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1.
We inferred late Holocene lake-level changes from a suite of near-shore gravity cores collected in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area, northwestern Ontario. Results were reproduced across all cores. A gravity core from the deep central basin was very similar to the near-shore cores with respect to trends in the percent abundance of the dominant diatom taxon, Cylcotella stelligera. The central basin, however, does not provide a sensitive site for reconstruction of lake-level changes because of the insensitivity of the diatom model at very high percentages of C. stelligera and other planktonic taxa. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several depth transects in Lake 239. The lake-level reconstructions during the past ~3,000 years indicate that lake depth varied on average by ±2 m from present-day conditions, with maximum rises of ~3–4 m and maximum declines of ~3.5–5 m. The diatom-inferred depth record indicates several periods of persistent low levels during the nineteenth century, from ~900 to 1100 AD, and for extended periods prior to ~1,500 years ago. Periods of inferred high lake levels occurred from ~500 to 900 AD and ~1100 to 1650 AD. Our findings suggest that near-shore sediments from small drainage lakes in humid climates can be used to assess long-term fluctuations in lake level and water availability.  相似文献   

2.
Water levels in the Lake Erie basin are inferred from glacial lake times to present. An era of early to middle Holocene lowstands is defined below outlets by a submerged paleo-beach, and truncated reflectors in glaciolacustrine sediment beneath a mud-covered wave-cut terrace. Also, the glacial clay surface above the paleo-shore level has elevated shear strength because of porewater drainage during subaerial exposure. Below the paleo-shore where exposure did not occur, clay strength remained normal. Sedimentation rates were reduced during the lowstands. The distortion of once-level shore zone indicators by differential glacial rebound was removed by computing original elevations of the indicators using an empirical model of rebound based on observations of upwarped former lake shorelines. Erie water-level history was inferred from a plot of the original elevations of lake-level constraints and outlets versus age. The lake history was validated by reference to ~83 water-level indicators, not used as constraints. During the deglaciation, lake-crossing moraines were likely eroded by fluvial drainage into low-level Lake Ypsilanti and a subsequent unnamed low lake to produce the Lorain Valley and Pennsylvania Channel. Once inflow from the upper Great Lakes basins was directed to Ottawa Valley about 10,400 (12,270 cal BP), Erie water levels descended in a dry, evaporative climate to a closed lowstand during which ostracode δ18O increased ~2‰ above present values. Lake level began to rise 6,000 to 7,000 (6,830 to 7,860 cal) BP in response to increased atmospheric moisture and later, to northern inflow as the Nipissing Transgression returned upper Great Lakes drainage to Lake Erie by about 5,200 (6,000 cal) BP. At that time, the lake overflowed the uplifted Lyell–Johnson Sill north (downstream) of the present Niagara Falls at higher-than-present levels. After recession of the Falls breached this sill about ~3,500 (~3,770 cal) BP, Lake Erie fell 3–4 m to its present Fort Erie–Buffalo Sill. The extended low-water phase with its isolated sub-basins could have restricted migration of aquatic fauna. The early to middle Holocene closed-basin response highlights the sensitivity of Lake Erie to climatic reductions in its water budget.  相似文献   

3.
Abstract We present here the initial results of a high-resolution (sparker) reflection seismic survey in Northern Lake Tanganyika, East African Rift system. We have combined these results with data from earlier multichannel reflection seismic and 5-kHz echosounding surveys. The combination of the three complementary seismic investigation methods has allowed us to propose a new scenario for the late Aliocene to Recent sedimentary evolution of the North Tanganyika Basin. Seismic sequences and regional tectonic information permit us to deduce the palaeotopography at the end of each stratigraphic sequence. The basin history comprises six phases interpreted to be responses to variations in regional tectonism and/or climate. Using the reflection seismic-radiocarbon method (RSRM), the minimum ages for the start of each phase (above each sequence boundary) are estimated to be: ?7.4 Ma, ? 1.1 Ma, ?393–363 ka, ?295–262 ka, ? 193–169 ka, ?40–35 ka. Corresponding lowstand lake elevations below present lake level for the last five phases are estimated to have been: ?650–700 m, ?350 m, ?350 m, ?250 m and ? 160 m, respectively. The latest phase from ?40–35 ka until the present can be subdivided into three subphases separated by two lowstand periods, dated at ?23 ka and ? 18 ka. From the late Miocene until the mid Pleistocene, large-scale patterns of sedimentation within the basin were primarily controlled by tectonism. In contrast, from the mid Pleistocene to the present, sedimentation in Lake Tanganyika seems to have responded dramatically to climatic changes as suggested by repeated patterns of lake level fluctuations. During this period, the basin infill history is characterized by the recurrent association of three types of deposits: ‘basin fill’ accumulations; lens-shaped ‘deep lacustrine fans’; and ‘sheet drape’ deposits. The successive low-lake-level fluctuations decreased in intensity with time as a consequence of rapid sedimentary filling under conditions of declining tectonic subsidence. The climate signal has thus been more pronounced in recent sedimentary phases as tectonic effects have waned.  相似文献   

4.
Serpent River Bog lies north of North Channel, 10 m above Lake Huron and 15 m below the Nipissing Great Lake level. A 2.3 m Holocene sequence contains distinct alternating beds of inorganic clastic clay and organic peat that are interpreted as evidence of successive inundation and isolation by highstands and lowstands of the large Huron-Basin lake. Lowstand phases are confirmed by the presence of shallow-water pollen and plant macrofossil remains in peat units. Twelve 14C dates on peat, wood and plant macrofossils combined with previously published 14C ages of lake-level indicators confirm much of the known early Holocene lake-level history with one notable exception. A new Late Mattawa highstand (8,390 [9,400 cal]–8,220 [9,200 cal] BP) evidenced by a sticky blue-grey clay bed is tied to outburst floods of glacial Lake Minong during erosion of the Nadoway drift barrier in the eastern Lake Superior basin. A subsequent Late Mattawa highstand (8,110 [9,040 cal]–8,060 [8,970 cal] BP) is attributed to enhanced meltwater inflows that first had deposited thick varves throughout Superior Basin. Inundation by the Nadoway floods and possibly the last Mattawa flood were likely responsible for termination of the Olson Forest (southern Lake Michigan). A pollen diagram supports the recognized progression of Holocene vegetation, and defines a subzone implying a very dry, cool climate about 7.8–7.5 (8.6–8.3 cal) ka BP based on the Alnus crispa profile during the Late Stanley lowstand. A new date of 9,470 ± 25 (10,680–10,750 cal) BP on basal peat over lacustrine clay at Espanola West Bog supports the previous interpretation of the Early Mattawa highstand at ca. 9,500 (10,740 cal) BP. The organic and clastic sediment units at these two bogs are correlated with other records showing coherent evidence of Holocene repeated inundation and isolation around northern Lake Huron. Taken together the previous and new lake-level data suggest that the Huron and Georgian basin lakes were mainly closed lowstands throughout early Holocene time except for short-lived highstands. Three of the lowstands were exceptionally low, and likely caused three episodes of offshore sediment erosion which had been previously identified as seismo-stratigraphic sequence boundaries.  相似文献   

5.
The sedimentology of an 8.22-m long core of late-Holocene deposits in the submerged Crescent Island Crater basin of Lake Naivasha, Kenya, is used to reconstruct decade-scale fluctuations in lake-surface elevation during the past 1800 yrs. Lake-depth inference for the past 1000 yrs is semi-quantitative, based on (1) relationships between lake level and bottom dynamics predicted by wave theory, and (2) historical validation of the effects of lake-level fluctuation and hydrologic closure on sediment composition in Crescent Island Crater and nearby Lake Oloidien. In these shallow fluctuating lakes, organic-carbon variation in a lithological sequence from clayey mud to algal gyttja is positively correlated with lake depth at the time of deposition, because the focusing of oxidized littoral sediments which dilute autochthonous organic matter before burial is reduced during highstands. The lake-level reconstruction for Lake Naivasha agrees with other adequately dated lake-level records from equatorial East Africa in its implication of dry climatic conditions during the Mediaeval Warm Period and generally wet conditions during the Little Ice Age. Crescent Island Crater survived widespread aridity in the early-19th century as a fresh weedy pond, while the main basin of Lake Naivasha and many other shallow East African lakes fell dry and truncated their sediment archive of Little Ice Age climatic variability.  相似文献   

6.
J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 14C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.314C ka, determined from the inferred 14C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.  相似文献   

7.
Paleorecords from multiple indicators of environmental change provide evidence for the interactions between climate, human alteration of watersheds and lake ecosystem processes at Lake Tanganyika, Africa, a lake renowned for its extraordinary biodiversity, endemism and fisheries. This paper synthesizes geochronology, sedimentology, paleoecology, geochemistry and hydrology studies comparing the history of deltaic deposits from watersheds of various sizes and deforestation disturbance levels along the eastern coast of the lake in Tanzania and Burundi. Intersite differences are related to climate change, differences in the histories of forested vs. deforested watersheds, differences related to regional patterns of deforestation, and differences related to interactions of deforestation and climate effects. Climate change is linked to variations in sediment accumulation rates, charcoal accumulation, lake level and water chemistry, especially during the arid-humid fluctuations of the latter part of the Little Ice Age. Differences between forested and deforested watersheds are manifested by major increases in sediment accumulation rates in the latter (outside the range of climatically driven variability and for the last 40 years unprecedented in comparison with other records from the lake in the late Holocene), differences in eroded sediment and watershed stream composition, and compositional or diversity trends in lake faunal communities related to sediment inundation. Variability in regional patterns of deforestation is illustrated by the timing of transitions from numerous sedimentologic, paleoecologic and geochemical indicators. These data suggest that extensive watershed deforestation occurred as early as the late-18th to the early-19th centuries in the northern part of the Lake Tanganyika catchment, in the late-19th to early-20th centuries in the northern parts of modern-day Tanzania, and in the mid-20th century in central Tanzania. Rapid increases in sediment and charcoal accumulation rates, palynological and lake faunal changes occurred in the early-1960s. We interpret this to be the result of greatly enhanced flushing of sediments in previously deforested watersheds triggered by extraordinary rainfall in 1961/62. Regional differences in deforestation histories can be understood in light of the very different cultural and demographic histories of the northern and central parts of the lake shoreline. Incursion of slaving and ivory caravans from the Indian Ocean to the central coast of Lake Tanganyika by the early-19th century, with their attendant diseases, reduced human and elephant populations and therefore maintained forest cover in this region through the late-19th to early-20th centuries. In contrast, the northeastern portion of the lakeshore did not experience the effects of the caravan trades and consequently experienced high human population densities and widespread deforestation much earlier. These studies demonstrate the importance of paleolimnological data for making informed risk assessments of the potential effects of watershed deforestation on long-term lake ecosystem response in the Lake Tanganyika catchment. Differences in sediment yield and lake floor distribution of that yield, linked to factors such as watershed size, slope, and sediment retention, must be accounted for in management plans for both human occupation of currently forested watersheds and the development of future underwater reserves.  相似文献   

8.
Sediment lithology and mineralogy, as well as ostracode, plant macrofossil and stable isotope stratigraphies of lake sediment cores, are used to reconstruct late Holocene hydrologic changes at Kenosee Lake, a relatively large, hyposaline lake in southeastern Saskatchewan. Chronological control is provided by AMS radiocarbon ages of upland and shoreline plant macrofossils. All indicators outline an early, low-water, saline phase of lake history (4100–3000 BP), when the basin was occupied by a series of small, interconnected, sulfate-rich brine pools, as opposed to the single, topographically-closed lake that exists today. A rapid rise in lake-level (3000–2300 BP) led to the establishment of carbonate-rich, hyposaline lake conditions like those today. Lithostratigraphic data and ostracode assemblages indicate peak salinities were attained early in this period of lake infilling, suggesting that the lake-level rise was initially driven by an influx of saline groundwater. Lake-level and water chemistry have remained relatively stable over the last 2000 years, compared to earlier events. Because of a lack of datable organic material in sediments deposited during the last 2000 years, the chronology of recent events is not well resolved. Plant macrofossil, lithostratigraphic and ostracode evidence suggests that lake draw-down, accompanied by slightly higher than present salinites, occurred sometime prior to 600 BP, followed by peak lake-level and freshwater conditions. This most recent high lake stand, indicative of a high water table on the surrounding upland, may also have led to the establishment of an extensive cover of Betula in the watershed, possibly in response to paludification. Ostracode assemblages indicate that peak freshwater conditions occurred within the last 100 years. Since historically documented lake-level fluctuations correlate with decadal scale climatic fluctuations in the meteorological record, and late-Holocene hydrologic dynamics correspond to well documented climatic excursions of the Neoglacial and Little Ice Age, Kenosee Lake dynamics offer insight into the susceptibility of the region's water resources to climate change.  相似文献   

9.
The character and impact of climate change since the last glacial maximum (LGM) in the eastern Mediterranean region remain poorly understood. Here, two new diatom records from the Ioannina basin in northwest Greece are presented alongside a pre-existing record and used to infer past changes in lake level, a proxy for the balance between precipitation and evaporation. Comparison of the three records indicates that lake-level fluctuations were the dominant driver of diatom assemblage composition change, whereas productivity variations had a secondary role. The reconstruction indicates low lake levels during the LGM. Late glacial lake deepening was underway by 15.0 cal kyr BP, implying that the climate was becoming wetter. During the Younger Dryas stadial, a lake-level decline is recorded, indicating arid climatic conditions. Lake Ioannina deepened rapidly in the early Holocene, but long-term lake-level decline commenced around 7.0 cal kyr BP. The pattern of lake-level change is broadly consistent with an existing lake-level reconstruction at Lake Xinias, central Greece. The timing of the apparent change, however, is different, with delayed early Holocene deepening at Xinias. This offset is attributed to uncertainties in the age models, and the position of Xinias in the rain shadow of the Pindus Mountains.  相似文献   

10.
The post-glacial history of the Great Lakes has involved changes in lake levels that are equivalent in vertical extent to the Pleistocene changes in global sea level and changes in sediment accumulation by at least two orders of magnitude. In the sediments of the northern Lake Michigan basin, these radical changes in base level and sediment supply are preserved in detailed records of changing depositional environment and the impact of these changes on depositional architecture. The seismic sequences of the sediment fill previously described in Lake Huron have been carried into northern Lake Michigan and used to map the history and architecture of basinal deposition. As the Laurentide Ice Sheet retreated northward in the early Holocene, it opened progressively deeper channels to the east that allowed the larger lakes to drain through the North Channel, Huron, and Georgian Bay basins. At the end of the Main Algonquin highstand, about 10,200 (radiocarbon) yrs ago, the eastern drainage passage deepened in a series of steps that defined four seismic sequences and lowered lake levels by over 100 m. Near the same time a new source of sediment and meltwaters poured across the Upper Peninsula of Michigan and into the northern Lake Michigan basin from the Superior basin ice lobe. A marked increase in deposition is seen first in the northern part of the study area, and slightly later in the Whitefish Fan area at the southern end of the study area. Accumulation rates in the area gradually decreased even as lake levels continued to fall. Drainage directly from the Superior basin ended before the beginning of the main Mattawa phase about 9,200 (radiocarbon) yrs ago.Although individual lowstand systems tracts are at the most a few hundred yrs in duration, their geometries and seismic character are comparable to marine systems tracts associated with sea level falls of similar magnitudes. In some of the thicker lowstand deposits a second order cyclicity in sedimentation can be detected in the high resolution seismic records.  相似文献   

11.
Fishing and Farming at Lake Chad: Responses to Lake-level Fluctuations   总被引：5，自引：0，他引：5  
Lake Chad lies at the southern extreme of the Sahara Desert and is well known for large fluctuations in its surface area this century. Seasonal fluctuations, however, have received much less attention. This paper presents the results of two complimentary research efforts on the south-west shore of the lake. These illustrate how important both inter and intra-annual fluctuations in the level of the lake are, both in terms of their impact on the environment and in the response of the communities living on the lake shore. The paper compares a time series of the fluctuations in the level of Lake Chad as monitored by the TOPEX/POSEIDON satellite with findings from participatory research with the communities of the south-west lake shore. It shows how the communities of the lake have responded to lake-level fluctuations with their livelihood choices. These results are used to show that although vastly different in scope, a high degree of complimentarity exists between remotely-sensed information and community-based research and that they are of potentially great value to development initiatives on the shores of Lake Chad.  相似文献   

12.
We report here on the first detailed ostracode stratigraphic record to be obtained from late Holocene sediments of Lake Tanganyika. We analyzed four cores, three from the northern lake region and a fourth from a more southern lake locality, that collectively record ostracode assemblages under a variety of disturbance regimes. These cores provide a stratigraphic record of ostracode abundance and diversity, as well as depositional changes over time periods of decades to millennia. We have investigated the fossil ostracodes in these cores by looking at temporal changes of species diversity and population structure for the species present. All four cores provided distinct patterns of ostracode diversity and abundance. BUR-1, a northern lake core obtained close to the Ruisizi River delta, yielded a sparse ostracode record. Karonge #3, another northern core from a site that is closely adjacent to a river delta with high sediment loading, yielded almost no ostracodes. The third core 86-DG-14, taken from a somewhat less disturbed area of the lake, suggests that there have been recent changes in ostracode populations. Through most of the lower portion of this core, ostracode abundance is low and species richness is relatively constant. Above 7 cm there is a marked increase in ostracode abundance and a corresponding decrease in species richness, probably signaling the onset of a major community disturbance, perhaps due to human activities. The southernmost core, 86-DG-32, is from a site that is well removed from influent rivers. Ostracode abundance varies erratically throughout the core, whereas species richness is relatively constant and high throughout the core. The temporal variation evident in ostracode community makeup both within and between the studied cores may be a result of naturally patchy distributions among ostracodes, coupled with local extinctions and recolonizations, or it may reflect inadequate sampling of these high diversity assemblages. In either case, these cores illustrate the potential to obtain high resolution ostracode records from the rich, endemic fauna of Lake Tanganyika that can be used to address questions about the history of community structure and human impacts in this lake.  相似文献   

13.
The vegetation history and development of three different types of lakes, lakes Valday, Kubenskoye and Vishnevskoye (northwest of the East European Plain) were reconstructed using paleolimnological techniques. Watershed vegetation demonstrates a close connection with climate fluctuations: gradual expansion of the southern broad-leaved trees to the North during the Holocene with the maximum extent during the climate optimum (8000–5000 BP); and their subsequent retreat afterwards; followed by the extension of spruce during the cold and dry Subboreal time; and dominance of pine-spruce-birch forests in the Subatlantic time. The Late Pleistocene and Holocene climate changes resulted in lake-level fluctuations and other ecosystem changes. Valday Lake was formed ca. 12,500 BP as an oligotrophic, deep water basin. The lake level decreased during the dry Boreal, then increased again during the humid Atlantic period. The large shallow Kubenskoye Lake was formerly a part of an ice margin lake, which was then separated (ca. 13,000 BP) and developed into the Sukhona Basin with an outflow to the northwest. During the Atlantic, the outflow direction changed to the east. As a result, the ancient Sukhona Lake disappeared and Kubenskoye Lake formed in its modern size and shape. Vishnevskoye Lake, on the Karelian Isthmus, was formed at the beginning of the Preboreal after the disappearance of the Baltic Ice Lake. It was flooded by waters of the Boreal Ancylus transgression of the Baltic Basin and had become a small eutrophic lake by the time.  相似文献   

14.
This paper seeks to arrive at a consistent interpretation of (1) the age model, (2) the grain size record, and (3) seismic reflection data from Lake Hovsgol (a.k.a Khubsugul or Hövsgöl), Mongolia, reported by Fedotov et al. (2007, earlier by Fedotov et al. 2002, 2004). In their most recent contribution, the grain size record of the KDP-01 drill core is interpreted as a climatic signal while little consideration is given to lake-level changes and hence to basin-wide changes in depositional setting evident from seismic profiles; also, a nearly linear age model is at odds with the seismic evidence for a major angular unconformity in the sediment strata. The lack of regional seismic stratigraphic analysis has thus led to an improbable interpretation of the Lake Hovsgol sediment grain size record and ultimately to an improbable scenario of Mongolian glaciation history. Using the available seismic profiles, here we show that the drill core penetrated several transgressive/regressive sedimentary sequences and a major angular unconformity. Therefore, the drilled sediment section cannot represent continuous sediment accumulation and the Brunhes age model across the unconformity cannot be nearly linear; the time interval representing a hiatus remains to be determined. The assumed nearly linear age/depth relationship in the upper 23 m above the angular unconformity is also an unlikely relationship, given the evidence of repeated changes in lake level, and hence in the depositional setting and sedimentation rates. We further propose a qualitative reference model for changes in the Lake Hovsgol depositional setting (presented as a step-by-step animation – see supplementary material) based on manually ‘backstripping and rebuilding’ the seismic pattern. We argue that this model provides a useful template of the likely sediment facies changes in the deep axial part of the Hovsgol basin: our crude model in fact captures the major depositional trends in the KDP-01 drill core section located some 10 km NW along the seismic line. We contend that changes in the depositional setting provide the first-order control on sediment grain size in the Hovsgol record. Our study provides important new constraints on the nature of sedimentary proxy records in Lake Hovsgol and on their interpretation as a record of Mongolian glaciation history.  相似文献   

15.
《Basin Research》2018,30(4):587-612
  相似文献   

16.
Documenting the history of catchment deforestation using paleolimnological data involves understanding both the timing and magnitude of change in the input of erosional products to the downstream lake. These products include both physically-eroded soil and the byproducts of burning, primarily charcoal, which arise from both intentional and climatically-induced changes in fire frequency. As a part of the Lake Tanganyika Biodiversity Projects special study on sedimentation, we have investigated the sedimentological composition of seven dated cores from six deltas or delta complexes along the east coast of Lake Tanganyika: the Lubulungu River delta, the Kabesi River delta, the Nyasanga/Kahama River delta, and the Mwamgongo River delta in Tanzania, and the Nyamusenyi River delta and Karonge/Kirasa River delta in Burundi. Changes in sediment mass accumulation rates, composition, and charcoal flux in the littoral and sublittoral zones of the lake that can be linked to watershed disturbance factors in the deltas were examined. Total organic carbon accumulation rates, in particular, are strongly linked to higher sediment mass accumulation from terrestrial sources, and show striking mid-20th century increases at disturbed watershed deltas that may indicate a connection between increased watershed erosion and increased nearshore productivity. However, changes in sedimentation patterns are not solely correlated with the 20th century period of increasing human population in the basin. Fire activity, as recorded by charcoal accumulation rates, was also elevated during arid intervals of the 13th–early 19th centuries. Some differences between northern and southern sedimentation histories appear to be correlated with different histories of human population in central Tanzania in contrast with northern Tanzania and Burundi.  相似文献   

17.
New intermediate-resolution, normal-incidence seismic reflection profiles from Lake Tanganyika’s central basin capture dramatic evidence of base-level change during two intervals of the late Pleistocene. Four seismically-defined stratigraphic sequences (A–D) tied to radiocarbon-dated sediment cores provide a chronology for fluctuating environmental conditions along the Kalya Platform. Stacked, oblique clinoforms in Sequence C are interpreted as prograding siliciclastic deltas deposited during a major regression that shifted the paleo-lake shore ∼21 km towards the west prior to ∼106 ka. The topset-to-foreset transitions in these deltas suggest lake level was reduced by ∼435 m during the period of deposition. Mounded reflections in the overlying sequence are interpreted as the backstepping remnants of the delta system, deposited during the termination of the lowstand and the onset of transgressive conditions in the basin. The youngest depositional sequence reflects the onset of profundal sedimentation during the lake level highstand. High amplitude reflections and deeply incised channels suggest a short-lived desiccation event that reduced lake level by ∼260 m, interpreted as a product of Last Glacial Maximum (32–14 ka) aridity. Paleobathymetric maps constructed for the two interpreted regressions reveal that despite the positive lake-floor topography created by the Kavala Island Ridge Accommodation Zone, Lake Tanganyika remained a large, mostly connected water body throughout the late Pleistocene. The results of this analysis further imply that Lake Tanganyika is the most drought resistant water body in the East African tropics, and may have acted as a refuge for local and migrating fauna during periods of prolonged aridity.  相似文献   

18.
Prokopenko and Kendall (J Paleolimnol doi:, 2008) criticise the work presented in Fedotov et al. (J Paleolimnol 39:335–348, 2008), and instead propose an alternative interpretation for the grain-size evolution recorded in the KDP-01 core, retrieved from the central part of Lake Khubsugul. Their interpretation is based (i) on a seismic-stratigraphic re-interpretation of sparker seismic profile khub012 (which they copied from Fedotov et al. (EOS Trans 87:246–250, 2006)), (ii) on the presupposition that changes in lake level are the dominant control on facies distribution in Lake Khubsugul, and (iii) on the invalidation of our age-depth model. In this reply to their comment, we demonstrate that they interpreted seismic artefacts and geometries caused by changes in profile orientation as true stratigraphic features and that the lake-level reconstruction they derive from this interpretation is therefore incorrect. We also demonstrate that their grain-size predictions, which they consider to be predominantly driven by changes in lake level, are inconsistent with the measured sulphate concentration, which is a demonstrated proxy of lake level in Lake Khubsugul, and with the measured grain-size record. Finally, we point out that even if there would be a problem with the age-depth model, this problem would not affect the part of the sedimentary sequence discussed in Fedotov et al. (J Paleolimnol 39:335–348, 2008).  相似文献   

19.
The seismically and volcanically active Kivu Rift, in the western branch of the East African Rift System, is a type locale for studies of high‐elevation, humid‐climate rift basins, as well as magmatic basin development. Interpretations of offshore multi‐channel seismic (MCS) reflection data, terrestrial radar topography, lake bathymetry and seismicity data recorded on a temporary array provide new insights into the structure, stratigraphy and evolution of the Kivu rift. The Kivu rift is an asymmetric graben controlled on its west side by a ca. 110 km‐long, N‐S striking border fault. The southern basins of the lake and the upper Rusizi river basin are an accommodation zone effectively linking 1470 m‐high Lake Kivu to 770 m‐high Lake Tanganyika. MCS data in the eastern Kivu lake basin reveal a west‐dipping half graben with at least 1.5 km of sedimentary section; most of the ca. 2 km of extension in this sub‐basin is accommodated by the east‐dipping Iwawa normal fault, which bounds an intrabasinal horst. Lake Kivu experienced at least three periods of near desiccation. The two most recent of these approximately correlate to the African Megadrought and Last Glacial Maximum. There was a rapid lake level transgression of at least 400 m in the early Holocene. The line load of the Virunga volcanic chain enhances the fault‐controlled basin subsidence; simple elastic plate models suggest that the line load of the Virunga volcanic chain depresses the basin by more than 1 km, reduces flank uplift locally and broadens the depocentre. Not only do the voluminous magmatism and degassing to the lake pose a hazard to the riparian population, but our studies demonstrate that magmatism has important implications for short‐term processes such as lake levels, inflow and outlets, as well as long term modification of classic half‐graben basin morphology.  相似文献   

20.
Land-use history, soil erosion, lake trophy and lake-level fluctuations during the last 3000 years were reconstructed through a multidisciplinary palaeolimnological study (pollen, plant macrofossils, diatoms, physical and chemical analysis, magnetic measurements and radiometric methods) of a small eutrophic lake in southern Sweden (Bjäresjösjön, Scania). There are striking responses in diatom, chemical, sediment yield and magnetic records to land-use changes documented by pollen analysis or historical sources, and to lake-level changes identified from sedimentary changes. Our multidisciplinary approach assists interpretation of the processes controlling long-term changes and separation of the effects of different factors (land-use changes, lake-level fluctuations) on individual biostratigraphical records. Climate has controlled processes in the lake indirectly, through lake-level fluctuations, from the Late Bronze Age to the Viking Age (700 BC-AD 800). Since the Viking Age, land-use controlled most of the changes observed in the lake's development and soil erosion processes. Major changes in lake development occurred during the last 200 years, due to a drastic increase in soil erosion and water eutrophication during a period of agricultural modernization.  相似文献   

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