首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 781 毫秒
1.
The Amery Ice Shelf is the largest ice shelf in East Antarctica.A new DEM was generated for this ice shelf,using kriging to interpolate the data from ICESat altimetry and the AIS-DEM.The ice thickness distribution map is conver ted from the new DEM,assuming hydrostatic equilibrium.The Amery Ice Shelf mari ne ice,up to 230 m thick,is concentrated in the northwest of the ice shelf.Th e volume of the marine ice is 2.38×10～3 km～3 and accounts for about 5.6% of t he shelf volume.  相似文献   

2.
As a contribution to the International Polar Year program MERGE (Microbiological and Ecological Responses to Global Environmental change in polar regions), studies were conducted on the terrestrial and aquatic microbial ecosystems of northern Canada (details at: http://www.cen.ulaval.ca/merge/). The habitats included permafrost soils, saline coldwater springs, supraglacial lakes on ice shelves, epishelf lakes in fjords, deep meromictic lakes, and shallow lakes, ponds and streams. Microbiological samples from each habitat were analysed by HPLC pigment assays, light and fluorescence microscopy, and DNA sequencing. The results show a remarkably diverse microflora of viruses, Archaea (including ammonium oxidisers and methanotrophs), Bacteria (including filamentous sulfur-oxidisers in a saline spring and benthic mats of Cyanobacteria in many waterbodies), and protists (including microbial eukaryotes in snowbanks and ciliates in ice-dammed lakes). In summer 2008, we recorded extreme warming at Ward Hunt Island and vicinity, the northern limit of the Canadian high Arctic, with air temperatures up to 20.5 °C. This was accompanied by pronounced changes in microbial habitats: deepening of the permafrost active layer; loss of perennial lake ice and sea ice; loss of ice-dammed freshwater lakes; and 23% loss of total ice shelf area, including complete break-up and loss of the Markham Ice Shelf cryo-ecosystem. These observations underscore the vulnerability of Arctic microbial ecosystems to ongoing climate change.  相似文献   

3.
Observation of the retreat and disintegration of ice shelves around the Antarctic Peninsula during the last three decades and associated changes in air temperature, measured at various meteorological stations on the Antarctic Peninsula, are reviewed. The climatically induced retreat of the northern Larsen Ice Shelf on the east coast and of the Wordie, George VI, and Wilkins ice shelves on the west coast amounted to about 10 000 km2 since the mid-1960s. A summary is presented on the recession history of the Larsen Ice Shelf and on the collapse of those sections north of Robertson Island in early 1995. The area changes were derived from images of various satellites, dating back to a late 1963 image from the recently declassified US Argon space missions. This photograph reveals a previously unknown, minor advance of the northern Larsen Ice Shelf before 1975. During the period of retreat a consistent and pronounced warming trend was observed at the stations on both east and west coasts of the Antarctic Peninsula, but a major cause of the fast retreat and final collapse of the northernmost sections of the Larsen Ice Shelf were several unusually warm summers. Temperature records from the nearby station Marambio show that a positive mean summer temperature was reached for the first time in 1992-93. Recent observations indicate that the process of ice shelf disintegration is proceeding further south on both sides of the Antarctic Peninsula.  相似文献   

4.
The change in dissolved inorganic δ13C in the ocean resulting from the change in δ13C in atmospheric CO2 owing to anthropogenic activities (the Suess effect) is well known. The need to correct for the Suess effect when applying δ13C in organic matter in lacustrine sediment deposited during the anthropocene as a productivity proxy, is widely although not universally acknowledged. This paper reviews conceptions about the Suess effect in lacustrine δ13Corg and methods to adjust for the Suess effect when δ13Corg is used to infer recent changes in aquatic productivity. Lake Tanganyika is used as an example to illustrate the necessity of the correction. When the Suess effect is not considered, interpretations of sediment core data can result that are opposite to those achieved with the correction applied, as is here shown in Lake Tanganyika and in other lakes. A new method to correct for the Suess effect is provided which has the advantage of being applicable to data for a larger period (1700–2000) than methods currently available. In addition, Lake Tanganyika is shown to be a net sink for CO2.  相似文献   

5.
We investigated how the history of local disturbances in a watershed can influence the regional coherence of ecosystem properties in lakes that have similar morphometry and climatic conditions. We measured sedimentary δ13C, δ15N, C:N and %BSiO2 in Sooke Lake Reservoir (SOL) and Shawnigan Lake (SHL), which are located within 4 km of each other on Vancouver Island, Canada. SOL is an impounded lake whose watershed has been fully protected over the last century, although the lake level has been raised 3 times via impoundment during this period. SHL has a similar limnological regime, but the surrounding watershed has been developed extensively for residential uses. We investigated how a pulse disturbance regime in SOL (i.e. repeated dam raising) and a press disturbance regime in SHL (i.e. persistent development) influenced the variability of paleoindicators in each system over time. We found that these contrasting disturbance regimes reduced the regional temporal coherence of aquatic productivity between the two lakes (indicated by %BSiO2), but did not influence the regional coherence of nutrient status or the main carbon sources of the lakes (indicated by %C, %N and δ13C). In contrast, an indicator of the sources and cycling of nitrogen (δ15N) showed increased coherence. Local disturbances also affected the variability of the paleoindicators within each system over time. In SOL, impoundments led to both declines (%N, δ15N) and increases (δ13C) in the variability of paleoindicators. In SHL, persistent watershed development led to lower variability of two paleoindicators (%N, %BSiO2). Overall, our data suggest that local disturbances can influence the %BSiO2 and C:N ratio of lake sediments, but are less likely to alter the regional coherence of %C, %N and δ13C between lakes. Biplob Das and Anita Narwani contributed equally to this work.  相似文献   

6.
Stable isotopes (δ18O and δD) are useful tracers for investigating hydrologic and climatic variability on a variety of temporal and spatial scales. Since the early isotopic studies on mountainous glaciers in the late 1960s, a great deal of information has been generated on the isotopic composition of rainfall, snow, ice, surface waters, and lake carbonate sediments across the Tibetan Plateau. However, measurements of δ18O and δD values of lake water are scarce. Here we present a new dataset of δ18O and δD values of lake waters collected from 27 lakes across the plateau during a reconnaissance survey in summer 2009. δ18O and δD values of lake water range from −19.9 to 6.6‰ and from −153 to −16‰, respectively. The average values of δ18O and δD are −6.4 and −72‰, considerably greater than those of precipitation observed in this region. The derived Tibetan lake water line, δD = 5.2δ18O − 38.9, is significantly different from the global meteoric water line. Most of the lakes, including some freshwater lakes, contain water with negative values of d-excess (d). There is a negative correlation between d and total dissolved solids (TDS). Each of these findings indicates that evaporation-induced isotopic enrichment prevails in Tibetan lakes. Moreover, we develop an isotope modeling scheme to calculate E/P ratios for Tibetan lakes, using a combination of existing isotopic fractionation equations and the Rayleigh distillation model. We use the intersection of the local evaporation line and GMWL as a first approximation of δ18O and δD values of lake water inputs to infer an E/P ratio for each lake. Our modeling calculations reveal that although variable from lake to lake, the water budget across the plateau is positive, with an average E/P of 0.52. This is in good agreement with other observational and model data that show varying degrees of increases in lake size from satellite imagery and significant decreases in lake salinity in many lakes on the plateau over the last several decades. Together with the new isotopic dataset, the proposed modeling framework can be used to examine and quantify past changes in a lake’s hydrologic balance from the isotopic record of downcore carbonate sediments in the region.  相似文献   

7.
Lake sediments contain archives of past environmental conditions in and around water bodies and stable isotope analyses (δ13C and δ15N) of sediment cores have been used to infer past environmental changes in aquatic ecosystems. In this study, we analyzed organic matter (OM), carbon (C), nitrogen (N), phosphorus (P), and δ13C and δ15N values in sediment cores from three subtropical lakes that span a broad range of trophic state. Our principal objectives were to: (1) evaluate whether nutrient concentrations and stable isotope values in surface deposits reflect modern trophic state conditions in the lakes, and (2) assess whether stratigraphic changes in the measured variables yield information about shifts in trophic status through time, or alternatively, diagenetic changes in sediment OM. Three Florida (USA) lakes of very different trophic status were selected for this study. Results showed that both δ13C and δ15N values in surface sediments of the oligo-mesotrophic lake were relatively low compared to values in surface sediments of the other lakes, and were progressively lower with depth in the sediment core. Sediments of the eutrophic lake had δ13C values that declined upcore, whereas δ15N values increased toward the sediment surface. The eutrophic lake displayed δ13C values intermediate between those in the oligo-mesotrophic and hypereutrophic lakes. Sediments of the hypereutrophic lake had relatively higher δ13C and δ15N values. In general, we found greater δ13C and δ15N values with increasing lake trophic state.  相似文献   

8.
We conducted multi-proxy geochemical analyses (including measurements of organic carbon, nitrogen and sulphur stable isotope composition, and carbonate carbon and oxygen isotope composition) on a 13.5 m sediment core from Lake Bliden, Denmark, which provide a record of shifting hydrological conditions for the past 6,700 years. The early part of the stratigraphic record (6,700–5,740 cal year BP) was wet, based on δ18Ocarb and lithology, and corresponds to the Holocene Thermal Maximum. Shifts in primarily δ18Ocarb indicate dry conditions prevailed from 5,740 to 2,800 cal year BP, although this was interrupted by very wet conditions from 5,300 to 5,150, 4,300 to 4,050 and 3,700 to 3,450 cal year BP. The timing of the latter two moist intervals is consistent with other Scandinavian paleoclimatic records. Dry conditions at Lake Bliden between 3,450 and 2,800 cal year BP is consistent with other paleolimnological records from southern Sweden but contrasts with records in central Sweden, possibly suggesting a more northerly trajectory of prevailing westerlies carrying moisture from the North Atlantic at this time. Overall, fluctuating moisture conditions at Lake Bliden appear to be strongly linked to changing sea surface temperatures in the Greenland, Iceland and Norwegian seas. After 2,800 cal year BP, sedimentology, magnetic susceptibility, δ13CORG, δ13Ccarb and δ18Ocarb indicate a major reduction on water level, which caused the depositional setting at the coring site to shift from the profundal to littoral zone. The Roman Warm Period (2,200–1,500 cal year BP) appears dry based on enriched δ18Ocarb values. Possible effects of human disturbance in the watershed after 820 cal year BP complicate attempts to interpret the stratigraphic record although tentative interpretation of the δ18Ocarb, magnetic susceptibility, δ13CORG, δ13Ccarb and δ18Ocarb records suggest that the Medieval Warm Period was dry and the Little Ice Age was wet.  相似文献   

9.
A 7.6-m lake sediment core from a marl lake, Lough Inchiquin, records variation in landscape evolution from 16,800 cal yrs B.P. to 5,540 cal yrs B.P. We observe significant variations (up to 12‰) in δ 13Corg and δ 13Ccalcite values that are interpreted to reflect secular changes in lake water δ 13CDIC values that result from a regional landscape transition from barren limestone bedrock to a forested ecosystem. Lake water δ 13CDIC values are therefore influenced by two isotopically distinct sources of carbon: terrestrial organic material (−27.1 to −31.2‰VPDB) via oxidized soil organic matter and weathered limestone bedrock (+3.4‰VPDB). Isotope excursions in lacustrine sediment records are forced not only by changes in productivity but also by changes in the terrestrial environment. This has profound implications for the interpretation of paleoclimate records derived from lacustrine sediment and suggests that selection of appropriate lakes can provide records of terrestrial change where other related records are not available.  相似文献   

10.
  相似文献   

11.
We developed a new method for reconstructing millennia-long hurricane records from coastal environments that uses Organic Geochemical Proxies (OGPs) of organic carbon and nitrogen concentrations and their δ13C and δ15N compositions. The new method is independent of presence/absence of sand layers and improves significantly the severe-storm history resolution. The subject of this investigation is a 1.5 m long sediment core raised at 2.8 m water depth from the center of Lake Shelby, Alabama, a freshwater lake located approximately 250 m from the Gulf of Mexico, from which an overwash sand-layer based record was previously derived. The core contains two distinct sediment units; an upper 62 cm thick, fine-grained, organic-rich lacustrine sapropel (gyttja) that shows no visible structures except one sand lamina at 23.7 cm depth, and an underlying 90 cm thick, organic-poor lagoon/estuary clay unit. The sapropel unit was deposited over a 682 ± 30 cal year time interval (1320–2002 A.D.) with a mean sedimentation rate of 0.79 ± 0.04 mm/year. Lake Shelby’s water column exhibits two contrasting states based on water chemistry surveys (i) an “isolated”, stratified, mode under calm weather conditions with a relatively low trophic state, and (ii) a “flooded” mode occurring during storm surges when nutrient-rich seawater floods the lake. Statistically significant δ13C and δ15N positive excursions in organic matter, up to maximum values of −25 (‰ PDB) and 4 (‰ Air N2), respectively, are interpreted as geochemical responses to the marine intrusions that fertilize the lake, increase light availability, and cause eutrophication spikes. Detailed OGPs analyses crossing a sand layer that offers visual evidence of a catastrophic hurricane overwash event at 1717 A.D. exhibit large δ13C and δ15N positive shifts bounded by rapid returns to base values, thus confirming the validity of the hurricane identification by the OGPs model. Our data indicate that 11 catastrophic hurricanes hit the Alabama coast over the past 682 years with a rough recurrence interval of one in 62 years.  相似文献   

12.
The oxygen isotope ratios of diatoms (δ18Odiatom), and the oxygen and hydrogen isotope ratios of lake water (δW) of lakes in south Alaska provide insight into past changes in atmospheric circulation. Lake water was collected from 31 lakes along an elevation transect and diatoms were isolated from lake sediment from one lake (Mica Lake) in south Alaska. In general, δW values from coastal lakes overlap the global meteoric water line (GMWL). δW values from interior lakes do not lie on the GMWL; they fall on a local evaporation line trajectory suggesting source isotopes are depleted with respect to maritime lakes. Sediment cores were recovered from 58 m depth in Mica Lake (60.96° N, 148.15° W; 100 m asl), an evaporation-insensitive lake in the western Prince William Sound. Thirteen calibrated 14C ages on terrestrial macrofossil samples were used to construct an age-depth model for core MC-2, which spans 9910 cal years. Diatoms from 46, 0.5-cm-thick samples were isolated and analyzed for their oxygen isotope ratios. The analyses employed a newly designed, stepwise fluorination technique, which uses a CO2 laser-ablation system, coupled to a mass spectrometer, and has an external reproducibility of ±0.2‰. δ18Odiatom values from Mica Lake sediment range between 25.2 and 29.8‰. δ18Odiatom values are relatively uniform between 9.6 and 2.6 ka, but exhibit a four-fold increase in variability since 2.6 ka. High-resolution sampling and analyses of the top 100 cm of our lake cores suggest large climate variability during the last 2000 years. The 20th century shows a +4.0‰ increase of δ18Odiatom values. Shifts of δ18Odiatom values are likely not related to changes in diatom taxa or dissolution effects. Late Holocene excursions to lower δ18Odiatom values suggest a reduction of south-to-north storm trajectories delivered by meridional flow, which likely corresponds to prolonged intervals when the Aleutian Low pressure system weakened. Comparisons with isotope records of precipitation (δP) from the region support the storm-track hypothesis, and add to evidence for variability in North Pacific atmospheric circulation during the Holocene.
Zachary SharpEmail:
  相似文献   

13.
Earlier work in northeast Greenland has suggested a limited advance of the Greenland Ice Sheet during the Last Glacial Maximum (LGM). However, this concept has recently been challenged by marine geological studies, indicating grounded ice on the continental shelf at this time. New 10Be‐ages from the Store Koldewey island, northeast Greenland, suggest that unscoured mountain plateaus at the outer coast were covered at least partly by cold‐based ice during the LGM. It is, however, still inconclusive whether this ice was dynamically connected to the Greenland Ice Sheet or not. Regardless of the LGM ice sheet extent, the 10Be results from Store Koldewey add to a growing body of evidence suggesting considerable antiquity of crystalline unscoured terrain near present and Pleistocene ice sheet margins.  相似文献   

14.
Variations in the oxygen-isotope composition of paleo-water bodies in the Lake Superior Basin provide information about the timing and pathways of glacial meltwater inflow into and within the Lake Superior Basin. Here, the oxygen-isotope compositions of Lake Superior have been determined using ostracodes from four sediment cores from across the Basin (Duluth, Caribou and Ile Parisienne sub-basins, Thunder Bay trough). The δ18O values indicate that lake water (Lake Minong) at ~10,600–10,400 cal [~9,400–9,250] BP was dominated by glacial meltwater derived from Lake Agassiz and the Laurentide Ice Sheet (LIS). From that time to ~9,000 cal [~8,100] BP, a period associated with formation of thick varves across the Lake Superior Basin, the δ18O values of Lake Minong decreased even further (−24 to −28‰), symptomatic of an increasing influx of glacial meltwater. Its supply was reduced between ~9,000 and ~8,900 cal [~8,100–8,000] BP, and lake water δ18O values grew higher by several per mil during this period. Between ~8,900 and ~8,800 cal [~8,000–7,950] BP, there was a return to δ18O values as low as −29‰ in some parts of the Lake Superior Basin, indicating a renewed influx of glacial meltwater before its final termination at ~8,800–8,700 cal [~7,950–7,900] BP. The sub-basins in the Lake Superior Basin generally displayed very similar patterns of lake water δ18O values, typical of a well-mixed system. The final stage of glacial meltwater input, however, was largely expressed near its input (Thunder Bay trough) and recognizable in dampened form mainly in the Duluth sub-basin to the west. Water in the easternmost Ile Parisienne sub-basin was enriched in 18O relative to the rest of the lake, particularly after ~10,000 cal [~8,900] BP, probably because of a strong influence of local precipitation/runoff, and perhaps also enhanced evaporation. By ~9,200 cal [~8,250] BP, lake water δ18O values in the Ile Parisienne sub-basin were similar to the adjacent Lake Huron Basin, suggesting a strong hydraulic connection between the two water bodies, and common responses to southern Ontario’s shift to warmer and dry climatic conditions after ~9,000 cal [~8,100] BP.  相似文献   

15.
Hydrographic and current measurements obtained during the Norwegian Antarctic Research Expedition 1978/79 to the southern Weddell Sea are presented. Cold, dense Ice Shelf Water circulating under the floating ice shelves is observed to leave the shelf as a concentrated bottom flow. From moored current metres this discharge is estimated at 0.7 106 m3/s at -2.0°C (one year average) and with no appreciable seasonal variation. This contribution to the Weddell Sea Bottom Water is clearly identified through extreme temperature gradients at our deepest stations (below 2500 m). The core of Weddell Deep Water shows a considerable (T ∼ 0.5°C) warming up since 1977, presumably due to the lack of polynya activity in the intervening period. Measurements in the coastal current at the ice shelf (70°S, 2°W) show step structures which are probably due to cooling and melting at the vertical ice barrier. Slight supercooling due to circulation under the ice shelf is also seen. The net effect of the ice shelf boundary seems to be a deep reaching cooling and freshening of the coastal current providing the low salinity, freezing point Eastern Shelf Water. This process is considered a preconditioning which enhances production of the saline Western Shelf Water which in turn is transformed to Ice Shelf Water.  相似文献   

16.
Hydrographic (CTD), current and water level measurements obtained in the vicinity of the floating Ronne and Filchner Ice Shelves are presented. The distribution of Western Shelf Water (S > 34.7) and Ice Shelf Water (T<−1.9°C) are discussed. The general circulation in the area seems to consist of two large cyclonic gyres, one in the Filchner Depression and one north of the Ronne Ice Shelf. Each gyre shows a'warm'(T 1°C) southgoing flow of Modified Weddell Deep Water and a cold northward flow of Ice Shelf Water. The mean surface current was found to be 8cm/s towards the north-west along the barrier. The mean flow below the ice shelf shows significant components normal to the barrier, and mixing seems to be very efficient here. Well mixed layers down to more than 150 m were observed. North of Berkner Island the water level shows a typical mixed tide with tidal range ∼3m. In the tidal currents the semidiurnal constituents dominate (∼30cm/s) and with the largest current components normal to the barrier.  相似文献   

17.
Bulk organic matter (OM) in sediments that accumulated in Lakes Victoria, Albert and Tanganyika during the major transgression that transformed these basins from a desiccated or lowstand condition at the time of the Last Glacial Maximum to highstand with overflow in the terminal Pleistocene and early Holocene, show some striking compositional trends. Sediment OM content (TOC) and Rock-Eval Hydrogen Index (HI) both rise markedly, reflecting increasing primary productivity during the transgression and enhanced conditions for the preservation of labile OM, In Lake Victoria rising sedimentary sulphur content tracks the change in HI. Over the same time interval in all three lakes, δ13COM shows a falling trend of 5–8‰ . Nitrogen isotopic and C/N trends are, in contrast, more variable. Relatively high δ15N values at an early stage in the transgression may reflect isotopic effects associated with ammonia loss from an alkaline waterbody, while high δ15N later in the transgression was probably due to periods when stratification-related denitrification had a particularly strong impact upon the lakes' N cycle. The observed changes are attributed to the influence of the landscape drowned during the transgression upon the nutrient budget of the lake, a rise in river-supplied nutrients as the climate became wetter, and an increasing tendency to stable chemical stratification as the water column deepened. In the case of δ13C, additional contributory factors were changes in the amount and isotopic composition of river-supplied dissolved inorganic carbon (DIC) as runoff and soil–moisture turnover rates increased, and the replacement of a C4-grassland dominated flora by mixed woodland-grassland and forest with a significant presence of C3 plants. Distinct geochemical changes are also apparent in Lakes Victoria and Albert sediments at the end of the transgression which are consistent with a reduction in nutrient loading. In the case of N this resulted in biological N fixation becoming a major part of the lakes' N cycle, Some of the observed geochemical variations may be generally applicable as proxy indicators of major transgressions in large lakes. IDEAL Contribution No. 152.  相似文献   

18.
Walker Lake, a hydrologically closed, saline, alkaline lake located along the western margin of the Great Basin of western United States, has experienced a 77% reduction in volume and commitment drop in lake level as a result of anthropogenic perturbations and climatic fluctuations over the last century. The history of lake-level change in Walker Lake has been recorded instrumentally since 1860. A high-resolution multi-proxy sediment core record from Walker Lake has been generated through analysis of total inorganic carbon (TIC), total organic carbon (TOC), and oxygen and carbon isotope ratios (δ18O and δ13 C) of both downcore bulk TIC and ostracods over the last 200 yr. This allows us to examine how these sediment indices respond to actual changes in this lake’s hydrologic balance at interannual to decadal timescales. In Walker Lake sediments, changes in %TIC, %TOC, and δ13C and δ18O of TIC and ostracods are all associated to varying degrees with changes in the lake’s hydrologic balance, with δ18O of the TIC fraction (δ18OTIC) being the most highly correlated and the most effective hydrologic indicator in this closed-basin lake. The δ18OTIC record from Walker Lake nearly parallels the instrumental lake-level record back to 1860. However, comparison with sporadic lake-water δ18O and dissolved inorganic carbon δ13C (δ13CDIC) results spanning the last several decades suggests that the isotopic values of downcore carbonate sediments may not be readily translated into absolute or even relative values of corresponding lake-water δ18O and δ13CDIC. Changes in the lake’s hydrologic balance usually lead to changes in isotopic composition of lake waters and downcore sediments, but not all the variations in downcore isotopic composition are necessarily caused by hydrologic changes.  相似文献   

19.
Late Quaternary Lakes in the McMurdo Sound Region of Antarctica   总被引：2，自引：0，他引：2  
Lake levels within the enclosed drainage basins of the Dry Valleys adjacent to McMurdo Sound have fluctuated widely during the Late Quaternary due to (a) local climate change and the consequent variation in the evaporation–precipitation regime, and (b) glacial fluctuations, resulting in changes in the catchment and meltwater drainage areas of the glaciers and, in some cases, in the volumes of the available lake basins. Three types of lakes can be distinguished on the basis of their water source: (1) lakes receiving the bulk of their water from melting of local alpine glaciers; (2) proglacial lakes associated with outlet glaciers from the East Antarctic Ice Sheet; (3) proglacial lakes associated with the marine oxygen-isotope stage 2 Ross Sea ice sheet and its precursors. The Dry Valleys contain an exceptionally long lacustrine record, extending back at least 300,000 years. Lacustrinesedimentation is cyclical, occurring over periods of about 100,000 years. During the last such cycle, relatively small lakes, both adjacent to East Antarctic outlet glaciers and fed by meltwater from alpine glaciers, existed during stage 5. However, these local lakes gave way to large proglacial lakes adjacent to the Ross Sea ice sheet in stage 2. The same relationship apparently occurred during the previous 100,000-year cycle. Dating of lacustrine sediments suggests that lakes proglacial to the Ross Sea ice sheet have existed during episodes of sea-level lowering during global glaciations. Lakes proglacial to outlet glaciers from the East Antarctic Ice Sheet have formed coincident with episodes of high eustatic sea level during interglacial periods.  相似文献   

20.
Stable isotopic compositions and concentrations of total sedimentary sulphur (S) were determined in cores from 6 lakes in the acid-sensitive Muskoka-Haliburton region of south-central Ontario. The isotopic composition of S in deep sediment (> ~ 20 cm) was approximately constant in all lakes, and indicated a pre-industrial δ 34S value between +4.0 and +5.3‰, which is similar to current bulk deposition. Similarly, total S concentrations in deep sediment were relatively low (1.9–5 mg S g−1 dwt) and approximately constant with depth within cores. All lakes exhibited up-core increases in total S and decreases in δ 34S at a depth corresponding to the beginning of industrialization in the Great Lakes region ( ~ 1900), resulting in a generally reciprocal depth pattern between total S concentration and δ 34S ratios. While initial shifts in total S and δ 34S were likely due to enhanced SO4 reduction of newly available anthropogenic SO4, both the magnitude and pattern of up-core S enrichment and shifts in δ 34S varied greatly among lakes, and did not match changes in S deposition post 1900. Differences between lakes in total S and δ 34S were not related to any single hydrologic (e.g., residence time) or physical (e.g., catchment-area-to-lake area ratio) lake characteristic. This work indicates that sediment cores do not provide consistent records of changes in post-industrial S deposition in this region, likely due to redox-related mobility of S in upper sediment.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号