Download Relationships between dinoflagellate cyst assemblages in surface

JOURNAL OF QUATERNARY SCIENCE (2001) 16(7) 667–680
Copyright  2001 John Wiley & Sons, Ltd.
DOI: 10.1002/jqs.652
Relationships between dinoflagellate cyst
assemblages in surface sediment and
hydrographic conditions in the Bering and
Chukchi seas
TAOUFIK RADI,* ANNE DE VERNAL and ODILE PEYRON
Centre de recherche en géochimie isotopique et en géochronologie (GEOTOP), Université du Québec à Montréal, P.O. Box 8888,
Montréal, QC H3C 3P8, Canada
Radi, T., de Vernal, A. and Peyron, O. 2001. Relationships between dinoflagellate cyst assemblages in surface sediment and hydrographic conditions in the Bering and
Chukchi seas. J. Quaternary Sci., Vol. 16 pp. 667–680. ISSN 0267-8179.
Received 3 April 2001; Revised 23 July 2001; Accepted 27 July 2001
ABSTRACT: Palynological analyses were performed on 52 surface sediment samples from the
eastern part of the Bering Sea and Chukchi Sea in order to document the regional distribution of
dinoflagellate cyst assemblages and their relationship with sea-surface conditions. The assemblages
present a relatively high species diversity (20 taxa are recovered routinely), especially in the Bering
Sea, where they are dominated by Operculodinium centrocarpum and the cyst of Pentapharsodinium
dalei accompanied mainly by Spiniferites elongatus s.l., Spiniferites ramosus, Impagidinium
pallidum, Brigantedinium spp., Islandinium minutum, Selenopemphix quanta, Selenopemphix Journal of Quaternary Science
nephroides, Quinquecuspis concreta and the cyst of Polykrikos kofoidii. The percentages of the main
taxa vary with latitude, and principal component analysis shows that the distribution of assemblages
is closely related to hydrographic conditions, notably the seasonal duration of sea-ice cover and
the sea-surface temperature in February. The dinoflagellate cyst assemblages from the Bering Sea
differ significantly from those of subarctic seas of the North Atlantic, with respect to their species
composition and relationships with sea-surface conditions. In particular, the occurrence of the
cyst of Polykrikos kofoidii and Quinquecuspis concreta and the positive correlation between the
percentages of Operculodinium centrocarpum and the extent of sea-ice, constitute peculiar features
in the Bering Sea assemblages. Copyright  2001 John Wiley & Sons, Ltd.
KEYWORDS:
Bering Sea; Chukchi Sea; dinocysts; sea-ice cover; sea-surface temperature.
Introduction
Palynological studies of surface sediment samples from middle
to high-latitude marine environments revealed that dinoflagellate cysts (dinocysts) are excellent tracers of hydrographic
conditions, including the sea-surface temperature, salinity and
seasonal duration of sea-ice cover (e.g. Mudie, 1992; Rochon
and de Vernal, 1994; Matthiessen, 1995; Marret and de Vernal, 1997; de Vernal et al., 1997, 2000). These studies led
us to hypothesise that dinocysts also could be good tracers of
sea-surface conditions in subpolar regions of the North Pacific
Ocean, such as the Bering Sea, and in adjacent Arctic seas,
such as the Chukchi Sea. To date, however, only a few studies
on high-latitude dinocyst assemblages from the North Pacific
Ocean and western Arctic Ocean are available. They deal with
* Correspondence to: T. Radi, GEOTOP, Université du Québec à Montréal, P.O.
Box 8888, Montréal, Québec, H3C 3P8, Canada. E-mail: [email protected]
Contract/grant sponsor: Fonds FCAR of Quebec.
Contract/grant sponsor: Natural Sciences and Engineering Council of Canada.
modern assemblages from marine environments around Japan
(cf. Matsuoka, 1985, 1987, 1992), the Cenozoic biostratigraphy of the northwest North Pacific and Bering Sea (Bujak,
1984; Bujak and Matsuoka, 1986; Matsuoka and Bujak, 1988),
and the Quaternary ecostratigraphy of the Gulf of Alaska (de
Vernal and Pedersen, 1997; Marret et al., 2001).
Here, we present the results of palynological analyses
performed on surface sediment samples from 52 sites of the
eastern Bering Sea and Chukchi Sea (Fig. 1 and Table 1).
Our objective is to determine the relationships between the
distribution of dinocyst assemblages and the hydrographical
parameters of the upper water layer, notably the salinity
and temperature and the seasonal duration of sea-ice cover.
We also examined possible links between the dinocyst
assemblages and the primary productivity. This study is a
contribution to a collective project aiming at the establishment
of a hemispheric data base (cf. de Vernal et al., this issue). Such
a data base should permit wider domain application of the
transfer functions developed initially for palaeoceanographic
reconstruction in the northern North Atlantic Ocean and
adjacent subpolar basins (e.g. de Vernal et al., 1994, 1997,
2000; Rochon et al., 1999).
668
JOURNAL OF QUATERNARY SCIENCE
70°N
160°E
140°E
180°
−160°W
−140°W
−130°W
−120°W
60°N
−120°W
130°E
0m
10000m
2
Chukchi Sea
140°E
−130°W
Siberia
60°N
Alaska
m
20000m
10
150°E
50°N
−140°W
Bering Sea
s
nd
50°N
n
a
Isl
tia
eu
Al
160°E
170°E
180°
−170°W
−160°W
−150°W
Figure 1 Map showing locations of study sites. The bathymetry is represented by isobaths at 200 and 1000 m
Oceanographic context
Hydrography
A latitudinal gradient of sea-surface temperature is observed
in the study area, which includes the Bering Sea and Chukchi
Sea (Table 2). From the north to the south, temperature varies
between freezing point (−1.9 ° C) and 5 ° C in February, and
between 7 and 12 ° C in August (cf. NODC, 1994). A gradient
of summer salinity also is observed from the Chukchi Sea (26.7
to 28) to the southern Bering Sea (30 to 33).
The seasonal duration of sea-ice cover (>50% of sea-ice
coverage) ranges from zero to five months per year in the
Bering Sea and from six to twelve months per year in the
Chukchi Sea. The extension of sea-ice cover is controlled
by atmospheric circulation, which initiates considerable
interannual fluctuations of the surface coverage and thickness
of sea-ice (Cavalieri and Parkinson, 1987; Niebauer, 1988;
Cavalieri and Martin, 1994; Roach et al., 1995).
In the northeast Bering Sea, three hydrographic domains
can be distinguished depending upon freshwater inputs and
surface currents (Cooney and Coyle, 1982; Walsh and McRoy,
1986; see Fig. 2).
1 The Anadyr Water (AW) to the west is characterised by
low summer temperature (ca. 3 ° C) and salinity of about 33.
It circulates northeastward through the Anadyr Strait and
constitutes an important source of nutrients (20 to 40 µM
of nitrates) owing to the upwelling from the Aleutian basin
on to the Bering Sea shelf (Walsh et al., 1989; Springer
and McRoy, 1993; Grebmeier and Cooper, 1995). The
annual productivity of this water mass is about 300 gC m−2
(Grebmeier, 1993; Hansell et al., 1993).
2 The Alaska Coastal Water (ACW) is relatively warm (about
11 ° C) and characterised by low salinity (about 32) during
summer because of freshwater inputs from the Yukon River,
which also contributes to some nutrient enrichment (Walsh
et al., 1989; Chen, 1993; Chen et al., 1997). However, the
Copyright  2001 John Wiley & Sons, Ltd.
nitrate content of the water, which circulates northwards
along the Alaskan coast, considerably decreases (<1 µM)
after the spring bloom. As a consequence, the annual
productivity associated with this water mass is relatively
low (<60 gC m−2 ; Grebmeier, 1993; Hansell et al., 1993).
3 The Bering Shelf Water (BSW) formed by waters from the
North Pacific Current spreads on the shelf, which constitutes
a more open ocean environment characterised by a salinity
of approximately 32.5 in summer (Thomson, 1981; Reed
and Stabeno, 1997). This water mass mixes with the AW
in the north, where productivity is relatively low (Maynard
and Clark, 1987; Hansell et al., 1993).
In the Chukchi Sea, surface waters result from the mixing
of Arctic waters with a subpolar component originating from
the Bering Sea. As a consequence, in summer, the surface
water is characterised by salinity in the range 26.5–27, and by
a northward gradient of decreasing temperature from 7 ° C to
freezing conditions. The northward circulation of the Bering
Sea water (Fig. 2) through the Bering Strait (85 km wide and
50 m deep) is controlled by the sea-level difference of about
0.5 m between the Pacific and Arctic Oceans (Overland and
Roach, 1987). The northward flow of surface water on the
Chukchi shelf is further determined by atmospheric circulation
patterns (Coachman, 1993). The mean annual water flux
towards the Arctic through the Bering Strait is estimated to
be about 0.83 Sv (Roach et al., 1995), but presents strong
interannual and seasonal variations (Aagaard et al., 1985;
Overland and Roach, 1987; Coachman and Aagaard, 1988;
Roach et al., 1995). The circulation of waters rich in nutrients
towards the Arctic Ocean, notably that coming from the
Anadyr Gulf neighbourhood towards the Bering Strait, plays
a role by bringing nutrients to the Arctic Ocean (Springer
and McRoy, 1993). However, the concentration of nutrients
strongly decreases as the current circulates northward (Walsh
et al., 1989; Hansell et al., 1993; Cooper et al., 1997) and
the resulting productivity is low (Maynard and Clark, 1987;
Hansell et al., 1993).
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
DINOCYSTS IN SURFACE SEDIMENT, BERING AND CHUKCHI SEAS
Table 1
669
Sampling site number and location
Site
number
Laboratory
number
Coring
devise
Latitude
Longitude
Water
depth(m)
1565-5
1322-3
1322-2
1322-1
1565-6
1566-1
1566-2
1570-1
1565-4
1566-6
1570-6
1565-3
1565-2
1565-1
1570-2
1455-4
1461-5
1566-5
1461-4
1566-4
1461-6
1455-1
1461-1
1461-3
1461-2
1455-2
1566-3
1477-1
1477-2
1455-3
1477-3
1454-1
1477-4
1454-2
1465-1
1454-6
1465-2
1455-5
1454-3
1477-5
1477-6
1465-3
1530-4
1530-3
1454-5
1465-4
1454-4
1465-6
1530-5
1465-5
1455-6
1530-6
Gravity
Box
Box
Box
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
Gravity
76° 15.00N
75° 44.03N
74° 30.06N
74° 00.00N
71° 45.00N
67° 52.00N
63° 03.00N
63° 00.70N
63° 00.00N
62° 39.80N
62° 32.10N
63° 00.00N
62° 29.50N
62° 00.00N
61° 08.90N
61° 18.00N
61° 07.60N
61° 07.20N
60° 59.00N
60° 49.00N
60° 45.00N
60° 38.50N
60° 01.40N
60° 33.60N
60° 17.50N
59° 58.90N
59° 54.80N
59° 45.20N
59° 35.70N
59° 29.30N
59° 14.00N
58° 53.70N
58° 34.20N
58° 27.30N
58° 22.00N
58° 04.50N
57° 18.20N
57° 52.30N
57° 07.30N
57° 02.50N
56° 38.20N
56° 23.80N
56° 20.80N
56° 28.90N
57° 15.00N
56° 03.50N
55° 24.00N
55° 39.00N
55° 18.70N
55° 12.20N
54° 55.00N
53° 45.10N
167° 00.00W
160° 51.63W
159° 58.06W
161° 23.07W
156° 30.00W
167° 55.00W
174° 27.00W
173° 01.90W
175° 00.00W
175° 38.70W
174° 43.00W
177° 00.00W
177° 02.00W
178° 00.00W
172° 18.30W
176° 29.80W
175° 12.10W
173° 25.60W
176° 44.00W
177° 03.00W
174° 36.00W
175° 08.00W
170° 37.70W
176° 31.80W
175° 54.30W
171° 01.50W
176° 19.50W
174° 24.70W
173° 05.30W
173° 46.10W
172° 34.00W
170° 26.30W
172° 01.00W
169° 13.10W
168° 56.00W
167° 51.00W
166° 03.50W
171° 39.40W
165° 41.30W
171° 18.00W
171° 08.00W
165° 18.20W
167° 37.10W
170° 03.40W
168° 21.20W
165° 18.90W
165° 54.00W
166° 32.20W
167° 04.50W
165° 17.90W
166° 58.50W
169° 06.00W
1335
2135
585
447
81
61
71
67
80
84
73
93
98
117
66
111
96
75
120
126
94
104
64
129
120
69
138
113
96
107
93
70
99
70
66
66
68
101
68
107
119
85
127
103
166
97
118
129
143
114
200
1464
SI689-01
BC-15
B-5
BC-4
SI689-22
TT20-079
TT20-225
TT42-239
NW262-30
TT42-092
TT42-294
NW262-23
NW262-22
NW262-20
TT42-255
TT51-237
TT51-240
TT42-054
TT51-234
TT42-045
TT51-242
TT51-031
TT51-204
TT51-230
TT51-227
TT51-207
TT42-030
TT51-222
TT51-244
TT51-219
TT51-246
TT51-045
TT51-250
TT51-049
TT51-051
TT51-186
TT51-060
TT51-253
TT51-062
TT51-257
TT51-259
TT51-149
TT51-295
TT51-267
TT51-173
TT51-152
TT51-160
TT51-164
TT51-299
TT51-157
TT51-301
TT52-001
Productivity and biogenic flux
The continental shelf of the Bering Sea covers an area of
1.2 × 106 km2 and is one of the largest in the world. The
primary productivity varies between 50 and 450 gC m−2 yr−1
Copyright  2001 John Wiley & Sons, Ltd.
(Antoine et al., 1996) depending upon the characteristics of
the mixed layer, which are regionally controlled by the wind
and the sea-ice cover (Walsh et al., 1989). A first bloom,
called sea-ice edge bloom, develops along the thawing icepack during spring, in April–May (Niebauer et al., 1990,
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
670
JOURNAL OF QUATERNARY SCIENCE
Table 2 Sea-surface temperature and salinity around the sampling sites (NODC, 1994), sea-ice cover with more than 50% of concentration,
productivity (Antoine et al., 1996), dinocyst counts and concentrations
Location
Site
number
Chukchi
Sea
Bering
Sea
Temperature
February (° C)
Temperature
August (° C)
Salinity
August
Sea-ice
cover
(months yr−1 )
Annual
productivity
(gC cm−2 )
Dinocyst
count
Dinocyst
concentration
(cysts cm−3 )
SI689-01
BC-15
B-5
BC-4
SI689-22
TT20-079
−1.9
−1.7
−1.6
−1.6
−1.4
−1.7
−1
−0.7
0
1
2.4
6.6
27.3
27.9
27.7
27.3
26.7
30
12
12
11.8
11.7
11.1
6.9
—
132.8
132.3
156.4
140.8
248.7
25
21
100
144
355
352
271
179
797
5459
23 843
11 548
TT20-225
TT42-239
NW262-30
TT42-092
TT42-294
NW262-23
NW262-22
NW262-20
TT42-255
TT51-237
TT51-240
TT42-054
TT51-234
TT42-045
TT51-242
TT51-031
TT51-204
TT51-230
TT51-227
TT51-207
TT42-030
TT51-222
TT51-244
TT51-219
−1.7
−1.6
−1.3
−1.3
−1.4
−0.9
−0.9
−0.2
−1.5
−1.2
−1.3
−1.3
−1
0.1
−1.5
−1.4
−1.7
−0.8
−1.5
−1.7
0.5
−0.7
−1.4
−1.4
8.2
8.4
8.5
8.5
8.4
8.7
8.7
9.1
8.5
9.2
9.1
8.6
9.2
9.2
8.5
9.5
9.1
9.3
9.5
9.1
9.3
9.9
10.2
10.7
31.5
31.2
31.7
31.7
31.6
31.7
31.7
32
31.5
32.3
32.2
31.7
32.5
32.3
31.6
32.2
31.3
32.3
32.9
31.4
31.7
31.9
32.1
31.9
4.5
5.1
5.1
4.5
4.5
4.5
3.4
4
4.5
3.5
3.6
3.8
3.5
2.3
2.6
3.6
3.9
2.5
2.6
2.9
2.5
2.6
2.8
2.6
130.9
133.9
108.2
104.5
98.9
159.9
101.8
101.6
162.6
97
116.1
183.5
97.3
100.7
84
76.4
131.6
110.5
107.8
116.3
83.02
99.6
88.4
98.9
307
125
331
305
150
341
309
306
347
333
363
328
316
323
358
347
320
331
309
302
312
335
325
269
9845
7100
8497
6191
11 608
12 018
10 853
7166
8823
6898
2356
12 162
4718
4587
5163
9802
6968
2834
6221
8507
7785
11 294
12 006
5767
TT51-246
TT51-045
TT51-250
TT51-049
TT51-051
TT51-186
TT51-060
TT51-253
TT51-062
TT51-257
TT51-259
TT51-149
TT51-295
TT51-267
TT51-173
TT51-152
TT51-160
TT51-164
TT51-299
TT51-157
TT51-301
TT52-001
−1.4
−1
−0.9
−1.3
−1.6
−1
0.3
—
1.4
2.1
2.8
2.9
2.1
3.1
2.7
2.9
2.3
3.2
3.3
3.5
3.4
3.6
10.3
8.5
9.6
8.6
8.6
8.4
9.5
10.1
9.5
9.4
8.9
9.5
9.4
8
9
9.4
8.9
9.3
9.1
8.7
8.6
8.1
32.2
31.7
32.2
31.4
31.5
31.4
31.6
32.1
31.6
32.1
32.1
31.6
31.9
32.1
32.1
31.7
32.2
32.1
32.3
32.1
32.5
32.8
2.8
2.8
1.3
2.7
2.6
2.2
1.8
1.3
1.1
0.3
0.3
0.2
0.6
0.1
0
0.2
0.2
0
0
0
0
0
100.9
216.9
128.4
159.6
192
183.2
175.8
103.8
180.7
112.7
106.8
143.3
121.5
107.6
135.2
131.1
123.6
126.3
136.4
131.3
122.6
98.9
361
327
332
334
210
308
341
245
333
329
329
384
314
316
69
339
348
462
323
447
341
317
19 024
4774
7118
5056
3243
6520
5957
8305
4143
7019
7323
5768
19 725
19 967
439
7640
16 165
27 592
17 076
11 972
5890
4263
1995; Grebmeier, 1993; Stabeno et al., 1998). This bloom,
dominated by diatoms (e.g. Walsh et al., 1989; Fukuchi
et al., 1993), represents approximately 65% of the annual
primary production according to measurements made in the
late 1970s (Niebauer et al., 1990). During the sea-ice edge
Copyright  2001 John Wiley & Sons, Ltd.
bloom, chlorophyll concentrations reach 3 to 5 mg m−2 ,
i.e. 3.6 gC m−2 day−1 (Codispoti et al., 1982; Maynard and
Clark, 1987). The remaining part of the vegetative season
is characterised by a decrease in nutrients and reduced
phytoplankton growth (Maynard and Clark, 1987; Hanssel
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
DINOCYSTS IN SURFACE SEDIMENT, BERING AND CHUKCHI SEAS
671
Bering
Strait
Chukchi Sea
r
Siberia
Gulf of
Anadyr
Yukon
Anady
dyr
Ana it
Stra
AW
Alaska
St. Lawrence Shpanberg
Strait
Island
ACW
m
200 0m
100
Nunivak
Island
Bering Sea
BSW
s
nd
la
n
Is
ia
ut
e
Al
Figure 2 Schematic illustration of the surface water circulation in the Bering Sea: AW, Anadyr Water; BSW, Bering Shelf Water; ACW, Alaskan
Coastal Water
et al., 1993) with a productivity of about 0.5 gC m−2 day−1 ,
the flora being dominated by flagellates and small diatoms
(Springer and McRoy, 1993).
Method
Sampling and laboratory procedures
Surface sediment samples analysed in the present study were
taken with a gravity corer, with the exception of samples
B4, B5 and B15, which were collected with a box corer
(see Table 1). Sediments for palynological analyses were
subsampled in the upper few centimetres (between 0 and
3 cm) of material recovered in the gravity and box corers. This
interval is thought to cover the biological mixing zone and is
considered representative of recent sedimentation. Depending
upon the sedimentation and biological mixing rates, however,
the subsampling interval may represent hundreds to thousand
of years of sediment accumulation.
The samples were prepared for palynological analyses following the standard procedure of the micropaleontological
laboratory of GEOTOP (de Vernal et al., 1999). A microwave
digestion technique adapted to the palynological preparation
of Quaternary marine sediments also was used (cf. Loucheur,
1999). This technique proved extremely useful for preparing
sediment samples from the Bering and Chukchi seas, which
contain abundant silica particles. Palynomorphs were identified and counted using a transmitted light microscope at
250× to 1000× magnification. More than 300 dinocysts were
counted in each sample for calculation of percentages and
statistical analyses, with the exception of a few samples containing a sparse palynoflora (Table 2). The concentrations of
palynomorphs were evaluated using the marker-grains method
(Matthews, 1969), which yields results accurate to about 10%
for a 0.95 confidence interval (de Vernal et al., 1987).
Copyright  2001 John Wiley & Sons, Ltd.
Taxonomy of the dinocyst taxa from Bering and
Chukchi seas
The taxonomical nomenclature used in this study conforms
with that presented in Rochon et al. (1999), with a few
exceptions. Cysts designated previously as Algidasphaeridium?
minutum var. minutum and var. cezare are now called
Islandinium minutum and Islandinium? cezare, respectively,
following the systematics proposed by Head et al. (this issue).
Two additional taxa also were distinguished and counted.
They include the cyst of Polykrikos kofoidii and an arctic
morphotype of Operculodinium centrocarpum, which are
described briefly below (see also de Vernal et al., this issue).
The cyst of Polykrikos kofoidii recovered in Bering Sea
is characterised by a fibrous ectophragm, with extensions
forming fan-shaped processes that are not linked distally (see
1–6 in Fig. 3), unlike the specimens of the cyst of Polykrikos
schwartzii Bütschli 1873 as illustrated in Rochon et al. (1999).
The cyst of Polykrikos kofoidii from the Bering Sea differs
from Polykrikos kofoidii Chatton 1914 described by MoreyGrains and Ruse (1980) with respect to the morphology of
the processes, which are fan-shaped rather than tubular. The
cysts from surface sediments of Bering Sea are similar to
those described by Matsuoka (1987) from the Akkeshi Bay
and Saroma Lake North of Japan. However, none of the
specimens from the Bering Sea exhibit shelf-like or reticulate
ornamentation, as observed to occur in the Japan Sea. The
cysts of Polykrikos kofoidii that were recovered in the Bering
Sea show some morphological variations with respect to the
size and texture of the fan-shaped processes (see 1–6 of Fig. 3).
They could be associated with the Type 1 cyst of Polykrikos
kofoidii described by Matsuoka and Cho (2000), but differ
significantly from the Types 2 and 3 in not having separate
rows of lumina between the processes. They also differ from
the Type 4 Polykrikos kofoidii cyst, which is characterised by a
complete ornamentation network (Matsuoka and Cho, 2000).
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
672
JOURNAL OF QUATERNARY SCIENCE
1
4
3
2
5
7
8
10
11
6
9
12
Figure 3 Scale bar is 10 µm. Sample number is followed by the England Finder coordinates. (1–6). Cyst of Polykrikos kofoidii: (1) TT51-295,
D8/4; (2) TT51-164, W54/2; (3) TT51-164, O47; (4) TT51-164, U35/1-3; (5) TT51-164, H43/3-4; (6) TT51-164, W16/1-2; (7) Selenopemphix
nephroides: TT52-001, T44/2, apical surface. (8) Selenopemphix quanta: TT51-257, X51/2, antapical view. (9) Islandinium minutum:
TT51-295, M17. (10) Lejeunecysta oliva: TT51-257, U15/2, ventral surface. (11) Brigantedinium cariacoence: TT51-295, P44/4, dorsal surface.
(12) Brigantedinium simplex: TT51-164, O22/3, dorsal surface
In the Bering and Chukchi seas, Operculodinium centrocarpum also presents some particularity with regard to its
morphology. Several morphotypes can be differentiated on
the basis of the ornamentation of the cyst wall, and the density and length of the processes (Fig. 4). In general, the cysts
Copyright  2001 John Wiley & Sons, Ltd.
of Operculodinium centrocarpum in the sediments of the
Bering and Chukchi seas are dominated by specimens with
a morphology intermediate between Operculodinium centrocarpum sensu Wall & Dale 1966 and the short processes form
(cf. Rochon et al., 1999). In the Bering Sea samples, the length
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
DINOCYSTS IN SURFACE SEDIMENT, BERING AND CHUKCHI SEAS
1
4
2
5
673
3
6
9
Figure 4 Scale bar is 10 µm. Sample number is followed by the England Finder coordinates. (1 and 2) Operculodinium centrocarpum: TT51-60,
P14/4; (1) optical cross-section; (2) lateral view. (3) Operculodinium centrocarpum —short processes: TT51-149, D8/4, lateral view.
(4–6) Operculodinium centrocarpum —Arctic morphotype: TT20-225, H49; (4) ventral surface; (5) dorsal surface; (6) mid-focus. (7) Spiniferites
intergrade elongatus-frigidus: TT51-227, O39/3, optical section. (8 and 9) Spiniferites frigidus: (8) TT51-60, S12/1; (9) TT51-60, F55/2 ventral
surface
of processes ranges from 4 to 7 µm (Fig. 4), which is less than
that reported for northern North Atlantic specimens (7–14 µm;
Rochon et al., 1999). However, it is of note that Operculodinium centrocarpum bearing relatively short processes were
reported to occur in coastal environments of the western
Pacific Ocean (Matsuoka, 1987; Matsuoka et al., 1997) and
the Baltic Sea (Matthiessen and Brenner, 1996; Nehring, 1997;
Ellegaard, 2000). Apart from Operculodinium centrocarpum
sensu Wall & Dale 1966, several specimens exhibit a morphology intermediate with that of the type cezare described
by de Vernal et al. (1989) from the post-glacial Champlain Sea
material. These specimens show morphology similar to that
of the cyst informally called Operculodinium centrocarpum
type B by de Vernal et al. (1989). They are characterised by
the presence of very few processes, which are imperfectly
developed and heterogeneously distributed around the cyst.
Such specimens were counted as the arctic morphotype of
Operculodinium centrocarpum (see also de Vernal et al., this
issue).
Owing to difficulties of identification at species level, some
taxa were grouped together for statistical analyses. This is the
case for Brigantedinium spp., which includes Brigantedinium
simplex and Brigantedinium cariacoence in addition to
Copyright  2001 John Wiley & Sons, Ltd.
specimens with archeopyle difficult to examine. This also is the
case for Spiniferites elongatus s.l., which groups Spiniferites
elongatus, Spiniferites frigidus and intergrade morphotypes (cf.
Rochon et al., 1999). Specimens belonging to Protoperidium,
for which determination at the genus level was difficult owing
to bad orientation and/or preservation, were grouped under
the label Peridinioids.
Source of oceanographical data
The temperature and salinity data of the surface water (0 m)
are from the National Oceanographic Data Center (NODC)
data sets, which includes measurements made between 1930
and 1994 (NODC, 1994). The data were compiled within
a circle of 30 nautical miles around the study sites, using
ORACLE data base managing system. The monthly averages of
temperature and salinity for the coldest and warmest months
(February and August respectively) have been used because
they provide information on the annual hydrographical cycle,
which seems to be more of a determinant for the dinocyst
distribution than seasonal or annual means (de Vernal et al.,
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
674
JOURNAL OF QUATERNARY SCIENCE
1994, 1997). Sea-ice-cover data were provided by the National
Climate Data Center. The original data consist of 1° × 1° grid
scale measurements for the 1953–1990 period. We express
the sea-ice cover as the mean number of months per year
with more than 50% of sea-ice coverage. Productivity data are
from Antoine et al. (1996). They are estimates from satellite
observation of the chlorophyll distribution. Productivity is
expressed as the annual production of carbon per surface area
(gC cm−2 yr−1 ).
The distribution of dinocyst assemblages
Concentrations
Most sediment samples contain abundant dinocysts, with
concentrations ranging from 2000 to 25 000 cysts cm−3
(Fig. 5). Such concentrations are comparable to those recorded
along the continental margins of the North Atlantic (e.g.
de Vernal et al., 1997; Rochon et al., 1999) and reflect a
relatively high productivity of dinoflagellates. A slight trend
of decreasing concentration, however, is observed from south
to north. Samples from northernmost sites located at the edge
of the permanent ice-pack in the Chukchi Sea (Si689-01, BC15 and B-5) are characterised by concentrations lower than
800 cysts cm−3 . Low cyst concentrations in the Chukchi Sea
may result from a high sedimentation rate, low cyst fluxes
owing to low productivity, or both. Extreme hydrographic
conditions with quasi-permanent sea-ice together with low
nutrient input (e.g. Walsh et al., 1989; Hansell et al., 1993;
Cooper et al., 1997) may explain low productivity.
The dinocyst assemblages
On the whole, the dinocyst assemblages of the Bering Sea
are characterised by relatively high species diversity, with
more than 20 taxa recovered routinely. However, only six
taxa represent more than 90% of the assemblages. They
are Operculodinium centrocarpum, the cyst of Pentapharsodinium dalei, Brigantedinium spp., Spiniferites elongatus
s.l., Spiniferites ramosus and the cyst of Polykrikos kofoidii
(Figs 3–5). The other accompanying taxa that are commonly
recovered include Impagidinium pallidum, Islandinium minutum, Lejeunecysta oliva, Selenopemphix quanta, Selenopemphix nephroides and Quinquecuspis concreta (Figs 3–5). In
the Chukchi Sea, the species diversity is much lower than
that of the Bering Sea. Operculodinium centrocarpum and the
cyst of Pentapharsodinium dalei largely dominate the assemblages, whereas Spiniferites elongatus s.l., Brigantedinium
spp., Spiniferites ramosus and Impagidinium pallidum are
observed occasionally.
Relationships between the assemblages and the
environmental parameters
The relative abundance of the main dinocyst taxa in the
assemblages shows important variations with latitude, thus
suggesting a direct relationship with hydrographic and climatic
conditions. In general, a decrease of the species diversity
is observed from south to north. Moreover, in the Bering
Sea, a linear relationship can be established between the
Copyright  2001 John Wiley & Sons, Ltd.
percentage of Operculodinium centrocarpum and the extent
of sea-ice cover, whereas an inverse relationship is established
with the percentage of the cyst of Pentapharsodinium dalei
(Fig. 6). It is noteworthy that such relationships seem to
be valid exclusively at the regional scale of the Bering
Sea because they are not in evidence when the spectra
from the Chukchi Sea are included or at the scale of the
northern North Atlantic (de Vernal et al., 1997; Rochon et al.,
1999).
Principal component analyses were performed on taxa percentages using the software of Guiot and Goeury (1996).
Logarithmic transformations were carried out in order to give
more weight to accompanying taxa. The analyses reveal that
the first component (PC1), which explains 51.7% of the
variance, is determined by the opposition between Operculodinium centrocarpum and the other taxa (Fig. 7). Principal component 1 shows a latitudinal distribution (Fig. 8)
and a significant correlation with sea-surface temperature
in February and the seasonal duration of sea-ice cover
(Fig. 9). The second component (PC2), which explains 15.1%
of the variance, is determined by an opposition between
Spiniferites elongatus s.l. and the cyst of Pentapharsodinium
dalei (Fig. 7). It shows some relationship with the temperature and salinity in August (Fig. 10). There are no apparent relationships between the two first components and
productivity.
Definition of a regional zonation
The relative proportion of taxa and the principal component
analyses permitted the identification of two distinct assemblages associated with two geographical zones in the Bering
Sea, and one in the Chukchi Sea (Fig. 5).
The first zone (I) is characterised by high concentrations of
dinocysts (up to 28 000 cysts cm−3 ) and by the dominance of
the cyst of Pentapharsodinium dalei and Brigantedinium spp. It
also is characterised by sparse occurrence of Operculodinium
centrocarpum and the absence of its Arctic morphotype. This
zone corresponds to the BSW in the southern part of the
Bering Sea, adjacent to the Aleutian Islands, where yearround ice-free conditions prevail. In this area, sea-surface
temperatures range from 1° to 4 ° C in February and from
8° to 11 ° C in August, and the salinity fluctuates from 31.6
to 32.8.
The second zone (II) is characterised by the dominance of Operculodinium centrocarpum and its Arctic morphotype, accompanied by Spiniferites elongatus
s.l. and Spiniferites ramosus. Relatively high dinocyst
concentrations reaching up to 20 000 cysts cm−3 , are
observed in this zone. Zone II corresponds to the northern BSW and includes the sites in the AW zone near
the Anadyr Gulf, where February temperatures fluctuate
from −1.6° to 0.5 ° C and sea-ice cover occurs for 2–4
months yr−1 .
The Chukchi Sea represents a third zone, characterised
by low species diversity. The assemblages are dominated by
Operculodinium centrocarpum and the cyst of Pentapharsodinium dalei. High proportions of the Arctic morphotype of
Operculodinium centrocarpum are recorded. Dinocyst concentrations are low, especially to the north, near the edge
of the permanent ice pack. This zone is characterised by
freezing winter temperature and extensive sea-ice cover for
7 to 12 months yr−1 . During summer, in August, temperature
and salinity range −1.0° to 2.4 ° C and from 26.7 to 27.9,
respectively.
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
Copyright  2001 John Wiley & Sons, Ltd.
SI689-01
BC-15
B-5
BC-4
SI689-22
TT20-079
TT20-225
TT42-239
NW262-30
TT42-092
TT42-294
NW262-23
NW262-22
NW262-20
TT42-255
TT51-237
TT51-240
TT42-054
TT51-234
TT42-045
TT51-242
TT51-031
TT51-204
TT51-230
TT51-227
TT51-207
TT42-030
TT51-222
TT51-244
TT51-219
TT51-246
TT51-045
TT51-250
TT51-049
TT51-051
TT51-186
TT51-060
TT51-253
TT51-062
TT51-257
TT51-259
TT51-149
TT51-295
TT51-267
TT51-173
TT51-152
TT51-160
TT51-164
TT51-299
TT51-157
TT51-301
TT52-001
0
40
80 0
10
20 0
40
80 0 5 10 15 0
5 1015 0
50
20 40 60 0 5 10 0
4 0 2 020
5
10 0
14000 28000 −5
0
5 −5
a
p
ci
in
Pr
PC2
0
ne
po
om
lc
50
s
nt
a
Se
pe
2
m
Te
o
(m
12−2 0
e
-ic
r
ve
co
pe
b
m
Te
Fe
ru
r
(˚C
l
Sa
ity
˚C
)
Assemblage
zone
I
Assemblage
zone
II
Assemblage
zone
III
t
us
t(
us
g
Au
g
Au
)
in
e
ur
at
y
ar
4 −20 5 10 25 28 31
r
r)
e
ur
at
a
ye
s/
h
nt
Figure 5 Diagram of percentages of the main dinocyst taxa. Taxa that are not represented in the diagram are those occurring in low percentages. They include Impagidinium aculeatum, Impagidinium patulum,
Lejeunecysta oliva and Lejeunecysta sabrina. The concentrations of dinocysts, the principal components (PC1 and PC2) and hydrographical means for each site also are indicated
South
Bering
Sea
Bering Strait
Chukchi
Sea
North
i
le
3)
m
m
da
m
pu
pu
m
/c
es
r
r
i
i
.
s
u
l
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a
a
i
t
i
.
d
6
c
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s
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o ta i
a
in
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ro 96
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cy
um p.
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us
ua
ep on s k
um
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no
rs
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l
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m ty
m D
in
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iu &
m
um
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ph
ph sp lyk
in rph
in all
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nt
um
ni
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i
u
i
m
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o
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tr
ed
pe
pe ec f P
rit
rit
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ul W cul c m
in
nt
en
gi
no
no qu o
ife
ife
r ti
rc su
s
to
nd
nc
le
le uin yst
in
in
ga
pa
i
a
te
pe arc
l
pe sen
ys
o
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r
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i
S
S Q C
S
B
O
O
C
S
Im
Is
S
C
PC1
DINOCYSTS IN SURFACE SEDIMENT, BERING AND CHUKCHI SEAS
675
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
JOURNAL OF QUATERNARY SCIENCE
Cyst of Pentapharsodinium dalei (%)
676
Discussion: the particularities of the
assemblages from the Bering and Chukchi
seas
80
70
60
50
The dinocyst assemblages from the eastern Bering Sea and
Chukchi Sea are composed of taxa commonly observed in
the subpolar environments of the North Atlantic. These taxa
include Brigantedinium spp., Operculodinium centrocarpum,
the cyst of Pentapharsodinium dalei, Islandinium minutum,
Spiniferites elongatus s.l., Spiniferites ramosus and Impagidinium pallidum (de Vernal et al., 1997; Rochon et al., 1999).
However, the assemblages of the Bering Sea present some particularities when compared with those of Arctic and subarctic
seas adjacent to the North Atlantic.
40
30
R = 0.702
20
10
0
Operculodinium centrocarpum
sensu Wall & Dale 1966 (%)
80
70
1 In high-latitude settings, the presence of the cyst of
Polykrikos kofoidii seems to be a feature specific to the
North Pacific (Matsuoka, 1985, 1987) and Bering Sea (this
study). In the northern North Atlantic, the most similar
taxon is the cyst of Polykrikos kofoidii sensu reported to
occur in low latitudes (e.g. Marret, 1994; Zonneveld, 1997;
Zonneveld et al., 1997). This leads us to hypothesise that the
two taxa are taxonomically distinct from each other, unless
they represent phenotypes adapted to different ecological
niches.
60
R = 0.912
50
40
30
20
10
0
0
2
4
6
8
10
12
Sea ice cover (months/year)
Figure 6 Relationship between sea-ice cover and percentages of the
two dominant taxa from the Bering Sea (filled circles),
Operculodinium centrocarpum sensu Wall & Dale 1966 and cyst of
Pentapharsodinium dalei. Percentages of these taxa in the Chukchi
Sea are indicated by open circles
2 Selenopemphix nephroides and Quinquecuspis concreta
also seem to record a more boreal limit in the North
Pacific and Bering Sea than in the North Atlantic (Rochon
et al., 1999). Such an observation suggests that temperature
probably is not the most determinant parameter for the
distribution of these taxa.
3 Amongst the particularities of the Bering Sea assemblages,
it is worth mentioning the morphological variations of
Operculodinium centrocarpum, which bears generally short
PC 2 (15.1%)
60
40
Cyst of Pentapharsodinium dalei
20
Lejeunecysta sabrina
Impagidinium pallidum
Impagidinium aculeatum
Quinquecuspis concreta
Lejeunecysta oliva
0
Spiniferites ramosus
Selenopemphix quanta
Selenopemphix nephroides
PC 1 (51.7%)
Impagidinium patulum
Cyst of Polykrikos kofoidii
−20
Islandinium minutum
Peridinioids
Islandinium? cezare
Brigantedinium spp.
Operculodinium centrocarpum - arctic morphotype
Operculodinium centrocarpum sensu Wall & Dale 1966
Spiniferites spp.
−40
Spiniferites elongatus
−60
−40
−30
−20
−10
0
10
20
30
40
Figure 7 Weighting of taxa according to principal components PC1 and PC2. The open diamonds represent taxa associated with an autotrophic
production, and the filled diamonds represent taxa associated with an heterotrophic production
Copyright  2001 John Wiley & Sons, Ltd.
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
DINOCYSTS IN SURFACE SEDIMENT, BERING AND CHUKCHI SEAS
677
A
PC 1
+3 to +6
0 to +3
−3 to 0
−6 to −3
B
PC 2
+2 to +5
0 to +2
−2 to 0
−5 to −2
Figure 8 Maps showing spatial distribution of principal components PC1 and PC2
processes (<7 µm), and the occurrence of its Arctic morphotype characterised by a reduced density of imperfectly
developed processes. Such variations in the morphology of
Operculodinium centrocarpum are not exclusive to the
Bering Sea. Morphological variations similar to that of
the Arctic morphotype were reported in a study of the
post-glacial Champlain Sea (de Vernal et al., 1989), which
occupied the St Lawrence lowlands from about 12 000 to
9800 yr BP during the retreat of the Laurentide ice-sheet
(Gadd, 1988). A similar morphology for Operculodinium
centrocarpum also is illustrated from sediments of the Beaufort Sea in the western Canadian Arctic (Harland et al.,
1980), and the southern Baltic Sea (Matthiessen and Brenner, 1996; Ellegaard, 2000). As suggested from the study
of the Champlain Sea sediments, the morphological variations of Operculodinium centrocarpum could represent
phenotypic adaptation to low salinity conditions (e.g. de
Vernal et al., 1989). Such a hypothesis would be consistent
with the relative abundance of the Arctic morphotype of
Operculodinium centrocarpum in the Bering and Chukchi
seas, which are characterised by a salinity lower than
that of the North Atlantic Ocean. However, we failed to
define a relationship between the salinity and the relative
proportion of the Arctic morphotype of Operculodinium
Copyright  2001 John Wiley & Sons, Ltd.
centrocarpum from the assemblages of the Chukchi and
Bering seas.
4 In the Bering Sea, there is a clear relationship between
the seasonal extent of sea-ice or the February temperature, and the percentages of the dominant taxa, Operculodinium centrocarpum and the cyst of Pentapharsodinium dalei. However, such a relationship cannot
be extrapolated on the scale of the Arctic Ocean or
North Atlantic (Rochon et al., 1999; de Vernal et al., this
issue), pointing towards some regionalism in the distribution of dinocyst assemblages in surface sediments of the
Bering Sea.
5 In many subpolar environments marked by cold conditions with seasonal sea-ice cover, dinocyst assemblages are
dominated by Peridiniales, which relate to a heterotrophic
production. In the subarctic seas adjacent to the North
Atlantic Ocean, Brigantedinium spp. or Islandinium minutum often dominate the assemblages (e.g. de Vernal et al.,
1997, this issue; Rochon et al., 1999); in the southern
Indian Ocean, Selenopemphix antarctica is characteristic
of circum-Antarctic environments (Marret and de Vernal,
1997). Thus, the assemblages of the northern Bering Sea and
the Chukchi Sea are peculiar because they are dominated
by Operculodinium centrocarpum, the cyst of PentapharJ. Quaternary Sci., Vol. 16(7) 667–680 (2001)
678
JOURNAL OF QUATERNARY SCIENCE
32
9
R = 0.838
7
5
3
Salinity August
Sea-ice cover (months/year)
33
11
31
R = 0.730
30
29
28
27
1
26
4
3
10
2
1
R = 0.819
0
−1
Temperature August (°C)
Temperature February (°C)
12
8
6
R = 0.845
4
2
0
−2
−6
−4
−2
0
2
4
6
PC1
Figure 9 Correlation between PC1 and the sea-surface temperature
in February and sea-ice cover. Samples from Bering Sea are
represented by filled circles and those from Chukchi Sea are
represented by open circles. The coefficients of correlation are based
on a logarithmic relationship
sodinium dalei and Spiniferites spp., which are all related
to an autotrophic production. Nevertheless, the dominance
of Operculodinium centrocarpum allows comparison with
offshore dinocyst assemblages from the central Baffin Bay
(Mudie and Short, 1985; Rochon and de Vernal, 1994),
where the surface water is marked by the inflow of the
West Greenland Current fed by a westward branch of the
North Atlantic Drift (NAD). Similarly, the co-dominance
of Operculodinium centrocarpum and the cyst of Pentapharsodinium dalei in the Bering Sea assemblages allows
comparison with dinocyst assemblages from the Nordic and
Barents seas, where surface waters also are under the influence of northward branches of the NAD (Rochon et al.,
1999; Voronina et al., this issue). As the BSW is under the
influence of northern branches of the North Pacific Current
(Thomson, 1981; Reed and Stabino, 1997), we are tempted
to associate the dominance of Gonyaulacales in the eastern Bering Sea to the impact of current inflow from an
offshore origin. Currents no doubt play a determinant role
on the dispersion of nutrient and plankton populations, and
ultimately on the distribution of microfossil assemblages in
sediments.
6 Some species occurring frequently in modern sediment of
the northern North Atlantic Ocean, such as Ataxiodinium
choane, Bitectatodinium tepikiense and Nematosphaeropsis labyrinthus (e.g. Rochon et al., 1999), have not been
recorded in surface sediment of the Bering Sea despite
comparable temperature and salinity conditions. The differences in species composition of assemblages of the
Copyright  2001 John Wiley & Sons, Ltd.
−2
−3
−2
−1
0
1
2
3
4
5
PC2
Figure 10 Relationship between PC2 and sea-surface temperature
and salinity in August. Samples from Bering Sea are represented by
filled circles and those from Chukchi Sea are represented by open
circles. The coefficients of correlation are based on a polynomial
equation (order 3)
Bering Sea versus that of subarctic seas of equivalent
latitude in the North Atlantic may reflect some endemism
owing to limited exchanges between the North Atlantic
and North Pacific. The disparity in dinocyst assemblages
might be explained by different phenotypic adaptations
of species to the North Pacific and the North Atlantic
oceans, respectively. It might be related to differences
between the two oceans, notably with respect to the
concentration of nutrients or the various dissolved compounds.
Conclusion
The present study demonstrates that the dinocyst assemblages
in the Bering and Chukchi seas are closely related to
hydrographic conditions on a regional scale. In particular,
the sea-surface temperature and the seasonal extent of
sea-ice cover seem to exert a determinant role on the
regional distribution of dinocysts assemblages, which results
in a latitudinal zonation. These data indicate that dinocyst
assemblages could be used for the development of transfer
functions with the aim of reconstructing palaeoceanographic
conditions in the northern North Pacific. However, because
of differences in species composition between the Bering
Sea assemblages and those of high-latitude marine basins
J. Quaternary Sci., Vol. 16(7) 667–680 (2001)
DINOCYSTS IN SURFACE SEDIMENT, BERING AND CHUKCHI SEAS
of the North Atlantic, the use of an hemispheric scale
Pacific–Arctic–Atlantic dinocyst data base for a transfer
function relying on extrapolation techniques should be
considered with caution.
Acknowledgements Special thanks are due to B. Conard from the
Oregon State University for the subsampling of core tops. We are
grateful to Maryse Henry and Virginie Loucheur for their help in
laboratory studies. This study has been supported by the Fonds
FCAR of Quebec and the Natural Science and Engineering Council
(NSERC) of Canada, notably through the project Climate System
History and Dynamics (CSHD). The comments by J. Matthiessen and
K. Matsuoka were most helpful in preparing the revised version of the
manuscript.
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