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1.
A small lake, Kaksoislammi (60° 3830N, 24° 4550E), in southern Finland was studied for Cladocera, diatoms and pollen from a core which covers the entire Holocene. The diatom remains indicate a steady development from alkaliphilous taxa towards the dominance of acidophilous forms and lowering pH in the late Holocene. About 1800–1700 BP, dramatic changes took place in the microfauna, mainly the planktonic Cladocera. Bosmina longirostris, the dominant species, suddenly disappeared, and Daphnia, Chydorus sphaericus and Chaoborus increased. The change is simultaneous with a decline of the diatom-inferred pH to 4.8. It is probable that there was a sudden, profound change in predator-prey relationships. The acidity of the lake water probably increased to such a low level that it led to the disappearance of even the most acid-tolerant fish. Consequently invertebrate predators increased and quickly altered the species composition in the lake. There is also pollen evidence of the onset of Iron Age cultivation and grazing almost simultaneously with the faunal change. Therefore, it cannot be ruled out that the sudden lowering of pH was indirectly caused by prehistoric human activity; possibly the acidic peatland surrounding the lake was disturbed.  相似文献   

2.
Palaeolimnological and palynological records from climatically variable central Alberta, Canada, document periods of hypersaline lake conditions indicative of late glacial and early Holocene drought. The sensitivity of palaeolimnological indicators for inferring palaeoclimates is examined by comparing records from two sites at opposite ends of the regional precipitation gradient. Palaeosalinity is identified by the presence of Ruppia pollen, a hypersaline aquatic plant not presently growing in either lake, and diatom assemblages comprising both saline epipelic and planktonic species. Goldeye Lake (52° 27 N; 116° 12 W), in the relatively moist Rocky Mountain Foothills remained saline from its inception before ca 14500 years BP until ca 10400 years BP by which time pioneering forests had replaced tundra vegetation; however, freshwater planktonic diatoms dominated ca 12500 to 11500 years BP. However, dating problems endemic to the Foothills region make this chronology only tentative. Moore Lake (54° 30 N; 110° 30 N), in dry, east-central Alberta contained Ruppia only between ca 9000 and 6000 years BP. Freshwater diatoms dominated until ca 10000 years BP when they were succeeded by taxa characteristic of saline water. The lake remained saline until ca 6000 years BP. The late glacial period of palaeosalinity at Goldeye Lake occurred because the lake was surrounded by Cordilleran and Laurentide glacial ice, and therefore, cut off from moisture sources until the early Holocene by which time significant ice recession had occurred. Factors causing the second period of salinity remain unknown at this time. In contrast, by the early Holocene, Moore Lake was influenced by drought caused by high summer insolation induced by orbital fluctuations. Freshwater conditions were maintained through the Holocene in the Foothills region of west-central Alberta, but occurred consistently only over the last 4000 years in central and east-central Alberta. The warmer, drier climate during the early Holocene did affect lake levels in at least one headwater Foothills lake (Fairfax Lake — 52° 58 N; 116° 34 W). The severity of the drought increased in an easterly direction across the province.This publication is the fourth of a series of papers presented at the Conference on Sedimentary and Palaeolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr. Evans is serving as Guest Editor.  相似文献   

3.
The paleohydrological evolution of several high altitude, saline lakes located in the southernmost Altiplano (El Peinado and San Francisco basins, Catamarca province, NW Argentina) was reconstructed applying sedimentological, geochemical and isotopic techniques. Several playa lakes from the San Francisco basin (26° 56 S; 68° 08 W, 3800-3900 m a.s.l.) show evidence of a recent raise in the watertable that led to modern deposition of carbonate and diatomaceous muds. A 2 m - long core from El Peinado Lake (26° 29 59 S, 68°05 32 W, 3820 m a.s.l.) consists of calcitic crusts (unit 3), overlaid by an alternation of macrophyte-rich and travertine clast- rich, laminated muds (unit 2), and topped by travertine facies (unit 1). This sedimentary sequence illustrates a paleohydrological evolution from a subaerial exposure (unit 3) to a high lake stand (unit 2), and a subsequent smaller decrease in lake level (unit 1). The 13Corganic matterrecord also reflects the lake transgression between units 3 and 2. Although there is a general positive correlation between 18Ocarbonate and salinity proxies (Na, Li and B content), the large data dispersion indicates that other factors besides evaporation effects control chemical and isotopic composition of lakewater. Consequently, the oxygen isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO2 during groundwater discharge can explain the enriched 13C values for primary carbonates precipitated. The carbon budget in these high altitude, saline lakes seems to be controlled by physical rather than biological processes.The Altiplano saline lakes contain records of environmental and climatic change, although accurate 14C dating of these lacustrine sediments is hindered by the scarcity of terrestrial organic material, and the large reservoir effects. Sedimentologic evidence, a 210Pb-based chronology, and a preliminary U/Th chronology indicate a very large reservoir effect in El Peinado, likely as a result of old groundwaters and large contributions of volcanic and geothermal 14C-free CO2 to the lake system. Alternative chronologies are needed to place these paleorecords in a reliable chronological framework. A period of increased water balance in the San Francisco basin ended at about 1660 ± 82 yr B.P. (calendar yr U/Th age), and would correlates with the humid phase between 3000 and 1800 yr B.P detected in other sites of the southern Altiplano. Both, 210Pb and preliminary U/Th dating favor a younger age for the paleohydrological changes in El Peinado. The arid period reflected by subaerial exposure and low lake levels in unit 3 would have ended with a large increase in effective moisture during the late 17th century. The increased lake level during deposition of unit 2 would represent the period between AD1650 - 1900, synchronous to the Little Ice Age. This chronological framework is coherent with other regional records that show an abrupt transition from more arid to more humid conditions in the early 17th century, and a change to modern conditions in the late 19th century. Although there are local differences, the Little Ice Age stands as a significant climatic event in the Andean Altiplano.  相似文献   

4.
A sedimentary blue-green algal record has been investigated through measurement of myxoxanthophyll and oscillaxanthin in two cores taken from deep and shallow sites in Lake Wabamun, Alberta, Canada (Longitudes 114° 26 and 114° 44 W; Latitudes 50° 30 and 50° 35 N). Blue-green algae have been a component of the algal flora of this lake throughout the Holocene period. Myxoxanthophyll and oscillaxanthin maxima occur in early Holocene sediments (ca. 9000 years BP), whereas oscillaxanthin concentrations are high between 7000 and 3800 years BP. High oscillaxanthin levels suggest that a phytoplankton assemblage, which included Oscillatoria spp., existed during this latter period and the lake was more eutrophic than at present. Decreases in the number of planktonic diatoms in the core from the deep site (Seba core) appear to be related to increased eutrophy, increased salinity, and sediment redistribution as well as possible competition with Oscillatoria. That the lake has been less productive during the last 2500 years in supported by the diatom record, the diatom: chrysophyte statospore (stomatocyst) ratio and concentrations of the blue-green algal pigments. In the core from the shallow site (Moonlight Bay) concentrations of blue-green algal pigments are initially high, which along with the diatom assemblage, indicates a younger basal age of the sediments. It is possible that benthic blue-green algae contributed significantly to sedimentary pigment concentrations in the Moonlight Bay core. Major fluctuations in the Osc: Myx ratio, particularly in the Seba core, casts some doubt upon the usefulness of this ratio, and suggests that it is not degradation-independent.  相似文献   

5.
Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenköllesee (Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a.s.l.) and the geographical position (47° 1346N, 11° 0051E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20–30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition (chironomids, pigments) or changes were small and independent of temperature (cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly.  相似文献   

6.
Diatoms preserved in the sediments of Lake C2 (82°50 N, 76°00 W), a high arctic meromictic lake, track changes in the lake's salinity which have occurred as the basin was isolated from the sea. An assemblage dominated by marine taxa, such as Chaetoceros species, Nitzschia cylindrus and Diploneis spp., was replaced by a Cyclotella kuetzingiana var. planetophora dominated freshwater flora. A brief brackish period separates the two assemblages. Relatively little floristic change occurred within either the marine or freshwater periods, indicating rather stable environmental conditions, except that rheophilous diatoms fluctuated in relative abundance during the lacustrine phase, perhaps tracking past changes in discharge from inflowing streams. These may reflect periods of warmer, wetter environmental conditions.This is the eighth in a series of papers published in this issue on the Taconite Lakes Project. These papers were collected by Dr R. S. Bradley.  相似文献   

7.
Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a 18O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a 18O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the 18O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the 18O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell 18O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell 18O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the 13C of shells against predicted values based solely on the 13C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle.  相似文献   

8.
Lake Chen Co, situated at 90°33–39E, 28°53–59N with a lake level of 4420 m asl, is an enclosed lake with 148 km2 of catchment area and 40 km2 of lake surface. It is mainly supplied by glacier melt water either from surface inflow or groundwater. Atmospheric precipitation is mainly concentrated in June–September. A 216-cm long lake sediment core was obtained at a site with 8 m of water depth, 800 m from the lakeshore and 1.5% of the bottom slope in this lake. The sediment core was taken by a piston sampler and was sliced with an interval of 1 cm each. 210Pb dating measurement suggested that the average sedimentary rate was 0.16 cm yr–1, which also was confirmed by 137Cs peak occurrence. Magnetic analyses included low-frequency dependent susceptibility (LF), susceptibility of anhysteretic remanent magnetism (ARM), the saturation isothermal remanent magnetism (SIRM), the isothermal remanent magnetism (IRM) reverse and Soft and Hard contents were performed for the sediment core. Results showed that LF was an index for reflecting the environmental conditions, but was not sufficient to reveal details of magnetic features. This had been proved by measurements of IRM Reverse percentage and Soft and Hard magnetic minerals values. The log(SIRM/LF) had much more information to reveal environmental changes. The ARM/LF might be more sensitive to the local environmental conditions because it was well able to indicate the grain-size variations of magnetic particles. In the past ca. 1400 years, the warm stages were ca. 620–740 AD, 1120–1370 AD and since ca. 1900 AD. After an intensively cold stage during ca. 1550–1690 AD, a cold-humid stage from ca. 1690–1900 AD and a warm-dry stage since ca. 1900 AD followed. Among these stages, the warmest one occurred in ca. 1120–1370 AD and the coldest stage was between ca. 1550 and 1690 AD. This result might be compared with many other research results from lake cores, ice cores and the Chinese historical documents.  相似文献   

9.
With the purpose of studying the vegetation and climatic changes in the last millenia of the Coastal Plain of Rio Grande do Sul, Brazil, a palynological study was made of the sediments of the northern part of Lagoa dos Patos lagoon. Twenty-four samples from a 2.26 m core taken at a depth of 7.70 m (30° 50 50 S and 50° 59 05 W) were collected.The analyses revealed marine transgression at 5170±120 years B.P., giving rise to local vegetation consisting chiefly of xerophytes and halophytes. Vegetation characteristic of a humid environment was present along adjacent portions of the Coastal Plain at this time. Transgression increased at about 4080±110 years B.P., when the greatest level of tidewater was reached. This coincided with the beginning of forest vegetation development along the inner portions of the Coastal Plain. These data suggest that marine transgression may have been a consequence of higher temperatures and more humidity. After 4000 years B.P., regression occurred, resulting in fresh waters characteristics in the northern portion of the lagoon. The development of forest vegetation began at this time.This is the fourth in a series of papers published in this issue on Paleolimnology in Southern South America. Dr. C. A. Fernández served as guest editor for these papers.  相似文献   

10.
Yuanyang Lake (24°35N, 121°24E), located at an altitude of 1,670 m within a nature preserve in northern Taiwan, is an acidic lake. Remains of diatoms and pollen from a 3.72-m sediment core were used to elucidate the relationships between the vegetation of the watershed and the paleolimnological environment. Past pH, saprobity level, and total P of the lake were inferred from the diatom assemblages and were analyzed with respect to changes in the terrestrial vegetation. The inferred pH values fluctuated only slightly, whereas the inferred saprobity level increased markedly towards the sediment surface. In the topmost sediment, a slight drop in the inferred pH was associated with a lowering in the saprobity index. This was interpreted as a possible result of recent anthropogenic acidification and changes in productivity related to changes in acidity. Based on pollen analyses, we conclude that Chamaecyparis persisted over at least the last four thousand years in the watershed. The vegetation in the watershed changed little during this period of time, which is consistent with the constancy of inferred pH values. A positive correlation between the inferred pH and 13C values of the sediments was found.  相似文献   

11.
We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (18Op) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (18Oc). Comparison of the calcite oxygen isotope proxy for 18Op in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This 18Oc/temperature relationship is calculated to be 0.39/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8 anomalous negative shift in calcite 18O values since the 1960s. Possible explanations for this recent 18Oc shift, as it is not related to mean annual air temperature, include changes in 18Op due to synoptic circulation patterns. In particular, the 0.8 negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8 offset in 18Oc data.  相似文献   

12.
Reconnaissance 18O,, D, and 87Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of 87Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the 87Sr values. Based on the mutual variations of 18O and D, the lakes can be divided into three groups. In Group 1, both 18O and D values increase from spring to fall. The 18O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.  相似文献   

13.
Late Quaternary environments have been studied by pollen analysis of lake sediments from the savannas of the Colombian Llanos Orientales at 180 m elevation. The pollen record form Laguna El Pinal (4°08N, 70°23W), dated by 6 AMS radiocarbon dates, starts at 18,290 14C yr B.P. The record from Laguna Carimagua (4°04N, 70°14W), also dated by 6 AMS dates, starts at 8270 14C yr B.P. Both records show a landscape dominated by grassland savanna with only few woody savanna taxa, such as Curatella and Byrsonima, frequent fires, and little occurrence of forest and/or gallery forest along the rivers. The savanna ecosystem at the studied sites was relatively stable during the last 18,000 yrs, but minor changes in floral composition, and in the proportion of savanna/forest, have been recorded. Very little gallery forest and the non permanent lake conditions of Laguna El Pinal reflect the driest period, interpreted to reflect low rainfall rates and long dry seasons during the Last Glacial Maximum until 10,690 14 C yr B.P. During the Late Glacial, Laguna El Pinal was a permanent shallow lake, and changed into a lake with higher water levels during the Holocene, indicating wetter conditions. Expansion of regional gallery forest also started at around 10,690 14C yr B.P. Little vegetational change observed in Laguna Carimagua at 5570 14 C yr B.P., in combination with a simultaneous decrease of savanna observed in previously studied lakes, suggest a change to regional wetter conditions. Thus, the Holocene before 5500 14 C yr B.P. was somewhat drier than the following period until about 3850 14C yr B.P. In both records, Late Holocene lake deposits are incomplete. Shore vegetation of Laguna Carimagua always included a minor contribution of the palms Mauritia and Mauritiella. The marked increase of palms during the last c. 3800 yrs points to increased human impact on the vegetation under the wettest Holocene climate regime.  相似文献   

14.
Late Quaternary Palaeoenvironment of Spring Lake,Alberta, Canada   总被引：1，自引：0，他引：1  
Palaeoenvironmental investigations based upon sediment cores taken from Spring Lake in the Peace River District of Alberta, Canada (latitude 55° 31 N; longitude 119° 35 W) show that the sedimentary record spans the Holocene period. Chemical and diatom changes coincide with regional climatic change since deglaciation (about 11 000 yr. B.P.). Calcite laminations in the basal 3 metres of the cores are evident, and were probably formed through elevated water temperature although photosynthetic removal of CO2 undoubtedly contributed. The disappearance of the laminations, and concurrent decrease in calcite X-ray diffraction peaks ca. 5000 yr. B.P. may have been caused by a change from a partially meromictic to a dimictic lake as the climate changed. Benthic and alkaline diatoms dominate before 5000 yr. B.P., while the planktonic Stephanodiscus hantzschii dominated between 5100 and 4200 yr. B.P. probably owing to increased nutrient levels. From ca. 4200 yr. B.P. until the present, benthic Fragilaria spp. and more circumneutral diatoms were dominant. Palaeoproduction, as measured by chlorophyll derivates, was highest in the early developmental stages of the lake, decreased coinciding with a major disturbance of the sediments and lowered water levels, and then gradually increased again until present. Only during the earlier period of peak production does biogenic calcite formation appear more important than calcite deposition caused by high water temperatures.  相似文献   

15.
The sedimentary chironomid stratigraphy in short-core samples covering approx. the past 150 years was studied in the northernmost basin of Lake Päijänne, southern Finland (62° 11 N, 25° 48 E). The basin has received effluent loading from the wood-processing industry and municipal waste water. Four developmental stages were distinguished based on the changes in chironomid assemblages: 1. Pre-industrial stage (dated by the210Pb method as covering approximately the period 1838–1936), 2. Stage of increasing pollution (approx. 1944–1973), 3. The black decade, or the period of worst pollution (approx. 1973–1983), and 4. Water protection stage (approx. 1983 onwards).During the first stage the basin suffered very little human interference and was oligotrophic, with values of 4.00–4.28 for Wiederholm's Benthic Quality Index (BQI). During the second stage it altered between weak mesotrophy and moderate mesotrophy, and the former profundal assemblages characterized byHeterotrissocladius subpilosus andMicropsectra were replaced bySergentia coracina andChironomus anthracinus gr. The BQI ranged from 2.75 to 3.50. This process led to the extirpation ofH. subpilosus. Eutrophication of the basin reached its climax during the black decade, representing moderate eutrophy with a BQI of 2.15. The profundal assemblages were composed mainly ofChironomus anthracinus gr. andC. plumosus gr. The adoption of more effective waste water purification processes in both the wood-processing industry and the municipal treatment plant markedly reduced effluent loading, especially BOD loading during the fourth stage, and this led to a recovery in the basin. At present it is mesotrophic, with a BQI of 2.90–3.00, and its biological conditions resemble those of the second stage in the 1950's and 1960's.  相似文献   

16.
The volcanogenic lake Laguna Potrok Aike, Santa Cruz, Argentina, reveals an unprecedented continuous high resolution climatic record for the steppe regions of southern Patagonia. With the applied multi-proxy approach rapid climatic changes before the turn of the first millennium were detected followed by medieval droughts which are intersected by moist and/or cold periods of varying durations and intensities. The total inorganic carbon content was identified as a sensitive lake level indicator. This proxy suggests that during the late Middle Ages (ca. AD 1230–1410) the lake level was rather low representing a signal of the Medieval Climate Anomaly in southeastern Patagonia. At the beginning of the Little Ice Age the lake level rose considerably staying on a high level during the whole period. Subsequently, the lake level lowered again in the course of the 20th century.  相似文献   

17.
The study was undertaken as part of a wider palaeoecological investigation of Late glacial and Holocene lake sediments from a site on the exposed Atlantic coast of the Shetland Islands. The diatom data presented here define a sequence of assemblages, commencing at c. 15.8 cal ka BP, which reflects lithological variation in the section, in particular the Late glacial alternation of minerogenic and more organic horizons. Cliff retreat caused drainage of the lake sometime after c. 4.0 cal ka BP. Almost all taxa recorded are small benthic and tychoplanktonic diatoms: Fragilaria (sensu lato), Achnanthes (s.l.) and some Navicula spp. predominate in the Late glacial. Different benthos become dominant in the Holocene, but no plankton developed. Stauroforma was the commonest genus present, and results indicate a relationship between the occurrence of two types, Stauroforma A and Stauroforma B, and the severity of prevailing environmental conditions. The lithology and associated assemblages suggest a sequence including the classic north European Bølling and Allerød' warmer periods, followed by the Loch Lomond Stadial. Subsequently, the temporal diatom succession resembles the pattern described in modern linear transects across the circumpolar treeline in north America and Asia, both in type of assemblage and some dominant species.  相似文献   

18.
The diatom stratigraphy of Holocene sediment cores from two Antarctic lakes (Mondsee, Tiefersee; 62° 10 S/58° 50 W) of King George Island was investigated. The diatom assemblages were dominated by cosmopolitan species. The flora was composed of three main components: 1) taxa of submersed bryophytic habitats, which also occur in the bryophilic diatom flora of southern South America; 2) species of various terrestrial habitats, including some specific subantarctic taxa; 3) species distributed in coastal inland waters influenced by sea-spray.Changes in sediment pattern and diatom species composition seemed to reflect climatic changes. At least three different core sections were distinguished in both lakes: Two sections rich in mosses occur between approx. 7000–4700 B.P. and from 3200 B.P. to present, in general resembling present day conditions. In between is a section of high allochthonous content with generally low diatom concentrations and rising percentages of aerophilic, halophilic and alkaliphilic diatoms. An increase of melt water discharge, possible sea-spray influence, and lake- level fluctuations are discussed.Three new taxa are described: Achnanthes metakryophila nov. spec., Achnanthes renei nov. spec., Navicula australomediocris nov. spec. The taxonomy of selected taxa is discussed.  相似文献   

19.
As part of the international cooperative Baikal Drilling Project, siliceous microfossil assemblage succession was analyzed in two short ( 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55°15N, 109°30E), a second from between the central and southern basins (Core 316, 52°28N, 106°5E). The northern core had higher amounts of biogenic silica (40 g SiO2 per 100 g dry weight sediment) compared to the southern core, and increased deposition in the more recent sediments. Weight percent biogenic silica was lower in the southern core, ranging from approximately 20–30 g SiO2 per 100 g dry weight sediment throughout the entire core. Trends in absolute microfossil abundance mirror those of biogenic silica, with generally greater abundance in the northern core (86–275×106 microfossils g–1 dry sediment) compared to the southern core (94–163×106 microfossils g–1 dry sediment).Cluster analyses using relative abundance of the dominant diatom and chrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be reflecting shifts in nutrient supply and cycling driven by climatic changes. The most recent sediments in the northern basin (Zone 1,c. 1890's–1991 A.D.) were characterized by an increased abundance ofAulacoseira baicalensis andAulacoseira spore. Zone 3 (c. 1630's–1830's A.D.) was dominated by the endemicCyclotella spp. and reduced abundance of theAulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zones 1 and 3 (Zone 2,c. 1830's–1890's A.D.) and below Zone 3 (Zone 4,c. 830's–1430's A.D.) are similar to one another suggesting they represent transitional intervals between warm and cold periods. Southern basin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g.Stephanodiscus binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late 1700's) suggests evidence for anthropogenic induced changes in southern Lake Baikal.  相似文献   

20.
Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (12–9 ka), 18O of ostracodes ranged from –16 to –5 (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative 18O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the 18O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.18O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from –4 at 8 ka to –11 at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (–20 SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The 18O of this inorganic calcite abruptly shifts to higher values (–6) after 4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After 2 ka, the 18O of the Mg-calcite fluctuates between –13 and –7, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The 13C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.This is the sixth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.  相似文献   

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