50 research outputs found
Community and trophic responses of benthic Foraminifera to oxygen gradients and organic enrichment
Global warming and eutrophication are driving an expansion of hypoxia in the World Ocean. This will favour organisms, such as Foraminifera (testate protists), that tolerate low-oxygen conditions and may lead to an overall decline in marine biodiversity. With this in mind, community and trophic responses of benthic Foraminifera were investigated at two contrasting sites in the upper boundary (140 m water depth; bottom-water oxygen concentrations = 2.05 mll-1 during the spring intermonsoon and 0.11 mll-1 during the SW monsoon) and the core (300 m water depth; bottom-water oxygen concentration consistently ~ 0.11 mll-1) of an intense, natural, mid-water oxygen minimum zone (OMZ) on the Pakistan Margin, NE Arabian Sea. Live macrofaunal (>300 µm fraction) Foraminifera (including softwalled species) and metazoans were examined at each site during the 2003 spring intermonsoon (April) and SW monsoon (October) seasons (4 replicate multicores/site/season, 25.5cm2 surface area, 0-5 cm depth). Wet-sorting revealed a low diversity assemblage dominated (> 60 %) by calcareous Foraminifera at both sites. A total of 36 species was recognised and diversity was not greatly affected by water depth or season. At both sites, >86 % of Foraminifera were restricted to the upper 0-1 cm layer of sediment and the Average Living Depth (ALD) decreased from the spring intermonsoon to the SW monsoon (140 m, ALD5 = 0.41 to 0.33; 300 m, ALD5 = 0.65 to 0.44). Foraminifera increased in mean abundance from 124 to 153 individuals per 10 cm2 from the spring intermonsoon to the SW monsoon at 140 m and from 86 to 122 individuals per 10 cm2 at 300 m. The calcareous species Uvigerina ex. gr. semiornata dominated communities and increased in mean abundance from 54 to 118 individuals (140 m) and from 41 to 69 individuals (300 m) per 10 cm2 following the SW monsoon. At 140 m, Foraminifera were 3.6 times more abundant than metazoans during the spring intermonsoon, rising to 13.9 times during the SW monsoon. The corresponding proportions at 300 m, where metazoans were rare, were 12.4 and 14.5. Fatty acid biomarkers suggest that foraminiferal diets vary between species. The calcareous species U. ex. gr. semiornata, Bolivina aff. dilatata and Globobulimina cf. G. pyrula selectively ingested phytodetrital material, whereas the agglutinated species, Ammodiscus aff. cretaceus, Bathysiphon sp. nov. 1, and Reophax dentaliniformis favoured bacteria. Moreover, U. ex. gr. semiornata, rapidly ingested (within two days) 13C-labelled diatoms in shipboard laboratory and in situ pulse-chase experiments at the 140-m site following the SW monsoon. This enabled the uptake and processing of organic matter (OM) to be tracked in the foraminiferal cell into individual fatty acids, using Gas Chromatography - Mass Spectrometry (selective ion scan). These results suggest that calcareous Foraminifera, in particular U. ex. gr. semiornata, play a central role in OM cycling on the sea-floor in the upper part of the Pakistan margin OMZ
The trophic and metabolic pathways of foraminifera in the Arabian Sea: evidence from cellular stable isotopes
The Arabian Sea is a region of elevated productivity with the highest globally recorded fluxes of particulate organic matter (POM) to the deep ocean, providing an abundant food source for fauna at the seafloor. However, benthic communities are also strongly influenced by an intense oxygen minimum zone (OMZ), which impinges on the continental slope from 100 to 1000 m water depth. We compared the trophic ecology of foraminifera on the Oman and Pakistan margins of the Arabian Sea (140–3185 m water depth). These two margins are contrasting both in terms of the abundance of sedimentary organic matter and the intensity of the OMZ. Organic carbon concentrations of surficial sediments were higher on the Oman margin (3.32 ± 1.4%) compared to the Pakistan margin (2.45 ± 1.1%) and sedimentary organic matter (SOM) quality estimated from the Hydrogen Index was also higher on the Oman margin (300–400 mg HC mg TOC−1) compared to the Pakistan margin (< 250 mg HC mg TOC−1). The δ13C and δ15N values of sediments were similar on both margins (−20 and 8‰, respectively). Stable isotope analysis (SIA) showed that foraminiferal cells had a wide range of δ13C values (−25.5 to −11.5‰), implying that they utilise multiple food sources; indeed δ13C values varied between depths, foraminiferal types and between the two margins. Foraminifera had broad ranges in δ15N values (−7.8 to 27.3‰). The enriched values suggest that some species may store nitrate to utilise in respiration; this was most notable on the Pakistan margin. Depleted foraminiferal δ15N values, particularly at the Oman margin, may reflect feeding on chemosynthetic bacteria. We suggest that differences in productivity regimes may be responsible for the differences observed in foraminiferal isotopic composition. In addition, at the time of sampling, whole jellyfish carcasses (Crambionella orsini) and a carpet of jelly detritus were observed across the Oman margin transect. Associated chemosynthetic bacteria may have provided an organic-rich food source for foraminifera at these sites. Our data suggest that foraminifera in OMZ settings can utilise a variety of food sources and metabolic pathways to meet their energetic demands
Current noise in long diffusive SNS junctions in the incoherent MAR regime
Spectral density of current fluctuations at zero frequency is calculated for
a long diffusive SNS junction with low-resistive interfaces. At low
temperature, T << Delta, the subgap shot noise approaches linear voltage
dependence, S=(2/ 3R)(eV + 2Delta), which is the sum of the shot noise of the
normal conductor and voltage independent excess noise. This result can also be
interpreted as the 1/3-suppressed Poisson noise for the effective charge q =
e(1+2Delta/eV) transferred by incoherent multiple Andreev reflections (MAR). At
higher temperatures, anomalies of the current noise develop at the gap
subharmonics, eV = 2Delta/n. The crossover to the hot electron regime from the
MAR regime is analyzed in the limit of small applied voltages.Comment: improved version, to be published in Phys. Rev.
Long-Range Coulomb Interaction and the Crossover between Quantum and Shot Noise in Diffusive Conductors
Frequency-dependent nonequilibrium noise in quantum-coherent diffusive
conductors is calculated with account taken of long-range Coulomb interaction.
For long and narrow contacts with strong external screening the crossover
between quantum and shot noise takes place at frequencies much smaller than the
voltage drop across the contact. We also show that under certain frequency
limitations, the semiclassical and quantum-coherent approaches to shot noise
are mathematically equivalent.Comment: 13 pages, RevTex, 7 ps figures, more details of derivation give
Theory of Interaction Effects in N-S Junctions out of Equilibrium
We consider a normal metal - superconductor (N-S) junction in the regime,
when electrons in the normal metal are driven out of equilibrium. We show that
the non-equilibrium fluctuations of the electron density in the N-layer cause
the fluctuations of the phase of the order parameter in the S-layer. As a
result, the density of states in the superconductor deviates from the BCS form,
most notably the density of states in the gap becomes finite. This effect can
be viewed as a result of the time reversal symmetry breaking due to the
non-equilibrium, and can be described in terms of a low energy collective mode
of the junction, which couples normal currents in N-layer and supercurrents.
This mode is analogous to the Schmid-Sch\"{o}n mode. To interpret their
measurements of the tunneling current, Pothier {\em et. al} [Phys. Rev. Lett.
{\bf 79}, 3490 (1997)] had to assume that the energy relaxation rate in the
normal metal is surprisingly high. The broadening of the BCS singularity of the
density of states in the S-layer manifest itself similarly to the broadening of
the distribution function. Mechanism suggested here can be a possible
explanation of this experimental puzzle. We also propose an independent
experiment to test our explanation.Comment: 16 pages, 2 .eps figure
Overscreening Diamagnetism in Cylindrical Superconductor-Normal Metal-Heterostructures
We study the linear diamagnetic response of a superconducting cylinder coated
by a normal-metal layer due to the proximity effect using the clean limit
quasiclassical Eilenberger equations. We compare the results for the
susceptibility with those for a planar geometry. Interestingly, for
the cylinder exhibits a stronger overscreening of the magnetic field, i.e., at
the interface to the superconductor it can be less than (-1/2) of the applied
field. Even for , the diamagnetism can be increased as compared to the
planar case, viz. the magnetic susceptibility becomes smaller than
-3/4. This behaviour can be explained by an intriguing spatial oscillation of
the magnetic field in the normal layer
Inelastic Scattering Time for Conductance Fluctuations
We revisit the problem of inelastic times governing the temperature behavior
of the weak localization correction and mesoscopic fluctuations in one- and
two-dimensional systems. It is shown that, for dephasing by the electron
electron interaction, not only are those times identical but the scaling
functions are also the same.Comment: 10 pages Revtex; 5 eps files include
Superconductive proximity effect in interacting disordered conductors
We present a general theory of the superconductive proximity effect in
disordered normal--superconducting (N-S) structures, based on the recently
developed Keldysh action approach. In the case of the absence of interaction in
the normal conductor we reproduce known results for the Andreev conductance G_A
at arbitrary relation between the interface resistance R_T and the diffusive
resistance R_D. In two-dimensional N-S systems, electron-electron interaction
in the Cooper channel of normal conductor is shown to strongly affect the value
of G_A as well as its dependence on temperature, voltage and magnetic field. In
particular, an unusual maximum of G_A as a function of temperature and/or
magnetic field is predicted for some range of parameters R_D and R_T. The
Keldysh action approach makes it possible to calculate the full statistics of
charge transfer in such structures. As an application of this method, we
calculate the noise power of an N-S contact as a function of voltage,
temperature, magnetic field and frequency for arbitrary Cooper repulsion in the
normal metal and arbitrary values of the ratio R_D/R_T.Comment: RevTeX, 28 pages, 18 PostScript figures; added and updated reference
Dynamic ordering and frustration of confined vortex rows studied by mode-locking experiments
The flow properties of confined vortex matter driven through disordered
mesoscopic channels are investigated by mode locking (ML) experiments. The
observed ML effects allow to trace the evolution of both the structure and the
number of confined rows and their match to the channel width as function of
magnetic field. From a detailed analysis of the ML behavior for the case of
3-rows we obtain ({\it i}) the pinning frequency , ({\it ii}) the onset
frequency for ML ( ordering velocity) and ({\it iii}) the
fraction of coherently moving 3-row regions in the channel. The
field dependence of these quantities shows that, at matching, where is
maximum, the pinning strength is small and the ordering velocity is low, while
at mismatch, where is small, both the pinning force and the ordering
velocity are enhanced. Further, we find that , consistent
with the dynamic ordering theory of Koshelev and Vinokur. The microscopic
nature of the flow and the ordering phenomena will also be discussed.Comment: 10 pages, 7 figure, submitted to PRB. Discussion has been improved
and a figure has been adde