Bioturbation depths, rates and processes in Massachusetts Bay sediments inferred from modeling of 210Pb and 239 + 240Pu profiles

Author Posting. © The Author(s), 2004. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Estuarine, Coastal and Shelf Science 61 (2004): 643-655, doi:10.1016/j.ecss.2004.07.005.Profiles of 210Pb and 239+240Pu from sediment cores collected throughout Massachusetts Bay (water depths of 36-192 m) are interpreted with the aid of a numerical sedimentmixing model to infer bioturbation depths, rates and processes. The nuclide data suggest extensive bioturbation to depths of 25-35 cm. Roughly half the cores have 210Pb and 239+240Pu profiles that decrease monotonically from the surface and are consistent with biodiffusive mixing. Bioturbation rates are reasonably well constrained by these profiles and vary from ~0.7 to ~40 cm2 yr-1. As a result of this extensive reworking, however, sediment ages cannot be accurately determined from these radionuclides and only upper limits on sedimentation rates (of ~0.3 cm yr-1) can be inferred. The other half of the radionuclide profiles are characterized by subsurface maxima in each nuclide, which cannot be reproduced by biodiffusive mixing models. A numerical model is used to demonstrate that mixing caused by organisms that feed at the sediment surface and defecate below the surface can cause the subsurface maxima, as suggested by previous work. The deep penetration depths of excess 210Pb and 239+240Pu suggest either that the organisms release material over a range of >15 cm depth or that biodiffusive mixing mediated by other organisms is occurring at depth. Additional constraints from surficial sediment 234Th data suggest that in this half of the cores, the vast majority of the presentday flux of recent, nuclide-bearing material to these core sites is transported over a timescale of a month or more to a depth of a few cm below the sediment surface. As a consequence of the complex mixing processes, surface sediments include material spanning a range of ages and will not accurately record recent changes in contaminant deposition.This work was conducted under a joint funding agreement between the USGS and the Massachusetts Water Resources Authority

The cyclo-synchrotron process and particle heating through the absorption of photons

We propose a new approximation for the cyclo-synchrotron emissivity of a single electron. In the second part of this work, we discuss a simple application for our approximation, and investigate the heating of electrons through the self-absorption process. Finally, we investigate the self-absorbed part of the spectrum produced by a power-law population of electrons. In comparison to earlier approximations, our formula provides a few significant advantages. Integration of the emissivity over the whole frequency range, starting from the proper minimal emitting frequency, gives the correct cooling rate for any energy particle. Further, the spectrum of the emission is well approximated over the whole frequency range, even for relatively low particle energies (beta << 0.1), where most of the power is emitted in the first harmonic. In order to test our continuous approximation, we compare it with a recently derived approximation of the first ten harmonics. Finally, our formula connects relatively smooth to the synchrotron emission at beta=0.9. We show that the self-absorption is a very efficient heating mechanism for low energy particles, independent of the shape of the particle distribution responsible for the self-absorbed synchrotron emission. We find that the energy gains for low energy particles are always higher than energy losses by cyclo-synchrotron emission. We show also that the spectral index of the self-absorbed part of the spectrum at very low frequencies differs significantly from the well known standard relation I(nu) ~ nu^(5/2).Comment: 9 pages, 4 figures, accepted for publication in A&

Energetics of Tev Blazars and Physical Constraints on their Emission Regions

Using multi-frequency spectra from TeV blazars in quiescent states, we obtain the physical parameters of the emission region of blazars within the framework of the one-zone synchrotron self-Compton (SSC) model. We numerically calculate the steady-state energy spectra of electrons by self-consistently taking into account the effects of radiative cooling with a proper account of the Klein-Nishina effects. Here electrons are assumed to be injected with a power-law spectrum and to escape on a finite time scale, which naturally leads to the existence of a break energy scale. Although we do not use time variabilities but utilize a model of electron escape to constrain the size of the emission region, the resultant size turns out to be similar to that obtained based on time variabilities. Through detailed comparison of the predicted emission spectra with observations, we find that for Mrk 421, Mrk 501, and PKS 2155--304, the energy density of relativistic electrons is about an order of magnitude larger than that of magnetic fields with an uncertainty within a factor of a few.Comment: Accepted for publication in Ap

X-ray variability patterns in blazars

We study the expected variability patterns of blazars within the two-zone acceleration model putting special emphasis on flare shapes and spectral lags. We solve semi-analytically the kinetic equations which describe the particle evolution in the acceleration and radiation zone. We then perturb the solutions by introducing Lorentzian variations in its key parameters and examine the flaring behavior of the system. We apply the above to the X-ray observations of blazar 1ES 1218+304 which exhibited a hard lag behavior during a flaring episode and discuss possibilities of producing it within the context of our model. The steady-state radio to X-rays emission of 1ES 1218+304 can be reproduced with parameters which lie well within the ones generally accepted from blazar modeling. Additionally, we find that the best way to explain its flaring behavior is by varying the rate of particles injected in the acceleration zone.Comment: accepted by A&

Submarine groundwater discharge to a small estuary estimated from radon and salinity measurements and a box model

Author Posting. © 2005 Author(s). This work is licensed under a Creative Commons License. The definitive version was published Biogeosciences 2 (2005): 141-157, doi:10.5194/bg-2-141-2005.Submarine groundwater discharge was quantified by a variety of methods for a 4-day period during the early summer of 2004, in Salt Pond, adjacent to Nauset Marsh, on Cape Cod, USA. Discharge estimates based on radon and salinity took advantage of the presence of the narrow channel connecting Salt Pond to Nauset Marsh, which allowed constructing whole-pond mass balances as water flowed in and out due to tidal fluctuations. The data suggest that less than one quarter of the discharge in the vicinity of Salt Pond happened within the pond itself, while three quarters or more of the discharge occurred immediately seaward of the pond, either in the channel or in adjacent regions of Nauset Marsh. Much of this discharge, which maintains high radon activities and low salinity, is carried into the pond during each incoming tide. A box model was used as an aid to understand both the rates and the locations of discharge in the vicinity of Salt Pond. The model achieves a reasonable fit to both the salinity and radon data assuming submarine groundwater discharge is fresh and that most of it occurs either in the channel or in adjacent regions of Nauset Marsh. Salinity and radon data, together with seepage meter results, do not rule out discharge of saline groundwater, but suggest either that the saline discharge is at most comparable in volume to the fresh discharge or that it is depleted in radon. The estimated rate of fresh groundwater discharge in the vicinity of Salt Pond is 3000-7000 m3 d-1. This groundwater flux estimated from the radon and salinity data is comparable to a value of 3200-4500 m3 d-1 predicted by a recent hydrologic model (Masterson, 2004; Colman and Masterson, 2004), although the model predicts this rate of discharge to the pond whereas our data suggest most of the groundwater bypasses the pond prior to discharge. Additional work is needed to determine if the measured rate of discharge is representative of the long-term average, and to better constrain the rate of groundwater discharge seaward of Salt Pond.Financial support was provided by the US Geological Survey and by National Science Foundation grant #OCE-0346933 to MAC

Multiwavelength observations of Mkn 501 during the 1997 high state

During the observation period 1997, the nearby Blazar Mkn 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using radio, optical, soft and hard X-ray and TeV data. We concentrate on the medium-timescale variability of the broadband spectra, averaged over weekly intervals. We confirm the previously found correlation between soft and hard X-ray emission and the emission at TeV energies, while the source shows only minor variability at radio and optical wavelengths. The non-linear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet, and flux variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with a Synchrotron Self-Compton (SSC) dominated leptonic jet model, using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet, accounting for gamma-gamma absorption and pair production within the source as well as due to the intergalactic infrared background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found to be negligible at TeV energies. We find that high levels of the X-ray and TeV fluxes can be explained by a hardening of the energy spectra of electrons injected at the base of the jet, in remarkable contrast to the trend found for gamma-ray flares of the flat-spectrum radio quasar PKS 0528+134.Comment: accepted for publication in ApJ, 31 pages, 11 figure

On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field

Here we investigate some aspects of stochastic acceleration of ultrarelativistic electrons by magnetic turbulence. In particular, we discuss the steady-state energy spectra of particles undergoing momentum diffusion due to resonant interactions with turbulent MHD modes, taking rigorously into account direct energy losses connected with different radiative cooling processes. For the magnetic turbulence we assume a given power spectrum of the type $W(k) \propto k^{-q}$. In contrast to the previous approaches, however, we assume a finite range of turbulent wavevectors $k$, consider a variety of turbulence spectral indexes $1 =< q =< 2$, and concentrate on the case of a very inefficient particle escape from the acceleration site. We find that for different cooling and injection conditions, stochastic acceleration processes tend to establish a modified ultrarelativistic Maxwellian distribution of radiating particles, with the high-energy exponential cut-off shaped by the interplay between cooling and acceleration rates. For example, if the timescale for the dominant radiative process scales with the electron momentum as $\propto p^r$, the resulting electron energy distribution is of the form $n_e(p) \propto p^2 exp[ - (1 / a) (p / p_eq)^a]$, where $a = 2-q-r$, and $p_eq$ is the equilibrium momentum defined by the balance between stochastic acceleration and energy losses timescales. We also discuss in more detail the synchrotron and inverse-Compton emission spectra produced by such an electron energy distribution, taking into account Klein-Nishina effects. We point out that the curvature of the high frequency segments of these spectra, even though being produced by the same population of electrons, may be substantially different between the synchrotron and inverse-Compton components.Comment: 42 pages, 14 figures included. Slightly modified version, accepted for publication in Ap

Genetic Association Analysis of the Functional c.714T>G Polymorphism and Mucosal Expression of Dectin-1 in Inflammatory Bowel Disease

Contains fulltext : 80614.pdf (publisher's version ) (Open Access)BACKGROUND: Dectin-1 is a pattern recognition receptor (PRR) expressed by myeloid cells that specifically recognizes beta-1,3 glucan, a polysaccharide and major component of the fungal cell wall. Upon activation, dectin-1 signaling converges, similar to NOD2, on the adaptor molecule CARD9 which is associated with inflammatory bowel disease (IBD). An early stop codon polymorphism (c.714T>G) in DECTIN-1 results in a loss-of-function (p.Y238X) and impaired cytokine responses, including TNF-alpha, interleukin (IL)-1beta and IL-17 upon in vitro stimulation with Candida albicans or beta-glucan. The aim of the present study was to test the hypothesis that the DECTIN-1 c.714T>G (p.Y238X) polymorphism is associated with lower disease susceptibility or severity in IBD and to investigate the level of dectin-1 expression in inflamed and non-inflamed colon tissue of IBD patients. METHODOLOGY: Paraffin embedded tissue samples from non-inflamed and inflamed colon of IBD patients and from diverticulitis patients were immunohistochemically stained for dectin-1 and related to CD68 macrophage staining. Genomic DNA of IBD patients (778 patients with Crohn's disease and 759 patients with ulcerative colitis) and healthy controls (n = 772) was genotyped for the c.714T>G polymorphism and genotype-phenotype interactions were investigated. PRINCIPAL FINDINGS: Increased expression of dectin-1 was observed in actively inflamed colon tissue, as compared to non-inflamed tissue of the same patients. Also an increase in dectin-1 expression was apparent in diverticulitis tissue. No statistically significant difference in DECTIN-1 c.714T>G allele frequencies was observed between IBD patients and healthy controls. Furthermore, no differences in clinical characteristics could be observed related to DECTIN-1 genotype, neither alone, nor stratified for NOD2 genotype. CONCLUSIONS: Our data demonstrate that dectin-1 expression is elevated on macrophages, neutrophils, and other immune cells involved in the inflammatory reaction in IBD. The DECTIN-1 c.714T>G polymorphism however, is not a major susceptibility factor for developing IBD

Parsec-Scale Images of Flat-Spectrum Radio Sources in Seyfert Galaxies

We present high angular resolution (~2 mas) radio continuum observations of five Seyfert galaxies with flat-spectrum radio nuclei, using the VLBA at 8.4 GHz. The goal of the project is to test whether these flat-spectrum cores represent thermal emission from the accretion disk, as inferred previously by Gallimore et al. for NGC 1068, or non-thermal, synchrotron self-absorbed emission, which is believed to be responsible for more powerful, flat-spectrum nuclear sources in radio galaxies and quasars. In four sources (T0109-383, NGC 2110, NGC 5252, Mrk 926), the nuclear source is detected but unresolved by the VLBA, indicating brightness temperatures in excess of 10^8 K and sizes, on average, less than 1 pc. We argue that the radio emission is non-thermal and synchrotron self-absorbed in these galaxies, but Doppler boosting by relativistic outflows is not required. Synchrotron self-absorption brightness temperatures suggest intrinsic source sizes smaller than ~0.05-0.2 pc, for these four galaxies, the smallest of which corresponds to a light-crossing time of ~60 light days or 10^4 gravitational radii for a 10^8 M_sun black hole. We also present MERLIN and VLA observations of NGC 4388, which was undetected by the VLBA, and argue that the observed, flat-spectrum, nuclear radio emission in this galaxy represents optically thin, free-free radiation from dense thermal gas on scales ~0.4 to a few pc. It is notable that the two Seyfert galaxies with detected thermal nuclear radio emission (NGC 1068 and NGC 4388) both have large X-ray absorbing columns, suggesting that columns in excess of \~10^{24} cm^{-2} are needed for such disks to be detectable. (Abridged)Comment: 36 pages including 5 tables and 4 figures; accepted for publication in Ap

Radiation from the Relativistic Jet: a Role of the Shear Boundary Layer

Recent radio and optical large scale jets' observations suggest a two-component jet morphology, consisting of a fast central spine surrounded with a boundary layer with a velocity shear. We study radiation of electrons accelerated at such boundary layers as an option for standard approaches involving internal shocks in jets. The acceleration process in the boundary layer yields in a natural way a two component electron distribution: a power-law continuum with a bump at the energy, where energy gains equal radiation losses, followed by a cut-off. For such distributions we derive the observed spectra of synchrotron and inverse-Compton radiation, including comptonization of synchrotron and CMB photons. Under simple assumptions of energy equipartition between the relativistic particles and the magnetic field, the relativistic jet velocity at large scales and a turbulent character of the shear layer, the considered radiation can substantially contribute to the jet radiative output. In the considered conditions the synchrotron emission is characterized by a spectral index of the radio-to-optical continuum being approximately constant along the jet. A characteristic feature of the obtained broad-band synchrotron spectrum is an excess at X-ray frequencies, similar to the one observed in some objects by Chandra. As compared to the uniform jet models, the velocity shear across the radiating boundary region leads to decrease and frequency dependence of the observed jet-counterjet radio brightness asymmetry. We conclude that a careful investigation of the observational data looking for the derived effects can allow to evaluate the role of the boundary layer acceleration processes and/or impose constraints for the physical parameters of such layers in large scale jets.Comment: 30 pages, 4 figures included. Modified version, accepted for publication in Astrophysical Journa