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1.
A 36 Ky record of iceberg rafting and sedimentation from north-west Iceland   总被引：2，自引：0，他引：2  
Evidence from north-west Iceland's shelf and fjords is used to develop a scenario for environmental change during the last 36 cal Ky. The retreat history of the Iceland Ice Cap during the last deglaciation is delineated through lithofacies studies, carbon analyses and magnetic susceptibility, and studies of ice-rafted debris (IRD) in sediment cores. Sedimentological data from lake Efstadalsvatn, Vestfirdir peninsula, trace the glacier retreat on land. In two of the high resolution shelf cores we detect near continuous IRD accumulation from 36 to 11 cal Kya. However, IRD is absent in the cores from ca. 22 to 19 cal Kya, possibly indicating more extensive landfast sea ice conditions. All cores show intensified IRD during the Younger Dryas chronozone; the fjord cores show a continuous IRD record until 10 cal Kya. Magnetic susceptibility and carbon analyses from Efstadalsvatn reveal the disappearance of local ice in the basin just before 10.5 cal Kya. No IRD was detected in the sediment cores during 10 to Ø4 cal Kya. Some indication of cooling occurs between 4 and 3 cal Kya, with a fresh input of IRD in fjord cores after 1 cal Kya.  相似文献   

2.
The last British Ice Sheet: growth, maximum extent and deglaciation   总被引：2，自引：0，他引：2  
The growth, maximum lateral extent and deglaciation of the last British Ice Sheet (BIS) has been reconstructed using sediment, faunal and stable isotope methods from a sedimentary record recovered from the Barra Fan, north-west Scotland. During a phase of ice sheet expansion postdating the early "warmth" of Marine Isotope Stage 3 (MIS 3), ice rafting events, operating with a cyclicity of approximately 1500 years, are interspersed between warm, carbonate-rich interstadials operating with a strong Dansgaard-Oeschger (D-O) cyclicity. The data suggest that the BIS expanded westwards to the outer continental shelf break shortly after 30 Ky BP (before present) and remained there until about 15 Ky BP. Within MIS 2, as the ice sheet grew to its maximum extent, the pronounced periodicities which characterize MIS 3 are lost from the record. The exact timing of the Last Glacial Maximum is difficult to define in this record; but maxima in Neogloboquadrina pachyderma (sinistral) Ø18O are observed between 21-17 Ky BP. A massive discharge of ice-rafted detritus, coincident with Heinrich event 1, is observed at about 16 Ky BP. Deglaciation of the margin is complete by about 15 Ky BP and surface waters warm rapidly after this date.  相似文献   

3.
ABSTRACT. Two well dated Holocene sediment records bordering the Denmark Strait region have been used to reconstruct past climate variability. The content of biogenic silica, classic and organic material and moss in a lacustrine record from Lake N14 has been used to infer past variability in precipitation and temperature in southern Greenland. Sedimentologic and petrologic composition of sand in a shelf sediment record from the Djúpáll trough is used to infer past variability in the northwestern storm activity on northwestern Iceland, which probably also affected the inflow of polar waters from the East Greenland Current. Our evaluation of these records with a number of previous studies from the region documents Holocene climatic optimum conditions peaking between 8000 and 6500 calendar years before present (cal yr BP). Mid-Holocene climate deterioration set in around 5000 cal yr BP followed by a further marked setback around 3500 cal yr BP. A stacking of climate variability on a centennial timescale from previous studies in the area shows a fairly good correspondence to the timing of marked cold and warm events as evidenced from the Lake N14 and the Djúpáll trough records. Cooler periods are explained as the response to marked incursions of ice-laden polar water from the Arctic Ocean to the Denmark Strait region. Cool northerly and northwesterly winds along the East Greenland coast in relation to frequent strong atmospheric low pressure in the Barents Sea, coupled with strong high pressure over Greenland, would have favoured southward export of polar waters. A comparison with the proxy records of nuclide production (14C and 10Be) suggests that solar activity may have had some influence on the atmospheric pressure distribution in the Denmark Strait region.  相似文献   

4.
A high resolution study of early Holocene climate and palaeoceanography has been performed on two combined sediment cores from Malangenfjord, northern Norway. The fjord provides a regional oceanographic climatic signal reflecting changes in the North Atlantic heat flux at this latitude because of its deep sill and the relatively narrow adjoining continental shelf. Fauna and stable oxygen and carbon isotopes indicate cool, meltwater-depleted water masses in the fjord from 12000 to 11400 cal. yr BP followed by a warming between 11400-10300 cal. yr BP. The climatic variability can be explained partly by freshwater forcing hampering the North Atlantic heat conveyor, and partly by changing solar irradiance. A major cooling event at 11500-11400 cal. yr BP, followed by a rapid warming, is correlated to the Preboreal Oscillation, a widespread signal in the North Atlantic region which is probably linked to the increased meltwater flux to the northern North Atlantic at this time. Brief and small-scale cooling events between 10 300 and 10100 cal. yr BP, correlated to the onset of increased 10Be flux in the Greenland ice cores, suggest a response to solar forcing.  相似文献   

5.
Late‐middle Miocene to Pliocene siliciclastics in the Northern Carnarvon Basin, Northwest Shelf of Australia, are interpreted as having been deposited by deltas. Some delta lobes deposited sediments near and at the shelf break (shelf‐edge deltas), whereas other lobes did not reach the coeval shelf break before retreating landward or being abandoned. Shelf‐margin mapview morphology changes from linear to convex‐outward in the northern part of the study area where shelf‐edge deltas were focused. Location and character of shelf‐edge deltas also had significant impact on along‐strike variability of margin progradation and shelf‐edge trajectory. Total late‐middle and late Miocene margin progradation is ca. 13 km in the south, where there were no shelf‐edge deltas, vs. ca. 34 km in the north where shelf‐edge deltas were concentrated. In the central area, the deltas were arrested and accumulated a few kilometres landward of the shelf break, resulting in an aggradational shelf‐edge trajectory, in contrast to the more progradational trajectory farther north. This illustrates a potential limitation of shelf‐edge trajectory analysis: only where shelf‐edge deltas occur, there is sufficient sediment available for the shelf‐edge trajectory to record relative sea‐level fluctuations reliably. Small‐scale (ca. 400 m wide) incisions were already conspicuous on the coeval slope even before deltas reached the shelf break. However, slope gullies immediately downdip from active shelf‐edge deltas display greater erosion of underlying strata and are wider and deeper (>1 km wide, ca. 100 m deep) than coeval incisions that are laterally offset from the deltaic depocenter (ca. 0.7 km wide, ca. 25 m deep). We interpret this change in slope‐gully dimensions as the result of greater erosion by sediment gravity flows sourced from the immediately adjacent shelf‐edge deltas. Similarly, gullies also incised further (up to 6 km) into the outer shelf in the region of active shelf‐edge deltas.  相似文献   

6.
The deglaciation history of Balsfjord, northern Norway, and post-glacial mass movement events were investigated. Radiocarbon dates indicate that the Balsfjord glacier retreated from the Tromsø–Lyngen moraines about 10.4 14C Ky BP. Between ca. 10.3 14C Ky BP and 9.9 14C Ky BP, deposition of a distinct end moraine–the Skjevelnes moraine–in the central part of Balsfjord occurred. The transition from glacimarine to open marine sedimentary environment took place before 9.6 14C Ky BP. Between ca. 9.5 14C Ky BP and 8.4 14C Ky BP, at least one local and three regional mass movement events occurred. After this period, no gravity flow activity is preserved in the cores. The high frequency of mass movements in the early post-glacial period is presumed to be due to fast sea level changes and/or tectonic activity induced by rapid isostatic uplift.  相似文献   

7.
The extent of the Barents-Kara Ice Sheet during the eastern Last Glacial Maximum (LGM) is not yet fully known. A detailed echo-sounding survey performed during the Boris Petrov Expedition 2001 permitted the detailed mapping of part of it. Based on the profiling results, a southern connection between the LGM Barents-Kara Ice Sheet and a local ice sheet on Taymyr Peninsula appears to be unlikely. Based on sediment core data and profiling results, most of the terrigenous river-derived material accumulated in the estuaries during late Holocene times, whereas during early Holocene times of lowered sea level major amounts were transported further offshore and accumulated on the shelf. During the post-glacial sea level rise, the main depocentre migrated southward, reaching its present position no earlier than about 6 cal. Ky BP (or 5.2 Kya). Future studies of accelerator mass spectrometry (AMS) 14C-dated sediment cores will allow a detailed reconstruction of the variability of fluvial sediment discharge and the history of glaciation in the Kara Sea during late Quaternary times.  相似文献   

8.
We studied mineral magnetic properties of a 6-m-long, late Pleistocene through Holocene sediment sequence from Lake Aibi in Dzungaria (Zunggary, Junggar), northern Xinjiang, China. Results were used to infer environmental changes and are compared with previously studied cores from Lake Manas. Both water bodies occupy the deepest parts of the Dzungarian Basin and are remnants of large Holocene lakes. During the Late Pleistocene, the magnetic mineralogy in both lakes was dominated by detrital, iron oxide minerals. Oxic conditions, which dominated during sedimentation and early diagenesis, persisted over the Pleistocene–Holocene transition. Later, during the middle Holocene, lake bottom conditions enabled authigenic formation of iron sulphide minerals such as pyrite (FeS2) in Lake Aibi, and pyrite and greigite (Fe3S4) in Lake Manas. This iron sulphide mineralogy suggests increased biological activity in stagnant, anoxic bottom waters. Anoxic bottom conditions started about 9.8 cal kyr BP in Lake Manas and at about 7.2 cal kyr BP in Lake Aibi. A short dry event recorded in Lake Manas between 6.8 and 5.2 cal kyr BP is not clearly observed in Lake Aibi. In the late Holocene, i.e. the last 2.8 cal kyr, sediments of both lakes are again characterised by iron oxides, suggesting well-mixed, shallow water bodies. For this recent period, it seems that the detrital material in the two lakes had a common origin. Magnetic properties of sediments in Lakes Aibi and Manas show broadly similar environmental evolution during the late Pleistocene and Holocene. Nevertheless, despite the close proximity of the two lakes (~200 km) in the same basin, they display some different magnetic properties and record environmental changes at different times.  相似文献   

9.
A multi-proxy analysis of two sediment cores from Rantin Lake are used to reconstruct past lake-level changes and to make inferences about millennial-scale variations in precipitation/evaporation (P/E) balance in the southern Yukon, Canada between 10,900 and 3,100?cal?yr BP. Analyses of calcium carbonate and organic matter concentration, magnetic susceptibility, titanium content, dry bulk density, and macrofossils are used to reconstruct water-level changes. The development of sand layers and deformed sediments at the deep-water core site (i.e. Core A-06) prior to ~10,900?cal?yr BP suggest that lake level was lower at this time. Fine-grained organic sediment deposited from 10,600 to 9,500?cal?yr BP indicates a rise in lake level. The formation of an unconformity at the shallow cores site (Core C-06) and the deposition of shallow-water calcium carbonate-rich facies at the Core A-06 site between ~9,500 and ~8,500?cal?yr BP suggest lower lake levels at this time. Shallow-water facies gradually transition into a sand layer that likely represents shoreline reworking during an extreme lowstand that occurred at ~8,400?cal?yr BP. Following this low water level, fine-grained organic-rich sediment formed by ~8,200?cal?yr BP, suggesting deeper water conditions at core site A-06. Calcium carbonate concentrations are relatively low in sediment deposited from ~6,300 to 3,100?cal?yr BP in Core A-06, indicating that lake level was comparatively higher during the middle and late Holocene. In general, results from this study suggest that the early Holocene was characterized by high P/E from ~10,500 to 9,500?cal?yr BP, low P/E from ~9,500 to 8,400?cal?yr BP, and return to higher P/E from ~8,200 to 3,100?cal?yr BP.  相似文献   

10.
Four sediment cores and twenty-five 14C ages from Ritterbush Pond in northern Vermont provide a detailed and continuous temporal record of Holocene lake and watershed dynamics. Using visual logs, carbon content, magnetic susceptibility, stable isotope signatures, and X-radiography, all measured at 1-cm scale, we identify and date discrete layers of terrestrially-derived sediment in the organic-rich, lacustrine gyttja. These inorganic layers range in thickness from <1 mm to >10 cm and range in grain size and sorting from homogeneous silt to graded sand. AMS radiocarbon ages both from macrofossils within the thickest layers, and gyttja bracketing these layers, provide the basis for correlation among the cores, the dating of 52 basin-wide sedimentation events, and the development of a detailed sedimentation chronology for the Holocene.Physical, chemical, and isotopic analyses suggest the inorganic layers are terrestrially derived and result from hydrologic events large enough to erode and transport sediment from the watershed into the pond. The temporal and spatial distribution of the inorganic layers suggests changing basin-wide sedimentation and thus erosion dynamics since deglaciation over 12,000 years ago. Specifically, for intervals lasting 400 to 1000 years, during the early (>8600 cal yBP), middle (6400 to 6800 cal yBP) and late Holocene (1800 to 2600 cal yBP), the Ritterbush Pond watershed eroded more rapidly than at other times and terrestrially derived material poured into the pond. Analysis of Ritterbush Pond sediments demonstrates the potential for North American lakes to preserve a record of drainage basin dynamics.  相似文献   

11.
The tephrochronology of the last 3000 years has been investigated in soil sections in north Iceland and in a marine sediment core from the north Icelandic shelf, 50 km offshore. Tephra markers, identified with major element geochemical analysis of volcanic glass shards, serve to correlate the marine and terrestrial records. Hekla 3, the largest Holocene tephra marker from the volcano Hekla, in south Iceland, dated to 2980 years BP, is used as the basal unit in the tephra stratigraphy. AMS 14C dating of molluscs in the sediment core shows variable deviation from the tephrochronological age model, indicating that the reservoir age of the seawater mass at the coring site has varied with time. A standard marine reservoir correction of 400 14C years appears to be reasonable at the present day in the coastal and shelf waters around Iceland, which are dominated by the Irminger Current. However, values over 500 years are observed during the last 3000 years. We suggest that the intervals with increased and variable marine reservoir correction reflect incursions of Arctic water masses derived from the East Greenland Current to the area north of Iceland.  相似文献   

12.
An 8 m core from the central plain of the Petit Lac d'Annecy, France, two floodplain cores, river bedload sediments and several hundred soil samples from the catchment have been studied using magnetic techniques. The soils, mainly developed on limestones and local glacial tills, show widespread magnetic enhancement with higher ferrimagnetic concentrations and contents of SP grains than found in the lake sediments. Some soils show significant concentrations of canted antiferromagnetic minerals (mainly haematite). Using magnetic quotient parameters the surface soils are classified into four mineralogical types. The lake and floodplain sediment properties over the past 6000 yrs can largely be explained by the erosion and deposition of these sources, with a smaller superimposed biogenic (magnetosomes) signal. Derived sediment-source linkages allow the construction of several hypotheses about geomorphological changes in the catchment system: (i) the long-term erosion of high altitude unweathered substrates has gradually increased towards the present day; (ii) the erosion of high altitude soils has increased within the last 1000 yrs, possibly during the period of the 'Little Ice Age'; (iii) shifts towards an increased erosion of surface lowland soil occurred ~2000 and 1000 yrs ago and may be linked to an accelerated accretion of floodplain overbank deposits; (iv) there has been a significant storage of surface soil within floodplains, which leads to an underestimation of the importance of soil erosion in the lake sediment records; (v) the sediment transported by high magnitude, low frequency flood events has shifted in source from high altitude soils before ~1000 cal. yr BP to lowland and mid-altitude free draining soils after ~1000 cal. yr BP.  相似文献   

13.
A sediment record is used, in combination with shallow landslide soil redistribution and sediment-yield modelling, to reconstruct the incidence of high-magnitude/low-frequency landslide events in the upper part of a catchment and the history of a wetland in the lower part. Eleven sediment cores were obtained from a dune-impounded wetland at Te Henga, west Auckland, northern New Zealand. Sediment stratigraphy and chronology were interpreted by radiocarbon dating, foraminiferal analysis, and provisional tephrochronology. Gradual impoundment of the wetland began c. 6000 cal yr BP, coinciding with the start of a gentle sea-level fall, but complete damming and initial sedimentation did not begin until c. 1000 cal yr BP. After damming, four well-defined sediment pulses occurred and these are preserved in the form of distinct clay layers in most of the sediment cores. For interpreting the sediment pulses, a physically based landslide model was used to determine spatially distributed relative landslide hazard, applicable at the catchment scale. An empirical landslide-soil redistribution component was added and proved able to determine the volumes and spatial pattern of eroded and deposited soil material, sediment delivery ratio and the impact on total catchment sediment yield. Sediment volumes were calculated from the wetland cores and corresponding landslide scenarios are defined through back-analysis of modelled sediment yield output. In general, at least four major high-magnitude landslide events, both natural and intensified by forest clearance activities, occurred in the catchment upstream of Te Henga Wetland during the last c. 1000 years. The spatial distribution of modelled critical rainfall values for the catchment can be interpreted as an expression of shallow landslide hazard. The magnitude of the sediment pulses represented in the wetland can be back-calculated to critical rainfall thresholds representing a shallow landslide model scenario.  相似文献   

14.
Two sediment cores from the Archipelago Sea in the northern Baltic Sea were examined for their siliceous microfossils in order to study the Holocene palaeoenvironmental history of the area. The diatom record was divided into local diatom assemblage zones (LDAZ). An age model was constructed using independent palaeomagnetic and AMS-14C methods. The early history of the Archipelago Sea was freshwater. Initial brackish-water influence is observed at 7,950 ± 80 cal. BP (LDAZ4), but fully brackish conditions were established at 7,700 ± 80 cal. BP (LDAZ5). Diatom assemblages indicate increasing salinity, warming climate, and possible increasing trophic state during the transition from lacustrine to brackish-water conditions. The decreasing abundance of Pseudosolenia calcar-avis (Schultze) Sundström and the increasing abundance of the ice-cover indicator species Pauliella taeniata (Grunow) Round and Basson indicate reduced salinity and climatic cooling after ～5,000 cal. BP. LDAZ boundaries do not always correlate with changes in the sediment appearance, which underlines the importance of defining biostratigraphic boundaries independently to the sediment visual character, in contrast with the conventional practice for classifying the Baltic Sea sediments.  相似文献   

15.
Ice-rafted detritus (IRD) layers in the Arctic Ocean not only indicate the source of this detrital sediment, but give insights into the ice drift and ice sheet history. Detrital sand-sized Fe oxide mineral grains that are matched to precise sources using the microprobe chemical fingerprint of each grain, along with elevated coarse IRD abundance and radiocarbon ages, are used to define IRD peaks from the Innuitian and Arctic portions of the Laurentide ice sheets. Because grains from these two areas can be entrained by sea ice from the shelves just offshore of the calving areas, peaks in these grains must be correlated to coarse IRD to identify iceberg calving events, and to distinguish them from sea-ice rafting. The sequence of IRD peaks deposited by icebergs from these two ice sheets indicate that both ice sheets calved bergs at accelerated numbers, six or seven times, from 11 to 36 Kya. The relatively short times between most of these IRD events suggest that the ice sheets did not completely collapse with each IRD event, except the last event. Although there is some indication that one ice sheet may have begun calving bergs before the other, the resolution of the Arctic cores does not allow definitive determination of this. This emphasizes the need for higher resolution cores from the central Arctic, as well as from near the terminus of large Pleistocene ice sheets. Sea-ice rafting occurs throughout the last glacial stage, even during some glacial IRD events, as indicated by Fe grains from non-glacial sources.  相似文献   

16.
Three lake sediment sequences (lakes Nero, Chashnitsy, Zaozer’e) from the Rostov-Jaroslavl’ region north of Moscow were studied to provide information on palaeoclimatic and palaeoenvironmental changes during the past 15,000 cal yr. The multi-proxy study (i.e., pollen, macrofossils, mineral magnetic measurements, total carbon, nitrogen and sulphur) is chronologically constrained by AMS 14C measurements. Lake Nero provided the longest sedimentary record back to ca. 15,000 cal yr BP, while sediment accumulation began around ca. 11,000 cal yr BP in the two other lakes, possibly due to melting of permafrost. Limnic plant macrofossil remains suggest increased lake productivity and higher mean summer temperatures after 14,500 cal yr BP. While the late glacial vegetation was dominated by Betula and Salix shrubs and various herbs, it appears that Betula sect. Albae became established as early as 14,000 cal yr BP. Major hydrological changes in the region led to distinctly lower lake levels, starting 13,000 cal yr BP in Lake Nero and ca. 9000 cal yr BP in lakes Chashnitsy and Zaozer’e, which are situated at higher elevations. These changes resulted in sedimentary hiatuses in all three lakes that lasted 3500–4500 cal yr. Mixed broad-leaved – coniferous forests were widespread in the area between 8200 and 6100 cal yr BP and developed into dense, species-rich forests between 6100 and 2500 cal yr BP, during what was likely the warmest interval of the studied sequences. Agricultural activity is documented since 500 cal yr BP, but probably began earlier, since Rostov was a major capital by 862 A.D. This apparent gap may be caused by additional sedimentary hiatuses around 2500 and 500 cal yr BP.  相似文献   

17.
We use coupled numerical models (HydroTrend and SedFlux) to investigate the dispersal and accumulation of sediment on Poverty Shelf, North Island, New Zealand, during the past 3 kyr. In this timeframe, we estimate that the Waipaoa River system delivered ∼10 Gt of sediment to Poverty Shelf,  5–10% of which was transported to the outer shelf and continental slope. The domain of the two-dimensional model (SedFlux) is representative of a 30 km traverse across the shelf. Comparing the model output with seismic reflection data and a core obtained from the middle shelf shows that, without extensively modifying the governing equations or imposing unrealistic conditions on the model domain, it is possible to replicate the geometry, grain size and accumulation rate of the late Holocene mud deposit. The replicate depositional record responds to naturally and anthropogenically induced vegetation disturbance, as well as to storms forced by long-period climatic events simulated entirely within the model domain. The model output also suggests that long-term fluctuations in the amount and caliber of river sediment discharge, promoted by wholesale changes in the catchment environment, may be translated directly to the shelf depositional record, whereas short-term fluctuations conditioned by event magnitude and frequency are not. Thus on Poverty Shelf, as well as in depocenters on other active continental margins which retain a much smaller proportion of the terrigeneous sediment delivered to them, flood-generated event beds are not commonplace features in the high-resolution sedimentary record. This is because the shelf sedimentary record is influenced more by the energy available to the coastal ocean which helps keep the sediment in suspension and facilitates its dispersal, than by basin hydrometeorology which determines the turbidity and velocity of the river plume.  相似文献   

18.
We have conducted a palaeomagnetic study on three sediment cores obtained from a single site in Lake Biwa, central Japan. A total of 1430 discrete samples from three cores show strong and stable remanence, which is carried by pseudo-single-domain magnetite. Palaeomagnetic records from individual cores show little scatter of directions, and there is excellent correlation among the records from the site. A composite palaeomagnetic secular variation (PSV) record for the Holocene was constructed after fine-scale adjustment and stacking of the three records. The age model is based on two wide-spread tephra layers: Kawagodaira (3150 yr BP, calendar years) and Kikai-Akahoya (7250 yr BP). Our results show that the declinations and inclinations are strikingly similar to archaeomagnetic data from southwest Japan for the last 2000 years. The PSV record from Lake Biwa shows good correlation in directions with that of Beppu Bay, located about 500 km west of Lake Biwa, although the Beppu Bay record shows relatively smaller amplitudes of both inclination and declination, probably due to heavy smoothing of the initial results. When the Lake Biwa record is compared with the proposed type curve for southwest Japan, there is considerable disagreement throughout the Holocene. A re-analysis of the data sets and other information used to construct the type curve revealed that the discrepancy in the results was due to inadequate age determination for the cores used in previous studies. We conclude that the stacked PSV record from Lake Biwa meets the criteria required for a reliable curve. Moreover, our results resolve the complications found in the correlation of PSV records in southwest Japan and thereby contribute to a better understanding of Holocene PSV.  相似文献   

19.
Late glacial palaeoceanography of Hinlopen Strait, northern Svalbard   总被引：3，自引：0，他引：3  
Timing and structure of the Late and post-glacial development of the northern Svalbard margin, together with the initial influx of Atlantic water into the Arctic Ocean are still very poorly constrained. We investigated a sediment core (NP94-51) from a high accumulation area on the continental shelf north of Hinlopen Strait with the purpose of resolving the timing and structure of the last deglaciation. Detailed analyses of ice-rafted detritus, benthic and planktonic foraminiferal fauna, diatom flora, grain size and radiocarbon dates are used to reconstruct the palaeoceanographic evolution of the area. Our results indicate that the disintegration of Hinlopen Strait ice and possibly the northern margin of the Svalbard Ice Sheet commenced between 13.7 and 13.9 14C Ky BP. Influx of subsurface Atlantic waters into the area (12.6 14C Ky BP) and the retreat of the sea ice cover, with the accompanying opening of the surface waters (10.8 14C Ky BP), happened at different times and both much later than the disintegration of the ice sheets. The transition into the Holocene shows a two-step warming.  相似文献   

20.
A 20-m sediment core from Rano Kau, Easter Island provides plant microfossil, arthropod fossil and high-resolution 14C sampling evidence for late Quaternary environments, ant (Formicidae) distributions and human activity. The record commences prior to or during the LGM, providing the two oldest dates for Rano Kau thus far, 20,340 ± 160 and 34,260 ± 440 BP. The vegetation at the time was mostly Arecaceae (palm)-dominated grassland-woodland, suggesting cooler/drier conditions than present. Near the start of the Holocene c. 12,500 cal BP, climate alleviation is indicated by Asteraceae shrubland increasing at the expense of grassland. There is an early to mid-Holocene sediment hiatus. The record recommences c. 3,500 cal BP, with in-washed clayey soils bearing charcoal and phytoliths of Polynesian-introduced Musa (banana) and mixed with wetland detritus, reflecting slumping as a result of forest clearance and gardening. Dates of material of other plants from the clay/detritus layers containing the Musa phytoliths are older than expected, within the range 3,680–2,750 cal BP, well before settlement of Eastern Polynesia. This could reflect horticultural material settling in an older part of the sediment column or age increase by reservoir effects. A long section of overlying unconsolidated detritus provides a progressively younger upward sequence from 2,840–2,870 to 930–810 cal BP, but dates are variable after 1,290–1,180 cal BP, where charcoal, disturbance-related pollen and older than expected ant exoskeletons demonstrate the inclusion of pre-aged material from the floating vegetation mat or upslope erosion in the sediment column. Arecaceae pollen declines sharply after 930–810 cal BP. Arthropod analysis reveals two new weevil species for Easter Island, and two ant taxa, Tetramorium bicarinatum and Pheidole sp., the latter of which is also new for the island. Three of the four age determinations on ant remains suggest that ants were present prior to c. 2,500 cal BP, and probably associated with the period following the resumption of sediment accumulation c. 3,500 cal BP. Unless the apparent ant 14C ages have been increased by reservoir effects, these results question the long-accepted assumption that all Eastern Polynesia’s ant species have been introduced.  相似文献   

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