ARTICLE IN PRESS
Deep-Sea Research I 50 (2003) 1353–1373
Role of colloidal material in the removal of 234Th in the
Canada basin of the Arctic Ocean
M. Baskarana,*, P.W. Swarzenskib, D. Porcellic
a
b
Department of Geology, Wayne State University, 0224 Old Main Bldg, Detroit, MI 48202, USA
US Geological Survey, Center for Coastal and Watershed Studies, St. Petersburg, FL 33701, USA
c
Department of Earth Sciences, Oxford University, OX1 3PR, UK
Received 4 September 2002; received in revised form 23 July 2003; accepted 23 July 2003
Abstract
The phase partitioning of 234Th between dissolved (o10-kiloDalton, kD), colloidal (10 kD—0.4 mm), and particulate
(X0.5 mm) matter across a horizontal transect, from a coastal station to the deep Canada Basin, and a vertical profile in
the deep Canada Basin of the western Arctic Ocean was investigated. Concentrations of suspended particulate matter
(SPM), dissolved, colloidal and particulate organic carbon, particulate organic nitrogen and nutrients (silicate,
phosphate and nitrate) were also measured to assess transport and scavenging processes.
Total 234Th (colloidal+particulate+dissolved) indicated deficiencies relative to secular equilibrium with its parent,
238
U in the upper 100 m, which suggests active scavenging of 234Th onto particle surfaces. In contrast, at depths
>200 m, general equilibrium existed between total 234Th and 238U. The inventory of SPM and the specific activity of
particulate 234Th in the Canada Basin was about an order of magnitude higher than the profile reported for the Alpha
Ridge ice camp station. This higher concentration of SPM in the southwestern Canada Basin is likely derived from icerafted sedimentary particles. Inventories of nutrients, and dissolved organic carbon and nitrogen in the upper 100 m of
the Canada Basin are comparable to the other estimates for the central Arctic Ocean. Comparison of the mass
concentrations of colloidal and filter-retained particulate matter as well as the activity of 234Th in these phases indicates
that only a very small component of the colloidal material is actively involved in Th scavenging. Lower values of the
conditional partition coefficient between the colloidal and dissolved phase indicate that the Arctic colloids are less
reactive than colloidal material from other regions. The conditional partition coefficient between the filter-retained and
dissolved phases (Kf ) is generally higher than that for other regions, which is attributed to the higher complexation
capacity of glacio-marine sedimentary particles in these waters. The 234Th-derived export of POC for the shelf and deep
Canada Basin ranges between 5.6 and 6.5 mmol m2 d1, and is in agreement with other estimates reported for the
central Arctic Ocean and Beaufort Sea.
r 2003 Elsevier Ltd. All rights reserved.
Keywords: Marine colloids; Thorium scavenging; Arctic Ocean; Ultrafiltration; POC export; DOM; Residence times
1. Introduction
*Corresponding author. Tel.: +1-313-577-3262; fax: +1313-577-0517.
E-mail address: [email protected] (M. Baskaran).
The uniqueness of the Arctic Ocean, with such
features as seasonal and permanent ice-cover, ice
rafting (which describes that transport of sea-ice
0967-0637/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0967-0637(03)00140-7
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
laden sedimentary particulate matter), and pulse
primary productivity during ice-free months,
makes a study on the biogeochemical cycling of
particle-reactive elements, including radionuclides,
particularly interesting. Earlier studies indicate
that sedimentation rates in certain regions of the
Arctic Ocean are about 1–2 orders of magnitude
lower than in other arctic and world oceans, and
these areas have been generally regarded as areas
of low scavenging (Ku and Broecker, 1967; Finkel
et al., 1977; Moore and Smith, 1986; Bacon et al.,
1989). For example, increased removal rates of
particle-reactive radionuclides were observed in
the western Arctic (Edmonds et al., 1997) relative
to Alpha Ridge, (Moore and Smith, 1986; Bacon
et al., 1989). Recent studies also indicate that
sedimentation rates are highly variable in the
western Arctic Ocean (Darby et al., 1997; Huh
et al., 1997), and are likely due to the variability in
particle fluxes.
The concentration and nature of suspended
particulate matter (SPM) plays a dominant role in
the removal of particle-reactive radionuclides from
the water column (Turekian, 1977; Honeyman and
Santschi, 1989). In areas where SPM concentrations
are low, removal fluxes of particle-reactive radionuclides are also expected to be reduced. In such
areas, if colloidal mass concentrations (colloids are
defined here as macromolecules from 1 nm to 1 mm;
Buffle et al., 1992) are comparable to other major
ocean basins, then the relative importance of
colloidal material in the removal of particle-reactive
radionuclides is likely significant.
Contrary to studies carried out in the 1960s that
showed that primary productivity is near-zero
during non-summer months in the Arctic Ocean
(English, 1961), Wheeler et al. (1996) have recently
shown that the Central Arctic Ocean is not a
biological desert. Although generally less productive than oligotrophic ocean regions not covered
by ice, these waters support an active biological
community that contributes to the cycling of
dissolved and particulate organic carbon (POC).
As most of the colloidal matter is biogenic, and a
major component of SPM is derived from the
coagulation of this colloidal material, and SPM is
therefore also biogenic. Thus, certain regions in
the Arctic Ocean could serve as ideal sites for
investigating the relative importance of colloids
and SPM in the removal of particle-reactive
radionuclides.
Over the last 10 years, much work has been
focused on utilizing 234Th as a tracer to investigate
the role of colloids in marine scavenging in a wide
variety of environments. Earlier studies showed
that colloidal mass concentrations are much higher
than the corresponding filter-retained particle
concentrations, and that a significant fraction of
Th is associated with colloidal material. The
residence time of colloidal 234Th is usually very
short, B1–10 days (Baskaran et al., 1992; Moran
and Buesseler, 1992, 1993; Huh and Prahl, 1995;
Santschi et al., 1995; Guo et al., 1997; Dai and
Benitez-Nelson, 2001). At present, there is still no
data on colloidal Th in high-latitudes, so the
relative importance of colloid and filter-retained
particle concentrations on the removal of Th
remains unknown. The objective of the present
investigation is to understand the role of colloids
in the removal of 234Th in a unique high-latitude
marine setting.
2. Materials and methods
A suite of surface water samples as well as
samples from one vertical profile in the deep
Canada Basin were collected from the USCGC
Polar Star (Fig. 1), starting offshore from Barrow,
Alaska, and extending to the southeastern part of
the deep Canada Basin. Sample locations, water
column depths and surface salinities are given in
Table 1. In waters less than 85 m, samples for
filter-retained and filter-passing fractions of 234Th
were collected with a submersible pump, while for
depths >85 m with 30-l Niskin bottles. At all
stations 1–7 l aliquots were also collected from the
Niskin bottles for the determination of SPM,
particulate and dissolved organic carbon (DOC)
and nutrients.
2.1. Filtration of samples for SPM, DOC and POC
analyses
For the determination of SPM concentrations,
water samples were filtered through pre-weighed
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Fig. 1. Sampling stations on the shelf, slope and deep basin of the Canada Basin during Western Arctic, POLAR STAR 1998.
Table 1
Sample locations, maximum water depth and surface water salinitya and temperaturea
Sample code
Latitude (N)
Longitude (W)
Salinity
Water depth (m)
Temperature ( C)
Station-1
Station-2
Station-3
Station-4
Station-5
Station-6
Station-7
71
72
74
73
71
71
72
156
153
145
149
155
155
155
29.29
27.32
29.41
28.96
28.85
26.52
26.48
50
170
3000
1100
190
25
22
4.31
3.75
3.66
0.87
4.25
4.34
4.32
a
260
330
580
430
590
590
000
320
160
510
590
190
260
310
Values taken from uncalibrated CTD profiles for the upper 100 m.
Nuclepore membrane filters (pore size 0.4 mm cutoff). The filters were subsequently repeatedly
washed with double-distilled water to rinse off
any residual salts. These filters were then dried at
90 C for B24 h and stored for B24 h in a
desiccator prior to weighing on a microbalance.
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
Duplicate blank filters confirmed that there was no
gain or loss of weight during filtration. Three
duplicate samples were also collected to evaluate
the reproducibility of the SPM measurements.
For POC and particulate organic nitrogen
(PON) determinations, 1–5 l water samples from
the 30-l Niskin bottles were filtered through
precombusted 25 mm, 0.7-mm glass fiber filters.
Two samples were filtered sequentially through
two filters to assess potential DOC or dissolved
organic nitrogen (DON) losses during filtration.
As the second filter did not contain POC or PON
concentrations above blank levels, it is unlikely
that DOC and DON were retained on the filter (a
slight possibility does exist that whatever can be
adsorbed was removed on the first filter). In
addition, one blank filter was placed inside a precleaned glass beaker containing pre-filtered water
(o0.4 mm) for about 30 min after which the POC
concentration was determined. Again, the POC
concentration was indistinguishable from the
blank value. The POC and PON measurements
were conducted at the Central Instrumentation
Facility of Tulane University. Duplicate analyses
of POC and PON agreed to within 10%.
Samples for nutrient analyses were also drawn
from 30-l Niskin bottles. An aliquot of the water
that passed through 0.4 mm Nuclepore filter was
collected and immediately frozen. Nutrient concentrations were measured at the shore-based
laboratory with a 6-channel Alpkem IWA-6
industrial auto-analyzer, following standard methods (i.e., Baskaran et al., 1996). DOC was
measured on a Shimadzu total organic carbon
(TOC)-5000 analyzer by a high temperature
catalytic oxidation (HTCO) method, as described
in Guo et al. (1994).
finally through a flow meter to record the volume
of the water filtered (Baskaran et al., 1993). SPM
was removed by the pre-filter while the filterpassing fraction of Th was adsorbed by the two
MnO2-coated cartridge filters. The flow rates
varied between 10 and 15 l/min. After filtration,
the cartridges were brought to the shore-based
laboratory and were rinsed with distilled water to
remove salts. The amount of U associated with the
salt retained by the cartridge filter is very small
compared to the volume of water filtered such that
234
Th produced from this U is assumed to be
negligible. Santschi et al. (1999) showed that there
was no desorption of radionuclides during these
distilled water rinses. The cartridges were subsequently cut and the fibers were ashed at 550 C for
4 h. The ash was then quantitatively weighed and a
known amount of this ash was transferred to a 10
or 20 ml vial for radiochemical assay. The activity
of 234Th was determined by the 63 keV gamma
energy line. The dissolved and filter-passing
fractions were determined from the activity of
234
Th collected on the first (F1 ) and second (F2 )
MnO2-coated filter cartridges assuming that both
cartridges extract Th with the same efficiency. This
assumption is critical, and when the extraction
efficiency is less than B60%, the errors introduced
by this assumption can be significant (Cochran
et al., 1995a). In two samples where the extraction
efficiency was o65%, the data were discarded. In
the remaining 25 samples, the activity of 234Th was
calculated as follows (Baskaran et al., 1993):
2.2. Partitioning of filter-retained and filter-passing
fractions of 234Th
2.3. Ultrafiltration experiments
Water samples from the submersible pump (up
to 85 m from surface, 528–1185 l) or from the
Niskin bottles in a CTD-Rosette (for depths
>85 m, 351–491 l) were directly pumped through
a manifold consisting of three cartridges connected
in series: a pre-cleaned pre-filter (median pore size
of 0.5 mm), two MnO2-coated cartridges and
234
Th ¼ F1 =ð1 F2 =F1 Þ;
ð1Þ
234
where Th is the specific activity in the sample, F1
is the 234Th activity in the first cartridge and F2 is
the 234Th activity in the second cartridge.
For each ultrafiltration procedure, 120–190 l of
water from Niskin bottles was first filtered through
a pre-cleaned 0.4 mm Nuclepore filter cartridge to
remove SPM. The filtered water was then pumped
through an AMICON cross-flow hollow fiber
ultrafiltration cartridge (Model H10P10–20; molecular-weight cut-off 10 kD, polysulfone). The
cartridge was tested for integrity, as recommended
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
by the manufacturer prior to field sampling. The
ultrafiltration cartridge was calibrated with Cytochrome C (12 kD) standard dissolved in seawater
prior to the field sampling and the recovery was
found to be 8874%. The final retentate solution
(‘colloidal soup’) ranged from 1.3 to 2.4 l and the
concentration factor (=initial volume/final volume) ranged from 36 to 143 (mean=64).
Subsamples were also collected for later determinations of colloidal (colloidal organic carbon,
COC) and dissolved (p10 kD) organic carbon
concentrations. After each ultrafiltration experiment, the ultrafiltration cartridge and all tubing
were cleaned by re-circulating 2 l of 1N HCl for
B20 min, and this solution were also processed for
234
Th and OC measurements. Subsequently, the
cartridge was circulated with 10 l of nano-pure
water for 20 min twice and OC measurements were
made on the first batch of the circulated water.
After a set of 3 samples, the ultrafiltration
cartridge was cleaned rigorously as follows: 5 l of
1% Micro solution, 10 l of distilled water, 2 l of
1-M HCl, 10 l distilled water, 2 l of 0.5 M NaOH,
10 l of distilled water, 2 l of 1 M HCl, and 10 l of
distilled water. The organic carbon concentration
in the 1 M HCL washing as well as in the following
nano-pure water wash of the CFF cartridge was
indistinguishable from the OC levels in the nanopure water used for the experiment (2–3 mM).
The ultrafiltered water was subsequently
pumped through a manifold consisting of two
Mn-impregnated polypropylene cartridges connected in series. To the acidified colloidal concentrate and cartridge wash solutions, 50 mg of Fe3+
(in the form of FeCl3) was added and Fe(OH)3 was
precipitated twice by the addition of NH4OH to
ensure that all Th was quantitatively removed. The
dried Fe(OH)3 precipitate was quantitatively
transferred to a 10 ml counting vial and gamma
counted for 234Th (63 keV peak). The high-resolution Ge-well detector was calibrated with a
NIST-238U standard solution and IAEA RGU
standards. Since some amount of 238U would also
have precipitated along with Th, the samples
were recounted after about 6 months and the
results were used to correct for the contribution
from 238U. The decay (and growth from 238U)
corrections for 234Th were applied and the
1357
final activities of dissolved (o10-kD), colloidal
(10-kD—0.4 mm) and particulate (X0.5 mm)
phases were calculated.
2.4. Mass balance for
234
Th and organic carbon
Previous studies indicated that the mass balance
of carbon and 234Th were always very good using
10-kD cartridges, as compared to 1-kD cartridges.
For example, Guo et al. (1997) reported losses of
1–6% for 234Th in the 10-kD and 15–29% in the
1-kD cartridge systems. Wen et al. (1996) showed
that the mass balance is much better for Fe and Hg
with a 10-kD cartridge (93710% for Fe and
96716% for Hg) rather than 1-kD (7672% for
Fe and 60710% for Hg). Swarzenski et al. (1995)
reported 87 to >95% mass balance with 10-kD
for U in the Amazon shelf. Since a 10-kD cartridge
was used for this work, we will only compare our
results with those obtained using 10-kD cartridge,
as mass balance issue is a major concern with 1-kD
cartridges. In over half of the samples (8 out of 13)
the total recovery was within two-sigma of the
propagated errors (Fig. 2). In the remaining
samples, losses are slightly outside the two-sigma
propagated errors, as the sum of the Th in the
separate fractions is less than the total ultrafiltered. This loss is most likely due to absorption of
dissolved 234Th onto plastic liners used in the 200-l
3
y = 0.53 + 0.75 x R = 0.97
Summation
2
of
Fractions
1
-1
(dpm L )
Ideal line with perfect mass balance
0
0
1
Filter-passing
3
2
234
-1
Th (dpm L )
Fig. 2. Mass balance results for all the 13 samples that were
utilized for the ultrafiltration studies from the shelf, slope and
deep waters of the Canada Basin. Along the 45 line, there is no
loss or gain of Th (ideal line, with perfect mass balance).
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
1358
water tanks. While we did not attempt a carbon
mass balance, we did measure DOC in the acid
rinses of three samples and they were all below
detection limits. The total recovery efficiency for
234
Th varied between 83% and 138%, with a mean
value of 98%. This is similar to 74–135% range by
the same cleaning procedures (Guo et al., 1997).
Note that the 234Th in the wash of the ultrafiltration cartridge was added to the colloidal pool
(details given in Baskaran et al., 1992). Based on
lab experiments, it was demonstrated that colloidbound Th is not adsorbed on the wall surfaces
while inorganic Th is. On three samples, we
separately measured 234Th in the cartridge wash
and found less than a third of the colloidal 234Th in
the cartridge wash (in one 30% and other two,
below detection limit, Table 3). In the remaining
samples, we combined the colloidal and cartridge
wash fraction.
3. Results and discussion
3.1. Concentrations of SPM, and dissolved,
colloidal and particulate organic carbon
The concentrations of SPM from all the stations
varied between 24 and 4674 mg l1; highest concentrations were found in the coastal waters off
Barrow (Table 2, Fig. 3c). Duplicate analyses
indicate that SPM concentrations are reproducible
to within 10% (Table 2). In the deep Canada
Basin, the concentrations varied between 24 and
102 mg l1, with a mean value of 55 mg l1. In the
margins and deep basins, ice-rafting can play a
significant role in the re-distribution and concentration of SPM in the water column which could
affect the particle flux and rates of sedimentation.
Extremely low sedimentation rates in certain
regions of the Canada Basin (Ku and Broecker,
Table 2
Concentrations of suspended particulate matter (SPM), dissolved (DOC) and colloidal carbon (COC) and particulate organic carbon
(POC) and nitrogen (PON) in the Western Arctic Ocean, 1998
Sample code
SPM (mg l1)
DOC (mM)
COC (mM)
Concentration
factor
POC (mM)
PON (mM)
C/N
(mol/mol)
ST-1–5 m
ST-2–5 m
ST-2–15 m
ST-3–5 m
ST-3–25 m
ST-3–50 m
ST-3–100 m
ST-3–200 m
ST-3–500 m
ST-3–1000 m
ST-3–1500 m
ST-3–2000 m
ST-3–2860 m
ST-4–100 m
ST-5–5 m
ST-5–25 m
ST-5–75 m
ST-5–100 m
ST-6–10 M
ST-6–20 m
ST-7–5 m
ST-7–10 m
ST-7–20 m
275
105
122
64
30
43
102
62
NM
36 (39)
87
NM
24
NM
106 (110)
195
138
91
4090
4674
3422
768
1134
NM
118
86.0
109
NM
88.0
80.0
77.0
79.0
87.1
74.0
NM
68.3
95.6
116
NM
NM
NM
NM
NM
174
NM
NM
NM
13.8
10.8
12.0
NM
7.1
14.8
10.2
8.6
3.1
5.1
NM
6.2
8.8
13.6
NM
NM
NM
NM
NM
13.9
NM
NM
––
66
69
52
––
51
63
56
36
101
43
––
50
143
54
––
––
––
––
––
53
––
––
11.3
4.33
3.34
1.16
0.92
1.06
0.79
0.36
NM
0.45
2.94
3.30
0.41
NM
4.36
2.95
2.57
2.89
19.5
25.8
12.8
8.05
21.1
2.6
0.58
0.42
0.22
0.10
0.11
0.06
0.036
NM
0.029
0.16
0.15
––
NM
0.51
0.26
0.30
0.27
1.57
2.36
3.94
0.80
6.34
4.3
7.4
7.9
5.3
9.2
9.6
13.2
10.0
––
15.3
18.4
22.6
––
––
8.5
11.6
8.6
10.8
12.4
10.9
3.2
10.1
3.3
Numbers in parenthesis denote duplicate measurement.
NM: Not measured.
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3-
PO4 Concentration (µ M)
Nutrient Concentration ( µ M)
0
10
20
30
40
50
0.5
0
60
0
1.5
2
2.5
3-
ST-5-PO 4
-
Depth (m)
Depth (m)
1
50
50
100
1359
2-
ST-5-NO3
ST-5-SiO3
150
100
150
-
ST-3-NO3
200
200
2-
3-
ST-3-SiO2
ST-3-PO 4
(a) 250
(b)
250
-1
Suspended Particle Concentration (µg L )
Depth (m)
0
1000
(c)
3000
2000
0
20
40
60
80
100
120
CESAR Station
C anada Basin
Fig. 3. Vertical profiles of: (a) nitrate and silica; (b) phosphate and (c) suspended particulate matter concentration (SPM) in the
Canada Basin of the Western Arctic Ocean. The SPM data from the CESAR Ice Camp Station is plotted for comparison.
1967; Finkel et al., 1977) were attributed to low
particle flux and ultimately to low SPM concentrations. The total inventory of SPM for the
1500 m water depth is calculated to be 83.4 g m2,
and this can be compared to the value of 7.3 g m2
for the CESAR Ice Camp water column (Fig. 3c;
Bacon et al., 1989). This one order of magnitude
higher value will also lead to active scavenging of
Th and other particle-reactive species in this region
of the Canada Basin (discussed below). It appears
that there are isolated regions in the Arctic Ocean
(especially in areas of permanent ice cover) where
the SPM concentrations are very low and it is
expected that in these regions the sedimentation
rate also to be low (1–2 order of magnitude lower
than other basins). We speculate ice-rafting plays a
major role in the transport of SPM in the open
waters all the way to the North Pole.
3.2. Nutrient profiles
The biological activity in surface waters is
controlled primarily by the availability of nutrients. As will be discussed later, most of the
colloidal material is derived from biological
activity, and hence the concentrations of dissolved
organic matter (DOM) and colloidal organic
matter (COM) are closely tied to the availability
of nutrients. The concentrations of silicate
3
(SiO2
3 ), phosphate (PO4 ) and nitrate (NO3 ) are
considerably higher in the continental slope waters
than those in the deep basin (Figs. 3a and b). This
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
1360
3.3. Dissolved, colloidal and particulate organic
carbon
is likely due to input from the shelf waters of the
Chukchi Sea. Silicate concentrations are highest at
depths X100 m (32–37 mM) in the deep basin,
while phosphate concentrations increase with
depth (Fig. 3b). High silicate concentrations at
>100 m are typical of the Canada Basin, and this
increased concentration arises from shelf contributions into the boundary regions followed by
diffusion, mixing, and lateral injection (Swift
et al., 1997). On the continental shelf, large
component of the water budget is likely to have
been derived from the Chukchi shelf. Our values
are comparable to values published recently for
the Chukchi shelf (Wheeler et al., 1997) and the
Chukchi–Mendeleyev boundary region (Swift
et al., 1997).
Concentrations of DOC and COC (X10 kD–
0.4 mM) in the samples varied between 68
and 173 mM, and 3.1 and 14.8 mM, respectively
(Table 2), and these DOC values are comparable
to the values reported for the central Arctic Ocean
(Wheeler et al., 1997). In the deep Canada Basin,
DOC and COC concentrations in the upper 100 m
ranged between 80 and 109 mM, and 7.1 and
14 mM, respectively (Fig. 4a). The highest concentration of DOC was found in the coastal waters off
Barrow. POC and PON ranged between 0.4 and
26 mM and 0.03 and 6.3 mM, respectively. In the
upper 100 m of the deep Canada Basin, these
Concentrations of POC & PON ( µ M)
Organic Carbon Concentration ( µ M)
0
0
20
40
60
80
100
0
120
1
2
3
4
0
1000
Depth (m)
Depth (m)
500
1500
2000
1000
2000
PON
2500
(a)
COC
POC
DOC
(b) 3000
3000
POC/
Depth (m)
0
0
5
234
Th p ( mol dpm -1)
10
15
20
1000
2000
(c)
3000
Fig. 4. Vertical profiles of: (a) dissolved and COC; (b) POC and PON; and (c) POC/234Thp ratios in the deep Canada Basin of the
Western Arctic Ocean.
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values ranged between 0.8 and 1.2 mM and 0.11
and 0.22 mM, respectively (Fig. 4b). The C/N (mol/
mol) ratios varied between 3.3 and 22.6 in all these
samples and between 5.3 and 13.2 in the upper
100 m of the deep Canada Basin (Table 2).
Concentrations of DOC in surface waters
provide information on the biological activity.
The DOC concentrations are highest in the coastal
waters off Barrow, with values up to 174 mM. In
the mixed layer of the deep basin, the DOC
concentration was 86 mM, which is within the
range (34–107 mM) reported for other deep Arctic
basins (Makarov, Amundsen, Nansen basins and
Chukchi Abyssal Plain: Wheeler et al., 1997). The
concentration of DOC (77 mM) in the Atlantic
layer (200–300 m) appears to be slightly higher
than the value (56 mM) reported by Wheeler et al.
(1997). Wheeler et al. (1997) showed that the DOC
concentrations in the surface mixed layer of the
central Arctic are typically X100 mM, which is
considerably higher than those found in other
oligotrophic regions (40–60 mM), such as the
Sargasso and Ross Sea. The total inventory of
DOC in the upper 100 m was found to be
110 g m2, and the corresponding COC value was
found to be 12.5 g m2, equivalent to an average of
11.3% (range: 3.6–18.5%, Table 2) of the DOC
pool. Our value is comparable to the range of
values (Basin: 94.4714.4 g m2) reported for
Chukchi Abyssal Plain, Makarov Basin, Amundsen and Nansen Basins (Wheeler et al., 1997). The
ratio of COC to total DOC is similar to other
major ocean basins where on the order of 10% of
the DOM was extracted on a 10-kD ultrafiltration
cartridge (e.g., Amon and Benner, 1994; Guo et al.,
1994). Thus, it is likely that the molecular size
distribution of DOC in the deep Canada basin is
similar to other major ocean basins.
The highest POC was found in the coastal
station samples and ranged from 4 to 26 mM. In
the deep basin, the vertical profiles of POC and
PON (Table 2, Fig. 4b) indicated that there was a
general decrease in the concentrations of POC and
PON below 100 m and that the C/N ratios
monotonically increased with depth below 200 m.
At two depths, 1500 and 2000 m, the POC
concentrations were X3 mM, while the C/N ratio
continued to increase. This increase is likely due to
1361
preferential remineralization of biogenic components. High C/N ratios generally indicate terrigenous sources of organic matter. High values of
C/N ratios in the bottom waters (>200 m) indicated
that these materials are likely refractory, and are
derived from the discharge of Arctic rivers. Lower
values of C/N (o10) ratios in the upper 200 m of
the water column indicate that the sources are
more likely marine, rather than terrestrial (Wheeler et al., 1997). Krishnamurthy et al. (2001)
showed that 50% of the sedimentary organic
carbon at >700 km offshore in the Kara Sea was
derived from the terrigenous input discharged
from the Ob and Yenisey Rivers. Wheeler et al.
(1997) compiled the C/N ratios in Russian Arctic
Rivers, which are estimated to contribute up to
25% of the DOC to the central Arctic area. Based
on the concentrations of lignin oxidation products
and a depletion of d13C values in >1-kD colloidal
material from the surface Arctic Ocean waters,
Opsahl et al. (1999) concluded that the terrigenous
colloidal material accounts for a much greater
fraction of the colloidal material in the surface
Arctic (5–33%) than in the Pacific and Atlantic
Oceans (0.7–2.4%). The fraction of POC in TOC
varied between 0.5% and 6.8%, with a mean value
of 2.6%, making it a minor component. Moran
et al. (1997) reported POC concentrations in the
range of B0.5–1 mM in the deeper waters of the
Makarov, Amundsen and Nansen Basins (a total
of three samples) and the difference between our
data set and that of Moran et al. (1997) could
likely be due to the differences in the sampling
method. The concentrations of DOC, COC, and
POC in the study area indicate that the biological
activity is significantly higher than in other
oligotrophic regions.
3.4. Relationship between COC and SPM
Colloids are primarily organic-rich matter produced mainly in the euphotic zone biologically,
especially by exudates of phytoplankton (e.g.,
Niven et al., 1995; Wells and Goldberg, 1992).
Based on the colloidal 232Th activities in Gulf
of Mexico waters, Baskaran et al. (1992) estimated
that the terrigenous silicate-derived colloids (>10kD—0.5 mm) are p6% of the total colloidal mass
ARTICLE IN PRESS
1362
M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
concentration, leaving X94% to be of biogenic.
The colloidal material can undergo coagulation,
which results in particle sizes that exist at the
larger end of the particle size spectrum. If most of
the filter-retained particulate matter is derived
from the coagulation of sub-micron particulate
matter, then a relationship between these two is
expected (Honeyman and Santschi, 1989; Baskaran et al., 1992; Moran and Buesseler, 1993).
Attempts have been made to predict the relationship between the SPM concentration and colloidal
concentration (Honeyman and Santschi, 1989;
Baskaran et al., 1992; Moran and Buesseler, 1993).
In order to determine the relative amounts of
SPM derived from atmospheric deposition and
biological activity in the deep basin, we have
estimated dust deposition on the surface of the
Arctic Ocean from the land. Darby et al. (1989)
estimated a range of 3–14 mg cm2 yr1 for the
accumulation in snow samples from the Arctic
pack ice. The mean residence time of filter-retained
particulate matter in the upper 50 m in the water
column is B15 days (mean value of the particulate
234
Th residence time in five samples from stations
2, 3 and 5, Table 4; the residence times of particles
and particulate 234Th are assumed to be the same).
Assuming that all the aeolian dust particles are
greater than 0.4 mm size and do not undergo
degradation while they are in the mixed layer, the
atmospheric dust input would yield a SPM
concentration of B0.2 mg l1, which is much less
than 1% of the total mass concentration. Assuming then that most of the particles are of marine
origin, as the above discussion implies, and that
particulate matter enters the particle size spectrum
towards the fine end (colloidal) and leaves at the
top (filter-retained larger particles), the production
rate of filter-retained particles is expected to be
proportional to the concentration of sub-micron
particles, assuming first-order rate process. The
colloidal mass concentration (Cc ) was calculated
from COC and a stoichiometric conversion factor
of 2.5 between organic carbon and organic matter
(Buffle et al., 1992). The Cc =Cp (Cp : concentration
of SPM) ratios varied between 1.8 and 7.8, with a
mean value of 4.2 (sample in ST-7 is excluded, as
the particle concentration is B2 orders of magnitude higher than typical value). This is significantly
higher than the value used by Moran and
Buesseler (1993) for the continental shelf waters
off New England. The vertical profiles of Cc =Cp
ratios show a decrease with depth, as a relatively
large portion of the Cc is removed in the upper
100 m (Fig. 7a) by coagulation of sub-micron
particles. The increase in the ratio near the
sediment–water interface (bottom nepheloid layer)
could be due to release of colloidal material from
the porewaters and/or from the breakdown of
biogenic particles and subsequent release of
colloidal particles into the water column.
The widespread occurrence of particles entrained within the ice has been reported in several
regions in the Arctic Ocean. (e.g., Barnes et al.,
1982; Reimnitz et al., 1993). How much of the icerafted particulate material would be transported to
the sampling area remains unknown. Although
there were no major ice floes during our sampling,
how much would have come from adjoining areas
by lateral transport and how much of it is biogenic
in origin remains unknown. However, the C/N
ratios of the filter-retained particulate matter do
indicate a marine, rather than a riverine source. If
the sedimentary particles were trapped and retained within the sea ice for several months in the
marine environment by the annual ice-rafting
mechanism, it is likely that the sediments would
exhibit a more marine signature due to constant
interaction with dissolved organic matter produced from biological activity, although they were
originally derived from rivers.
3.5. Activities of dissolved, colloidal and particulate
Th
234
As expected, particulate 234Th activities (Table 3)
are highest in the coastal waters where concentrations of SPM are highest there. There is considerable removal of Th in the surface continental
margin waters, where the particulate 234Th activities are also quite high. Uranium activities were
calculated using the well-known salinity-U relationship of Chen et al. (1986):
238
Uðdpm=lÞ ¼ 0:0709 salinity:
The salinity of the surface waters in the deep
Canada Basin is significantly lower (Table 1) than
Table 3
Size-fractionated
234
Th activitiesa, salinityb and
238
U activity
U activityc
(dpm l1)
Total filter passing
(p0.5 mm)
Colloidal (10-kD0.4 mm)d
Particulate
(X0.5 mm)
Ultrafilter passing
(p10-kD)
ST-1–5 m
ST-2–5 m
ST-2–15 m
29.29
27.32
30.23
2.08
1.94
2.14
0.4170.03
0.7970.03
1.3170.01
0.7370.10
0.4370.02
0.1370.01
NM
0.6770.04
1.1670.08
ST-2–40 m
ST-3–5 m
ST-3–25 m
ST-3–50 m
ST-3–80 m
ST-3–100 m
ST-3–200 m
ST-3–500 m
ST-3–1000 m
ST-3–1500 m
ST-3–2000 m
ST-3–2860 m
ST-4–100 m
ST-5–5 m
ST-5–25 m
ST-5–50 m
ST-5–75 m
ST-5–100 m
ST-6–3 m
ST-6–20 m
ST-7–5 m
ST-7–20 m
32.14
29.41
31.29
32.08
32.76
33.09
33.70
34.83
34.88
34.92
34.88
34.88
32.44
28.85
31.02
31.82
32.53
32.87
26.52
27.36
26.48
27.18
2.28
2.09
2.22
2.27
2.32
2.35
2.39
2.47
2.47
2.48
2.47
2.47
2.30
2.05
2.20
2.26
2.31
2.33
1.88
1.94
1.88
1.93
0.7370.04
1.3470.02
1.2370.08
1.9770.03
1.8670.06
1.7970.02
1.8570.03
2.2070.03
2.1870.02
2.4970.04
2.3670.17
2.8170.10
1.7670.01
1.4270.03
0.7870.11
0.6370.01
0.6170.03
0.5570.03
0.1070.01
0.1370.01
0.2470.01
0.2470.03
NM
0.1670.03
0.3470.03
(0.0870.02)
NM
0.1970.02
NM
0.3070.03 (BD)
NM
0.2170.01
0.1270.02 (BD)
0.1370.03
0.0970.02
0.0970.01
NM
0.0870.01
0.1870.03
0.2270.02
NM
NM
NM
NM
NM
NM
0.0670.01
NM
0.3170.02
0.1170.01
0.0970.02
0.2070.02
0.0670.01
0.1970.01
0.5370.02
0.2570.03
0.1870.01
0.2370.02
0.1870.03
0.1870.02
0.3570.01
0.2570.01
0.3870.02
0.3770.01
0.4870.01
0.6870.02
0.5470.05
0.3170.03
0.2170.01
0.3270.01
NM
1.1770.08
NM
1.4370.07
NM
1.3770.09
1.6470.10
1.8470.06
1.7370.07
2.3470.06
NM
2.4170.19
1.3770.11
1.0270.19
NM
NM
NM
NM
NM
NM
0.2770.01
NM
NM: Not measured.
a
The standard errors quoted above are the propagated errors due to the counting statistics arising from the 234Th activities in the sample and calibration of gamma-ray
spectrometer with standards.
b
Salinity values for the upper 100 m were obtained from the uncalibrated CTD profiles. For deeper waters, salinity values were taken from the CTD profiles obtained
in AWS 2000 cruise (CTD was lost during 1998 cruise).
c 238
U activities were calculated using the relationship: 238U (dpm/l)=0.0709 salinity (Chen et al., 1986).
d
On three samples, 234Th was measured separately in the cartridge wash (numbers in parenthesis); in one sample, 234Th concentrations was about 30% of the colloidal
fraction and the remaining two, 234Th in cartridge wash was below detection limit.
ARTICLE IN PRESS
Salinity (psu)
M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
Station and water
depths
238
1363
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
1.870.2
1.070.2
1.770.3
0.370.3
0.770.2
1.470.2
0.570.2
0.670.6
1.270.3
1.670.6
3.9070.5
2.170.2
1.670.2
2.270.3
1.870.3
1.470.2
1.970.2
0.170.2
0.0570.5
2.270.3
2.370.6
4.570.4
Th in the cartridge wash (data in Table 3).
Number in parenthesis for ST-2–15 m was calculated without adding
a
1.970.2
1.570.2
3.270.1
1.970.2
234
2.670.2
2.470.2
2.870.3
2.270.3
1.870.2
2.170.2
0.470.2
0.270.5
2.770.3
3.070.5
4.670.3
ST-35 m
ST-350 m
ST-3100 m
ST-3200 m
ST-3500 m
ST-31000 m
ST-31500 m
ST-32860 m
ST-4100 m
ST-55 m
ST-75 m
44710
9.171.4
6.271.0
59734
1973
2075
49714
9.571.2
1272
767601
6.671.5
1857194
102776
9.072.4
35711
81720
4.871.1
1372
5987154
68791
––
13287414 16371823 32731
5175
8.371.9
3079
35725
9.572.4
1675
5.870.3
1.470.2
5.570.3
81
1371
176
2773
102 11.870.9
723
4973
282
2573
143
2772
966
1171
––
3074
154
2472
89
1973
14 18607119
1572
3374
1471
5274
2773
2972
1171
3174
2772
2374
22707137
5.871.1
9.071.0
(6.971.0)
4.570.6
9.871.1
3.570.3
2.470.4
2.770.6
5.671.3
2.570.3
1.870.3
5.070.9
5.371.1
5.370.9
6175
9.270.9
4974
7.170.7
63
105
2272
8.871.6
5.071.0
1873
3.670.1
2.870.2
ST-25 m
ST-215 m
1873
41714
Kd
Kp
Kc a
5
1
5
1
(10 ml g ) (10 ml g ) (105 ml g1)
ttot
(d)
tp (d)
tc (d)
JCTh
JPTh
to10 kD
JDTh
( 103 dpm l1 d1) ( 103 dpm l1 d1) ( 103 dpm l1 d1) (d)
Station and
water depth
Table 4
Residence time of dissolved (td ), particulate (tp ), colloidal (tc ), and total
phases
234
Th(ttot ), and partition coefficients of
234
Th between dissolved, particulate, and colloidal
1364
in other major ocean basins. During the annual ice
formation, the brine formation leads to higher
salinity values just below the ice cover and when
this ice melts during early summer months, less
saline water is likely added to the underlying water
mass. It is assumed that when salt is rejected
during ice formation, U is also rejected and thus
the U-salinity relationship remains valid.
In the deep Canada Basin, there is a deficiency
of 234Th {[238U—(234Thp+234Thfp)] >0 where ‘p’
refers particulate and ‘fp’ refers filter-passing,
p0.5 mm} in the upper 100 m, and it reaches
equilibrium below 100 m (Fig. 5a). At 1500 m, the
total 234Th is >238U and the SPM concentration is
significantly higher than it is in the sampled layers
below and above this depth. Increased concentrations of POC and PON are also seen at this depth
and it is likely that this could be from advected
sedimentary particles released from the sea ice. It
could also be argued, however, that this observation is not statistically significant. The deepest
sample has significantly higher 234Th than 238U
which could be attributed to the bottom nepheloid
layer where fine sedimentary particles are resuspended and adsorb/desorb dissolved 234Th. Such
excess of 234Th over 238U at the bottom-most
depths in open oceans has been reported in other
places (e.g., Gulf of Mexico, Baskaran et al.,
1996). The SPM concentrations at the shelf station
are considerably higher than those at the deep
station, and the fraction of particulate 234Th
increased with depth from 15% to 55% in the
upper 100 m, which is unusually high for the
continental shelf region.
3.6. Modeling
The distribution of colloidal, particulate and
filter-passing 234Th can be modeled to determine
the residence time of 234Th associated with these
three phases. The basic approach is similar to
Coale and Bruland’s (1985) irreversible, scavenging box model, except one more compartment is
added to the box to represent the colloidal pool.
Similar approaches have already been attempted
to determine the rate constants and residence times
of particulate, colloidal and dissolved 234Th
(Moran and Buesseler, 1993; Huh and Prahl,
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
Concentration of
0
234
Th and
238
-1
U (dpm L )
(a)
500
Depth (m)
1000
1500
238
U
Colloidal234 Th
2000
Particulate 234 Th
Dissolved (<10-kD) 234 Th
2500
234
Total (Σ fractions)
Th
3000
(a)
0
1
Concentration of
0
1
0
2
234
3
238
-1
Th and U (dpm L )
2
3
Depth (m)
20
40
60
80
238
Particulate
U
100
(b)
120
Colloidal
Dissolved (<10-kD) Total (Σ fractions)
Fig. 5. Vertical profiles of: (a) dissolved (o 10-kD), particulate
(>0.5 mm), colloidal (>10-kD–0.4 mm) and total 234Th (o 0.5
mm+>0.5 mm) in the entire water column and (b) dissolved,
particulate, colloidal and total 234Th in the upper 100 m of the
in the deep Canada Basin of the Arctic Ocean.
P
238
U
Dissolved 234Th
J DTh
1365
1995). This box model (Fig. 6) assumes the
following: (i) the system is at steady state; (ii) the
horizontal and vertical advection as well as
diffusion are negligible; (iii) the removal of 234Th
is only a serial process, and the removal of
dissolved 234Th directly onto SPM is negligible,
and all the removal of dissolved 234Th goes
through the colloidal loop; and (iv) there is neither
disaggregation of colloidal or particulate 234Th,
nor any desorption of 234Th either from the colloidal
or particulate pool into the dissolved phase. These
important assumptions are evaluated below.
In general, most open oceans may be considered
to function as steady-state systems, except during
the spring plankton bloom (Buesseler et al., 1992).
In the Canada Basin, there was no evidence for a
bloom during late August and so we assume that
this system was in steady state during our sampling
period. The vertical movement of water masses
containing 234Th has been reported to be significant in regions of high upwelling, such as the
equatorial Pacific (Buesseler et al., 1995; Bacon
et al., 1996) as well as at sites where coastal
upwelling is dominant during certain times of the
year, as in the Arabian Sea during the Southwest
Monsoon (Lee et al., 1998). There was no evidence
of upwelling in the deep Canada Basin during our
sampling, and hence we assume that vertical
advection is negligible. Horizontal advection has
been found to be significant only in near-shore
waters where intensified horizontal scavenging has
been reported (Gustafsson et al., 1998). It is likely
that this factor could affect our coastal station
Colloidal 234Th
JCTh
Particulate 234 Th
Cd
Cc
Cp
λCd
λCc
λCp
JPTh
Fig. 6. Scavenging box model used to determine the net removal fluxes from solution (o10-kD) to colloids (JDTh ), from colloids to
filter-retained particles (JCTh ), and removal of these filter-retained particles (JPTh ) and residence time of colloidal, particulate, and
dissolved residence time; l is the decay constant (=0.0288 d1) of 234Th.
ARTICLE IN PRESS
M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
1366
samples (ST-5 and ST-7). The diffusion of 234Th is
likely to be negligible, as there is no steep activity
gradient in 234Th. With our data, it is not possible
to address assumptions (iii) and (iv) as this would
require measurements of 234Th in various classsizes of particles.
The rates of change of 234Th in each of the
reservoirs, shown in Fig. 6, are
qCd =qt ¼ P lCd JDTh ¼ 0;
ð2Þ
qCc =qt ¼ JDTh lCc JCTh ¼ 0;
ð3Þ
qCp =qt ¼ JCTh lCp JPTh ¼ 0;
ð4Þ
of 234Th; P is the production rate (=l activity of
238
U) of 234Th from its parent, 238U; and
JDTh ; JCTh ; and JPTh are the removal fluxes from
solution (o10-kD) to colloids, from colloids to
filter-retained particles, and removal of these filterretained particles. The scavenging residence times
of dissolved, colloidal, and particulate 234Th are
td ¼ Cd =JDTh
2
4
Th and Ratios of C /C
c
6
8
10
12
14
1000
[
234
234
Th ] / [
c
Th ] Ratio
fp
2000
Net removal Fluxes of
p
16
0
Depth (m)
Depth (m)
0
0
234
tp ¼ Cp =JPTh :
ð5Þ
In this three-reservoir system, 234Th produced in
the water column is removed by colloidal material,
which is then subsequently removed by the
filter-retained particulate matter. The relationship between residence time of dissolved (td ),
colloidal (tc ), and particulate (tp ) 234Th and filterpassing 234Th (tfp ) is (Kim et al., 1999; Alleau
where Cd ; Cc and Cp are activities of 234Th
(dpm l1) in the dissolved, colloidal and particulate phases; l (=0.0288 d1) is the decay constant
Fraction of Colloidal
tc ¼ Cc =JCTh
0
5
10
234
-2
-1
-1
Th (x10 dpm L d )
15
20
25
30
1000
J
2000
DTh
J
CTh
J
PTh
Cc / Cp
(b) 3000
3000
Specific Activity of Particulate
0
0
2000
4000
6000
234
-1
Th (dpm g )
8000
4
1 10
Depth (m)
CESAR Ice Camp Station
1000
AWS 98 Canada Basin Station
2000
(c) 3000
Fig. 7. Vertical profiles of: (a) ratios of mass concentration of colloidal material (Cc ) to that of the filter-retained SPM (Cp ) and ratio
of colloidal 234Th (234Thc) to that of filter-passing 234Th (234Thfp); (b) the net removal fluxes of 234Th from dissolved, particulate and
colloidal phases and (c) specific activities of 234Th in the deep Canada Basin of the Arctic Ocean. Data from the CESAR Ice Camp
Station (Bacon et al., 1989) are plotted for comparison.
ARTICLE IN PRESS
M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
et al., 2001)
tfp ¼ td þ tc þ tp þ lðtd tc þ tc tp þ tp td Þ
þ l2 t d t c t p :
ð6Þ
3.7. Activities, fluxes and residence times of
colloidal, dissolved and particulate 234Th
If the concentration of SPM is very low, as was
observed in the CESAR Ice Camp water column,
then the slow removal of 234Th could lead to nearequilibrium values with its parent, 238U, in the
surface waters. Analyses of particulate and filterpassing 234Th in the upper 100 m of the water
column of the coastal, shelf, and deep Canada
Basin indicate that there are deficiencies of 234Th
relative to equilibrium with its parent, 238U, due to
active scavenging on time scales of days to months
(Table 3, Figs. 5a and b). Similar observations
have been reported by Moran et al. (1997) and
Moran and Smith (2000) for the shelf, slope, and
basin of the Chukchi plateau, and the Amundsen
and Makarov basins and by Cochran et al. (1995b)
for the Northeast Water Polyna, Greenland.
Particulate 234Th activities in the deep basin are
significantly higher than the values reported by
Bacon et al. (1989) for the CESAR Ice Camp
water column, following the trend in SPM
observed at these two stations. The removal fluxes
of 234Th from solution (o10-kD) to colloids
(JDTh ), from colloids to filter-retained particles
(JCTh ), and of these filter-retained particles (JPTh )
to sediments are the highest in the coastal station
(ST-7) where the SPM concentrations are the
highest. These higher values of removal fluxes in
the water column with high concentrations of
SPM are likely due to high rates of removal by
adsorption–desorption reactions on particles and
coagulation–disaggregation processes. Active coagulation of metals and organic matter in estuaries
and coastal waters has been reported in the
literature since the 1970s, and these coastal
processes could lead to active removal of various
size-fractions of 234Th. Honeyman et al. (1988)
suggested that particle concentration serves as a
surrogate parameter for surface site concentration
and thus serves as the master variable controlling
Th and other trace metal scavenging in marine
1367
systems. The lowest removal fluxes of dissolved,
particulate and colloidal 234Th are found in the
deeper waters of the Canada Basin (Table 4 and
Fig. 7b), where SPM is lowest.
It can be seen in the vertical profile that the
specific activity (=activity of 234Th per gram of
particulate matter) of 234Th varies by a factor of 4
(Fig. 7c), as opposed to a near-constancy reported
by Bacon et al. (1989). At mid-depths (1000 and
1500 m), the specific activity decreases as SPM
concentration increases. The highly variable specific concentrations are likely due to varying
concentrations of SPM. Particulate matter trapped
in porous sea-ice is likely transported farther from
its source and could serve as a ‘sieve’; when surface
seawater freely flows through the porous particleladen sea ice, more 234Th can be scavenged by the
particulate matter during the transport of sea-ice.
When the particles are released, their specific
activities of 234Th are likely higher. Since we find
higher specific activities only at intermediate
depths, it is likely that particles in these layers
were derived from ice floes in an adjacent water
mass by non-steady-state pulse. However, observed equilibrium between 234Th and 238U at
depths >1000 m in the deep basin indicates that
lateral scavenging time scales are much longer
than the mean life of 234Th.
The [234Thc]/[234Thfp] ratios varied between
B15% in the surface waters to B3% in the
bottom water, with a mean value of B14% in the
upper 100 m. Generally, these values are similar to
the COC/DOC ratios extracted using the 10-kD
ultrafiltration cartridge. Because the >10 kD–
0.4 mm colloidal mass concentration is B4–8 times
higher than the filter-retained particulate matter, it
is likely that only a small portion of the extracted
colloidal material complexes with thorium and a
major fraction of the colloidal Th is inert with
respect to Th scavenging. It is possible that a small
but variable fraction of the colloidal material (i.e.,
organic ligands) complexes with Th. Rich et al.
(1997) reported concentrations of dissolved combined neutral sugars (including polysaccharides) in
the western Arctic that ranged from 0.2 to 17 mM
C (mean value of 4.5 mM C) and made up o10%
of the total DOC pool. Polysaccharides have high
affinity for 234Th (Nash and Choppin, 1980), and
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
Quigley et al. (2002) proposed that the observed
variability of POC/234Th ratios in the upper 100 m
of the oceanic water column is caused by the
variability of the polysaccharide content.
The residence times of dissolved, particulate and
colloidal 234Th (Table 4) indicate the following: (i)
the colloidal and particulate 234Th residence times
are the shortest at the coastal site (1.4 days) and
increase off-shore (9.1 days in the surface waters);
(ii) the colloidal residence times generally increase
with depth, with the highest values (598 days at
1500 m) occurring in the deepest waters and (iii)
the dissolved and particulate residence times of
234
Th increase with depth in the deep basin. Our
observations are similar to those for the particlerich waters of the Buzzards Bay by Moran and
Buesseler (1993), who reported low colloidal 234Th
residence times, averaging 0.3 days and longer
residence times of dissolved and particulate 234Th
in surface waters, averaging 2 and 4 d, respectively.
Santschi et al. (1995) reported residence times of
colloidal 234Th ranging between 3 and 30 d in the
Gulf of Mexico and the Middle Atlantic Bight,
which suggests rapid turnover of high-molecularweight fractions that are traced by 234Th. The slow
removal of colloidal 234Th in the deep waters in
this study is attributed to the low colloidal mass
concentration in the deep waters as well as the
refractory nature of DOM in the deep waters
(Druffel et al., 1989; Santschi et al., 1995). Rich
et al. (1997) reported rapid turnover rates of
glucose and amino acids, averaging 0.23 d1
(residence time of 4.3 d) in the upper waters of
the western Arctic. It is worth noting that the
values of residence times and removal fluxes of
colloidal, particulate and dissolved 234Th were
calculated under the assumption that 234Th always
goes through the colloidal pool serially and there is
no direct removal of 234Th from the solution to the
filter-retained particulate matter. As will be
discussed later, the reaction sorption sites appear
to be low, as compared to the filter-retained
suspended particles and hence there could be some
amount of parallel removal (i.e., dissolved 234Th
directly removed onto filter-retained particulate
matter). Under such a scenario, the dissolved
removal flux will remain the same while the
removal fluxes of colloidal and particulate matter
will increase; correspondingly, the residence time
of dissolved 234Th will remain the same, while the
residence times of particulate and colloidal 234Th
will decrease. Controlled experiments using Th
tracers do indicate serial reactions, with X50% of
the total uptake of ionic Th onto suspended
matter, regardless of the mass ratio of colloids to
filter-retained particulate matter (Honeyman and
Santschi, 1989). With our data, we are unable to
determine whether there is any parallel removal
occurring. Honeyman and Santschi (1989) and
Baskaran et al. (1992) used another approach
using the sedimentation model of Farley and
Morel (1986) to determine the particle and
colloidal removal rates. The model these authors
employed is a semi-empirical description of the
various coagulation mechanisms (such as shear,
Brownian motion, differential settling and aggregation) yielding particle sedimentation. Both the
coagulation/sedimentation model and the box
model used here yield the same residence times.
In the coagulation/sedimentation model, it is also
assumed that 234Th enters the particle size
spectrum at the fine end (sub-micron size) and
leave at the top end by coagulation/sedimentation
and the direct transfer of the dissolved to filterretained particulate pool was not considered.
Simply, the dissolved 234Th produced from the
decay of 238U attaches to a small sized colloidal
particle and is pumped up the particle size
spectrum by coagulation mechanisms (Honeyman
and Santschi, 1989).
3.8. Conditional partition coefficients, Kc and Kf,
and distribution coefficient, Kd
The partitioning study of Th provides insight
into the association of Th with colloidal and filterretained particulate matter. The mass concentration of colloidal material extracted with 10-kD
cross-flow polysulfone filter is B4 times the mass
of the filter-retained particulate matter. The mean
sizes of these two pools are unknown, but if it is
reasonably assumed that the difference between
diameter of the filter-retained SPM and the
equivalent spherical diameter of the macromolecules extracted with the 10-kD ultrafiltration
cartridge is a factor of 10, then the sorption site
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
density of colloid should be about 400 times that
of Cp : If the behaviors of sorption sites of colloids
and filter-retained particles were similar, then we
would expect most of the Th to be associated with
colloidal material. However, our results show that
only about 10% of the total 234Th is colloidal, so it
appears that complexation capacity and hence
binding site density of colloidal material and filterretained particulate matter are different. The
implication is that the site density of colloidal
particles for Th may not increase in proportion to
the size of the colloidal pool in the submicron
range, despite the increase in surface area per unit
mass. Similar observations were reported by
Baskaran et al. (1992) and Moran and Buesseler
(1993).
The two conditional partition coefficients (conditional because these partition coefficients depend
on solution conditions such as pH, ionic strength,
type and concentration of Th-complexing ligands,
etc.) are defined as follows (Honeyman and
Santschi, 1989; Moran and Moore, 1989; Baskaran et al., 1992; Moran and Buesseler, 1993):
Kc ¼ ½Thc =f½Thd Cc g;
ð7Þ
Kf ¼ ½Thf =f½Thd Cp g;
3 1
ð8Þ
3 1
where Kc (cm g ) and Kf (cm g ) are the
conditional partitioning coefficients between the
colloidal and dissolved phases (cm3 g1), and
between particulate and dissolved phases; [Thc],
[Thd] and [Thf] are the activities (dpm cm3) of
234
Th in colloidal, dissolved and filter-retained
particulate phases; Cc and Cp are the mass
concentrations of colloidal and filter-retained
particulate matter (g cm3).
The Kc values range between 1.8 and
9.8 105 cm3 g1, with a mean value of
4.8 105 cm3 g1. This is about an order of
magnitude lower than the values reported for the
Gulf of Mexico (Baskaran et al., 1992), and this
difference is likely due to the variability in the
nature of colloidal material, i.e., the composition
and structure of macromolecules could be significantly different. Opsahl et al. (1999), using the
concentrations of lignin and d13C isotopic compositions in the colloidal fractions (>1-kD) of Arctic
samples, estimated that 5–33% of the colloids are
1369
terrigenous, much higher than in the Pacific and
Atlantic Oceans (0.7–2.4%). Although the colloidal size ranges of Opsahl et al. (1999) are different
than those measured in this study, it appears that
Arctic colloids are generally less reactive than the
colloidal material from other regions. Rich et al.
(1997) concluded that the heterotrophic bacterial
production is relatively high compared to primary
production and is substantiated by high turnover
and uptake rates of dissolved free amino acids and
glucose. Thus, the colloidal composition in the
Arctic could be significantly different than in other
regions. Our values of Kc are higher than the
values reported for the continental shelf waters
off New England, 0.2–3.8 105 cm3 g1 (mean:
1 105 cm3 g1; Moran and Buesseler, 1993). The
difference between our values and those of Moran
and Buesseler’s is likely due to the assumed Cc =Cp
ratio of 2 as our field-based data indicate that this
ratio is closer to 4. The values of Kf and Kd for
the Arctic are quite similar, but about six times
higher than the Kc values. The Kf and Kd values
ranged between 9 and 61 105 cm3 g1 (mean:
28 105 cm3 g1) and between 7 and 40 105 cm3 g1 (mean: 24 105 cm3 g1). These values
are comparable to the values of 15 and
17 105 cm3 g1, for Kd and Kf for the New
England continental shelf waters. Comparing the
values of Kc ; Kf ; Kd for the Arctic water with those
from two other regions for which published data
are available, we can infer that suspended particles
in the Arctic have a higher complexation capacity,
while the colloidal material has a considerably
lower complexation capacity. Previous studies
have shown that the glacio-marine sediments have
lower ion-exchange capacity (Naidu and Mowatt,
1983); thus our results contradict earlier observations (based on the total concentrations of trace
metals on sediments) on the ion-exchange capacity
of high-latitude sedimentary particles.
From previous studies on the distribution of
colloidal, particulate and dissolved 234Th in the
Gulf of Mexico and continental shelf waters off
New England, it was concluded that only a small
fraction of the colloidal pool actively participates
in short-term Th scavenging (Baskaran et al., 1992;
Moran and Buesseler, 1993). In the present study,
the average Kc is B6 times smaller than Kp ; while
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
the mass concentrations of colloids are about 4
times higher so that o1% of the colloidal pool
actively participates in scavenging of metals. We
speculate that the coagulation of colloids involves
primarily these reactive colloids and that rest of
the colloids probably do not readily participate in
the removal of any species. A recent study by
Quigley et al. (2002) indicates that polysaccharides
actively participate in complexing thorium, as does
humic material. Additional studies need to address
the role of colloids in metal scavenging in various
regions of the Arctic, in order to be able to
determine if this observation is applicable to other
regions in the Arctic.
POC/234Th ratios are expected to vary with
particle size, as has been reported (e.g., Buesseler
et al., 1995; Moran et al., 1997). In the upper 25 m,
POC/234Th ratios remain constant (10.4–12.7
mmol dpm1, calculated based on the inventory
of these values) for all three stations. However,
this ratio varied from 7.8 to 25.7 at discrete depths,
with the highest value occurring at the coastal
station. The vertical profile indicates that
POC/234Thp ratio remains constant in the mixed
layer of the deep basin (Fig. 4c). The export flux of
organic carbon is given by
3.9. POC export
where [CU ] and [CTTh ] are the total activities of
U and 234Th, in the upper 25 m, respectively; l is
the decay rate constant for 234Th (0.0288 d1).
[POC] and [234Thp] are the mean concentrations of
POC and particulate 234Th activity in the upper
25 m, respectively. The calculated estimates of the
export fluxes of POC varied between 5.6 and
6.5 mmol m2 d1. This range is comparable to the
POC flux estimates derived by Moran et al. (1997)
and Moran and Smith (2000) for the central
Beaufort Sea.
Corg export flux ¼ lð½CU ½CTTh Þ
f½POC
=½234 Thp g;
ð9Þ
238
Two common methods have been employed to
determine POC fluxes from the upper ocean. The
first method utilizes the activities of particulate
234
Th and organic carbon while the second method
uses shallow water sediment traps. A relationship
between the 234Th scavenging rate and primary
and new production was first shown by Coale and
Bruland (1985) and Bruland and Coale (1986).
Subsequently, Eppley (1989) suggested that the
residence time of 234Th in the euphotic zone could
be applied to the POC stocks to determine the
residence time of POC. Buesseler et al. (1992)
utilized 234Th to determine the export fluxes of
POC. The fundamental assumptions in utilizing
234
Th as a tracer to determine the export of POC
are: (i) particulate thorium and carbon behave in a
similar way, and there is no preferential recycling
within the euphotic zone; (ii) the sampling devices
that are employed to obtain POC and 234Th collect
representative populations of the particulate matter and there is no discrimination of the particle
size population in sample collection and (iii) the
particulate matter sampled represents the particle
population that exports POC and Th out of the
euphotic zone. However, preferential recycling of
POC in the euphotic zone was reported by Murray
et al. (1989). 234Th is primarily sorbed onto the
particulate pool (and so is related to surface area)
while the POC is derived from the coagulation of
colloidal material so that its concentration depends on the volume of the particles. Thus,
4. Conclusions
From our study, we draw the following conclusions:
(i) In coastal, shelf and deep basin samples there
is disequilibrium between 234Th and 238U,
indicating that the residence time of 234Th is
in the range of days to months. There is
enhanced scavenging in the upper 100 m even
in the deep Canada Basin.
(ii) The depth-normalized inventory of SPM as
well as the specific activity of particulate
234
Th in the deep basin is about an order of
magnitude higher than those reported from
the Alpha Ridge station in the deep Arctic
Ocean. Considerable variation in the specific
activity of particulate 234Th is attributed to
variations in the SPM derived from ice rafting
in adjoining water masses. In open waters
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M. Baskaran et al. / Deep-Sea Research I 50 (2003) 1353–1373
(which in some areas may extend all the way
to the North Pole) and areas where there is
annual pack ice cover the geochemical cycling
of particle-reactive elements is governed
primarily by the amount of sedimentary
particles released from ice-rafted sediments.
In areas where there is permanent ice-cover
(such as Alpha Ridge), the influence of icerafted sedimentary particles is minimal.
(iii) The residence times of colloidal 234Th in the
deep ocean appear to be significantly higher
than those in the coastal waters and only a
very small portion of the 10-kD–0.4 mm
colloidal material actively participates in
thorium scavenging.
(iv) The conditional partition coefficient between
the colloidal and dissolved phases, Kc ; is
about an order of magnitude lower than the
values reported earlier from other regions and
it is likely that the nature of the colloidal
material in the Arctic is significantly different
than those found in other world oceans. It
appears that the Kf values are higher than
those reported from other regions, which
suggests that the glacio-marine sediments
have higher ion-exchange capacity, in contrast to earlier studies.
(v) POC export estimates ranged between 5.6 and
6.5 mmol m2 d1 and are comparable to
other estimates in the central Arctic Ocean.
Acknowledgements
We kindly thank the captain and the crew of the
USCGC POLAR STAR for their expert help in
obtaining these samples during the AWS 1998
Cruise. We acknowledge the dedicated help of
Andrew Walter in the field as well as in the
laboratory. The Coastal and Marine Geology
Program of the US Geological Survey provided
funding for PWS. Constructive comments and
suggestions from three anonymous reviewers and
Mike Bacon improved this manuscript. This work
was supported by research grants (NSF-OPP9709003 and NSF-OPP-9996337) and an instrumentation grant (NSF-OCE-9732536) to MB from
the National Science Foundation.
1371
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