The chronology and long term dynamics of a low energy river system: the Kelvin Valley,central Scotland     

R. Tipping  A. P. Jones  S. Carter  T. Holden  M. Cressey 《地球表面变化过程与地形》2008,33(6):910-922

The long term (Holocene) channel and floodplain dynamics of a low gradient, low energy, fine grained aggradational fluvial system within a formerly glaciated landscape in central Scotland, the Kelvin Valley, are described from a series of sediment stratigraphic transects and 12 ¹⁴C assays in a headwater reach between Kirkintilloch and Kilsyth. The ¹⁴C assays and dated archaeological sites on the floodplain together suggest that the River Kelvin ceased to aggrade more than 2000 years ago, probably much more, so the 4–6 m of channel and floodplain deposits are almost entirely of early to mid‐Holocene age. The Kelvin Valley is characterized, despite its low flow characteristics, by a highly variable floodplain architecture, in which some transects suggest long term channel stability and strong partitioning of floodplain sedimentation and others indicate high channel mobility. This variation makes the application of general models of fluvial evolution difficult. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Developing probabilistic eruption forecasts for dormant volcanoes: a case study from Mt Taranaki,New Zealand     

Michael B. Turner  Shane J. Cronin  Mark S. Bebbington  Thomas Platz 《Bulletin of Volcanology》2008,70(4):507-515

The majority of continental arc volcanoes go through decades or centuries of inactivity, thus, communities become inured to their threat. Here we demonstrate a method to quantify hazard from sporadically active volcanoes and to develop probabilistic eruption forecasts. We compiled an eruption-event record for the last c. 9,500 years at Mt Taranaki, New Zealand through detailed radiocarbon dating of recent deposits and a sediment core from a nearby lake. This is the highest-precision record ever collected from the volcano, but it still probably underestimates the frequency of eruptions, which will only be better approximated by adding data from more sediment core sites in different tephra-dispersal directions. A mixture of Weibull distributions provided the best fit to the inter-event period data for the 123 events. Depending on which date is accepted for the last event, the mixture-of-Weibulls model probability is at least 0.37–0.48 for a new eruption from Mt Taranaki in the next 50 years. A polymodal distribution of inter-event periods indicates that a range of nested processes control eruption recurrence at this type of arc volcano. These could possibly be related by further statistical analysis to intrinsic factors such as step-wise processes of magma rise, assembly and storage.  相似文献   

Postglacial growth history of a French Polynesian barrier reef tract, Tahiti, central Pacific     

Cabioch  Camoin  & Montaggioni 《Sedimentology》1999,46(6):985-1000

The internal structure and growth pattern of Tahiti reefs over the last 14 ka is reconstructed using sedimentological, morphological and palaeobiological data coupled with radiometric dates in drill cores through the modern barrier reef. Flooding of the volcaniclastic deposits or the karst surface of a Pleistocene reef started at ≈ 14 ka BP, and coral growth began shortly after inundation. The sequence in the Tahiti barrier-reef edge has formed predominantly through long-term keep-up growth controlled by stable environmental conditions, while the adjacent backreef deposits did not start to accumulate before sea-level stabilization, around 6 ka. The dominance of Porites communities and the coeval occurrence of branching gracile Lithophyllum in the lowermost part of the postglacial reef sequence (14–11 ka) suggest the prevalence of uniformly moderate- to low-energy conditions and/or growth in slightly deeper waters all over the drilled area during the early reef stages. During the last 11 ka, the reef frameworks developed in a high-energy environment, at maximum water depths of 5–6 m, and were dominated by an Acropora robusta/danai–Hydrolithon onkodes association; the local interlayering of other coralgal assemblages (dominated by tabular Acropora or domal Porites ) reflects distinct diversification stages, resulting either from the palaeotopographic control of the substrate or from slight and episodic environmental changes.  相似文献   

Classification of paraglacial barrier systems: coastal New England, USA   总被引：1，自引：0，他引：1  

Duncan M. Fitzgerald  & Sytze Van Heteren 《Sedimentology》1999,46(6):1083-1108

The New England coast harbours a wide variety of barrier forms, which we organize into six barrier-coastline types. The barriers develop in response to the relative importance of several spatially and temporally variable parameters, particularly antecedent topography and geology, sediment abundance and size, exposure to wave and tidal energy and sea-level history. The six coastline types can also be identified in other paraglacial regions. Existing barrier-coastline classification schemes do not allow consistent subdivision of paraglacial barrier coasts. This paper presents a new scheme that is applicable to paraglacial and non-paraglacial barrier coasts alike. Aside from hydrodynamic regime, which forms the basis of the barrier classification most commonly used to date, it includes a compartmentalization factor. Sediment-starved ‘isolated’ (‘type 1’) barrier coastlines are characterized by short, widely spaced barriers. Small, localized updrift and offshore sources have provided sediment for short barriers along ‘clustered headland-separated’ (‘type 2’) barrier coastlines. Various amounts of sediment from larger updrift and offshore glaciofluvial deposits or directly from rivers have formed the longer barriers along ‘wave-dominated mainland-segmented’ (‘type 3a’), ‘mixed-energy mainland-segmented’ (‘type 3b’), ‘wave-dominated inlet-segmented’ (‘type 4a’) and ‘mixed-energy inlet-segmented’ (‘type 4b’) barrier coastlines. Geomorphic form, grain size and stratigraphy can be used to characterize individual barriers along barrier coastlines. Most paraglacial barriers form as spits, but many are transformed subsequently. Welded barriers are common along ‘type 1’ and ‘type 2’ coasts. Baymouth barriers are characteristic of ‘type 3’ coasts, and barrier islands occur exclusively along ‘type 4’ coasts. The coarsest grained barriers are located along ‘type 1’ and ‘type 2’ coasts. Progradational barrier sections are concentrated along ‘type 3’ and ‘type 4’ coasts with abundant sediment supply, but are also present along ‘type 2’ coasts. Temporary increases in sediment supply, common in paraglacial regions, result in transitions between retrogradational and progradational barrier behaviour, which may be recognized on shore-perpendicular stratigraphic cross-sections.  相似文献   

钱塘江下切河谷SE2孔全新世沉积物黏土矿物特征及其意义^*     

李鑫  张霞  林春明  邓程文  李绪龙  冯旭东  夏长发  黄舒雅  赵雪培 《古地理学报》1999,24(2):332-343

通过X射线衍射系统分析了杭州湾地区SE2孔全新世沉积物的黏土矿物组成,结果显示研究层段黏土矿物主要由伊利石、绿泥石、高岭石和蒙脱石组成;伊利石结晶度较好,化学风化指数普遍大于0.5,表明以化学风化为主,且风化趋势自下而上呈递减趋势。通过对比中国东南部主要河流沉积物的黏土矿物组成,认为钱塘江下切河谷全新世沉积物的黏土矿物组成具有较好的物源指示意义: 全新世 Ⅰ 段(即古河口湾和河漫滩)沉积物主要来自钱塘江上游,特征黏土矿物为高岭石,河口外物质贡献不大;Ⅱ 段(即现代河口湾和近岸浅海)沉积物包含较多蒙脱石,表明不仅包括钱塘江上游物质,河口外长江物质也开始进入钱塘江河口。黏土矿物中,高岭石对气候有较好的指示作用:中全新世高岭石含量达到最高值,反映气候最为湿热,化学风化程度最高;晚全新世含量逐渐降低,反映气候逐渐回冷,化学风化强度降低。  相似文献   

Land Use and Lake Sedimentation on the New England Tablelands of New South Wales,Australia     

R. J. HAWORTH  S. J. GALE  S. A. SHORT  H. HEIJNIS 《The Australian geographer》1999,30(1):51-73

Lead-210 methods have been used to establish a chronology of sedimentation extending back almost 80 years in Black Mountain Lagoon near Guyra on the New England Tablelands of north-eastern New South Wales, Australia. Estimates of the direct atmospheric fallout of unsupported ²¹⁰Pb and historical records of the pattern of phosphorus input to the lake provide support for the ²¹⁰Pb chronology. The sediments in the lake record an episode of disturbance which took place prior to c .1916, but after c .1790-1860. Since then, however, the site-specific rate of minerogenic sedimentation has been maintained at a relatively low and constant level of 0.93 kg m^-2 a^-1. [Note: the following symbols are used in this paper: a = year (annum), and d min^-1 g^-1 = radioactivity (spontaneous nuclear disintegrations per minute per gram of material).] This is despite major shifts in land use and intensification of agriculture, despite the occurrence of significant floods and droughts, and in the absence until recent times of the application of soil conservation practices. These rates may be contrasted with rates of 0.023 kg m^-2 a^-1 from the period 12.3-5.6 ka. These are at least an order of magnitude lower than those of the twentieth century. The lake sediments preserve evidence of the use of agricultural chemicals on the catchment during the twentieth century. These include phosphorus, zinc, arsenic and lead. Soils and sediments may act as long-term toxic stores for such chemicals, a problem often overlooked in rural areas.  相似文献   

Stable isotopes and soil-geomorphology as indicators of Holocene climate change, northern Chihuahuan Desert     

Brenda J. Buck  H. Curtis Monger 《Journal of Arid Environments》1999,43(4):357

Coeval δ¹³C shifts recorded in buried soils at both piedmont slope and basin floor sites in the northern Chihuahuan Desert indicate a major shift from C₄grasses to C₃desert-scrub between 7 and 9 ka. The age assignments are based on stratigraphic correlations to charcoal dates and carbon-14 dates of carbonate. This shift is synchronous with a period of cooling in the North Atlantic that may have triggered a period of drought in the south-western United States. Coinciding with this vegetation change, geomorphic evidence in Rio Grande, piedmont, and basin floor eolian environments indicates a major period of erosion. Subsequent gradual enrichment of pedogenic carbonateδ¹³ C values in younger deposits suggests that C₄grasses rebounded in the late Holocene (approximately 4 ka), which is consistent with other evidence of increased moisture regionally. A period of less severe aridity at approximately 2·2 ka is indicated by erosion and subsequent deposition along the alluvial fans and within the basin, and correlates with depleted pedogenic carbonate δ¹³C values suggesting a decrease in C₄grasses. Isotope and packrat midden records should be used together to infer past environmental conditions at different elevations.  相似文献   

Diatom-derived palaeoconductivity estimates for Lake Awassa, Ethiopia: evidence for pulsed inflows of saline groundwater   总被引：2，自引：2，他引：0  

Richard J. Telford  Henry F. Lamb  Mohammed Umer Mohammed 《Journal of Paleolimnology》1999,21(4):409-422

A 6,500-year diatom stratigraphy has been used to infer hydrochemical changes in Lake Awassa, a topographically closed oligosaline lake in the Ethiopian Rift Valley. Conductivity was high from ~6400-6200 BP, and from 5200-4000 BP, with two brief episodes of lower conductivity during the latter period. Although the timing of the conductivity changes is similar to the timing of lake-level change in the nearby Zwai-Shalla basin, their directions are the reverse of that expected from a climatic cause. Dissolution of the tephras which precede both phases of high conductivity cannot explain the increases in salinity, because rhyolitic tephras are only sparingly soluble. Instead, the pulsed input of groundwater made saline by the reaction of silicate minerals and volcanic glass with carbonic acid, formed from the solution of carbon dioxide degassed from magma under the Awassa Caldera, is suggested as a plausible mechanism for the observed change in lake chemistry. Diatom-inferred hydrochemistry cannot therefore be used to reconstruct climate change in Lake Awassa.  相似文献   

A late Holocene palaeolimnological record from central Mexico, based on faunal and stable-isotope analysis of ostracod shells     

N.D. Bridgwater  T.H.E. Heaton  S.L. O'Hara 《Journal of Paleolimnology》1999,22(4):383-397

A late Holocene palaeolimnological record for central Mexico has been obtained from Lake Pátzcuaro, using recent and fossil ostracods. Lake Pátzcuaro, Michoacán, is a closed-basin lake which responds rapidly to changes in the ratio of precipitation/evaporation in the region. The record from a single lake-sediment core, dated by AMS radiocarbon method, covers the last ~3,530 yrs, and is based on ostracod faunal palaeoecology coupled with analysis of the stable-isotope (¹⁸O/¹⁶O and ¹³C/¹²C) composition of ostracod valves. The faunal distribution is determined by the presence or absence of aquatic vegetation and, to a lesser extent, salinity. The ¹⁸O/¹⁶O and ¹³C/¹²C ratios in ostracod calcite show good agreement with palaeolimnological inferences from the faunal assemblages, principally recording changing precipitation/evaporation and primary-productivity levels, respectively. Wetter conditions existed in central Mexico between approximately ~3,600 and ~2,390 yr BP, between ~1,330 to ~1,120 yr BP, and from ~220 yr BP to present, characterised by fluctuating lake levels. A dilution of the sediment load in the lake reduced turbidity levels allowing for a marked increase in productivity. During these phases, the combination of a deeper lake and increased macrophyte cover reduced the degree of mixing of the waterbody. In the earliest of these phases there was sufficient stratification of the waterbody for methanogenesis to occur in the sediment interstices. The wet phases were separated by prolonged dry periods, during which time the climatic conditions were relatively stable. Good agreement was found between the findings of this study and others from the central Mexican/Caribbean region suggesting that abrupt climate changes occurred at least at a regional scale.  相似文献   

Lake evolution in a semi-arid montane environment: response to catchment change and hydroclimatic variation     

Henry Lamb  Neil Roberts  Melanie Leng  Philip Barker  Abdelfattah Benkaddour  Sander van der Kaars 《Journal of Paleolimnology》1999,21(3):325-343

Pollen, geochemical and sedimentological data from Sidi Ali, a montane Moroccan lake, provide a 7000 yr record of changes in climate, catchment vegetation and soil erosion intensity. Diatoms, non-silicious algae, macrophyte fossils and ostracods from the same core record the dynamics of the lake ecosystem. Oxygen isotope and trace-element ratios of benthic ostracods appear to be relatively insensitive to climatic variation in this open lake with low water-residence time, but diatom plankton / periphyton (P/L) ratios show lake-level variations that are probably climate controlled. At least two superimposed processes are recorded, but at different timescales: catchment vegetation and soils show long-term changes due to climate and human impact, whereas P/L ratios suggest century-scale oscillations in lake depth. The timing of changes in algal and macrophyte productivity and carbon cycling within the lake broadly corresponds to changes in terrestrial vegetation, suggesting either that lake nutrient status is linked to catchment vegetation and soils, or that both were influenced by climate. The lack of a sensitive and independent (non-biological) climate proxy makes it more difficult to assess the lake's ecological response to short-term climate variation. Overall, the lake's evolution has been influenced both by catchment-mediated nutrient flux and by changes in water balance, thus having characteristics in common with both temperate and arid zone lakes.  相似文献