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Many pre‐Mesozoic records of Earth history are derived from shallow water carbonates deposited on continental shelves. While these carbonates contain geochemical proxy records of climate change, it is the stratal architecture of layered carbonate units that often is used to build age models based on the idea that periodic astronomical forcing of sea‐level controls the layering. Reliable age models are crucial to any interpretation of rates and durations of environmental change, but the physical processes that actually control this stratal architecture in shallow water carbonates are controversial. In particular, are upward‐shallowing stacks of carbonate beds bounded by flooding surfaces (‘parasequences’) truly a record of relative sea‐level change? The purpose of this study is to examine a tidal flat that is actively accumulating carbonate stratigraphy, and to determine the relative importance of tidal channel migration (poorly known, but investigated here) and Holocene sea‐level rise (well‐known) in controlling post‐glacial parasequence architecture. This work represents a field study of peritidal carbonate accumulation at Triple Goose Creek, north‐west Andros Island. By integrating surface facies maps with differential global positioning system topographic surveys, a quantitative relationship between facies and elevation is derived. Sedimentary facies are sensitive to elevation changes as small as 5 cm, and are responding to both internal (distance to nearest tidal channel) and external (sea‐level rise) controls. The surface maps also are integrated with 187 sediment cores that each span the entire Holocene succession. While flooding of the Triple Goose Creek area should have occurred by ca 4500 years ago, preservation of Holocene sediment did not begin until 1200 years ago. The tidal channels are shown to be stationary, or to migrate sluggishly at up to 6 cm per year. Therefore, while the location of tidal channels is responsible for the modern mosaic of surface facies, these facies and the channels that control them have not migrated substantially during the ca 1200 years of sediment accumulation at Triple Goose Creek. Once the region was channellized, vertical and lateral shifts in facies, such as the landward retreating shoreline, expanding mangrove ponds and seaward advancing inland algal marsh, are driven by changes in relative sea‐level and sediment supply, not migrating channels. While stratigraphic columns look different depending on the distance to the nearest tidal channel, the overall parasequence architecture everywhere at Triple Goose Creek records an upward‐shallowing trend controlled by the infilling of accommodation space generated by post‐glacial sea‐level rise. 相似文献
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Simulated Holocene sediment discharges from a high‐resolution cellular model of river evolution in northern England, driven by a peat‐bog proxy climate and palynological based land‐cover record, show a very close similarity to alluviation phases evident in British river valleys. Independent validation of the model indicates that British river systems have been surprisingly sensitive to short term (ca. 102 yr) climate fluctuations, although river dynamics also have been modulated by land‐use change and sediment supply. This has important implications for understanding alluvial system evolution, establishing controlling factors and, most importantly, forecasting river response to future climate and land‐use changes. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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A detailed molluscan succession from a 10 m thick deposit of Holocene tufa at St Germain‐le‐Vasson, Normandy, provides the most complete record from northern France and has shed new light on the historical biogeography of several species of land snail. The succession has been reconstructed from four profiles and a chronology provided by accelerator mass spectrometry radiocarbon dating of charcoal, wood and shell. The onset of tufa formation occurred after 9700±90 yr BP and persisted until 4213±77 yr BP. The tufa appears to have accumulated at a remarkably constant rate (14.4–16.5 cm 100 yr?1), except for the upper levels, where the rate increases fourfold. The succession has been divided into six local molluscan zones. An early assemblage consisting of ecologically tolerant species and those indicative of marshy grassland is replaced by a sequence of shade‐demanding taxa, reflecting the encroachment of woodland. Shaded conditions persist until the end of the sequence but the most hygrophilous elements decline after 5422±60 yr BP (zone 5), a change also reflected in the tufa lithology by the development of silty grey horizons. Notable species recovered from the tufa include Acicula fusca, Vertigo substriata, V. alpestris, V. moulinsiana (all rare or unknown living in northern France). Hygromia limbata, a twentieth century introduction to Britain, was previously thought to be a relatively recent arrival in northern France, but its record at St Germain shows that it has been present in Normandy since 6500 yr BP. Azeca goodalli, another shade‐demanding species, appeared at St Germain much later, just after 4420±65 yr BP. Several other species present in the tufa, such as Pomatias elegans, no longer live on the site, adding to the evidence for a distributional decline in Normandy and elsewhere. Perhaps the most noteworthy record is that of Leiostyla anglica, between about 8500 yr BP and 5000 yr BP, because this constitutes its only Holocene occurrence from mainland Europe. Following this discovery it seems likely that further Holocene sites with L. anglica may be found along the western seaboard of Europe connecting populations in Iberia with those of the British Isles. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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A wide range of palaeoenvironmental evidence from the Holocene has suggested periodicities in the Earth's climate of 10s to 1000s of years. Identifying these millennial‐, century‐ and decadal periodicities, and their impacts, is critical in developing a fuller understanding of natural climate variability. Any solar‐induced climatic change needs to be distinguished from other causes of natural climate variability and from short‐term catastrophic events induced either by external or internal processes. Such events might themselves generate a periodicity, or in combination with other forcing factors they may contribute towards a periodicity (and so spuriously imply a universal and continuing periodicity in the climate record), or they may resonate with a solar‐induced periodicity. Here, evidence from peat records for periodicity in climate change over the mid to late Holocene is reviewed and this is followed by a test of the replicability of claimed periodicities using blanket peat data covering the past 2000 yr from four sites in the British Isles. Results suggest that the mires studied do go through phases of being responsive to periodic forcing factors, with ca. 200, ca. 80 and 60–50 yr wavelengths reflected in some data sets. However, the patterns shown are not consistent. This could be the result of local conditions at individual mires (human impact, sensitivity and vegetation succession) or of changes in the strength or nature of global forcing factors. Assessing a solar–mire link remains difficult because the century‐scale variations of the Sun show different intervals between solar minima, the durations of which are themselves unequal, and because the proxy‐climate data‐sets from peat profiles may themselves not be dated with sufficient precision and/or accuracy. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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Michel Magny Odile Peyron Laura Sadori Elena Ortu Giovanni Zanchetta Boris Vannière Willy Tinner 《第四纪科学杂志》2012,27(3):290-296
Pollen‐based quantitative estimates of seasonal precipitation from Lake Pergusa and lake‐level data from Lake Preola in Sicily (southern Italy) allow three successive periods to be distinguished within the Holocene: an early Holocene period before ca. 9800 cal a BP with rather dry climate conditions in winter and summer, a mid‐Holocene period between ca. 9800 and 4500 cal a BP with maximum winter and summer wetness, and a late Holocene period after 4500 cal a BP with declining winter and summer wetness. This evolution observed in the south‐central Mediterranean shows strong similarities to that recognized in the eastern Mediterranean. But, it contrasts with that reconstructed in north‐central Italy, where the mid‐Holocene appears to be characterized by a winter (summer) precipitation maximum (minimum), while the late Holocene coincided with a decrease (increase) in winter (summer) precipitation. Maximum precipitation at ca. 10 000–4500 cal a BP may have resulted from (i) increased local convection in response to a Holocene insolation maximum at 10 000 cal a BP and then (ii) the gradual weakening of the Hadley cell activity, which allowed the winter rainy westerlies to reach the Mediterranean area more frequently. After 4500 cal a BP, changes in precipitation seasonality may reflect non‐linear responses to orbitally driven insolation decrease in addition to seasonal and inter‐hemispheric changes of insolation. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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Reconstructions of past climatic changes on the northeastern Tibetan Plateau(NETP) can provide insights into the Asian summer monsoon(ASM) variability. However, the regional climate changes on both orbital and suborbital time scales since the late Glacial remain controversial. Here we present the results of high-resolution geochemical and grain-size analyses of a sediment core from Genggahai Lake, a small, shallow lake in this area. Changes of the accumulation rate of the core sediments show a similar trend with variations of the coarse fraction. Accordingly, the chronological framework is constructed using a grain-size age model. In addition, the histories of chemical weathering and aeolian activity since the late Glacial are reconstructed based on the Al/Ti ratios and coarse fractions, respectively. The results suggest that an enhanced chemical weathering and a weakened aeolian activity occurred on the NETP under a warmer, wetter climate during the early to mid-Holocene(11.3–6.3 ka cal BP), compared with the late Glacial(17.1–11.3 ka cal BP) and the late Holocene(6.3 ka cal BP to present), which responded mainly to the strengthened ASM on orbital time scale. In addition, the synchronous occurrences of weakened chemical weathering, low lake level and intense aeolian activity on suborbital time scale reflect several episodes of weakened ASM. Furthermore, these episodes largely coincide with the centennial-to millennial-scale cold events in the North Atlantic, which demonstrates the close connection between the ASM and the cooling at high latitudes. 相似文献
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Changes in tree density are estimated for the northern forest‐limit region of Finnish Lapland over the past seven and a half millennia. This is done using dendrochronologically dated Scots pine megafossils and their tree‐ring series. Direct and indirect estimates of past tree density are derived from chronology sample size (CSS) and growth trend modelling (GTM) respectively. The latter is a new method, where the past levels of growth competition (the influence of nearby trees) are extracted from the behaviour of growth trends in cross‐dated tree‐ring series, trends that are expected to be driven by tree‐density control. Two records constructed from the same original data set but by different means (independently) correlate significantly. Both records show a tree‐density maximum around 3000–1750 BC , indicating relative warmth during that time. Another positive tree‐density anomaly occurred in accordance with the ‘Medieval Warm Period’, preceding the thinning at the forest‐limit due to the ‘Little Ice Age’. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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Asfawossen Asrat Andy Baker Mohammed Umer Mohammed Melanie J. Leng Peter Van Calsteren Claire Smith 《第四纪科学杂志》2007,22(1):53-63
An annually laminated stalagmite from Southeastern Ethiopia grew for 443±8 yr starting at 5023 yr BP (±160 yr) as determined by lamina‐tuned U‐Th dating. The speleothem shows three growth phases: (1) an initial phase where the stalagmite has a candlestick shape, deposited by relatively slow rate drip water; (2) a middle growth phase where laminae grow on the flanks, suggesting faster drip rates; and (3) a final growth phase where the speleothem becomes narrower owing to a decrease in drip water supply towards the end of deposition. Morphometry, annual growth rate, fluorescence index, and 234U/238U isotope ratio show marked differences between the growth phases, while δ13C and δ18O do not show significant variation, except within the third phase towards the top of the stalagmite, where they have higher values. The study indicates that the multiproxy approach is crucial as the different proxies respond to single climate forcing in different manners. Our results can be widely applied to speleothem studies distinguishing the relative importance of the various proxies in recording cave ‘external’ (climate, hydrology) and cave ‘internal’ (e.g. evaporation) processes. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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Palynological and sedimentological analyses of a sedimentary sequence sampled at Hauterive/Rouges‐Terres, Lake Neuchâtel (Switzerland) provide documentation of changes in vegetation and lake‐level during the Bølling, Younger Dryas and Preboreal pollen zones, and have allowed a comparison with sequences covering the same period from other sites located in the western part of the Swiss Plateau. The Juniperus–Hippophaë zone (regional pollen assemblage zone (RPAZ) CHb‐2, first part of the Bølling, ca. 14 650–14 450 cal. yr BP) was characterised by a generally low lake‐level. A weak rise occurred during this zone. The Juniperus–Hippophaë to Betula zone transition coincided with a lake‐level lowering, interrupted by a short‐lived but marked phase of higher lake‐level recorded at the neighbouring site of Hauterive‐Champréveyres, but not present at Hauterive/Rouges‐Terres owing to an erosion surface. Shortly after the beginning of the Betula zone (RPAZ CHb‐3, second part of the Bølling, ca 14 450–14 000 cal. yr BP), a marked rise in lake‐level occurred. It was composed of two successive periods of higher level, coinciding with high values of Betula, separated by a short episode of relatively lower lake‐level associated with raised values in Artemisia and other non‐arboreal pollen. The last part of RPAZ CHb‐3 saw a fall in lake‐level. The lower lake‐levels during RPAZ CHb‐2 to early RPAZ CHb‐3 can be correlated with the abrupt warming at the beginning of the Greenland Interstadial (GI) 1e thermal maximum. The successive episodes of higher lake‐level punctuating the GI 1e might be linked to the so‐called Intra‐Bølling Cold Oscillations identified from several palaeoclimatic records in the North Atlantic area, and also documented in oxygen‐isotope data sets from Swiss Plateau lakes. The Hauterive/Rouges‐Terres lake‐level record provides evidence for marked climatic drying through the second part of the Younger Dryas event (GS1), during the GS1–Preboreal (RPAZ CHb‐4b–4c) transition (except for a rise at ca. 11 450–11 400 cal. yr BP), and at the RPAZ CHb‐4c–5 (Preboreal–Boreal) transition, following the Preboreal Oscillation (after 11 150 cal. yr BP). The Preboreal Oscillation coincided with higher lake‐levels, its end being followed by a rapid expansion of Corylus, Quercus, Ulmus and Tilia. The Hauterive/Rouges‐Terres lake‐level record suggests that radiocarbon plateau at 12 600, 10 000 and 9500 14C yr BP corresponded to periods of generally lower lake‐level. This suggests that an increase in solar activity may have contributed to both climatic dryness and a decrease in atmospheric radiocarbon content. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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Pollen and macrofossil analyses of a sediment core from Beaver Pond (60° 37′ 14″ N, 154° 19′ W, 579 m a.s.l.) reveal a record of regional and local postglacial vegetation change in south‐western Alaska. The chronology is based on five AMS (accelerator mass spectrometry) 14C ages obtained from terrestrial plant macrofossils. Pollen and macrofossil records suggest that open herb and shrub tundra with e.g. Poaceae, Cyperaceae, Artemisia, Vaccinium and Salix prevailed on the landscape before ca. 14 000 cal a BP. The shift from herb‐ to shrub‐dominated tundra (Salix, subsequent Betula expansion) possibly reflects climatic warming at the beginning of the Bølling period at ca. 14 700–14 500 and around 13 500 cal a BP. Vegetation (Betula shrub tundra) remained relatively stable until the early Holocene. Macrofossil influx estimates provide evidence for greater biomass in Betula shrub tundra during the early postglacial period than today. Charcoal accumulation rates suggest tundra fire activity was probably greater from ca. 12 500 to 10 500 cal a BP, similar to results from elsewhere in Alaska. The pollen and macrofossil records of Beaver Pond suggest the prevalence of low shrub tundra (shrub Betula, Betula nana, Vaccinium, Ledum palustre, Ericaceae) and tall shrub tundra (Alnus viridis ssp. crispa, Salix) between 10 000 and 4000 cal a BP. This Holocene vegetation type is comparable with that of the modern treeless wet and moist tundra in south‐western Alaska. The expansion of Picea glauca occurred ～4000 cal a BP, much later than that of A. viridis (ssp. crispa), whereas in central and eastern Alaska Picea glauca expanded prior to or coincident with Alnus (viridis). At sites located only 200–400 km north‐east of Beaver Pond (Farewell and Wien lakes), Picea glauca and Betula forests expanded 8000–6000 cal a BP. Unfavourable climatic conditions and soil properties may have inhibited the expansion and establishment of Picea across south‐west Alaska during the mid and late Holocene. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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Robin J. Edwards 《第四纪科学杂志》2001,16(3):221-235
A foraminiferal transfer function for mean tide level (MTL) is used in combination with AMS radiocarbon dated material to construct a record of relative sea‐level (RSL) change from Poole Harbour, southern Britain. These new data, based on multiple cores from duplicate sites, indicate four phases of change during the last 5000 cal. (calendar) yr: (i) rising RSL between ca. 4700 cal. yr BP and ca. 2400 cal. yr BP; (ii) stable to falling RSL from ca. 2400 cal. yr BP until ca. 1200 cal. yr BP; (iii) a brief rise in RSL from ca. 1200 cal. yr BP to ca. 900 cal. yr BP, followed by a period of stability; (iv) a recent increase in the rate of RSL rise from ca. 400–200 cal. yr BP until the present day. In addition, they suggest that the region has experienced long‐term crustal subsidence at a rate of 0.5 mm C14 yr?1. Although this can account for the overall rise in MTL observed during the past 2500 yr, it fails to explain the changes in the rate of rise during this period. This implies that the phases of RSL change recorded in the marshes of Poole Harbour reflect tidal range variations or ‘eustatic’ fluctuations in sea‐level. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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Very little is known about the temporal pattern and the palaeoenvironmental implications of Holocene colluvial processes (debris‐flow and water‐flow processes) in eastern Norway. In this study, well‐dated sedimentary successions from 19 deep excavations are used to reconstruct Holocene colluvial activity in upper Gudbrandsdalen, eastern Norway. Following deglaciation, debris‐flow and water‐flow events have been common in upper Gudbrandsdalen throughout the Holocene, with 62% of the recognised debris‐flow and water‐flow units deposited prior to 5000 cal. yr BP. Relatively high colluvial activity is recorded at ca. 8600–7400, 2400–1900 and 800–400 cal. yr BP, with a conspicuous peak at ca. 8500–8100. Periods of relatively low colluvial activity are recorded at ca. 7100–6500, 5900–5300 and 3500–2500 cal. yr BP. Two different weather situations, unusually heavy rains and warm periods during the snowmelt season, are responsible for triggering colluvial processes in this area. These different weather situations may in turn be related to different climatic conditions. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
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Research on abrupt paleoclimatic and paleoenvironmental change provides a scientific basis for evaluating future climate. Because of spatial variability in monsoonal rainfall, our knowledge about climate change during the mid-to lateHolocene in southern China is still limited. We present a multi-proxy record of paleoclimatic change in a crater lake, Lake Shuangchi. Based on the age-depth model from 210 Pb, 137 Cs and AMS14 C data, high-resolution mid-to late-Holocene climatic and environmental records were reconstructed using multiple indices(TOC, TN, C/N, δ13 C and grain size). Shuangchi underwent a marked change from a peat bog to a lake around 1.4 kaBP. The C3 plants likely dominated during 7.0–5.9 ka and 2.5–1.4 kaBP, while C4 plants dominated between 5.9–3.2 and 3.0–2.5 kaBP. Algae were dominant sources of organic matter in the lake sediments after 1.4 kaBP. Several intervals with high concentrations of coarser grain sizes might be due to flood events. These results reveal that several abrupt paleoclimatic events occurred around 6.6 ka, 6.1 ka, 5.9 ka, 3.0 ka, 2.5 ka and 1.4 kaBP. The paleoclimatic change recorded in the lake may be related to the migration of the Intertropical Convergence Zone(ITCZ) and El Ni?o-Southern Oscillation(ENSO) activity. 相似文献
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Two ca 8000 year long sediment cores from the Gotland Deep, the central sub‐basin of the Baltic Sea, were studied by means of digital images, X‐radiographs and scanning electron microscopy–energy‐dispersive X‐ray mineralogical analysis to gain understanding of the physicochemical and biological influences on sedimentary‐fabric formation in modern and ancient seas with a high flux of organic carbon, and associated oxygen stress and depauperate ichnofauna. Four lithofacies were recognized: (i) sharply laminated mud; (ii) biodeformed mud; (iii) burrow‐mottled mud; and (iv) sedimentation‐event bed. The sharply laminated and burrow‐mottled facies dominate the cores as alternating long intervals, whereas the biodeformed and sedimentation‐event facies occur as thin interbeds within the sharply laminated intervals. The sharply laminated mud comprises alternating diatom‐rich and lithic laminae, with occasional Mn‐carbonate laminae. Lamination discontinuity horizons within the laminites, where the regular lamination is overlain sharply by gently inclined lamination, challenge the traditional view of mud accumulation by settling from suspension, but indicate localized accumulation by particle‐trapping microbial mats and, potentially, by the rapid lateral accretion of mud from bedload transport. The biodeformed interbeds record brief (few years to few decades) oxic–dysoxic conditions that punctuated the anoxic background conditions and permitted sediment‐surface grazing and feeding by a very immature benthic community restricted to the surface mixed tier. The likely biodeformers were meiofauna and nectobenthic pioneers passively imported with currents. The sedimentation‐event interbeds are distal mud turbidites deposited from turbidity currents probably triggered by severe storms on the adjacent coastal areas. The turbidite preservation was favoured by the anoxic background conditions. The long burrow‐mottled intervals are characterized by intensely bioturbated fabrics with discrete Planolites, rare Arenicolites/Polykladichnus and very rare Lockeia trace fossils, as well as bivalve biodeformational structures which represent shallowly penetrating endobenthic feeding and grazing strategies and permanent dwellings. These burrowed intervals represent longer periods (several years to few centuries) of oxic–dysoxic conditions that permitted maturation in the benthos by means of larval settling of opportunistic worm‐like macrofauna and bivalves, resulting in the development of a transition tier. These observations imply more dynamic and oxic depositional conditions in Gotland Deep than previously thought. Comparison to previous zoobenthic studies in the area allowed discussion of the benthic dynamics, and the identification of probable biodeforming and trace‐producing species. Implications for current biofacies and trace‐fossil models are discussed. 相似文献