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1.
The well known Clovis and Plainview archaeological sites of New Mexico and Texas have yielded new data on regional late Quaternary geologic, paleoclimatic, and pedologic histories. Eolian sedimentation at the Clovis site from about 10,000 to less than 8500 yr B.P. was followed by the formation of a cumulic soil between 8500 and 5000 yr B.P. Episodic eolian and slope wash deposition then culminated in massive eolian sedimentation about 5000 yr B.P. after which a Haplustalf formed then was subsequently buried by part of a dune system within the last 1000 yr. At the Plainview site, a basal stream gravel contains Plainview cultural material (ca. 10,000 yr B.P.), which is followed by a localized early Holocene lacustrine deposit, two eolian deposits (the younger dating to about 5000 yr B.P.), and a marsh deposit which slowly accreted as an Argiustoll formed in the younger eolian unit. The data indicate that on the Southern High Plains (1) between 12,000 and 8500 yr B.P. sedimentation varied from site to site, (2) there was a regional climate change toward warming and drying in the early Holocene, (3) two episodes of severe drought apparently occurred in the middle Holocene (6500 to 4500 yr B.P.), (4) between 4500 yr B.P. and the present an essentially modern climate existed, but with several shifts toward aridity within the last 1000 yr, (5) argillic horizons have developed in late Holocene soils, (6) clay illuviation can occur in calcareous soils, and (7) long-distance correlation of Holocene stratigraphy in the region is possible, particularly with the aid of soil morphology.  相似文献   

2.
Deposits of the Tamanduá River contain evidence for four major paleohydrologic stages in the last 33,000 years. A wet period between 33,000 and 20,00014C yr B.P. produced a high water table that allowed organic-rich deposition in the Tamanduá valley. A dry interval 17,000-10,00014C yr B.P. produced sandy deposits of braided channels and alluvial fans. River aggradation during this period probably resulted from a high sediment load promoted by intense slope erosion and from flash floods. A wet period after 10,000 and before 600014C yr B.P. was marked by reduced slope erosion and by high discharge that led to erosion of the valley fill. During that time forests developed widely in Brazil. A drier climate after 600014C yr B.P. caused a reduction of discharge but allowed a high water table to be maintained.  相似文献   

3.
The geoarchaeological study of the Palaeoeskimo Tayara site on Qikirtaq Island (Nunavik) has led to a better understanding of archaeological site formation in the arctic periglacial environment. The surrounding geomorphology (extra‐site) is characterized by fine‐grained, low plastic and leached postglacial glaciomarine sediments that have been reworked by sheet‐like solifluction. This process buried the northern part of the Tayara site with mean annual rates between 1.68 and 2.86 cm/yr over approximately 350 years (1330–980 yr B.P.). The physicochemical and mineralogical properties of the frost‐susceptible glaciomarine sediments may explain their susceptibility to solifluction. This process was probably enhanced by longer thawing periods or warmer/moister summer months that induced active layer thickening or rapid soil thawing. The dates we obtained in the downstream valley show that solifluction occurred during short warm periods in the Late Holocene between ca. 1500 and 1000 yr B.P., after 1000 yr B.P. (or after 500 yr B.P.) and recently (90–60 yr B.P.). Our data provide insights on the site factors and climate factors that govern site burial by solifluction. Solifluction promoted the preservation of the three superposed archaeological levels in the Tayara site; however, the waterlogging of the site related to solifluction also likely caused the subsequent abandonment of the site by the Palaeoeskimo people. © 2008 Wiley Periodicals, Inc.  相似文献   

4.
The object of this study is to document how the Inuit on the northern coast of Labrador, Canada used terrestrial resources such as peat and wood during the Little Ice Age (LIA; A.D. 1500–1870). Paleoecological investigations consisting of pollen and macrofossil analyses were undertaken in conjunction with archaeological excavations at the Inuit winter settlement sites of Oakes Bay 1, located in the Nain region of north‐central Labrador. Our data indicate that the major changes in terrestrial ecosystems of this coastal region were triggered by climate change. From ca. 5700 to 3000 cal. yr B.P., climatic conditions were relatively warm and moist. At ca. 3000 cal. yr B.P. conditions became significantly drier and colder, which corresponds to broader climatic trends during the Neoglacial period. At ca. 1000 cal. yr B.P., the reappearance of hygrophilic species and the establishment of Larix laricina provide evidence of a return to more humid conditions that in turn triggered the onset of the paludification of sandy terraces in the Dog Island region. Peat accumulation persisted after ca. 580 cal. yr B.P. likely due to the elevation of the frost table during the LIA. Elevated frost tables contributed to water saturation of the surface during the spring, creating conditions that were conducive to the preservation of organic material. Natural resources such as trees and peat were therefore readily available and more abundant during the LIA and extensively used by the Inuit for house construction and heating in the Dog Island region.  相似文献   

5.
Sedimentological, malacological, and pollen analyses from 14C-dated alluvial sections from the Luján River provide a detailed record of environmental changes during the Holocene in the northeastern Pampas of Argentina. From 11,200 to 9000 14C yr B.P., both sedimentary and biological components suggest that the depositional environment was eutrophic, alkaline, and freshwater to brackish shallow water bodies without significant water circulation. During this time, bioclastic sedimentation was dominant and the shallow water bodies reached maximum development as the climate became more humid, suggesting an increase in precipitation. Short-term fluctuations in climate during the last stage of this interval may have been sufficient to initiate changes in the water bodies, as reduction of the volume alternated with periods of flooding. The beginning of the evolution of shallow swamps in the wide floodplain or huge wetlands was contemporaneous with a sea level lower than the present one. From 9000 and 7000 14C yr B.P., mesotrophic, alkaline, brackish, probably anoxic swamps existed. Between 7000 and 3000 14C yr B.P., anoxic calcareous swamps were formed, with subaerial exposure and development of the Puesto Berrondo Soil (3500-2900 14C yr B.P.). A trend to a reduction of water bodies is recorded from 9000 to ca. 3000 14C yr B.P., with a significant reduction after ca. 7000 14C yr B.P. A shift to subhumid-dry climate after 7000 14C yr B.P. appears to be the main cause. During this time, an additional external forcing toward higher groundwater levels was caused by Holocene marine transgression causing changes in the water bodies levels. The climate became drier during the late Holocene (ca. 3000 yr B.P.), when clastic sedimentation increased, under subhumid-dry conditions. Flood events increased in frequency during this time. From ca. A.D. 1790 to present, the pollen record reflects widespread disturbance of the vegetation during the European settlement.  相似文献   

6.
In the Tigray region at the northern Ethiopian Highlands, paleoenvironmental reconstruction based on several infilled valley deposit sequences suggests that the past 4000 yr comprised three major wetter periods (ca. 4000–3500 yr B.P., 2500–1500 yr B.P., and 1000–960 yr B.P.), during which soils were formed, and two degradation episodes (ca. 3500–2500 yr B.P. and 1500–1000 yr B.P.), during which there was an increase of sediment yield from the slopes into the valleys. For the past 1000 yr, and in particular since the early 17th century, stratigraphic records together with historic chronicles suggest increasing aridity. Although difficulties arise in distinguishing between natural and human impacts, particularly in a region with a long established agricultural background, stratigraphical and proxy paleoclimatic data have indicated climate as the main controlling factor responsible for the environmental changes in the Tigray.  相似文献   

7.
Abandoned fields in Colorado River alluvium in the eastern Grand Canyon show signs of primitive agriculture. Presence of maize pollen in association with buried soils near Comanche Creek suggests that farming began prior to 3130 yr B.P. Cotton pollen, identified in buried soils near Nankoweap Creek, dates to 1310 yr B.P., approximately 500 years earlier than previously reported anywhere on the Colorado Plateau. Farming spanned three millennia in this reach of the canyon. Entrenchment, starting approximately 700 yr B.P., making water diversion to fields infeasible, was likely responsible for field abandonment. © 2000 John Wiley & Sons, Inc.  相似文献   

8.
Tephra layers near Glacier Peak in the North Cascade Range provide limiting dates for four periods of alpine glacier advance. Field relations suggest that late Wisconsin alpine glaciers last advanced prior to the eruption of tephra layers from Glacier Peak about 11,250 yr B.P. Late Wisconsin deglaciation in the central North Cascades was complete prior to the Glacier Peak tephra eruptions. Glaciers again expanded in the early Holocene about 8400 – 8300 yr B.P. Soil formed in alpine meadows during an episode of mild climate in the middle Holocene prior to at least two intervals of glacier expansion: an older episode between 5100 and 3400 yr B.P., and a younger episode within the last 1000 yr.  相似文献   

9.
Analyses of pollen and land snails from rocksheter sites in the Osage Hills of northeastern Oklahoma indicate that the period 2000-1000 yr B.P. was moister than today. During that time, colonies of the prairie vole Microtus ochrogaster were present in the Texas Panhandle. About 1000 yr B.P. the climate changed to dry conditions that have persisted to the present. Disjunct colonies of small mammals in Texas became extinct at the beginning of the dry episode, thereby establishing the composition of the modern fauna. The climatic model for the origin of the Panhandle Aspect (A.D. 1200–1500) is questioned on the grounds that the Southern Plains experienced a long period of dry climate commencing A.D. 950.  相似文献   

10.
The post-Hypsithermal history of Waldsea Lake, a saline meromictic lake located in south-central Saskatchewan, has been deduced from a study of the changes in physical, mineralogical, and paleobiological parameters in sediment cores from the basin. Six lithostratigraphic units and three palynological zones are identified in the most recent sediment. These units and zones indicate that a shallow hypersaline lake with extensive mudflats existed about 4000 yr B.P. In response to the subsequent trend toward a cooler and wetter climate, deeper water conditions ensued, and by about 3000 yr ago a relatively deep stratified lake occupied the Waldsea Basin. A short climatic reversal about 2500 yr B.P. again caused low-water and mudflat conditions, but by 2000 yr ago the lake had regained its higher levels. The past 2000 yr of Waldsea's history have been relatively uneventful, except for a minor lowering of the lake about 700 yr B.P.  相似文献   

11.
The study place is in the Barreirinhas region, Maranhão State, northeastern Brazil. A vegetation transect of 78 km was studied among four vegetation types: Restinga (coastal vegetation), Cerrado (woody savanna), Cerradão (dense woody savanna), and Forest, as well as three forested sites around Lagoa do Caçó, located approximately 10 km of the transect. Soil profiles in this transect were sampled for δ13C analysis, as well as buried charcoal fragments were used for 14C dating. The data interpretation indicated that approximately between 15,000 and ∼9000 14C yr B.P., arboreal vegetation prevailed in the whole transect, probably due to the presence of a humid climate. Approximately between ∼9000 and 4000-3000 14C yr B.P., there was the expansion of the savanna, probably related to the presence of drier climate. From ∼4000-3000 14C yr B.P. to the present, the results indicated an increase in the arboreal density in the area, due to the return to a more humid and probably similar climate to the present. The presence of buried charcoal fragments in several soil depths suggested the occurrence of palaeofires during the Holocene. The vegetation dynamic inferred in this study for northeastern Brazil is in agreement with the results obtained in areas of Amazon region, based on pollen analysis of lake sediments and carbon isotope analysis of soil organic matter (SOM), implying than similar climatic conditions have affected these areas during the late Pleistocene until the present.  相似文献   

12.
Palynological and sedimentological data from Lake Telmen, in north-central Mongolia, permit qualitative reconstruction of relative changes in moisture balance throughout the mid to late Holocene. The climate of the Atlantic period (7500–4500 yr ago) was relatively arid, indicating that Lake Telmen lay beyond the region of enhanced precipitation delivered by the expanded Asian monsoon. Maximum humidity is recorded between 4500 and 1600 cal yr B.P., during the Subboreal (4500–2500 yr ago) and early Subatlantic (2500 yr–present) periods. Additional humid intervals during the Medieval Warm Epoch (1000–1300 A.D. or 950–650 ago) and the Little Ice Age (1500– 1900 A.D. or 450–50 yr B.P.) demonstrate the lack of long-term correlation between temperature and moisture availability in this region. A brief aridification centered around 1410 cal yr B.P. encompasses a decade of cold temperatures and summer frost between A.D. 536 and 545 (1414–1405 yr B.P.) inferred from records of Mongolian tree-ring widths. These data suggest that steppe vegetation of the Lake Telmen region is sensitive to centennial- and decadal-scale climatic perturbations.  相似文献   

13.
Sedimentological, faunal, and archaeological investigations at the Sunshine Locality, Long Valley, Nevada reveal a history of human adaptation and environmental change at the last glacial–interglacial transition in North America's north-central Great Basin. The locality contains a suite of lacustrine, alluvial, and eolian deposits associated with fluvially reworked faunal remains and Paleoindian artifacts. Radiocarbon-dated stratigraphy indicates a history of receding pluvial lake levels followed by alluvial downcutting and subsequent valley filling with marsh-like conditions at the end of the Pleistocene. A period of alluvial deposition and shallow water tables (9,800 to 11,000 14C yr B.P.) correlates to the Younger Dryas. Subsequent drier conditions and reduced surface runoff mark the early Holocene; sand dunes replace wetlands by 8,000 14C yr B.P. The stratigraphy at Sunshine is similar to sites located 400 km south and supports regional climatic synchroneity in the central and southern Great Basin during the terminal Pleistocene/early Holocene. Given regional climate change and recurrent geomorphic settings comparable to Sunshine, we believe that there is a high potential for buried Paleoindian features in primary association with extinct fauna elsewhere in the region yet to be discovered due to limited stratigraphic exposure and consequent low visibility.  相似文献   

14.
A proxy climate record from a raised bog in County Fermanagh, Northern Ireland, is presented. The record spans the interval between 2850 cal. yr BC and cal. yr AD 1000 and chronological control is achieved through the use of tephrochronology and 14C dating, including a wiggle‐match on one section of the record. Palaeoclimatic inferences are based on a combination of a testate amoebae‐derived water table reconstruction, peat humification and plant macrofossil analyses. This multiproxy approach enables proxy‐specific effects to be identified. Major wet shifts are registered in the proxies at ca. 1510 cal. yr BC, 750 cal. yr BC and cal. yr AD 470. Smaller magnitude shifts to wetter conditions are also recorded at ca. 380 cal. yr BC, 150 cal. yr BC, cal. yr AD 180, and cal. yr AD 690. It is hypothesised that the wet shifts are not merely local events as they appear to be linked to wider climate deteriorations in northwest Europe. Harmonic analysis of the proxies illustrates statistically significant periodicities of 580, 423–373, 307 and 265 years that may be related to wider Holocene climate cycles. This paper illustrates how the timing of climate changes registered in peat profiles records can be precisely constrained using tephrochronology to examine possible climatic responses to solar forcing. Relying on interpolated chronologies with considerable dating uncertainty must be avoided if the climatic responses to forcing mechanisms are to be fully understood. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

15.
Shallowly buried archaeological sites are particularly susceptible to surface and subsurface disturbance processes. Yet, because cultural deposition often operates on short time scales relative to geologic deposition, vertical artifact distributions can be used to clarify questions of site formation. In particular, patterns in artifact distributions that cannot be explained by occupation histories must be explained by natural processes that have affected sites. Buried only 10–50 cm beneath the ground surface for 10,450 14C yr, the Folsom component at Barger Gulch Locality B (Middle Park, Colorado) exhibits many signs of post‐depositional disturbance. Through examination of variation in the vertical distribution of the artifact assemblage, we are able to establish that only a Folsom component is present. Using vertical artifact distributions, stratigraphy, and radiocarbon dating, we are able to reconstruct the series of events that have impacted the site. The Folsom occupation (˜10,450 14C yr B.P.) was likely initially buried in a late‐Pleistocene eolian silt loam. Erosion brought the artifacts to rest on a deflation surface at some time prior to 9400 14C yr B.P. A mollic epipedon formed in sediments that accumulated between 9400 and 7000 14C yr B.P. Some time after 5200 14C yr B.P., this soil was partially truncated, and artifacts that had previously dispersed upward created a secondary lag at its upper contact. This surface was buried again and artifact dispersal continued. © 2005 Wiley Periodicals, Inc.  相似文献   

16.
Lithology, pollen, macrofossils, and stable carbon isotopes from an intermontane basin bog site in southern New Zealand provide a detailed late-glacial and early Holocene vegetation and climate record. Glacial retreat occurred before 17,000 cal yr B.P., and tundra-like grassland–shrubland occupied the basin shortly after. Between 16,500 and 14,600 cal yr B.P., a minor regional expansion of forest patches occurred in response to warming, but the basin remained in shrubland. Forest retreated between 14,600 and 13,600 cal yr B.P., at about the time of the Antarctic Cold Reversal. At 13,600 cal yr B.P., a steady progression from shrubland to tall podocarp forest began as the climate ameliorated. Tall, temperate podocarp trees replaced stress-tolerant shrubs and trees between 12,800 and 11,300 cal yr B.P., indicating sustained warming during the Younger Dryas Chronozone (YDC). Stable isotopes suggest increasing atmospheric humidity from 11,800 to 9300 cal yr B.P. Mild (annual temperatures at least 1°C higher than present), and moist conditions prevailed from 11,000 to 10,350 cal yr B.P. Cooler, more variable conditions followed, and podocarp forest was completely replaced by montane Nothofagus forest at around 7500 cal yr B.P. with the onset of the modern climate regime. The Cass Basin late-glacial climate record closely matches the Antarctic ice core records and is in approximate antiphase with the North Atlantic.  相似文献   

17.
Major Holocene monsoon changes in continental Southeast Asia are reconstructed from analysis of 14C-dated changes in pollen and organic/inorganic carbon in sediment cores taken from permanent, closed-basin, volcanic lakes in Ratanakiri Province, northeastern Cambodia. Analysis focuses on the nature and timing of monsoon changes, inferred from changes in vegetation and lake conditions. These data provide the first well-dated palynological record, covering most of the Holocene and continuous up to the present, from a terrestrial site in mainland Southeast Asia. The record from a 15-m core retrieved from Kara Lake, representing the last 9300 years, shows that the late Glacial conditions ended about 8500 14C yr B.P., more than 1000 years later than sites in southwest China. Summer monsoon intensity increased over the period ca. 8400–5300 14C yr B.P., similar to most other sites in the Asian monsoon region. A subsequent expansion of secondary forests at the expense of dense semievergreen forests suggest a drier climate leading to more frequent fire disturbance. After ca. 3500 14C yr B.P. disturbance frequency may have increased further with increasing seasonality. From ca. 2500 14C yr B.P. to the present, dense forest has recovered in a mosaic with annually burned dry forest, but climate may not be the main control on local vegetation dynamics in the late Holocene.  相似文献   

18.
Pollen spectra from cores of organic spring deposits from the Transvaal provide evidence for the climatic evolution of the province during the last 35,000 yr B.P. or more. The past climatic phases are derived from palynological reconstructions of past vegetation types by comparison of fossil pollen data with modern surface pollen spectra from various localities. Evidence is provided for an early moist, cool phase with relatively mesic bushveld and expanded montane forest in the central Transvaal, followed by a drier period with drier bushveld which probably lasted until approximately 25,000 yr B.P. During the next phase, which at the latest ended about 11,000 yr B.P., the temperatures were probably 5°–6°C cooler than at present. At that time bushveld vegetation in the central Transvaal was replaced by open grassland with macchia elements. Climatic amelioration came and semiarid savanna returned to the plains, at first gradually and then developing into a warm Kalahari thornveld-type vegetation. After 6000 yr B.P. it apparently became slightly wetter and a more broad-leafed bushveld developed. About 4000 yr B.P. it again became cooler and slightly wetter and the bushveld vegetation on the central and northern plains was comparable to present open upland types. After 2000 yr B.P. conditions gradually became warner until about 1000 yr B.P., when the modern climate of the central Transvaal bushveld originated.  相似文献   

19.
Pollen evidence from Lake Shayema, Mianning County, was obtained to examine postglacial vegetation and climatic change in southwestern Sichuan, China. The sclerophyllous character of the region's warm temperate vegetation today is a reflection of extreme drought in spring before the onset of the Asian monsoons. The pollen record displays several changes in the vegetation over the last 11,000 yr. From 11,000 to 9100 yr B.P., cold-tolerant species, such as Abies , Betula, and deciduous oaks, dominated the vegetation. Between 9100 and 7800 yr B.P., the abundance of deciduous oaks decreased and evergreen oaks increased, as did Tsuga and mesic deciduous species. This change suggests a warming climate with increased precipitation. From 7800 to 4000 yr B.P., sclerophyllous species increased at the expense of mesic deciduous species, an indication that precipitation was becoming more seasonal. Except for increased disturbance starting ca. 1000 yr B.P., the predominance of sclerophyllous vegetation continued until today. The pollen results are compatible with proposed global circulation hypotheses of a strengthened monsoon system during the early to mid Holocene.  相似文献   

20.
The location of South Georgia (54°S, 36°W) makes it a suitable site for the study of the climatic connections between temperate and polar environments in the Southern Hemisphere. Because the mass balance of the small cirque glaciers on South Georgia primarily responds to changes in summer temperature they can provide records of changes in the South Atlantic Ocean and atmospheric circulation. We use grey scale density, weight-loss-on-ignition, and grain size analyses to show that the proportion of glacially eroded sediments to organic sediments in Block Lake was highly variable during the last 7400 cal yr B.P. We expect that the glacial signal is clearly detectable above noise originating from nonglacial processes and assume that an increase in glacigenic sediment deposition in Block Lake has followed Holocene glacier advances. We interpret proglacial lake sediment sequences in terms of summer climate warming and cooling events. Prominent millennial-scale features include cooling events between 7200 and 7000, 5200 and 4400, and 2400 and 1600 cal yr B.P. and after 1000 cal yr B.P. Comparison with other terrestrial and marine records reveals that the South Georgian record captures all the important changes in Southern Hemisphere Holocene climate. Our results reveal a tentative coupling between climate changes in the South Atlantic and North Atlantic because the documented temperature changes on South Georgia are anti-phased to those in the North Atlantic.  相似文献   

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