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1.
The paper deals with the implementation of a levelling and Global Positioning System (GPS) network to control land subsidence in the coastal area north of the Venice Lagoon. About 480 km of levelling lines with 527 benchmarks, 45 of which suited for Differential GPS measurements, were established in 2004. A complete survey of the net was carried out in 2004 soon after its establishment. The 2004 records have been compared with previous scattered data obtained by the use of levelling surveys, DGPS and SAR interferometry. The results show a trend in land settlement that increases from the lagoon margin to the north and jeopardize the Venice coastland. Groundwater withdrawals for domestic, agricultural, and health spas uses, peat oxidation of reclaimed marshlands for farming, natural consolidation of the Holocene deposits, and tectonics of the pre-Quaternary basement are the causes of land subsidence in the study area. Since most of the area lies below the mean sea level and on account of the expected sea level rise due to global change, a detailed monitoring of land displacements in the near future will be of paramount importance to plan necessary works for coastland protection.  相似文献   

2.
A major development programme comprising 15 gas fields of the northern Adriatic Sea has recently been submitted to the Ministry of the Environment, VIA Committee for the assessment of the environmental impact, by ENI-Agip, the Italian national oil company. One of the largest reservoirs is Chioggia Mare, located about 10 km offshore of the Venetian littoral, with a burial depth of 1000–1400 m. The planned gas production from this field is expected to impact the shoreline stability with a potential threat to the city of Venice, 25 km northwest of the center of Chioggia Mare. To evaluate the risk of anthropogenic land subsidence due to gas withdrawal, a numerical model was developed that predicts the compaction of both the gas-bearing formations and the lateral/bottom aquifer (water drive) during a 13-year producing and a 12-year post-production period, and the transference of the deep compaction to the ground surface. To address the uncertainty of a few important hydromechanical parameters, several scenarios are simulated and the most pessimistic predictions obtained. The modeling results show that at most 1 cm of land subsidence over 25 years may be expected at the city of Chioggia, whereas Venice is not subject to settlement. If aquifer drawdown is mediated by water injection, land subsidence is arrested 5 km offshore, with the Chioggia littoral zone experiencing a rebound of 0.6–0.7 cm. Electronic Publication  相似文献   

3.
Cross-valley fills composed cf waste rock from mining operations pose environmental concerns related to their long-term stability. A large (2.06×107 m3) phosphate waste dump in southeastern Idaho was monitored from 1981 to 1988 for subsidence and horizontal displacement in conjunction with different surficial rock types and long-term precipitation patterns. Most of the 19 surveyed benchmarks on the dump face moved in the northeast direction during the 7-yr study period, roughly the same orientation as the aspect of the fill face. Average vertical subsidence rate decreased exponentially with time from a high of 0.15 m/yr in 1981–1982 to a fairly stable level of 0.008 m/yr by 1988. Net displacement similarly decreased from 0.19 m/yr shortly after dump construction to 0.023 m/yr by 1988. Both subsidence and net displacement were initially higher in the shale cap on the west side of the chert-filled dump; however, by 1986–1988 subsidence and displacement rates were actually lower in the shale material. Subsidence and net displacement appeared unaffected by temporal precipitation patterns; both subsidence and net displacement rates declined significantly during the very wet snowmelt seasons of 1983 and 1984. The temporal patterns of movement at the dump site suggest that the breakdown of the rock (especially shale) occurs primarily during the first 1.5 yr following dump construction. Periodic saturation of the upper 6 m of spoil occurred in nine of 60 neutron access tubes, normally during the spring snowmelt seasons. Saturation occurred at depths 1.8 m and is thought to be associated with uneven compaction or heterogeneities in the fill. Overall, the dump appeared to be internally well drained with rates of rock creep unaffected by seasonal water inputs.  相似文献   

4.
We use beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park. Although beaver damming has been considered a viable mechanism for major aggradation of mountain stream valleys, this has not been previously tested with stratigraphic and geochronologic data. Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~ 2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought. Maximum channel gradient for reaches with Holocene beaver-pond deposits decreases with increasing basin area, implying that stream power limits beaver damming and pond sediment preservation. In northern Yellowstone, the patchy distribution and cumulative thickness of mostly < 2 m of beaver-pond deposits indicate that net aggradation forced by beaver damming is small, but beaver-enhanced aggradation in some glacial scour depressions is greater. Although 20th-century beaver loss and dam abandonment caused significant local channel incision, most downcutting along alluvial reaches of the study streams is unrelated to beaver dam abandonment or predates historic beaver extirpation.  相似文献   

5.
The last ca. 20,000 yr of palaeoenvironmental conditions in Podocarpus National Park in the southeastern Ecuadorian Andes have been reconstructed from two pollen records from Cerro Toledo (04°22'28.6"S, 79°06'41.5"W) at 3150 m and 3110 m elevation. Páramo vegetation with high proportions of Plantago rigida characterised the last glacial maximum (LGM), reflecting cold and wet conditions. The upper forest line was at markedly lower elevations than present. After ca. 16,200 cal yr BP, páramo vegetation decreased slightly while mountain rainforest developed, suggesting rising temperatures. The trend of increasing temperatures and mountain rainforest expansion continued until ca. 8500 cal yr BP, while highest temperatures probably occurred from 9300 to 8500 cal yr BP. From ca. 8500 cal yr BP, páramo vegetation re-expanded with dominance of Poaceae, suggesting a change to cooler conditions. During the late Holocene after ca. 1800 cal yr BP, a decrease in páramo indicates a change to warmer conditions. Anthropogenic impact near the study site is indicated for times after 2300 cal yr BP. The regional environmental history indicates that through time the eastern Andean Cordillera in South Ecuador was influenced by eastern Amazonian climates rather than western Pacific climates.  相似文献   

6.
A sediment core 7.2 m long from Lake Mirabad, Iran, was examined for loss-on-ignition, mineralogy, oxygen-isotopic composition of authigenic calcite, and trace-element composition of ostracodes to complement earlier pollen and ostracode-assemblage studies. Pollen, ostracode-inferred lake level, and high Sr/Ca ratios indicate that the early Holocene (10000 to 6500 cal yr BP) was drier than the late Holocene. Low δ18O values during this interval are interpreted as resulting from winter-dominated precipitation, characteristic of a Mediterranean climate. Increasing δ18O values after 6500 cal yr BP signal a gradual increase in spring rains, which are present today. A severe 600-yr drought occurred at ca. 5500 cal yr BP, shortly after the transition from pistachio-almond to oak forest. During the late Holocene, two milder droughts occurred at about 1500 and 500 cal yr BP. Within the resolution of the record, no drought is evident during the collapse of the Akkadian empire (4200–3900 cal yr BP). Rather, a decrease in δ18O values to early-Holocene levels may indicate the return to a Mediterranean precipitation regime.  相似文献   

7.
During 1992–2007, excessive pumping of groundwater caused large-scale aquifer-system compaction and land subsidence in the Choshui River Alluvial Fan, especially in the area of Yunlin county. The subsidence impedes surface-water runoff and endangers the operation of Taiwan High Speed Rail. Leveling, Global Positioning System (GPS), multi-level compaction monitoring well, and Differential Interferometric Synthetic Aperture Radar (DInSAR) are used to study the extent of subsidence in Yunlin and its mechanism. These sensors complement each other in spatial and temporal resolutions. A leveling network totaling 434 km in length was deployed to derive subsidence at every 1.5 km along the routes, and the result is accurate to few mm and shows a basin-like subsidence pattern centering at Tuku Township. Four multi-level compaction monitoring wells, co-located with GPS pillars, detect compactions at different depths, showing that the aquifer-system compaction (the cause of subsidence) occurs mostly below depths >200 m, where reduction of groundwater pumping is most needed. The vertical displacements from GPS and leveling agree to within 1 cm, and are larger than the cumulative compaction detected by the compaction-monitoring wells, suggesting that compaction also occurs below 300 m (the depth of the wells). The vertical displacements derived using DInSAR and 8 ENVISAT SAR images agree with the leveling result to 1–2 cm.  相似文献   

8.
Relic carbonate deposits along the western margin of India occur as dolomite crusts, aragonite sands (pelletal / oolitic) and aragonite-cemented limestones, oyster shells, corals, encrusted coralline algal and foraminiferal-dominated nodules. The petrology and mineralogy of the deposits indicate that except for aragonite sands and foraminiferal nodules, the others were formed in shallow marine conditions and serve as sea level indicators. Radiocarbon dates were measured for 62 relic deposits covering the entire margin. The age of these deposits on the continental shelf off Cape Comorin and Mangalore, between 110 and 18 m depth, ranges between 12, 61014C yr BP and 6,39014C yr BP. On the northwestern margin of India, especially on the carbonate platform (between 64 and 100 m), the age ranges from 17,250 to 6,73014C yr BP. The relic deposits of the Gulf of Kachchh at depths between 35 and 25 m are dated at 12,550–9,63014C yr BP. The age vs. depth plot of the relic deposits further indicates that the Gulf of Kachchh was inundated much early, atleast by 15 ka, after the Last Glacial Maximum, and was subjected to uplift and subsidence during the Holocene. The carbonate platform subsided during the early Holocene. Some of the relic deposits between Cape Comorin and Mangalore plot on or, closely follow the glacio-eustatic sea level curve. Despite abundant siliciclastic flux discharged by the Narmada and Tapti during the early Holocene, the platform off these rivers is largely devoid of this flux and carbonate sedimentation continued until 6,70014C yr BP. We suggest that the river-derived sediment flux diverted southwards under the influence of the SW monsoon current and, thereby, increased the turbidity on the shelf and slope southeast of the carbonate platform and facilitated the formation of deeper water foraminiferal nodules off Vengurla-Goa.  相似文献   

9.
Pollen analysis from a peat core 7.0 m in length, taken from a bog near Bisoca, in a mid-altitude area of the Buzăului Subcarpathian mountains, is used to reconstruct the postglacial vegetation history of the region. The vegetation record, which is supported by twelve 14C dates, starts at the end of the Late Glacial period. At the Late Glacial/Holocene transition, open vegetation was replaced by forest, suggesting a fast response to climatic warming. The Holocene began with the expansion of Betula, Pinus and Ulmus, followed, after 11,000 cal yr BP, by Fraxinus, Quercus, Tilia and Picea. The rapid expansion of these taxa may be due to their existence in the area during the Late Glacial period. At ca. 9200 cal yr BP, Corylus expanded, reaching a maximum after 7600 cal yr BP. The establishment of Carpinus occurred at ca. 7200 cal yr BP, with a maximum at ca. 5700 cal yr BP. Fagus pollen is regularly recorded after 7800 cal yr BP and became dominant at ca. 2000 cal yr BP. The first indications of human activities appear around 3800 cal yr BP.  相似文献   

10.
天津滨海地区晚新生代地层自然固结与地面沉降研究   总被引：1，自引：0，他引：1  
天津滨海地区地处渤海湾西岸,晚新生代沉积了巨厚的松散沉积物。地下水位下降、地层自然固结、地表载荷的加速增长等复合因素造成了严重的地面沉降。利用在天津滨海新区塘沽地区施工的一眼1 226 m全取芯钻孔,通过原状样品测试分析,系统研究了晚新生代土层的物理力学性质、黏性土固结特征,并结合欠固结黏性土层沉降量计算等方法阐述了土层固结状态空间特征,探讨了土层固结特征与地面沉降的相关关系。结果表明：该地区0～100 m深度土层具有低天然密度、高孔隙比、高含水率、高压缩性等特点,表现出软土的性质,在地表荷载增大的情况下,易发生地面沉降;100～550 m的黏性土大都处于超固结和微超固结状态,主要是由于过去地下水的大量开采造成的;550 m以下的黏性土多为正常固结,局部存在欠固结黏性土夹层。钻孔中存在合计约218 m的欠固结黏性土夹层,这些欠固结黏性土夹层在自重应力下的最终沉降量为1 985 mm,沉降量最大的土层对应于第1、6含水组,分别达614 mm和665 mm,这一沉降过程完成所需时间为数十年甚至上百年。  相似文献   

11.
This paper presents new evidence from the Dhamar highlands, Yemen, of paleohydrologic response to fluctuations in Holocene climate. Stratigraphic, geochemical, and chronological analyses of highland peat and lacustrine deposits contribute to knowledge of the timing of early Holocene moisture changes on the Arabian Peninsula, providing a backdrop to understanding early cultural development in the Arabian highlands. The location of the Dhamar highlands, characterized by intermontane valleys surrounded by the highest mountains on the Arabian Peninsula and adjacent to the Indian Ocean is ideal for examining the influence of the Indian Ocean Monsoon (IOM) on the moisture history of this region. Fluctuations in the lacustrine and paleosol records of the Dhamar highlands reflect both local changes in paleohydrology and regional influences on the Holocene paleoclimatic conditions in southwest Arabia. In addition, a peat deposit with a radiocarbon age of 10,253 – 10,560 cal yr BP documents some of the earliest Holocene high moisture conditions on the Arabian Peninsula.  相似文献   

12.
This synthesis integrates recently acquired archaeological and geological data with earlier documented observations to shed light on the subsidence of ancient Greek coastal facilities in southern Italy. These are now positioned between former shorelines and inner shelf sectors at five Calabrian margin localities. Submergence of coastal to inner shelf facilities has resulted in part from sea‐level rise by about 2 m associated with glacio‐hydro‐isostatic factors since archaic to classic Greek time. This phenomenon alone, however, does not explain the wide variation of measured subsidence rates from site‐to‐site. The marked lowering of coastal site substrates by seismo‐tectonic activity (including extensional fault motion), stratal readjustments at depth, and compaction of underlying sediment sequences is significant. Four of the subsided facilities are positioned near emerged Calabrian areas where prevailing Holocene average annual land uplift rates range to ˜1.0 mm/yr; at the fifth, near Hipponion, terrains have risen by nearly 2 mm/yr. In marked contrast, submerged and/or buried structures record the following late Holocene long‐term average rates of coastal margin subsidence: Sybaris‐Thuri on the Taranto Gulf margin (˜0.5–1.0 mm/yr); Hipponion‐Vibo Valentia along the Tyrrhenian coast (˜0.8 to ˜3.2 mm/yr); and Locri‐Epizefiri, Kaulonia, and Capo Colonna on Calabria's Ionian margin (˜1.6, ˜1.6–2.4, and ˜4.0 mm/yr, respectively). © 2012 Wiley Periodicals, Inc.  相似文献   

13.
Precise measurements of submerged archaeological markers in the Siracusa coast (Southeastern Sicily, Italy) provide new data on relative sea-level change during the late Holocene. Four submerged archaeological sites have been studied and investigated through direct observations. Two of them are Greek archaic in age (2.5–2.7 ka) and are now 0.98–1.48 m below sea level; the other two developed during the Bronze age (3.2–3.8 ka) and are now 1.03–1.97 m below sea level. These archaeological data have been integrated with information derived from a submerged speleothem collected in a cave located along the Siracusa coast at − 20 m depth. The positions of the archaeological markers have been measured with respect to present sea level, corrected for tide and pressure at the time of surveys. These data were compared with predicted sea-level rise curves for the Holocene using a glacio-hydro-isostatic model. The comparison with the curve for the southeastern Sicily coast yields a tectonic component of relative sea-level change related to regional uplift. Uplift rates between 0.3 and 0.8 mm/yr have been estimated.  相似文献   

14.
Kenai, located on the west coast of the Kenai Peninsula, Alaska, subsided during the great earthquake of AD 1964. Regional land subsidence is recorded within the estuarine stratigraphy as peat overlain by tidal silt and clay. Reconstructions using quantitative diatom transfer functions estimate co‐seismic subsidence (relative sea‐level rise) between 0.28±0.28 m and 0.70±0.28 m followed by rapid post‐seismic recovery. Stratigraphy records an earlier co‐seismic event as a second peat‐silt couplet, dated to ～1500–1400 cal. yr BP with 1.14±0.28 m subsidence. Two decimetre‐scale relative sea‐level rises are more likely the result of glacio‐isostatic responses to late Holocene and Little Ice Age glacier expansions rather than to co‐seismic subsidence during great earthquakes. Comparison with other sites around Cook Inlet, at Girdwood and Ocean View, helps in constructing regional patterns of land‐level change associated with three great earthquakes, AD 1964, ～950–850 cal. yr BP and ～1500–1400 cal. yr BP. Each earthquake has a different spatial pattern of co‐seismic subsidence which indicates that assessment of seismic hazard in southern Alaska requires an understanding of multiple great earthquakes, not only the most recent. All three earthquakes show a pre‐seismic phase of gradual land subsidence that marked the end of relative land uplift caused by inter‐seismic strain accumulation. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

15.
Fabric and stable isotopic composition of a Holocene stalagmite (CR1) from a cave in northern Sicily record changes in paleorainfall in the early Holocene. High δ13C stable isotope values in the calcite deposited from ca. 8500 to ca. 7500 yr ago are interpreted as reflecting periods of high rainfall. The wet phase was interrupted by two periods of multi-century duration characterized by relatively cool and dry winters centered at ca. 8200 and ca. 7500 yr ago, highlighted by low δ13C and δ18O values. A high variability of δ13C values is recorded from ca. 7500 to ca. 6500 yr ago and indicates that the transition from a pluvial early Holocene to the present-day climate conditions was punctuated by decadal-scale periods of relatively dry winters. In northern Sicily, the traditional elements of the Neolithic appear at ca. 7700 yr ago. It is possible that changes in rainfall influenced the passage from hunter-gathering to farming and sheep-herding economies.  相似文献   

16.
Peat is most compressible of all natural soils. Compaction of peat layers potentially leads to substantial amounts of land subsidence. Peat is common in many distal parts of Holocene deltas, which are often densely populated. It is known that land subsidence due to peat compaction may have serious societal implications in such areas, as it may cause damage to construction works and lead to land inundation. Effects of peat compaction on the natural evolution of deltas are however poorly understood, whereas this might be an important control on delta evolution at both local and regional scales.The main objective of this paper is to review current knowledge concerning the peat compaction process and its effect on delta evolution in Holocene settings, and to identify gaps in this knowledge. An overview is given regarding: 1) the compaction process, 2) presumed and potential effects of peat compaction on delta evolution, 3) field methods to quantify peat compaction and 4) numerical models to calculate the amount and rate of peat compaction.Peat compaction and formation influence channel belt elevation, channel belt geometry and channel belt configuration. Last-mentioned aspect mostly concerns the influence of peat compaction on avulsion, which is one of the most important processes controlling delta evolution. Interactions between peat compaction, peat formation and avulsion have seldom been studied and remain unclear, partly because factors such as peat type, organic matter content, sediment sequence composition and groundwater table fluctuation are so far not taken into account. Peat compaction and formation potentially influence avulsion as 1) a decrease in accommodation space created by peat compaction underneath a channel causes superelevation and/or an increase in lateral migration, 2) the high cohesiveness of peat banks inhibits lateral migration, which increases bed aggradation, decreases sediment transport capacity and hence increases crevassing frequencies, which possibly evolve into an avulsion, although the low regional gradient in peatlands will hinder this, and 3) peat compaction and oxidation in flood basins following groundwater table lowering leads to relief amplification of channel belts. At delta scale, variations in compaction rates might stimulate the occurrence of nodal avulsions.To quantify effects of peat compaction on delta evolution, and to determine the relative importance of different factors involved, field research should be combined with numerical models describing peat compaction and formation. The model should be validated and calibrated with field data.  相似文献   

17.
Plant macrofossils from 33 rodent middens sampled at three sites between 2910 and 3150 m elevation in the main canyon of the Río Salado, northern Chile, yield a unique record of vegetation and climate over the past 22,000 cal yr BP. Presence of low-elevation Prepuna taxa throughout the record suggests that mean annual temperature never cooled by more than 5°C and may have been near-modern at 16,270 cal yr BP. Displacements in the lower limits of Andean steppe and Puna taxa indicate that mean annual rainfall was twice modern at 17,520–16,270 cal yr BP. This pluvial event coincides with infilling of paleolake Tauca on the Bolivian Altiplano, increased ENSO activity inferred from a marine core near Lima, abrupt deglaciation in southern Chile, and Heinrich Event 1. Moderate to large increases in precipitation also occurred at 11,770–9550 (Central Atacama Pluvial Event), 7330–6720, 3490–2320 and at 800 cal yr BP. Desiccation occurred at 14,180, 8910–8640, and 4865 cal yr BP. Compared to other midden sites in the region, early Holocene desiccation seems to have happened progressively earlier farther south. Emerging trends from the cumulative midden record in the central Atacama agree at millennial timescales with improved paleolake chronologies for the Bolivian Altiplano, implying common forcing through changes in equatorial Pacific sea-surface temperature gradients.  相似文献   

18.
We study the aggradation and incision of the Alaknanda River Valley during the late Pleistocene and Holocene. The morphostratigraphy in the river valley at Deoprayag shows the active riverbed, a cut terrace, and a fill terrace. The sedimentary fabric of the fill terrace comprises four lithofacies representing 1) riverbed accretion, 2) locally derived debris fan, 3) the deposits of waning floods and 4) palaeoflood records. The sedimentation style, coupled with geochemical analysis and Optically Stimulated Luminescence (OSL) dating, indicate that this terrace formed in a drier climate and the river valley aggraded in two phases during 21–18 ka and 13–9 ka. During these periods, sediment supply was relatively higher. Incision began after 10 ka in response to a strengthened monsoon and aided by increase of the tectonic gradient. The cut terrace formed at ~ 5 ka during a phase of stable climate and tectonic quiescence. The palaeoflood records suggest wetter climate 200–300 yr ago when the floods originated in the upper catchment of the Higher Himalaya and in the relatively drier climate ~ 1.2 ka when locally derived sediments from the Lesser Himalaya dominated flood deposits. Maximum and minimum limits of bedrock incision rate at Deoprayag are 2.3 mm/a and 1.4 mm/a.  相似文献   

19.
Holocene variations in annual precipitation (Pann) were reconstructed from pollen data from southern Argentinian Patagonia using a transfer function developed based on a weighted-averaging partial least squares (WA-PLS) regression. The pollen–climate calibration model consisted of 112 surface soil samples and 59 pollen types from the main vegetation units, and modern precipitation values obtained from a global climate database. The performance (r2 = 0.517; RMSEP = 126 mm) of the model was comparable or slightly lower than in other comparable pollen–climate models. Fossil pollen data were obtained from a sediment core from Cerro Frias site (50°24'S, 72°42'W) located at the forest-steppe ecotone. Reconstructed Pann values of about 200 mm suggest dry conditions during the Pleistocene–Holocene transition (12,500–10,500 cal yr BP). Pann values were about 300–350 mm from 10,500 to 8000 cal yr BP and increased to 400–500 mm between 8000 and 1000 cal yr BP. An abrupt decrease in Pann at about 1000 cal yr BP was associated with a Nothofagus decline. The reconstructed Pann suggests a weakening and southward shift of the westerlies during the early Holocene and intensification, with no major latitudinal shifts, during the mid-Holocene at high latitudes in southern Patagonia.  相似文献   

20.
The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr− 1 in the Little Andaman to 2.80 mm yr− 1 in South Andaman and 2.45 mm yr− 1 in the North Andaman. Our radiocarbon dates converge on  600 yr and  1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region.  相似文献   

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