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1.
The varved sediments of the Santa Barbara Basin off southern California, offer a unique opportunity to study the changes in oceanographic conditions of this nearshore area during the last 8000 yr. Quantitative analysis of Radiolaria found in recent surface sediment samples from the eastern North Pacific allows the identification of four “assemblages” which can be related to the physical oceanography of the California Current. Two assemblages are associated with the southerly flowing California Current, one with the main stream of the current (California Current Assemblage) and the other with the offshore flow along northern California (Central Assemblage). The two other assemblages are associated with the subtropical region of the eastern North Pacific (Subtropical Assemblage) and one found mostly off the coast of Baja California (Baja Assemblage). Analysis of the Radiolaria found in the varved sediments of a core from the Santa Barbara Basin give an 8000-yr continuous record of these four assemblages. The California and Baja Assemblages show only minor fluctuations in their importance in the sediments of the Santa Barbara Basin. The California Assemblage, however, shows a steady increase during this time period. Prior to 5400 yr B.P. the Radiolaria were predominately subtropical in character, whereas after 5400 yr B.P. the Central Assemblage becomes more important. Since 5400 yr B.P. most of the changes in the radiolarian fauna consist of fluctuations in the importance of these two assemblages. Past sea-surface temperatures for the month of February were calculated using the transfer function technique of J. Imbrie and N. G. Kipp (1971, In “The Late Cenozore Glacial Ages” L. K. Turekian, Ed.), (Chap. 5, Yale Univ. Press, New Haven, Conn.). The time series of paleotemperature estimates show major changes in the average February temperature of Santa Barbara Basin waters. The range of estimated temperatures (12°C) exceeds that of the historical observations of February temperatures in the Santa Barbara Basin but does not exceed the observed range for the California Current region. The intervals from 800 to 1800 yr B.P. 3600 to 3800 yr B.P. and 5400 to the end of the record appear to have been generally warmer than today. Comparison of the Holocene record of alpine glacial advances with the radiolarian assemblage and paleotemperature time series shows that the initiations of advances was coincident with a decrease in sea-surface temperatures and an increase in the importance of the Central Assemblage in the Santa Barbara Basin. The terminations of these advances were not marked by any consistent characteristic in the Santa Barbara Basin time series.  相似文献   

2.
Particulate matter was collected during September–October, 1977, in particle traps suspended 30–60 m above the floor of San Nicolas, Santa Barbara, Santa Monica and San Pedro Basins, off the coast of southern California. The trap particulates were analyzed for C15–C35 hydrocarbons using gas chromatography (GC) and GC-mass spectrometry. Kerogens and humic acids were characterized by H/C, N/C, δ13C, δ15N and δ34S ratios, and by electron-spin resonance. Hydrocarbons arising from fresh and weathered petroleum, marine autochthonous and terrestrial sources were identified. The rates of petroleum deposition during the collection period followed the order: San Nicolas Basin < Santa Barbara Basin ~ Santa Monica Basin < San Pedro Basin, with the largest amount of weathered petroleum being deposited in San Pedro Basin. The rates of petroleum deposition are correlated more strongly with human activities such as shipping, and the discharge of municipal and industrial wastes, than with natural submarine oil seepage. Analyses of kerogens and humic acids indicate that the majority of the organic matter in the trap particulates is of marine origin. The water column overlying Santa Barbara Basin appears to have the highest marine productivity of the four basins studied.  相似文献   

3.
Multiple geochemical tracers [ion chemistry, stable isotopes of water, chlorofluorocarbons (CFC), tritium] and a 25-year-long record of discharge were used to understand residence times and flow paths of groundwater seeps in the fractured rock aquifer surrounding the Mission Tunnel, Santa Barbara, California. Tritium data from individual seeps indicate that seep waters are a mixture of >45-year-old (recharged prior to the nuclear bomb tests) and young groundwater. CFC data support this interpretation, however, a two-end member mixing model cannot completely explain the age tracer data. Microbial degradation and partial re-equilibration complicate the CFC signal. Spectral analysis of precipitation and groundwater seepage records shows that seepage lags precipitation by 3 months. This delay is related to the advancement of the wetting front and increasing the number of active flow paths. Additionally, the amount of seepage produced by precipitation is less during extended periods of drought than during normal or wet periods, suggesting antecedent conditions strongly affect flow through this fractured rock aquifer.  相似文献   

4.
Charcoal particles are widespread in terrestrial and lake environments of the northern temperate and boreal biomes where they are used to reconstruct past fire events and regimes. In this study, we used botanically identified and radiocarbon-dated charcoal macrofossils in mineral soils as a paleoecological tool to reconstruct past fire activity at the stand scale. Charcoal macrofossils buried in podzolic soils by tree uprooting were analyzed to reconstruct the long-term fire history of an old-growth deciduous forest in southern Québec. Charcoal fragments were sampled from the uppermost mineral soil horizons and identified based on anatomical characters. Spruce (Picea spp.) fragments dominated the charcoal assemblage, along with relatively abundant wood fragments of sugar maple (Acer saccharum) and birch (Betula spp.), and rare fragments of pine (Pinus cf. strobus) and white cedar (Thuja canadensis). AMS radiocarbon dates from 16 charcoal fragments indicated that forest fires were widespread during the early Holocene, whereas no fires were recorded from the mid-Holocene to present. The paucity of charcoal data during this period, however, does not preclude that a fire event of lower severity may have occurred. At least eight forest fires occurred at the study site between 10,400 and 6300 cal yr B.P., with a dominance of burned conifer trees between 10,400 and 9000 cal yr B.P. and burned conifer and deciduous trees between 9000 and 6300 cal yr B.P. Based on the charcoal record, the climate at the study site was relatively dry during the early Holocene, and more humid from 6300 cal yr B.P. to present. However, it is also possible that the predominance of conifer trees in the charcoal record between 10,400 and 6300 cal yr B.P. created propitious conditions for fire spreading. The charcoal record supports inferences based on pollen influx data (Labelle, C., Richard, P.J.H. 1981. Végétation tardiglaciaire et postglaciaire au sud-est du Parc des Laurentides, Québec. Géographie Physique et Quaternaire 35, 345-359) of the early arrival of spruce and sugar maple in the study area shortly after deglaciation. We conclude that macroscopic charcoal analysis of mineral soils subjected to disturbance by tree uprooting may be a useful paleoecological tool to reconstruct long-term forest fire history at the stand scale.  相似文献   

5.
The record of charcoal in lake sediments indicates that fire has always been an important ecological factor in the forest history of northeastern Minnesota. The annually laminated sediments of Lake of the Clouds permit precise dating of the charcoal peaks and record the changes in the influx of various pollen types. A detailed record of the past 1000 yr shows that the average frequency of fire is approximately 60–70 yr, with a range of about 20–100 yr. The amount of charcoal in sediments dating between 1000-500 y.a. is consistently higher than that for the last 500 yr, although the fire frequency for the two periods was not appreciably different. Pollen analysis shows no change or only short-term changes in the percentages of major pollen types following charcoal peaks.  相似文献   

6.
Understanding the natural mechanisms that control fire occurrence in terrigenous ecosystems requires long and continuous records of past fires. Proxies, such as sedimentary charcoal and tree-ring fire scars, have temporal or spatial limitations and do not directly detect fire intensity. We show in this study that polycyclic aromatic hydrocarbons (PAHs) produced during wildfires record local fire events and fire intensity. We demonstrate that high performance liquid chromatography with fluorescence detector (HPLC-FLD) is superior to gas chromatography–mass spectrometry (GC–MS) for detecting the low concentrations of sedimentary PAHs derived from natural fires. The HPLC-FLD is at least twice as sensitive as the GC–MS in selective ion monitoring (SIM) mode for parent PAHs and five times as sensitive for retene. The annual samples extracted from varved sediments from Swamp Lake in Yosemite National Park, California are compared with the observational fire history record and show that PAH fluxes record fires within 0.5 km of the lake. The low molecular weight (LMW) PAHs (e.g., fluoranthene, pyrene and benz[a]anthracene) are the best recorders of fire, whereas the high molecular weight (HMW) PAHs likely record fire intensity. PAHs appear to resolve some of the issues inherent to other fire proxies, such as secondary deposition of charcoal. This study advances our understanding of how PAHs can be used as markers for fire events and poses new questions regarding the distribution of these compounds in the environment.  相似文献   

7.
Two independent data sets are used to develop a model for reconstructing sea-surface temperature and dynamic height anomaly distributions for the California Current during the last 8000 years. The first data set, all hydrographic data available for the California Current region, was used to determine the statistical relationships between the historical record of sea-surface conditions in the Santa Barbara Basin and all one-degree-square grid points of the California Current area. Given these relationships and the second data set, an 8000-year record of sea-surface temperatures and dynamic height anomalies from the Santa Barbara Basin, past sea-surface conditions throughout the California Current can be estimated for times before historical observations. The 8000-year record of sea-surface conditions was estimated by analysis of the radiolarian fauna found in a varved sediment core from the Santa Barbara Basin (Pisias, N. G., 1978, Quaternary Research 10, 366–384). The reconstructions of sea-surface temperature and dynamic height anomalies indicate that at times of cold sea-surface conditions in the Santa Barbara Basin, the flow of the California Current was much stronger than it is today or was during the times of the warmest sea-surface conditions during the last 8000 years. The atmospheric circulation during the winter of 1950, the period of the coldest recorded sea-surface temperatures in the Santa Barbara Basin based on the historical data set, contained a strong northerly component in the winds which is consistent with the inferred increase in the California Current at times of cold sea-surface temperatures. Times of warm sea-surface temperatures in the Santa Barbara Basin are characterized by decreased southward flow of the California Current and a marked increase in northward flow into the Santa Barbara Basin itself. In the historical record, times of warm sea-surface temperatures are often associated with high precipitation in southern California. The atmospheric circulation during the winter of 1968–1969 is characterized by strong eastward flow over southern California and a northward transport of warm humid air from the tropics into the region of southern California producing the high rainfall observed. The persistence of this atmospheric circulation could produce the more zonal flow predicted for the California Current during times of warmer average conditions in the Santa Barbara Basin.  相似文献   

8.

In arid and semiarid regions from the southwestern USA and vast areas of northwestern Mexico, Santa Ana wind events modify the environment with high temperatures, very low humidity, and dust storms representing a recurrent phenomenon that triggers asthma and other respiratory diseases. While research has emphasized Santa Ana wind effects on the USA side, northwestern Mexico has been less investigated. Numerical modeling of a severe dust storm in November 2018, applying the Weather Research and Forecasting model coupled with a chemistry module (WRF-Chem), revealed that erosion, transport, and dust storms extend along the peninsula and the Gulf of California. Santa Ana winds eroded large areas, transported desert conditions to urban zones, causing high dust concentrations and reducing the relative humidity below 10%, deteriorating climatic conditions favorable to wellness. In Tijuana, Mexicali, Ensenada, San Diego, and Los Angeles, PM10 and PM2.5 concentrations (particle matter with diameter below 10 µm and 2.5 µm) reached values over 2000 µg/m3 for PM10, with daily mean concentrations well above national standards, leading to poor air quality and representing a health threat even in short-term exposure. This Santa Ana event transported dust particles several hundreds of kilometers over urban areas, the Gulf of California, and the Pacific Ocean. Severe soil deterioration was simulated within the study area, reaching dust emissions above 700,000 t, including croplands from the northern part of Baja California and Sonora's coastal area.

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9.
Quantitative analyses of variations in morphological features of charcoal were undertaken in a 210Pb-dated sediment core from Prosser Lake (British Columbia, Canada). Seven morphological types of charcoal were defined by particle shape, major-minor axis ratio, apparent porosity and progradation to unburned material. The distribution of morphotypes and total charcoal abundances were assessed as a proxy for fire events recorded between 1919 and 2000 and to subsequent mechanisms of transportation-sedimentation to lake sediments. Charcoal morphotypes showed distinct relationships to recorded area burned by fires. Fragile charcoal fragments with highly irregular porosity (termed Type M) displayed the strongest correlation to burned area (r2 = 0.51; P = 0.0001) and did not produce any false-positive signal for fires recorded within a radius of 20 km around the lake. We infer that high porosity and low density Type M fragments are aerially transported and directly deposited on the lake, and that the fragility of Type M charcoal prevents significant quantities from being secondarily transported and incorporated into the sedimentary record. We propose that charcoal morphology is an important but underutilized technique that can yield important insights into fire type, proximity and transportation-sedimentation processes.  相似文献   

10.
We examined long-term charcoal records spanning the glacial–interglacial cycles that are evident in two cores collected from Lake Biwa in central Japan. We found that the records of the two cores have a similar long-term variation pattern of charcoal concentrations and abundant large charcoal fragments in postglacial sediments, which indicates that frequent fires occurred near the shores of Lake Biwa during the postglacial period. Analogous natural conditions in the early postglacial period and the early part of the last interglacial period strongly suggest that the frequent fires that occurred only during the postglacial period were anthropogenic. A comparison between the charcoal records of Lake Biwa sediments and the cultural changes and human populations in this district suggests that anthropogenic fires in this district were influenced by the lifestyle and culture of each era rather than by the populations. Humans tended to use more fire at the start of the settlement during the early Neolithic era in this region, in spite of the small population size.  相似文献   

11.
《Quaternary Science Reviews》2007,26(19-21):2631-2643
In order to link the charcoal record from sedimentary archives with the combustion processes that reflect past anthropogenic activity, a novel method based on automated image analysis was developed. It allows a detailed quantification and morphological analysis of the combustion-derived products that were emitted in the area of Lake Lucerne (Central Europe) throughout the last 7200 years.Charcoal-particle distribution reconstructed from the composite sedimentary record shows that the charcoal input is primarily linked to redistribution of detrital μm-size charcoal degradation products from surface runoff into the large lake basin. However, the independent distribution of the coarser charcoal fraction (>38 μm) exhibits four major periods of large-scale fire activity around 5500, 3300, 2400, and 530 cal. BP. These events are synchronous with major anthropogenic changes (lake-dwellings, land-use changes, technological innovations), although it is possible that these major fire episodes could have been indirectly triggered by climatic deterioration and unfavorable environmental conditions. During the late-nineteenth-century, a great increase in slag particles and magnetic spherules of fly-ash occurred due to the steamboat navigation on Lake Lucerne. The successive burning of wood (after AD 1838), coal (after AD 1862), and diesel (after AD 1931) by the steamboat traffic produced specific particle shapes, providing valuable chronological markers for dating the recent sediments and a proxy for fossil fuel combustion.  相似文献   

12.
At the close of the Pleistocene, fire regimes in North America changed significantly in response to climate change, megafaunal extinctions, anthropogenic burning and, possibly, even an extraterrestrial impact. On California's Channel Islands, researchers have long debated the nature of late Pleistocene “fire areas,” discrete red zones in sedimentary deposits, interpreted by some as prehistoric mammoth-roasting pits created by humans. Further research found no evidence that these red zones were cultural in origin, and two hypotheses were advanced to explain their origin: natural fires and groundwater processes. Radiocarbon dating, X-ray diffraction analysis, and identification of charcoal from six red zones on Santa Rosa Island suggest that the studied features date between ~ 27,500 and 11,400 cal yr BP and resulted from burning or heating, not from groundwater processes. Our results show that fire was a component of late Pleistocene Channel Island ecology prior to and after human colonization of the islands, with no clear evidence for increased fire frequency coincident with Paleoindian settlement, extinction of pygmy mammoths, or a proposed Younger Dryas impact event.  相似文献   

13.
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14.
To aid interpreting the source area of charcoal in lake-sediment records, we compare charcoal deposition from an experimental fire to predictions from a particle dispersal model. This provides both a theoretical framework for understanding how lake sediments reflect fire history and a foundation for simulating sediment-charcoal records. The dispersal model captures the two-dimensional patterns in the empirical data (predicted vs. observed r2 = 0.67, p < 0.001). We further develop the model to calculate the potential charcoal source area (PCSA) for several classes of fires. Results suggest that (1) variations in airborne charcoal deposition can be explained largely by the size of PCSAs relative to fire sizes and (2) macroscopic charcoal travels many kilometers, longer than suggested by dispersal data from experimental fires but consistent with dispersal data from uncontrolled fires.  相似文献   

15.
We show how sedimentary charcoal records from multiple sites within a single landscape can be used to compare fire histories and reveal small scale patterns in fire regimes. Our objective is to develop strategies for classifying and comparing late-Holocene charcoal records in Midwestern oak- and pine-dominated sand plain ecosystems where fire regimes include a mix of surface and crown fires. Using standard techniques for the analysis of charcoal from lake sediments, we compiled 1000- to 4000-yr-long records of charcoal accumulation and charcoal peak frequencies from 10 small lakes across a sand plain in northwestern Wisconsin. We used cluster analysis to identify six types of charcoal signatures that differ in their charcoal influx rates, amount of grass charcoal, and frequency and magnitude of charcoal peaks. The charcoal records demonstrate that while fire histories vary among sites, there are regional patterns in the occurrence of charcoal signature types that are consistent with expected differences in fire regimes based on regional climate and vegetation reconstructions. The fire histories also show periods of regional change in charcoal signatures occurring during times of regional climate changes at ~ 700, 1000, and 3500 cal yr BP.  相似文献   

16.
Holocene fires in the northern Amazon basin   总被引：3，自引：0，他引：3  
The ubiquitous occurrence of charcoal in the forest soils of the Upper Rio Negro region of Colombia and Venezuela indicates the presence of frequent and widespread fires in the Amazon Basin, possibly associated with extremely dry periods or human disturbances. Charcoal ranged from 3.12 to 24.76 mg/cm3 in the upper 50 cm of soil and was more abundant in Oxisols and Ultisols than in other soil types. Charcoal dates range from 6260 yr B.P. to the present. Several dates coincide with dry phases recorded during the Holocene. Ceramic shards were found at several sites, and thermoluminescence analysis indicates that their ages range from 3750 to 460 yr B.P. The age of charcoal and shards confirms that this region has been subjected to fire and human disturbances during the past 6000 yr.  相似文献   

17.
The brief, terminal Pleistocene archaeological site at Santa Julia (SJ, 31° 50′ S; 71° 45′ W) is the only one with fluted projectile preforms and megafauna consumption known from the Chilean semiarid coastline. Here, we present the climatic history at SJ during the early Holocene reconstructed from pollen and charcoal analyses spanning 13.2–8.6 ka (=103 calibrated 14C yr BP). Elevated charcoal concentrations confirm human activity by 13.2 ka. Human occupation decreased in intensity and charcoal practically disappears from the record after 10.6 ka, followed by wetland expansion at SJ between 10.5 and 9.5 ka. Local dominance of coastal shrubland reveals that dry phases occurred between >11.2–10.5 and 9.5–9.0 ka. Overall, these findings imply that by modulating available resources at both local and landscape levels climate change may have played an important role in explaining the peopling of semiarid coastal Chile. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

18.
《Quaternary Science Reviews》2007,26(13-14):1790-1809
Interpreting sediment-charcoal records is challenging because there is little information linking charcoal production from fires to charcoal accumulation in lakes. We present a numerical model simulating the major processes involved in this pathway. The model incorporates the size, location, and frequency of fires, primary and secondary charcoal transport, sediment mixing, and sediment sampling. We use the model as a tool to evaluate assumptions of charcoal dispersal and taphonomy and to assess the merits of inferring local and regional fire history by decomposing charcoal records into low-frequency (‘background’) and high-frequency (‘peak’) components. Under specific dispersal scenarios, the model generates records similar in appearance to sediment-charcoal records from Alaskan boreal forests. These scenarios require long-distance dispersal (e.g. 100–101 km), consistent with observations from wildfires but longer than previously inferred from experimental dispersal data. More generally, charcoal accumulation in simulated records mainly reflects area burned within the charcoal source area. Variability in charcoal peak heights is primarily explained by the size of charcoal source areas relative to the size of simulated fires, with an increase in this ratio resulting in increased variability in peak heights. Mixing and multi-year sampling add noise to charcoal records, obscuring the relationship between area burned and charcoal accumulation. This noise highlights the need for statistical treatments of charcoal records. Using simulated records we demonstrate that long-term averages of charcoal accumulation (>10×mean fire return interval) correlate well with area burned within the entire charcoal source area. We further demonstrate how decomposing simulated records to isolate the peak component emphasizes fire occurrence at smaller spatial scales (<1 km radius), despite the importance of long-distance charcoal dispersal in simulating charcoal records similar to observations. Together, these results provide theoretical support for the analysis of charcoal records using the decomposition approach.  相似文献   

19.
Forest fires play a key role in the global carbon cycle and thus, can affect regional and global climate. Although fires in extended areas of Russian boreal forests have a considerable influence on atmospheric greenhouse gas and soot concentrations, estimates of their impact on climate are hampered by a lack of data on the history of forest fires. Especially regions with strong continental climate are of high importance due to an intensified development of wildfires. In this study we reconstruct the fire history of Southern Siberia during the past 750 years using ice-core based nitrate, potassium, and charcoal concentration records from Belukha glacier in the continental Siberian Altai. A period of exceptionally high forest-fire activity was observed between AD 1600 and 1680, following an extremely dry period AD 1540–1600. Ice-core pollen data suggest distinct forest diebacks and the expansion of steppe in response to dry climatic conditions. Coherence with a paleoenvironmental record from the 200 km distant Siberian lake Teletskoye shows that the vegetational shift AD 1540–1680, the increase in fire activity AD 1600–1680, and the subsequent recovery of forests AD 1700 were of regional significance. Dead biomass accumulation in response to drought and high temperatures around AD 1600 probably triggered maximum forest-fire activity AD 1600–1680. The extreme dry period in the 16th century was also observed at other sites in Central Asia and is possibly associated with a persistent positive mode of the Pacific Decadal Oscillation (PDO). No significant increase in biomass burning occurred in the Altai region during the last 300 years, despite strongly increasing temperatures and human activities. Our results imply that precipitation changes controlled fire-regime and vegetation shifts in the Altai region during the past 750 years. We conclude that high sensitivity of ecosystems to occasional decadal-scale drought events may trigger unprecedented environmental reorganizations under global-warming conditions.  相似文献   

20.
We have compiled 223 sedimentary charcoal records from Australasia in order to examine the temporal and spatial variability of fire regimes during the Late Quaternary. While some of these records cover more than a full glacial cycle, here we focus on the last 70,000 years when the number of individual records in the compilation allows more robust conclusions. On orbital time scales, fire in Australasia predominantly reflects climate, with colder periods characterized by less and warmer intervals by more biomass burning. The composite record for the region also shows considerable millennial-scale variability during the last glacial interval (73.5–14.7 ka). Within the limits of the dating uncertainties of individual records, the variability shown by the composite charcoal record is more similar to the form, number and timing of Dansgaard–Oeschger cycles as observed in Greenland ice cores than to the variability expressed in the Antarctic ice-core record. The composite charcoal record suggests increased biomass burning in the Australasian region during Greenland Interstadials and reduced burning during Greenland Stadials. Millennial-scale variability is characteristic of the composite record of the sub-tropical high pressure belt during the past 21 ka, but the tropics show a somewhat simpler pattern of variability with major peaks in biomass burning around 15 ka and 8 ka. There is no distinct change in fire regime corresponding to the arrival of humans in Australia at 50 ± 10 ka and no correlation between archaeological evidence of increased human activity during the past 40 ka and the history of biomass burning. However, changes in biomass burning in the last 200 years may have been exacerbated or influenced by humans.  相似文献   

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