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1.
A detailed molluscan succession from a 10 m thick deposit of Holocene tufa at St Germain‐le‐Vasson, Normandy, provides the most complete record from northern France and has shed new light on the historical biogeography of several species of land snail. The succession has been reconstructed from four profiles and a chronology provided by accelerator mass spectrometry radiocarbon dating of charcoal, wood and shell. The onset of tufa formation occurred after 9700±90 yr BP and persisted until 4213±77 yr BP. The tufa appears to have accumulated at a remarkably constant rate (14.4–16.5 cm 100 yr?1), except for the upper levels, where the rate increases fourfold. The succession has been divided into six local molluscan zones. An early assemblage consisting of ecologically tolerant species and those indicative of marshy grassland is replaced by a sequence of shade‐demanding taxa, reflecting the encroachment of woodland. Shaded conditions persist until the end of the sequence but the most hygrophilous elements decline after 5422±60 yr BP (zone 5), a change also reflected in the tufa lithology by the development of silty grey horizons. Notable species recovered from the tufa include Acicula fusca, Vertigo substriata, V. alpestris, V. moulinsiana (all rare or unknown living in northern France). Hygromia limbata, a twentieth century introduction to Britain, was previously thought to be a relatively recent arrival in northern France, but its record at St Germain shows that it has been present in Normandy since 6500 yr BP. Azeca goodalli, another shade‐demanding species, appeared at St Germain much later, just after 4420±65 yr BP. Several other species present in the tufa, such as Pomatias elegans, no longer live on the site, adding to the evidence for a distributional decline in Normandy and elsewhere. Perhaps the most noteworthy record is that of Leiostyla anglica, between about 8500 yr BP and 5000 yr BP, because this constitutes its only Holocene occurrence from mainland Europe. Following this discovery it seems likely that further Holocene sites with L. anglica may be found along the western seaboard of Europe connecting populations in Iberia with those of the British Isles. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

2.
《Quaternary Science Reviews》2007,26(9-10):1236-1300
Multidisciplinary investigations of the sequence at Beeches Pit, West Stow (Suffolk, UK), have a direct bearing the age of the Hoxnian Interglacial and its correlation with the continental Holsteinian and with the global marine record. At this site, glacial deposits (till and outwash gravels) referable to the Anglian Lowestoft Formation fill a subglacial channel cut in Chalk bedrock. Above these glacial deposits a series of interglacial sediments occurs, consisting of limnic, tufaceous and colluvial silts, lacking pollen but rich in shells, ostracods and vertebrates. Lower Palaeolithic flint artefacts of Acheulian character have also been recovered, including refitting examples. Charred material is abundant at certain horizons and many of the bones have been burned. Several discrete areas of burnt sediment are interpreted as hearths. The molluscan fauna comprises some 78 taxa and includes species of considerable zoogeographical and biostratigraphical importance. The land snail assemblage from the tufa consists of woodland taxa with no modern analogue, including species that are either extinct (e.g. Zonitoides sepultus) or which no longer live in Britain (e.g. Platyla polita, P. similis, Neniatlanta pauli). This is also the type locality of Retinella (Lyrodiscus) skertchlyi, which belongs to a subgenus of zonitid land snail now living only on the Canary Islands. There are indications from this fauna (‘the Lyrodiscus biome’) that the climate was wetter and perhaps warmer than the present day. The vertebrate fauna is also noteworthy with species of open habitats, such as rabbit (Oryctolagus cf. cuniculus), and of closed forest, such as squirrel (Sciurus sp.) and garden dormouse (Eliomys quercinus) present at different times. The occurrence of southern thermophiles, such as Aesculapian snake (Zamenis longissimus), indicates temperatures warmer than those of eastern England today. The upper levels include much material reworked from the interglacial sediments, although there is clear faunal evidence for climatic deterioration. Both the molluscan and vertebrate faunas suggest correlation of the interglacial sediments with the Hoxnian. Uranium series dates from the tufa (∼455 ka BP), TL dates from burnt flints (414±30 ka BP) and a range of amino acid racemization data all support correlation of this interglacial with MIS 11. However, four OSL dates from sand beneath the interglacial sequence yield a mean age of 261±31 ka BP, far younger than all other age determinations and far younger than implied by the biostratigraphy. Archaeologically the site is unusual in showing prolonged human occupation within closed deciduous forest and evidence for controlled use of fire in a Lower Palaeolithic context. Biostratigraphical correlations with other Lower Palaeolithic sites support the suggestion that Acheulian and Clactonian industries both occurred in southern Britain during the same substage of the Hoxnian, although not necessarily at precisely the same time. The characteristics of the MIS 11 interglacial in Britain are discussed in the light of evidence from Beeches Pit and elsewhere.  相似文献   

3.
Radiocarbon dates are described from a section through Lateglacial and early Flandrian sediments at Llanilid, Mid-Glamorgan, South Wales. Comparisons between age determinations on the alkali soluble (humic) and alkali insoluble (humin) organic fractions from 12 biostratigraphic horizons reveal the extent of contamination by both older and younger carbon residues. The Llanilid time-scale suggests that for the Lateglacial, the earliest organic sediments date from around 13 200 yr BP, the early Interstadial Juniperus maximum occurred at ca. 12 400-12 500 yr BP with a marked decline some 200 years later, the main Betula phase lasted only from ca. 11 700 to 11 400 yr BP and the end of the Interstadial occurred around 11 100 yr BP. The beginning of the Flandrian dates from ca. 10 000 yr BP, the Juniperus maximum occurred approximately 200 years later, the expansion of birch woodland began around 9600 yr BP, while the first hazel arrived in the area at ca. 9300 yr BP. These age determinations are discussed in the context of radiocarbon dates from comparable biostratigraphic horizons in western Britain and the dating of Lateglacial events in the ocean core records from the North Atlantic.  相似文献   

4.
One of the thickest and most extensive tufa deposits in northern France occurs at Daours, about 8 km upstream of Amiens at the confluence of the River Somme and its tributary the Hallue. It covers an area of about 80 ha and reaches 10 m above the level of the present valley, probably owing to the construction of a tufa barrage, which caused periodic damming of the valley. The molluscan succession from a 4‐m section through the Daours tufa provides an unusually detailed record of environmental change during the Holocene. The faunal record, supported by four radiocarbon dates, indicates that the tufa sequence at this location does not extend back to the early Holocene but began c. 7340 cal. a BP and ceased to form shortly after c. 4938 cal. a BP, consistent with the notion of a NW European ‘late Holocene tufa decline’. The molluscan record is divisible into six zones that define three aquatic phases separated by three terrestrial episodes, when the tufa surface dried out sufficiently to allow colonization by dry‐ground species, including some shade‐demanding elements. These events are also reflected by differences in the lithology of the tufa. The earliest aquatic phase at the base of the sequence represents the most stable and permanent water‐body. The two subsequent aquatic phases appear to represent smaller, more ephemeral, water‐bodies surrounded by marsh. The land snail assemblages show ecological changes within each terrestrial episode, as well as faunal differences between them. These events, which reflect changes in the local hydrology, were short‐lived, each lasting for only a few hundred years. The tufa at Daours has also yielded flint artefacts and pottery, but it seems unlikely that the dynamic environmental record of the site results from anthropogenic activity.  相似文献   

5.
The presence of Eurasian lynx as a former native species in Britain during the Holocene is known from bones recovered from several sites. AMS radiocarbon dating of lynx bone recovered from two sites in the Craven area of northern England gave 1842 ± 35 14C yr BP and 1550 ± 24 14C yr BP, together representing the youngest dates for lynx from England, and in the case of the latter, the youngest for Britain as a whole. These dates support the view that the game animal whose occurrence in the nearby Lake District is described in the early 7th century Cumbric text Pais Dinogad, and whose translation to date has been problematic, is a lynx. The occurrence of lynx in early medieval Britain shows that earlier periods of climate change, previously blamed for the species' extinction in Britain, were not responsible. Instead, anthropogenic factors such as severe deforestation, declining deer populations, and persecution, are likely to have caused the extirpation of lynx in Britain. Consequently, the lynx qualifies as a candidate for reintroduction. Large‐scale reafforestation, the growth of deer populations, and more positive attitudes towards carnivores in modern society, could permit the restoration of lynx to Britain, particularly in Scotland. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

6.
We report radiocarbon dates that constrain the timing of the deposition of the late-glacial Puerto Bandera moraine system alongside the western reaches of Lago Argentino adjacent to the Southern Patagonian Icefield. Close maximum-limiting radiocarbon ages (n = 11) for glacier advance into the outer moraines, with a mean value of 11,100 ± 60 14C yrs BP (12,990 ± 80 cal yrs BP), were obtained from wood in deformation (soft) till exposed beneath flow and lodgment till in Bahía del Quemado on the northeast side of Brazo Norte (North Branch) of western Lago Argentino. Other exposures of this basal deformation till in Bahía del Quemado reveal incorporated clasts of peat, along with larger inclusions of deformed glaciofluvial and lacustrine deposits. Radiocarbon dates of wood included in these reworked peat clasts range from 11,450 ± 45 14C yrs BP to 13,450 ± 150 14C yrs BP (13,315 ± 60 to 16,440 ± 340 cal yrs BP). The implication is that, during this interval, glacier fronts were situated inboard of the Puerto Bandera moraines, with the peat clasts and larger proglacial deposits being eroded and then included in the basal till during the Puerto Bandera advance.Minimum-limiting radiocarbon ages for ice retreat come from basal peat in cores sampled in spillways and depressions generated during abandonment of the Puerto Bandera moraines. Glacier recession and subsequent plant colonization were initiated close behind different frontal sectors of these moraines prior to: 10,750 ± 75 14C yrs BP (12,660 ± 70 cal yrs BP) east of Brazo Rico, 10,550 ± 55 14C yrs BP (12,490 ± 80 cal yrs BP) in Peninsula Avellaneda, and 10,400 ± 50 14C yrs BP (12,280 ± 110 cal yrs BP) in Bahía Catalana. In addition, a radiocarbon date indicates that by 10,350 ± 45 14C yrs BP (12,220 ± 110 cal yrs BP), the Brazo Norte lobe (or former Upsala Glacier) had receded well up the northern branch of Lago Argentino, to a position behind the Herminita moraines. Furthermore, glacier termini had receded to just outboard of the outer Holocene moraines at Lago Frías and Lago Pearson (Anita) prior to 10,400 ± 40 14C yrs BP (12,270 ± 100 cal yrs BP) and 9040 ± 45 14C yrs BP (10,210 ± 50 cal yrs BP), respectively. The most extensive recession registered during the early Holocene was in Agassiz Este Valley, where the Upsala Glacier had pulled back behind the outer Holocene moraine, reaching close to the present-day glacier terminus before 8290 ± 40 14C yrs BP (9300 ± 80 cal yrs BP).The radiocarbon-dated fluctuations of the Lago Argentino glacier in late-glacial time, given here, are in accord with changes in ocean mixed layer properties, predominately temperature, derived from the isotopic record given here of ODP Core 1233, taken a short distance off shore of the Chilean Lake District. It also matches recently published chronologies of late-glacial moraines in the Southern Alps of New Zealand on the opposite side of the Pacific Ocean from Lago Argentino. Finally, the timing of the late-glacial reversal of the Lago Argentino glacier fits the most recent chronology for the culmination of the Antarctic Cold Reversal (ACR) in the deuterium record of the EPICA Dome C ice core from high on the East Antarctic Plateau. Therefore, we conclude that the climate signature of the ACR was widespread in both the ocean and the atmosphere over at least the southern quarter of the globe.  相似文献   

7.
New radiocarbon dates from Finnish subfossil mammoth material (Mammuthus sp.), transported by glacial ice, range in age from ca. 32000 to ca. 22500 yr BP. These results suggest that there was a larger ice-free area in Fennoscandia during the Middle Weichselian than previously assumed. In addition, two dates are also presented for bones found in clay with a different transport history. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

8.
《Quaternary Science Reviews》2007,26(5-6):732-742
The radiocarbon reservoir age of high latitude North Atlantic Ocean surface water is essential for linking the continental and marine climate records, and is expected to vary according to changes in North Atlantic deep water (NADW) production. Measurements from this region also provide important input and/or tests of oceanic radiocarbon using 3-D global ocean circulation models. Here, we present a surface water radiocarbon reservoir age record of the high latitude western North Atlantic for the deglacial period via the use of fossil cold-water corals growing in waters that are rapidly exchanged with nearby surface waters. The reservoir age of high latitude North Atlantic surface waters was computed from the radiocarbon age difference between our radiocarbon calibration record (http://radiocarbon.LDEO.columbia.edu) and our marine radiocarbon data. 230Th/234U/238U dates provide the absolute coral ages. Our high latitude North Atlantic Ocean reservoir age data combined with recalculated reservoir ages based on published coexisting terrestrial and marine material and Vedde ash radiocarbon dates from central and eastern North Atlantic show modern values (380±140 year, n=14) during the Bolling and Allerod warm period and a 200 year increase in reservoir age (590±130 year, n=10) during the entire Younger Dryas (YD) cold episode. The reservoir age then decreased to 270±20 year (n=2) at the Preboreal/YD transition, although the dates are too sparse for us to be confident in this estimate. We are not able to resolve the timing of the transition to increased reservoir ages from the mid-Allerod to the YD due to the relatively small change and correspondingly large uncertainty in the estimates. The atmospheric Δ14C record derived from our atmospheric radiocarbon record displays a 40 per mil increase from 12,900 to 12,650 cal years BP, coincident with the shift to high reservoir ages in the early YD cold event. Intrusion of 14C depleted Antarctic Intermediate Water (AAIW) to the high latitude North Atlantic and reduction of NADW formation are possible causes for the coincident shift to high reservoir ages in the North Atlantic surface ocean and increased atmospheric Δ14C during the beginning of the YD event.  相似文献   

9.
Abstract

The first numerical age determinations from radiocarbon dating establish the chronology of glacial events in Redes Natural Park (Cantabrian Mountain, NW Spain). A core drilled in an ice-dammed deposit provided a minimum age of 28 990 ± 230 years BP for the maximum glacial expansion (phase I). Another core from a cirque bottom-fill provided organic sediment with 20 640 ± 300 years BP, a minimum age for the first glacial retreat (phase II). Radiometric dating of proglacial deposits interpreted as synchronous with the last glacial maximum phase in neighbouring Cornelia basin (Picos de Europa), yield ages of 40 480 ± 820 years BP. The chronological data presented in this work are consistent with the model of glacier evolution established in the Pyrénées, with a glacial maximum phase for the last glacial period older than 18 ka. © 2002 Éditions scientifiques et médicales Elsevier SAS. AU rights reserved.  相似文献   

10.
The late‐glacial Bølling period was first identified by Johs. Iversen on the basis of pollen results from Lake Bølling Sø in Denmark. Because there were no radiocarbon dates from the sequence the Bølling Chronozone (12 000–13 000 14C yr BP) was later established on the basis of dates from other sites. A new project is reinvestigating the sediments from the Bølling Sø sequence with AMS radiocarbon dating and multiproxy analyses. Here we present results of AMS radiocarbon dating, macrofossil analyses, cladoceran analyses (Cladocera concentrations and chydorid ephippia) and Pediastrum analyses (concentrations). The AMS dates on land plant remains show that the lower part of the sequence is around 12 500 14C yr BP, and thus clearly pre‐dates the Allerød chronozone. However, construction of a chronology for the sequence was problematic, partly because of reworking of macroscopic plant remains. The climate ameliorated after glacial conditions to such an extent that growth of plants could begin at ca. 12 500 14C yr BP, but the results of multiproxy analyses show little evidence for a further warming period during the pre‐Allerød part of the sequence. Lake productivity was low, and tree birch rare or maybe absent. This may reflect widespread occurrence of dead ice, unstable soils, heavy in‐wash of minerogenic matter to the lake, resulting in turbid water and rapid sedimentation. The early pioneer vegetation was characterised by Salix polaris and Dryas octopetala, and by herbs. The Allerød Chronozone, and especially its initial part, appears to have been relatively warm but reduced cladoceran concentrations and increased proportion of chydorid ephippia suggest that climate cooled in the middle Allerød and that the late Allerød was colder than the early part. The early Younger Dryas was probably colder than the late Younger Dryas. Clear warming is apparent at the beginning of the Holocene, where the first macrofossil evidence of trees (Betula pubescens, Populus tremula) is found. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

11.
《Quaternary Science Reviews》2003,22(21-22):2303-2310
The present work revisits the chronology of the archaeologically controversial Pedra Furada Rock Shelter of Southeast Piauı́, Brazil, using an improved radiocarbon laboratory pre-treatment and measurements on charcoal samples. The procedure, known as ABOX-SC (acid–base–wet oxidation followed by stepped combustion), has previously been used to secure radiocarbon dates of >40 ka for the antiquity of human occupation of Australia and South Africa, and now has been applied to charcoal from the previously dated oldest occupation layer of the Pedra Furada site. Previous radiocarbon dating had obtained only lower limits of 40–45 ka BP for the Pedra Furada basal layer. Nine charcoal samples from well-structured hearths were subjected to the ABOX-SC procedure and their radiocarbon content determined by accelerator mass spectrometry. Measurements on five of the samples returned ages of greater than 56 ka BP, from graphites produced from ABOX pre-treated charcoal combusted at 910°C. Two other samples were greater than 50 ka BP. The remaining two samples were essentially completely combusted at 650°C, with no material surviving to make a 910°C CO2 fraction. Their ages were 41.3 and 47.2 ka BP. Ages obtained from graphites generated from the 650°C combusted fraction are considered minimum ages.  相似文献   

12.
Growth hiatuses in massive corals are usually indicative of past ecological or environmental stresses. Among 37 fossil Porites colonies surveyed from the reef flat of Dadonghai fringing reef at Sanya, Hainan Island, northern South China Sea, seven of them were found to show clear evidence of past mortality, representing a population of ～19%. Among these samples, two of them (SYO‐13 and SYO‐28) display clear growth hiatuses reflecting mortality followed by subsequent recruitment, and five others exhibit a well‐preserved mortality surface and no subsequent recruitment. The growth hiatuses were dated using high‐precision thermal ionisation mass spectrometry U‐series techniques. The age results suggest all the dated corals formed and died in the mid Holocene. Multiple dates below the growth hiatuses suggest that SYO‐13 and SYO‐28 died at 6298 ± 11 and 6929 ± 19 a BP (i.e. years before AD 1950), respectively. Multiple dates above the growth hiatuses indicate that growth in SYO‐13 and SYO‐28 resumed at 6257 ± 14 and 6898 ± 20 a BP, respectively. The calculated durations of growth hiatuses are therefore 41 ± 18 a for SYO‐13 and 31 ± 28 a for SYO‐28, respectively, implying growth resumed within decades after the mortality events. U‐series dating of four other samples with dead heads suggests that they died at 6035 ± 53, 6059 ± 23, 6127 ± 22 and 6474 ± 24 a BP, respectively. In addition, using solution inductively coupled plasma mass spectrometry (ICP‐MS), monthly resolution Sr/Ca and Mg/Ca ratios were determined for the annual growth bands below and above the growth hiatuses for three of the dated samples. The Sr/Ca and Mg/Ca profiles indicate that the three corals probably died in different seasons (from spring to autumn), and the mortality appears to be unrelated to anomalous sea surface temperature‐induced bleaching. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

13.
An extensive insect fauna is described from an organic deposit occurring in a gravel pit at Lea Marston, Warwickshire, England. This organic deposit was lying beneath approximately 2.5 m of alluvial clay and directly above gravels which had provided organic material 14C dated at 11,700 ± 200 yr BP. The insect fauna was indicative of a temperate oceanic climate similar to that found today in southern England or south Sweden and suggested the presence of deciduous woodland. The six radiocarbon dates, however, agreed in giving the deposit an age of around 9500 yr BP which is earlier than the arrival of the principal broad-leaved trees. The pollen assemblage was consistent with the radiocarbon dates in showing, in addition to herbaceous pollen, only the presence of Betula, Salix, and small amounts of Pinus. As the insect faunas known from midland Britain at the close of the Devensian period, barely 500 yr before, are aretic in aspect and entirely devoid of thermophilous species a very rapid climatic amelioration is postulated which permitted the immigration of the very mobile insect fauna well in advance of the more slowly migrating trees.  相似文献   

14.
The adoption of cereal cultivation is a key benchmark in the transition from Mesolithic hunter–gatherer foraging to Neolithic farming economies, but the nature, timing and ecological–cultural context of the earliest cereal use in the British Isles and northwest Europe is still uncertain. We present AMS radiocarbon dating and fine‐resolution pollen evidence from the Isle of Man for cereal growing in the latter stages of a distinct episode of forest disturbance at almost 6000 yr BP (uncalibrated). The coherent ecological structure of this phase at the fine resolution level suggests that it records cereal cultivation well before the Ulmus decline, rather than wild grass pollen grains. This example is one of a cluster of early dates for cereal‐type pollen near the start of the sixth millenium BP, including several around the Irish Sea, which indicate that the introduction of cereal agriculture probably occurred as early in the central British Isles as in the northern European plain. This early cereal phase is followed later by a probable phase of pre‐Ulmus decline pastoral activity. We also report Mesolithic age woodland disturbance around 7000 yr BP (uncalibrated) and the first radiocarbon dates for mid‐Holocene forest history of the Isle of Man. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

15.
《Quaternary Science Reviews》2003,22(8-9):943-947
We present 21 radiocarbon dates on 19 charcoal samples from the sedimentary sequence preserved in Border Cave, South Africa. The background radiocarbon activity for charcoal from the cave was determined to be 0.050±0.018 percent modern carbon, from the analysis of a radiocarbon-dead sample from unit 5WA. Radiocarbon ages for individual samples ranged from 25.2 to >58.2 ka BP.The error-weighted mean ages for successively older strata are 38.5+0.85/−0.95 ka BP for unit 1WA, 50.2+1.1/−1.0 ka BP for units 2BS.LR.A and 2BS.LR.B, 56.5+2.7/−2.0 ka BP for unit 2BS.LR.C and 59.2+3.4/−2.4 ka BP for unit 2WA. This radiocarbon chronology is consistent with independent chronologies derived from electron spin resonance and amino acid racemization dating. The results therefore provide further evidence that radiocarbon dating of charcoal by the ABOX-SC technique can yield reliable radiocarbon ages beyond 40 ka BP. They also imply that Border Cave 5, a modern human mandible, predates >58.2 ka BP and that the Middle Stone Age (Mode 3)—Later Stone Age (Mode 5) transition of Border Cave was largely effected between ∼56.5 and ∼41.6 ka ago.  相似文献   

16.
Appropriate marine–terrestrial reservoir offset (ΔR) values are essential for accurate calibration of marine radiocarbon dates. However, ΔR values are only valid for the specific calibration curve that their calculation is based on. Here, we present revised ΔR values for the Marine20 calibration curve from Arctic North America, based on previously published 14C dates on pre-bomb live-collected marine molluscs (n = 124) and cetaceans (beluga whales; tooth dentine; n = 12), and bowhead whale–driftwood age comparisons from the same glacio-isostatically uplifted shorelines (n = 18). Molluscan-based ΔR are: Chukchi/Beaufort sea coasts, 265±116 14C years; NW Canadian Arctic Archipelago, 188±91 14C years; NE Baffin Island, 81±18 14C years; SE Baffin Island, 14±58 14C years; Hudson Strait, −73±64 14C years; Ungava Bay, 0±86 14C years; Foxe Basin, 175±89 14C years; Hudson Bay, −21±72 14C years; James Bay, 209±114 14C years; West Greenland, −93±111 14C years. Species-specific marine mammal ΔR terms are 107±59 14C years for beluga and 24±58 14C years for bowheads. Our revised ΔR values are applicable for as long as the same broad oceanographic conditions (circulation, ventilation) have persisted, i.e. through the Holocene. While molluscan values are applicable to other marine carbonate (e.g. foraminifera), cetacean ΔR are valid only for the species they were calculated for and should not be applied to other marine mammals. Importantly, the ΔR terms calculated here are only valid for Marine20 and should not be used with earlier or later calibration curves.  相似文献   

17.
The deglaciation history of the Malangen‐Målselv fjord and valley area proximally to the Tromsø‐Lyngen (Younger Dryas) moraine at Bakkejord, Malangen, northern Norway, is reconstructed based on morphostratigraphic, lithostratigraphic and geophysical evidence, and 25 radiocarbon dates from marine shells and foraminifera. The results show that following the Skarpnes event c. 12 200 14Cyr BP, and prior to the Younger Dryas readvance, the area was deglaciated at least as far as Sandmo situated 22 km proximally to the Tromsø‐Lyngen moraine. Two moraine ridges crossing the fjord at Sandmo and buried beneath thick glaciomarine sediments are correlated with this period. The area was subsequently deglaciated between 10 300 and 9200 14Cyr BP, following the Tromsø‐Lyngen (Younger Dryas) readvance. Five ice‐front accumulations post‐dating the Tromsø‐Lyngen moraine and situated 19, 27, 42, 55 and 77 km behind it are identified and dated based on radiocarbon dates and correlation of marine limits: Målsnes (c. 10 050 14Cyr BP), Kjerresnes (c. 10 000 14Cyr BP), Solli (c. 9750 14Cyr BP), Bardufoss‐Brentmoen‐Storskogmoen (c. 9600–9700 14Cyr BP) and Alapmoen (c. 9200 Cyr BP). The largest of these, at Bardufoss‐Storskogmoen, possibly accumulated as a response to an ice advance. Fourteen dates of apparent late Allerød/Younger Dryas age (11 100–10 000 14Cyr BP), obtained from fossils in glaciomarine sediments in the Målselv valley up to 77 km proximally to the Tromsø‐Lyngen moraine, are interpreted as postdating rather than predating this moraine. Several of these are considered to be too old because of uncertain reservoir age, carbon‐dating plateaus and/or contamination. This highlights uncertainties associated with radiocarbon‐dating and the profound effect such uncertainties may have on interpreting geological events.  相似文献   

18.
Late Quaternary terrestrial climate records from the semi-arid zone of the Western Cape of South Africa are rare. However, palaeoenvironmental information may be inferred from ancient termite mounds of the region. Calcrete lenses in these mounds have δ13C and δ18O values that show systematic changes with radiocarbon dates, which range from 33,629–36,709 to 21,676–23,256 cal yr BP. These dates confirm that these heuweltjies had been present in the landscape since the last glacial period. The decrease in δ13C and δ18O from 33,629–36,709 to 21,676–23,256 cal yr BP indicates that climate information is recorded by the calcretes. It is suggested that a progressive decline in air temperature and an increase in moisture availability, and a decline in abundance of C4 or CAM plants, occurred in the region during the time heuweltjie calcite precipitated.  相似文献   

19.
In this paper we discuss the limitation of radiocarbon dates on the pedogenic calcic nodules formed in situ within the vertisols in the upland region of Coimbatore, Tamil Nadu. The radiocarbon ages were obtained using low-level scintillation counters and the dates range from ∼24 to 31 14C kyrs BP. The ages correlate with the marine isotope stage of Late MIS3. However, since the calcic nodules are pedogenised and formed in an open system, the 14C ages should only be considered as estimates and not absolute ages because of the possibility of open-system behaviour with respect to carbon. Thus, we express caution in the interpretation of these and other radiocarbon ages obtained on pedogenic carbonate nodules. Multiple sub-mm size subsamples could provide more reliable age estimates.  相似文献   

20.
Woolly rhinoceros bones, from a number of sites in Britain, have been AMS radiocarbon dated following ultrafiltration pre-treatment. These determinations give a coherent set of ages between >50 and c. 35 cal ka BP. The youngest (35,864–34,765 cal BP) come from the area around Bishopbriggs in western central Scotland and are derived from glaciofluvial sand and gravel overlain by till, both deposited during the Last Glacial Maximum (LGM) glaciation. A previous radiocarbon date from the site suggested that woolly rhinoceros lived c. 27 14C ka BP and the region was ice-free at the time. This date has had significant influence on the timing of extinction of woolly rhinoceros and the onset of glaciation over Britain during the LGM. The new dates revise this earlier determination and confirm that woolly rhinoceros became extinct in Britain after c. 35 cal ka BP, that central Scotland was ice-free at this time, and glaciation extended across this region sometime after 35 cal ka BP.  相似文献   

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