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Sabine K.J. Cochrane Stanislav G. Denisenko Paul E. Renaud Christopher S. Emblow William G. Ambrose Jr. Ingrid H. Ellingsen Jofrid Skarhamar 《Journal of Sea Research》2009,61(4):222-233
Benthic faunal assemblages were analysed from 47 stations in the central and southern parts of the Barents Sea, together with sedimentary and water column parameters, daily ice records and modelled integrated primary productivity. Sampling spanned areas influenced by Atlantic Water (AW) to those lying under Arctic Water (ArW), and included stations with mixed water masses. Ice cover suppressed water column productivity in the northern areas. Three main faunal groups were identified, based on similarity of numerical faunal composition. The northern and southern faunal groups were separated by the northernmost penetration of AW in the bottom water and the third group, the Hopen group, was influenced by modified bank water. Faunal abundances were significantly higher within the southern faunal group relative to the northern group, but the numbers of taxa present were similar. The particularly rich fauna of the Hopen group reflected sediment heterogeneity and tight pelagic–benthic coupling. These results suggest that a retreat and thinning of the ice cover in the Barents Sea likely will result in the northern parts of the Barents Sea becoming more Atlantic in character, with a higher productivity at the sea floor. 相似文献
2.
Lucien von Gunten Oliver Heiri Christian Bigler Jacqueline van Leeuwen Carlo Casty André F. Lotter Michael Sturm 《Journal of Paleolimnology》2008,39(3):283-299
We analysed a 42 cm long sediment record from Lej da la Tscheppa, a high-altitude lake (2,616 m a.s.l.) in the Upper Engadine
valley (Switzerland) for subfossil diatoms, chironomids and pollen. The chronology of the top 21 cm of the record was established
using 210Pb analysis using a constant-rate-of-supply model, and validated with 137Cs measurements and the content of spheroidal carbonaceous particles. A tentative chronology for the lower part of the core
was obtained through extrapolation of the sedimentation rates in the uppermost part of the record. Pollen assemblages in the
record reflect regional changes in forestation and land-use patterns in the Upper Engadine valley and show no evidence of
significant local human activity in the lake’s catchment. Diatom assemblages record a distinct increase in planktonic taxa
since the early 19th century, suggesting a decrease in the duration of ice-cover. In contrast, chironomid assemblages remained
stable during a large part of the record. We applied an established chironomid-based July air temperature transfer function
and a newly developed diatom-based spring air temperature transfer function to reconstruct past seasonal air temperature changes
at Lej da la Tscheppa. The reconstructions indicate a diatom-inferred warming trend in spring temperatures during the past
ca. 400 years, whereas chironomid-inferred summer temperatures suggest a slight cooling trend. These biota-based reconstructions
are in good agreement with the centennial-scale temperature trend in an independent reconstruction of regional temperatures
in the Upper Engadine region based on instrumental records and documentary proxy evidence from the Alps. Our results suggest
that, in high-altitude lakes, independent chironomid- and diatom-based seasonal temperature reconstruction is possible and
can be successfully used to track seasonal temperature trends. 相似文献
3.
Diatom and chrysophyte cyst-based reconstructions of the dates of spring and autumn lake-mixing enabled us to estimate spring
(STanom) and autumn (ATanom) temperature anomalies as well as ice-cover of the last ca. 4,000 years in a lake sediment core (Oberer Landschitzsee, 2,076
m a.s.l.) from the southern slopes of the Austrian Central Alps. The two independently inferred temperature anomalies were
significantly correlated. On average, spring and autumn temperatures were lower during the two millennia B.C than during 0–1,300
A.D. Marked spring and autumn temperature minima occurred at about 1,300 and 600 B.C. At about 1,300 A.D, STanom declined again. Spring-temperature anomalies during Roman and Medieval times equaled or slightly exceeded the modern values
and paralleled tree-line and glacier fluctuations. The de-coupling of autumn and spring climates, which began during the Medieval
period, might indicate changes in major circulation modes. It was assumed that the North-Atlantic influence, triggering winter-rain
climate in the Northern Mediterranean, became weaker during Medieval times, resulting in a trend towards warmer autumns and
overall more continental climate conditions in the study area. Four pulses of land use, inferred from indicator pollen, occurred
during (1) the Early to Late Bronze, (2) the transition from Late Bronze to Early Iron Age (Hallstatt), (3) Late Iron Age
(La Tène, Celtic time) to Roman times, and (4) during high to late Medieval times. Climate seemed to be an important, though
complex, trigger of Alpine land use. 相似文献
4.
S. Mohapatra 《地球物理与天体物理流体动力学》2013,107(5):347-374
Scattering of oblique incident waves by small bottom undulation in a two-layer fluid, where the upper layer has a thin ice-cover while the lower one has the undulation, is investigated within the framework of linearized water wave theory. The ice-cover is being modeled as an elastic plate of very small thickness. There exist two modes of time-harmonic waves–one with lower wave number propagating along the ice-cover (ice-cover mode) and the other with higher wave number along the interface (interfacial mode). A perturbation analysis is employed to solve the corresponding boundary value problem governed by modified Helmholtz equation and thereby evaluating the reflection and transmission coefficients approximately up to first order for both modes. A patch of sinusoidal ripples, having two different wave numbers over two consecutive stretches, is considered as an example and the related coefficients are determined. It is observed that when the wave is incident on the ice-cover surface we always find energy transfer to the interface, but for interfacial incident waves there are parameter ranges for which no energy transfer to the ice-cover surface is possible. Also it is observed that for small angles of incidence, the reflected energy is more as compared to the other angles of incidence. These results are demonstrated in graphical form. From the derived results, the solutions for problems with free surface can be obtained as particular cases. 相似文献
5.
The scattering of plane surface waves by bottom undulations in an ice-covered ocean modelled as a two-layer fluid consisting of a layer of fresh water of lesser density above a deep layer of salt water, is investigated here by using a simplified perturbation analysis. In such a two-layer fluid there exist waves of two different modes, one with higher mode propagates along the interface and the other with lower mode propagates along the ice-cover. An incident wave of a particular mode gets reflected and transmitted by the bottom undulations into waves of both the modes so that transfer of wave energy from one mode to another takes place. The first-order reflection and transmission coefficients of two different modes are obtained due to incident waves of again two different modes by employing Fourier transform technique in the mathematical analysis. For sinusoidal bottom topography these coefficients are depicted graphically against the wavenumber. These figures show how the transfer of energy from one mode to another takes place. 相似文献
6.
During the last four decades, several numerical formulations and specialized software have been developed in response to studies about dam break (DB) wave propagation and its hydraulic and environmental impacts on downstream hydraulic structures and valleys. These methods cannot, however, be used to predict wave propagation within partially covered channels or reservoirs located upstream of hydraulic structures. In fact, such problems require the modelling of the complex transition from a free surface flow into a pressurized one. Because rivers or channels partially covered with ice sheets are typical examples commonly met in winter in northern climates, it is vitally important to assess ice-cover effects on the DB wave propagation and develop appropriate tools to predict resulting hydrodynamic loads on hydraulic structures downstream. This paper proposes an original numerical formulation to model wave propagation and hydrodynamic pressure in partially covered channels. The proposed formulation uses one-dimensional St. Venant equations to simulate open-water flow and water hammer equations to simulate pressure flow within the partially covered channel. To illustrate the use of the hydrodynamic pressures obtained, a case study is presented where a channel cover and a dam located downstream are modelled using finite elements to investigate their dynamic structural response. 相似文献
7.
Roland Schmidt Monika Roth Richard Tessadri Kaarina Weckström 《Journal of Paleolimnology》2008,40(1):453-469
Major and trace elements, minerals and grain-size were analysed from a sediment core covering the last 4,000 years of an Austrian
Alpine lake (Oberer Landschitzsee, ObLAN, 2,076 m a.s.l.). These analyses were combined with autumn and spring temperature
anomalies and ice-cover estimated from a diatom and chrysophyte cyst, thermistor-based regional calibration dataset and selected
pollen markers published previously. Diatom-inferred pH (Di-pH) and DOC (Di-DOC) completed the multi-proxy approach, which,
together with ordination techniques (PCA and RDA), helped (1) to confirm proposed climatic patterns and hypotheses, and (2)
to disentangle the complex interactions between climatic and anthropogenic impacts. Shifts in chemical and physical weathering,
erosion, production, lake stratification, redox potentials and air pollution were the major processes that have affected Oberer
Landschitzsee in relation to climate, lake/catchment interactions, and human impact. Geochemistry supported the finding that
the four waves of high-altitude land use (Early to Middle Bronze, ca. 1,800–1,300 B.C.; Late Bronze to Hallstatt, ca. 1,000–500
B.C.; Celtic to Roman, ca. 300 B.C.–400 A.D.; and Medieval, ca. 1,000–1,600 A.D.) were coupled mainly with warm periods. Increased
production, onset of lake stratification, and the deterioration of hypolimnetic oxygen conditions were the major changes governing
in-lake processes during climate warming. They resulted in specific element assemblages associated with organic matter accumulation
(Br), oxygen depletion (As, Ga, Cu, S), and changes in redox (Fe/Mn). The Se/Di-DOC ratio was introduced to track shifts between
in-lake production and allochthonous sources. Nutrient loading from pastures, coupled with climate warming, could explain
that in-lake production was highest during Roman and Medieval land use. Lithogenic elements mainly originated from chemical
weathering of silicate bedrock and they increased when intense land-use was coupled with climate deterioration and/or increased
humidity. These perturbations were highest during a High Medieval climate fluctuation around 1,000 A.D. The association of
sand with LOI and C/N and the decoupling of sand from quartz and feldspar separates erosion from physical weathering. Di-DOC,
S, and C/N showed hybrid characters in relation to climate and human impact. The mineral proportions indicated gradients in
relation to weathering, snow-cover and running waters, as well as vegetation. Air pollution by metallurgic industries, starting
during the High Medieval and culminating during the Late Medieval, caused lead accumulation and could have contributed to
As and S enrichment. Corresponding features in the distribution of selected elements and pollen tracers, as well as changes
in mineral proportions, supported the hypotheses of shifts in seasonal climate and an overall trend towards more continental
climate conditions since Medieval times. 相似文献
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