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1.
A displacement history and slip rates were determined for the Reelfoot fault in the New Madrid seismic zone from a seismic reflection profile and trench data. Based on calculations from the seismic reflection line the average slip rate over the last 80 million years is 0.0009 mm year −1. Slip rate during the Late Cretaceous was 0.0007 mm year −1, 0.002 mm year −1 during the Paleocene Midway Group, 0.001 mm year −1 during Paleocene–Eocene Wilcox Formation time, 0.0003 mm year −1 during the post-Wilcox/pre-Holocene period, and a Holocene slip rate of 1.8 mm year −1. Based on trench data, slip rate on the Reelfoot fault has been 4.4 mm year −1 over the last 2400 years and a maximum of 6.2 mm year −1 during the two most recent earthquake cycles between AD 900 and AD 1812. The Holocene slip rate is at least four orders of magnitude higher than the average Late Cretaceous and Cenozoic slip rates for the Reelfoot fault. It would appear that there has been a Quaternary change in the stress field in the central United States or the Reelfoot fault is experiencing a short-lived burst of seismic activity. 相似文献
2.
The isotopic composition and mass balances of sources and sinks of sulfur are used to constrain the limnological–hydrological evolution of the last glacial Lake Lisan (70–14 ka BP) and the Holocene Dead Sea. Lake Lisan deposited large amounts of primary gypsum during discrete episodes of lake level decline. This gypsum, which appears in massive or laminated forms, displays δ 34S values in the range of 14–28‰. In addition, Lake Lisan’s deposits (the Lisan Formation) contain thinly laminated and disseminated gypsum as well as native sulfur which display significantly lower δ 34S values (−26 to 1‰ and −20 to −10‰, respectively). The calculated bulk isotopic compositions of sulfur in the sources and sinks of Lake Lisan lacustrine system are similar (δ 34S ≈ 10‰), indicating that freshwater sulfate was the main source of sulfur to the lake. The large range in δ 34S found within the Lisan Formation (−26 to +28‰) is the result of bacterial sulfate reduction (BSR) within the anoxic lower water body (the monimolimnion) and bottom sediments of the lake. Precipitation of primary gypsum from the Ca-chloride solution of Lake Lisan is limited by sulfate concentration, which could not exceed 3000 mg/l. The Upper Gypsum Unit, deposited before ca. 17–15 ka, is the thickest gypsum unit in the section and displays the highest δ34S values (25–28‰). Yet, our calculations indicate that no more than a third of this Unit could have precipitated directly from the water column. This implies that during the lake level decline that instigated the precipitation of the Upper Gypsum Unit, significant amounts of dissolved sulfate had to reach the lake from external sources. We propose a mechanism that operated during cycles of high-low stands of the lakes that occupied the Dead Sea basin during the late Pleistocene. During high-stand intervals (i.e., Marine Isotopic Stages 2 and 4), lake brine underwent BSR and infiltrated the lake’s margins and adjacent strata. As lake level dropped, these brines, carrying 34S-enriched sulfate, were flushed back to the shrinking lake and replenished the water column with sulfate, thereby promoting massive gypsum precipitation. The Holocene Dead Sea precipitated relatively small amounts of primary gypsum, mainly in the form of thin laminae. δ34S values of these laminae and disseminated gypsum are relatively constant (15 ± 0.7‰) and are close to present-day lake composition. This reflects the lower supply of freshwater to the lake and the limited BSR activity during the arid Holocene time and possibly during former arid interglacials in the Levant. 相似文献
3.
The Gulf of Corinth is a graben, which has undergone extension during the Late Quaternary. The subsidence rate is rapid in the currently marine part whereas uplift now affects a large part of the initially subsiding area in the North Peloponnese. In this paper, we document the rates of subsidence/uplift and extension based on new subsurface data, including seismic data and long piston coring in the deepest part of the Gulf. Continuous seismic profiling data (air gun) have shown that four (at least) major oblique prograding sequences can be traced below the northern margin of the central Gulf of Corinth. These sequences have been developed successively during low sea level stands, suggesting continuous and gradual subsidence of the northern margin by 300 m during the Late Quaternary (last 250 ka). Subsidence rates of 0.7–1.0 m kyr − 1 were calculated from the relative depth of successive topset to foreset transitions. The differential total vertical displacement between the northern and the southern margins of the Corinth graben is estimated at about 2.0–2.3 m kyr − 1. Sequence stratigraphic interpretation of seismic profiles from the basin suggests that the upper sediments (0.6 s twtt thick) in the depocenter were accumulated during the last 250 ka at a mean rate of 2.2–2.4 m kyr− 1. Long piston coring in the central Gulf of Corinth basin enabled the recovery of lacustrine sediments, buried beneath 12–13.5 m of Holocene marine sediments. The lacustrine sequence consists of varve-like muddy layers interbedded with silty and fine sand turbidites. AMS dating determined the age of the marine–lacustrine interface (reflector Z) at about 13 ka BP. Maximum sedimentation rates of 2.4–2.9 m kyr− 1 were calculated for the Holocene marine and the last glacial, lacustrine sequences, thus verifying the respective rates obtained by the sequence stratigraphic interpretation. Recent accumulation rates obtained by the 210Pb-radiometric method on short sediment box cores coincide with the above sedimentation rates. Vertical fault slip rates were measured by using fault offsets of correlated reflector Z. The maximum subsidence rate of the depocenter (3.6 m kyr− 1) exceeds the maximum sedimentation rate by 1.8 m kyr− 1, which, consequently, corresponds to the rate of deepening of the basin's floor. The above rates indicate that the 2.2 km maximum sediment thickness as well as the 870 m maximum depth of the basin may have formed during the last 1 Ma, assuming uniform mean sedimentation rate throughout the evolution of the basin. 相似文献
4.
Sediments deposited in two small ice-contact lakes with low rates of sediment input have been studied in subaerial exposures. Sediment characteristics are a function of the water source (glacial meltwater versus non-meltwater), proximity to the glacier margin and lake shore, amount of supraglacial debris, and lake duration. Calving Lake expanded (and later partially drained) as a calving ice margin retreated. Nearshore deltas contain 1 × 10 5 m 3 stratified sand and gravel deposited at rates up to 1 m/yr during a 9-yr interval. Deltaic sediment contains types A and B ripple-drift cross-lamination, draped lamination, and scour surfaces caused by variations in water-flow velocity and the amount of sediment settling from suspension. Most water inflow came from non-subglacial meltwater sources and was sediment-poor, so overflow and interflow sedimentation processes dominated the offshore environment. Offshore sediment generally contains massive silt or silt interbedded with fine-grained sand deposited at rates of 1.3-1.5 cm/yr. Iceberg gravity craters observed on the lake plain were formed when icebergs impacted the lake floor during calving events. In Bruce Hills Lake, proximity to glacier ice and the presence of supraglacial sediment formed coarsening-upward successions when debris fell directly from an ice ledge onto silty lacustrine sediment. 相似文献
5.
自20世纪70年代初以来，采用无线电回波探测（RES）等技术，在南极冰盖发现了70多个冰下湖泊，最近，随着Vostok冰芯钻孔逼近Vostok湖面，对南极冰盖下伏湖泊的研究提上日程，Vostok湖因其面积大以及有距离湖面最近的钻孔，成为冰下湖群中被优选考虑的“靶子”湖泊，主要研究目标定位于古气候学，原始生命科学两方面，目前工作尚处于技术研制阶段，技术关键点在于研制先进的湖泊沉积物钻探技术，无污染和自动化湖水，湖芯取样技术等，技术上的高难度使得该计划成为一项系统工程。 相似文献
6.
Upper and Middle Waterton lakes fill a glacially scoured bedrock basin in a large (614 km 2) watershed in the eastern Front Ranges of the Rocky Mountains of southern Alberta, Canada and northern Montana, U.S.A. The stratigraphic infill of the lake has been imaged with 123 km of single-channel FM sonar (‘chirp') reflection profiles. Offshore sonar data are combined with more than 2.5 km of multi-channel, land-based seismic reflection profiles collected from a large fan-delta. Three seismic stratigraphic successions (SSS I to III) are identified in Waterton Lake resting on a prominent basal reflector (bedrock) that reaches a maximum depth of about 250 m below lake level. High-standing rock steps (reigels) divide the lake into sub-basins that can be mapped using lake floor reflection coefficients. A lowermost transparent to poorly stratified seismic succession (SSS I, up to 30 m thick) is present locally between bedrock highs and has high seismic velocities (1750–2100 m/s) typical of compact till or outwash. A second stratigraphic succession (SSS II, up to 50 m thick), occurs throughout the lake basin and is characterised by continuous, closely spaced reflectors typical of repetitively bedded and rhythmically laminated silts and clays most likely deposited by underflows from fan-deltas; paleo-depositional surfaces identify likely source areas during deglaciation. Intervals of acoustically transparent seismic facies, up to 5 m thick, are present within SSS II. At the northern end of Upper Waterton Lake, SSS II has a hummocky surface underlain by collapse structures and chaotic facies recording the melt of buried ice. Sediment collapse may have triggered downslope mass flows and may account for massive facies in SSS II. A thin Holocene succession (SSS III, <5 m) shows very closely spaced reflectors identified as rhythmically laminated fine pelagic sediment deposited from interflows and overflows. SSS III contains Mt. Mazama tephra dated at 6850 yr BP. 相似文献
7.
Holocene lacustrine sediments from two isolated lakes in north China are investigated. Based on palaeoclimatic significance of independent proxies in lake sediments, Holocene chemical weathering, and hence climate change, has been reconstructed for dated sediment cores from Daihai Lake and Aibi Lake. During early to mid-Holocene, higher weathering intensity occurred in the Daihai catchment under warm and humid climate conditions, and this reached a maximum at ∼5 kyr BP. However, synchronous proxy shifts from the two widely separated, isolated lake sediments indicate that there was a cool climate event during the early to mid-Holocene transition. This is characterized by reduced weathering in each catchment, low δ 13 C and δ 18 O of authigenic carbonate, and by lake level fluctuations. These might correspond to a global cooling signal identified in lakes, oceans, mollusc sequences, and polar ice cores, typically centred between ∼8.0 and 8.5 kyr BP. Dry conditions were experienced in Greenland, the North Atlantic and surrounding regions, and in broad monsoonal regions including Daihai at this time. However, recent extensive evidences as well as our data from the Aibi Lake sediments show that cool but wet conditions occurred in the central Eurasian continent at this time. After ∼2.5 kyr BP, a significant shift of independent sediment proxies indicates the beginning of the Neoglaciation with a higher frequency of fluctuations, including both the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Our continental records provide new evidence of the Holocene climate variability with global significance and highlight the different spatial nature of the response to oscillations associated with different climate patterns. 相似文献
8.
The Loire River, with one of the largest watersheds in France, has been monitored just outside the city of Orleans since 1994. Physico-chemical parameters and major and trace elements were measured between 2-day and 1-week intervals according to the river flow. The sampling site represents 34% of the total Loire watershed with 76% silicate rocks and 24% carbonate rocks. Elements are transported mainly in the dissolved phase with the ratio of total dissolved salts (TDS) to suspended matter (SM) ranging between 1.6 and 17.4. Chemical weathering of rocks and soils are thus the dominant mechanisms in the Loire waters composition. The highest TDS/SM ratios are due to dissolved anthropogenic inputs. The database shows no link between NO3− content and river flow. The Na+, K+, Mg2+, SO42−, and Cl− concentrations are seen to decrease with increasing discharge, in agreement with a mixing process involving at least two components: the first component (during low flow) is concentrated and may be related with input from the groundwater and sewage station water, the second component (during high flow) is more dilute and is in agreement with bedrock weathering and rainwater inputs. A geochemical behaviour pattern is also observed for HCO3− and Ca2+ species, their concentrations increase with increasing discharge up to 300 m3/s, after which, they decrease with increasing discharge. The Sr isotopic composition of the dissolved load is controlled by at least five components — a series of natural components represented by (a) waters draining the silicate and carbonate bedrock, (b) groundwater, and (c) rainwaters, and two kinds of anthropogenic components. The aim of this study is to describe the mixing model in order to estimate the contribution of each component. Finally, specific export rates in the upper Loire watershed were evaluated close to 12 t year−1 km−2 for the silicate rate and 47 t year−1 km−2 for the carbonate rate. 相似文献
9.
We determine seismic strain rate of tectonic earthquakes along the Central America Volcanic Arc. We then compare this result to those obtained from earthquakes related to the convergence of the Cocos and Caribbean plates and to earthquakes in the back-arc region of northern Central America. The seismic strain-rate tensor for shallow-focus earthquakes along the Central America volcanic arc since 1700, has a compressive eigenvector with a magnitude of 0.7 × 10−8 year−1, and oriented in a 357° azimuth. The extensive eigenvector is oriented in a 86° azimuth, with a magnitude of 0.82 × 10−8 year−1. When only Centroid Moment-tensor solutions (CMT) are considered, the respective eigenvectors are 1.2 × 10−8 year−1 and 1.0 × 10−8 year−1. The compressive eigenvector from the seismic strain-rate tensor for earthquakes along the Cocos-Caribbean convergent margin is 2.0 × 10−8 year−1, plunging at 25°, and oriented in a 29° azimuth. Its magnitude and direction are similar to those of the compressive eigenvector for earthquakes along the volcanic arc. The extensive eigenvector along the convergent margin, on the other hand, has a large vertical component. The compressive and extensive eigevenvectors are 4.9 × 10−8 year−1 and 4.6 × 10−8 year−1, using only CMTs as the database. Earthquakes along the grabens of northern Central America yield a seismic strain-rate tensor whose extensive eigenvector has a magnitude of 2.4 × 10−8 year−1, oriented in a 109° azimuth. Magnitude and direction are similar to those of the extensive eigenvector for earthquakes along the volcanic arc. The compressive eigenvector along the grabens is practically vertical. Similarities in magnitudes and directions for compressive and extensive eigenvectors suggest to us that the strain field along the Central America volcanic arc is the result of compression along the convergent Cocos-Caribbean margin, and extension in the back-arc region, along the grabens of northern Central America. This field is resolved as strike-slip faulting along the arc. 相似文献
10.
Lake Zürich occupies a glacially overdeepened perialpine trough in the northern Middlelands of Switzerland. A total of 154.4
m of Quaternary sediments and 47.3 m of Tertiary Molasse bedrock has been cored from the deepest part of the lake, some 10
km south of the city of Zürich. Some 16.8 m of gravels and sands directly overlying the bedrock include basal till and probably
earliest subglacial fluvial and lacustrine deposits. These are overlain by 98.6 m of fine-grained, glacial-aged sediments
comprising completely deformed proglacial and/or subglacial lacustrine muds, separated by four basal mud tills. The lack of
interglacial sediments, fossils, and other datable material, and the presence of severe sediment deformation and unknown amounts
of erosion prevent the establishment of an exact chronostratigraphy for sediments older than the upper mud till. Above it
some 8.6 m of lacustrine muds were deposited, folded, faulted, and tilted during the final opening of the lake at about 17,500–17,000
years ago. Superimposed are 30.4 m of final Würm and post-glacial sediments comprising (from oldest): cyclic proglacial mud,
thick-bedded and laminated mud, a complex transition zone, laminated carbonate, laminated marl, and diatom-calcite varves.
These sediments reflect changing catchment and lacustrine conditions including: glacial proximity, catchment stability, lake
inflow characteristics, thermal structure, chemistry, and bed stability. Average sedimentation rates ranged from 11 cm yr −1 immediately after glacier withdrawal, to as low as 0.4 mm yr −1 as the environment stabilized.
The lack of coarse outwash deposits separating the fine-grained glaciolacustrine sediments from a corresponding underlying
basal till suggests that deglaciation of the deep northern basin of Lake Zürich was by stagnation-zone retreat rather than
by retreat of an active ice-front. 相似文献
11.
It is proposed that a lake, here named “Subglacial Lake McGregor”, existed beneath the Laurentide Ice Sheet at, or near, the last glacial maximum. The lake resided in the ancient buried McGregor and Tee Pee preglacial valleys, which are now mostly filled with glacigenic deposits. The greatest thickness of sediment in the valleys is in the form of chaotically deposited lake beds that were laid down in a subaqueous environment by a number of process: gravity flow, water transport, and suspension settling. Topographic, sedimentary, and stratigraphic evidence point to a subglacial, not a proglacial, origin for the beds. During the early stages of lake existence, ice movement was significant as there are numerous sets of shear planes in the sedimentary beds. This indicates that the lake filled (lake sedimentation) and drained (shearing of the beds by overlying ice when ice contacted the bed) often. Thus, early in its history, the lake(s) was/were ephemeral. During the later stages of lake existence, the lake was relatively stable with no rapid draining or influx of sediment. Gradual drainage of the lake resulted in lowering of the ice onto the lake beds resulting in subglacial till deposition. Drainage was not a single continuous event. Rather it was characterized by multiple phases of near total drainage (till deposition), followed by water accumulation (lake sedimentation). Water accumulation events became successively less significant reflected by thinning of lake beds and thickening of till beds higher in the stratigraphic sequence. Since subglacial lake sedimentation appears to be restricted to the subglacial valleys, it is suggested that the valleys acted as a large-scale interconnected cavity system that both stored and transported water. It is also suggested that these acted as the main routes of water flow beneath the Laurentide Ice Sheet. 相似文献
12.
Direct exploration of subglacial lakes buried deep under the Antarctic Ice Sheet has yet to be achieved. However, at retreating margins of the ice sheet, there are a number of locations where former subglacial lakes are emerging from under the ice but remain perennially ice covered. One of these lakes, Hodgson Lake (72°00.549′S, 068°27.708′W) has emerged from under more than 297–465 m of glacial ice during the last few thousand years. This paper presents data from a multidisciplinary investigation of the palaeolimnology of this lake through a study of a 3.8 m sediment core extracted at a depth of 93.4 m below the ice surface. The core was dated using a combination of radiocarbon, optically stimulated luminescence, and relative palaeomagnetic intensity dating incorporated into a chronological model. Stratigraphic analyses included magnetic susceptibility, clast provenance, organic content, carbonate composition, siliceous microfossils, isotope and biogeochemical markers. Based on the chronological model we provisionally assign a well-defined magnetic polarity reversal event at ca 165 cm in the lake sediments to the Mono Lake excursion (ca 30–34 ka), whilst OSL measurements suggest that material incorporated into the basal sediments might date to 93 ± 9 ka. Four stratigraphic zones (A–D) were identified in the sedimentological data. The chronological model suggests that zones A–C were deposited between Marine Isotope Stages 5–2 and zone A during Stage 1, the Holocene. The palaeolimnological record tracks changes in the subglacial depositional environment linked principally to changing glacier dynamics and mass transport and indirectly to climate change. The sediment composition in zones A–C consists of fine-grained sediments together with sands, gravels and small clasts. There is no evidence of overriding glaciers being in contact with the bed reworking the stratigraphy or removing this sediment. This suggests that the lake existed in a subglacial cavity beneath overriding LGM ice. In zone D there is a transition to finer grained sediments characteristic of lower energy delivery coupled with a minor increase in the organic content attributed either to increases in allochthonous organic material being delivered from the deglaciating catchment, a minor increase in within-lake production or to an analytical artefact associated with an increase in the clay fraction. Evidence of biological activity is sparse. Total organic carbon varies from 0.2 to 0.6%, and cannot be unequivocally linked to in situ biological activity as comparisons of δ13C and C/N values with local reference data suggest that much of it is derived from the incorporation of carbon in catchment soils and gravels and possibly old CO 2 in meteoric ice. We use the data from this study to provide guidelines for the study of deep continental subglacial lakes including establishing sediment geochronologies, determining the extent to which subglacial sediments might provide a record of glaciological and environmental change and a brief review of methods to use in the search for life. 相似文献
13.
Far-from-equilibrium batch dissolution experiments were carried out on the 2000–500, 500–250, 250–53 and 53–2 μm size fractions of the mineral component of the B horizon of a granitic iron humus podzol after removal of organic matter and secondary precipitates. The different size fractions were mineralogically and chemically similar, the main minerals present being quartz, alkali and plagioclase feldspar, biotite and chlorite. Specific surface area increased with decreasing grain size. The measured element release rates decreased in the order 53–2>>>2000–500>500–250>250–53 μm. Surface area normalised element release rates from the 2000–500, 500–250 and 250–53 μm size fractions (0.6–77×10 −14 mol/m 2/s) were intermediate between literature reported surface area normalised dissolution rates for monomineralic powders of feldspar (0.1–0.01×10 −14 mol/m 2/s) and sheet silicates (100×10 −14 mol/m 2/s) dissolving under similar conditions. Element release rates from the 53–2 μm fraction (400–3000×10 −14 mol/m 2/s) were a factor of 4–30 larger than literature reported values for sheet silicates. The large element release rate of the 53–2 μm fraction means that, despite the small mass fraction of 53–2 μm sized particles present in the soil, dissolution of this fraction is the most important for element release into the soil. A theoretical model predicted similar (within a factor of <2) bulk element release rates for all the mineral powders if observed thicknesses of sheet silicate grains were used as input parameters. Decreasing element release rates with decreasing grain size were only predicted if the thickness of sheet silicates in the powders was held constant. A significantly larger release rate for the 53–2 μm fraction relative to the other size fractions was only predicted if either surface roughness was set several orders of magnitude higher for sheet silicates and several orders of magnitude lower for quartz and feldspars in the 53–2 μm fraction compared to the other size fractions or if the sheet silicate thickness input in the 53–2 μm fraction was set unrealistically low. It is therefore hypothesised that the reason for the unpredicted large release rate from the 52–3 μm size fraction is due to one or more of the following reasons: (1) the greater reactivity of the smaller particles due to surface free energy effects, (2) the lack of proportionality between the BET surface area used to normalise the release rates and the actual reactive surface area of the grains and, (3) the presence of traces quantities of reactive minerals which were undetected in the 53–2 μm fraction but were entirely absent in the coarser fractions. 相似文献
14.
The central trough of the Bolivian Altiplano is occupied by two wide salt crusts: the salar of Uyuni, which is probably the largest salt pan in the world (10,000 km 2) and the salar of Coipasa (2,500 km 2). Both crusts are essentially made of porous halite filled with an interstitial brine very rich in Li, K, Mg, B (up to 4.7 g/l Li, 4.3 g/l B, 30 g/l K and 75 g/l Mg). Lithium reserves are the highest known in the world, around 9 × 10 6 tons. Potassium, magnesium and boron reserves in brines are also important (around 194 × 10 6 tons K, 8 × 10 6 tons B and 211 × 10 6 tons Mg). The crusts are the remnant of saline Lake Tauca (13,000–10,000 yr BP). Its salinity was estimated approximately at 80 g/l. Its paleochemistry was derived in two ways: (1) by dissolving the present amounts of all chemical components in the former lake volume, and (2) by simulating the evaporation of the major inflows to the basin. The resulting chemical compositions are quite different. The dissolution-derived one is 5 to 50 times less concentrated in Li, K, Mg, B than the evaporation-simulated ones. However all compositions present the same Na and Cl contents. This suggests either a removal of bittern salts or an enrichment of the former lake water in Na and Cl. The most probable interpretation is that Lake Tauca redissolved a salt crust akin to that existing today. Several older lakes have been detected on the Altiplano. Nevertheless, such an explanation only pushes the problem back. It is likely that the anomaly was transferred from one lake to an other. Three hypotheses may be put forward: (1) bittern seepage through bottom sediments, (2) uptake of the missing components by minerals, and (3) leaching of ancient evaporites from the catchment area at the beginning of the lacustrine history of the basin. The excess halite could have been recycled from lake to lake. This latter process seems to be the most effective to explain the large excess of Na and Cl over the bittern solutes — Li, K, Mg and B. The occurrence of almost pure Na/1bCl saline springs flowing out from a gypsum diapir in the northern Altiplano gives substantial support to this hypothesis. 相似文献
15.
The hydrochemical interaction between groundwater and lakewater influences the composition of water that percolates downward from the surficial aquifer system through the underlying intermediate confining unit and recharges the Upper Floridan aquifer along highlands in Florida. The 87Sr/ 86Sr ratio along with the stable isotopes, D, 18O, and 13C were used as tracers to study the interaction between groundwater, lakewater, and aquifer minerals near Lake Barco, a seepage lake in the mantled karst terrane of northern Florida. Upgradient from the lake, the 87Sr/ 86Sr ratio of groundwater decreases with depth (mean values of 0.71004, 0.70890, and 0.70852 for water from the surficial aquifer system, intermediate confining unit, and Upper Floridan aquifer, respectively), resulting from the interaction of dilute oxygenated recharge water with aquifer minerals that are less radiogenic with depth. The concentrations of Sr 2+ generally increase with depth, and higher concentrations of Sr 2+ in water from the Upper Floridan aquifer (20–35 μg/L), relative to water from the surficial aquifer system and the intermediate confining unit, result from the dissolution of Sr-bearing calcite and dolomite in the Eocene limestone. Dissolution of calcite [δ 13C = −1.6 permil (% o)] is also indicated by an enriched δ 13C DIC(-8.8 to -11.4% o) in water from the Upper Floridan aquifer, relative to the overlying hydrogeologic units (δ 13C DIC < -16% o). Groundwater downgradient from Lake Barco was enriched in 18O and D relative to groundwater upgradient from the lake, indicating mixing of lakewater leakage and groundwater. Downgradient from the lake, the 87Sr/86Sr ratio of groundwater and aquifer material become less radiogenic and the Sr2+ concentrations generally increase with depth. However, Sr2+ concentrations are substantially less than in upgradient groundwaters at similar depths. The lower Sr2+ concentrations result from the influence of anoxic lakewater leakage on the mobility of Sr2+ from clays. Based on results from mass-balance modeling, it is probable that cation exchange plays the dominant role in controlling the 87Sr/86Sr ratio of groundwater, both upgradient and downgradient from Lake Barco. Even though groundwater from the three distinct hydrogeologic units displays considerable variability in Sr concentration and isotopic composition, the dominant processes associated with the mixing of lakewater leakage with groundwater, as well as the effects of mineral-water interaction, can be ascertained by integrating the use of stable and radiogenic isotopic measurements of groundwater, lakewater, and aquifer minerals. 相似文献
16.
Concentrations of suspended solids in lakes can affect the latter’s primary productivity and reflect changes in sediment deposition. Determining the temporal and spatial distribution of suspended solid concentrations has important significance in lake water environmental management; this is particularly urgent for Poyang Lake, the largest freshwater lake in China. In this study, suspended solid concentration inversion models for Poyang Lake were created using a semi-empirical method with regression analysis between continuously measured suspended solid concentration data and multi-band moderate-resolution imaging spectroradiometer images for spring, summer, autumn, and winter from 2009 to 2012. The coefficient of determination ( R2) is from 0.6 to 0.9 and the average relative error for the accuracy verification was between 10 and 30%. The seasonal distributions of suspended solid concentrations in Poyang Lake from 2000 to 2013 were then obtained using optimal reversal models. The results showed that the seasonal variation in suspended solid concentrations had a “W” shape in which high spring and autumn and low summer and winter values. The suspended solid concentrations increased annually from 2000 to 2013 and were mainly distributed in the northern and central portions of the lake, with lower values along the shorelines. Further analysis indicated that the large difference in water level between the wet and dry seasons is an important factor in explaining these seasonal variations. Moreover, the suspended solid concentrations were poorly correlated with water temperature and chlorophyll- a concentration but more highly correlated with the deferred chlorophyll- a concentration. 相似文献
17.
受全球气候变暖的影响, 冰川退缩, 冰湖数量增多和面积增大被认为指示气候变化的重要依据, 冰湖面积增大导致其潜在危险性增大. 因此, 研究冰湖的变化对于气候变化和冰湖灾害研究具有重要意义. 基于Landsat TM/ETM+遥感影像采用人工解译的方法, 获取了喜马拉雅山地区科西河流域1990年前后、2000年和2010年的冰湖数据, 并对冰湖面积>0.1 km 2且一直存在的199个冰湖的面积和长度变化进行对比分析. 结果表明: 科西河流域内面积>0.1 km 2的冰湖的面积呈现增加趋势, 1990年冰湖面积为73.59 km 2, 2010年冰湖面积增加至86.12 km 2. 科西河流域内喜马拉雅山南北坡冰湖变化存在差异, 喜马拉雅山北坡变化较大的冰湖主要分布在海拔4 800~5 600 m之间, 而南坡变化较大的冰湖主要分布在海拔4 300~5 200 m之间; 喜马拉雅山北坡的冰湖有65%的冰湖表现扩张, 且扩张冰湖的面积主要是由冰湖在靠近终碛垅的一端基本不发生变化, 而仅在靠近冰川一端发生变化贡献的; 喜马拉雅山南坡的冰湖有32%的冰湖变化表现扩张, 且扩张的冰湖面积主要来自于冰面湖扩张. 在科西河流域内, 位于喜马拉雅山北坡的冰湖平均变化速度略高于南坡的冰湖平均变化速度. 相似文献
18.
Lake Chicot is an oxbow lake located along the western side of the Mississippi River in southeastern Arkansas. A major flood
in 1927, levee construction, land use changes from bottomland hardwood to agriculture, a large increase in drainage area,
and stream channelization have altered the appearance of contributing watersheds and the lake. The lake often has high suspended
sediment concentrations making it undesirable for recreation and aquatic production. As part of a coordinated study to determine
changes in the lake, sediment accumulation patterns and rates were determined in Lake Chicot using the 137Cs technique. Major sediment accumulation is occurring near the major inlet and along the thalweg of the oxbow. Average sediment
accumulation of 1.8 cm/yr since 1963 was measured for twelve samples. Calculations show sediment accumulation is decreasing.
This study has shown that the 137Cs technique can provide useful planning information for the environmental geologist or reservoir manager. Results of the
current study will be used to develop better sediment and water management strategies for Lake Chicot.
Contribution of the U.S. Department of Agriculture, Agricultural Research Service, in cooperation with the Vicksburg District
of the U.S. Army Corps of Engineers. 相似文献
19.
The Huanghe is noted for its high transport rate of silt and clay, which may reach depth-averaged values of 200 kg m −3 during peak discharge. The sediment load transported through the river on entering the delta plain, amounts to 10 12 kg per year. In contrast to most other large deltas only one distributary channel is active at any one time. The high sediment load causes the rivermouth to prograde at a yearly rate of 1–4 km into the shallow (less than 20 m deep) Bohai gulf. The vertical aggradation of the channel belt and mouth bar complex is also rapid (decimetres per year on average), so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe. A series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front. A newly developed distributary passes from a multichannel to a single, straight channel system, and ends with the formation of meanders. The protruding mature delta lobe shows a radiating pattern of crevasse channels. Overbank/ crevasse deposits are made of vertically stacked dm-scale waning flow sequences, structurally characterized by (from bottom to top) small scour-and-fills, even (parallel) lamination, and climbing-ripple crosslamination. Accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge. It can be concluded that each sequence has been deposited within a few hours, and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence. 相似文献
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