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1.
《Quaternary Science Reviews》1999,18(4-5):671-695
Percentage, concentration and accumulation pollen data together with diatom and non-siliceous microfossil data are presented for the site of Gölhisar Gölü (37°8′N, 29°36′E; elevation 930 m), a small intramontane lake in Burdur Province, southwest Turkey. Microfossil assemblages from the longest sediment core (GHA: 813 cm) record changes in local and regional vegetation and lake productivity over the last ∼9500 years. Pollen spectra indicate that vegetation progressed from an open landscape with an increase in arboreal pollen occurring ∼8500 bp to mixed forest comprising oak, pine and juniper until around 3000 bp (Cal ∼1240 BC) when a human occupation phase becomes discernible from the pollen spectra. This occurs shortly after the deposition of a volcanic tephra layer which originated from the Minoan eruption of Santorini (Thera) and radiocarbon dated to 3330±70 yr bp (Cal ∼1600 BC). This human occupation phase is comparable to the Beyşehir Occupation phase recorded at other sites in southwest Turkey and involved forest clearance and the cultivation of fruit trees such as Olea, Juglans, Castanea and Vitis together with arable cereal growing and pastoralism. The presence of pollen types associated with the Beyşehir Occupation phase in deposits above the Santorini tephra layer confirms a Late Bronze Age/early Anatolian Dark Age date for its commencement. Since ∼3000 bp notable changes in aquatic ecology associated with tephra deposition and subsequent nutrient and sediment flux from the lake catchment are recorded. The Beyşehir Occupation phase at Gölhisar Gölü came to an end around 1300 bp (Cla AD ∼700) when pine appears to have become the dominant forest tree.  相似文献   

2.
《Quaternary Science Reviews》2005,24(12-13):1479-1498
Multiple peat-silt couplets preserved in tidal marsh sediment sequences suggest that numerous great plate boundary earthquakes caused the coast around Cook Inlet, Alaska, to subside over the past 3500 years. Field and laboratory analyses of the two youngest couplets record the well-documented earthquake of AD 1964 and the penultimate one, approximately 850 cal yr BP. Diatom assemblages from a range of modern day estuarine environments from tidal flat through salt marsh to acidic bog produce quantitative diatom transfer function models for elevation reconstructions based on fossil samples. Only nine out of 124 fossil assemblages analysed, including previously published data for the AD 1964 earthquake, have a poor modern analogue. Calibration of fossil samples indicate co-seismic subsidence of 1.50±0.32 m for AD 1964, similar to measurements taken after the earthquake, and 1.45±0.34 m for the ∼850 cal yr BP earthquake. Elevation standard errors for individual fossil samples range from ∼0.08 m in peat layers to ∼0.35 m in silt units. Lack of a chronology within fossil silt units prevents identification of changes in the rate of recovery and land uplift between the post-seismic and inter-seismic periods. However, preservation of multiple peat-silt couplets indicates no net emergence over multiple earthquake cycles. Glacio-isostatic movements from Little Ice Age glacier advance and retreat explains a ∼0.15 m relative sea-level oscillation recorded within the peat layer subsequently submerged as a result of the AD 1964 earthquake. Before both this and the ∼850 cal yr BP earthquake, diatom assemblages suggest pre-seismic relative sea-level rise of ∼0.12±0.13 m, representing possible precursors to great earthquakes.  相似文献   

3.
Chemical proxies are useful analogs for reconstructing physical properties of sea water, such as sea surface temperature (SST) and sea surface salinity (SSS). Time series of these inferred properties would allow for reconstructions of past El Niño–Southern Oscillation (ENSO) events, where no instrumental records exist. In this study, a monthly oxygen isotope record from a Porites coral is used to explain how past ENSO events are recorded in the coral skeletons. The sample covers a 12 year period and was collected from Nanwan Bay, Taiwan. During El Niño events the coral skeleton is shown to produce a δ18O–SST correlation with a slope of −0.12 ± 0.04‰ °C−1. During other times, this value is significantly different, with a slope of −0.21 ± 0.04‰ °C−1. Coral that grew during El Niño summers have δ18O values which are enriched by ∼0.2‰, relative to other times. A possible mechanism to explain this difference may be enhanced penetration of Kuroshio Current waters into the South China Sea during summer. The observed contrast in the correlation of δ18O–SST variability in this sample supports the influence of El Niño in eastern Asia.  相似文献   

4.
《Journal of Structural Geology》2001,23(6-7):1007-1013
The phenomenon of shear-heating is generally difficult to recognise from petrologic evidence alone. Establishing that shear zones attain higher temperatures than the surrounding country rocks requires independent evidence for temperature gradients. In the Musgrave Block, central Australia, there is a clear spatial association between shear zones and interpreted elevated temperatures. Eclogite facies shear zones that formed at ∼550 Ma record temperatures of ∼650–700°C. Outside the high-pressure shear zones, minerals with low closure temperatures such as biotite (∼450°C in the 40Ar–39Ar and Rb–Sr systems), preserve ages >800 Ma, suggesting that these rocks did not experience temperatures greater than about 450°C at ∼550 Ma for any extended period. Thus, the shear zones record temperatures that are ∼200°C higher than the surrounding country rocks. Simple calculations show that the combination of relatively high shear stresses (∼100 MPa) and high strain rates (∼10−11 s−1) for short durations (<1 Ma) can account for the observed apparent temperature variations. The evidence indicates that shear heating is the dominant mechanism for localised temperature increases in the shear zones, while the country rock remained at relatively lower temperatures.  相似文献   

5.
Multiple proxies from a 319-cm peat core collected from the Hudson Bay Lowlands, northern Ontario, Canada were analyzed to determine how carbon accumulation has varied as a function of paleohydrology and paleoclimate. Testate amoeba assemblages, analysis of peat composition and humification, and a pollen record from a nearby lake suggest that isostatic rebound and climate may have influenced peatland growth and carbon dynamics over the past 6700 cal yr BP. Long-term apparent rates of carbon accumulation ranged between 8.1 and 36.7 g C m? 2 yr? 1 (average = 18.9 g C m? 2 yr? 1). The highest carbon accumulation estimates were recorded prior to 5400 cal yr BP when a fen existed at this site, however following the fen-to-bog transition carbon accumulation stabilized. Carbon accumulation remained relatively constant through the Neoglacial period after 2400 cal yr BP when pollen-based paleoclimate reconstructions from a nearby lake (McAndrews et al., 1982) and reconstructions of the depth to the water table derived from testate amoeba data suggest a wetter climate. More carbon accumulated per unit time between 1000 and 600 cal yr BP, coinciding in part with the Medieval Climate Anomaly.  相似文献   

6.
The study of a 300-cm-thick exposed lacustrine sediment section in the Hedong village in Zhaoqing area which is located in sub-tropical west Guangdong Province in South China, demonstrates that the lacustrine sedimentary sequence possibly contains evidence for exploring variation of Asian monsoon climate. Multi-proxy records, including the humification intensity, total organic carbon, and grain size fractions, reveal a general trend towards dry and cold conditions in the late Holocene that this is because of a decrease in solar insolation on an orbital scale. Three intensified Asian summer monsoon (ASM) intervals (∼3300–3000 cal yr BP, ∼2600–1600 cal yr BP, and ∼900–600 cal yr BP), and three weakened ASM intervals (∼4000–3300 cal yr BP, ∼3000–2600 cal yr BP, and ∼1600–900 cal yr BP) are identified. Our humification record (HDcal) shows a good correlation on multi-centennial scale with the tree ring Δ14C record, a proxy of solar activity. A spectral analysis of HDcal reveals four significant cycles, i.e., ∼1250 yr, 300 yr, 110 yr, and 70 yr, and most of these cycles are related to the solar activity. Our findings indicate that solar output and oceanic–atmospheric circulation probably have influenced the late Holocene climate variability in the study region.  相似文献   

7.
Uranium-lead (U–Pb) dating was conducted on zircons in tuff layers and sandstone samples from the uppermost Liantuo Formation and in a tillite sample from the lowermost Nantuo Formation in Three Gorges area, South China, using SHRIMP and LA-ICP-MS techniques. Zircons from these samples yielded age spectrum (within 1000 Ma) of ∼890, ∼830, ∼780, ∼730, and 646 Ma. Zircons from the Liantuo tuffs have a weighted mean 206Pb/238U age of 734.1 ± 8.1 Ma (2σ, n = 7, MSWD = 0.48), which was regarded as the best estimation of the upper boundary age of the Liantuo Formation. Combining with previous geochronologic data, the Liantuo Formation was proposed to be a pre-Chang′an glaciation unit, and it is comparable to the middle-upper Banxi/Danzhou Group in South China.  相似文献   

8.
The Balkhash Metallogenic Belt (BMB) in Kazakhstan, Central Asia, with the occurrence of the super-large Kounrad and Aktogai, the large Borly porphyry Cu–Mo deposits, and the large Sayak skarn polymetallic ore-field, is one of the central regions of the Paleozoic Central Asian metallogenic domain and orogenic belt. In this study, newly obtained SHRIMP zircon U–Pb ages of nine samples and 40Ar/39Ar ages of six mineral samples (inclding hornblende, biotite and K-feldspar) give more detailed constraints on the timing of the granitic intrusions and their metallogeny. Porphyritic monzonite granite and tonalite porphyry from the Kounrad deposit yield U–Pb zircon SHRIMP ages of 327.3 ± 2.1 Ma and 308.7 ± 2.2 Ma, respectively. Quartz diorite and porphyritic granodiorite from the Aktogai deposit yield U–Pb SHRIMP ages of 335.7 ± 1.3 Ma and 327.5 ± 1.9 Ma, respectively. Porphyritic granodiorite and granodiorite from the Borly deposit yield U–Pb SHRIMP ages of 316.3 ± 0.8 Ma and 305 ± 3 Ma, respectively. Diorite, granodiorite, and monzonite from the Sayak ore-field yield U–Pb SHRIMP ages of 335 ± 2 Ma, 308 ± 10 Ma, and 297 ± 3 Ma, respectively. Hornblende, biotite, and K-feldspar from the Aktogai deposit yield 40Ar/39Ar cooling ages of 310.6 Ma, 271.5 Ma, and 274.9 Ma, respectively. Hornblende, biotite, and K-feldspar from the Sayak ore-field yield 40Ar/39Ar cooling ages of 287.3 ± 2.8 Ma, 307.9 ± 1.8 Ma, and 249.8 ± 1.6 Ma, respectively. The new ages constrain the timing of Late Paleozoic felsic magmatism to ∼336 to ∼297 Ma. Skarn mineralization in the Sayak ore-field formed at ∼335 and ∼308 Ma. Porphyry Cu–Mo mineralization in the Kounrad deposit and the Aktogai deposit formed at ∼327 Ma, and in the Borly deposit at ∼316 Ma. The Late Paleozoic regional cooling in the temperature range of ∼600 °C to ∼150 °C occurred from ∼307 to ∼257 Ma.  相似文献   

9.
The Reed Bank Basin in the southern margin of the South China Sea is considered to be a Cenozoic rifted basin. Tectono-thermal history is widely thought to be important to understand tectonics as well as oil and gas potential of basin. In order to investigate the Cenozoic tectono-thermal history of the Reed Bank Basin, we carried out thermal modeling on one drill well and 22 pseudo-wells using the multi-stage finite stretching model. Two stages of rifting during the time periods of ∼65.5–40.4 Ma and ∼40.4–28.4 Ma can be recognized from the tectonic subsidence rates, and there are two phases of heating corresponding to the rifting. The reconstructed average basal paleo-heat flow values at the end of the rifting events are ∼60 and ∼66.3 mW/m2, respectively. Following the heating periods, this basin has undergone a persistent thermal attenuation phase since ∼28.4 Ma and the basal heat flow cooled down to ∼57.8–63.5 mW/m2 at present. In combination with the radiogenic heat production of the sedimentary sequences, the surface heat flow of the Reed Bank Basin ranges from ∼60.4 to ∼69.9 mW/m2.  相似文献   

10.
《Quaternary Science Reviews》2005,24(10-11):1217-1222
Tide gauge data at seven sites of the Permanent Service for Mean Sea Level (PSMSL), with information for relative sea-level during the past 140–200 yr, were analyzed to examine the rates and causes of the global sea-level rise (GSLR) during the twentieth century. By subtracting linear trends for relative sea-level rise during the past 100 yr from the observed data, we get the apparent GSLRs of ∼1 mm yr−1 for five sites around the Baltic Sea and Brest. The rate for San Francisco is significantly larger than this, with an optimum value ∼2 mm yr−1. The spatial difference of ∼1 mm yr−1 between these sites is reasonably explained by the recent melting of the Greenland ice sheet with an equivalent sea-level rise of ∼1 mm yr−1. The predicted relative sea-level change for this melting scenario is 0.5 mm yr−1 at sites around the Baltic Sea and Brest, and 1.5 mm yr−1 for San Francisco. The residuals between observations and predictions, ∼0.5 mm yr−1 at all sites, may be contributed by thermal expansion of seawater and/or other melting sources. These results suggest the rate of twentieth-century GSLR to be 1.5 mm yr−1.  相似文献   

11.
《Journal of Structural Geology》2001,23(6-7):1031-1042
The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende 40Ar/39Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The 40Ar/39Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The 40Ar/39Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones.  相似文献   

12.
Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO2 (∼36–39 wt.%), high in MgO (∼27–30 wt.%) and notably high in FeO (∼26–37 wt.%), and low in Al2O3 (∼0.2–2.5 wt.%) and combined alkalis Na2O and K2O (∼0–0.7 wt.%). They consist mostly of olivine (∼71–96 vol.%; ∼Fo64–73), major clinopyroxene (minor to ∼15 vol.%; ∼En40–63Wo36–48), with variable small amounts of plagioclase (0 to ∼10 vol.%; ∼An15–33), and minor to trace amounts of orthopyroxene (none to ∼20 vol.%; En69–73Wo2–4), Fe-sulfides (trace to ∼7 vol.%), chromite (none to ∼5 vol.%), phosphates (none to ∼3 vol.%) and metallic Fe,Ni (trace to ∼2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (∼0.1–1.5 mm), with olivine commonly displaying triple junctions. Brachina has near-chondritic lithophile element abundances, whereas other brachinites show variable depletions in Al, Ca, Rb, K, Na, and LREE. Siderophile element abundance patterns vary and range from ∼0.01 to ∼0.9 CI. Oxygen isotope composition (Δ17O) ranges from ∼−0.09 to −0.39‰, with the mean = −0.23 ± 0.14‰. Brachinites are ancient rocks, as was recognized early by the detection, in some brachinites, of excess 129Xe from the decay of short-lived 129I (half-life 17 Ma) and of fission tracks from the decay of 244Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using 53Mn–53Cr systematics, relative to the Pb–Pb age of the angrite LEW 86010, and yielded an age of 4563.7 ± 0.9 Ma. Thus, Brachina is at most ∼4 Ma younger that the CAIs whose age is 4567.2 ± 0.6 Ma. There is no consensus on the origin of brachinites, but they most likely are primitive achondrites, i.e., ultra-mafic residues from various low degrees of partial melting. Partial melting experiments suggest that they possibly formed from a parent lithology chemically similar but not identical to the Rumuruti (R) chondrites, although the different oxygen isotopic compositions of the R chondrites and the brachinites put a serious constraint on this hypothesis. The apparent lack of abundant rocks representing the partial melts suggests that brachinites may have formed on a parent body <∼100 km in radius, where early partial melts were removed from the parent body by explosive pyroclastic volcanism. Graves Nunataks 06128 and 06129 are felsic, andesitic basalts which have properties that suggest a relationship to brachinites and thus, may be samples of the elusive partial melts.  相似文献   

13.
We used a new sedimentary record to reconstruct the Holocene vegetation and fire history of Gorgo Basso, a coastal lake in south-western Sicily (Italy). Pollen and charcoal data suggest a fire-prone open grassland near the site until ca 10,000 cal yr BP (8050 cal BC), when Pistacia shrubland expanded and fire activity declined, probably in response to increased moisture availability. Evergreen Olea europaea woods expanded ca 8400 to decline abruptly at 8200 cal yr BP, when climatic conditions became drier at other sites in the Mediterranean region. Around 7000 cal yr BP evergreen broadleaved forests (Quercus ilex, Quercus suber and O. europaea) expanded at the cost of open communities. The expansion of evergreen broadleaved forests was associated with a decline of fire and of local Neolithic (Ficus carica–Cerealia based) agriculture that had initiated ca 500 years earlier. Vegetational, fire and land-use changes ca 7000 cal yr BP were probably caused by increased precipitation that resulted from (insolation-forced) weakening of the monsoon and Hadley circulation ca 8000–6000 cal yr BP. Low fire activity and dense coastal evergreen forests persisted until renewed human activity (probably Greek, respectively Roman colonists) disrupted the forest ca 2700 cal yr BP (750 BC) and 2100 cal yr BP (150 BC) to gain open land for agriculture. The intense use of fire for this purpose induced the expansion of open maquis, garrigue, and grassland-prairie environments (with an increasing abundance of the native palm Chamaerops humilis). Prehistoric land-use phases after the Bronze Age seem synchronous with those at other sites in southern and central Europe, possibly as a result of climatic forcing. Considering the response of vegetation to Holocene climatic variability as well as human impact we conclude that under (semi-)natural conditions evergreen broadleaved Q. ilex–O. europaea (s.l.) forests would still dominate near Gorgo Basso. However, forecasted climate change and aridification may lead to a situation similar to that before 7000 cal yr BP and thus trigger a rapid collapse of the few relict evergreen broadleaved woodlands in coastal Sicily and elsewhere in the southern Mediterranean region.  相似文献   

14.
《Quaternary Science Reviews》2005,24(12-13):1375-1389
High-resolution analyses of the elemental composition of calcite and biogenic silica (BSi) content in piston cores from Lake Edward, equatorial Africa, document complex interactions between climate variability and lacustrine geochemistry over the past 5400 years. Correlation of these records from Lake Edward to other climatically-forced geochemical and lake level records from Lakes Naivasha, Tanganyika, and Turkana allows us to develop a chronology of drought events in equatorial East Africa during the late Holocene. Major drought events of at least century-scale duration are recorded in lacustrine records at about 850, 1500, ∼2000, and 4100 cal year BP. Of these, the most severe event occurred between about 2050 and 1850 cal year BP, during which time Lake Edward stood about 15 m below its present level. Numerous additional droughts of less intensity and/or duration are present in the Lake Edward record, some of which may be correlated to other lacustrine climate records from equatorial East Africa. These events are superimposed on a long-term trend of increasingly arid conditions from 5400 to about 2000 cal year BP, followed by a shift toward wetter climates that may have resulted from an intensification of the winter Indian monsoon. Although the causes of decade- to century-scale climate variability in the East African tropics remain obscure, time-series spectral analysis suggests no direct linkage between solar output and regional rainfall. Rather, significant periods of ∼725, ∼125, 63–72, 31–25, and 19–16 years suggest a tight linkage between the Indian Ocean and African rainfall, and could result from coupled ocean-atmosphere variability inherent to the tropical monsoon system.  相似文献   

15.
A 7-cm long aragonite stalagmite, FR0510-1, from Furong Cave, Chongqing, was dated by 210Pb and 230Th methods, revealing a-2000-year record of climate history under the influence of the East Asian Monsoon. The FR0510-1 record resembles Dongge Cave DA record on 10–100-year scales, but quite different from the Wanxiang Cave WX42B record, indicating that while stalagmite δ18O record represents local/regional moisture change, spatial variability of the monsoonal rainfall over eastern China must take into account. During the past 2000 years, climate in Chongqing was relatively wet in the intervals of 50 BC–AD 250, AD 1150–1450 and AD 1600–1950, and relatively dry during the periods of AD 250–1150 and AD 1450–1600. Dry conditions were prevailing over the Medieval Warm Period, whereas wet climates were dominant during the most time of the Little Ice Age in Chongqing area.  相似文献   

16.
A peat cellulose δ18O record spanning around 14,000 years from the Hani peat mire in northeastern China reveals several abrupt temperature anomalies in the period from the last deglaciation through the Holocene. The timing of these anomalies coincides well with the notable cooling events recorded respectively using the GISP2 ice core and ice-rafted sediment of the North Atlantic Ocean, such as the Older Dryas, Inter-Allerød, Younger Dryas, and the nine ice-rafted debris events. The results demonstrate that this repeating pattern of abrupt temperature deterioration is not limited to the North Atlantic area at high latitude but also exists in the western North Pacific region at middle latitude. The synchronous temperature anomalies possibly are resulted from the joint effects of meltwater discharge into the North Atlantic Ocean and reduced solar activity. In the period from around 8600 to 8200 cal. yrs BP the Hani peat record shows a broad δ18O peak that may reflect compound climate signals resulting from the two kinds of forcing factors: the temperature drop related to reduced solar activity at around 8600–8250 cal. yrs BP, and the temperature anomaly attributed to the meltwater effect at around 8220 ± 70 cal. yrs BP. This result may provide palaeo-temperature evidence for existence of the sharp “8.2 k” event in the western North Pacific region. In addition, our results have revealed that in the period from the last deglaciation through the Holocene the synchronous temperature anomalies before and after the “8.2 k” event seem to be related to meltwater outflow and reduced solar activity, respectively. It is important that the all temperature anomalies—whether because of reduced solar activity in the late Holocene or from meltwater discharge in the early Holocene—are accompanied by an abrupt decline in the Indian Ocean summer monsoon and abrupt strengthening of the East Asian summer monsoon. It is likely that reduced solar activity and meltwater outflow appear to modulate Earth system changes in the same direction. The influences could be compounded. Reduced solar activity and meltwater outburst both appear to act as triggers for occurrence of the El Niño phenomenon in the equatorial Pacific Ocean, which may result in broad teleconnections between the temperature anomaly in the Northern Hemisphere and abrupt variation of the Asian monsoon.  相似文献   

17.
《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   

18.
High-resolution sedimentological and rock magnetic analyses from sediment cores collected in the Seine estuary record changes in coastal sedimentary dynamics linked to climatic variations during the late Holocene. Using AMS 14C and paleomagnetic data we present a first attempt in developing a reliable age model on macrotidal estuarine archives, with a decadal resolution. Correlations between sedimentary successions from the outer Seine estuary document the main sedimentary infilling phases of the system during the last 3000 years. Between 3000 and 1150 cal. BP sedimentary patterns reveal that sequence deposition and preservation are predominantly controlled by marine and tidal hydrodynamics while severe storm events are recorded at ca. 2700 and 1250 cal. BP in the outermost estuary. Conversely, the Medieval Warm Period (MWP; 900–1200 AD) is characterized by a drastic waning of the influence of marine hydrodynamics on sediment preservation. Pronounced episodes of Seine river floods indicate a much stronger impact of continental inputs on sedimentary patterns during this period. The onset of the Little Ice Age marks a diminishing influence of continental inputs while tidal and open marine hydrodynamics again exerted a primary control on the sedimentary evolution of the system during 1200–2003 AD. Coastal sedimentary dynamics as preserved within sedimentary successions appear to have been largely influenced by changes in storminess during the last 3000 years. We have matched the preservation of MWP Seine river floods, as revealed by sedimentological and rock magnetic proxy data, to a prolonged interval of weakened storminess in Normandy permitting the preservation of estuarine flood deposits within a context of reduced coastal erosion in northern Europe. The preservation of sedimentary successions in the Seine estuary is therefore maximal when climate conditions resembled those of the preferred low phase of the NAO on multidecadal timescales such as during 800–1200 AD (MWP). In contrast, increased removal and transport of estuarine sediments occur when winter storm activity greatly intensified over northwestern France. We report four prominent centennial-scale periods of stronger storminess, occurring with a pacing of ～1500 years, which are likely to be related to the last four Bond's Holocene cold events. Our results documenting a close link between coastal sedimentary dynamics, millennial-scale variations in Holocene climate and North Atlantic atmospheric circulation are fairly consistent with other records from Scandinavia, central Greenland and southern Europe.  相似文献   

19.
《Quaternary Science Reviews》2007,26(1-2):130-141
Analyses of sediment cores from Marcella Lake, a small, hydrologically closed lake in the semi-arid southwest Yukon, provides effective moisture information for the last ∼4500 years at century-scale resolution. Water chemistry and oxygen isotope analyses from lakes and precipitation in the region indicate that Marcella Lake is currently enriched in 18O by summer evaporation. Past lake water values are inferred from oxygen isotope analyses of sedimentary endogenic carbonate in the form of algal Charophyte stem encrustations. A record of the δ18O composition of mean annual precipitation at Jellybean Lake, a nearby evaporation-insensitive system, provides data of simultaneous δ18O variations related to decade-to-century scale shifts in Aleutian Low intensity/position. The difference between the two isotope records, Δδ, represents 18O-enrichment in Marcella Lake water caused by summer effective moisture conditions. Results indicate increased effective moisture between ∼3000 and 1200 cal BP and two marked shifts toward increased aridity at ∼1200 and between 300 and 200 cal BP. These prominent late Holocene changes in effective moisture occurred simultaneously with changes in Aleutian Low circulation patterns over the Gulf of Alaska indicated by Jellybean Lake. The reconstructed climate patterns are consistent with the topographically controlled climatic heterogeneity observed in the coastal mountains and interior valleys of the region today.  相似文献   

20.
《Quaternary Science Reviews》2007,26(7-8):894-919
This study presents relative sea level (RSL) curves for seven coastal areas in Akarnania and the northwestern Peloponnese (NW Greece) since the mid-Holocene. RSL fluctuations are deduced from 48 14C-AMS dated sedimentological sea level markers from 27 vibracores drilled in near-coast geological archives as well as from six geoarchaeological sea level indicators of known ages. Seven palaeo sea level curves including uncertainty bands are reconstructed for a coastal zone spanning a distance of 150 km. Considerable intra-regional differences in sea level evolution exist. These differences are mainly due to tectonic reasons. In general, RSL in northwestern Greece has never been higher than today. Rates of local sea level rise were highest until 5500–5000 cal BC (up to 12.3 m/ka) and lowest during 4000–500 cal BC (0.2–1.4 m/ka). During the past 2500 or so years, RSL has accelerated anew (0.7–2.7 m/ka). Calculating differences between local mean sea level curves provides quantitative information on intra-regional differences of tectonic activity. The coastal plains of Palairos and Elis show signs of uplift, whereas the Mytikas and Boukka plains are strongly subsiding. Compared to other areas of the eastern Mediterranean, northwestern Greece has been subject to significant net long-term subsidence. Regional tectonic events (RTEs) were detected for the time around 4000, 2500, 500 and 250 cal BC as well as around 250 and 1250 cal AD. RTEs are characterized by changes of uplift/subsidence rates or by the redirection of local tectonic movements. The question if some of the RTEs were of a supra-regional nature is still open. From a geodynamic point of view, the results presented show that Akarnania's southwestern fringe is being downwarped while the tectonic block as a whole is moving towards the southwest. Strongest subsidence rates are observed for central Akarnania. At Akarnania's fringes, subsidence is reduced by the influence of strong uplift of adjacent areas such as around Preveza and the northern Peloponnese.  相似文献   

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