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91.
New pollen, micro-charcoal, sediment and mineral analyses of a radiocarbon-dated sediment core from the Serra Sul dos Carajás (southeast Amazonia) indicate changes between drier and wetter climatic conditions during the past 25,000 yr, reflected by fire events, expansion of savanna vegetation and no-analog Amazonian forest communities. A cool and dry last glacial maximum (LGM) and late glacial were followed by a wet phase in the early Holocene lasting for ca. 1200 yr, when tropical forest occurred under stable humid conditions. Subsequently, an increasingly warm, seasonal climate established. The onset of seasonality falls within the early Holocene warm period, with possibly longer dry seasons from 10,200 to 3400 cal yr BP, and an explicitly drier phase from 9000 to 3700 cal yr BP. Modern conditions with shorter dry seasons became established after 3400 cal yr BP. Taken together with paleoenvironmental evidence from elsewhere in the Amazon Basin, the observed changes in late Pleistocene and Holocene vegetation in the Serra Sul dos Carajás likely reflect large-scale shifts in precipitation patterns driven by the latitudinal displacement of the Inter-Tropical Convergence Zone and changes in sea-surface temperatures in the tropical Atlantic. 相似文献
92.
Oxygen isotopes from Chinese caves: records not of monsoon rainfall but of circulation regime 总被引:1,自引:0,他引:1
Oxygen isotope variations in Chinese stalagmites have been widely interpreted as a record of the amount of East Asian summer monsoonal rainfall. This interpretation infers decreasing monsoonal rainfall from the mid‐Holocene and large, dipolar rainfall oscillations within glaciations. However, the speleothem δ18O variations conflict with independent palaeoclimate proxies (cave δ13C, loess/palaeosol magnetic properties, δ13C alkanes), which indicate no systematic decline in rainfall from the mid‐Holocene, and no glacial rainfall maxima. Using mass balance calculations (which incorporate seasonality effects in both δ18O concentration and amount of precipitation), we demonstrate that the cave δ18O variations cannot be accounted for by summer rainfall changes, or rainfall seasonality or winter cooling, but instead reflect changes in moisture source. A possible driver of the δ18O variations in Chinese stalagmites is precessional forcing of inter‐hemispheric temperature gradients, and resultant shifts in the position and intensity of the subtropical pressure cells. Through such forcing, Indian monsoon‐sourced δ18O may have dominated at times of high boreal summer insolation, and local Pacific‐sourced moisture at low insolation. Suppression of summer monsoonal rainfall during glacial stages may reflect diminished sea and land surface temperatures and the radiative impacts of increased regional dust fluxes. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
93.
Lynda Yorke Barbara T. Rumsby Richard C. Chiverrell 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(4):608-625
This paper provides sedimentological and morphological data from an investigation of the Late Devensian glacigenic deposits along the Tyne valley, northeast England. The area lies in the central sector of the British-Irish Ice Sheet, with the lowlands influenced by both the Tyne Gap and Tweed-Cheviot ice streams. The sequences here provide insights into the existence of complex, multi-phase activity within the British-Irish Ice Sheet. Field mapping of the area reveals kamiform topography in the Tyne lowlands and lower South Tyne valley, whilst the mid Tyne is characterised by high-level sandur terraces. Inset below the glacial features are river terraces. The sedimentary sequence comprises diamicton overlain by gravel and sandy gravels; sands, muddy sands and gravels; laminated silty sands and muds; and well sorted sands and gravel. The depositional environments indicate ice-contact, subaqueous and terrestrial sedimentation, with supraglacial, proglacial, subaquatic and paraglacial landsystems. Following the onset of deglaciation, westward retreat of Tyne Gap ice resulted in land to the east and southeast of its margin becoming ice-free. Continued/renewed southward flow of ice along the North Sea coast formed a persistent barrier to sediment-charged meltwaters draining the Tyne Gap ice margin. The separation of these two ice masses allowed a glacial lake to develop in the lower Tyne fed by a large proglacial sandur system, which with ice marginal retreat subsequently merged with Glacial Lake Wear. The sediment sequences record the final waning of the Tyne Gap ice stream, and are contiguous with sediments that extend west through the Tyne Gap and into the Cumbrian lowlands. 相似文献
94.
Christopher Buzzelli Rebecca Robbins Peter Doering Zhiqiang Chen Detong Sun Yongshan Wan Barbara Welch Arthur Schwarzschild 《Estuaries and Coasts》2012,35(6):1401-1415
Decreased salinity and submarine light associated with hurricanes of 2004?C2005 impacted seagrass habitats in the Florida coastal zone. A combination of salinities ??20 and light attenuation ??1.5?m?1 resulting from the freshwater discharge in 2005 were among the drivers for a widespread decrease in the coverage and biomass of Syringodium filiforme (manatee grass) in 2006. These observations provided an opportunity to develop and apply a modeling framework to simulate responses of S. filiforme to variable water quality. The framework connects water column variables to field monitoring of seagrass abundance and salinity growth response experiments. The base model was calibrated with macrophyte abundance observed in southern Indian River Lagoon (IRL) from 2002 to 2007 and tested against shoot data from a different time (1997?C2002) and nearby location in the IRL. Model shoot biomass (gC?m?2) was similar to field observations (r 2?=?0.70) while responding to monthly seasonal fluctuations in salinity and light throughout the 6-year simulations. Field and model results indicated that S. filiforme growth and survival were sensitive to, and increased with, rising salinity throughout 2007. This modeling study emphasizes that discharge, salinity, and submarine light are inter-dependent variables affecting South Florida seagrass habitats on seasonal to inter-annual time scales. 相似文献
95.
Barbara S. Jessup W. Jesse Hahm Scott N. Miller James W. Kirchner Clifford S. Riebe 《Applied Geochemistry》2011
The dynamics of granitic landscapes are modulated by bimodal weathering, which produces patchy granular soils and expanses of bare rock ranging from meter-scale boulders to mountain-scale domes. We used terrain analysis and with cosmogenic nuclide measurements of erosion rates to quantitatively explore Wahrhaftig’s decades-old hypothesis for the development of “stepped topography” by differential weathering of bare and soil-mantled granite. According to Wahrhaftig’s hypothesis, bare granite weathers slower than soil-mantled granite; thus random erosional exposure of bare rock leads to an alternating sequence of steep, slowly weathering bedrock “steps” and gently sloped, but rapidly weathering, soil-mantled “treads.” Our investigation focused on the terrain surrounding the Southern Sierra Critical Zone Observatory (CZO), which is underlain by granitic bedrock and lies outside the limits of recent glaciation, in the heart of the stepped topography described by Wahrhaftig. Our digital terrain analysis confirms that steep steps often grade into gentle treads, consistent with Wahrhaftig’s hypothesis. However, we observe a mix-and-match of soil and bare rock on treads and steps, contrary to one of the hypothesis’ major underpinnings – that bare rock should be much more common on steps than on treads. Moreover, the data show that bare rock is not as common as expected at step tops; Wahrhaftig’s hypothesis dictates that step tops should act as slowly eroding base levels for the treads above them. The data indicate that, within each landscape class (i.e., the steps and treads), bare rock erodes more slowly than surrounding soil. This suggests that the coupling between soil production and denudation in granitic landscapes harbors a tipping point wherein erosion rates decrease when soils are stripped to bedrock. Although broadly consistent with the differential weathering invoked by Wahrhaftig, the data also show that steps are eroding faster than treads, undermining Wahrhaftig’s explanation for the origins of the steps. The revised interpretation proposed here is that the landscape evolves by back-wearing of steps in addition to differential erosion due to differences in weathering of bare and soil-mantled granite. 相似文献
96.
A review of research on seismic behaviour of irregular building structures since 2002 总被引:1,自引:1,他引:0
The paper presents an overview of the progress in research regarding seismic response of plan and vertically irregular building
structures. Three areas of research are surveyed. The first is the study of the effects of plan-irregularity by means of single-storey
and multi-storey building models. The second area encompasses passive control as a strategy to mitigate torsional effects,
by means of base isolation and other types of devices. Lastly, the third area concerns vertically irregular structures and
setback buildings. Although fewer papers have been published in this last area with respect to the former ones, this state-of-the-art
reports extensively on research efforts and progress into the seismic behaviour of irregular buildings in elevation to show
the growing interest among specialists in the field. 相似文献
97.
Broxton W. Bird Mark B. Abbott Bruce P. Finney Barbara Kutchko 《Journal of Paleolimnology》2009,41(1):25-41
Varved minerogenic sediments from glacial-fed Blue Lake, northern Alaska, are used to investigate late Holocene climate variability.
Varve-thickness measurements track summer temperature recorded at Atigun Pass, located 41 km east at a similar elevation (r
2 = 0.31, P = 0.08). Results indicate that climate in the Brooks Range from 10 to 730 AD (varve year) was warm with precipitation inferred
to be higher than during the twentieth century. The varve-temperature relationship for this period was likely compromised
and not used in our temperature reconstruction because the glacier was greatly reduced, or absent, exposing sub-glacial sediments
to erosion from enhanced precipitation. Varve-inferred summer temperatures and precipitation decreased after 730 AD, averaging
0.4°C above the last millennial average (LMA = 4.2°C) from 730 to 850 AD, and 0.1°C above the LMA from 850 to 980 AD. Cooling
culminated between 980 and 1030 AD with temperatures 0.7°C below the LMA. Varve-inferred summer temperatures increased between
1030 and 1620 AD to the LMA, though the period between 1260 and 1350 AD was 0.2°C below the LMA. Although there is no equivalent
to the European Medieval Warm Period in the Blue Lake record, two warm intervals occurred from 1350 to 1450 AD and 1500 to
1620 AD (0.4 and 0.3°C above the LMA, respectively). During the Little Ice Age (LIA; 1620 to 1880 AD), inferred summer temperature
averaged 0.2°C below the LMA. After 1880 AD, inferred summer temperature increased to 0.8°C above the LMA, glaciers retreated,
but aridity persisted based on a number of regional paleoclimate records. Despite warming and glacial retreat, varve thicknesses
have not achieved pre-730 AD levels. This reflects limited sediment availability and transport due to a less extensive retreat
compared to the first millennium, and continued relative aridity. Overall, the Blue Lake record is similar to varve records
from the eastern Canadian Arctic that document a cool LIA and twentieth century warming. However, the occurrence and timing
of events, such as the LIA and Medieval Warm Period, varies considerably among records, suggesting heterogeneous climatic
patterns across the North American Arctic.
相似文献
Broxton W. BirdEmail: |
98.
99.
Microbial populations in contaminant plumes 总被引:4,自引:1,他引:3
Received, May 1999/Revised, October 1999/Accepted, October 1999 相似文献
100.
We consider in detail the evolutionary patterns of few white-light flares observed by Yohkoh. The following data have been used in the analysis: sequences of de-convolved SXT images in X-ray and optical filters, MEM reconstructed HXT images and the other supporting data. The resolution in the de-convolved images is below 1 arc sec. Working with sequences of de-convolved images makes it also possible to investigate the dynamics of these structures with high spatial accuracy. Comparison of the morphology of flare brightenings as observed in hard, soft and optical ranges reveals that these emissions are not co-spatial and are most probably related to different plasma volumes at any instant. These observations cannot be easily accommodated within standard flare scenarios. Traditionally, the hard and optical flare emissions are expected to be co-spatial and the soft X-ray emission is presumed to fill the coronal portion of flaring loop(s) during rise phase. Present observations do not easily fit to such scenario. 相似文献