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1.
A lake sediment core spanning 9900 years, collected from a small lake on western Victoria Island, NWT, Canada, provides a high-resolution record of diatom community dynamics over the Holocene. Ten radiocarbon dates and 210Pb dating provided the core chronology. Loss-on-ignition (LOI), magnetic susceptibility, biogenic silica content, and diatom concentrations provided information on changes in the sedimentary environment. LOI gradually increased over the Holocene whereas magnetic susceptibility showed an inverse trend. Biogenic silica content showed three distinct peaks spaced approximately 3000 years apart. Major shifts in diatom assemblages occurred at 8100–8000, 5800–5700, and 3800–3500 cal yr BP. There is evidence of diatom community response to centennial-scale variations such as the ‘Medieval Warm Period’ (1000–700 cal yr BP), ‘Little Ice Age’ (800–150 cal yr BP) and recent warming. Although recent changes in diatom community composition, productivity, and species richness are apparent they were surpassed at other periods throughout the Holocene. Variations of the taxa within the genera Staurosira, Pseudostaurosira, Fragilaria, and Staurosirella, usually combined into one genus in Arctic lake sediment studies, suggest these taxa provide useful insight into paleonvironmental questions.  相似文献   

2.
Two sediment cores from Mauritia flexuosa palm swamps have been studied by pollen and charcoal analysis. The cores Fazenda Cigana (FC) and Terra Indígena Aningal (TIA) were taken from a savanna-forest ecotone area in the Roraima State, northern Brazilian Amazon. Based on 5 radiocarbon dates, these records allow the reconstruction of the vegetation fire and climate dynamics during the past 1550 years. At the FC site was recorded a higher proportion of forest cover, suggesting local wetter climatic conditions favorable for forest expansion, especially by gallery forests, between 1550 and 1400 cal yr BP. Stands of M. flexuosa started to establish on the site indicating sufficient soil moisture. From 1400 to 1050 cal yr BP, forest cover retreated while savanna, and the Mauritia palm swamp expanded considerably. The FC site was marked by savanna and Mauritia cover with a slight increase of forest between ca. 1050 and 900 cal yr BP. From 900 to 300 cal yr BP the savanna and palm swamp taxa became dominant and the forest area decreased. At the TIA site the savanna cover was dominant between 1200 and 1000 cal yr BP. From 1000 to 700 forest expanded while savanna and Mauritia palm swamp reduced. Between 700 and 300 cal yr BP savanna and Mauritia palm swamp increased and forest area decreased. The high amount of charred particles found in the sediments, indicate fires with a marked increase between 1400 to 1000 cal yr BP (FC site) and 700 to 300 cal yr BP (TIA site), and probably caused the retreat of forest cover during these two time intervals. The relatively lower fire activity after 300 cal yr BP until present-day favored the increase of forested area at both TIA and FC sites. The arrival of the European settler and the subsequent introduction of cattle, is suggested as the main reason for the decrease of fire in the study region. The results point the fire caused by indigenous people as the principal controlling factor for forest and savanna dynamics during the past 1550 years.  相似文献   

3.
We reconstruct the Holocene range fluctuations of the European pond turtle (Emys orbicularis) for northern and Central Europe. Based on 529 subfossil records and radiocarbon dating of critical finds, we provide evidence that E. orbicularis experienced a rapid range expansion with Holocene warming. Phylogeographic data argue for a colonization of northern and Central Europe from a glacial refuge located in the south-eastern Balkans. Already in the Late Preboreal (9100–8600 cal BC) the species occurred in northern Central Europe and was widespread in Denmark and southern Sweden (Skåne) during the Boreal (8600–7100 cal BC). The maximum range extension occurred during the Holocene climatic optimum (Atlantic: 7100–3750 cal BC), with records in southern England, middle Sweden, and northern Estonia. After the early Subboreal (3750–1750 cal BC) northern populations collapsed, coinciding with a climatic cooling and oceanization. Northern populations were most likely established via long distance dispersal (rafting using rivers debouching in the North and Baltic Seas); after northern Europe was reached, a minimum dispersal of 50 km per 100 years may be hypothesized for the Boreal.  相似文献   

4.
In this paper we present geological evidence from the Larsemann Hills (Lambert Glacier – Amery Ice Shelf region, East Antarctica) of marine sediments at an altitude of c. 8 m a.s.l., as revealed by diatom, pigment and geochemical proxies in a lake sediment core. The sediments yielded radiocarbon dates between c. 26 650 and 28 750 14C yr BP (31 366–33 228 cal yr BP). This information can be used to constrain relative sea level adjacent to the Lambert Glacier at the end of Marine Isotope Stage 3. These data are compared with the age and altitude of Marine Isotope Stage 3 marine deposits elsewhere in East Antarctica and discussed with reference to late Quaternary ice sheet history and eustatic sea-level change.  相似文献   

5.
Paleoclimatic records from the climatically sensitive Canadian prairies are relatively rare due to the scarcity of study sites with continuous Holocene stratigraphic sequences. Oro Lake, a meromictic lake in the dry grasslands of Saskatchewan (Canada), contains a continuous Holocene diatom record spanning the last 10,000 years. Here we present analyses at three different time scales and resolution: (1) 1–3 yr resolution of the past 80 years, (2) century-scale analysis of the Holocene, and (3) decadal-scale analysis of the past 7000 years. Recent changes in the diatom assemblages and their respective salinity inferences were significantly related to measured effective moisture (precipitation minus evaporation, P−ET). The droughts of the 1930s, and a wet period during the 1950s are clearly evident in the diatom record, suggesting the Oro Lake record contains a sensitive archive of past climatic conditions. Century-scale analysis of the diatom record during the Holocene is consistent with a cool and moist climate in the early Holocene (prior to ca 9700 cal yr BP, 8600 14C yr BP). An abrupt increase in diatom-inferred salinity at 9600 cal yr BP (8500 14C yr BP) indicates the onset of an arid climate, with continuing arid conditions throughout the mid-Holocene. Decadal-scale analysis of the past 7000 years suggests that the mid-Holocene was more complex, with extended periods of increased variability in precipitation, particularly between ca 5800–3600 cal yr BP (5000–3200 14C yr BP) which is characterized by intervals of increased effective moisture. The past 2000 years is characterized by reduced salinities and generally wetter conditions in comparison to the mid-Holocene. The combination of the different scales of analyses in this study provides a detailed account of the dynamic nature of climate from sub-decadal to millennial scale in the Oro Lake region within the Palliser Triangle. Climate model predictions suggest that the Canadian prairie region may see a higher frequency of extreme droughts under projected global warming, potentially similar to the most arid periods seen during the mid-Holocene when many lake basins completely dried out.  相似文献   

6.
High resolution analyses of a sediment sequence from an in-filled lake on Nightingale Island at 37°S in the South Atlantic have revealed hydrological variations during the last 10 700 cal yr BP, which are linked to shifts in the Southern Hemisphere westerlies and the Meridional Overturning Circulation (MOC) in the Atlantic. The lowermost part of the sequence, 10 700–8600 cal yr BP, consists of peat. The peat is overlaid by gyttja, which continues up to the final overgrowing of the basin around 500 years ago. The transition from peat to gyttja at 8600 cal yr BP implies a significant increase in effective humidity, probably caused by a northward shift of the Southern Hemisphere westerlies after the end of the Antarctic thermal optimum. Recurring phases of increased erosion with high deposition of minerogenic particles, terrestrial organic matter (high C/N ratios), and pollen grains from plant taxa growing in more distal parts of the catchment, indicate periods with higher precipitation. These high precipitation periods correspond to periods of southward shifted Southern Hemisphere Westerlies inferred from South America and with periods of weaker MOC in the North Atlantic. Therefore we conclude that these periods, also characterized by decreased influence from marine aerosols, were not caused by the stronger westerlies, but by weaker MOC and less heat transport from the South to the North Atlantic causing warmer sea surface temperatures (SSTs) and increased precipitation in the South Atlantic. This is the expression of the bipolar seesaw mechanism and implies that it may have been operational also during interglacial conditions.  相似文献   

7.
《Quaternary Science Reviews》2007,26(9-10):1416-1431
Accumulating palaeobotanical evidence points to agricultural activity in Central Europe well before the onset of the Neolithic, commonly dated at ca 5500–5200 cal BC. We reinvestigated an existing pollen profile from Soppensee with refined taxonomical resolution by further subdividing the Cerealia pollen type into Triticum t. and Avena t. because the sediments at this site currently provide the highest temporal resolution and precision for the period of interest among all sites in Switzerland. Our new results are in agreement with previous high-resolution investigations from Switzerland showing scattered but consistent presence of pollen of Cerealia, Plantago lanceolata, and other cultural plants or weeds during the late Mesolithic period (6700–5500 cal BC). Chronologically, this palynological evidence for sporadic agricultural activities coincides with a major break in material culture at ca 6700 cal BC (i.e. the transition from early to late Mesolithic). Here, we review possible arguments against palaeobotanical evidences of Mesolithic agriculture (e.g. chronological uncertainties, misidentification, contamination, long-distance transport) and conclude that none of these can explain the consistent pollen pattern observed at several sites. The palynological evidence can, of course, not prove the existence of pre-ceramic agriculture in Central Europe. However, it is so coherent that this topic should be addressed by systematic archaeobotanical analyses in future archaeological studies. If our interpretation should turn out to be true, our conclusions would have fundamental implications for the Neolithic history of Europe. Currently, it is intensely debated whether Central European agriculture developed locally under the influence of incoming ideas from areas where Neolithic farming had already developed earlier (e.g. southeastern Europe) or whether it was introduced by immigrating farmers. On the basis of our results, we suggest that agriculture developed locally throughout the late Mesolithic and Neolithic. Mesolithic trading networks connecting Southern and Central Europe also support the hypothesis of a slow and gradual change towards sessile agriculture, probably as a result of incoming ideas and regional cultural transformation.  相似文献   

8.
Atmospheric radiocarbon variations over the Younger Dryas interval, from 13,000 to 11,600 cal yr BP, are of immense scientific interest because they reveal crucial information about the linkages between climate, ocean circulation and the carbon cycle. However, no direct and reliable atmospheric 14C records based on tree rings for the entire Younger Dryas have been available. In this paper, we present (1) high-precision 14C measurements on the extension of absolute tree-ring chronology from 12,400 to 12,560 cal yr BP and (2) high-precision, high-resolution atmospheric 14C record derived from a 617-yr-long tree-ring chronology of Huon pine from Tasmania, Australia, spanning the early Younger Dryas. The new tree-ring 14C records bridge the current gap in European tree-ring radiocarbon chronologies during the early Younger Dryas, linking the floating Lateglacial Pine record to the absolute tree-ring timescale. A continuous and reliable atmospheric 14C record for the past 14,000 cal yr BP including the Younger Dryas is now available. The new records indicate that the abrupt rise in atmospheric Δ14C associated with the Younger Dryas onset occurs at 12,760 cal yr BP, 240 yrs later than that recorded in Cariaco varves, with a smaller magnitude of 40‰ followed by several centennial Δ14C variations of 20–25‰. Comparing the tree-ring Δ14C to marine-derived Δ14C and modelled Δ14C based on ice-core 10Be fluxes, we conclude that changes in ocean circulation were mainly responsible for the Younger Dryas onset, while a combination of changes in ocean circulation and 14C production rate were responsible for atmospheric Δ14C variations for the remainder of the Younger Dryas.  相似文献   

9.
Tufa domes and towers are common around the margins of Winnemucca Dry Lake, Nevada, USA, a desiccated sub‐basin of pluvial Lake Lahontan. A 2·5 m diameter concentrically‐layered tufa mound from the southern end of the playa was sampled along its growth axis to determine timing, rate and geochemical conditions of tufa growth. A radiocarbon‐based age model indicates an 8200‐year tufa depositional record that begins near the end of the Last Glacial Maximum (ca 23 400 cal yr bp ) and concludes at the end of the most recent Lahontan highstand (ca 15 200 cal yr bp ). Petrography, stable isotopes and major and minor elemental compositions are used to evaluate the rate and timing of tufa growth in the context of the depositional environment. The deposit built radially outward from a central nucleation point, with six decimetre‐scale layers defined by variations in texture. Two distinct tufa types are observed: the inner section is composed of two layers of thinolite pseudomorphs after ikaite, with the innermost layer comprised of very small pseudomorphs (<0·25 cm) and an outer layer composed of larger, ca 3 cm long pseudomorphs, followed by a transitional layer where thinolite pseudomorphs grade into calcite fans. The outer section consists of three distinct layers of thrombolitic micrite with a branching mesofabric. The textural change occurred as lake levels began to rise towards the most recent Lahontan highstand interval and probably was prompted by warming of lake waters caused by increased groundwater flux during highstand lake levels. The Mg/Ca and Sr/Ca variations suggest a warming trend in the tufa growth environment and may also reflect increasing growth rates of tufa associated with increased fluxes of groundwater. This systematic study of tufa deposition indicates the importance of the hydrology of the lacustrine tufa system for reconstructing palaeoenvironmental records, and particularly the interaction of ground and surface waters.  相似文献   

10.
《Sedimentology》2018,65(5):1777-1799
Sequences of lake sediments often form long and continuous records that may be sensitive recorders of seismic shaking. A multi‐proxy analysis of Lake Bohinj sediments associated with a well‐constrained chronology was conducted to reconstruct Holocene seismic activity in the Julian Alps (Slovenia). A seismic reflection survey and sedimentological analyses identified 29 homogenite‐type deposits related to mass‐wasting deposits. The most recent homogenites can be linked to historical regional earthquakes (i.e. 1348 ad , 1511 ad and 1690 ad ) with strong epicentral intensity [greater than ‘damaging’ (VIII ) on the Medvedev–Sponheuer–Karnik scale]. The correlation between the historical earthquake data set and the homogenites identified in a core isolated from local stream inputs, allows interpretation of all similar deposits as earthquake related. This work extends the earthquake chronicle of the last 6600 years in this area with a total of 29 events recorded. The early Holocene sedimentary record is disturbed by a seismic event (6617 ± 94 cal yr bp ) that reworked previously deposited sediment and led to a thick sediment deposit identified in the seismic survey. The period between 3500 cal yr bp and 2000 cal yr bp is characterized by a major destabilization in the watershed by human activities that led to increases in erosion and sedimentation rates. This change increased the lake's sensitivity to recording an earthquake (earthquake‐sensitivity threshold index) with the occurrence of 72 turbidite‐type deposits over this period. The high turbidite frequency identified could be the consequence of this change in lake earthquake sensitivity and thus these turbidites could be triggered by earthquake shaking, as other origins are discarded. This study illustrates why it is not acceptable to propose a return period for seismic activity recorded in lake sediment if the sedimentation rate varies significantly.  相似文献   

11.
Biodiversity loss, climate change, and increased freshwater consumption are some of the main environmental problems on Earth. Mountain ecosystems can reduce these threats by providing several positive influences, such as the maintenance of biodiversity, water regulation, and carbon storage, amongst others. The knowledge of the history of these environments and their response to climate change is very important for management, conservation, and environmental monitoring programs. The genesis of the soil organic matter of the current upper montane vegetation remains unclear and seems to be quite variable depending on location. Some upper montane sites in the very extensive coastal Sea Mountain Range present considerable organic matter from the late Pleistocene and other from only the Holocene. Our study was carried out on three soil profiles (two cores in grassland and one in forest) on the Caratuva Peak of the Serra do Ibitiraquire (a sub-range of Sea Mountain Range – Serra do Mar) in Southern Brazil. The δ13C isotopic analyses of organic matter in soil horizons were conducted to detect whether C3 or C4 plants dominated the past communities. Complementarily, we performed a pollen analysis and 14C dating of the humin fraction to obtain the age of the studied horizons. Except for a short and probably drier period (between 6000 and 4500 cal yr BP), C3 plants, including ombrophilous grasses and trees, have dominated the highlands of the Caratuva Peak (Pico Caratuva), as well as the other uppermost summits of the Serra do Ibitiraquire, since around 9000 cal yr BP. The Caratuva region represents a landscape of high altitude grasslands (campos de altitude altomontanos or campos altomontanos) and upper montane rain/cloud forests with soils that most likely contain some organic matter from the late Pleistocene, as has been reported in Southern and Southeastern Brazil for other sites. However, our results indicate that the studied deposits (near the summit) are from the early to late Holocene, when somewhat wetter and warmer conditions (since around 9000 cal yr BP) enabled a stronger colonization of the ridge of Pico Caratuva by mainly C3 plants, especially grassland species. However, at the same time, even near the summit, the soil core from the forest site already presented the current physiognomy (or a shrubby/elfin or successional forest), indicating that the colonization of the neighboring uppermost saddles and valleys were probably populated mainly by upper montane forest species.  相似文献   

12.
Fossil Chironomidae assemblages (with a few Chaoboridae and Ceratopogonidae) from Zagoskin and Burial Lakes in western Alaska provide quantitative reconstructions of mean July air temperatures for periods of the late-middle Wisconsin (～39,000–34,000 cal yr B.P.) to the present. Inferred temperatures are compared with previously analyzed pollen data from each site summarized here by indirect ordination. Paleotemperature trends reveal substantial differences in the timing of climatic warming following the late Wisconsin at each site, although chronological uncertainty exists. Zagoskin Lake shows early warming beginning at about 21,000 cal yr B.P., whereas warming at Burial Lake begins ～4000 years later. Summer climates during the last glacial maximum (LGM) were on average ～3.5 °C below the modern temperatures at each site. Major shifts in vegetation occurred from ～19,000 to 10,000 cal yr B.P. at Zagoskin Lake and from ～17,000 to 10,000 cal yr B.P. at Burial Lake. Vegetation shifts followed climatic warming, when temperatures neared modern values. Both sites provide evidence of an early postglacial thermal maximum at ～12,300 cal yr B.P. These chironomid records, combined with other insect-based climatic reconstructions from Beringia, indicate that during the LGM: (1) greater continentality likely influenced regions adjacent to the Bering Land Bridge and (2) summer climates were, at times, not dominated by severe cold.  相似文献   

13.
The multi-proxy analysis of sediment cores recovered in karstic Lake Estanya (42°02′ N, 0°32′ E; 670 m a. s. l., NE Spain), located in the transitional area between the humid Pyrenees and the semi-arid Central Ebro Basin, provides the first high-resolution, continuous sedimentary record in the region, extending back the last 21 000 years. The integration of sedimentary facies, elemental and isotopical geochemistry and biogenic silica, together with a robust age model based on 17 AMS radiocarbon dates, enables precise reconstruction of the main hydrological and environmental changes in the region during the last deglaciation.Arid conditions, represented by shallow lake levels, predominantly saline waters and reduced organic productivity occurred throughout the last glacial maximum (21–18 cal kyrs BP) and the lateglacial, reaching their maximum intensity during the period 18–14.5 cal kyrs BP (including Heinrich event 1) and the Younger Dryas (12.9–11.6 cal kyrs BP). Less saline conditions characterized the 14.5–12.6 cal kyrs BP period, suggesting higher effective moisture during the Bölling/Allerød. The onset of more humid conditions started at 9.4 cal kyrs, indicating a delayed hydrological response to the onset of the Holocene which is also documented in several sites of the Mediterranean Basin. Higher, although fluctuating, Holocene lake levels were punctuated by a mid Holocene arid period between 4.8 and 4.0 cal kyrs BP. A major lake-level rise occurred at 1.2 cal kyrs BP, conducive to the establishment of conditions similar to the present and interrupted by a last major water level drop, occurring around 800 cal yrs BP, which coincides with the Medieval Climate Anomaly.The main hydrological stages in Lake Estanya are in phase with most Western Mediterranean and North Atlantic continental and marine records, but our results also show similarities with other Iberian and northern African reconstructions, emphasizing peculiarities of palaeohydrological evolution of the Iberian Peninsula during the last deglaciation.  相似文献   

14.
Neolithic seafaring across the Taiwan Strait began approximately 5000 years ago and involved open-sea voyages over distances of at least 130 km. Rapid sea-level rise preceded the emergence of open-sea voyaging, but the possible role of environmental change as a stimulus for the development of seafaring is poorly understood. We investigate this problem by presenting a record of Holocene sea-level change and coastal transformation based on sediment cores obtained from the Fuzhou Basin on the coast of Fujian, China. The cores are located in direct proximity to archaeological sites of the Tanshishan Neolithic culture (5000–4300 cal BP), which is significant for its similarity to the earliest Neolithic cultures of Taiwan. Multiple lines of evidence record the early Holocene inundation of the Fuzhou Basin around 9000 cal BP, the mid-Holocene sea-level highstand, and the final Holocene marine transgression. This final transition is precisely documented, with AMS dates showing the change occurred close to 1900 cal BP. Our paleogeographic reconstruction shows that a large estuary filled the Fuzhou Basin during the mid-Holocene. Tanshishan and Zhuangbianshan, two of the major Fuzhou Basin Neolithic sites, are located today on hills nearly 80 km from the modern coastline. However, when the sites were settled around 5500–5000 cal BP, the marine transgression had transformed these hills into islands in the upper estuary. We suggest that the Neolithic era estuary setting, together with the lack of land suitable for rice paddy agriculture, inhibited intensive food production but favored a maritime orientation and the development of seafaring.  相似文献   

15.
Atle Nesje   《Quaternary Science Reviews》2009,28(21-22):2119-2136
During the early Holocene abrupt, decadal to centennial-scale climate variations caused significant glacier variations in Norway. Increased freshwater inflow to the North Atlantic and Arctic Oceans has been suggested as one of the most likely mechanisms to explain the abrupt and significant Lateglacial and early Holocene climatic events in NW Europe. The largest early Holocene glacier readvances occurred 11,200, 10,500, 10,100, 9700, 9200 and 8400–8000 cal. yr BP. The studied Norwegian glaciers apparently melted away at least once during the early/mid-Holocene. The period with the most contracted glaciers in Scandinavia was between 6600 and 6000 cal. yr BP. Subsequent to 6000 cal. yr BP the glaciers started to advance and the most extensive glaciers existed at about 5600, 4400, 3300, 2300, 1600 cal. yr BP, and during the ‘Little Ice Age’. Times with overall less glacier activity were apparently around 5000, 4000, 3000, 2000, and 1200 cal. yr BP. It has been proposed that several glacier advances occurred in Scandinavia (including northern Sweden) at 8500–7900, 7400–7200, 6300–6100, 5900–5800, 5600–5300, 5100–4800, 4600–4200, 3400–3200, 3000–2800, 2700–2000, 1900–1600, 1200–1000, and 700–200 cal. yr BP. Glaciers in northern Sweden probably reached their greatest ‘Little Ice Age’ extent between the 17th and the beginning of the 18th centuries. Evidence for early Holocene glacier advances in northern Scandinavia, however, has been questioned by more recent, multi-disciplinary studies. The early to mid-Holocene glacier episodes in northern Sweden may therefore be questioned.Most Norwegian glaciers attained their maximum ‘Little Ice Age’ extent during the mid-18th century. Cumulative glacier length variations in southern Norway, based on marginal moraines dated by lichenometry and historic evidence, show an overall retreat from the mid-18th century until the 1930s–40s. Subsequently, most Norwegian glaciers retreated significantly. Maritime outlet glaciers with short frontal time lags (<10–15 years) started to advance in the mid-1950s, whereas long outlet glaciers with longer frontal time lags (>15–20 years) continued their retreat to the 1970s and 1980s. However, maritime glaciers started to advance as a response to higher winter accumulation during the first part of the 1990s. After 2000 several of the observed glaciers have retreated remarkably fast (annual frontal retreat > 100 m) mainly due to high summer temperatures. The general glacier retreat during the early Holocene and the Neoglacial advances after 6000 cal. yr BP are in line with orbital forcing, due to the decrease of Northern Hemisphere summer solar insolation and the increase in winter insolation. In addition, regional weather modes, such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), play a significant role with respect to decadal and multi-decadal climate variability.  相似文献   

16.
Fire and vegetation records at the City of Rocks National Reserve (CIRO), south-central Idaho, display the interaction of changing climate, fire and vegetation along the migrating front of single-leaf pinyon (Pinus monophylla) and Utah juniper (Juniperus osteosperma). Radiocarbon dating of alluvial charcoal reconstructed local fire occurrence and geomorphic response, and fossil woodrat (Neotoma) middens revealed pinyon and juniper arrivals. Fire peaks occurred ~ 10,700–9500, 7200–6700, 2400–2000, 850–700, and 550–400 cal yr BP, whereas ~ 9500–7200, 6700–4700 and ~ 1500–1000 cal yr BP are fire-free. Wetter climates and denser vegetation fueled episodic fires and debris flows during the early and late Holocene, whereas drier climates and reduced vegetation caused frequent sheetflooding during the mid-Holocene. Increased fires during the wetter and more variable late Holocene suggest variable climate and adequate fuels augment fires at CIRO. Utah juniper and single-leaf pinyon colonized CIRO by 3800 and 2800 cal yr BP, respectively, though pinyon did not expand broadly until ~ 700 cal yr BP. Increased fire-related deposition coincided with regional droughts and pinyon infilling ~ 850–700 and 550–400 cal yr BP. Early and late Holocene vegetation change probably played a major role in accelerated fire activity, which may be sustained into the future due to pinyon–juniper densification and cheatgrass invasion.  相似文献   

17.
The Mediterranean climate and its variability depend on global-scale climate patterns. Close correlations appear when comparing Holocene palaeoenvironmental data (lake levels, fluvial activity, Mediterranean surface temperature and salinity, marine sedimentation) with the main stages of the history of the circum-Mediterranean vegetation. They indicate an evolution of the Mediterranean biome controlled by the climate and emphasize the teleconnections between the climate of the Mediterranean area and the global climatic system. In the circum-Mediterranean area, the Holocene can be divided into three periods: a lower humid Holocene (11 500–7000 cal BP) interrupted by dry episodes; a transition phase (7000–5500 cal BP) during which occurred a decrease in insolation as well as the installation of the present atmosphere circulation in the northern hemisphere; and an upper Holocene (5500 cal BP—present) characterized by an aridification process. Throughout the Holocene, humans used and modified more or less strongly the environment but the climatic changes were the determining factors of the evolution of the Mediterranean biome. Societies had to adapt to natural environmental variations, their impact on the environment increasing the ecological consequences of the global changes.  相似文献   

18.
This geoarcheological study investigates soil stratigraphy and geochronology of alluvial deposits to determine Holocene landscape evolution within the Hot Creek, La Jara Creek, and Alamosa River drainage basins in the San Juan Mountains of Colorado. Geomorphic mapping and radiocarbon dating indicate synchronicity in patterns of erosion, deposition, and stability between drainage basins. In all three basins, the maximum age of mapped alluvial terraces and fans is ~ 3300 cal yr BP. A depositional period seen at both Hot Creek and the Alamosa River begins ~ 3300 to 3200 cal yr BP. Based on soil development, short periods of stability followed by alluvial fan aggradation occur in the Alamosa River basin ~ 2200 cal yr BP. A period of landscape stability at Hot Creek before ~ 1100 cal yr BP is followed by a period of rapid aggradation within all three drainages between ~ 1100 and 850 cal yr BP. A final aggradation event occurred between ~ 630 and 520 cal yr BP at La Jara Creek. These patterns of landscape evolution over the past ~ 3300 yr provide the framework for an archeological model that predicts the potential for buried and surficial cultural materials in the research area.  相似文献   

19.
The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holocene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Holocene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid-Holocene shorelines in the small lakes (4–110 ha) were as much as 10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca 7000–4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability.  相似文献   

20.
The Sierra Nevada of southern Spain is a landscape with a rich biological and cultural heritage. The range was extensively glaciated during the late Pleistocene. However, the postglacial paleoecologic history of the highest range in southern Europe is nearly completely unknown. Here we use sediments from a small lake above present treeline – Laguna de Río Seco at 3020 m elevation – in a paleoecological study documenting over 11,500 calendar years of vegetation, fire and climate change, addressing ecological and paleoclimatic issues unique to this area through comparison with regional paleoecological sequences. The early record is dominated by Pinus pollen, with Betula, deciduous Quercus, and grasses, with an understory of shrubs. It is unlikely that pine trees grew around the lake, and fire was relatively unimportant at this site during this period. Aquatic microfossils indicate that the wettest conditions and highest lake levels at Laguna de Río Seco occurred before 7800 cal yr BP. This is in contrast to lower elevation sites, where wettest conditions occurred after ca 7800. Greater differences in early Holocene seasonal insolation may have translated to greater snowpack and subsequently higher lake levels at higher elevations, but not necessarily at lower elevations, where higher evaporation rates prevailed. With declining seasonality after ca 8000 cal yr BP, but continuing summer precipitation, lake levels at the highest elevation site remained high, but lake levels at lower elevation sites increased as evaporation rates declined. Drier conditions commenced regionally after ca 5700 cal yr BP, shown at Laguna de Río Seco by declines in wetland pollen, and increases in high elevation steppe shrubs common today (Juniperus, Artemisia, and others). The disappearance or decline of mesophytes, such as Betula from ca 4000 cal yr BP is part of a regional depletion in Mediterranean Spain and elsewhere in Europe from the mid to late Holocene. On the other hand, Castanea sativa increased in Laguna de Río Seco record after ca 4000 cal yr BP, and especially in post-Roman times, probably due to arboriculture. Though not as important at high than at low elevations, fire occurrence was elevated, particularly after ca 3700 years ago, in response to regional human population expansion. The local and regional impact of humans increased substantially after ca 2700 years ago, with the loss of Pinus forest within the mountain range, increases in evidence of pasturing herbivores around the lake, and Olea cultivation at lower elevations. Though human impact was not as extensive at high elevation as at lower elevation sites in southern Iberia, this record confirms that even remote sites were not free of direct human influence during the Holocene.  相似文献   

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