ICES Journal of Marine Science (2011), 68(3), 427 –435. doi:10.1093/icesjms/fsr005
Sediment accumulation and bioturbation rates in the deep
Northeast Atlantic determined by radiometric techniques
Fernando P. Carvalho*, João M. Oliveira, and António M. M. Soares
Instituto Tecnológico e Nuclear, E.N. 10, 2686-953 Sacavém, Portugal
*Corresponding author: tel: +351 219 946332; fax: +351 219 941995; e-mail: [email protected].
Carvalho, F. P., Oliveira, J. M., and Soares, A. M. M. 2011. Sediment accumulation and bioturbation rates in the deep Northeast Atlantic
determined by radiometric techniques. – ICES Journal of Marine Science, 68: 427 –435.
Received 1 September 2009; accepted 8 January 2011.
The upper layers of boxcore bottom-sediment samples from the Porcupine Abyssal Plain (five cores) and from the Iberian Abyssal
Plain (eight cores) in the Northeast Atlantic were analysed for porosity, grain size, organic carbon, calcium carbonate, and radionuclides. Radiometric ages of sediment layers were determined using 230Th excess (the 230Thexc/232Th ratio method) and 14C radionuclides. Sediment accumulation rates were 0.14 and 3.2 cm kyear21 at the Porcupine and Iberian Abyssal Plains, respectively.
Sediment mixing, determined through 210Pb excess, was in the upper 5– 11 cm sediment layer, likely as a result of infauna activity.
Biodiffusion coefficients averaged 0.40 + 0.37 and 1.05 + 1.02 cm2 year – 1 and were not significantly different in the sediments of
either abyssal plain. The low rates of sediment accumulation and bioturbation in the abyssal depths of the Northeast Atlantic
suggested that immobilization of sediment-reactive man-made radionuclides released near the seafloor will take place very slowly
and with limited sediment burial.
Keywords: biodiffusion,
14
C sediment dating,
210
Pb, sediment mixing,
Introduction
Over several decades, low- and medium-activity-level radioactive
wastes were dumped into the Northeast Atlantic. Disposal of
radioactive waste ceased with the 1982 revision of the London
Sea Dumping Convention that banned the practice (NEA/
OECD, 1984; Livingston and Povinec, 2000). Drums containing
radioactive waste are not expected to retain their content for
ever and may leak. Particle reactive radionuclides released into
the seawater should be adsorbed by the downward flux of particles
and buried in the seafloor around the dumpsites (NEA/OECD,
1984; Povinec et al., 2000). The extent of contaminant burial vs.
long-range transport in the water column may depend on the
balance between, on one hand, sedimentation rate and sediment
mixing rate and, on the other hand, ocean currents. The abyssal
depths, below 2000 m deep, cover .60% of the Earth surface,
but their biology, geology, and hydrography are poorly known.
The supply of sediments and organic matter to the deep ocean
and their recycling on the seafloor have been a subject of continued research closely related to the fate of radionuclides and other
contaminants in the deep sea (Fowler and Knauer, 1986; Ståhl
et al., 2004; Lampitt et al., 2008). The flux of particles exported
from the euphotic zone of the ocean to the abyss is a key factor
in these oceanographic processes. The settling of these particles
on the seafloor is part of the sedimentation processes that may
counteract the release and diffusion of radioactive contaminants
into the water column and, ultimately, may bury and immobilize
pollutants contributing to their removal from the biosphere. As an
essential part of these processes, sediment accumulation and
mixing were investigated using radionuclide-based geochronology
in two areas of the Northeast Atlantic Ocean in which radioactive
# 2011
230
Th excess sediment dating.
wastes were dumped. These areas are the Porcupine Abyssal Plain,
at the NEA dumpsite, and the Iberian Abyssal Plain off the
Portuguese coast. Results of these geochronologies are presented
herein, and implications related to the fate of dumped radionuclides are discussed.
Material and methods
Sediment sampling
Sampling areas are shown in Figure 1. Sediment samples were collected during oceanographic cruises on board the German RV
“Walther Herwig” in 1989 to the NEA dumpsite in the
Porcupine Abyssal Plain and the Portuguese RV “A. Carvalho”
in 1988 to the Iberian Abyssal Plain, using large surface–sediment
boxcorers. Two sets of sediment cores were selected for analysis
based on the on-board inspection of the core sediment surface
and their assessment as undisturbed core samples.
Sediments were subsampled from the boxcorer by inserting a
stainless steel tube 12 cm in diameter by hand. Sediment subsamples were extruded on board, sliced in 1- or 2-cm layers, and
frozen until analysis. Following the determination of sediment
water content, samples were analysed for porosity, grain size,
organic matter, and carbonate content, according to wellestablished analytical procedures (Strickland and Parsons, 1968).
Radionuclide analyses
Aliquots of sediment layers were analysed for several radionuclides
by radiochemistry and a-spectrometry. Thorium isotopes
were determined to use the activity ratio of 230Th excess, the
230
Thexc (230Th physical half-life T1/2 ¼ 8.0 × 104 years) over
232
Th (T1/2 ¼ 1.41 × 1010 years) as a geochronometer. Uranium
International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved.
For Permissions, please email: [email protected]
428
F. P. Carvalho et al.
Figure 1. The Northeast Atlantic and the areas investigated: the NEA dumpsite is in the Porcupine Abyssal Plain, and the “Cecir X” area in the
Iberian Abyssal Plain.
isotopes (238U and 234U), radium 226Ra (T1/2 ¼ 1600 years), and
radioactive lead 210Pb (T1/2 ¼ 22.2 years), determined through
the a-emitter granddaughter polonium isotope 210Po (T1/2 ¼
138.4 d), were also analysed. For the purpose, each sample was
spiked with known quantities of isotopic tracer (232U, 229Th,
224
Ra, and 209Po) to be used as internal standards for the determination of radiochemical yield (Carvalho et al., 2005). After the
addition of tracers, samples were totally dissolved with HNO3,
HCl, HF, and a few drops of H2O2. Following separation and purification of radioelements, each was electroplated on stainless steel
discs, or a silver disc for Po, and measured by a-spectrometry with
silicon surface-barrier detectors ORTEC EG&G (Carvalho and
Oliveira, 2007).
Aliquots of sediment layers were subject to radiocarbon dating
using the carbonate fraction. To eliminate any contamination,
30% by weight of each sample was discarded by controlled acid
leaching (0.5 M HCl at 258C). The 14C content was measured by
the liquid scintillation technique, using low background Liquid
Scintillation Counters (Beckman and Packard) as described elsewhere (Soares, 1989, 2005). Stable isotope enrichment values
(d 13C) were determined for the CO2 gas produced at the initial
stage of benzene synthesis. Radiocarbon ages were calculated in
accord with the definitions recommended by Stuiver and Polach
(1977).
14
C sediment dating
14
C of cosmogenic origin enters the ocean at the surface, is incorporated in authigenic CaCO3, and settles with carbonate particles
from decaying planktonic organisms (Kershaw, 1985; Thomson
et al., 1993, 2000). These particles end on the seafloor, and their
cosmogenic 14C content allows for the age determination of sediment layers. For the Iberian Abyssal Plain, core C5 was used for
14
C carbonate dating. Experimental 14C age points below the sediment surface mixed layer (SML) were used to fit a straight line formally similar to Equation (1) to determine the sedimentation rate.
230
Thexc/232Th sediment dating
232
Th, the parent radionuclide of the thorium radioactive natural
series, exists everywhere in the seafloor as well as in the Earth crust
(Ivanovich and Harmon, 1992). 230Th also exists as a member of
the uranium (238U) radioactive natural series. Through sediment
layers, the activity ratio 230Th/232Th could be expected to be constant, reflecting the 238U/232Th activity ratios. However, in the
upper layers of seafloor sediments, an excess of 230Th exists as a
result of the deposition of sedimentary particles carrying 230Th
from the 234U decay in the water column. The 230Thexc/232Th
activity ratio in the sediments can be used to determine sedimentation rates (Ivanovich and Harmon, 1992). In sediments of the
429
Sediment accumulation and bioturbation rates in the deep NE Atlantic
Porcupine Abyssal Plain, the 230Thexc/232Th ratio method was
applied to determine the sedimentation rate in the core WH2.
The 230Thexc/232Th ratio experimental points located below the
SML, defined by subjective graphic analysis, were fitted by least
squares to the equation
AR(x)
l
x ,
= AR(0) exp −
v
(1)
where AR(x) is the 230Thexc/232Th activity ratio at sediment depth x
below the sediment surface, l the 230Th radioactive decay constant, and v the sedimentation rate (cm year21; Lalou, 1982;
Thomson et al., 2006).
The age of the SML (TML) and the extrapolated age of the sediment at the surface (T0) were determined according to the
expression
T0 = TML −
Pbexc and sediment bioturbation
C(x)
l 1/2
= C(0) exp −
x ,
DB
(2)
with C(x) equal to 210Pbexc activity at depth x (cm) below the
water–sediment interface, l the 210Pb radioactive decay constant,
and DB the biodiffusion rate (Erlenkeuser, 1980; De Master and
Cochran, 1982; Kershaw et al., 1986).
Table 1. Location, depth,
210
(3)
where v is the sedimentation rate (cm year21) calculated from the
14
C profile below the mixed layer, and L (cm) is the mixed layer
depth determined by the 210Pbexc.
As the activity of 210Pbexc accumulated in the sediments is
maintained by the continuous supply of 210Pb with the sedimenting particles that compensates 210Pbexc radioactive decay, the
steady-state inventory of 210Pbexc can be expressed through
210
All sediment samples were analysed for 210Pb and 226Ra. Without
210
Pb accretion and removal in seafloor sediments, the 226Ra and
210
Pb activities should be in radioactive equilibrium. However,
owing to the downward transport of 210Pb originated from the
226
Ra decay in the water column, there is a net excess of 210Pb relative to 226Ra in the upper layers of sediments. If sediments are not
disturbed, freshly deposited 210Pb would remain in the top sediment layer, and the layers underneath would display an exponential decrease in 210Pbexc through a layer of which the thickness
would depend on the sediment accumulation rate. In undisturbed
abyssal sediments, the 210Pbexc would remain in the most superficial layer. The 210Pbexc over 226Ra, i.e. the unsupported 210Pb in
the upper layers of bottom sediments, allowed for determining
the biologically induced mixing of sediment, also called biodiffusion. Biodiffusion rates were calculated from the equation
L
,
v
IPbexc = A[1 − exp(−lPb )],
(4)
where IPbexc is the inventory of unsupported 210Pb in the sediment
cores (Bq m22), A the annual flux of unsupported 210Pb in the
particle flux arriving at the seafloor (Bq m22 year21), and lPb
the radioactive decay constant of 210Pb.
Results and discussion
Characteristics of sediments and radionuclide profiles
Two sets of sediment cores from the Iberian and Porcupine
Abyssal Plains were analysed for radionuclides and sedimentological parameters (Table 1). Sediments from the Porcupine Abyssal
Plain were calcareous ooze with high carbonate content, 82 –
88% dry weight, of pelagic origin. The organic matter content of
these sediments was very low, 0.28 –0.40% dry weight in the top
1 cm, decreasing rapidly in deeper sediment layers. Sediments
from the Iberian Abyssal Plain had a greater contribution of
material from the continental margin, with fine sands and clays
accounting, respectively, for 5 –51 and 17– 45% dry weight of
the top 1 cm. In those sediments, carbonates accounted only for
25 –40% dry weight and organic matter for 0.20 –0.60% dry
weight in the upper 1 cm.
The main features in the abyssal sediments are illustrated with
two core profiles in Figure 2. In the upper 6 –8 cm of sediment, the
Pbexc inventory, 210Pb deposition rate, and biodiffusion rates in the sediment cores.
Core
Position
Iberian Abyssal Plain
C1
41840′ N 11855′ W
C2
41840′ N 10855′ W
C3
41840′ N 10805′ W
C5
40850′ N 11855′ W
C6
40850′ N 10855′ W
C7
40850′ N 10805′ W
C9
40800′ N 11855′ W
C11
40800′ N 10855′ W
x + 1 s.d.
Porcupine Abyssal Plain
WH2
45807.8′ N 17812.3′ W
WH2B
45807.8′ N 17812.3′ W
WH1A
46807.0′ N 16842.1′ W
WH22B
46803.9′ N 16802.2′ W
WH26B
46824.5′ N 16841.3′ W
x + 1 s.d.
Depth (m)
I 210Pbexc
(Bq m22)
210
Biodiffusion coefficient
Pb annual deposition
(Bq m22 year21)
DB (cm2 year21)
r
n
3 700
2 750
3 000
5 100
4 500
2 300
5 150
2 350
2 150
3 650
3 260
4 450
3 180
2 220
5 607
3 750
3 533 + 1 063
66
112
100
136
98
67
172
115
108 + 33
0.64
0.29
0.46
0.09
1.26
0.14
0.16
0.16
0.40 + 0.37
20.89
20.90
20.92
20.73
20.70
20.87
20.85
20.81
5
6
5
6
7
4
6
4
4 730
4 730
4 725
4 236
4 540
2 706
3 340
4 397
2 524
3 416
3 242 + 732
83
102
135
78
105
100 + 20
0.14
1.03
0.08
2.91
1.10
1.05 + 1.02
20.85
20.61
20.87
20.86
20.92
9
5
5
6
6
430
F. P. Carvalho et al.
Figure 2. Profiles of sediment porosity, organic carbon (Corg), calcium carbonate (CaCO3), 210Pb total, and 226Ra in two sediment cores from
the Iberian Abyssal Plain. Top, station C5 (5100 m); bottom, station C9 (5150 m).
porosity decreased exponentially from values around 0.85 –0.60 or
less, depending on the sediment grain size. The calcium carbonate
content was generally lower in the sediment upper layers than in
the deeper ones owing to the dissolution of carbonates below
the lisocline, which in the Northeast Atlantic is at around
4700 m deep (van der Loeff and Lavaleye, 1984; Rabouille et al.,
2001). The organic matter in the sediments was always low, but
still comparatively higher in the top few centimetres of sediment
than in deeper sediment layers. The radium (226Ra) activity in
the upper sediment layers was clearly lower than in deeper sediment layers, probably because of the partial dissolution of
radium with carbonates below the lisocline depth, and to
changes in the carbonate flux during recent centuries (Rabouille
et al., 2001). 210Pb activity in the upper sediment layers was systematically higher than radium activity (Figure 2 and
Supplementary Tables S1 and S2). Depending on the core, 210Pb
activity became lower than 226Ra activity in sediment layers
between 5 and 11 cm below the water–sediment interface and
stayed lower in deeper sediment layers (Figures 3 and 4).
Recent oceanographic research in the Northeast Atlantic has
revealed a significant flux of organic and inorganic debris
varying with primary productivity and exported from the euphotic
zone to the abyssal floor. The seasonality of this supply is notable
and may drive the abundance of populations of benthic organisms
(Fabiano et al., 2001; Vanucci et al., 2001; Brunnegård et al., 2004;
Smart, 2008; Soto et al., 2010). This particle flux carries particlebound radionuclides, such as 210Pb, 230Th, and 14C, to the seafloor,
as demonstrated by sediment traps (Fowler and Knauer, 1986;
Rouch, 1987; Lampitt et al., 2008). In abyssal depths, at the
water–sediment interface, 94% of the organic particles including phytoplankton debris are used by benthic fauna and rapidly
oxidized; just 6% of the deposited organic matter is buried
and not used (Heip et al., 2001; Ståhl et al., 2004). Therefore,
most of the particulate matter arriving on the abyssal seafloor is
431
Sediment accumulation and bioturbation rates in the deep NE Atlantic
Figure 3.
210
Pb excess in sediment cores from the Iberian Abyssal Plain.
reprocessed and, with it, probably most of the associated flux of
inorganic materials and radioactive elements is processed too,
then redistributed in the sediment mixed layer. As the abundance
of organic matter in the deposition flux seems to control benthic
fauna activity and biomass, it may play an indirect role in controlling sediment mixing and contaminant biodiffusion rates in the
seafloor sediment.
14
C sediment dating
Sediment dating using 14C of the sediment carbonate fraction was
performed in core C5 from the Iberian Basin (Figure 5). The age of
core layers yielded a sedimentation rate of 3.2 cm kyear21. The top
5 cm of sediment, not used in fitting the regression, indicated sediment mixing that was in line with and provided further
confirmation that the 210Pbexc present in the surface layer was
indeed the result of sediment reworking.
230
Thexc/232Th sediment dating
In the sediments of the Porcupine Abyssal Plain, the
230
Thexc/232Th ratio method was used in the dating core WH2
(Figure 6 and Supplementary Table S3). The sedimentation rate
calculated by the best fit equation was 0.14 cm kyear21. Again,
the top sediment layers (6 cm) did not follow the decreasing
trend with increasing sediment depth, likely the result of sediment
reworking, as seen in the 210Pbexc profile (Figure 4).
Both sediment cores, C5 from the Iberian and WH2 from the
Porcupine Abyssal Plain, indicated that the rates of sediment
accumulation were very low. For comparison, sedimentation
432
Figure 4.
F. P. Carvalho et al.
210
Pb excess in sediment cores from the Porcupine Abyssal Plain.
Figure 5. Conventional age (years before present, BP) in sediment
core C5 from the Iberian Abyssal Plain. The best fitting line (solid
line) to experimental points below the SML is y ¼ 24.91 +
0.00316 x, where y is the sediment depth (cm) and x the 14C age
(years). Correlation coefficient r ¼ 0.965; v, sedimentation rate.
rates at the outer continental shelf off the Portuguese coast at
depths of 75 –230 m are 0.16 –0.55 cm year21 and at the continental rise at depths of 520– 2860 m are 0.04 –0.38 cm year21, some
Figure 6. Sedimentation rate (v) in sediment core WH2 from the
Porcupine Abyssal Plain, computed from the best fit of the 230Th
excess/232Th activity ratio against depth (cm) in sediment layers
below the SML. Regression equation: AR(x) ¼ 17.2 exp(20.0641 x).
Correlation coefficient r ¼ 20.96.
three orders of magnitude greater than in the abyssal plains
(Carvalho and Ramos, 1990). The sedimentation rate in the
Iberian Basin was higher than in the Porcupine Basin and this,
in conjunction with the sediment grain size composition,
suggested terrigenous contributions to the Iberian Abyssal Plain,
probably with material from the Iberian continental margin.
433
Sediment accumulation and bioturbation rates in the deep NE Atlantic
Actually, sediment slumps in continental shelf deposits have been
documented in the sediment records of 210Pbexc and 137Cs from
atmospheric fallout. In the area, sediment slumps seemed to be
driven along the Nazaré canyon that extends from the west coast
of Portugal to the deep sea, and this explained the higher rates
of sedimentation measured at the continental rise (Carvalho and
Ramos, 1990). These sediment slumps might have a wider effect
than initially thought, supplying sediments to the adjacent
Iberian Abyssal Plain too.
With such extremely low rates of sedimentation in both
abyssal plains, the thickness of sediment layers sampled down
to 10 –30 cm below the water–sediment interface encompassed
the entire Holocene epoch (10 kyears). Moreover, the sediment
layers below the top 0.5 cm were too old to contain unsupported 210Pb carried with settling sediment particles. Indeed,
the 210Pbexc signal of seafloor sediments fades through radioactive decay in about a century (i.e. five 210Pb half-lives).
This interval is not compatible with the presence of 210Pbexc
in the much older 0.5 –11 cm sediment layers, unless freshly
deposited unsupported 210Pb had been buried into the sediment by the biological activity of infauna through mechanisms
of diffusive type (biodiffusion). This activity is likely carried
out by the small Foraminifera and Sipunculidae that contribute
to most of the infaunal biomass in the region (van der Loeff
and Lavalaye, 1984; Smith et al., 1986; Thomson et al.,
2000). Biological activity on the seafloor can therefore create
a sediment SML 5– 11 cm thick in the cores of both abyssal
basins (Figures 3 and 4). Further, the irregular vertical distribution of 210Pbexc in the SML between cores argues in favour
of biologically mediated transport based on particle mixing,
rather than on a geochemical, passive pore-water diffusion
mechanism.
210
Pbexc biodiffusion and inventory
Biodiffusion rates (DB) calculated from Equation (2) and using
210
Pb excess data ranged from 0.14 to 1.26 cm2 year21 (average
0.40 cm2 year21) in the Iberian Abyssal Plain and from 0.12 to
2.91 cm2 year21 (average 1.05 cm2 year21) in the Porcupine
Abyssal Plain. Because of the wide ranges of values, the average
rates of biodiffusion are not significantly different between
basins and are in the range of values reported in the literature
for the Northeast Atlantic (Table 1). In general, and based on
extensive pioneer work on biodiffusion in sediments, the values
of DB generally are in the range 0.03 –0.3 cm2 year21 in ocean
sediments and between 3.1 and 38 cm2 year21 in coastal environments (Turekian et al., 1978).
The inventory of 210Pb excess in the upper sediment layers averaged 3533 + 1063 and 3442 + 732 Bq m22 in the Iberian and
Porcupine Abyssal Plains, respectively (Table 1). Assuming that
these inventories are maintained in steady state by a continuous
supply of unsupported 210Pb flux from the oceanic water
column to the seafloor, the average annual 210Pbexc flux calculated
using Equation (4) was 100 Bq m22 year21 at both sites
(Table 1). The deposition fluxes are comparable with values determined in other studies at similar latitudes of the Northeast
Atlantic: 94 + 38 Bq m22 year21 (Kershaw, 1985; Kershaw et al.,
1986) and 67 + 22 Bq m22 year21 (Smith et al., 1986).
The 14C age of the sediment SML in core C5 from the Iberian
Abyssal Plain, TML ¼ 3132 years, is similar to the results for NE
Atlantic abyssal sediments. In general, however, the extrapolated
ages for the sediment surface (T0) vary far more than TML
values (Table 2). The variation suggests different mixing extent
of new and old particles in the sediment cores, but all of them corroborate the fact that 210Pbexc present below the sediment surface
is likely the result of sediment reworking and burial of 210Pbexc
(Table 2).
Conclusions
Rates of sediment accumulation in the NE Atlantic abyssal plains
were very low, about three orders of magnitude less than those
determined on the continental shelf and slope off the coast of
Portugal. For the abyssal plains, the average rate of sedimentation
was one order of magnitude greater in the Iberian Abyssal Plain
(3.2 cm kyear21), likely the result of the advection of sediments
from the continental margin, than in the Porcupine Abyssal
Plain (0.14 cm kyear21), where the sediments are almost exclusively of oceanic origin. Despite the differences in sedimentation
rates between basins, it is clear that sediment accumulation in
the abyssal floor of the NE Atlantic is very slow.
The sediment mixing or bioturbation (DB) in the surface
layers of the abyssal seafloor may contribute to the mixing of
sediment-bound contaminants and their removal from the
water–sediment interface into the sediment. In these abyssal
regions, the sediment SML has a thickness of some 5 – 11 cm.
Nevertheless, in both basins contaminant burial will also be
very slow, as shown from the 210Pbexc profiles and biodiffusion
values. These sediment processes may be too slow to effectively
counteract the dispersal of contaminants released near the
abyssal floor in the long term.
Table 2. Sedimentation rate (v), SML thickness (L), age of the mixed layer (TML), age of the sediment at the surface (T0), and biodiffusion
coefficient (DB) of NE Atlantic sediments.
Abyssal basin
Iberian
Madeira
Porcupine
Porcupine
Iberian
Golfe de Gascogne
Porcupine
Iberian
Porcupine
a
Depth (m)
5 448
5 161
4 768
4 758
5 255
4 800
4 300
5 150
4 730
v (cm kyear21)
0.8a
1.8a
2.1a
2.2a
2.14a
3.25a
3.13a
3.2a
0.14b
Sedimentation rates estimated using 14C.
Sedimentation rates estimated using 230Thexc/232Th.
b
L (cm)
4.1
3.7
6.1
5.8
–
4.0
–
5
7
TML (years)
4 800
3 370
3 050
3 250
2 890
2 500
2 600
3 132
–
T0 (years)
988
2 015
673
1 014
1 441
945
890
1 406
–
DB (cm2 year21)
–
0.13
–
–
–
–
–
0.16
0.14
Source
Kershaw (1985)
Kershaw (1985)
Kershaw (1985)
Kershaw (1985)
Rouch (1987)
Rouch (1987)
Rouch (1987)
This study
This study
434
Supplementary material
Supplementary material is available at ICESJMS online.
Supplementary Table S1 lists concentrations of 210Pb and 226Ra
(Bq kg21 dry weight) in sediment cores from the Iberian
Abyssal Plain, and Supplementary Table S2 the same parameters
for the Porcupine Abyssal Plain. Supplementary Table S3 lists
uranium and thorium concentrations (Bq kg21+1s dry
weight) in sediment core WH2 from the Porcupine Abyssal Plain.
Acknowledgements
Thanks are due to W. Feldt, Labör für Radiöoekologie der
Gewasser, Hamburg, and the crew of the RV “Walther Herwig”
and the Instituto Hidrográfico, Lisbon, and the crew of the RV
“A. Carvalho” for collaboration with sample collection.
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