174 research outputs found
Top-down, bottom-up and physical controls on diatom-diazotroph assemblage growth in the Amazon River plume
The nutrient-rich waters of the Amazon River plume (ARP) support dense blooms
of diatom-diazotroph assemblages (DDAs) that introduce large quantities of
new nitrogen to the planktonic ecosystem and, unlike other nitrogen-fixers,
are likely to directly fuel vertical carbon flux. To investigate the factors
controlling DDA blooms, we develop a five phytoplankton (cyanobacteria,
diatoms, unicellular microbial diazotrophs, DDAs, and
Trichodesmium), two zooplankton model and embed it within a
1/6° resolution physical model of the tropical and subtropical
Atlantic. The model generates realistic DDA blooms in the ARP and also
exhibits basin-wide primary production, nitrogen fixation, and grazing rates
consistent with observed values. By following ARP water parcels with
synthetic Lagrangian drifters released at the river mouth we are able to
assess the relative impacts of grazing, nutrient supply, and physical forcing
on DDA bloom formation. DDA bloom formation is stimulated in the
nitrogen-poor and silica-rich water of the ARP by decreases in grazing
pressure when mesozooplankton (which co-occur in high densities with coastal
diatom blooms) concentrations decrease. Bloom termination is driven primarily
by silica limitation of the DDAs. In agreement with in situ data, this net
growth niche for DDAs exists in a salinity range from ∼20–34 PSU,
although this co-occurrence is coincidental rather than causative. Because
net growth rates are relatively modest, bloom formation in ARP water parcels
depends critically on the time spent in this ideal habitat, with high DDA
biomass only occurring when water parcels spent >23 days in the optimal
habitat niche
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Livelihood diversification and the expansion of artisanal mining in rural Tanzania: drivers and policy implications
This paper provides an extended analysis of livelihood diversification in rural Tanzania, with special emphasis on artisanal and small-scale mining (ASM). Over the past decade, this sector of industry, which is labour-intensive and comprises an array of rudimentary and semi-mechanized operations, has become an indispensable economic activity throughout Sub-Saharan Africa, providing employment to a host of redundant public sector workers, retrenched large-scale mine labourers and poor farmers. In many of the region’s rural areas, it is overtaking subsistence agriculture as the primary industry. Such a pattern appears
to be unfolding within the Morogoro and Mbeya regions of southern Tanzania, where findings from recent research suggest that a growing number of smallholder farmers are turning to ASM for employment and financial support. It is imperative that national rural development programmes take this trend into account and provide support to these people
The band structure of BeTe - a combined experimental and theoretical study
Using angle-resolved synchrotron-radiation photoemission spectroscopy we have
determined the dispersion of the valence bands of BeTe(100) along ,
i.e. the [100] direction. The measurements are analyzed with the aid of a
first-principles calculation of the BeTe bulk band structure as well as of the
photoemission peaks as given by the momentum conserving bulk transitions.
Taking the calculated unoccupied bands as final states of the photoemission
process, we obtain an excellent agreement between experimental and calculated
spectra and a clear interpretation of almost all measured bands. In contrast,
the free electron approximation for the final states fails to describe the BeTe
bulk band structure along properly.Comment: 21 pages plus 4 figure
Precision measurement of Zn electron-capture decays with the KDK coincidence setup
Zn is a common calibration source, moreover used as a radioactive
tracer in medical and biological studies. In many cases, -spectroscopy
is a preferred method of Zn standardization, which relies directly on
the branching ratio of via electron capture (EC*). We measure the relative
intensity of this branch to that proceeding directly to the ground state
(EC) using a novel coincidence technique, finding
. Re-evaluating the decay
scheme of Zn by adopting the commonly evaluated branching ratio of
we obtain , and
I_\text{EC^0} = (48.50 \pm 0.06) \%. The associated 1115 keV gamma intensity
agrees with the previously reported NNDC value, and is now accessible with a
factor of ~2 increase in precision. Our re-evaluation removes reliance on the
deduction of this gamma intensity from numerous measurements, some of which
disagree and depend directly on total activity determination. The KDK
experimental technique provides a new avenue for verification or updates to the
decay scheme of Zn, and is applicable to other isotopes.Comment: Uses similar methodology to the 40K measurement by the KDK
Collaboration (Stukel et al PRL 2023, arXiv:2211.10319; Hariasz et al PRC
2023, arXiv:2211.10343), as such there may be some similarity in figures and
tex
Trade-offs between risk of predation and starvation in larvae make the shelf break an optimal spawning location for Atlantic Bluefin tuna
Atlantic bluefin tuna (ABT) (Thunnus thynnus) travel long distances to spawn in oligotrophic regions of the Gulf of Mexico (GoM) which suggests these regions offer some unique benefit to offspring survival. To better understand how larval survival varies within the GoM a spatially explicit, Lagrangian, individual-based model was developed that simulates dispersal and mortality of ABT early life stages within realistic predator and prey fields during the spawning periods from 1993 to 2012. The model estimates that starvation is the largest cumulative source of mortality associated with an early critical period. However, elevated predation on older larvae is identified as the main factor limiting survival to late postflexion. As a result, first-feeding larvae have higher survival on the shelf where food is abundant, whereas older larvae have higher survival in the open ocean with fewer predators, making the shelf break an optimal spawning area. The modeling framework developed in this study explicitly simulates both physical and biological factors that impact larval survival and hence could be used to support ecosystem based management efforts for ABT under current and future climate conditions.Postprin
Trade-Offs Between Risks of Predation and Starvation In Larvae Make the Shelf Break an Optimal Spawning Location For Atlantic Bluefin Tuna
Atlantic bluefin tuna (ABT) (Thunnus thynnus) travel long distances to spawn in oligotrophic regions of the Gulf of Mexico (GoM) which suggests these regions offer some unique benefit to offspring survival. To better understand how larval survival varies within the GoM a spatially explicit, Lagrangian, individual-based model was developed that simulates dispersal and mortality of ABT early life stages within realistic predator and prey fields during the spawning periods from 1993 to 2012. The model estimates that starvation is the largest cumulative source of mortality associated with an early critical period. However, elevated predation on older larvae is identified as the main factor limiting survival to late postflexion. As a result, first-feeding larvae have higher survival on the shelf where food is abundant, whereas older larvae have higher survival in the open ocean with fewer predators, making the shelf break an optimal spawning area. The modeling framework developed in this study explicitly simulates both physical and biological factors that impact larval survival and hence could be used to support ecosystem based management efforts for ABT under current and future climate conditions
BAs and boride III-V alloys
Boron arsenide, the typically-ignored member of the III-V arsenide series
BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma
conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an
X_1c-like indirect band gap, and its bond charge is distributed almost equally
on the two atoms in the unit cell, exhibiting nearly perfect covalency. The
reasons for these are tracked down to the anomalously low atomic p orbital
energy in the boron and to the unusually strong s-s repulsion in BAs relative
to most other III-V compounds. We find unexpected valence band offsets of BAs
with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is
significantly higher than that of AlAs, despite the much smaller bond length of
BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects
result from the unusually strong mixing of the cation and anion states at the
VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and
composition-independent band gap bowing. This means that while addition of
small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of
boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the
conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing
enthalpies which are smaller than in GaN-GaAs alloys. The unique features of
boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for
publication in Phys. Rev. B. Scheduled to appear Oct. 15 200
Evidence for ground-state electron capture of K
Potassium-40 is a widespread isotope whose radioactivity impacts estimated
geological ages spanning billions of years, nuclear structure theory, and
subatomic rare-event searches - including those for dark matter and
neutrinoless double-beta decay. The decays of this long-lived isotope must be
precisely known for its use as a geochronometer, and to account for its
presence in low-background experiments. There are several known decay modes for
K, but a predicted electron-capture decay directly to the ground state
of argon-40 has never been observed, while theoretical predictions span an
order of magnitude. The KDK Collaboration reports on the first observation of
this rare decay, obtained using a novel combination of a low-threshold X-ray
detector surrounded by a tonne-scale, high-efficiency -ray tagger at
Oak Ridge National Laboratory. A blinded analysis reveals a distinctly nonzero
ratio of intensities of ground-state electron-captures () over
excited-state ones () of
(68% CL), with the null hypothesis rejected at 4 [Stukel et al.,
DOI:10.1103/PhysRevLett.131.052503]. This unambiguous signal yields a branching
ratio of
,
roughly half of the commonly used prediction. This first observation of a
third-forbidden unique electron capture improves understanding of low-energy
backgrounds in dark-matter searches and has implications for nuclear-structure
calculations. A shell-model based theoretical estimate for the
decay half-life of calcium-48 is increased by a factor of . Our
nonzero measurement shifts geochronological ages by up to a percent;
implications are illustrated for Earth and solar system chronologies.Comment: This is a companion submission to Stukel et al (KDK collaboration)
"Rare K decay with implications for fundamental physics and
geochronology" [arXiv:2211.10319; DOI: 10.1103/PhysRevLett.131.052503]. As
such, both texts share some figures and portions of text. This version
updates the text following its review and production proces
Deep-underground dark matter search with a COSINUS detector prototype
Sodium iodide (NaI) based cryogenic scintillating calorimeters using quantum
sensors for signal read out have shown promising first results towards a
model-independent test of the annually modulating signal detected by the
DAMA/LIBRA dark matter experiment. The COSINUS collaboration has previously
reported on the first above-ground measurements using a dual channel readout of
phonons and light based on transition edge sensors (TESs) that allows for
particle discrimination on an event-by-event basis. In this letter, we outline
the first underground measurement of a NaI cryogenic calorimeter read out via
the novel remoTES scheme. A 3.67 g NaI absorber with an improved silicon light
detector design was operated at the Laboratori Nazionali del Gran Sasso, Italy.
A significant improvement in the discrimination power of /-events
to nuclear recoils was observed with a five-fold improvement in the nuclear
recoil baseline resolution, achieving = 441 eV. Furthermore, we
present a limit on the spin-independent dark-matter nucleon elastic scattering
cross-section achieving a sensitivity of (pb) with an exposure of
only 11.6 g d.Comment: 11 pages, 14 figure
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