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1.
Macrofossil evidence indicates that the mid-Holocene hemlock [Tsuga canadensisL. (Carr.)] decline that occurred over a wide area in eastern North America was associated with phytophagous insect activity.In situhemlock macrofossils and insect remains found in a paludified dunefield at the northern limit of hemlock testify that two defoliation events occurred at 4910 ± 90 and 4200 ± 100 yr B.P., respectively. The sharp coincidence of remains from hemlock needles with chewing damage typical of hemlock looper feeding, head capsules from the hemlock looper (Lambdina fiscellaria) and the spruce budworm (Choristoneura fumiferana), absence of hemlock fruiting remains, and tree-ring anomalies in fossil hemlocks that died prematurely (<165 yr) suggest that defoliation affected hemlock reproductive capacity and pollen productivity, or more likely caused mass mortality. Our findings indicate that defoliation can affect ecosystems for centuries, especially when long-lived tree species are involved.  相似文献   

2.
This research aims at uncovering the stand-scale Holocene fire history of balsam fir forest stands from two bioclimatic zones of the boreal forest and assessing the existence of a sub-continental shift in past fire activity that could have triggered a change in the Holocene zonal pattern. In eastern Canada, the extant closed-crown boreal forest corresponds to two ecological regions separated along 49°N, the northern black spruce zone and the southern balsam fir zone. We sampled balsam fir stands from the southern fir zone (n = 7) and among the northernmost patches of fir forest located far beyond the fir zone boundary, into the spruce zone (n = 6). Macrofossil analysis of charcoal in mineral soils was used to reconstruct both the stand-scale and regional Holocene fire histories. Data were interpreted in the context of published palaeoecological evidence. Stands of the balsam fir zone were submitted to recurrent fire disturbances between c. 9000 and 5000 cal. yr B.P. Local fire histories suggested that four sites within the fir zone escaped fire during the Holocene. Such fire protected sites allowed the continuous maintenance of the balsam fir forest in the southern boreal landscape. Stands of the spruce zone have been affected by recurrent fires from 5000 cal. yr B.P. to present. Local fire histories indicated that no site escaped fire in this zone. Published palaeoecological data suggested that balsam fir migrated to its current northern limit sometime between 7300 and 6200 cal. yr B.P. A change of the fire regime 5000 years ago caused the regional decline of an historical northern balsam fir forest and its replacement by black spruce forest. The consequence was a sub-continental reshuffling of the fir and spruce zones within the closed-crown boreal forest. The macrofossil analysis of charcoal in mineral soils was instrumental to the reconstruction of stand-scale Holocene fire history at sites where no other in situ fire proxies were available.  相似文献   

3.
Quaternary deposits on the Pacific slope of Washington range in age from the earliest known interglaciation, the Alderton, through the Holocene. Pollen stratigraphy of these deposits is represented by 12 major pollen zones and is ostensibly continuous through Zone 8 over more than 47,000 radiocarbon yr. Before this, the stratigraphy is discontinuous and the chronology less certain. Environments over the time span of the deposits are reconstructed by the comparison of fossil and modern pollen assemblages and the use of relevant meteorological data. The Alderton Interglaciation is characterized by forests of Douglas fir (Pseudotsuga menziesii), alder (Alnus), and fir (Abies). During the next younger interglaciation, the Puyallup, forests were mostly of pine, apparently lodgepole (Pinus contorta), except midway in the interval when fir, western hemlock (Tsuga heterophylla), and Douglas fir temporarily replaced much of the pine. Vegetation outside the limits of Salmon Springs ice (>47,00034,000 yr BP) varied chiefly between park tundra and forests of western hemlock, spruce (Picea), and pine. The Salmon Springs nonglacial interval at the type locality records early park tundra followed by forests of pine and of fir. During the Olympia Interglaciation (34,00028,000 yr BP), pine invaded the Puget Lowland, whereas western hemlock and spruce became manifest on the Olympic Peninsula. Park tundra was widespread during the Fraser Glaciation (28,00010,000 yr BP) with pine becoming more important from about 15,000 to 10,000 yr BP. Holocene vegetation consisted first of open communities of Douglas fir and alder; later, closed forests succeeded, formed principally of western hemlock on the Olympic Peninsula and of western hemlock and Douglas fir in the Puget Lowland. Over the length of the reconstructed environmental record, climate shifted between cool and humid or relatively warm, semihumid forest types and cold, relatively dry tundra or park tundra types. During times of glaciation, average July temperatures are estimated to have been at least 7°C lower than today. Only during the Alderton Interglaciation and during the Holocene were temperatures higher for protracted periods that at present.  相似文献   

4.
This paper documents a continuous  44,000-yr pollen record derived from the Mfabeni Peatland on the Maputaland Coastal Plain. A detailed fossil pollen analysis indicates the existence of extensive Podocarpus-abundant coastal forests before  33,000 cal yr BP. The onset of wetter local conditions after this time is inferred from forest retreat and the development of swampy conditions. Conditions during the last glacial maximum ( 21,000 cal yr BP) are inferred to have been colder and drier than the present, as evidenced by forest retreat and replacement of swampy reed/sedge communities by dry grassland. Forest growth and expansion during the Holocene Altithermal ( 8000–6000 cal yr BP) indicates warm, relatively moist conditions. Previous records from Maputaland have suggested a northward migration of Podocarpus forest during the late Holocene. However, we interpret a mid-Holocene decline in Podocarpus at Mfabeni as evidence of deforestation. Forest clearance during the mid-Holocene is supported by the appearance of Morella serrata, suggesting a shift towards more open grassland/savanna, possibly due to burning. These signals of human impact are coupled with an increase in Acacia, indicative of the development of secondary forest and hence disturbance.  相似文献   

5.
High-resolution pollen analyses ( 50 yr) from sediment cores retrieved at Chernyshov Bay in the NW Large Aral Sea record shifts in vegetational development from subdesertic to steppe vegetation in the Aral Sea basin during the late Holocene. Using pollen data to quantify climatic parameters, we reconstruct and date for the first time significant changes in moisture conditions in Central Asia during the past 2000 yr. Cold and arid conditions prevailed between ca. AD 0 and 400, AD 900 and 1150, and AD 1500 and 1650 with the extension of xeric vegetation dominated by steppe elements. These intervals are characterized by low winter and summer mean temperatures and low mean annual precipitation (Pmm < 250 mm/yr). Conversely, the most suitable climate conditions occurred between ca. AD 400 and 900, and AD 1150 and 1450, when steppe vegetation was enriched by plants requiring moister conditions (Pmm  250–500 mm/yr) and some trees developed. Our results are fairly consistent with other late Holocene records from the eastern Mediterranean region and the Middle East, showing that regional rainfall in Central Asia is predominantly controlled by the eastern Mediterranean cyclonic system when the North Atlantic Oscillation (NAO) is in a negative phase.  相似文献   

6.
A laminated sequence (core BAP96-CP 24°38.12′N, 110°33.24′W; 390 m depth) from the Alfonso Basin in Bay of La Paz, southern Gulf of California, contains a record of paleoceanographic and paleoclimatic changes of the past 7900 yr. Radiolarian assemblages and magnetic susceptibility are used as proxies of oceanographic and climatic variability. The records provide a regional scenario of the middle and late Holocene, suggesting two major climatic regimes and several millennial-scale events. Conditions relatively warmer and drier than today occurred from 7700 to 2500 cal yr BP, promoting the intensification of evaporation processes and the prevalence of the Gulf of California water in the Basin. These conditions correlate with strong droughts in the middle Holocene of North America and with minimal incursion of tropical waters into the Gulf of California. Proxies indicate a warm scenario and the dominance of the Equatorial Surface Water in the Alfonso Basin from 2400 to 700 cal yr BP, suggesting the intensification of ENSO cycles. A climatic signal between 1038 and 963 cal yr BP may be correlated with global signal of the “Medieval Warm Period.” Several cooling events are recognized at 5730, 3360, 2700, 1280 and 820 cal yr BP and are associated with intensification of northwest winds leading to upwellings and enhanced productivity in the Basin.  相似文献   

7.
Pollen and organic-walled dinoflagellate cyst assemblages from core GeoB 9503-5 retrieved from the mud-belt ( 50 m water depth) off the Senegal River mouth have been analyzed to reconstruct short-term palaeoceanographic and palaeoenvironmental changes in subtropical NW Africa during the time interval from ca. 4200 to 1200 cal yr BP. Our study emphasizes significant coeval changes in continental and oceanic environments in and off Senegal and shows that initial dry conditions were followed by a strong and rapid increase in humidity between ca. 2900 and 2500 cal yr BP. After ca. 2500 cal yr BP, the environment slowly became drier again as indicated by slight increases in Sahelian savannah and desert elements in the pollen record. Around ca. 2200 cal yr BP, this relatively dry period ended with periodic pulses of high terrigenous contributions and strong fluctuations in fern spore and river plume dinoflagellate cyst percentages as well as in the fluxes of pollen, dinoflagellate cysts, fresh-water algae and plant cuticles, suggesting “episodic flash flood” events of the Senegal River. The driest phase developed after about 2100 cal yr BP.  相似文献   

8.
The late Quaternary paleoclimate of eastern Beringia has primarily been studied by drawing qualitative inferences from vegetation shifts. To quantitatively reconstruct summer temperatures, we analyzed lake sediments for fossil chironomids, and additionally we analyzed the sediments for fossil pollen and organic carbon content. A comparison with the δ18O record from Greenland indicates that the general climatic development of the region throughout the last glaciation–Holocene transition differed from that of the North Atlantic region. Between  17 and 15 ka, mean July air temperature was on average 5°C colder than modern, albeit a period of near-modern temperature at  16.5 ka. Total pollen accumulation rates ranged between  180 and 1200 grains cm− 2 yr− 1. At  15 ka, approximately coeval with the Bølling interstadial, temperatures again reached modern values. At  14 ka, nearly 1000 yr after warming began, Betula pollen percentages increased substantially and mark the transition to shrub-dominated pollen contributors. Chironomid-based inferences suggest no evidence of the Younger Dryas stade and only subtle evidence of an early Holocene thermal maximum, as temperatures from  15 ka to the late Holocene were relatively stable. The most recognizable climatic oscillation of the Holocene occurred from  4.5 to 2 ka.  相似文献   

9.
The mid-Holocene eastern hemlock [Tsuga canadensisL. (Carr.)] decline has been recently attributed to the activity of insect defoliators. N. Bihiry and L. Filion,Quaternary Research45,312–320 (1996). In this study, soil hydromorphic conditions were investigated for the period 6800–3200 yr B.P. using micromorphological data from a peat section from a swale in a paludified dunefield in southern Québec. After a short period of plant colonization in shallow pools between 6800 and 6400 yr B.P., mesic conditions predominated in the interdune before the decline (6400–4900 yr B.P.), as evidenced by strong bioturbation and abundance of excrements from the soil fauna. During the decline, a shift from mesic to wet conditions occurred (4900–4100 yr B.P.), although xeric to mesic conditions persisted on dune ridges until at least 4200 yr B.P. Wetness culminated when beaver occupied the site (4100–3750 yr B.P.). Hemlock needles with chewing damage typical of hemlock looper (Lambdina fiscellaria) feeding were identified at levels dated 4900, 4600, and 4200 yr B.P., respectively, implying that the hemlock decline was associated with at least three defoliation events. The ca. 400-yr interval between these events likely represents the time required for this late-sucessional tree species to recover.  相似文献   

10.
Holocene occurrences of conifer needle endophytes have not previously been reported. We report the fossil remains of Lophodermium piceae (Fckl.) Hoehn., a fungal endophyte of black spruce (Picea mariana (Mill.) B.S.P.) needles, in macrofossils dating back to 8000 cal yr BP. Spruce budworm head capsules and L. piceae remains were found preceding charcoal layers delineating the transformation of four spruce−moss forest sites to spruce−lichen woodland. As L. piceae is found solely on senescent needles, its increased presence during these transformation periods likely indicates that the forests were in decline due to the spruce budworm (Choristoneura fumiferana (Clem.)) when they burned. Future paleoecological studies incorporating needle fungi observations could be used to investigate the historical occurrence of tree disease and the role of fungi in forest health and decline.  相似文献   

11.
A high-resolution pollen and Pediastrum record, spanning 12,500 yr, is presented for Lake Bayanchagan (115.21°E, 41.65°N, and 1355 m a.s.l.), southern Inner Mongolia. Individual pollen taxa (PT-MAT) and the PFT affinity scores (PFT-MAT) were used for quantitative climatic reconstruction from pollen and algal data. Both techniques indicate that a cold and dry climate, similar to that of today, prevailed before 10,500 cal yr B.P. The wettest climate occurred between 10,500 and 6500 cal yr B.P., at which time annual precipitation was up to 30–60% higher than today. The early Holocene increases in temperature and precipitation occurred simultaneously, but mid-Holocene cooling started at approximately 8000 cal yr B.P., 1500 yr earlier than the drying. Vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type. The results suggest that this region was dominated by steppe vegetation throughout the Holocene, except for the period 9200 to 6700 cal yr B.P., when forest patches were relatively common. Inner Mongolia is situated at the limit of the present East Asian monsoon and patterns of vegetation and climate changes in that region during the Holocene probably reflect fluctuations in the monsoon's response to solar insolation variations. The early to middle Holocene monsoon undoubtedly extended to more northern latitudes than at present.  相似文献   

12.
The last ca. 20,000 yr of palaeoenvironmental conditions in Podocarpus National Park in the southeastern Ecuadorian Andes have been reconstructed from two pollen records from Cerro Toledo (04°22'28.6"S, 79°06'41.5"W) at 3150 m and 3110 m elevation. Páramo vegetation with high proportions of Plantago rigida characterised the last glacial maximum (LGM), reflecting cold and wet conditions. The upper forest line was at markedly lower elevations than present. After ca. 16,200 cal yr BP, páramo vegetation decreased slightly while mountain rainforest developed, suggesting rising temperatures. The trend of increasing temperatures and mountain rainforest expansion continued until ca. 8500 cal yr BP, while highest temperatures probably occurred from 9300 to 8500 cal yr BP. From ca. 8500 cal yr BP, páramo vegetation re-expanded with dominance of Poaceae, suggesting a change to cooler conditions. During the late Holocene after ca. 1800 cal yr BP, a decrease in páramo indicates a change to warmer conditions. Anthropogenic impact near the study site is indicated for times after 2300 cal yr BP. The regional environmental history indicates that through time the eastern Andean Cordillera in South Ecuador was influenced by eastern Amazonian climates rather than western Pacific climates.  相似文献   

13.
Pollen analysis of sediments from a high-altitude (4215 m), Neotropical (9°N) Andean lake was conducted in order to reconstruct local and regional vegetation dynamics since deglaciation. Although deglaciation commenced 15,500 cal yr B.P., the area around the Laguna Verde Alta (LVA) remained a periglacial desert, practically unvegetated, until about 11,000 cal yr B.P. At this time, a lycopod assemblage bearing no modern analog colonized the superpáramo. Although this community persisted until 6000 cal yr B.P., it began to decline somewhat earlier, in synchrony with cooling following the Holocene thermal maximum of the Northern Hemisphere. At this time, the pioneer assemblage was replaced by a low-diversity superpáramo community that became established 9000 cal yr B.P. This replacement coincides with regional declines in temperature and/or available moisture. Modern, more diverse superpáramo assemblages were not established until 4600 cal yr B.P., and were accompanied by a dramatic decline in Alnus, probably the result of factors associated with climate, humans, or both. Pollen influx from upper Andean forests is remarkably higher than expected during the Late Glacial and early to middle Holocene, especially between 14,000 and 12,600 cal yr B.P., when unparalleled high values are recorded. We propose that intensification of upslope orographic winds transported lower elevation forest pollen to the superpáramo, causing the apparent increase in tree pollen at high altitude. The association between increased forest pollen and summer insolation at this time suggests a causal link; however, further work is needed to clarify this relationship.  相似文献   

14.
Timpanogos Cave, located near the Wasatch fault, is about 357 m above the American Fork River. Fluvial cave sediments and an interbedded carbonate flowstone yield a paleomagnetic and U–Th depositional age of 350 to 780 ka. Fault vertical slip rates, inferred from calculated river downcutting rates, range between 1.02 and 0.46 mm yr− 1. These slip rates are in the range of the 0–12 Ma Wasatch Range exhumation rate ( 0.5–0.7 mm yr− 1), suggesting that the long-term vertical slip rate remained stable through mid-Pleistocene time. However, the late Pleistocene (0–250 ka) decelerated slip rate ( 0.2–0.3 mm yr− 1) and the accelerated Holocene slip rate ( 1.2 mm yr− 1) are consistent with episodic fault activity. Assuming that the late Pleistocene vertical slip rate represents an episodic slowing of fault movement and the long-term (0–12 Ma) average vertical slip rate, including the late Pleistocene and Holocene, should be  0.6 mm yr− 1, there is a net late Pleistocene vertical slip deficit of  50–75 m. The Holocene and late Pleistocene slip rates may be typical for episodes of accelerated and slowed fault movement, respectively. The calculated late Pleistocene slip deficit may mean that the current accelerated Wasatch fault slip rate will extend well into the future.  相似文献   

15.
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique,  NW–SE convergence between Africa and Eurasia at a rate of  5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran–Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (MW 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ1 at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW–ENE extension (σ3 at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)–Eurasian plate convergence are currently accommodated at the N-Algerian margin,  2 mm/yr in the Moroccan Atlas, and  2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as  2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran–Betic–Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the  3.5 mm/yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.  相似文献   

16.
Numerous cirques of the Lofoten–Vesterålen archipelago in northern Norway have distinct moraine sequences that previously have been assigned to the Allerød-Younger Dryas ( 13,400 to 11,700 yr BP) interval, constraining the regional distribution of the equilibrium-line altitude (ELA) of cirque and valley glaciers. Here we present evidence from a once glacier-fed lake on southern Andøya that contests this view. Analyses of radiocarbon dated lacustrine sediments including rock magnetic parameters, grain size, organic matter, dry bulk density and visual interpretation suggest that no glacier was present in the low-lying cirque during the Younger Dryas-Allerød. The initiation of the glacial retreat commenced with the onset of the Bølling warming ( 14,700 yr BP) and was completed by the onset of Allerød Interstade ( 13,400 yr BP). The reconstructed glacier stages of the investigated cirque coincide with a cool and dry period from  17,500 to 14,700 yr BP and a somewhat larger Last Glacial Maximum (LGM) advance possibly occurring between  21,050 and 19,100 yr BP.  相似文献   

17.
Stratigraphic studies of pollen and macrofossils from six sites at different elevations in the White Mountains of New Hampshire demonstrate changes in the distributions of four coniferous tree species during the Holocene. Two species presently confined to low elevations extended farther up the mountain slopes during the early Holocene: white pine grew 350 m above its present limit beginning 9000 yr B.P., while hemlock grew 300–400 m above its present limit soon after the species immigrated to the region 7000 yr. B.P. Hemlock disappeared from the highest sites about 5000 yr B.P., but both species persisted at sites 50–350 m above their present limits until the Little Ice Age began a few centuries ago. The history of the two main high-elevation conifers is more difficult to interpret. Spruce and fir first occur near their present upper limits 9000 or 10,000 yr B.P. Fir persisted in abundance at elevations similar to those where it occurs today throughout the Holocene, while spruce became infrequent at all elevations from the beginning of the Holocene until 2000 yr B.P. These facts suggest a more complex series of changes than a mere upward shift of the modern environmental gradient. Nevertheless, we conclude that the minimum climatic change which would explain the upward extensions of hemlock and white pine is a rise in temperature, perhaps as much as 2°C. The interval of maximum warmth started 9000 yr B.P. and lasted at least until 5000 yr B.P., correlative with the Prairie Period in Minnesota.  相似文献   

18.
Pollen analysis from a peat core 7.0 m in length, taken from a bog near Bisoca, in a mid-altitude area of the Buzăului Subcarpathian mountains, is used to reconstruct the postglacial vegetation history of the region. The vegetation record, which is supported by twelve 14C dates, starts at the end of the Late Glacial period. At the Late Glacial/Holocene transition, open vegetation was replaced by forest, suggesting a fast response to climatic warming. The Holocene began with the expansion of Betula, Pinus and Ulmus, followed, after 11,000 cal yr BP, by Fraxinus, Quercus, Tilia and Picea. The rapid expansion of these taxa may be due to their existence in the area during the Late Glacial period. At ca. 9200 cal yr BP, Corylus expanded, reaching a maximum after 7600 cal yr BP. The establishment of Carpinus occurred at ca. 7200 cal yr BP, with a maximum at ca. 5700 cal yr BP. Fagus pollen is regularly recorded after 7800 cal yr BP and became dominant at ca. 2000 cal yr BP. The first indications of human activities appear around 3800 cal yr BP.  相似文献   

19.
A comprehensive record of lake level changes in the Dead Sea has been reconstructed using multiple, well dated sediment cores recovered from the Dead Sea shore. Interpreting the lake level changes as monitors of precipitation in the Dead Sea drainage area and the regional eastern Mediterranean palaeoclimate, we document the presence of two major wet phases ( 10–8.6 and  5.6–3.5 cal kyr BP) and multiple abrupt arid events during the Holocene. The arid events in the Holocene Dead Sea appear to coincide with major breaks in the Near East cultural evolution (at  8.6, 8.2, 4.2, 3.5 cal kyr BP). Wetter periods are marked by the enlargement of smaller settlements and growth of farming communities in desert regions, suggesting a parallelism between climate and Near East cultural development.  相似文献   

20.
This paper explores the environmental conditions that faced the people of ancient Jawa during the Holocene, as well as previous prehistoric periods of the mid-late Pleistocene. Calcite speleothems in a lava tube are dated using the U-Th method, to marine oxygen isotope stage 7 from  250 to 240 ka and from  230 to  220 ka; and the stage 5/4 transition between  80 and 70 ka. The available evidence indicates general aridity of the Black Desert during most of the mid-late Quaternary, punctuated by short wetter periods, when the Mediterranean cyclonic systems intensified and penetrated the north Arabian Desert. These Mediterranean systems had a longer and more intense effect on the desert fringe closer to the Mediterranean and only rarely penetrated the Black Desert of Jawa. The results do not exclude some increase of rainfall which did not change water availability dramatically during the warm Holocene. The ancient Jawa city appears to have depended on technological ability to build elaborate runoff-collection systems, which became the prime condition for success.  相似文献   

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