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1.
Five Neotoma spp. (packrat) middens are analyzed from Sand Canyon Alcove, Dinosaur National Monument, Colorado. Plant remains in middens dated at approximately 9870, 9050, 8460, 3000, and 0 14C yr B.P. are used to estimate Holocene seasonal temperature and precipitation values based on modern plant tolerances published by Thompson et al. (1999a, 1999b). Early Holocene vegetation at the alcove shows a transition from a cool/mesic to a warmer, more xeric community between 9050 and 8460 14C yr B.P. Picea pungens, Pinus flexilis, and Juniperus communis exhibit an average minimum elevational displacement of 215 m. Picea pungens and Pinus flexilis are no longer found in the monument.Estimates based on modern plant parameters (Thompson et al., 1999a) suggest that average temperatures at 9870 14C yr B.P. may have been at least 1° to 3°C colder in January and no greater than 3° to 10°C colder in July than modern at this site. Precipitation during this time may have been at least 2 times modern in January and 2 to 3 times modern in July. Discrepancies in estimated temperature and precipitation tolerances between last occurrence and first occurrence taxa in the midden record suggest that midden assemblages may include persisting relict vegetation.  相似文献   

2.
High-resolution charcoal and pollen analyses were used to reconstruct a 4600-yr-long history of fire and vegetation near Taylor Lake in the wettest forests of coastal Oregon. Today, fires in these forests are rare because the season of ignition does not coincide with months of dry fuels. From ca. 4600 to 2700 cal yr B.P. fire episodes occurred at intervals of 140±30 yr while forest vegetation was dominated by disturbance-adapted taxa such as Alnus rubra. From ca. 2700 cal yr B.P. to the present, fire episodes have become less common, occurring at intervals of 240±30 yr, and fire-sensitive forest taxa, such as Tsuga heterophylla and Picea sitchensis, have become more prominent. Fire occurrence during the mid-Holocene was similar to that of the more xeric forests in the eastern Coast Range and suggests that summer drought was widespread. After ca. 2700 cal yr B.P., a decrease in fire episode frequency suggests that cooler conditions and possibly increased summer fog allowed the establishment of present-day Picea sitchensis forests within the watershed. These results provide evidence that fire has been an important disturbance agent in the Coast Range of Oregon, and variations in fire frequency and climate have led to the establishment of present-day forests.  相似文献   

3.
The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, 14C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (Uk′37) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the Uk′37 SST estimates show an average late glacial–interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial–interglacial SST change, respectively. Both the Uk′37 and the FP-12E SST estimates, as well as the planktonic foraminiferal δ18O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the Uk′37 SST estimates show a cooling of ca. 0.2–0.6°C compared to the Bølling–Allerød period. These Uk′37 SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period.  相似文献   

4.
Analysis of a buried deposit in the Diamond Valley of southern California has revealed well-preserved pollen, wood, and diatom remains. Accelerator mass spectrometry dates of 41,200±2100 and 41,490±1380 14C yr B.P. place this deposit in marine isotope stage 3. Diatoms suggest a shallow lacustrine environment. Pollen data suggest that several plant communities were present near the site, with grassland, scrub, chaparral, forest, and riparian communities represented. Comparison with modern pollen suggests similarities with montane forests in the nearby San Bernardino and San Jacinto ranges, indicating vegetation lowering by at least 900 m elevation and temperatures 4°–5°C cooler than today. An increase in high-elevation conifer pollen documents climatic cooling near the profile top. Early-profile diatoms are typical of warm water with high alkalinity and conductivity, whereas later diatoms suggest a higher flow regime and input of cooler water into the system. We suggest that the sequence is part of the cooling phase of an interstadial Dansgaard–Oeschger cycle. Records of the middle Wisconsin period are rare in southern California, but the Diamond Valley site is similar to records from Tulare Lake in the San Joaquin Valley and the ODP Site 893A record from Santa Barbara Basin. It is probable that the Diamond Valley assemblage is a local expression of a vegetation type widespread in the ranges and basins of southwestern California during the middle Wisconsin.  相似文献   

5.
The muskrat (Ondatra zibethicus) is presumed to have undergone a rapid phyletic size decrease near the end of the Pleistocene. Evolutionary changes in the size of middle to late Wisconsinan (ca. 32,000–12,300 14C yr B.P.) muskrats from the Aucilla River, Jefferson County, Florida, were reconstructed by examining length and width of the lower first molar (m1). Body mass, estimated from m1 length, was relatively stable from 32,000 to 16,000 14C yr B.P. and decreased only slightly by 12,300 14C yr B.P. If the size trend found in the Aucilla River material is characteristic of the southeastern United States, a body size decrease after 12,300 14C yr B.P. is needed to explain the smaller size of modern populations. It was previously thought that the length/width (l/w) ratio of the muskrat m1 was a paleoenvironmental indicator based on its presumed correlation with latitude in modern populations. We examined the length and width of modern muskrats from several geographic regions and found only a very weak trend in the size of the m1 between northern and southern populations; however, highly significant differences were found between regions of similar latitude. Our data indicate that chronoclines in the m1 of the Aucilla muskrat material and other such documented trends among fossil muskrats have paleoenvironmental significance, but it is not yet clear which environmental variables can best be predicted from them.  相似文献   

6.
The 36Cl dating method is increasingly being used to determine the surface-exposure history of Quaternary landforms. Production rates for the 36Cl isotopic system, a critical component of the dating method, have now been refined using the well-constrained radiocarbon-based deglaciation history of Whidbey and Fidalgo Islands, Washington. The calculated total production rates due to calcium and potassium are 91±5 atoms 36Cl (g Ca)−1 yr−1 and are 228±18 atoms 36Cl (g K)−1 yr−1, respectively. The calculated ground-level secondary neutron production rate in air, Pf(0), inferred from thermal neutron absorption by 35Cl is 762±28 neutrons (g air)−1 yr−1 for samples with low water content (1–2 wt.%). Neutron absorption by serpentinized harzburgite samples of the same exposure age, having higher water content (8–12 wt.%), is 40% greater relative to that for dry samples. These data suggest that existing models do not adequately describe thermalization and capture of neutrons for hydrous rock samples. Calculated 36Cl ages of samples collected from the surfaces of a well-dated dacite flow (10,600–12,800 cal yr B.P.) and three disparate deglaciated localities are consistent with close limiting calibrated 14C ages, thereby supporting the validity of our 36Cl production rates integrated over the last 15,500 cal yr between latitudes of 46.5° and 51°N. Although our production rates are internally consistent and yield reasonable exposure ages for other localities, there nevertheless are significant differences between these production rates and those of other investigators.  相似文献   

7.
Luminescence geochronology, especially infrared stimulated luminescence analyses on marsh mud, shows that a relatively deep lake reached its peak (1340 m above sea level) in the Bonneville basin 59,000±5000 yr ago. The age is consistent with nonfinite 14C ages and with amino acid geochronology on ostracodes. The Cutler Dam Alloformation was deposited during this lake cycle, which, like the subsequent Bonneville lake cycle, appears to have reached its maximum highstand following the peak of a global glacial stage (marine oxygen-isotope stage 4) but at a time when other records from North America show evidence for cold climate and expanded glacier ice.  相似文献   

8.
Despite the presence of numerous active volcanoes in the northern half of Ecuador, few, if any, distal tephras have been previously recognized in the southern one third of the country. In this article, we document the presence of thin (0.1–1.0-cm-thick) distal tephras comprising glass and/or phenocrysts of hornblende and feldspar in sediment cores from five glacial lakes and one bog in Las Cajas National Park (2°40′–3°00′S, 79°00′–79°25′W). The lake cores contain from 5 to 7 tephras, and each has a diagnostic major element geochemistry as determined from electron microprobe analysis of 710 glass shards and 440 phenocrysts of feldspar and hornblende. The loss of sodium with exposure to the electron microbeam causes a 10±7 wt.% (±1σ) reduction in Na content, which we empirically determined and corrected for before correlating tephras among the sediment cores. We use a similarity coefficient to correlate among the sediment cores; pair-wise comparison of all tephras generally yields an unambiguous correlation among the cores. Six tephras can be traced among all or most of the cores, and several tephras are present in only one or two of the cores. Twenty-six accelerator mass spectrometry 14C dates on macrofossils preserved in the sediment cores provide the basis for establishing a regional tephrochronology. The widespread tephras were deposited 9900, 8800, 7300, 5300, 2500, and 2200 cal yr B.P. The oldest tephras were deposited 15,500 and 15,100 cal yr B.P., but these are not found in all cores. Two of the tephras appear correlative with volcaniclastic strata on the flanks of Volcán Cotopaxi and one tephra may correlate with strata on the flanks of Volcán Ninahuilca; both volcanoes are in central Ecuador. The absence of tephras in sediment cores correlative with the numerous eruptions of active volcanoes of the past two millennia implies that the earlier eruptions, which did deposit tephras in the lakes, must have been either especially voluminous, or southerly winds must have prevailed at the time of the eruption, or both.  相似文献   

9.
Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 10Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 10Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.  相似文献   

10.
Magnetic susceptibility (MS) of surface sediment varies systematically across the Loess Plateau in central China, decreasing exponentially from >200×10−8 m3/kg at the northern margin of the Qinling Shan to ≤30×10−8 m3/kg near the southern margin of the Mu Us Desert. MS correlates highly with loess median grain size (r2=0.79), which decreases south-southeastward across the plateau. It also correlates with mean annual temperature (MAT) and mean annual precipitation (MAP) (r2=0.58 and 0.60, respectively), and with their product MAT×MAP (r2=0.83), which is considered a measure of potential pedogenic activity. Because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel, it is difficult to determine their relative influence on MS. A simple MS model, based on the observed spatial variation in loess thickness, permits quantitative assessment of the effect of the dust accumulation rate on the MS signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals. The model suggests that 84% of the loess MS variance is dictated by the diluting effect of dust and 10–11% is associated with meteorological factors, primarily precipitation. The observed and modeled relationships support hypotheses that attribute variations in MS in the loess-paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils, both of which are controlled by monsoon climate.  相似文献   

11.
High-Resolution Climate Simulations of Oxygen Isotope Stage 3 in Europe   总被引：1，自引：0，他引：1  
Oxygen isotope stage 3 (OIS 3) climate and its variations are the focus of the Stage 3 Project. The objective of the OIS 3 modeling effort is twofold: (1) to explore the importance of different boundary conditions on the climate of Europe and (2) to develop climate simulations that best reproduce the wealth of OIS 3 observations. Given the complexity of the topography and coastlines, the modeling effort is based on a “nested” General Circulation Model (GCM) and mesoscale model (RegCM2) with climate simulations for Europe on a 60-km grid spacing. The key conclusions are as follows: (1) The mesoscale model, driven by GCM output, does a reasonable job of reproducing the modern European climate. (2) OIS 3 variations in orbit, CO2, and ice-sheet size are of little significance in explaining the observed climate variability. (3) The model results focus attention on North Atlantic sea-surface temperatures (SST) as a major factor in explaining OIS 3 climates. (4) Experiments for different SST values capture a number of systematic changes in sea-level pressure and precipitation. (5) Climate models simulate substantial European cooling and significant changes in precipitation, but they do not explain large differences between OIS 3 warm and cold episodes.  相似文献   

12.
Paleoecological records from two Holocene peat bogs in northern Germany are linked by two microscopic volcanic ash layers, correlated by petrology and geochemistry to explosive volcanism on Iceland. The younger “Microlite tephra” cannot be correlated to any known eruption, while the older tephra layer is identified as a deposit of the Hekla 3 eruption. The tephra layers are dated by an age–depth regression of accelerator mass spectrometry 14C ages that have been calibrated and combined in probability distributions. This procedure gives an age of 730–664 cal yr B.C. for the “Microlite tephra” event and 1087–1006 cal yr B.C. for the Hekla 3 event. Accordingly, the tephra layers were deposited during the late Bronze Age. At this time, human settlement slowly increased pressure on the environment, as indicated by changes in woodland pollen composition at the two bogs. The tephra-marker horizons further show that the palynologically defined transition from the Subboreal to the Subatlantic Period is synchronous in the investigated area. However, the macroscopic visible marker in peat, the change from fibrous to sapric peat, the “Schwarztorf-Weißtorf-Kontakt,” is asynchronous. Bog vegetation did not immediately react in unison to a climatic change at this pollen zone boundary; instead, the timing of vegetation change depended on the location within the bog.  相似文献   

13.
Archaeological investigations in Camels Back Cave, western Utah, recovered a series of small-mammal bone assemblages from stratified deposits dating between ca. 12,000 and 500 14C yr B.P. The cave's early Holocene fauna includes a number of species adapted to montane or mesic habitats containing grasses and/or sagebrush (e.g., Lepus townsendii, Marmota flaviventris, Reithrodontomys megalotis, and Brachylagus idahoensis) which suggest that the region was relatively cool and moist until after 8800 14C yr B.P. Between ca. 8600 and 8100 14C yr B.P. these mammals became locally extinct, taxonomic diversity declined, and there was an increase in species well-adapted to xeric, low-elevation habitats, including ground squirrels, Lepus californicus and Neotoma lepida. The early small-mammal record from Camels Back Cave is similar to the 11,300–6000 14C yr B.P. mammalian sequence from Homestead Cave, northwestern Utah, and provides corroborative data on Bonneville Basin paleoenvironments and mammalian responses to middle Holocene desertification.  相似文献   

14.
A sediment core from Smorodinovoye Lake (SML), northeastern Siberia (area to the east of the Verkhoyansk Range) spanning the last 24,000 14C yr indicates that vegetational and climatic changes in the upper Indigirka basin resemble those in eastern Siberia (Lena basin and westward). For example, maximum postglacial summer temperatures at SML probably occurred 6000–4000 14C yr B.P., an age more in accordance with eastern than northeastern records. Larix arrived near the lake by 9600 14C yr B.P., approximately when forests expanded in the east but ca. 1500 14C yr later than forests were established in the neighboring upper Kolyma basin. Paleobotanical data further suggest that Larix possibly migrated southward from populations in the arctic lowlands of eastern Siberia and did not originate from interior refugia of the upper Kolyma basin. Although a Younger Dryas cooling has been noted in eastern Siberia, SML provides the first evidence from the northeast for a similar climatic reversal. Climatic variations seemingly have persisted between the Indigirka and Kolyma basins over at least the last 11,000 14C yr, despite the proximity of the two drainages and the occurrence of major changes in boundary conditions (e.g., seasonal insolation, sea levels) that have influenced other regional climatic patterns.  相似文献   

15.
Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,000 14C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,000 14C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,300 14C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,300 14C yr B.P.; the Twin Creeks 1 advance, 19,100–18,300 14C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.  相似文献   

16.
Approximately 70 km of new decimeter-resolution seismic reflection profile data from Owasco Lake, New York define a middle Holocene (4600 14C yr B.P.) erosion surface in the north end of the lake at water depths as great as 26 m. Beneath the lake, post-glacial sediments are up to 9 m thick and represent about 10% of the total sediment fill. Early to middle Holocene sediments, 6 m thick, contain biogenic gas at the south end of the basin and a large (4 km×300 m×15 m) subaqueous slide deposit along the east-central portion of the lake. Late Holocene sediments are thinner or absent, particularly at the north end of the lake. The middle Holocene erosion surface may have been produced by a drop in lake level. Alternatively, it may represent a change in climate during the transition between the relatively warm Holocene hypsithermal and cool neoglacial. At this time (4600 14C yr B.P.) circulation in Owasco Lake appears to have evolved from sluggish to active. The increased circulation, which persists today, probably resulted from atmospheric cold fronts with strong southwesterly winds that piled up water at the north end of the lake. The increased water circulation may have been ultimately driven by decreasing insolation, which produced an increased pole-to-equator thermal gradient and, thus, stronger global winds that began at the transition between the hypsithermal and neoglacial.  相似文献   

17.
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   

18.
High-resolution lithostratigraphy, mineral magnetic, carbon, pollen, and macrofossil analyses, and accelerator mass spectrometry 14C measurements were performed in the study of a sediment sequence from Lake Tambichozero, southeastern Russian Karelia, to reconstruct late-glacial and early Holocene aquatic and terrestrial environmental changes. The lake formed ca. 14,000 cal yr B.P. and the area around the lake was subsequently colonized by arctic plants, forming patches of pioneer communities surrounded by areas of exposed soil. A minor rise in lake productivity and the immigration of Betula pubescens occurred ca. 11,500 cal yr B.P. The rise in summer temperatures probably led to increased melting of remnant ice and enhanced erosion. The distinct increase in lake productivity and the development of open Betula-Populus forests, which are reconstructed based on plant macrofossil remains, indicate stable soils from 10,600 cal yr B.P. onward. Pinus and Picea probably became established ca. 9900 cal yr B.P.  相似文献   

19.
More than 30 fossil coleopteran (beetle) assemblages have been recorded from oxygen isotope stage 3 in northern Europe, comprising several hundred identified species. Using the mutual climatic range method for quantifying palaeotemperatures, these assemblages show that the climates of the times can be divided into two distinct interstadial types. One, a short phase of temperate/oceanic climate occurred between 43,000 and 42,000 14C yr B.P. when conditions were not much different from those of the present day. In spite of the temperate climate, trees did not colonize northwestern Europe at this time. Two, both before and after this temperate interlude there were phases of cold/continental climate during which summers were too cold for trees to grow and winters were of Siberian intensity. It is possible that periods of even colder conditions intervened from time to time between the interstadial but, because of the intensity of the cold, biological systems were so inhibited that no fossil evidence for them can be found in northern Europe. Evidence for them must be sought in the fossil records from the warmer parts of southern Europe or in the physical evidence in the Greenland ice cores.  相似文献   

20.
The Magnitude and Proximate Cause of Ice-Sheet Growth Since 35,000 yr B.P.   总被引：1，自引：0，他引：1  
The magnitude of late Wisconsinan (post-35,000 yr B.P.) ice-sheet growth in the Northern Hemisphere is not well known. Ice volume at 35,000 yr B.P. may have been as little as 20% or as much as 70% of the volume present at the last glacial maximum (LGM). A conservative evaluation of glacial–geologic, sea level, and benthic δ18O data indicates that ice volume at 35,000 yr B.P. was approximately 50% of that extant at the LGM (20,000 yr B.P.); that is, it doubled in about 15,000 yr. On the basis of literature for the North Atlantic and a sea-surface temperature (SST) data compilation, it appears that this rapid growth may have been forced by low-to-mid-latitude SST warming in both the Atlantic and Pacific Oceans, with attendant increased moisture transport to high latitudes. The SST ice-sheet growth notion also explains the apparent synchroneity of late Wisconsinan mountain glaciation in both hemispheres.  相似文献   

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