Spatial variations of particle scavenging rates within the central and northern Adriatic Sea: use of U--Th disequilibria Page: 3 of 21
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Spatial variations of particle scavenging rates within the central and northern
Adriatic Sea: use of U-Th disequilibria
T.F. Hamilton a,* , J. Heilmann b, S.W. Fowler a , J.-C. Miquel a
a IAEA Marine Environment Laboratory, B.P. 800, MC-98012, Monaco
b The Danish Institute for Fisheries Research, Department of Marine and Coastal
Ecology, Charlottenlund Castle, DK-2920 Charlottenlund, Denmark
* Present address: Health & Ecological Assessment Division, Lawrence Livermore National
Laboratory, P.O. Box 808, L-453, Livermore, CA 94551-0808, USA; FAX: 925-422-6619;
Dissolved and particlulate Thorium-234 (234Th) were measured in near surface waters from
the Central and Northern Adriatic Sea in order to gain an insight into the intensity and variability
of active scavenging and particle removal processes. Dissolved 234Th to Uranium-238 (.3.U)
activity ratios vary from 0.21 to 0.75 and clearly imply that 234Th is being actively scavenged
from sea water on a timescale of <3 months. The scavenging rate of dissolved 234Th with respect
to scavenging onto particles appears to correlate with primary productivity measurements.
Scavenging and rapid removal of particulate 234Th from the water column is best explained by a
mechanism of zooplankton grazing and fecal pellet production. At one sight in the Jabuka Pit,
particulate 234Th residence times below the pycnocline are long (30-40 days) suggesting that
particles are being more readily recycled at these depths. By comparison, in a seawater profile
collected near the Po outflow region, 234Th is depleted with respect to z3U through the entire
water column. We conclude from this assessment that particles in waters near the Po River
outflow will be more efficiently transported to bottom sediments compared with those in deeper
waters over the Jabuka Pit.
Key words: '34Th, Adriatic Sea, scavenging, residence times
The half-lives and biogeochemistry of 234Th (t1 = 24.1 days), Polonium-210 (""Po) (t1 =
138.4 days) and Lead-210 (""Pb) (t1 = 22.3 years) are ideal for studying particle dynamics in the
upper water column (Aller and Cochran, 1976; Bacon et al., 1976; Coale and Bruland, 1985).
Models based on 234Th: 238U disequilibria have been successfully used to estimate scavenging
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Hamilton, T. F. Spatial variations of particle scavenging rates within the central and northern Adriatic Sea: use of U--Th disequilibria, article, April 1, 1998; California. (digital.library.unt.edu/ark:/67531/metadc702260/m1/3/: accessed November 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.