1,905 research outputs found
Rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal-bearing sediments offshore Shimokita Peninsula, Japan (IODP Site C0020)
Sediments recovered at Integrated Ocean Drilling Program (IODP) Site C0020, in a foreâarc basin offshore Shimokita Peninsula, Japan, include numerous coal beds (0.3â7 m thick) that are associated with a transition from a terrestrial to marine depositional environment. Within the primary coalâbearing unit (âŒ2 km depth below seafloor) there are sharp increases in magnetic susceptibility in close proximity to the coal beds, superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence. We investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record, using isothermal remanent magnetization (IRM), thermal demagnetization, anhysteretic remanent magnetization (ARM), iron speciation, and iron isotopes. Magnetic mineral assemblages in all samples are dominated by very lowâcoercivity minerals with unblocking temperatures between 350 and 580°C that are interpreted to be magnetite. Samples with lower unblocking temperatures (300â400°C), higher ARM, higherâfrequency dependence, and isotopically heavy ÎŽ56Fe across a range of lithologies in the coalâbearing unit (between 1925 and 1995 mbsf) indicate the presence of fineâgrained authigenic magnetite. We suggest that ironâreducing bacteria facilitated the production of fineâgrained magnetite within the coalâbearing unit during burial and interaction with pore waters. The coal/peat acted as a source of electron donors during burial, mediated by humic acids, to supply ironâreducing bacteria in the surrounding siliciclastic sediments. These results indicate that coalâbearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time, within the subsequent deep biosphere
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The dynamic switch mechanism that leads to activation of LRRK2 is embedded in the DFGÏ motif in the kinase domain.
Leucine-rich repeat kinase 2 (LRRK2) is a large multidomain protein, and LRRK2 mutants are recognized risk factors for Parkinson's disease (PD). Although the precise mechanisms that control LRRK2 regulation and function are unclear, the importance of the kinase domain is strongly implicated, since 2 of the 5 most common familial LRRK2 mutations (G2019S and I2020T) are localized to the conserved DFGÏ motif in the kinase core, and kinase inhibitors are under development. Combining the concept of regulatory (R) and catalytic (C) spines with kinetic and cell-based assays, we discovered a major regulatory mechanism embedded within the kinase domain and show that the DFG motif serves as a conformational switch that drives LRRK2 activation. LRRK2 is quite unusual in that the highly conserved Phe in the DFGÏ motif, which is 1 of the 4 R-spine residues, is replaced with tyrosine (DY2018GI). A Y2018F mutation creates a hyperactive phenotype similar to the familial mutation G2019S. The hydroxyl moiety of Y2018 thus serves as a "brake" that stabilizes an inactive conformation; simply removing it destroys a key hydrogen-bonding node. Y2018F, like the pathogenic mutant I2020T, spontaneously forms LRRK2-decorated microtubules in cells, while the wild type and G2019S require kinase inhibitors to form filaments. We also explored 3 different mechanisms that create kinase-dead pseudokinases, including D2017A, which further emphasizes the highly synergistic role of key hydrophobic and hydrophilic/charged residues in the assembly of active LRRK2. We thus hypothesize that LRRK2 harbors a classical protein kinase switch mechanism that drives the dynamic activation of full-length LRRK2
Remote Sensing Data Assimilation in Dynamic Crop Models Using Particle Swarm Optimization
A growing world population, increasing prosperity in emerging countries, and shifts in energy and food demands necessitate a continuous increase in global agricultural production. Simultaneously, risks of extreme weather events and a slowing productivity growth in recent years has caused concerns about meeting the demands in the future. Crop monitoring and timely yield predictions are an important tool to mitigate risk and ensure food security. A common approach is to combine the temporal simulation of dynamic crop models with a geospatial component by assimilating remote sensing data. To ensure reliable assimilation, handling of uncertainties in both models and the assimilated input data is crucial. Here, we present a new approach for data assimilation using particle swarm optimization (PSO) in combination with statistical distance metrics that allow for flexible handling of model and input uncertainties. We explored the potential of the newly proposed method in a case study by assimilating canopy cover (CC) information, obtained from Sentinel-2 data, into the AquaCrop-OS model to improve winter wheat yield estimation on the pixel- and field-level and compared the performance with two other methods (simple updating and extended Kalman filter). Our results indicate that the performance of the new method is superior to simple updating and similar or better than the extended Kalman filter updating. Furthermore, it was particularly successful in reducing bias in yield estimation
Structure and star formation in galaxies out to z=3: evidence for surface density dependent evolution and upsizing
We present an analysis of galaxies in the CDF-South. We find a tight relation
to z=3 between color and size at a given mass, with red galaxies being small,
and blue galaxies being large. We show that the relation is driven by stellar
surface density or inferred velocity dispersion: galaxies with high surface
density are red and have low specific star formation rates, and galaxies with
low surface density are blue and have high specific star formation rates.
Surface density and inferred velocity dispersion are better correlated with
specific star formation rate and color than stellar mass. Hence stellar mass by
itself is not a good predictor of the star formation history of galaxies. In
general, galaxies at a given surface density have higher specific star
formation rates at higher redshift. Specifically, galaxies with a surface
density of 1-3 10^9 Msun/kpc^2 are "red and dead" at low redshift,
approximately 50% are forming stars at z=1, and almost all are forming stars by
z=2. This provides direct additional evidence for the late evolution of
galaxies onto the red sequence. The sizes of galaxies at a given mass evolve
like 1/(1+z)^(0.59 +- 0.10). Hence galaxies undergo significant upsizing in
their history. The size evolution is fastest for the highest mass galaxies, and
quiescent galaxies. The persistence of the structural relations from z=0 to
z=2.5, and the upsizing of galaxies imply that a relation analogous to the
Hubble sequence exists out to z=2.5, and possibly beyond. The star forming
galaxies at z >= 1.5 are quite different from star forming galaxies at z=0, as
they have likely very high gas fractions, and star formation time scales
comparable to the orbital time.Comment: 20 pages, accepted for publication in ApJ, 2008, 68
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Determining the Absorption towards Classical T Tauri Stars from Hydrogen Emission Lines
Classical T Tauri stars (CTTS) are low-mass pre-main sequence objects surrounded by an accretion disk and still partially embedded in the parent circumstellar cloud. We observed a sample of 20 CTTS covering different ages and evolutionary stages with VLT/X-Shooter. Its wide wavelength coverage allows to simultaneously observe H I lines in the Balmer, Paschen, and Brackett series, which supposedly originate mainly from the accretion funnel. We present the results of our study of reddening in CTTS using ratios of common upper level lines. We determine new extinction values AV for several objects, but cannot find deviations from the standard parametrisation A=AV*lambda^-1.84 (Martin & Whittet 1990) of the reddening at infrared wavelengths.Astronom
Microbial Sulfate Reduction Potential in Coal-Bearing Sediments Down to ~2.5 km below the Seafloor off Shimokita Peninsula, Japan
Sulfate reduction is the predominant anaerobic microbial process of organic matter mineralization in marine sediments, with recent studies revealing that sulfate reduction not only occurs in sulfate-rich sediments, but even extends to deeper, methanogenic sediments at very low background concentrations of sulfate. Using samples retrieved off the Shimokita Peninsula, Japan, during the Integrated Ocean Drilling Program (IODP) Expedition 337, we measured potential sulfate reduction rates by slurry incubations with 35S-labeled sulfate in deep methanogenic sediments between 1276.75 and 2456.75 meters below the seafloor. Potential sulfate reduction rates were generally extremely low (mostly below 0.1 pmol cmâ3 dâ1) but showed elevated values (up to 1.8 pmol cmâ3 dâ1) in a coal-bearing interval (Unit III). A measured increase in hydrogenase activity in the coal-bearing horizons coincided with this local increase in potential sulfate reduction rates. This paired enzymatic response suggests that hydrogen is a potentially important electron donor for sulfate reduction in the deep coalbed biosphere. By contrast, no stimulation of sulfate reduction rates was observed in treatments where methane was added as an electron donor. In the deep coalbeds, small amounts of sulfate might be provided by a cryptic sulfur cycle. The isotopically very heavy pyrites (ÎŽ34S = +43â°) found in this horizon is consistent with its formation via microbial sulfate reduction that has been continuously utilizing a small, increasingly 34S-enriched sulfate reservoir over geologic time scales. Although our results do not represent in-situ activity, and the sulfate reducers might only have persisted in a dormant, spore-like state, our findings show that organisms capable of sulfate reduction have survived in deep methanogenic sediments over more than 20 Ma. This highlights the ability of sulfate-reducers to persist over geological timespans even in sulfate-depleted environments. Our study moreover represents the deepest evidence of a potential for sulfate reduction in marine sediments to date
Direct Measurements of the Stellar Continua and Balmer/4000 Angstrom Breaks of Red z>2 Galaxies: Redshifts and Improved Constraints on Stellar Populations
We use near-infrared (NIR) spectroscopy obtained with GNIRS on Gemini,
NIRSPEC on KECK, and ISAAC on the VLT to study the rest-frame optical continua
of three `Distant Red Galaxies' (having Js - Ks > 2.3) at z>2. All three galaxy
spectra show the Balmer/4000 Angstrom break in the rest-frame optical. The
spectra allow us to determine spectroscopic redshifts from the continuum with
an estimated accuracy dz/(1+z) ~ 0.001-0.04. These redshifts agree well with
the emission line redshifts for the 2 galaxies with Halpha emission. This
technique is particularly important for galaxies that are faint in the
rest-frame UV, as they are underrepresented in high redshift samples selected
in optical surveys and are too faint for optical spectroscopy. Furthermore, we
use the break, continuum shape, and equivalent width of Halpha together with
evolutionary synthesis models to constrain the age, star formation timescale,
dust content, stellar mass and star formation rate of the galaxies. Inclusion
of the NIR spectra in the stellar population fits greatly reduces the range of
possible solutions for stellar population properties. We find that the stellar
populations differ greatly among the three galaxies, ranging from a young dusty
starburst with a small break and strong emission lines to an evolved galaxy
with a strong break and no detected line emission. The dusty starburst galaxy
has an age of 0.3 Gyr and a stellar mass of 1*10^11 Msun. The spectra of the
two most evolved galaxies imply ages of 1.3-1.4 Gyr and stellar masses of
4*10^11 Msun. The large range of properties seen in these galaxies strengthens
our previous much more uncertain results from broadband photometry. Larger
samples are required to determine the relative frequency of dusty starbursts
and (nearly) passively evolving galaxies at z~2.5.Comment: Accepted for publication in the Astrophysical Journal. 12 pages, 6
figure
Mergers and Mass Accretion Rates in Galaxy Assembly: The Millennium Simulation Compared to Observations of z~2 Galaxies
Recent observations of UV-/optically selected, massive star forming galaxies
at z~2 indicate that the baryonic mass assembly and star formation history is
dominated by continuous rapid accretion of gas and internal secular evolution,
rather than by major mergers. We use the Millennium Simulation to build new
halo merger trees, and extract halo merger fractions and mass accretion rates.
We find that even for halos not undergoing major mergers the mass accretion
rates are plausibly sufficient to account for the high star formation rates
observed in z~2 disks. On the other hand, the fraction of major mergers in the
Millennium Simulation is sufficient to account for the number counts of
submillimeter galaxies (SMGs), in support of observational evidence that these
are major mergers. When following the fate of these two populations in the
Millennium Simulation to z=0, we find that subsequent mergers are not frequent
enough to convert all z~2 turbulent disks into elliptical galaxies at z=0.
Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at
z~2 into observed massive cluster ellipticals at z=0. We argue therefore, that
secular and internal evolution must play an important role in the evolution of
a significant fraction of z~2 UV-/optically and submillimeter selected galaxy
populations.Comment: 5 pages, 4 figures, Accepted for publication in Ap
The Rest-Frame Optical Luminosity Density, Color, and Stellar Mass Density of the Universe from z=0 to z=3
We present the evolution of the rest-frame optical luminosity density, of the
integrated rest-frame optical color, and of the stellar mass density for a
sample of Ks-band selected galaxies in the HDF-S. We derived the luminosity
density in the rest-frame U, B, and V-bands and found that the luminosity
density increases by a factor of 1.9+-0.4, 2.9+-0.6, and 4.9+-1.0 in the V, B,
and U rest-frame bands respectively between a redshift of 0.1 and 3.2. We
derived the luminosity weighted mean cosmic (U-B)_rest and (B-V)_rest colors as
a function of redshift. The colors bluen almost monotonically with increasing
redshift; at z=0.1, the (U-B)_rest and (B-V)_rest colors are 0.16 and 0.75
respectively, while at z=2.8 they are -0.39 and 0.29 respectively. We derived
the luminosity weighted mean M/LV using the correlation between (U-V)_rest and
log_{10} M/LV which exists for a range in smooth SFHs and moderate extinctions.
We have shown that the mean of individual M/LV estimates can overpredict the
true value by ~70% while our method overpredicts the true values by only ~35%.
We find that the universe at z~3 had ~10 times lower stellar mass density than
it does today in galaxies with LV>1.4 \times 10^{10} h_{70}^-2 Lsol. 50% of the
stellar mass of the universe was formed by $z~1-1.5. The rate of increase in
the stellar mass density with decreasing redshift is similar to but above that
for independent estimates from the HDF-N, but is slightly less than that
predicted by the integral of the SFR(z) curve.Comment: 19 pages, 12 figures, Accepted for Publication in the Dec. 20, 2003
edition of the Astrophysical Journal. Minor changes made to match the
accepted version including short discussions on the effects of clustering and
on possible systematic effects resulting from photometric redshift error
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