7,776 research outputs found
The marine geochemistry of thorium and protactinium
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Philosophy and the Woods Hole Oceanographic Institution November 1980Suspended particulate matter was collected by sediment traps deployed
in the Sargasso Sea (Site S2), the north equatorial Atlantic (Site E),
the north equatorial Pacific (Site P), and the Panama Basin (STIE Site).
Additional samples of suspended particles were obtained by in situ
filtration at Site F., at the STIE Site, and in the Guatemala Basin.
Concentrations of dissolved Th and Pa were determined by extraction onto
manganese dioxide adsorbers at Site P, at a second site in the Sargasso
Sea (Site D), at the STIE Site and in the Guatemala Basin. Sediment
samples were obtained from cores taken near Sites E and P.
Results have shown unequivocally that suspended particulate matter in
the open ocean preferentially scavenges Th relative to Pa. This behavior
could not have been predicted from the known physical chemistry of Th and
Pa. Dissolved 230Th/231Pa activity ratios were 3-5 at Sites P and D
and 3-8 at the STIE Site. In contrast, unsupported 230Th/231Pa
ratios were 22-35 (average 29.7 for 7 samples) in sediment-trap samples
from greater than 2000 m at Sites S2, E and P. Ratios were lower in
particulate matter sampled at shallower depths. Particles filtered at
3600 m and 5000 m at Site E had ratios of 50 and 40. In contrast to the
open ocean samples described above, samples collected by six sediment
traps at depths of 667-3791 m in the Panama Basin had unsupported
230Th/231Pa ratios of 4-8, and the deepest samples had the lowest
ratios. Fractionation of Th and Pa that was observed at the three open
ocean sites either does not occur or occurs to a very limited extent in
the Panama Basin.
Particulate 230Th/231Pa ratios were negatively correlated with
the concentration of suspended particles. However, variable scavenging
rates, as indicated by variable particle concentration, do not completely
control the ratio at which Th and Pa are scavenged from solution. Major
biogenic and inorganic components of trapped material were found in
approximately the same proportions in the STIE samples and in samples
from Sites E and S2. Lower 230Th/231Pa ratios found in the STIE
samples must therefore result from subtle changes in the chemical
properties of the particles. Consideration of 230Th/23lPa ratios in
several depositional environments indicates that no single factor
controls the ratio at which Th and Pa are adsorbed from seawater.
Fluxes of 210Th and 231Pa were less than their rates of
production in the overlying water column in every trap at Sites S2, E,
and P. In the Panama Basin, fluxes measured with the same traps were
greater than or equal to their rates of production. These results are a
strong indication that even extremely reactive elements such as Th and Pa
are redistributed within the oceans. Redistribution occurs because
variable scavenging rates in different environments set up horizontal
concentration gradients. Horizontal mixing processes produce a net
horizontal transport of Th and Pa from areas of 1ow scavenging rates to
areas of high scavenging rates. Protactinium is redistributed to a
greater extent than Th. Fluxes of 230Th can be used to set lower limits for horizonttal transport of Pa even when absolute trapping
efficiencies of the sediment traps are not known. Less than 50% of the
Pa produced at the open ocean sites is removed from the water column by
scavenging to settling particles. The remainder is removed by horizontal
transport to other environments.
At Sites E and P, 230Th/231Pa ratios were identical in the
deepest sediment trap sample and in surface sediments. However,
230Th/232Th and 231Pa/232Th ratios were 2.5 times higher in
trapped particles than in surface sediments. The 230Th/232Th ratios
were 5.5 times higher in particles filtered at 3600 m and 5000 m at Site
E than in surface sediments. This observation is best explained by
dissolution of most of the 230Th and 231Pa scavenged by settling
particles during remineralization of labile biogenic phases.
The behaviors of certain other radioisotopes were also studied.
232Th is present only in detrital mineral components of trapped
material. Concentrations of 232Th in trapped particles correlate
closely with Al and K, at ratios approaching that of average shale or
crustal abundances at Site E and P and basalts at the STIE Site. High
specifìc activities of 228Th and 239+240Pu were found in sediment
trap samples throughout the water column at Sites E and P and in the
Panama Basin. The dominant source of these isotopes is near the sea
surface and also near the sea floor in the case of 228Th. Thus it
appears that the bulk of the trapped material is recently derived from
the sea surface where it incorporates these isotopes,with little loss
during rapid transit through the water column. A bioauthigenic form of
particulate uranium is produced at the sea surface and remineralized in
the deep ocean along with its labile carrier phase(s). This flux of
uranium to the deep ocean is 0.25-1.0 dpm/cm2103 years, which is
insufficient to cause a measurable concentration gradient in the uranium
distribution within the mixing time of the oceans. Increased
concentrations and fluxes of particulate uranium were not found in the
eastern equatorial North Pacific under areas of an intense oxygen
minimum. Therefore, reduction of uranium to the tetravalent state with
subsequent scavenging to settling particles in oxygen minima is not a
mechanism removing uranium from the oceans.Financial support for parts of this work have come from many
sources, including: National Science Foundation Grants OCE-7826318, OCE-7825724, and OCE-7727004; Department of Energy Contract
EY-76-S-02-3566; a Cottrell Research Grant from the Research Corporation;
the WHOI Ocean Industries Program; a fellowship from the WHOI Education
Office, and the Paul Fye Fellowshi
Mapping Low-Density Intergalactic Gas: a Third Helium Lyman-alpha Forest
We present a new HST/STIS spectrum of the z=3.18 quasar PKS 1935-692 and
summarize the spectral features shortwards of 304A in the rest frame likely to
be caused by foreground HeII Lyman-alpha absorption. In accord with previous
results on two other quasars at similar redshifts, we demonstrate a correlation
with the HI Lyman-alpha forest absorption, and show that much of the helium
absorption is caused by a comparable quantity of more diffuse gas with
Omega~0.01, that is not detected in HI. The helium ionization zone around the
quasar is detected as well as a void seen in both HI and HeII. The properties
of the absorption are in broad agreement with those of the other quasars and
with models of the protogalactic gas distribution and ionization at this
redshift.Comment: 17 pages including 5 figures. As accepted for publication in The
Astronomical Journal (minor revisions
Improvements to 232-thorium, 230-thorium, and 231-protactinium analysis in seawater arising from GEOTRACES intercalibration
The GEOTRACES program requires the analysis of large numbers of seawater samples for ^(232)Th, ^(230)Th, and ^(231)Pa. During the GEOTRACES international intercalibration exercise, we encountered unexpected difficulties with recovery and contamination of these isotopes, ^(232)Th in particular. Experiments were carried out to identify the source of these issues, leading to a more streamlined and efficient procedure. The two particular problems that we identified and corrected were (1) frits in columns supplied by Bio-Rad Laboratories caused loss of Th during column chemistry and (2) new batches of AG1-X8 resin supplied by Bio-Rad Laboratories released more than 100 pg of ^(232)Th during elution of sample. To improve yields and blanks, we implemented a series of changes including switching to Eichrom anion exchange resin (100-200 μm mesh) and Environmental Express columns. All Th and Pa samples were analyzed on a Neptune multi-collector inductively-coupled-plasma mass spectrometer (MC-ICP-MS) using peak hopping of ^(230)Th and ^(229)Th on the central SEM, with either ^(232)Th, ^(236)U (or both) used to monitor for beam intensity. We used in-house laboratory standards to check for machine reproducibility, and the GEOTRACES intercalibration standard to check for accuracy. Over a 1-y period, the 2 s.d. reproducibility on the GEOTRACES SW STD 2010-1 was 2.5% for ^(230)Th, 1.8% for ^(232)Th, and 4% for ^(231)Pa. The lessons learned during this intercalibration process will be of value to those analyzing U-Th-Pa and rare earth elements as part of the GEOTRACES program as well as those using U-series elements in other applications that require high yields and low blanks, such as geochronology
Magnetic Instability in Strongly Correlated Superconductors
Recently a new phenomenological Hamiltonian has been proposed to describe the
superconducting cuprates. This so-called Gossamer Hamiltonian is an apt model
for a superconductor with strong on-site Coulomb repulsion betweenthe
electrons. It is shown that as one approaches half-filling the Gossamer
superconductor, and hence the superconducting state, with strong repulsion is
unstable toward an antiferromagnetic insulator an can undergo a quantum phase
transition to such an insulator if one increases the on-site Coulomb repulsion
Visualization of hydrogen injection in a scramjet engine by simultaneous PLIF imaging and laser holographic imaging
Flowfield characterization has been accomplished for several fuel injector configurations using simultaneous planar laser induced fluorescence (PLIF) and laser holographic imaging (LHI). The experiments were carried out in the GASL-NASA HYPULSE real gas expansion tube facility, a pulsed facility with steady test times of about 350 microsec. The tests were done at simulated Mach numbers 13.5 and 17. The focus of this paper is on the measurement technologies used and their application in a research facility. The HYPULSE facility, the models used for the experiments, and the setup for the LHI and PLIF measurements are described. Measurement challenges and solutions are discussed. Results are presented for experiments with several fuel injector configurations and several equivalence ratios
Antibodies to Surface IgM Can Accelerate Apoptosis of Mature B-Lymphocytes at Sub - Stimulatory Concentrations
Antibody to B-cell surface immunoglobulin D (IgD) or surface IgM results in crosslinking of Ig molecules and signal transduction. The function of these surface immunoglobulins has traditionally been investigated by extensive crosslinking experiments and interest has been focused on activation assays. We investigated the effects on apoptosis of culture with anti-(mathematical symbol) antibody (anti-(mathematical symbol)) concentrations ranging from 0.001 (mathematical symbol) mL-1 to 50 (mathematical symbol)g mL-1. Previous experiments have shown that weak dose anti-(mathematical symbol) antibody (anti-(mathematical symbol)) increases mature B-cell apoptosis at both 16- and 64-hour time points, while greater dose anti-(mathematical symbol) results in cell cycle entry at 64 hours. The question addressed is whether anti-(mathematical symbol) induces the same biphasic response. After 16 hours of culture, both a monoclonal and a polyclonal anti-(mathematical symbol). at weak concentrations caused much less of an increase in apoptosis than anti-(mathematical symbol), although cell cycle entry at 64 hours was similar. Together, these results suggest a mechanism for low-zone B-cell tolerance induction, a process that was previously thought only to occur in T-cells
Recommended from our members
Deep Ocean Carbonate Chemistry and Glacial-Interglacial Atmospheric CO₂ Changes
Changes in deep ocean carbonate chemistry have profound implications for glacial-interglacial atmospheric CO₂ changes. Here, we review deep ocean carbonate ion concentration ([CO₃²⁻]) changes based on the benthic foraminiferal boron-to-calcium ratio (B/Ca) and their links to global carbon reorganization since the last ice age. Existing deep ocean [CO₃²⁻] reconstructions are consistent with changes in the biological pump, in ocean stratification, and in the associated oceanic alkalinity inventory as key mechanisms for modulating atmospheric CO₂ on glacial-interglacial time scales. We find that the global mean deep ocean [CO₃²⁻] was roughly similar between the Last Glacial Maximum (LGM; 18,000–22,000 years ago) and the Late Holocene (0–5,000 years ago). In view of elevated glacial surface [CO₃²⁻], this indicates enhanced storage of respiratory carbon in a more alkaline deep ocean during the LGM. During early deglaciation, rising [CO₃²⁻] at three locations in the deep ocean suggests a release of deep-sea CO₂ to the atmosphere, probably via the Southern Ocean. Both increased late deglacial carbonate burial in deep-sea sediments due to elevated [CO₃²⁻] and Holocene expansion of coral reefs on newly flooded continental shelves depleted global ocean alkalinity, which reduced CO₂ solubility in seawater and contributed to atmospheric CO₂ rises at these times
Large deglacial shifts of the Pacific Intertropical Convergence Zone
The position of the Intertropical Convergence Zone (ITCZ) is sensitive to changes in the balance of heat between the hemispheres which has fundamental implications for tropical hydrology and atmospheric circulation. Although the ITCZ is thought to experience the largest shifts in position during deglacial stadial events, the magnitude of shifts has proven difficult to reconstruct, in part because of a paucity of high-resolution records, particularly those including spatial components. Here we track the position of the ITCZ from 150 to 110 ka at three sites in the central equatorial Pacific at sub-millennial time resolution. Our results provide evidence of large, abrupt changes in tropical climate during the penultimate deglaciation, coincident with North Atlantic Heinrich Stadial 11 (~136–129 ka). We identify this event both as a Northern Hemisphere increase in aeolian dust and as a shift in the mean position of the ITCZ a minimum of 4° southwards at 160° W
- …