14 research outputs found
Factors influencing the stable carbon isotopic composition of suspended and sinking organic matter in the coastal Antarctic sea ice environment
A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (&delta;<sup>13</sup>C<sub>POC</sub>) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting &delta;<sup>13</sup>C<sub>POC</sub> in the coastal western Antarctic Peninsula sea ice environment. The study covers two austral summer seasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shift in diatom species composition during the 2005/06 summer bloom to near-complete biomass dominance of <i>Proboscia inermis</i> is strongly correlated with a large ~10 &permil; negative isotopic shift in &delta;<sup>13</sup>C<sub>POC</sub> that cannot be explained by a concurrent change in concentration or isotopic signature of CO<sub>2</sub>. We hypothesise that the &delta;<sup>13</sup>C<sub>POC</sub> shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low &delta;<sup>13</sup>C<sub>POC</sub> in <i>P. inermis</i> may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher &delta;<sup>13</sup>C<sub>POC</sub> may employ CCMs. These short-lived yet pronounced negative &delta;<sup>13</sup>C<sub>POC</sub> excursions drive a 4 &permil; decrease in the seasonal average &delta;<sup>13</sup>C<sub>POC</sub> signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4 &permil; difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean &delta;<sup>13</sup>C<sub>POC</sub> variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary &delta;<sup>13</sup>C<sub>POC</sub>. We also find significantly higher &delta;<sup>13</sup>C<sub>POC</sub> in sea ice than surface waters, consistent with autotrophic carbon fixation in a semi-closed environment and possible contributions from post-production degradation, biological utilisation of HCO<sub>3</sub><sup>&minus;</sup> and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for &delta;<sup>13</sup>C<sub>POC</sub> in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based &delta;<sup>13</sup>C<sub>POC</sub> in the sedimentary record
First integrals of Ginzburg-Landau equations and stability criteria for vortex-free state in unconventional superconductors
The first integrals of the Ginzburg-Landau equations for a vortex-free state
of superconductors with different mixed symmetries of the order parameter are
found. The general boundary conditions for the order parameter at the ideal
interface between the superconductor and vacuum are derived. Based on these
integrals and boundary conditions, we analyze the stability criteria for
vortex-free state in unconventional superconductors. The threshold field above
which the Abrikosov vortices can enter the superconductor is found to be higher
or equal to the thermodynamic critical field for all states under study.Comment: 8 pages, pdf file, no figure
Silicon isotopes highlight the role of glaciated fjords in modifying coastal waters
Glaciers and ice sheets are experiencing rapid warming under current climatic change and there is increasing evidence that glacial meltwaters provide key dissolved and dissolvable amorphous nutrients to downstream ecosystems. However, large debate exists around the fate of these nutrients within complex and heterogenous fjord environments, where biogeochemical cycling is still often poorly understood. We combine silicon (Si) concentration data with isotopic compositions to better understand silicon cycling and export in two contrasting fjordic environments in south-west Greenland. We show that both fjords have isotopically light dissolved silicon (DSi) within surface waters, despite an apparently rapid biological drawdown of DSi with increasing salinity. We hypothesise that such observations cannot be explained by simple water mass mixing processes, and postulate that an isotopically light source of Si, most likely glacially-derived amorphous silica (ASi), is responsible for further modifying these coastal waters within the fjords and beyond. Fjord to coastal exchange is likely a relatively slow process (several months), and thus is less impacted by short-term (< seasonal) changes of glacial meltwater input into the fjord, which has implications when considering the role of glacial meltwaters on nutrient export beyond the shelf break. We highlight the need for isotopic studies combined with dissolved and particulate nutrient concentration analysis to provide a more detailed analysis into the biogeochemical cycles within these highly dynamic fjord environments
Interplay among critical temperature, hole content, and pressure in the cuprate superconductors
Within a BCS-type mean-field approach to the extended Hubbard model, a
nontrivial dependence of T_c on the hole content per unit CuO_2 is recovered,
in good agreement with the celebrated non-monotonic universal behaviour at
normal pressure. Evaluation of T_c at higher pressures is then made possible by
the introduction of an explicit dependence of the tight-binding band and of the
carrier concentration on pressure P. Comparison with the known experimental
data for underdoped Bi2212 allows to single out an `intrinsic' contribution to
d T_c / d P from that due to the carrier concentration, and provides a
remarkable estimate of the dependence of the inter-site coupling strength on
the lattice scale.Comment: REVTeX 8 pages, including 5 embedded PostScript figures; other
required macros included; to be published in Phys. Rev. B (vol. 54
Ecological influences on {delta}<sup>13</sup>C of particulate matter in seasonally ice-covered Ryder Bay, Antarctica
Carbon isotopes may be a useful paleoceanographic tool for reconstructing past pCO2 of surface water, but isotopic composition of particulate organic carbon has been shown in both field and laboratory studies to be affected by pCO2, growth rate, cell size, cell geometry, light availability, carbon metabolism and species composition. To date, field studies have not constrained
the dominant factors controlling surface water {delta}13CpOC. We present high-resolution time series data from a study in Ryder Bay,
Antarctica, which compares seasonal fluctuations in mixed-layer {delta}13CpOC with detailed ecological and morphological analysis of
phytoplankton communities, community productivity, {delta}13CDIC, nutrient dynamics, and hydrographic parameters. Preliminary results indicate that physical processes such as ocean-atmosphere gas exchange and upwelling do not significantly affect {delta}13CpOC signatures. In contrast, speciation shifts of diatom assemblages show strong correlation with changes in {delta} 13CpOC signals, supporting recent suggestions that taxonomic data are necessary for confident interpretation of sedimentary {delta}13C records