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1.
The oldest known fossil hominin in southern Asia was recovered from Hathnora in the Narmada Basin, central India in the early 1980's. Its age and taxonomic affinities, however, have remained uncertain. Current estimates place its maximum age at >236 ka, but not likely older than the early middle Pleistocene. The calvaria, however, could be considerably younger. We report recent fieldwork at Hathnora and associated Quaternary type-sections that has provided new geological and archaeological insights. The portion of the exposed ‘Boulder Conglomerate’ within the Surajkund Formation, which forms a relict terrace and has yielded the hominin fossils, contains reworked and stylistically mixed lithic artifacts and temporally mixed fauna. Three mammalian teeth stratigraphically associated with the hominin calvaria were dated by standard electron spin resonance (ESR). Assuming an early uranium uptake (EU) model for the teeth, two samples collected from the reworked surface deposit averaged 49 ± 1 ka (83 ± 2 ka, assuming linear uptake [LU]; 196 ± 7 ka assuming recent uptake [RU]). Another sample recovered from freshly exposed, crossbedded gravels averaged 93 ± 5 ka (EU), 162 ± 8 ka (LU) or 407 ± 21 ka (RU). While linear uptake models usually provide the most accurate ages for this environment and time range, the EU ages represent the minimum possible age for fossils in the deposit. Regardless, the fossils are clearly reworked and temporally mixed. Therefore, the current data constrains the minimum possible age for the calvaria to 49 ± 1 ka, although it could have been reworked and deposited into the Hathnora deposit any time after 160 ka (given the LU uptake ages) or earlier (given the RU ages). At Hathnora, carbonaceous clay, bivalve shells, and a bovid tooth recovered from layers belonging to the overlying Baneta Formation have yielded 14C ages of 35.66 ± 2.54 cal ky BP, 24.28 ± 0.39 cal ky BP, and 13.15 ± 0.34 ky BP, respectively. Additional surveys yielded numerous lithics and fossils on the surface and within the stratigraphic sequence. At the foot of the Vindhyan Hills 2 km from the river, we recovered a typologically Early Acheulean assemblage comprised of asymmetrical bifaces, large cleavers with minimal working, trihedral picks, and flake tools in fresh condition. These tools may be the oldest Acheulean in the Narmada Valley. Several lithics recovered from the Dhansi Formation may represent the first unequivocal evidence for an early Pleistocene hominin presence in India. In situ invertebrate and vertebrate fossils, pollen, and spores indicate a warm, humid climate during the late middle Pleistocene. High uranium concentrations in the mammalian teeth indicate exposure to saline water, suggesting highly evaporative conditions in the past. Late Pleistocene sediment dated between 24.28 ± 0.39 cal ky BP and 13.15 ± 340 ky BP has yielded pollen and spores indicating cool, dry climatic conditions corresponding to Oxygen Isotope Stage 2 (OIS 2). An early Holocene palynological assemblage from the type locality at Baneta shows evidence for relatively dry conditions and a deciduous forest within the region. The Dhansi Formation provisionally replaces the Pilikarar Formation as the oldest Quaternary formation within the central Narmada Basin. The Baneta Formation, previously dated at 70 ka to 128 ka, correlates with the late Pleistocene and early Holocene. Our results highlight the need for further Quaternary geological and paleoanthropological research within the Narmada Basin, especially because dam construction threatens these deposits.  相似文献   

2.
Meco, J., Muhs, D.R., Fontugne, M., Ramos, A.J.G., Lomoschitz, A. & Patterson, D. 2010: Late Pliocene and Quaternary Eurasian locust infestations in the Canary Archipelago. Lethaia, Vol. 44, pp. 440–454. The Canary Archipelago has long been a sensitive location to record climate changes of the past. Interbedded with its basalt lavas are marine deposits from the principal Pleistocene interglacials, as well as aeolian sands with intercalated palaeosols. The palaeosols contain African dust and innumerable relict egg pods of a temperate‐region locust (cf. Dociostaurus maroccanus Thunberg 1815 ). New ecological and stratigraphical information reveals the geological history of locust plagues (or infestations) and their palaeoclimatic significance. Here, we show that the first arrival of the plagues to the Canary Islands from Africa took place near the end of the Pliocene, ca. 3 Ma, and reappeared with immense strength during the middle Late Pleistocene preceding MIS (marine isotope stage) 11 (ca. 420 ka), MIS 5.5 (ca. 125 ka) and probably during other warm interglacials of the late Middle Pleistocene and the Late Pleistocene. During the Early Holocene, locust plagues may have coincided with a brief cool period in the current interglacial. Climatically, locust plagues on the Canaries are a link in the chain of full‐glacial arid–cold climate (calcareous dunes), early interglacial arid–sub‐humid climate (African dust inputs and locust plagues), peak interglacial warm–humid climate (marine deposits with Senegalese fauna), transitional arid–temperate climate (pedogenic calcretes), and again full‐glacial arid–cold climate (calcareous dunes) oscillations. During the principal interglacials of the Pleistocene, the Canary Islands recorded the migrations of warm Senegalese marine faunas to the north, crossing latitudes in the Euro‐African Atlantic. However, this northward marine faunal migration was preceded in the terrestrial realm by interglacial infestations of locusts. □Locust plagues, Canary Islands, Late Pliocene, Pleistocene, Holocene, palaeoclimatology.  相似文献   

3.
This study examines trends in stone tool reduction technology at Liang Bua, Flores, Indonesia, where excavations have revealed a stratified artifact sequence spanning 95 k.yr. The reduction sequence practiced throughout the Pleistocene was straightforward and unchanging. Large flakes were produced off-site and carried into the cave where they were reduced centripetally and bifacially by four techniques: freehand, burination, truncation, and bipolar. The locus of technological complexity at Liang Bua was not in knapping products, but in the way techniques were integrated. This reduction sequence persisted across the Pleistocene/Holocene boundary with a minor shift favoring unifacial flaking after 11 ka. Other stone-related changes occurred at the same time, including the first appearance of edge-glossed flakes, a change in raw material selection, and more frequent fire-induced damage to stone artifacts. Later in the Holocene, technological complexity was generated by “adding-on” rectangular-sectioned stone adzes to the reduction sequence. The Pleistocene pattern is directly associated with Homo floresiensis skeletal remains and the Holocene changes correlate with the appearance of Homo sapiens. The one reduction sequence continues across this hominin replacement.  相似文献   

4.
A high resolution marine pollen record from site GeoB1023, west of the northern Namib desert provides data on vegetation and climate change for the last 21 ka at an average resolution of 185 y. Pollen and spores are mainly delivered to the site by the Cunene river and by surface and mid-tropospheric wind systems. The main pollen source areas are located between 13°S and 21°S, which includes the northern Namib desert and semi-desert, the Angola-northern Namibian highland, and the north-western Kalahari. The pollen spectra reflect environmental changes in the region. The last glacial maximum (LGM) was characterised by colder and more arid conditions than at present, when a vegetation with temperate elements such as Asteroideae, Ericaceae, and Restionaceae grew north of 21°S. At 17.5 ka cal. B.P., an amelioration both in temperature and humidity terminated the LGM but, in the northern Kalahari, mean annual rainfall in the interval 17.5-14.4 ka cal. B.P. was probably 100–150 mm lower than at present (400–500 mm/y). The Late-glacial to early Holocene transition includes two arid periods, i.e. 14.4–12.5 and 10.9–9.3 ka cal. B.P. The last part of the former period may be correlated with the Younger Dryas. The warmest and most humid period in the Holocene occurred between 6.3 and 4.8 ka cal. B.P. During the last 2000 years, human impact, as reflected by indications of deforestation, enhanced burning and overgrazing, progressively intensified.  相似文献   

5.
Palynological and sedimentological data from a core extracted from Lake Eteza shed new light on the Holocene vegetation and climate history in KwaZulu-Natal and can be linked to regional and global climate change. A 2072 cm core with nineteen radiocarbon dates and chronological extrapolation to the bottom of the sequence suggests that sedimentation started ca. 10 200 cal yrs BP. Between ca. 10 200 and 6800 cal yrs BP pollen indicators point to a change from intermediately humid conditions to comparatively drier grassy environments. This is in good agreement with Sea Surface Temperature (SST) fluctuations from a core in the Mozambique Channel which influence precipitation in coastal KwaZulu-Natal, and the beginning of the Holocene Thermal Maximum ca. 10 500 cal yrs BP. The lower section of the core corresponds to gradually increasing Holocene sea levels along the coast and development of freshwater or estuarine conditions at Lake Eteza. The middle Holocene (ca. 6800-3600 cal yrs BP), when the sea level reached its highest stand and SST peak, indicate humid climatic conditions that favoured an increase of forest trees, e.g. Podocarpus, and undergrowth plants like Issoglossa. As a consequence of higher precipitation and increase of the water table, conditions were favourable for the spread of mangrove, swamp and possibly riverine forest. During the late Holocene after ca. 3600 cal yrs BP a decrease of Podocarpus and other trees as well as an increase of Chenopodiaceae/Amaranthaceae, grasses and Phoenix coincide with a return to lower sea levels and drier conditions. The decrease of all trees including Phoenix at ca. 700 cal yrs BP, accompanied by rapid sedimentation rates, possibly reflect forest clearing and upland erosion induced by activities of Iron Age settlers. A dry period at the globally recognized onset of the Little Ice Age might have contributed to these changes. Late Iron Age settlers have probably already introduced Zea mays, which was detected in the profile since ca. 210 BP. The appearance of neophytes like Pinus, Casuarina and pollen of Ambrosia-type in the youngest sediments indicates increased disturbance of European settlements and land use since ca. 100 cal yrs BP.  相似文献   

6.
Excavations at Liang Bua, a limestone cave on the island of Flores, East Indonesia, have yielded a well-dated archaeological and faunal sequence spanning the last 95 k.yr., major climatic fluctuations, and two human species - H. floresiensis from 95 to 17 k.yr.1, and modern humans from 11 k.yr. to the present. The faunal assemblage comprises well-preserved mammal, bird, reptile and mollusc remains, including examples of island gigantism in small mammals and the dwarfing of large taxa. Together with evidence from Early-Middle Pleistocene sites in the Soa Basin, it confirms the long-term isolation, impoverishment, and phylogenetic continuity of the Flores faunal community. The accumulation of Stegodon and Komodo dragon remains at the site in the Pleistocene is attributed to Homo floresiensis, while predatory birds, including an extinct species of owl, were largely responsible for the accumulation of the small vertebrates. The disappearance from the sequence of the two large-bodied, endemic mammals, Stegodon florensis insularis and Homo floresiensis, was associated with a volcanic eruption at 17 ka and precedes the earliest evidence for modern humans, who initiated use of mollusc and shell working, and began to introduce a range of exotic animals to the island. Faunal introductions during the Holocene included the Sulawesi warty pig (Sus celebensis) at about 7 ka, followed by the Eurasian pig (Sus scrofa), Long-tailed macaque, Javanese porcupine, and Masked palm civet at about 4 ka, and cattle, deer, and horse - possibly by the Portuguese within historic times. The Holocene sequence at the site also documents local faunal extinctions - a result of accelerating human population growth, habitat loss, and over-exploitation.  相似文献   

7.
A paleo-environmental record covering the last 16 ka (16,000 cal yrs BP) from the eastern areas of the summer rainfall region in South Africa is presented. This area is until now sparsely investigated due to the lack of well preserved natural archives. For this study, we used a peat section from a wetland situated close to the Drakensberg escarpment, where the high annual rainfall amounts supported a continuous peat accumulation since c. 16 ka. One peat core was analysed in terms of fossil pollen composition, carbon and nitrogen content, isotope composition (δ13C, δ15N) and microscopic charcoal concentration. The greatest degree of temporal resolution was achieved from the late Pleistocene and early Holocene section, where proxy-records indicate relatively dry conditions between ca. 16–13.7 ka, 12.8–10.5 ka, 9.5–8.2 ka, and wet conditions between c. 13.7–12.8 ka and 10.5–9.5 ka. A weak moisture signal is also evident at c. 8.2–7.5 ka. The late Pleistocene to early Holocene period was relatively cool, while conditions became generally warmer after 11–8 ka. The interpretation of the mid- and late-Holocene sequence is limited due to a slow accumulation and low sample resolution, but the available data suggest relatively dry conditions until c. 1.5 ka, followed by more humid conditions until c. 0.5 ka. We suggest that the millennial scale variability within the record can be attributed to shifts in the circulation systems dominating the region, i.e. the latitudinal movements of the inter-tropical convergence zone (ITCZ) and the dynamics of the mid-latitude low pressure belts.  相似文献   

8.
A high-resolution study of clay mineralogy and major element geochemistry has been carried out on a high sedimentation rate deep-sea core recovered off the Taitao Peninsula in southern Chili (46°S) to investigate climate and environmental changes in Northern Patagonia since the Late Glacial period (the last 22 ka BP). The chronology is based upon stable oxygen isotopes of planktonic foraminifera and AMS radiocarbon dating. Smectite/(illite + chlorite) and Ti/K ratios reveal strong changes of the sedimentary sources between the crustal rocks of the Coastal Range and the volcanic rocks of the Andean Cordillera over the last 22 ka BP. Compared to the Holocene, the Late Glacial period was characterized by reduced input of detrital material derived from the high relief of the Andean Cordillera, in agreement with a greater extension of the North Patagonian ice cap that prevented chemical weathering of basalt (smectite production) and induced physical erosion of illite and chlorite from glacial scours. The Glacial period is also characterized by rapid changes of smectite/(illite + chlorite) and Ti/K ratios indicating short-term (centennial-scale) glacier fluctuations. These changes could be the results of southern westerly shifts in the Aysen region alternating periods when glaciers were probably less fed by precipitation (mostly located in the North) and consequently less active to ice accumulation periods. The deglaciation was characterized by a trend towards warmer and more humid conditions, punctuated by a cold and arid period partly coeval with the Antarctic Cold Reversal event. Finally, the Holocene presents warmer and more humid conditions even though short term changes in the smectite/(illite + chlorite) ratio and the δ18O record could be attributed to Neoglacial events in the studied region.  相似文献   

9.
The Stuttgart Formation (Schilfsandstein) is approximately 50 m thick in Thuringia, representing deposition during the “Mid-Carnian Wet Intermezzo”. Stratigraphically it occurs between the Grabfeld and Weser formations, which formed under arid conditions. It comprises NNE-SSW-trending elongate, anastomosing channelized sand-rich bodies with erosional bases (channel belts) that are several kilometres wide and pass laterally into predominantly mudstones deposited in interfluve areas. The source area of these clastics was the uplifted Norwegian Caledonides. Muddy interfluve facies is dominant in exposures in Thuringia, Central Germany.The Lower Stuttgart Formation has an unconformable base that is locally overlain by metre-thick “Basal Beds”. These consist of grey mudstones and thin sandstones deposited under humid conditions in predominantly shallow brackish water environments after a marine ingression via the Eastern Carpathian/Upper Silesian Gate. The following 30-40 m-grey, fine-grained sandstones, siltstones and mudstones were deposited in fluvial environments in channel belts and interfluve areas under humid conditions. These are followed by predominantly reddish mudstones and sandstones of mainly fluvial origin, deposited under somewhat drier conditions with seasonal droughts. The Upper Stuttgart Formation may be more than 16 m thick; it comprises reddish and grey sandstones and mudstones that were mostly deposited in lake delta settings by recurring flash floods. During the deposition of this unit climate was weakly humid with less prominent seasonal draughts.The modern Ganga Plain of India is an analogue for the depositional setting of the Stuttgart Formation. Climatic conditions in Ganga Plain are humid monsoonal with seasonal droughts and roughly comparable with those interpreted for Mid-Carnian times in Germany. The sandy deposits of incised channel belts and channels and muddy deposits of interfluve areas in the Ganga Plain are comparable with the sandstone-dominated channelized facies and mudstone dominated interfluve facies of the Stuttgart Formation, respectively.  相似文献   

10.
Eolian deposits and landforms are ubiquitous in western Kansas, particularly south of the Arkansas River with the presence of a stabilized dune field. Stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late Quaternary. The oldest eolian deposits from ca. 16 to 12 ka completely or partially bury high terraces with ages of ca. 16 and 33 ka. It is unknown whether these eolian deposits reflect regional aridity and/or a change in sediment availability with the transition of the Arkansas River from a single channel to a braided system. There is pervasive evidence for episodic eolian erosion and deposition ca. 9.8 to 6.3 ka, generally coincident with loess deposition on upland surfaces [Olson, C.G., Nettleton, W.D., Porter, D.A., Brasher, B.R., 1997. Middle Holocene aeolian activity on the High Plains of west-central Kansas. Holocene, 7(3): 255–261], and thus it is inferred to reflect regional aridity. Sites within the dune field show a sequence of eolian sands and weak buried soils, reflecting either dune migration or sand sheet accretion at ca. 1490, 430, 380–320, 180, and 70 yr ago, which correspond well with continental-scale droughts in the tree-ring record. Eolian sand in the Arkansas River dune field may be derived principally with variability in fluvial activity and in climate during the late Pleistocene to the middle Holocene, with substantial reactivation of eolian systems during decadal-scale drought variability in the past 2000 yr.  相似文献   

11.
罗布泊地区第四纪环境演化   总被引:11,自引:0,他引:11  
新疆罗后干涸湖盆中的K1钻孔揭示,100、2m的沉积剖面共有92个宙积层,划归16同积旋回,包括了早更新世以来的各时期沉积。据划分的15个孢粉组这及古地磁、14^C、岩石地层资料综合分析,早更新世该区曾发育森林-草原植被,早更民末,强烈的构造运动保留我下更充变形,上覆的中更统与下更新统间呈不整合接触。中更新世,荒漠和荒漠草原交替出现,地层中有大量膏质泥岩和石膏。晚更新世以来,在干旱气候的控制之下,  相似文献   

12.
Optical dating of dune sands in the northeastern deserts of China   总被引:1,自引:0,他引:1  
Optical dating has been used to obtain the ages of fossil-stabilized sand dunes from four sections in the northeastern deserts of China. Our results indicate that the optically stimulated luminescence ages of the four sections correlate well, even though the samples were collected from different deserts about 600 km apart. Our results also indicate that active dune formation in this region lasted from the Last Glacial Maximum to about 10 ka, and that the warm climate of the Holocene was interrupted by a cold/dry dune formation episode about 3.5-1.7 ka. The Holocene Optimum in this region is between 10 and 3.6 ka, and a later warm/humid dune stabilization phase lasted from at least 1.6 to 1.0 ka. The youngest age on the uppermost sand unit yielded an age of only 40 yr, supporting the previous argument that the existence of modern active eolian sands in the regions with a mean annual precipitation of up to 450 mm is not mainly due to drought, but to extensive land cultivation over historic time. From the luminescence properties of the quartz grains, it is hypothesized that the sands in most of the sections are probably derived from more than one source, with a minor source of quartz having a different thermal history before deposition.  相似文献   

13.
The Rauer Group is an archipelago in Prydz Bay, East Antarctica. The ice-free islands and the surrounding shallow marine areas provide valuable archives for the reconstruction of the late Pleistocene and Holocene environmental and climatic history of the region. Two sediment records from two marine inlets of Rauer Group have been studied for their sedimentological, geochemical, and biological characteristics. Radiocarbon ages from one of the inlets indicate ice-free conditions within the last glacial cycle, probably during the second half of Marine Isotope Stage 3. Subsequent ice sheet coverage of Rauer Group during the Last Glacial Maxiumum (LGM) can be inferred from a till layer recovered in one of the basins. The inlets became ice-free prior to 11,200 cal yr BP, when biogenic sedimentation started. Deglacial processes in the catchments, however, influenced the inlets until ~ 9200 cal yr BP as evidenced by the input of minerogenic material. Marine productivity under relatively open water conditions indicates an early Holocene climate optimum until 8200 cal yr BP, which is followed by a cooler period with increased sea ice. Warmer conditions are inferred for the mid Holocene, when both basins experienced an input of freshwater between ~ 5700-3500 cal yr BP, probably due to ice-sheet melting and increased precipitation on the islands. Neoglacial cooling in the late Holocene since c. 3500 cal yr BP is reflected by an increase in sea ice in both inlets.  相似文献   

14.
The study was to clarify the events of monsoon climate recorded by the palaeovegetation, using the results of pollen analysis, organic carbon δ13C, together with high-resolution 14C dating in the desert-loess transition zone for the last 13 ka BP. The palaeovegetation went though in the order of 9 periods viz.:desert-grassland,steppe,semi-desert,humid-grassland,desert,sparse-wood-grassland, desert-grassland, grassland, and desert grassland. The semi-desert, humid-grassland, and desert corresponded with the Younger Dryas events of cold-dry, cool-humid, and cold-dry climate after the end of the last glaciation. The optimum of the Holocene and other humid period between 4.5-3.5 ka BP were of high significance in research area.  相似文献   

15.
A 5.4 m peat core from the Sandy Run Creek (SRC) valley in the upper Coastal Plain of Georgia shows that before 30 ka the valley had an aggradational sandy floor with shallow swales and low ridges of 1–2 m amplitude suggesting a braided stream pattern and a low water table. The climate from 30–25 ka was cool and dry and the vegetation open grassland with stands of pine and spruce that produced few fires. At ca. 16 ka a warmer, wetter climate caused SRC to meander and incise the valley fill removing some previously deposited sediment at the site of our peat core so that sediments of 13 ka rest directly on sediments deposited at 25 ka. After ca. 13 ka higher groundwater levels initiated vertical accretion of floodplains allowing peat accumulation in shallow depressions across the valley. Pollen from the Younger Dryas period (ca. 13–11 ka) indicates a cool, moist environment of open oak woodland, mesic trees, riparian populations of alder, and reduced levels of pine. By the early to mid Holocene, tupelo and oak dominated over pine indicating wetter conditions, while sand stringers in peat record periodic heavy rains, an elevated water table, and intervals of substantial runoff. Cooling after ca. 4.5 ka brought drier and more variable conditions. Fires increased and southern pine replaced tupelo and oak. The absence of sand stringers in peat younger than 4.5 ka indicates fewer intense rainfall events.  相似文献   

16.
Samples of alluvial and colluvial deposits from the coastal plain and coastal valleys north and south of Cairns (17°S) have been dated using optically stimulated luminescence (OSL) protocols, with additional thermoluminescence (TL) and Radiocarbon assays. Coarse fanglomerates from elevated coastal terraces date back to 81 ka, but most are clustered in oxygen isotope stage three (OIS3, 64-28 ka), indicating high-energy conditions during this period. Extensive fans and terraces of finer calibre sediments are widely represented grading from coarse gravels and cobbles in proximal zones fine sand and silt in distal areas. Dates show that vertical accumulation of 10-15 m of sediment occurred between ∼ 28 and 14 ka (OIS2), after which the fans were dissected and Holocene deposits become fragmentary. A number of deposits indicating hillslope instability were successfully dated and these fall mainly into OIS3 and post 12 ka. These results are interpreted as catchment responses to Late Pleistocene climate and vegetation changes, documented elsewhere from local pollen and ocean drilling sites. Correlation with these records and with evidence for regional climate change from the Austral-Asian region is good and indicates that these changes were sufficient to transform fluvial activity and slope processes.  相似文献   

17.
A newly discovered Paleolithic site in loess deposits in the Lushi Basin, South Luo River, central China, is dated using pedostratigraphic analysis, optically stimulated luminescence (OSL) and magnetostratigraphic analysis. This region is regarded as an important place for hominin occupation and settlement during the early to middle Pleistocene. Results indicate that the archaeological site dates from 600 ka to 620 ka, reinforcing the view that Homo erectus had occupied a large area of eastern Asia by 620 ka. The lithic assemblages of Lushi Basin is a flake and core technology, typical for this time period in north-central China. It may be compared with that at the Zhoukoudian locality 1 in north China and some sites in the Luonan Basin, and provides important data for understanding the behavior and stone tool technology of early Chinese hominins.  相似文献   

18.
Along the middle and lower reaches of the Kyichu River and its tributaries (Lhasa area, southern Tibet), a multidisciplinary study was carried out in order to investigate the areal distribution, sedimentological properties, ages and palaeoenvironmental implications of aeolian deposits including intercalated palaeosols. This research was initiated to investigate to what extent southern Tibet is influenced by past human activity, as even recent evaluations perceive the present treeless desertic environment as natural. Fifteen profiles were recorded at an altitude of 3540–4580 m a.s.l. with subsequent sedimentological, geochronological (OSL, AMS 14C) and palaeobotanical (charcoal) analyses. Sediment properties of both loesses and aeolian sands reveal an origin from aeolian sorting of nearby fluvial deposits. The calculated ages are the oldest obtained thus far on aeolian sediments from southern and interior Tibet, revealing natural aeolian sedimentation before and around the Last Glacial Maximum (c. 20 ka). However, a distinct portion of Late Holocene sandy aeolian sediments also occurs. Both the evidence for the aeolian dynamics (widespread Pleistocene loess and aeolian sand deposition, local Late Holocene aeolian sand deposition, modern reactivation of widespread Pleistocene aeolian sands) and the palaeobotanical findings (Late Holocene vegetation change from a tree-bearing to a widely treeless landscape) provide evidence that the Lhasa area was strongly influenced by human activity since at least the Late Neolithic (c. 4200 cal yrs BP). Thus the present-day desertic environment might not primarily be a result of the semiarid climate or the high-altitude conditions, but rather of activities of the humans and their collateral effects. However, once established, this semi-natural ecosystem persisted, controlled by strong grazing, firewood extraction, erosion and harsh edaphic conditions, preventing the recovery of trees.  相似文献   

19.
Despite high taxonomic diversity elsewhere in North America during the Pleistocene, vertebrate faunas are exceedingly rare in the region of northern Mexico. Térapa, a unique fossil site located in the present-day desert of Northcentral Sonora, Mexico (29°41′N, 109°39′W, 605 m elevation), contributes to our understanding of the paleoecology and paleoclimate of the region during the Late Pleistocene, ca. 43,000-40,000 cal. yr BP. At least 60 vertebrate taxa, including amphibians, turtles, a crocodilian, snakes, birds and many mammals, have been recovered from an 11-m thick sequence of fossiliferous sediments. The diversity and tropical affinity of these taxa suggest a more-forested environment than the thornscrub desert habitat present in this region today.Isotopic analyses of tooth enamel carbonate from ancient mammalian herbivores suggest that the Sonoran desert has undergone considerable climate change since the Late Pleistocene. Bulk carbon (δ13C) and oxygen (δ18O) isotopes from nine mammalian fossils indicate a habitat mosaic with variations in diet that include browsers, mixed feeders and C4 hyper-grazers (δ13C range of − 10‰ to 2‰). Unique to this site are δ13C tooth enamel values of − 6.1 and − 5.6 ‰ for the deer Odocoileus, which suggest a more variable diet than strict browsing, including possibly feeding on CAM and/or C4 plants. Serial sampling of carbon (δ13C) and oxygen (δ18O) isotopes for ancient mammal teeth with hypsodont dentitions (fossil Equus and Bison,) as well as δ18O meteroric water estimates from well-supported climatic models suggest a cooler and more equable environment at Térapa during the Late Pleistocene. These results also support previous habitat reconstructions inferred from the macrobotanical and packrat midden records of northern Sonora (Mexico). High-resolution stable isotope geochemistry indicates that: 1) ancient Térapa was covered with forest and grassland habitats that extended northward into Mexico by about 350 km relative to their present-day northern limits during the Late Pleistocene; and 2) an Amount Effect (AE) is demonstrated in the fossil record at Térapa even though the climate was less seasonal compared to the modern desert habitat.  相似文献   

20.
A 4 m long core taken from the freshwater Blue Lake crater near the township of Mount Gambier in southeastern South Australia provided a high-resolution palaeoclimatic record for the last six millennia. Accelerator Mass Spectrometry (AMS) radiocarbon (14C) dates were obtained from organic plant fibres and biogenic carbonates from the laminated sequence of the core and from a modern water sample. Large discrepancies between the radiocarbon ages determined from plant fibres and biogenic carbonates indicate the presence of a time-variable lacustrine reservoir, which is consistent with what is known of the lake's hydrology.Ostracod assemblages, associated with stable isotope (δ13C, δ18O) analyses and, in combination with Mg/Ca, Sr/Ca and Na/Ca analyses done on ostracod valves, infer salinity, temperature and water level changes in Blue Lake over the last 6 millenia. The influence of local aquifers through time has also been determined from the Na/Ca of ostracod valves. Approximately 900 year cycles are evident in the δ13C record from 5.4 ka to 1.8 ka.The history of Blue Lake records an initial period of high hydrological variability around 6 ka, becoming increasingly deeper as groundwater flowed into the basin. By 4 ka, the lake had reached steady state with the lake level fluctuating by as much as 9 m, although significant geochemical variations represent temperature fluctuations until European settlement near the lake in 1839.  相似文献   

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