1994 Arctic Ocean Section
During AOS-94 the naturally occurring, short-lived 234Th isotope (half-life
Cruise track of the
CCGS Louis S.
= 24.1 days) was used as a tracer to quantify scavenging and the export of sink-
St-Laurent and sta-
ing biogenic particulate matter and associated elements. In the surface ocean
tions occupied for
the disequilibrium between 234Th and its soluble parent 238U varies in response
vertical sampling of
234Th. Shown in
to short-term changes in particle export on a time scale of approximately 1
parentheses are
100 days. Because of its short half-life, it was essential to measure 234Th at sea
234Th/238U activity
ratios in the surface
using a low-energy pure germanium detector.
mixed layer (030 m).
The 234Th/238U activity ratios in the surface mixed layer (030 m) indi-
Activity ratios less
than one indicate
cates the amount of particle scavenging along the cruise track. The low
preferential removal
234Th/238U activity ratio of 0.22 at Station 1 is consistent with removal of
of 234Th by particle
234Th from the water column by particle scavenging within the particle-rich
scavenging, whereas
ratios equal to one
Chukchi Sea. In contrast, surface waters in the interior Arctic generally show
indicate 234Th/238U
no significant 234Th/238U disequilibrium and hence very low rates of particle
equilibrium and very
scavenging. An exception is the relatively low 234Th/238U activity ratio of
low rates of particle
scavenging.
0.68 at Station 23, which may be due to enhanced biological productivity and
associated biogenic particle scavenging of 234Th at this location.
Vertical profiles of 234Th, 238U and silicate are another way of viewing
particle scavenging. A striking observation is the subsurface deficit in total
234Th evident in the Makarov Basin (Station 26). The low 234Th activities
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