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41.
Mahboobeh FEREYDOONPOUR Hossein VAZIRI-MOGHADDAM and Azizollah TAHERI 《《地质学报》英文版》2014,88(6):1681-1695
Abstract: A rich assemblage of planktonic foraminifera has been studied from an outcrop of the Gurpi Formation, the hydrocarbon source rock in the southwest Iran, Deh Dasht area (Kuh-e Siah anticline). Based on the distribution of the planktonic foraminifera, eight biozones have been recognized that included: Dicarinella concavata Interval Zone (Earliest Santonian), Dicarinella asymetrica Total Range Zone (Santonian to Earliest Campanian), Globotruncanita elevata Partial Range Zone (Early Campanian), Globotruncana ventricosa Interval Zone (Middle to Late Campanian), Radotruncana calcarata Total Range Zone (Late Campanian), Globotruncanella havanensis Partial Range Zone (Late Campanian), Globotruncana aegyptiaca Interval Zone (Late to latest Campanian), Gansserina gansseri Interval Zone (Latest Campanian to Early Maastrichtian). These biozones indicates that the Gurpi Formation deposited during the Early Santonian- Early Maastrichtian. These biozones are compared to the most standard biozones defined in Tethysian domain. Based on distribution of morphotype groups of planktonic foraminifera, planktonic to benthic ratio (P/B) and content of carbonate, nine third-order sequences are recognized. 相似文献
42.
Bronwen S. Whitney Ruth Dickau Francis E. Mayle J. Daniel Soto José Iriarte 《Quaternary Research》2013
We present a multiproxy study of land use by a pre-Columbian earth mounds culture in the Bolivian Amazon. The Monumental Mounds Region (MMR) is an archaeological sub-region characterized by hundreds of pre-Columbian habitation mounds associated with a complex network of canals and causeways, and situated in the forest–savanna mosaic of the Llanos de Moxos. Pollen, phytolith, and charcoal analyses were performed on a sediment core from a large lake (14 km2), Laguna San José (14°56.97′S, 64°29.70′W). We found evidence of high levels of anthropogenic burning from AD 400 to AD 1280, corroborating dated occupation layers in two nearby excavated habitation mounds. The charcoal decline pre-dates the arrival of Europeans by at least 100 yr, and challenges the notion that the mounds culture declined because of European colonization. We show that the surrounding savanna soils were sufficiently fertile to support crops, and the presence of maize throughout the record shows that the area was continuously cultivated despite land-use change at the end of the earth mounds culture. We suggest that burning was largely confined to the savannas, rather than forests, and that pre-Columbian deforestation was localized to the vicinity of individual habitation mounds, whereas the inter-mound areas remained largely forested. 相似文献
43.
Cathy Whitlock Maria Martha Bianchi Vera Markgraf Megan Walsh 《Quaternary Research》2006,66(2):187-201
The history of the low-elevation forest and forest-steppe ecotone on the east side of the Andes is revealed in pollen and charcoal records obtained from mid-latitude lakes. Prior to 15,000 cal yr BP, the vegetation was characterized by steppe vegetation with isolated stands of Nothofagus. The climate was generally dry, and the sparse vegetation apparently lacked sufficient fuels to burn extensively. After 15,000 cal yr BP, a mixture of Nothofagus forest and shrubland/steppe developed. Fire activity increased between 13,250 and 11,400 cal yr BP, contemporaneous with a regionally defined cold dry period (Huelmo/Mascardi Cold Reversal). The early-Holocene period was characterized by an open Nothofagus forest/shrubland mosaic, and fire frequency was high in dry sites and low in wet sites; the data suggest a sharp decrease in moisture eastward from the Andes. A shift to a surface-fire regime occurred at 7500 cal yr BP at the wet site and at 4400 cal yr BP at the dry site, preceding the expansion of Austrocedrus by 1000-1500 yr. The spread of Austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late Holocene. The change to a surface-fire regime is consistent with increased interannual climate variability and the onset or strengthening of ENSO. The present-day mixed forest dominated by Nothofagus and Austrocedrus was established in the last few millennia. 相似文献
44.
Holocene fire-climate-vegetation linkages are mostly understood at individual sites by comparing charcoal and pollen records with other paleoenvironmental proxy and model simulations. This scale of reconstruction often obscures detection of large-scale patterns in past fire activity that are related to changes in regional climate and vegetation. A network of 31 charcoal records from southern South America was examined to assess fire history along a transect from subtropic to subantarctic biomes. The charcoal data indicate that fire activity was greater than present at ca. 12,000 cal yr BP and increased further and was widespread at 9500 cal yr BP. Fire activity decreased and became more spatially variable by 6000 cal yr BP, and this trend continued to present. Atmospheric circulation anomalies during recent high-fire years show a southward shift in westerlies, and paleoclimate model simulations and data syntheses suggest that such conditions may have prevailed for millennia in the early Holocene when the pole-to-equator temperature gradients were weaker and annual temperatures were higher than present, in response to orbital-time-scale insolation changes. 相似文献
45.
The environmental history of the Northern Rocky Mountains was reconstructed using lake sediments from Burnt Knob Lake, Idaho, and comparing the results with those from other previously published sites in the region to understand how vegetation and fire regimes responded to large-scale climate changes during the Holocene. Vegetation reconstructions indicate parkland or alpine meadow at the end of the glacial period indicating cold-dry conditions. From 14,000 to 12,000 cal yr B.P., abundant Pinus pollen suggests warmer, moister conditions than the previous period. Most sites record the development of a forest with Pseudotsuga ca. 9500 cal yr B.P. indicating warm dry climate coincident with the summer insolation maximum. As the amplification of the seasonal cycle of insolation waned during the middle Holocene, Pseudotsuga was replaced by Pinus and Abies suggesting cool, moist conditions. The fire reconstructions show less synchroneity. In general, the sites west of the continental divide display a fire-frequency maximum around 12,000–8000 cal yr B.P., which coincides with the interval of high summer insolation and stronger-than-present subtropical high. The sites on the east side of the continental divide have the highest fire frequency ca. 6000–3500 cal yr B.P. and may be responding to a decrease in summer precipitation as monsoonal circulation weakened in the middle and late Holocene. This study demonstrated that the fire frequency of the last two decades does not exceed the historical range of variability in that periods of even higher-than-present fire frequency occurred in the past. 相似文献
46.
We present here the first high-resolution pollen record of vegetation response to interactions of hurricane and fire disturbances over the past 1200 yr from a small lake in Alabama on the Gulf of Mexico coast. The paleotempestological record inferred from the overwash sand layers suggests that the Alabama coast was directly struck by Saffir-Simpson category 4 or 5 hurricanes twice during the last 1200 yr, around 1170 and 860 cal yr BP, suggesting an annual landfall probability of 0.17% for these intense hurricanes. The charcoal data suggest that intense fires occurred after each of these hurricanes. The pollen data suggest that populations of halophytic plants (Chenopodiaceae) and heliophytic shrubs (Myrica) expanded after the hurricane strikes, probably due to saltwater intrusion into the marshes and soil salinization caused by overwash processes. Populations of pines (Pinus sp.) decreased significantly after each intense hurricane and the ensuing intense fire, suggesting that repeated hurricane-fire interactions resulted in high tree mortality and probably impeded recruitment and recovery. Our data support the hypothesis that the likelihood and intensity of fire increased significantly after a major hurricane, producing responses by vegetation that are more complex and unpredictable than if the disturbance agents were acting singly and independently. 相似文献
47.
The environmental history of the Kootenay Valley in the southern Canadian Rockies was reconstructed using lake sediment from
Dog Lake, British Columbia, and compared to other paleoenvironmental studies in the region to understand how vegetation dynamics
and fire regimes responded to climate change during the Holocene. A pollen-based vegetation reconstruction indicates five
periods of vegetation change. At 10,300 cal yr B.P. Pinus-Juniperus parkland colonized the valley and by 7600 cal yr B.P. was replaced by mixed stands of Pinus, Picea and Pseudotsuga/Larix. Fire frequencies increased to their Holocene maximums during the 8200–4000 cal yr B.P. period. From 5500–4500 cal yr B.P.
Pseudotsuga/Larix reached its maximum extent in the Kootenay Valley under a more frequent fire regime. At 5000 cal yr B.P. Picea and Abies began to expand in the area and by 4500 cal yr B.P. the forest shifted to a closed montane spruce forest type with dramatically
reduced fire frequency. The shift to less frequent forest fires after 4500 cal yr B.P., along with a moisterPicea – dominated closed forest, corresponds to Neoglacial advances in the Canadian Rockies and Coast Mountains. Fire intervals
after 4000 cal yr B.P. are significantly longer than the shorter fire intervals of the early to mid Holocene. A return to
drier, more open forest condition occurs between 2400–1200 cal yr B.P. with a slight increase in fire activity and summer
drought events. Lower lake levels inferred by charophyte accumulation rates during the 2400–1200 cal yr B.P. interval support
this moisture regime shift. An abrupt shift toPicea dominated forest occurred from 1200–1000 cal yr B.P. and a final period of wet-closed forest cover reaches its maximum extent
from 700–150 cal yr B.P. that appears to be a response to Little Ice Age cooling. Present forests are within their natural
range of variability but are predicted to shift again to a drier more open structure with increased Pseudotsuga/Larix cover. More frequent stand replacing fires and increased area burned likely will accompany this change due to continued global
warming. 相似文献
48.
We investigate the potential of quartz-based OSL-dating for application to heated sandy sediments that are closely associated with the remains of charcoal production (charcoal kilns). This is particularly relevant for post-1650 CE features, where 14C-dating lacks resolution. We first document the general OSL characteristics using large aliquots of sand-sized quartz. Results from procedural tests indicate that the laboratory measurement procedure should allow reliable equivalent dose estimation. The scale of analysis is then reduced to smaller aliquots for all samples, each composed of 100–200 grains, as well as to single grains for eight heated samples. For one sample, both the small and single-grain ages are consistent with the 14C-ages obtained for charcoal fragments in the same sedimentary unit. Also for three other samples, single-grains yield ages consistent with 14C-dating; all other OSL-ages are older although, for three of these samples, single grains and small aliquots yield results that are not significantly different. We discuss potential sources of inaccuracy for the OSL-dates, such as dose rate determination and homogeneous incomplete resetting. The latter is the most difficult to assess and requires additional empirical data. 相似文献
49.
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
50.
The Yan'an Formation of the Ordos Basin is a sequence of four members,consisting of siliciclastic sediments deposited in alluvial,lacustrine and mire settings during the Middle Jurassic.Samples collect... 相似文献