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1.
Stratigraphic shifts in the oxygen isotopic (18O) and trace element (Mg and Sr) composition of biogenic carbonate from tropical lake sediment cores are often interpreted as a proxy record of the changing relation between evaporation and precipitation (E/P). Holocene 18O and Mg and Sr records from Lakes Salpetén and Petén Itzá, Guatemala were apparently affected by drainage basin vegetation changes that influenced watershed hydrology, thereby confounding paleoclimatic interpretations. Oxygen isotope values and trace element concentrations in the two lowland lakes were greatest between ~ 9000 and 6800 14C-yr BP, suggesting relatively high E/P, but pollen data indicate moist conditions and extensive forest cover in the early Holocene. The discrepancy between pollen- and geochemically-inferred climate conditions may be reconciled if the high early Holocene 18O and trace element values were controlled principally by low surface runoff and groundwater flow to the lake, rather than high E/P. Dense forest cover in the early Holocene would have increased evapotranspiration and soil moisture storage, thereby reducing delivery of meteoric water to the lakes. Carbonate 18O and Mg and Sr decreased between 7200 and 3500 14C-yr BP in Lake Salpetén and between 6800 and 5000 14C-yr BP in Lake Petén Itzá. This decline coincided with palynologically documented forest loss that may have led to increased surface and groundwater flow to the lakes. In Lake Salpetén, minimum 18O values (i.e., high lake levels) occurred between 3500 and 1800 14C-yr BP. Relatively high lake levels were confirmed by 14C-dated aquatic gastropods from subaerial soil profiles ~ 1.0–7.5 m above present lake stage. High lake levels were a consequence of lower E/P and/or greater surface runoff and groundwater inflow caused by human-induced deforestation.  相似文献   

2.
Pollen and diatoms preserved in the radiocarbon dated sediments of Two Frog Lake in the Seymour-Belize Inlet Complex of the central mainland coast of British Columbia document postglacial climate change. Two Frog Lake was isolated from the sea prior to 11,040 ± 50 yr BP (13,030 cal. yr BP) when the climate was cool and dry, and open Pinus contorta woodlands covered the landscape. These woodlands were replaced by a mixed conifer forest ca. 10,200 yr BP (ca. 12,300 cal. yr BP) when the climate became moister. A relatively dry and warm early Holocene climate allowed Pseudotsuga menziesii to migrate northward to this site where it grew with Picea, Tsuga heterophylla and Alnus. The climate became cooler and moister at ca. 8,000 yr BP (ca. 9,200 cal. yr BP), approximately 500–1,000 years prior to sites located south of Two Frog Lake and on the Queen Charlotte Islands, but contemporary with sites on the northern mainland coast of British Columbia and south coastal Alaska. Climate heterogeneity in central coastal British Columbia appears to have occurred on a synoptic scale, suggesting that atmospheric dynamics linked to a variable Aleutian Low pressure system may have had an important influence on early Holocene climate change in the Seymour-Belize Inlet Complex. The transition to cooler and moister conditions facilitated the expansion of Cupressaceae and the establishment of a modern-type coastal temperate rainforest dominated by Cupressaceae and T. heterophylla. This was associated with progressive lake acidification. Diatom changes independent of vegetation change during the late Holocene are correlative with the mid-Neoglacial period, when cooler temperatures altered diatom communities.  相似文献   

3.
Multiple proxy indicators are regularly used to present robust arguments for paleoenvironmental change. We use fossil pollen and diatoms from a 495-cm core taken from Cootes Paradise, a coastal wetland in the western end of Lake Ontario, to investigate ecological changes in the late Holocene. We use consensus analysis to demonstrate that pollen diagrams are best zoned after the data have been split into source area, in this case upland and wetland taxa, because each group responds differently to environmental change. We also use consensus analysis to demonstrate the sensitivity of clustering to the distance measure used. The record begins at 2400 14C years BP, when the wetland was dominated by shallow water emergents and epiphytic diatoms. At 2100 14C years BP, a decline in the epiphytic diatoms Epithemia spp., a rise in Poaceae (cf. Zizania aquatica) pollen and a coincident increase in pollen concentration suggest a water level rise at this time. At about 800 14C years BP, the diatom record shows a pulse in small benthic Fragilaria species; shortly after, a shift occurs in the upland tree pollen spectra involving an increase in Pinus and a decline in Fagus. This shift in upland forest trees has been associated elsewhere with the Little Ice Age and the diatom data present some evidence for impacts of this climatic event on aquatic systems. The diatom and pollen records both indicate large changes associated with the effects of European settlement, including rises in Ambrosia as well as Typha angustifolia pollen. Planktonic diatoms dominate post-settlement assemblages indicating large-scale hydrological and ecological changes, probably associated with the introduction of carp and alterations to the Iroquois bar which separates the wetland from Lake Ontario. Our work at Cootes Paradise is important for multi-proxy coastal wetland studies in general, in addition to the late Holocene environmental history and prehistory of the Great Lakes region.  相似文献   

4.
The oribatid mite assemblages found in late-glacial and early-Holocene sediments in Kråkenes Lake, western Norway, consist of 38 species within 24 genera. In accordance with known present habitat distribution we distinguish 4 true aquatic species, 6 species associated with wetland, 2 with mesic grassland, 12 with dry grassland and heathland, 3 with saxicolous and arboricolous lichens, 7 widely distributed species, and 4 with uncertainly known habitat preferences. The sediments from the pioneer phase (12,300-11,700 14C BP) contain 7 species, among them the 2 typical pioneer species Limnozetes ciliatus and Tectocepheus velatus.The sediments of the warmer Allerød interstadial (11,700-10,900 BP) contain 18 species, the cold Younger Dryas (10,900-10,000 BP) 9 species, and the early Holocene (10,000-9,000 BP) 36 species.The oribatid fossils of the pioneer phase indicate habitats and climate similar to the mid-alpine zone in western Norway today, the Allerod interstadial that of the low- to mid-alpine zone. L. ciliatus is the aquatic pioneer species in the Allerod. In the Younger Dryas the climate was more arctic, 60-70% of the species had presumably disappeared, and no aquatic species were found. In the early Holocene, 4 true aquatic species colonised the lake and showed a high productivity. The terrestrial oribatid fauna in this period contained a high diversity of more boreal species, while the alpine species declined or became extinct.  相似文献   

5.
We analysed a 620-cm-long sediment record from Lake Kotokel located in East Siberia (Russia) for subfossil diatoms, chironomids and pollen to provide a reconstruction of the climate history of the area for the last 12.2 kyr. The subfossil records show differing time lags in their responses to climate change; diatoms and chironomids were more sensitive to climate change than the pollen record. Changes in the biogenic proxies seem related with changes in insolation, the temperature of the North Atlantic and solar activity. The chironomids Chironomus plumosus-type and Einfeldia carbonaria-type and the diatom Aulacoseira granulata were interpreted as markers of warm climate condition. The proxy records were divided into four periods (A, B, C and D) suggesting differing climate in East Siberia during the Holocene. Period D (12.2–9.5 kyr BP) at the beginning of the Holocene, according to chironomid and diatom records, was characterized by warm climate with summer temperatures close to modern. However, forest vegetation had not become fully established yet. During Period C (9.5–5.8 kyr BP), the climate seemed to gradually become colder and wetter from the beginning of Period C to 7 kyr BP. From 7 to 5.8 kyr BP, the climate seemed to remain cold, but aridity increased. Period B (5.8–1.7 kyr BP) was characterised by frequent and sharp alternations between warm and cold conditions. Unstable conditions during this time are also registered in records from Lakes Baikal, Khubsugul and various other shallow lakes of the region. Optimal warm and wet conditions seemed to occur ca. 4 kyr BP. During Period A (the last 1.5 kyr) the diatom and chironomid records show evidence of cold conditions at 1.5–1 kyr BP, but the forest vegetation did not change significantly.  相似文献   

6.
Studies combining sedimentological and biological evidence to reconstruct Holocene climate beyond the major changes, and especially seasonality, are rare in Europe, and are nearly completely absent in Germany. The present study tries to reconstruct changes of seasonality from evidence of annual algal successions within the framework of well-established pollen zonation and 14C-AMS dates from terrestrial plants. Laminated Holocene sediments in Lake Jues (10°20.7′ E, 51°39.3′ N, 241 m a.s.l.), located at the SW margin of the Harz Mountains, central Germany, were studied for sediment characteristics, pollen, diatoms and coccal green algae. An age model is based on 21 calibrated AMS radiocarbon dates from terrestrial plants. The sedimentary record covers the entire Holocene period. Trophic status and circulation/stagnation patterns of the lake were inferred from algal assemblages, the subannual structure of varves and the physico-chemical properties of the sediment. During the Holocene, mixing conditions alternated between di-, oligo- and meromictic depending on length and variability of spring and fall periods, and the stability of winter and summer weather. The trophic state was controlled by nutrient input, circulation patterns and the temperature-dependent rates of organic production and mineralization. Climate shifts, mainly in phase with those recorded from other European regions, are inferred from changing limnological conditions and terrestrial vegetation. Significant changes occurred at 11,600 cal. yr. BP (Preboreal warming), between 10,600 and 10,100 cal. yr. BP (Boreal cooling), and between 8,400 and 4,550 cal. yr. BP (warm and dry interval of the Atlantic). Since 4,550 cal. yr. BP the climate became gradually cooler, wetter and more oceanic. This trend was interrupted by warmer and dryer phases between 3,440 and 2,850 cal. yr. BP and, likely, between 2,500 and 2,250 cal. yr. BP.  相似文献   

7.
Pollen and plant macrofossils were analysed at Sägistalsee (1935 m asl), a small lake near timber-line in the Swiss Northern Alps. Open forests with Pinus cembra and Abies alba covered the catchment during the early Holocene (9000–6300 cal. BP), suggesting subcontinental climate conditions. After the expansion of Picea abies between 6300 and 6000 cal. BP the subalpine forest became denser and the tree-line reached its maximum elevation at around 2260 m asl. Charcoal fragments in the macrofossil record indicate the beginning of Late-Neolithic human impact at ca. 4400 cal. BP, followed by a extensive deforestation and lowering of the forest-limit in the catchment of Sägistalsee at 3700 cal. BP (Bronze Age). Continuous human activity, combined with a more oceanic climate during the later Holocene, led to the local extinction of Pinus cembra and Abies alba and favoured the mass expansion of Picea and Alnus viridis in the subalpine area of the Northern Alps. The periods before 6300 and after 3700 cal. BP are characterised by high erosion activity in the lake's catchment, whereas during the phase of dense Picea-Pinus cembra-Abies forests (6300–3700 cal. BP) soils were stable and sediment-accumulation rates in the lake were low. Due to decreasing land-use at higher altitudes during the Roman occupation and the Migration period, forests spread beween ca. 2000 and 1500 cal. BP, before human impact increased again in the early Middle Ages. Recent reforestation due to land-use changes in the 20th century is recorded in the top sediments. Pollen-inferred July temperature and annual precipitation suggest a trend to cooler and more oceanic climate starting at about 5500 cal. BP.  相似文献   

8.
This study of fossils (pollen, plant macrofossils, stomata and fish) and sediments (lithostratigraphy and geochemistry) from the Wendel site in North Dakota, USA, emphasizes the importance of considering ground-water hydrology when deciphering paleoclimate signals from lakes in postglacial landscapes. The Wendel site was a paleolake from about 11,500 14C yr BP to 11,100 14C yr BP. Afterwards, the lake-level lowered until it became a prairie marsh by 9,300 14C yr BP and finally, at 8,500 14C yr BP, an ephemeral wetland as it is today. Meanwhile, the vegetation changed from a white spruce parkland (11,500 to 10,500 14C yr BP) to deciduous parkland, followed by grassland at 9,300 14C yr BP. The pattern and timing of these aquatic and terrestrial changes are similar to coeval kettle lake records from adjacent uplands, providing a regional aridity signal. However, two local sources of ground water were identified from the fossil and geochemical data, which mediated atmospheric inputs to the Wendel basin. First, the paleolake received water from the melting of stagnant ice buried under local till for about 900 years after glacier recession. Later, Holocene droughts probably caused the lower-elevation Wendel site to capture the ground water of up-gradient lakes.  相似文献   

9.
Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-1 throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake's history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.  相似文献   

10.
The paper focuses on early Holocene tree growth and alpine tree-limits in the northernmost Swedish Scandes (Lapland). Megafossil wood remnants in peats and lakes were searched for over a large area at elevations high above the modern tree-limits. Wood of Pinus sylvestris, Betula pubescens spp. tortuosa and Alnus incana was discovered near the shore of a small lake (999 m a.s.l.) c. 500 m higher than today's tree-limit of Pinus sylvestris in this region. Radiocarbon dating yielded values of unprecedented age, c. 8500–8100 14C years BP for all three species and in addition 5400–4500 14C years BP for Betula and Alnus. The highest position of the Pinus tree-limit occurred during the early Holocene, which compares well with the situation reconstructed by megafossils in the southern Scandes. It now appears that the long-term tree-limit and climate histories are broadly the same in entire mid- and northern Fennoscandia. Corrected for glacio-isostatic land uplift, the tree-limit record suggests that the summers were c. 2.4°C warmer than today at 8500 14C years BP . A dry continental climate with substantial seasonal contrasts is likely to have prevailed during this period, which restricted the occurrence of glaciers and glacier activity. Most circumstances point to the overriding importance of the Milankovitch orbital theory for pacing or forcing the long-term postglacial climate change. The results are inconsistent with most inferences based on pollen, certain macrofossil records and general circulation simulations. These proxy environmental histories have frequently advocated a mid-Holocene thermal optimum and an oceanic and humid climate in northern and western Fennoscandia during the early Holocene. The uncovered discrepancy between the outcome of the objective and factual megafossil method and more subjective/inferential microfossil methods should be important for Quaternary plant ecology in general, stressing the usefulness of megafossil studies.  相似文献   

11.
Pollen analysis of 3.25 m of late glacial and Holocene sediments gives a mid‐altitude (600 m) record of vegetation development after the last or Margaret Glaciation. Alpine herbfield, coniferous heath and Nothofagus gunnli scrub developed on the moraines until 11,400 BP. Wet montane forest and heath then developed with Phyllocladus aspleniifolius, Nothofagus cunninghamii and Eucalyptus until c. 10,000 BP. After 10,000 BP a mosaic of N. cunninghamii rainforest, Myrtaceae and Proteaceae scrub and Sprengelia incarnata heath occurred. The development of the vegetation from alpine communities to temperate rainforest, which is near its limit at 600 m, occurred under the influence of improving climatic conditions with rapid upslope migration or local expansion of taxa during the late glacial. Temperatures were warm enough for the development of rainforest at 600 m by 10,000 BP, if not earlier. The development of a mosaic of rainforest, scrub and heath vegetation rather than extensive rainforest after 10,000 BP reflects the influence of poor soils, bad drainage and fires. Comparison with similar pollen diagrams from western Tasmania suggests that the development of pollen/vegetation associations was time transgressive with altitude during the late glacial when climatic influences and migration rates were important, and that the mosaic of vegetation communities became more complex during the Holocene because of adjustment to or control by local ecological factors.  相似文献   

12.
Macrofossil analyses were carried out on the late-glacial and early-Holocene sediments of the radiocarbon-dated master core at Kråkenes Lake, western Norway, to investigate the aquatic vegetation changes. Ranunculus sect. Batrachium and Nitella were the earliest pioneers after deglaciation ca. 12,300 14C yr BP. The Allerød vegetational succession was very slow during ca. 1000 14C yrs in a cool climate and conditions that were similar to those above tree-line in Norway today. The rapid cooling at the start of the Younger Dryas stadial caused extensive disturbance, and with the development of an active cirque glacier in the catchment, plants and animals were almost exterminated from the lake by inflow of permanently cold and turbid water. Rising temperatures caused the glacier to melt at the end of the Younger Dryas. The biotic response to the rapid warming was immediate, with pioneer Ranunculus sect. Batrachium and Nitella expanding within 1-3 decades, closely followed by other elodeids. The lake witnessed a remarkable isoetid succession, with phases dominated by Limosella aquatica, Subularia aquatica, Elatine hydropiper, Isoetes echinospora, and, later, I. lacustris. About 800 yrs into the Holocene most of the macrophytes declined. The short-lived isoetids became extinct, but other taxa probably survived vegetatively. The reasons for this decline are unknown, but are probably related to nutrient depletion in combination with other factors. About 550 yrs later, I. lacustris and Nymphaea colonised, and a stable flora and vegetation developed. This study illustrates the large and rapid changes that occurred over the first 1400 yrs of the Holocene in the macrophyte flora and vegetation in Kråkenes Lake before stability was attained, pointing to the value of a palaeoecological study in tracing aquatic successions over time, and highlighting our lack of knowledge of the underlying ecological factors responsible for such rapid and marked changes.  相似文献   

13.
The late Quaternary diatom record from subalpine Crowfoot Lake, Banff National Park, Alberta (lat. 51° 61N; long. 116° 31W) has been analyzed. Results are related to independently inferred vegetation and climate changes. No diatoms were found in the basal diamict that predates 11330 14C yr BP. Very few occur until ca. 10 10014 C yr BP probably due to the short time between de-glaciation and an advance of the Crowfoot Glacier during the Younger Dryas Chron. Initial pioneering species were characteristic of alkaline water and calcareous organic sediments. They appeared as sediments became organic and laminated suggesting increasing water clarity, and as the Pinus-dominated forest expanded and the climate warmed. After ca. 9060 14C yr BP diatom numbers increased rapidly, reaching a maximum prior to the Mazama tephra; they remained high until ca. 3500 14C yr BP. The period between ca. 9060 and 3500 14C yr saw timberline elevation increase and the dominance of xerophytic taxa. These are consistent with early to mid-Holocene warmth and aridity. Diatom productivity reflects the warm climate and presumably longer ice-free season, a stable catchment and transparent water. Decreases in diatom productivity coincide with a vegetation change with reduction of xerophytic taxa and the appearance of a closed Picea-Abies forest, hence a cooler, wetter climate at ca. 4100 to 3500 14C yr BP. The diatom numbers during the Neoglacial were of the same magnitude as prior to ca. 9060 14C yr BP. Small species of Fragilaria (overwhelmingly Fragilaria construens v. venter) became extremely dominant during the period of high diatom productivity, and remained so thereafter. Recovery of the lake appears to have been rapid after deposition of the Mazama tephra. Maximum occurrence of Cyclotella radiosa occurred ca. 8000 14C yr BP during the warm early Holocene and may reflect this warmer climate, a longer ice-free season than presently, perhaps less turbid water, or it may reflect a subtly higher nutrient status of the lake water. The diatom record of Crowfoot Lake has responded with sensitivity, particularly in terms of productivity, to the Holocene vegetation and climate changes.  相似文献   

14.
A 61-cm core was obtained from 4 m below the water table in Cenote Aktun Ha, on the Yucatan Peninsula, Mexico. The cenote is 8.6 km from the Caribbean coast and its formation and evolution have been largely affected by sea-level change. The base of the core dates to 6,940–6,740 cal year BP and overlying sediments were deposited rapidly over the subsequent ~200 years. The pollen record shows that the cenote evolved from a marsh dominated by red mangrove (Rhizophora mangle) and fern (Polypodiaceae) to an open-water system. These vegetation changes were controlled by water level and salinity and are thus useful indicators of past sea level. At the base, the δ13Corg isotopic ratios reveal the influence of terrestrial vegetation (−29‰ VPDB), but shift to more negative values up-core (−33‰), indicating an influence from particulate matter in the flooded cenote pool. Although microfossil populations were nearly absent through most of the core, the microfossil assemblage in the upper 6 cm of the core is dominated by the juvenile foraminifer Ammonia tepida and the thecamoebian genus Centropyxis. These populations indicate open-water conditions in the cenote and a major environmental shift around 6,600 cal year BP, which is related to sea-level rise in the Caribbean basin. These data fit well with previously established sea-level curves for the Caribbean Sea. Our reconstruction of the environmental history of Cenote Aktun Ha helps elucidate the floral and hydrological history of the region, and highlights the utility of cenote sediments for studying the Holocene sea-level history of the Caribbean Sea.  相似文献   

15.
A 341 cm long sediment sequence was recovered from the unofficially named Raffles Sø on Raffles Ø, outer Scoresby Sund region, East Greenland. The sediment sequence consists in the upper part (0–230 cm) of a stratified gyttja enriched in organic carbon and biogenic silica whereas the lower core part (235–341 cm) is composed of terrigenous, consolidated glacio-limnic sediments. 14C-AMS measurements indicate that the sediment sequence represents the entire Holocene lake history from 10,030 calibrated radiocarbon years.The geochemical parameters (opal, total organic carbon (TOC), total nitrogen (TN)) and the total diatom concentration show similar developments during the Holocene, and reflect changes in biological production and nutrient input into the lake. These records clearly reveal a broad Holocene TOC-opal-maximum interval between 5200 and 1800 cal. yrs BP.The diatom flora consisted of 66 taxa representing 20 genera but only seven taxa were abundant and, sometimes, these were monospecifically dominant during the Holocene. In the sediment core from Raffles Sø four successive stratigraphical zones can be distinguished. Accumulation of diatom valves began at 9900 cal. yrs BP with a Stephanodiscus minutulus (Kütz.) Cleve and Möller dominated assemblage (stratigraphic zone 1) followed by a diatom flora dominated by Cyclotella pseudostelligera Hustedt and, less frequently, by Diatoma tenuis Agardh (9400 until 5900 cal. yrs BP, zone 2). Cyclotella sp. A, a taxon which belongs to the Cyclotella rossii-comensis-tripartita-complex, was the dominant floral element between 5200 and 1800 cal. yrs BP (zone 3). From 1800 cal. yrs BP, the periphytic taxa Fragilaria capucina var. gracilis (Østr.) Hustedt and F. capucina var. rumpens (Kütz.) Lange-Bertalot attained highest relative abundances, also almost monospecifically (zone 4).The distribution and composition of the diatom assemblages in the sediment record from Raffles Sø probably reflect past variations in the extent of the lake-ice cover during the growing season. More or less ice-free conditions during summer may have prevailed during the early Holocene until ca. 1800 cal. yrs BP, which allowed growth of planktonic diatoms (Cyclotella taxa) in the pelagic lake region. From 1800 cal. yrs BP, colder conditions lead to a perennial lake-ice cover with a small ice-free moat in summer which favored the growth of periphytic, littoral species (Fragilaria capucina varieties).  相似文献   

16.
A palynological investigation was conducted on two cores with Holocene sediments collected from the northeastern littoral part of the border Lake Doirani in northern Greece. The radiocarbon dates indicated that the analyzed sediments accumulated during the last 5000 yrs. The pollen-stratigraphic record revealed the environmental changes in the catchment area, starting from a natural undisturbed landscape to one modified by increasing anthropogenic influences. The tree vegetation dominated by Quercus woods in the lowlands and byPinus, Abies, and Fagus at higher altitudes, lasted for the period 2900 - 830 cal. B.C. Subsequently it was replaced by xerothermic herb and tree vegetation as a result of intensive human activity - and farming and stock-breeding. The accumulation of sediments with more sand and gravel in historical time was the result of increased erosion.  相似文献   

17.
This study presents a detailed analysis of geochemical and biotic proxies in a lake sediment profile to assess the effects of local and regional environmental drivers on the Holocene development of Lake Loitsana, situated in the northern boreal forest of NE Finland. Multi-proxy studies, in particular those that include a detailed plant macrofossil record, from the part of the northern boreal zone of Fennoscandia which has not been affected by treeline fluctuations, are scarce and few of these records date back to the earliest part of the Holocene. A 9-m sediment sequence of gyttja overlying silts representing the last c. 10,700 cal year, allowed for a high-resolution study with emphasis on the early to mid-Holocene lake history. The lacustrine sediments were studied using lithology, loss-on-ignition and C/N ratios, micro- and macro-fossils of aquatic and wetland taxa, diatoms, chironomids and accelerator mass spectrometry 14C dating on terrestrial plant macrofossils. Our study shows that the local development at Loitsana was complex and included a distinct glacial lake phase and subsequent drainage, a history of fluvial input affected by nearby wetland expansion, and lake infilling in an eventual esker-fed shallow lake. Enhanced trophic conditions, due to morphometric eutrophication, are recorded as Glacial Lake Sokli drained and open water conditions became restricted to a relatively small Lake Loitsana depression. pH appears to have been stable throughout the Holocene with a well-buffered lake due to the local carbonatite bedrock (Sokli Carbonatite Massif). The fossil assemblage changes are best explained by a complex mixture of drivers, including water-body conditions (i.e. depth, turbidity and turbulence), rate of sediment input, and the general infilling of the lake, highlighting the need to carefully evaluate the possible influence of such local factors as palaeoenvironmental conditions are reconstructed based on aquatic proxies.  相似文献   

18.
The post-glacial history of the Great Lakes has involved several changes in lake levels throughout the latest Pleistocene and Holocene, resulting from the changing position of the retreating Laurentide ice sheet, outlet incision and isostatic rebound. The final lowering of lake levels occurred at approximately 7600 14C yr BP, after which lake levels began to rise again to the Nipissing highstand at approximately 4700 14C yr BP. During this time of rising lake levels, black bands of iron sulfide were being formed in the sediments of all five of the Great Lakes. These bands signify suboxic to anoxic conditions, at least within the sediments and possibly at the sediment-water interface, during the middle Holocene warm interval. During this interval, the climate was warmer and drier than present, possibly resulting in the occasional absence of seasonal turnover in the lakes. We examined a series of piston cores from northern Lakes Michigan and Huron and found that the black bands are correlatable among cores taken from within the same basin. The observation that the banding can be correlated suggests a basin-wide cause, near-bottom or sub-bottom anoxia in the northern Michigan and northern Huron sediments during the mid-Holocene warm period. The sedimentary and geochemical processes in the Great Lakes during the middle Holocene warm interval are good indicators of possible future scenarios for the lakes as a result of global warming, as 21st-century temperatures are predicted to reach similar levels due to increased concentrations of greenhouse gases.  相似文献   

19.
Pollen records from 21 sites were used to reconstruct Holocene hydrological changes in northern Fennoscandia. The inferred dominance of maritime-type Betula-forests from c. 9600 BP onwards indicate an oceanic climate in the region during the early Holocene, with high amounts of precipitation and low seasonal contrasts. This interpretation is supported by oxygen isotope records obtained on lacustrine carbonates from Abisko, northern Sweden. Enrichment of 18O during the early Holocene suggests enhanced zonal circulation and a pronounced influence of Atlantic air-masses in northern Sweden. The pattern of Pinus expansion in the area was time-transgressive, ranging from 9200-8000 BP in the extreme north-east to 7900-5500 BP in the western and south-western parts of the region. The replacement of Betula-forests by Pinus-forests indicates a climatic change towards drier summers and increased seasonality. At Abisko, a close correlation between regional Pinus expansion and depletion of 18O in lacustrine carbonates suggests that the expansion of Pinus was triggered by a gradual attenuation of the influence of westerlies. Thus, in large-scale palaeohydrological terms, the early expansion of Pinus in the north-easternmost parts of Fennoscandia as compared to the successively later expansion in the west and south-west may reflect a gradually decreasing influence of Atlantic air-masses in northern Fennoscandia during the early and mid Holocene.  相似文献   

20.
A combination of pollen and macrofossil analyses from six lakes at altitudes between 370 and 999 m above sea level (a.s.l.) in the Torneträsk area reflect the Holocene vegetation history. The main field study area has been the Abisko valley at altitudes around 400 m a.s.l. The largest lake, Vuolep Njakajaure has annually laminated (varved) sediments. The chronology and sedimentation rates in the pollen-influx calculations are based on varve yrs in this lake and on radiocarbon dated terrestrial plant macrofossils in the other lakes. A strong increase of mountain birch (Betula pubescens ssp. tortuosa) during the early Holocene with a tree-line c. 300 m above the present, indicates that the summer temperature was c. 1.5 °C higher than today, assuming that the land uplift has been 100 m since then. Scattered stands of pine (Pinus sylvestris) may have been growing in the area immediately after the deglaciation but a forest consisting of pine and mountain birch expanded first at low elevations and reached the eastern parts of the Torneträsk area at c. 8300 cal BP and the western parts at c. 7600 cal BP. The highest pine-birch forest limit was not reached until 6300 cal BP (110 m above present pine limit). Warm and dry conditions during the pine forest maximum led to lowering of the water level documented in Lake Badsjön in the Abisko valley about 1-1.5 m lower than today. Pine and mountain birch were growing at the maximum altitude until c. 4500 cal BP. Assuming that land uplift has been in the range of 20-40 m since the mid-Holocene, this implies that the temperature was then c. 1.5-2 °C higher than today. Rising lake-levels and lowering limits of pine and mountain birch since c. 4500 cal BP indicate a more humid and cool climate during the late Holocene.  相似文献   

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