128 research outputs found
The last glacial-interglacial cycle in Lake Ohrid (Macedonia/Albania): testing diatom response to climate
Lake Ohrid is a site of global importance for palaeoclimate research. This study presents results of diatom analysis of a ca. 136 ka sequence, Co1202, from the northeast of the lake basin. It offers the opportunity to test diatom response across two glacial-interglacial transitions and within the Last Glacial, while setting up taxonomic protocols for future research. The results are outstanding in demonstrating the sensitivity of diatoms to climate change, providing proxy evidence for temperature change marked by glacial-interglacial shifts between the dominant planktonic taxa, Cyclotella fottii and C. ocellata, and exact correlation with geochemical proxies to mark the start of the Last Interglacial at ca. 130 ka. Importantly, diatoms show much stronger evidence in this site for warming during MIS3 than recorded in other productivity-related proxies, peaking at ca. 39 ka, prior to the extreme conditions of the Last Glacial maximum. In the light of the observed patterns, and from the results of analysis of early Holocene sediments from a second core, Lz1120, the lack of a response to Late Glacial and early Holocene warming from ca. 15-7.4 ka suggests the Co1202 sequence may be compromised during this phase. After ca. 7.4 ka, there is evidence for enhanced nutrient enrichment compared to the Last Interglacial, following by a post-Medieval cooling trend. Taxonomically, morphological variability in C. fottii shows no clear trends linked to climate, but an intriguing change in central area morphology occurs after ca. 48.7 ka, coincident with a tephra layer. In contrast, C. ocellata shows morphological variation in the number of ocelli between interglacials, suggesting climatically-forced variation or evolutionary selection pressure. The application of a simple dissolution index does not track preservation quality very effectively, underlining the importance of diatom concentration data in future studies
Complexity of diatom response to Lateglacial and Holocene climate and environmental change in ancient, deep and oligotrophic Lake Ohrid (Macedonia and Albania)
© Author(s) 2016. Lake Ohrid (Macedonia and Albania) is a rare example of a deep, ancient Mediterranean lake and is a key site for palaeoclimate research in the northeastern Mediterranean region. This study conducts the analysis of diatoms as a proxy for Lateglacial and Holocene climate and environmental change in Lake Ohrid at a higher resolution than in previous studies. While Lake Ohrid has the potential to be sensitive to water temperature change, the data demonstrate a highly complex diatom response, probably comprising a direct response to temperature-induced lake productivity in some phases and an indirect response to temperaturerelated lake stratification or mixing and epilimnetic nutrient availability in others. The data also demonstrate the possible influence of physical limnological (e.g. the influence of wind stress on stratification or mixing) and chemical processes (e.g. the influence of catchment dynamics on nutrient input) in mediating the complex response of diatoms. During the Lateglacial (ca. 12 300-11 800 cal yr BP), the low-diversity dominance of hypolimnetic Cyclotella fottii indicates low lake productivity, linked to low water temperature. Although the subsequent slight increase in small, epilimnetic C. minuscula during the earliest Holocene (ca. 11 800-10 600 cal yr BP) suggests climate warming and enhanced stratification, diatom concentration remains as low as during the Lateglacial, suggesting that water temperature increase was muted across this major transition. The early Holocene (ca. 10 600-8200 cal yr BP) is characterised by a sustained increase in epilimnetic taxa, with mesotrophic C. ocellata indicating high water-temperature-induced productivity between ca. 10 600-10 200 cal yr BP and between ca. 9500-8200 cal yr BP and with C. minuscula in response to low nutrient availability in the epilimnion between ca. 10 200-9500 cal yr BP. During the middle Holocene (ca. 8200-2600 cal yr BP), when sedimentological and geochemical proxies provide evidence for maximum Holocene water temperature, anomalously low C. ocellata abundance is probably a response to epilimnetic nutrient limitation, almost mimicking the Lateglacial flora apart from the occurrence of mesotrophic Stephanodiscus transylvanicus in the hypolimnion. During the late Holocene (ca. 2600 cal yr BP-present), high abundance and fluctuating composition of epilimnetic taxa are probably a response more to enhanced anthropogenic nutrient input, particularly nitrogen enrichment, than to climate. Overall, the data indicate that previous assumptions concerning the linearity of diatom response in this deep, ancient lake are invalid, and multi-proxy analysis is essential to improve understanding of palaeolimnological dynamics in future research on the long, Quaternary sequence
Complexity of diatom response to Lateglacial and Holocene climate and environmental change in ancient, deep and oligotrophic Lake Ohrid (Macedonia and Albania)
Lake Ohrid (Macedonia and Albania) is a rare example of a deep, ancient Mediterranean lake and is a key site for palaeoclimate research in the northeastern Mediterranean region. This study conducts the analysis of diatoms as a proxy for Lateglacial and Holocene climate and environmental change in Lake Ohrid at a higher resolution than in previous studies. While Lake Ohrid has the potential to be sensitive to water temperature change, the data demonstrate a highly complex diatom response, probably comprising a direct response to temperature-induced lake productivity in some phases and an indirect response to temperature-related lake stratification or mixing and epilimnetic nutrient availability in others. The data also demonstrate the possible influence of physical limnological (e.g. the influence of wind stress on stratification or mixing) and chemical processes (e.g. the influence of catchment dynamics on nutrient input) in mediating the complex response of diatoms. During the Lateglacial (ca. 12 300-11 800 cal yr BP), the low-diversity dominance of hypolimnetic Cyclotella fottii indicates low lake productivity, linked to low water temperature. Although the subsequent slight increase in small, epilimnetic C. minuscula during the earliest Holocene (ca. 11 800-10 600 cal yr BP) suggests climate warming and enhanced stratification, diatom concentration remains as low as during the Lateglacial, suggesting that water temperature increase was muted across this major transition. The early Holocene (ca. 10 600-8200 cal yr BP) is characterised by a sustained increase in epilimnetic taxa, with mesotrophic C. ocellata indicating high water-temperature-induced productivity between ca. 10 600-10 200 cal yr BP and between ca. 9500-8200 cal yr BP and with C. minuscula in response to low nutrient availability in the epilimnion between ca. 10 200-9500 cal yr BP. During the middle Holocene (ca. 8200-2600 cal yr BP), when sedimentological and geochemical proxies provide evidence for maximum Holocene water temperature, anomalously low C. ocellata abundance is probably a response to epilimnetic nutrient limitation, almost mimicking the Lateglacial flora apart from the occurrence of mesotrophic Stephanodiscus transylvanicus in the hypolimnion. During the late Holocene (ca. 2600 cal yr BP-present), high abundance and fluctuating composition of epilimnetic taxa are probably a response more to enhanced anthropogenic nutrient input, particularly nitrogen enrichment, than to climate. Overall, the data indicate that previous assumptions concerning the linearity of diatom response in this deep, ancient lake are invalid, and multi-proxy analysis is essential to improve understanding of palaeolimnological dynamics in future research on the long, Quaternary sequence
Tunneling in quantum cosmology: numerical study of particle creation
We consider a minisuperspace model for a closed universe with small and
positive cosmological constant, filled with a massive scalar field conformally
coupled to gravity. In the quantum version of this model, the universe may
undergo a tunneling transition through an effective barrier between regions of
small and large scale factor. We solve numerically the minisuperspace
Wheeler--De Witt equation with tunneling boundary conditions for the wave
function of the universe, and find that tunneling in quantum cosmology is quite
different from that in quantum mechanics. Namely, the matter degree of freedom
gets excited under the barrier, provided its interaction with the scale factor
is not too weak, and makes a strong back reaction onto tunneling. In the
semiclassical limit of small values of cosmological constant, the matter energy
behind the barrier is close to the height of the barrier: the system ``climbs
up'' the barrier, and then evolves classically from its top. These features are
even more pronounced for inhomogeneous modes of matter field. Extrapolating to
field theory we thus argue that high momentum particles are copiously created
in the tunneling process. Nevertheless, we find empirical evidence for the
semiclassical-type scaling with the cosmological constant of the wave function
under and behind the barrier.Comment: 29 pages, 17 figure
Quantum treatment of neutrino in background matter
Motivated by the need of elaboration of the quantum theory of the spin light
of neutrino in matter (), we have studied in more detail the exact
solutions of the Dirac equation for neutrinos moving in the background matter.
These exact neutrino wavefunctions form a basis for a rather powerful method of
investigation of different neutrino processes in matter, which is similar to
the Furry representation of quantum electrodynamics in external fields. Within
this method we also derive the corresponding Dirac equation for an electron
moving in matter and consider the electromagnetic radiation ("spin light of
electron in matter", ) that can be emitted by the electron in this case.Comment: 10 pages, in: Proceedings of QFEXT'05 (The Seventh Workshop on
Quantum Field Theory under the Influence of External Conditions, IEEC, CSIC
and University of Barcelona, Barcelona, Catalonia, Spain, 5-9 September
2005.), ed. by Emilio Elizalde and Sergei Odintsov; published in Journal of
Physics
One-loop corrections to the metastable vacuum decay
We evaluate the one-loop prefactor in the false vacuum decay rate in a theory
of a self interacting scalar field in 3+1 dimensions. We use a numerical
method, established some time ago, which is based on a well-known theorem on
functional determinants. The proper handling of zero modes and of
renormalization is discussed. The numerical results in particular show that
quantum corrections become smaller away from the thin-wall case. In the
thin-wall limit the numerical results are found to join into those obtained by
a gradient expansion.Comment: 31 pages, 7 figure
Wave function of the Universe in the early stage of its evolution
In quantum cosmological models, constructed in the framework of
Friedmann-Robertson-Walker metrics, a nucleation of the Universe with its
further expansion is described as a tunneling transition through an effective
barrier between regions with small and large values of the scale factor at
non-zero (or zero) energy. The approach for describing this tunneling consists
of constructing a wave function satisfying an appropriate boundary condition.
There are various ways for defining the boundary condition that lead to
different estimates of the barrier penetrability and the tunneling time.
In order to describe the escape from the tunneling region as accurately as
possible and to construct the total wave function on the basis of its two
partial solutions unambiguously, we use the tunneling boundary condition that
the total wave function must represent only the outgoing wave at the point of
escape from the barrier, where the following definition for the wave is
introduced: the wave is represented by the wave function whose modulus changes
minimally under a variation of the scale factor . We construct a new method
for a direct non-semiclassical calculation of the total stationary wave
function of the Universe, analyze the behavior of this wave function in the
tunneling region, near the escape point and in the asymptotic region, and
estimate the barrier penetrability. We observe oscillations of modulus of wave
function in the external region starting from the turning point which decrease
with increasing of and which are not shown in semiclassical calculations.
The period of such an oscillation decreases uniformly with increasing and
can be used as a fully quantum dynamical characteristic of the expansion of the
Universe.Comment: 19 pages, 21 files for 10 EPS figures, LaTeX svjour style. The Sec.2
(formalism of Wheeler-De Witt equation) is reduced. In Sec.3.1 definition of
the outgoing wave from barrier is defined more accurately. In Sec.4.1
semiclassical calculations of wavew function and penetrability are performed
and comparison with results in fully quantum approach is adde
Shapes of leading tunnelling trajectories for single-electron molecular ionization
Based on the geometrical approach to tunnelling by P.D. Hislop and I.M. Sigal
[Memoir. AMS 78, No. 399 (1989)], we introduce the concept of a leading
tunnelling trajectory. It is then proven that leading tunnelling trajectories
for single-active-electron models of molecular tunnelling ionization (i.e.,
theories where a molecular potential is modelled by a single-electron
multi-centre potential) are linear in the case of short range interactions and
"almost" linear in the case of long range interactions. The results are
presented on both the formal and physically intuitive levels. Physical
implications of the obtained results are discussed.Comment: 14 pages, 5 figure
Resonant structure of space-time of early universe
A new fully quantum method describing penetration of packet from internal
well outside with its tunneling through the barrier of arbitrary shape used in
problems of quantum cosmology, is presented. The method allows to determine
amplitudes of wave function, penetrability and reflection relatively the barrier (accuracy of the method: ), coefficient of penetration (i.e. probability of
the packet to penetrate from the internal well outside with its tunneling),
coefficient of oscillations (describing oscillating behavior of the packet
inside the internal well). Using the method, evolution of universe in the
closed Friedmann--Robertson--Walker model with quantization in presence of
positive cosmological constant, radiation and component of generalize Chaplygin
gas is studied. It is established (for the first time): (1) oscillating
dependence of the penetrability on localization of start of the packet; (2)
presence of resonant values of energy of radiation , at which the
coefficient of penetration increases strongly. From analysis of these results
it follows: (1) necessity to introduce initial condition into both
non-stationary, and stationary quantum models; (2) presence of some definite
values for the scale factor , where start of expansion of universe is the
most probable; (3) during expansion of universe in the initial stage its radius
is changed not continuously, but passes consequently through definite discrete
values and tends to continuous spectrum in latter time.Comment: 18 pages, 14 figures, 4 table
Particle creation in a tunneling universe
An expanding closed universe filled with radiation can either recollapse or
tunnel to the regime of unbounded expansion, if the cosmological constant is
nonzero. We re-examine the question of particle creation during tunneling, with
the purpose of resolving a long-standing controversy. Using a perturbative
superspace model with a conformally coupled massless scalar field, which is
known to give no particle production, we explicitly show that the breakdown of
the semiclassical approximation and the ``catastrophic particle production''
claimed earlier in the literature are due to an inappropriate choice of the
initial quantum state prior to the tunneling.Comment: 21 pages, 3 embedded figures, RevTeX
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