Radiochemical Analyses of U, Th Isotopes,
Robert F. Anderson
We will measure vertical profiles of naturally-occurring
uranium-series radionuclides (U, Th and Pa) as well as cosmogenic
Be in dissolved and particulate phases throughout the water column
at the location of each EqPac sediment trap mooring. Annual composites of
material collected by each sediment trap will be analyzed, as will be
fine-grained particles collected by in situ filtration.
Individual time-series samples will be analyzed from the site showing the
greatest seasonal variability of particle flux. Detailed profiles of
radionuclides will be measured in the uppermost 10 to 20 cm of sediments
collected by box cores, and additional samples from dated piston cores
will be analyzed, going back to ~200 ky BP.
Analysis of both filtered and trapped particles
will be coordinated with M. P. Bacon and J. K. Cochran to avoid unnecessary
Six 50-liter water samples will be collected near the
location of each sediment trap mooring from a depth of ~800 m to the
bottom by combining water from paired 30-liter go-flo bottles on a
12-bottle rosette. Unfiltered water is processed to obtain total
radionuclide activity. Water is transferred to a 100-liter polyethylene
tank and acidified to a pH of ~1.5 with redistilled HCl. Isotope yield
Be carrier, and~1000 mg of
iron as FeCl (cleaned by extraction into isopropyl ether and back
extraction into water) are added to acidified samples and allowed to sit
for ~24 hours. Ammonia is then added to raise the pH to between 7 and 8
to induce precipitation of FeO, which carries with it the radionuclides
of interest. Precipitates are concentrated by settling. Supernate
solutions are decanted and discarded. Precipitates are shipped back to
the lab for purification of the radionuclides.
Iron-hydroxide precipitates are freed of residual salts by
resuspension in high purity water, and further concentrated by
centrifugation. Precipitates are dissolved in redistilled HCl. Fe is
extracted into isopropyl ether and discarded. The acid phase,
containing the radionuclides, is evaporated by heating, with addition
of HNO3 and HF. Final purification of the radionuclides is by ion
exchange following a scaled-down version of the procedure described
by Anderson et al. (1990). Th and Pa are measured by mass
spectrometry. 10Be is measured by accelerator mass spectrometry
(AMS). Uranium is not measured because its concentration in
seawater can be calculated from the constant U/salinity ratio.
Particulate Matter and Sediments
Dried and weighed samples of particulate matter are totally
dissolved in the presence of
yield monitors and
Be carrier using a mixture of HCl, HNO , HF and HClO following the method of Fleisher and Anderson (1991). Individual nuclides are purified by ion exchange, following a
scaled-down version of the procedure described by Anderson et
al. (1990). U, Th and Pa are measured by mass spectrometry;
Be by AMS. Analysis of sediments follows the procedure of
Anderson et al. (1990), including total dissolution of
sediments and purification of nuclides by ion exchange.
be measured by AMS. At the time of this writing, it is uncertain
whether U, Th and Pa analyses will be by alpha spectrometry or by
All analyses will follow isotope dilution procedures,
which eliminates uncertainties associated with unknown or variable
chemical yield. Procedural blanks will be evaluated by carrying a
small volume of high purity water through all feasible steps of the
procedure; collection of water using go-flo bottles is the exception.
Our shallowest sample at each station is positioned to coincide with
the deepest sample collected using an adsorber cartridge attached to
J. K. B. Bishop's large-volume in situ filtration system. Cartridge samples
will be analyzed by T.-L. Ku's group for the same long-lived radionuclides.
This overlap of sampling will both provide an opportunity for
intercalibration and link our deep-water profiles with Ku's detailed
profiles of the upper water column. Aliquots of a homogenized
sediment sample will be distributed for intercalibration of methods
for solid phase analyses (i.e., both particles and sediments, since the
methods are essentially the same).
- Anderson, R. F., Y. Lao, W. S. Broecker, S. E. Trumbore, H. J. Hofmann
and W. Wolfli (1990).
- Boundary scavenging in the Pacific Ocean: A
Pa. Earth and Planetary Science Letters,
- Fleisher, M. Q. and R. Anderson (1991).
- Particulate matter digestion
(from mg to 10's of g) and radionuclide blanks. pp. 221--222, In: D. C.
- Hurd and D. W. Spencer (eds.),
- Marine Particles: Analysis and Characterization, Geophysical Monograph 63, Americal Geophysical Union, Washington, D. C.