by three main layers, considers the upper 10 km of the crust to better constrain the simulations with experimental data from borehole, fluid inclusion studies and hypocentral distributions. Several sets of simulations were carried out with different bottom boundary temperatures and different formation permeabilities for the two deeper layers. The results indicate that the present temperature (T) and pressure distributions in the Larderello field require deep reservoir rocks with higher permeability than the overlying capping units and underlying intermediate crust. Permeability values of 1 mDarcy for the reservoir rocks are enough to allow fluid convection, if the temperature at 10 km depth is as high as 500 ± 50°C. The presence of localized zones with formation permeability 50–100 times higher than the surrounding rocks strongly favours the migration of over-pressurized fluids, which episodically break through the overburden, feeding the presently exploited geothermal fields.     

Holocene vegetation and climate fluctuations in northwest Himalaya,based on pollen evidence from Surinsar Lake,Jammu region,India

Pollen analysis of a 33.21 m deep sediment core from Surinsar lake in Jammu region has revealed that between 9,500 and 7,700 yr BP (Pollen zone SL-I), the mixed oak-broad-leaved/chirpine forest occurred in the region under a warm and humid climate. The record of aquatic plants viz. Potamogeton, Typha and freshwater alga Botryococcus in appreciable numbers denotes the existence of the lake since the Early Holocene. Subsequently, mixed chirpine/oak-broad-leaved forests appeared in the area around 7,700 to 6,125 yr BP (Pollen zone SL-II) with the dominance of chirpine (Pinus cf. roxburghii) by the onset of cool and dry climate, attributed to decrease in monsoon rainfall. The expansion of oak and its broad-leaved associates between 6,125 and 4,330 yr BP (Pollen zone SL-III) suggests that the climate turned moderately humid and warm, presumably due to enhanced monsoonal effect. The region has witnessed a brief spell of pluvial environment between 4,330 and 4,000 yr BP (Pollen zone SL-IV) as evidenced by the presence of sandy layer at 15.4–14 m depths. The period of 4,000 to 2,100 yr BP (Pollen zone SL-V) is marked by the prevalence of cool and dry climate as depicted by a sharp decline in oak (Quercus cf. incana) and other broad-leaved taxa and a concurrent increase in chirpine. From 2,100 to 800 yr BP (Pollen zone SL-VI) no palaeofloristic inferences could be drawn due to paucity of pollen, however, the presence of sandy deposit at the corresponding level in lithocolumn implies a pluvial episode by this time. Since 800 yr BP to Present (Pollen zone SL-VII) the slight advance in the oak reflects the ameliorating trend of climate, despite the existing cool and dry climate.