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Transfer functions between diatom assemblages and surface hydrology in the southern ocean
Palaeogeography, Palaeoclimatology, Palaeoecology, 61 (1987): 79 95 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands
79
TRANSFER FUNCTIONS BETWEEN DIATOM ASSEMBLAGES AND SURFACE HYDROLOGY IN THE SOUTHERN OCEAN J. J. P I C H O N 1, M. L A B R A C H E R I E 1, L. D. L A B E Y R I E 2 a n d J. D U P R A T 1 ~Dgpartement de Gdologie et Ocdanographie, Unitg C.N.R.S. n.197, Universitg de Bordeaux 1, Avenue des Facultds, 33405 Talence (France) 2Centre des Faibles Radioactivitgs, Laboratoire mixte C.N.R.S.-C.E.A., Domaine du C.N.R.S.-91190, Gif-sur-Yvette (France)
(Received March, 10, 1986; revised and accepted February 18, 1987)
Abstract Pichon, J. J., Labracherie, M., Labeyrie, L. D. and Duprat, J., 1987. Transfer functions between diatom assemblages and surface hydrology in the southern ocean. Palaeogeogr., Palaeoclimatol., Palaeoecol., 61:79 95. Interpretation of the distribution of 31 marine diatom species and two silicoflagellategenera in 28 core tops from the Atlantic and West Indian sectors of the Southern Ocean by Q-mode factor analysis allows definition of three significantly different floral assemblages, one associated with Subantarctic and two with Antarctic waters. These three areas are clearly delineated by prominent oceanographic limits: the Antarctic Convergence and the maximum retreat of sea.ice during the summer season. A set of palaeoecological transfer functions is derived by comparison between the distribution of these associations and surface water hydrological parameters. Using these equations, estimated summer temperatures from core top floral assemblages range from 12c'C in the north of the study area to -2.0°C near the southernmost cores. The standard errors of estimate are + 1.1°C for summer temperatures, + 1.5 months/year for sea ice cover, +0.2 mg-atP/1 for sea-surface phosphate concentrations. Numerical tests of these paleoecological equations demonstrate their usefulness to reconstruct the evolution of the surface oceanography of the Austral Ocean at different periods of the last climatic cycle.
Introduction Large scale surface water paleotemperature reconstructions (CLIMAP project members, 1976, 1981) h a v e b e e n efficient a t low a n d intermediate latitudes. They are based upon t r a n s f e r f u n c t i o n s b e t w e e n specific d i s t r i b u t i o n of p e l a g i c m i c r o f o s s i l s a n d m o d e r n s u r f a c e w a t e r t e m p e r a t u r e . D e f i n i t i o n of e q u i v a l e n t t r a n s f e r f u n c t i o n s to e s t i m a t e t h e c h a n g e s of t e m p e r a t u r e a n d ice c o v e r a g e of h i g h l a t i t u d e surface waters during the last climatic cycles is a l s o v e r y i m p o r t a n t . H o w e v e r , , p l a n k t o n i c f o r a m i n i f e r a c a n n o t be u s e d as t h e y a r e monospecific (Neogloboquadrina pachyderma, left c o i l i n g v a r i e t y ) in s u r f a c e w a t e r s w i t h t e m p e r a t u r e s b e l o w 6°C. T r a n s f e r f u n c t i o n s for
r a d i o l a r i a n s in t h e S o u t h e r n O c e a n w e r e used e x t e n s i v e l y d u r i n g t h e C L I M A P P r o j e c t by L o z a n o a n d H a y s (1976), M o r l e y (1979) a n d M o l f i n o e t a l . (1982). Y e t i n t e r p r e t a t i o n of t h e s e d a t a is difficult b e c a u s e t h e l i v i n g r a d i o l a r i a h a v e a wide d e p t h r a n g e . M a n y s p e c i e s a r e r e l a t e d to i n t e r m e d i a t e w a t e r h y d r o l o g y ( H o w a r d a n d P r e l l , 1984). As b o t t o m s e d i m e n t s in t h e A n t a r c t i c O c e a n c o n t a i n a b u n d a n t s i l i c e o u s v a l v e s of d i a t o m s , s i n g l e - c e l l e d p h o t o s y n t h e t i c a l g a e l i v i n g in surface waters, these microorganisms appear as a g o o d s u p p o r t for t h e d e f i n i t i o n of p a l e o h y drological transfer functions. Defelice and W i s e (1981) s t u d i e d d i a t o m s in s u r f a c e sedim e n t s from t h e s o u t h e a s t A t l a n t i c O c e a n a n d i d e n t i f i e d five d i s t i n c t a s s e m b l a g e s , t h r e e of
80 which could be related to the overlying water masses. Burckle (1984) used diatom assemblages and preservational data to reconstruct paleoceanographic conditions during the Last Glacial Maximum. But the relationship existing between the phytoplankton assemblages in recent sediment and the surface water hydrology has not yet been quantitatively established in the high southern latitude. This is the main purpose of the present study. In the first step, the sediment core tops are examined to define and delineate independent microfossil species assemblages; in a second step, these assemblages are correlated with modern climate-related parameters (summer sea-surface temperatures, annual ice cover and phosphate concentrations).
strongly interdependent with the prevailing meteorological and ice conditions. In the Atlantic sector, the general circulation is part of the large cyclonic Weddell gyre (Deacon, 1979; Foster and Carmack, 1976) which extends from the Antarctic Peninsula to approximately longitude 30°E and northward from Antarctica to between latitude 55-60°S. The sea ice surrounding Antarctica undergoes large seasonal changes in areal extent. In the Atlantic and the Western Indian sectors, it extends as far north as 54-55°S in the winter and retreats close to the continent in summer (Antarctic Ice Charts 1975-1976 and 19771978). In the Weddell Sea, pack ice persists into the summer period and accounts for the highest amount of inter-annual variability around Antarctica (Ackley, 1981).
Physical oceanographic setting Data The oceanography of the entire Southern Ocean is governed by the West Wind Drift resulting in the clockwise Antarctic Circumpolar Current (Deacon, 1964; Gordon,, 1971). The subantarctic water mass occupies the zone between the Subtropical Convergence (STC) and the Antarctic Polar Front (APF) where flow of the Circumpolar Current is maximum. In this region, surface salinities range between 34.0%0 and 34.6%0 and surface temperatures fluctuate from 4 to 10°C (Deacon, 1937; Colborn, 1975). The Antarctic Polar Front (APF) marks the southern transition from warm to cold waters. This front is usually broad on the order of 50 to 100 km, and is characterized by high biological productivity to the south (Burckle, 1984). Antarctic surface waters exhibit a southward decrease in salinity from 34 to 33.8%o and a temperature decrease from about 4 to 2°C across the APF. The band of easterly winds near Antarctica and the irregular shape of the continent and the bathymetric effects, together produce several large cyclonic divergent cells characterized by upwelling of subsurface water: the Antarctic Divergence. Both the position of this divergence and its intensity are variable, being
Floral data
A total of 116 samples from the tops of piston cores and trigger cores have been studied. These samples are located in the Atlantic and West Indian sectors of the Southern Ocean, between latitude 29°S and 77.25°S and longitude 30°W and 70°E. Sources of the material include Marion Dufresne cores from Centre des Faibles Radioactivit~s and Universit~ de Bordeaux I, Vema and Conrad cores from the Lamont-Doherty Core Repository, Islas Orcadas cores from the FSU Antarctic Research Facility, and R. Gersonde (Alfred Wegener Institute for Polar Research) allowed us to do preliminary floral analyses of four samples from his own collection in the Weddell Sea. Cleaning and preparation of diatom slides followed the procedure of Riedel and Sanfilippo (1977) to extract radiolaria but sieving was replaced by successive decantations. In the siliceous deposits of the Southern Ocean, the diatoms range from 3 to 140#m. Some species e.g. Nitzschia curta or Nitzschia cylindrus which are lightly silicified and very small (15 #m) need a long decantation in the seven successive washings of distilled water. After
81
twelve hours of decantations of the frustules, some smaller species were at-ill floating. Therefore we added a step of seven minutes centrifugation after each washing at 1200 r.p.m. This allows preservation of all valves while eliminating the clay which stays in suspension. The diatom species population of each sample was quantified by counting at least 300 specimens encountered in random traverses. Specimens representing more than half of a valve were counted as one; for pennate diatoms, each pole was counted as one half.
1977-1978 constructed by the U.S. Navy from satellite imagery and conventional data. The summer and winter averaged sea-surface temperatures and the mean maximum and minimum extension of sea ice are shown in Figs.1 and 2. The chart constituting phosphate concentrations (inorganic dissolved phosphorus: Fig.3) is derived from Gorshkov (1978); the data for the Weddell Sea samples were drawn on the basis of investigations for February and March 1977 by E1-Sayed and Taguchi (1981).
Data analysis
Oceanographic data Phytoplankton assemblages Oceanographic parameters were derived by interpolation from various monthly or seasonal maps and documents. Sea-surface temperature values were taken from Wyrtki (1971), Gorshkov (1978) and different cruise reports for the summer temperatures (Antiprod I, 1978; Apsara I, 1982 and Apsara II, 1984). The present-day annual ice cover was averaged by area using Antarctic Ice Charts 1975-1976 and
90 °
6 0°
3 0"
One of the most difficult problems encountered was selecting samples truly representative of modern sediments, samples in which the thanatocoenosis may be associated to the present biocoenosis. From the 116 samples studied, 55 were rejected for several reasons: Seven of them contained extinct diatom species (Actinocyclus ingens, Hemidiscus karstenii), or
0°
30"
60"
90 °
40 °
Fig.1. M e a n s u m m e r sea-surface temperature and m i n i m u m sea-ice cover. The data for the Antarctic Polar Front are from Defant (1961) and Deacon (1983). Dots indicate position of surface sediment samples.
82
6 O"
90 °
WINTER o.r
.~
Cores used i n
90°
.
.
"'.".',"."I
0°
~
30'
'
60"
."".'"" 1~
;
90 °
,
/i T.: : /i Y:
.o
surf. . . . . diment
3 0°
6°°
.~: ~i".",'":'.'"'.".":":.".'"'.".' :':':":.".".'"':~ ~'..'.".:.'..'..'.'..'..'..'.'.."..""." ".,.'-.'.."" ~ . ~
'c
"~"
-~
0°
30"
"~:::~: ~g
60 °
90°
Fig.2. Mean winter sea-surface temperature and maximum sea-ice cover. The data for the Antarctic Polar F r o n t are from Defant (1961) and Deacon (1983). Dots indicate position of surface sediment samples.
~°
6 O"
3 0°
0°
30 °
60 °
90 °
,0 °
50 ~
Fig.3. Mean summer sea-surface phosphate concentrations (inorganic dissolved phosphates). Based on Gorshkov (1978). Dots indicate position of surface sediment samples.
83 TABLE I Positions and water depths of 28.~iston and trigger-top cores used in this study. Ship designations: MD = Marion Dufresne; RC = Robert Conrad; V = Vema; 1176 and 1277= Islas Orcadas; 1010 1223= Polarstern Core
Latitude
MD82 424 54°05.8'S MD82 436 6F'13.6'S MD80 301 54~00.0'S MD80 303 60~00.0'S MD80 304 5F04.0'S MD73 026 44~59.0'S MD24 KK63 5 F 5 6 . 0 ' S RCll-78 5ff 52.0'S RCll-80 46°45.0'S RC13-255 50°34.0'S RC13-263 53~'48.3'S RC13.273 55~04.5'S RC13- 274 53°09.0'S RC15.91 49°55.3'S
Longitude
Water depth (m)
Core
Latitude
Longitude
Water depth (m)
0°20.7'W 19°28.8'W 66°50.0'E 66°31.0'E 67°44.0'E 53°17.0'E 42°53.0'E 9°52.0'W 0°03.0'W 2°53.7'E 8°13.0'W 11°34.5'E 12°25.6'E 15°34.1'W
2350 3620 4540 4533 1950 3429 2550 3115 3656 3332 3389 4967 3372 3775
RC17-56 V16-60 V16-65 1176 82 1176 86 1176 88 1176 91 1277 2 1277 28 1277 41 1010 1192 1212 1223
65°24.0'S 49°59.5'S 45°00.0'S 49°31.2'S 48°02.6'S 46°57.8'S 44°56.7'S 45"02.1'S 61°28.0'S 69°59.9'S 77°20.0'S 77"25.0'S 75¢~30.0'S 76°00.0'S
37°43.0'E 36°45.5'E 45°46.0'E 13°11.5'E 13°49.0'E 14°18.2'E 15°02.9'E 22°28.2'E 9°ll.0'E 5°04.6'W 35°00.0'W 39°00.0'W 57~00.0'W 33°30.0'W
4794 4574 1618 4100 4338 5106 4649 4800 5322 1873 476 842 431 772
r a d i o l a r i a n species (Stylatractus universus). Others presented an a n o m a l o u s l y high perc e n t a g e of diatoms or r a d i o l a r i a clearly associated with the last glacial m a x i m u m (Eucampia antarctica - - B u r c k l e and Cooke, 1983; Cycladophora davisiana - - H a y s et al., 1976). At the opposite extreme, some o t h e r s had too m u c h c a l c i u m c a r b o n a t e for the latitude and w a t e r depth ' ( L o z a n o and Hays, 1976). We also rejected samples w h e r e the diatoms were largely dissolved or presented c o n s i d e r a b l e breakage. After f a c t o r analysis, we excluded the samples with low communalities. A total of 64 core-top samples were considered to r e p r e s e n t H o l o c e n e sedimentation. However, these samples were not homog e n e o u s l y d i s t r i b u t e d over the h y d r o g r a p h i c gradients: in p a r t i c u l a r , t h e s o u t h e r n limit of the p o l a r front was over-represented with 45 samples. To define an u n b i a s e d sample of the sediment p o p u l a t i o n as evenly d i s t r i b u t e d as possible, we kept only 28 samples (Table I) located a l o n g four b r o a d l y defined l a t i t u d i n a l t r a n s e c t s in the A t l a n t i c and West I n d i a n sectors of the S o u t h e r n O c e a n (Figs.l-3).
Statistical treatment The phytoplankton population of the S o u t h e r n Ocean is dominated by a few species, present in all the core tops, w h a t e v e r the surface w a t e r hydrology. A Q-mode factor analysis of the relative a b u n d a n c e of the different species in the 28 samples indicates the excessive role to the a b u n d a n c e of two domin a n t species: Nitzschia kerguelensis and Thalassiosira lentiginosa. Other species are associated in m u c h lower abundances. This ill-balance limits the possibility of i n t e r p r e t a t i o n of the floral changes in terms of surface w a t e r hydrology. To increase the weight of s e c o n d a r y species sensitive to local surface w a t e r hydrology, we have made several modifications to the data base: (1.) Of the 57 siliceous p h y t o p l a n k t o n species (55 diatom species and 2 silicoflagellate genera) identified in our set of samples, 24 have been eliminated because they are ubiquitous and in low relative abundance. (2) Several low relative a b u n d a n c e species are very specific to p a r t i c u l a r surface w a t e r hydrological conditions, for example: Hemidiscus cuneiformis,
84 TABLE II Conversion of each species percentage into abundance rank Species
Actinocyclus (charcotia) actinochilus Asteromphalus parvulus Azpeitia tabularis Chaetoceros sp. Cocconeis sp. Coscinodiscus furcatus Eucampia antarctica Hemidiscus cuneiformis Nitzschia angulata N. curia IV. cylindrus N. kerguelensis N. ritscherii IV. separanda N. sublineata Odontella weisflogii Porosira glacialis Rhizosolenia styliformis Roperia tesselata Schimperiella antarctica Thalassionema nitzschioides Thalassiosira antarctica T. decipiens T. delicatula T. eccentrica T. gracilis T. lentiginosa T. lineata T. oestrupii T. tumida Thalassiothrix sp.
Rank value 0
1
2
3
(in %)
(in %)
(in %)
(in %)
0
1
2-6
7-13
0
1
2
3
0 0 0
1 1 1
2 2 2-4
3-5 3 5-9
0
1
1
2-24
25-50
0 0
1 1
2
0 0
1 1
0 0
1 1
2
0 0 0 0
1 1 1 1
2-15 2-7 2-3 2
0 0
1 1
2 2
0 0
1 1
2-4 2-24
5 7 25-50
0 0 0
1 1 1
0
1
2-4 2-18
5 7 19-39
0 0 0
1 1 1
2
3
0
1
2
3
0 0
1 1
2 2
3 3
2-12 2-7 2-39
3
13-26 8-16 40-81 3
16-32 8-19 4 8 3-6 3 6 3
SILICOFLAGELLATES Dictyocha Distephanus
R o p e r i a t e s s a l a t a and the silicoflagellate genus D i c t y o c h a are only f o u n d n o r t h of the APF, and
a l w a y s in low p e r c e n t a g e s (Abbott, 1972; Defelice and Wise, 1981; Pichon, 1986). P r e s e n c e even in low a b u n d a n c e of these species is an i m p o r t a n t p a r a m e t e r . Therefore, we h a v e replaced the relative p e r c e n t a g e s of e a c h of these 33 species by a r a n k i n g in four a b u n d a n c e classes defined for e a c h species (Table II). We h a v e a r b i t r a r i l y s e p a r a t e d t o t a l absence ( r a n k
0), a n d t r a c e s or a b u n d a n c e less t h a n 2% ( r a n k 1), with the two last r a n k s c o v e r i n g the r e m a i n i n g r a n g e of the d i s t r i b u t i o n ( r a n k 2=2% - h a l f of the m a x i m u m a b u n d a n c e for e a c h species, r a n k 3 = m o r e t h a n h a l f of the m a x i m u m a b u n d a n c e for each species). This scale gives the same v a l u e for a species g o i n g from 5% relative a b u n d a n c e to its m a x i m u m of 7% (Thalassionema n i t z s c h i o i d e s ) as for a species t h a t goes from 40% to 81% ( N . k e r g u e l -
85
ensis). For the interpretation of the data we have to take into account that the resulting stochastic error within each rank is larger for the species of low relative abundance. The data base once modified was treated by the Q-mode factor analysis of Imbrie and Kipp (1971). Floral assemblages and their relationship to surface oceanography A factor analysis of 31 diatom species and 2 silieoflagellate genera from the 28 surface sediment samples produces three varimax factors which account for 86% of the distributional variance of the species data base. The varimax factor matrix (Table III) gives the TABLEIII V a r i m a x f a c t o r m a t r i x (28 s a m p l e s ) Communality
Assemblages 1
1 1277-2 2 RCl1-80 3 1176-91 4 MD73026 5 V16-65 6 1176-88 7 V16-60 8 RC15-91 9 1176-86 10 R C 1 3 - 2 5 5 11 1176-82 12 M D 8 0 3 0 4 13 R C l l - 7 8 14 M D 2 4 - K K 6 3 15 R C 1 3 - 2 7 4 16 M D 8 0 3 0 1 17 R C 1 3 - 2 7 3 18 R C 1 3 - 2 6 3 19 M D 8 2 4 2 4 20 M D 8 0 3 0 3 21 1277-28 22 R C 1 7 - 5 6 23 M D 8 2 4 3 6 24 1277-41 25 1010-W 26 1192-W 27 1212-W 28 1223-W
Variance
0.795 0.909 0.803 0.948 0.832 0.926 0.869 0.839 0.793 0.879 0.872 0.895 0.884 0.871 0.878 0.821 0.827 0.874 0.904 0.899 0.889 0.659 0.831 0.812 0.884 0.895 0.935 0.904
2
0.863 0.894 0.885 0.889 0.871 0.876 0.792 0.773 0.729 0.774 0.443 0.504 0.697 0.549 0.286 0.337 0.327 0.326 0.358 0.384 0.238 0.160 0.371 0.138 0.048 0.073 0.080 0.089
0.079 0.166 0.027 0.086 0.114 0.216 0.088 0.118 0.045 0.144 0.021 0.085 0.161 0.021 0.337 0.216 0.247 0.245 0.207 0.436 0.388 0.587 0.547 0.816 0.920 0.939 0.958 0.904
- 0.212 - 0.286 -0.139 - 0.387 -0.245 - 0.334 - 0.484 --0.478 - 0.510 -0.509 -0.821 - 0.796 - 0.593 - 0.755 - 0.826 - 0.813 - 0.812 - 0.841 - 0.857 - 0.749 - 0.826 - 0.537 - 0.628 - 0.358 - 0.074 - 0.092 - 0.104 - 0.281
32.788
20.062
33.148
composition of each sample in terms of the three varimax factors. The factor score matrix (Table IV) represents the relative contribution of each species to each factor. The communality, an index of how much of the compositional information in the original sample is accounted for by the varimax model is greater than 0.79 in all the samples, with the exception of two samples located near the Antarctic continent in the Western Indian sector. Interpretation of the analysis is facilitated because each of the three varimax factors is clearly associated with a specific hydrographic province. We have named the factors according to these zones.
Factor 1 (Subantarctic assemblage) This factor has an individual variance of 32.8%. It is dominated by Thalassionema nitzschioides with a varimax factor score of 0.50, Azpeitia tabularis (0.49), Thalassiosira lentiginosa (0.31) and the silicoflagellate genus Dictyocha (0.25). The dominant T. nitzschioides and A. tabularis species have been described as species living in the waters of the subantarctic region by Heiden and Kolbe (1928), Hustedt (1930), Hargraves (1968), Fenner et al. (1976). The abundance of A. tabularis remains very high in the plankton of the APF zone (Fenner et al., 1976). In sediments, Abbott (1972) finds these two species in his subtropical assemblage, Defelice and Wise (1981) also consider these species as characteristic of the sediments north of the APF. Burckle (1984) finds them in his factor 3 which overlaps the APF except in the Atlantic sector, in which T. lentiginosa is prevalent. In fact, this last species is found to be also present in antarctic and subantarctic waters where it is better represented (Hendey, 1937; Hustedt, 1958; Hargraves, 1968; Hasle, 1969). Among the associated species, H. cuneiformis and R. tesselata are present only in this assemblage; they have been described as tropical subtropical to transitional species by Hargraves (1968), Hasle (1969), Simonsen (1974), Fenner et al. (1976), Defelice and Wise (1981). Among the silicoflagellates, no Dictyocha specimen was found south of APF.
86 TABLE IV Varimax factor score matrix Species
Assemblages
1 Actinocyclus actinochilus Asteromphalus parvulus Azpeitia tabularis Chaetoceros sp. Cocconeis sp. Coscinodiscus furcatus Eucampia antarctica Hemidiscus cuneiformis Nitzschia angulata N. curta N. cylindrus N. kerguelensis N. ritscherii N. separanda N. sublineata Odontella weisflogii Porosira glacialis Rhizosolenia styliformis Roperia tesselata Schimperiella antarctica Thalassionema nitzschioides Thalassiosira antarctica T. decipiens T. delicatula T. eccentrica T. gracilis T. lentiginosa T. lineata T. oestrupii T. tumida Thalassiothrix sp. Dictyocha Distephanus
0.059 0.014 0.486 0.001 0.000 0.177 0.292 0.042 0.082 - 0.021 - 0.038 0.193 0.230 0.088 0.068 - 0.002 - 0.007 0.118 0.126 0.091 0.497 0.004 0.191 0.070 0.134 0.039 0.309 0.158 0.150 -0.014 - 0.031 0.251 0.148 -
-
-
-
2 0.468 - 0.021 - 0.055 0.003 0.000 0.004 0.503 0.002 0.057 0.378 0.329 - 0.031 - 0.025 - 0.066 0.494 0.032 0.077 0.005 0.006 - 0.001 - 0.013 0.064 - 0.007 0.002 0.004 - 0.073 0.054 0.008 0.004 0.018 - 0.064 0.022 0.010
3 -
0.067 0.017 0.010 0.058 0.000 0.081 - 0.016 0.027 - 0.141 0.055 0.017 - 0.507 0.041 - 0.165 - 0.121 0.006 0.011 - 0.050 0.081 - 0.148 0.187 0.026 0.069 0.025 0.072 - 0.373 -0.204 0.087 0.074 -0.039 - 0.530 0.151 - 0.232
T h i s a s s e m b l a g e p r e s e n t s it s m a x i m u m i n t h e a r e a n o r t h o f t h e A P F (Fig.4), w h e r e t h e temperatures of the surface waters range from
b l a g e is c h a r a c t e r i z e d b y five d i a t o m s p e c i e s :
12 t o 5°C i n s u m m e r , a n d f r o m 7 t o 2°C i n w i n t e r . T h e s a l i n i t i e s r a n g e f r o m 35%0 t o 34%0. T h e 0.75 f a c t o r l o a d i n g l i m i t c o r r e s p o n d s t o t h e actual position of the APF.
(0.32). A l l o f t h e s e s p e c i e s h a v e b e e n f o u n d t o be a s s o c i a t e d w i t h s e a - i c e ( V a n H e u r c k , 1909; H e n d e y , 1937; H a r t , 1942; K o z l o v a , 1964; B u n t a n d W o o d , 1963; B u y n i t s k y , 1977; K r e b s , 1977 a n d 1983; W h i t a k e r , 1977). B u r c k l e (1984) d e s c r i b e s t h i s a s s e m b l a g e n e a r t h e R o s s iceshelf, i n t h e P a c i f i c s e c t o r o f t h e S o u t h e r n O c e a n , and he a s s u m e s its o c c u r r e n c e in the Weddell Sea. Thus, this a s s e m b l a g e c a n be c o n n e c t e d w i t h
Factor 2 (South WeddeU assemblage) T h i s is l o c a t e d i n t h e s o u t h e r n study area, in the Weddell Sea A n t a r c t i c c o n t i n e n t (Fig.5). I t 20% of the total v a r i a n c e . T h e
portion of the and near the accounts for floral assem-
E. a n t a r c t i c a (0.50), N . s u b l i n e a t a (0.49), A . actin o c h i l u s (0.47), N . c u r t a (0.38), N . c y l i n d r u s
87 90 °
6
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,,----o°
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. 3o"
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Fig.4. Distribution of subantarctic assemblage in surface sediments. Values plotted are those given in varimax matrix (Table III).
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Fig.5. Distribution of South Weddell assemblage in surface sediments. Values plotted are those given in varimax matrix (Table III).
88 Hargraves, 1968; Hasle, 1969; Abbott, 1972; Fenner et al., 1976). Highest loadings for this factor are located between latitude 50°S and 55°S, and perfectly outline the well-preserved diatom ooze belt (Burckle, 1984). The geographical distribution of this assemblage is well defined between the APF north and the average limit of the maximum extension of the spring sea-ice, south (Fig.6). Factor 3 corresponds to the zone of medium salinity (33.80%0) and generally lower than 5°C sea-surface temperatures zones. It should be noted that the transition between the Subantarctic assemblage (factor 1) and the Open-ocean antarctic assemblage (factor 3) coincides with the main position of the APF, and that the South Weddell assemblage (factor 2) indicates the maximum retreat limit of sea-ice in the summer. Based upon the predominant variations between these three assemblages, these correlations may allow one to retrace the motions of the sea-ice, and the changes in the position of the APF.
a particular environment which, being ice-free during a very short period of the year (an average of 2 months), would only allow a quick development of some species in melting waters (E. antarctica), or within sea-ice (A. actinochilus, N. curta, N. cylindrus, N. sublineata). This South Weddell assemblage corresponds to a long annual sea ice cover.
Factor 3 (Open-ocean Antarctic assemblage) This has an individual variance of 33.1%. It is essentially determined by Thalassiothrix sp. (0.53) and N. kerguelensis (0.51) and to a lesser degree by Thalassiosira gracilis (0.37) and the silicoflagellate genus Distephanus (0.23). Among the accompanying species, the ones best represented are: Asteromphalus parvulus (0.17), Nitzschia separanda (0.16) and Schimperiella antarctica (0.14). The main species which define this assemblage are very common in waters and sediments of the subantarctic and antarctic zones (Heiden and Kolbe, 1928; Hendey, 1937; Hustedt, 1958; Kozlova, 1964;
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Fig.6. Distribution of "Open ocean" assemblage in surface sediments. Values plotted are those given in varimax matrix (Table III).
89
Surface hydrology
Relationship between floral assemblages and summer sea-surface temperatures
We h a v e tested the c o r r e l a t i o n s b e t w e e n the v a r i a t i o n s of six surface h y d r o l o g i c a l parameters: surface t e m p e r a t u r e s for summer, w i n t e r and spring, s u m m e r sea surface salinity, annual ice c o v e r and sea surface p h o s p h a t e c o n c e n t r a t i o n s for s u m m e r (Table V). The corr e l a t i o n coefficient r a n g e s from a high of 0.969 (summer versus w i n t e r surface t e m p e r a t u r e s ) to a low of - 0 . 0 0 2 (spring t e m p e r a t u r e s versus surface p h o s p h a t e c o n c e n t r a t i o n s ) . The surface t e m p e r a t u r e s form a g r o u p within w h i c h c o r r e l a t i o n s are high and related to the present h y d r o l o g y . The t r a n s f e r f u n c t i o n s t h e r e f o r e do not allow i n d e p e n d e n t e s t i m a t i o n s of summer, w i n t e r and spring t e m p e r a t u r e s . S u m m e r surface salinity, a n n u a l ice c o v e r and p h o s p h a t e c o n c e n t r a t i o n s show lower i n t e r c o r r e l a t i o n s . The d a t a base appears therefore specially well a d a p t e d for i n d e p e n d e n t e s t i m a t i o n s of summer t e m p e r a t u r e s , a n n u a l ice cover and phosphate concentrations.
Calibration of transfer function
The loadings of the t h r e e individual factors are plotted a g a i n s t observed s u m m e r seasurface t e m p e r a t u r e s (Fig.7). They exhibit an orderly succession of d o m i n a n c e between - 2 c C and 12°C: from " S o u t h Weddell" to " O p e n - o c e a n " and finally " S u b a n t a r c t i c " assemblages. The t r a n s f e r f u n c t i o n for the s u m m e r season presents a low s t a n d a r d e r r o r of estimate of __ 1.1~C and has a multiple c o r r e l a t i o n coefficient of 0.980 (Table VI). To m e a s u r e the robustness of the t r a n s f e r function, we h a v e plotted estimated a g a i n s t m e a s u r e d s u m m e r sea-surface t e m p e r a t u r e s for each of the 28 surface samples used for the regression. The dispersion of these v a r i o u s samples in the s c a t t e r d i a g r a m (Fig.8) is h o m o g e n e o u s . It implies t h a t the t r a n s f e r f u n c t i o n produces t e m p e r a t u r e estimates with similar reliability in all regions of the study area.
Relationship between floral assemblages and annual ice cover
We have used the multiple c u r v i l i n e a r regression m e t h o d of Imbrie and Kipp (1971) to q u a n t i f y the r e l a t i o n s h i p s between the dist r i b u t i o n s of the floral assemblages and s u m m e r t e m p e r a t u r e s , a n n u a l ice c o v e r and s u m m e r p h o s p h a t e c o n c e n t r a t i o n s of surface waters.
The plotted statistical weights a g a i n s t present day a n n u a l ice c o v e r in each sample show an obvious c o r r e l a t i o n for the S o u t h Weddell assemblage (Fig.9), a l t h o u g h samples are lacking for ice c o v e r b e t w e e n a few days and eight months/year.
TABLE V Correlation coefficients between all pairs of oceanographic parameters T-summer T-summer~ T-winter T-spring S-summerh Ice cover Phosphates
1.000 0.961 0.969 0.673 - 0.843 0.044
aT= Temperature. bS= Salinity.
T-winter 0.961 1.000 0.960 0.517 - 0.710 - 0.151
T-spring 0.969 0.960 1.000 0.630 - 0.825 - 0.002
S-summer 0.673 0.517 0.630 1.000 - 0.841 0.603
Ice cover -
0.843 0.710 0.825 0.841 1.000 - 0.469
Phosphates 0.044 - 0.151 - 0.002 0.603 - 0.469 1.000
90 100FACTOR I
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SUMMER SEA-SURFACE TEMPERATURE (°C)
Fig.7. General abundance trends for the three varimax assemblages related to summer sea-surface temperatures (dots = loadings for Factor 1, open triangles = loadings for Factor 2, black triangles = loadings for Factor 3).
TABLE VI Statistics of transfer function T-summer
Multiple correlation coefficienta Standard error of estimate Subantarctic South Weddell Open Ocean Subantarctic-South Weddell SubantarcticoOpen ocean South Weddell.Open ocean Subantarctic, squared South Weddell, squared Open ocean, squared Intercept
0.980 ± 1.1°C 31.288 38.620 - 38.819 - 31.220 31.175 36.782 2.904 - 21.873 - 11.529 - 19.186
Ice cover 0.945 ± 1.5 m./y. - 116.611 - 117.910 74.454 88.727 - 67.180 -82.077 52.206 64.669 18.576 65.565
PO4
0.943 ±0.2 mg-atP/1 17.889 17.160 - 11.880 - 13.323 9.544 9.574 - 8.590 - 8.101 - 2.819 - 8.938
'Adjusted for degrees of freedom.
The transfer function for this parameter s h o w s a s t a n d a r d e r r o r o f e s t i m a t e o f ___1.5 months/year and a multiple correlation coe f f i c i e n t o f 0.945 ( T a b l e VI). T h e e s t i m a t e d a g a i n s t m e a s u r e d a n n u a l s e a - i c e c o v e r is o n l y sensitive for total absence or a few days and, b e t w e e n 10 a n d 12 m o n t h s / y e a r of sea ice (Fig.10). The data base will be extended to increase this resolution.
Relationship between floral assemblages and sea-surface phosphate concentrations (January-March) F i g u r e 11 i l l u s t r a t e s t h e h i g h c o r r e l a t i o n s between the three varimax factors and the measured sea-surface phosphate concentrations for summer. The transfer function for this parameter has
9I STANDARD ERROR OF
STANDARD / ~
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J
ERROR OF 12--
/ ESTIMATE
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-
o - - MEASURED SUMMER SEA-SURFACE
Fig.8. Estimated versus measured temperature diagram.
2
TEMPERATURES
summer
6
MEASURED ANNUAL S E A - I C E
sea.surface
]
I J ,
J
8
10
12
COVER ( ' m o H t h s / y e ~ ' )
Fig.10. Estimated versus measured annual sea-ice cover diagram (14 samples represent total absence of measured estimated sea-ice cover).
Validity of the transfer functions
a multiple correlation of 0.943 and a standard error of estimate of +_0.2 mg-atP/1, i.e. 15% of the observed phosphate concentration range. The estimated versus measured concentrations using this transfer function are plotted in Fig.12. This equation allows estimations of one of the nutrient limiting factor for productivity.
The precision of these equations may be judged in part by scanning the multiple correlation coefficients and the standard errors of estimate (Table VI). The geographical analysis of residuals (difference between observed
100 -
/
80z H
4
/
60-
.J FACTOR
2
40-
20-
0
I 0
t 2
t
I
I
4 ANNUAL
1
I
6 SEA
ICE
I
I
8 COVER
I
10
I
l
12
(MONTHS/YEAR)
Fig.9. A b u n d a n c e of Factor 2 (South Weddell assemblage) versus annual sea-ice cover for the 28 surface sediment samples.
92 100-
i
FACTOR 1
/
~
-
•
//,
~8'
STANDARD ERROR OF
ESn.ATE
...
i, •'
or
0.5
,
,
~,
,
,
1.0 1.5 SUMMER SEA-SURFACE PHOSPHATE CONCENTRATIONS
~ &
~,
2.0 (mg-atP/l)
Fig.if. General abundance trends for the three varimax assemblages related to summer sea-surface phosphate concentrations (dots=loadings for Factor 1, open triangles = loadings for Factor 2, black triangles = loadings for Factor 3).
2,00 H
/!iiii:ii
S
~_1,00
i
,/: 1,00
.
.
.
.
,
,
,
MEASURED SUMMER SEA-SURFACE TEMPERATURES
Fig.13. Estimated versus measured summer sea-surface temperature diagram for the Holocene samples not used in the transfer function.
of the paleohydrological equation. This result indicates the reliability of the calculated summer temperatures ( _ 1.1°C).
S u m m a r y and c o n c l u s i o n s
,/;/
:
%J../
2,00
MEASURED PHOSPHATE CONCENTRATIONS
Fig.12. Estimated versus measured sea-surface phosphate concentration diagram.
and estimated parameters in the different zones) may be used as a test of accuracy if applied to samples which have not been used to establish the transfer function. Therefore, we have estimated summer temperatures for 34 other Holocene samples, eliminated from the data set to obtain an even geographic coverage. The scatter diagram of observed versus estimated summer SST is shown plotted in Fig.13. The distribution of errors is random in relation to geographic location. All the estimations are within the standard error of estimate
(1) Use of specific semiquantitative (ranked abundance) data is proposed to derive transfer functions from the phytoplankton distribution (diatoms and silicoflagellates) in the Southern Ocean. (2) Three floral assemblages have been defined and described in terms of regional oceanographic conditions. They exhibit succession of dominance along a latitudinal temperature gradient. (3) Paleoecological transfer functions are derived and indicate high correlations with various oceanographic parameters. The most highly reliable estimates are for summer sea surface temperatures. (4) A comparison between estimated temperatures for 34 South Ocean core tops not part of the data set with observed temperatures shows that these equations may be used with an accuracy better than +I.I°C. Paleotemperature calculations from fossil diatom assemblages covering the last climatic cycle are
93 r e a s o n a b l e ( P i c h o n , 1985; L a b e y r i e e t al., 1986; L a b r a c h e r i e et al., 1987).
Acknowledgments This work was supported by Programme National d'Etude de la Dynamique du Climat, Mission Scientifique des Terres Australes et Antarctiques Franqaises, Centre National de l a R e c h e r c h e S c i e n t i f i q u e , U n i v e r s i t ~ d e Bordeaux I and Institut G~ologique du Bassin d'Aquitaine. We gratefully acknowledge the generous contribution of samples from the Lamont-Doherty Geological Observatory core c o l l e c t i o n s , R. G e r s o n d e ( W e g e n e r I n s t i t u t ) a n d D . S . C a s s i d y o f t h e F S U A n t a r c t i c Res e a r c h F a c i l i t y . W e t h a n k P r o f . J. M o y e s , Drs. L . H . B u r c k l e , J . C . D u p l e s s y , A. P u j o s a n d J.L. Turon for much stimulating discussion a n d c o n s t a n t h e l p , M. F o n t u g n e a s c h i e f s c i e n t i s t o f t h e c r u i s e A P S A R A 2, a n d t h e Comm a n d i n g Officer o f t h e N a v a l P o l a r O c e a n ography Center (Washington) for providing t h e w e e k l y A n t a r c t i c S e a i c e c o v e r a g e s . Dr. C. Sancetta improved the manuscript. T h i s is C . F . R . c o n t r i b u t i o n no. 832.
Appendix -- Floral references Actinocyclus actinochilus (Ehrenberg) Simonsen. Hustedt
(1958), pp. 122 126, figs.57-80. Asteromphalus parvulus Karsten. Hustedt (1958), p. 128,
fig.91. Azpeitia (Coscinodiscus) tabularis (Grunow) Fryxell and
Sims (1986). Hustedt (1958), p. 119, figs.48-56. Chaetoceros sp.: Some species such as Chaetoceros atlanticure var. bulbosum Hargraves and others were lumped
together under this taxon. Cocconeis sp.: Both species Cocconeis costata Gregory and Cocconeis fasciolata (Ehrenberg) Brown were lumped together under this taxon. Coscinodiscus furcatus Karsten. Hustedt (1930), pp. 396-398, figs.207-208. Eucampia antarctica (Castracane) Mangin. Hendey (1937), pp. 285-286, pl. XIII, figs.8 10. Hemidiscus cuneiformis Wallich. Simonsen (1972), p. 267,, figs.7-11. Nitzschia angulata (O'Meara) Hasle. Hasle (1965), pp. 24 26, pl. 1, fig.6; pl. 9, figs.1 6. Nitzschia curta (Van Heurck) Hasle. Hasle (1965), pp. 32 33, pl. 12, figs.2 5.
cylindrus (Grunow) Hasle. Hasle (1965), pp. 34 37, pl. 12, figs.6-12. Nitzschia kerguelensis (O'Meara) Hasle. Hasle (1965),, pp. 14 18, pl. 4, figs.ll-18. Nitzschia ritscherii (Hustedt) Hasle. Hasle (1965), pp. 20-21, pl. 1, fig.20; pl. 4, figs.1 7. Nitzschia separanda (Hustedt) Hasle. Hasle (1965), pp. 26-27, pl. 2, figs.23 29. Nitzschia sublineata Hasle (Fragilaria sublinearis Van Heurck, Fragilariopsis sublinearis (Van Heurck) Heiden and Kolbe). Hasle (1965), pp. 27 30, pl. 7, fig.l; pl. 11, figs.1 10; pl. 12, fig.1. Odontella weisflogii (Janisch) Grunow. Van Heurck (1909), p. 39, pl. 10, figs.136-137. Porosira glacialis (Grunow) Jorgensen (1905). Hasle (1973), pp. 6-10, pl. 1, figs.2, 4, 5; pl. 3, figs.13-17; pl. 14, figs.19-25; pl. 15, figs.26-29. Rhizosolenia styliformis Brightwell. Hustedt (1930), pp. 584 588, fig.333. Roperia tesselata (Roper) Grunow. Hustedt (1930), pp. 523-524, fig.297. Schimperiella antarctica Karsten. Hendey (1937), p. 256. Thalassionema nitzschioides Grunow in Van Heurck. Hustedt (1959), p. 244, fig.725. Thalassiosira antarctica Comber. Hargraves (1968), p. 96, pl. 96 97, figs.182 184. Thalassiosira decipiens (Grunow) Jorgensen. Hustedt (1930), pp. 322-323, fig.158. Thalassiosira delicatula Hustedt (1958), p. 110, figs.8-10. Thalassiosira eccentrica (Ehrenberg) Cleve. Hustedt (1930), p. 388, fig.201. Thalassiosira gracilis (Karsten) Hustedt. Hustedt (1958), pp. 109 110, figs.4 7. Thalassiosira lentiginosa (Janisch) Fryxell. Hendey (1937), p. 248. Thalassiosira lineata Jouse. Simonsen (1974), p. 9, pl. 1, fig.6-7. Thalassiosira oestrupii Ostenfeld Proshkina-Lavrenko. Hustedt (1930), p. 318, fig.155. Thalassiosira tumida (Janisch) Hasle. Hasle, Heimda and Fryxell (1971), pp. 316-318. Thalassiothrix sp.: The two species Thalassiothrix antarctica and Thalassiothrix longissima were lumped together under this taxon. Nitzschia
SILICOFLAGELLATES Genus Dictyocha Ehrenberg 1839. Defelice and Wise (1981), p. 60, pl. II, fig.20. Genus Distephanus Stohr 1880. Defelice and Wise (1981), p. 60, pl. II, fig.19.
References Abbott, W. H., 1972. Vertical and lateral patterns of diatomaceous ooze found between Australia and Antarctica. Thesis. Dep. Geol., Univ. South Carolina, Columbia, 144 pp. Ackley, S. F., 1981. A review of sea ice weather relationships in the Southern Hemisphere. In: I. Allison (Editor),
94 Sea Level, Ice and Climate Change. IAHS Publ., 131: 127-159. Antarctic Ice Charts, 1975-1976 and 1977-1978. National Technical Information Service, Springfield, Va. Bunt, J. S. and Wood, E. J. F., 1963. Microalgae and antarctic sea ice. Nature,, 199: 1254-1255. Burckle, L. H., 1984. Diatom distribution and paleoceanographic reconstitution in the Southern Ocean - - Present and Last Glacial Maximum. Mar. Micropaleontol., 9: 241-261. Buynitsky, V. K., 1977. Organic life in sea ice. In: M . J . Dunbar (Editor), Polar Oceans. Arctic Institut of North America, pp. 301-306. Colborn, J. G., 1975. The Thermal Structure of the Indian Ocean. Univ. Hawaii Press, Honolulu, pp. 1-173. Deacon, G. E. R., 1937. The hydrology of the Southern Ocean. Discovery Rep. 15, 124 pp. Deacon, G. E. R., 1964. A discussion on the physical and biological changes across the Antarctic Convergence. Intro. Proc. R. Soc. Lond. Ser. A, 281: 1-6. Deacon, G. E. R., 1979. The Weddell Gyre. Deep-Sea Res., 26: 981--995. Defelice, D. R. and Wise,, S. W.,, 1981. Surface lithofacies, biofacies and diatom diversity patterns as models for delineation of climatic change in the Southeast Atlantic Ocean. Mar. Micropaleontol., 6:29 70. Fenner, J., Schrader, H. J. and Wienigk, H., 1976. Diatom phytoplankton studies in the Southern Ocean, composition and correlation to the Antarctic Convergence and its paleoecological significance. In: Initial Reports of the Deep Sea Drilling Project, leg 35. U.S. Government Printing Office, Washington, D.C., pp. 757-814. Foster, T. D. and Carmack, E. C., 1976. Frontal zone mixing and Antarctic Bottom Water formation in the Southern Weddell Sea. Deep-Sea Res., 23:30 317. Gordon, A. L., 1971. Oceanography of antarctic waters. In: J. L. Reid (Editor), Antarctic Oceanology I. Antarct. Res. Ser., 15: 196-203. Hargraves, P., 1968. Species composition and distribution of net plankton diatoms in the Pacific sector of the Antarctic Ocean. Thesis. Columbia Univ., New York, N.Y., 170 pp. Hart, T. J., 1942. Phytoplankton periodicity in antarctic surface waters. Discovery Rep., 21: 1-291. Hasle, G. R., 1969. An analysis of the phytoplankton of the Pacific Southern Ocean: abundance, composition and distribution during the Brategg Expedition 1947-1948. Hvalradets Skr. Sci. R. Mar. Biol. Res., 52: 6-168. Heiden, H. and Kolbe, R. W, 1928. Die marinen Diatomeen der Deutschen S(idpolar Expedition 1901 1903. Dtsch. S(idpolar Exped., 8:450 714. Hendey, N. I., 1937. The plankton diatoms of Southern seas. Discovery Rep., 16:154 364. Hustedt, F., 1930. Die kieselalgen Deutschlands, 0sterreichs und der Schweiz. In: L. Rabenhorst (Editor), Kryptogamen-flora yon Deutschland, 0sterreich und der Schweiz. Akademischer Verlagsgesellschaft, Leipzig, 7(1), pp. 1-920. Hustedt, F., 1958. Diatomeen aus der Antarktis und dem Sfidatlantik. Dtsch. Antarkt. Exped. 1938-1939, 2:103 191.
Imbrie, J. and Kipp, N. G., 1971. A new micropaleontological method for q ~ n t i t a t i v e paleoclimatology: application to a late l~leistocene Caribbean core. In: K. Turekian (Editor), Late Caenozoic Glacial Ages. Yale Univ. Press, New Haven, Conn., pp. 71-181. Imbrie, J., Van Donk, J. and Kipp, N. G., 1973. Paleoclimatic investigation of a Late Pleistocene Caribbean deepsea core: Comparison of isotopic and faunal methods. Quat. Res., 3: 10-38. Kipp, N. G., 1976. New transfer function for estimating past sea-surface conditions from sea-bed distribution of planktonic foraminiferal assemblages in the North Atlantic. In: R.M. Cline and J.D. Hays (Editors), Investigation of Late Quaternary Paleoceanography and Paleoclimatology. Geol. Soc. Am. Mem., 145:3 45. Kozlova, O. G., 1964. Diatoms of the Indian and Pacific sectors of the Antarctic. U.S. Dep. Comm. C.F.S.T.I., Springfield, TT 6651/54, pp. 191. Krebs, W. N., 1983. Ecology of neritic marine diatoms, Arthur Harbor, Antarctica. Micropaleontology, 29(3): 267 297. Labeyrie, L.D., Pichon, J.J., Labracherie, M.,, Ippolito, P.,, Duprat, J. and Duplessy,, J. C., 1986. Melting history of Antarctica during the past 60,000 years, nature,, 322: 701-706. Labracherie,, M., Pichon, J . J . and Labeyrie, L.D., 1987. R~ponse des diatom~es aux ~venements climatiques des 150,000 derni~res ann~es. Approche quantitative. Lozano, J.A. and Hays, J.D., 1976. The relationship of radiolarian assemblages to sediment types and physical oceanography in the Atlantic and Western Indian Ocean sectors of the Antarctic Ocean. In: R. M. Cline and J. D. Hays (Editors), Investigation of Late Quaternary Paleoceanography and Paleoclimatology. Geol. Soc. Am. mem., 145: 303-336. Morley,, J. J., 1979. A transfer function for estimating paleoceanographic conditions based on deep-sea surface sediment distribution of radiolarian assemblages in the South Atlantic. Quat. Res., 12: 381-395. Morley, J. J. and Hays, J. D., 1979. Comparison of Glacial or Interglacial oceanographic conditions in the South Atlantic from variations in calcium carbonate and radiolarian distribution. Quat. Res., 12: 396-408. Pichon, J. J., 1985, Les diatom~es traceurs de l'~volution climatique et hydrologique de l'Oc~an austral au cours du dernier cycle climatique. Thesis. Univ. de Bordeaux I, 279 pp. Pichon, J. J., 1986. Distribution des diatom~es dans les s~diments actuels des secteurs atlantique et ouestindien de l'Oc~an Austral: Interet hydrologique. In: M. Ricard (Editor), Eighth Int. Diatom Symp. Paris, August 27-September 1, 1984. Koeltz, Koenigstein, pp. 567-577. Riedel, W. R. and Sanfilipo, A., 1977. Cenozoic radiolaria. In: A. T. S. Ramsay (Editor), Oceanic Micropaleontology, Vol. II. Academic Press, London, pp. 847-912. Sachs, H. M., 1973a. North Pacific radiolarian assemblages and their relationship to oceanographic parameter. Quat. Res., 3:73 88.
95 Sachs, H. M., 1973b. Late Pleistocene history of the North Pacific: Evidence from a q u a n t i t a t i v e study of Radiolaria in Core V21-173. Quat. Res., 3: 89--98. Sancetta, C., 1979. Use of semiquantitative microfossil data for paleoceanography. Geology, 7:88 92. Simonsen, R., 1974. The diatom plankton of the Indian Ocean. Expedition of R/V "Meteor", 1964 1965. "Meteor" Forschungsereb., Reihe D., 19, pp. 66. Van Heurck, H.. 1909. Diatom~es R~sultats du voyage
References added in proof Burckle, L. H. and Cooke, D. W. 1983. Late Pleistocene Eucampia antarctica abundance stratigraphy in the Atlantic sector of the Southern Ocean. Micropaleontology, 29(1): 6 10. CLIMAP Project Members, 1976. The surface of the ice-age earth. Science, 191:1131 1137. CLIMAP Project Members, 1981. Seasonal reconstructions of the earth's surface at the last glacial maximum. In: (A. McIntyre, Leader) Map and Chart Ser. 36 (Text, Maps and Microfiche). Geol. Soc. Am. Deacon, G. E. R., 1983. Kerguelen, Antarctic and subam tarctic. Deep-Sea Res., 30(1): 77-81. Defant, A., 1961. Physical Oceanography. Pergamon, London, 729 pp. E1-Sayed, S. Z. and Taguchi, S., 1981. Primary production and standing crop of phytoplancton along the ice-edge in the Weddell Sea. Deep-Sea Res., 28A(9): 1017 1032. Gorshkov, S. G., 1978. World Ocean Atlas. Vol. 2 Atlantic and Indian Oceans. Pergamon, Oxford. Hasle, G. R., 1965. Nitzschia and Fragilariopsis species studied in the light and electron microscopes. III. The genus Fragilariopsis: Skr. Nor. Vidensk.-Akad. 1. Mat.Nat. Kl., 21:1 49. Hasle, G. R., 1973. The "mucilage pore" of pennate diatoms. Nova Hedwigia Beih., 45:167 186. Hasle, G. R., Heimdal, B. R. and Fryxell, G. A., 1971. Morphologic variability in fasciculated diatoms as exemplified by Thalassiosira tumida (Janisch) Hasle, comb. nov. Antarct. Res. Ser., 17 (Biol. Antarctic Seas IV): 313 333. Hays, J. D., Lozano, J. A., Schackleton, N. J. and Irving,
du S.Y. "Belgica" en 1897 1899. Rapp. Sci., Bot., pp. 3 126. Whitaker, T. M., 1977. Sea ice h a b i t a t s of Signy Island (South Orkneys) and their primary production. Adaptations within a n t a r c t i c ecosystem. In: G. Llano (Editor), Proc. Third SCAR Symp. Antarct. Biol., Smithsonian Inst., Washington, D.C., 1974, pp. 75-82. World Ocean Atlas Members, 1978. Atlantic and Indian Oceans. In: S. G. Gorshkov (Editor), World Ocean Atlas. Pergamon Press. Oxford.
G., 1976. Reconstruction of the Atlantic and West Indian Ocean sectors of the 18,000 years B.P. Antarctic Ocean. Geol. Soc. Am. Mere., 145: 337- 374. Howard, W. R. and Prell, W. L., 1984, A comparison of radiolarian and foraminiferal paleoecology in the southern Indian ocean: new evidence for the inerhemispheric timing of climatic change. Quat. Res., 21(2): 244 263. Hustedt, F., 1959. Die Kiselalgen Deutschland, Osterreich und der Schweiz u n t e r b e r h c k s i c h t i g u n g der hbrigen L/~nder Europas sowie der angrenzenden Meeresgebiete. In: Krytogamen-flora von Deutschland, Osterreich und der Scweiz. Akad. Verlagsgellschaft, Leipzig, 2(7): 1. Krebs, W. N., 1977. Ecology and preservation of neritic marine diatoms, A r t h u r Harbor, Antarctica. Thesis. Univ. Calif., Davis, 216 pp. Labracherie, M., Pichon, J. J. and Labeyrie, L. D., 1987. Les diatom~es et les ~v~nements climatiques des 150,000 derni~res ann~es. Application de fonctions de transfert aux diatom~es de l'oc~an Austral. Bull. Soc. Geol. Fr., 8 III(3): 103 114. Molfino, B , Kipp, N. G. and Morley, J. J., 1982. Comparison of foraminiferal, coccolithophorid, and radiolarian paleotemperature equations: assemblage coherency and estimate concordancy. Quat. Res., 17(3): 279 313. Simonsen, R., 1972. Uber die diatomeengattung Hemidiscus Wallich und andere angehSrigen der sogenannten Hemi. discaceae. Ver6fftl. Inst. Meeresforsch. Bremerhaven, 13: 265 273. Wyrtki, K., 1971. Oceanographic atlas of the International Indian Expedition. National Science Foundation, Washington, D.C., 53 pp.
79
TRANSFER FUNCTIONS BETWEEN DIATOM ASSEMBLAGES AND SURFACE HYDROLOGY IN THE SOUTHERN OCEAN J. J. P I C H O N 1, M. L A B R A C H E R I E 1, L. D. L A B E Y R I E 2 a n d J. D U P R A T 1 ~Dgpartement de Gdologie et Ocdanographie, Unitg C.N.R.S. n.197, Universitg de Bordeaux 1, Avenue des Facultds, 33405 Talence (France) 2Centre des Faibles Radioactivitgs, Laboratoire mixte C.N.R.S.-C.E.A., Domaine du C.N.R.S.-91190, Gif-sur-Yvette (France)
(Received March, 10, 1986; revised and accepted February 18, 1987)
Abstract Pichon, J. J., Labracherie, M., Labeyrie, L. D. and Duprat, J., 1987. Transfer functions between diatom assemblages and surface hydrology in the southern ocean. Palaeogeogr., Palaeoclimatol., Palaeoecol., 61:79 95. Interpretation of the distribution of 31 marine diatom species and two silicoflagellategenera in 28 core tops from the Atlantic and West Indian sectors of the Southern Ocean by Q-mode factor analysis allows definition of three significantly different floral assemblages, one associated with Subantarctic and two with Antarctic waters. These three areas are clearly delineated by prominent oceanographic limits: the Antarctic Convergence and the maximum retreat of sea.ice during the summer season. A set of palaeoecological transfer functions is derived by comparison between the distribution of these associations and surface water hydrological parameters. Using these equations, estimated summer temperatures from core top floral assemblages range from 12c'C in the north of the study area to -2.0°C near the southernmost cores. The standard errors of estimate are + 1.1°C for summer temperatures, + 1.5 months/year for sea ice cover, +0.2 mg-atP/1 for sea-surface phosphate concentrations. Numerical tests of these paleoecological equations demonstrate their usefulness to reconstruct the evolution of the surface oceanography of the Austral Ocean at different periods of the last climatic cycle.
Introduction Large scale surface water paleotemperature reconstructions (CLIMAP project members, 1976, 1981) h a v e b e e n efficient a t low a n d intermediate latitudes. They are based upon t r a n s f e r f u n c t i o n s b e t w e e n specific d i s t r i b u t i o n of p e l a g i c m i c r o f o s s i l s a n d m o d e r n s u r f a c e w a t e r t e m p e r a t u r e . D e f i n i t i o n of e q u i v a l e n t t r a n s f e r f u n c t i o n s to e s t i m a t e t h e c h a n g e s of t e m p e r a t u r e a n d ice c o v e r a g e of h i g h l a t i t u d e surface waters during the last climatic cycles is a l s o v e r y i m p o r t a n t . H o w e v e r , , p l a n k t o n i c f o r a m i n i f e r a c a n n o t be u s e d as t h e y a r e monospecific (Neogloboquadrina pachyderma, left c o i l i n g v a r i e t y ) in s u r f a c e w a t e r s w i t h t e m p e r a t u r e s b e l o w 6°C. T r a n s f e r f u n c t i o n s for
r a d i o l a r i a n s in t h e S o u t h e r n O c e a n w e r e used e x t e n s i v e l y d u r i n g t h e C L I M A P P r o j e c t by L o z a n o a n d H a y s (1976), M o r l e y (1979) a n d M o l f i n o e t a l . (1982). Y e t i n t e r p r e t a t i o n of t h e s e d a t a is difficult b e c a u s e t h e l i v i n g r a d i o l a r i a h a v e a wide d e p t h r a n g e . M a n y s p e c i e s a r e r e l a t e d to i n t e r m e d i a t e w a t e r h y d r o l o g y ( H o w a r d a n d P r e l l , 1984). As b o t t o m s e d i m e n t s in t h e A n t a r c t i c O c e a n c o n t a i n a b u n d a n t s i l i c e o u s v a l v e s of d i a t o m s , s i n g l e - c e l l e d p h o t o s y n t h e t i c a l g a e l i v i n g in surface waters, these microorganisms appear as a g o o d s u p p o r t for t h e d e f i n i t i o n of p a l e o h y drological transfer functions. Defelice and W i s e (1981) s t u d i e d d i a t o m s in s u r f a c e sedim e n t s from t h e s o u t h e a s t A t l a n t i c O c e a n a n d i d e n t i f i e d five d i s t i n c t a s s e m b l a g e s , t h r e e of
80 which could be related to the overlying water masses. Burckle (1984) used diatom assemblages and preservational data to reconstruct paleoceanographic conditions during the Last Glacial Maximum. But the relationship existing between the phytoplankton assemblages in recent sediment and the surface water hydrology has not yet been quantitatively established in the high southern latitude. This is the main purpose of the present study. In the first step, the sediment core tops are examined to define and delineate independent microfossil species assemblages; in a second step, these assemblages are correlated with modern climate-related parameters (summer sea-surface temperatures, annual ice cover and phosphate concentrations).
strongly interdependent with the prevailing meteorological and ice conditions. In the Atlantic sector, the general circulation is part of the large cyclonic Weddell gyre (Deacon, 1979; Foster and Carmack, 1976) which extends from the Antarctic Peninsula to approximately longitude 30°E and northward from Antarctica to between latitude 55-60°S. The sea ice surrounding Antarctica undergoes large seasonal changes in areal extent. In the Atlantic and the Western Indian sectors, it extends as far north as 54-55°S in the winter and retreats close to the continent in summer (Antarctic Ice Charts 1975-1976 and 19771978). In the Weddell Sea, pack ice persists into the summer period and accounts for the highest amount of inter-annual variability around Antarctica (Ackley, 1981).
Physical oceanographic setting Data The oceanography of the entire Southern Ocean is governed by the West Wind Drift resulting in the clockwise Antarctic Circumpolar Current (Deacon, 1964; Gordon,, 1971). The subantarctic water mass occupies the zone between the Subtropical Convergence (STC) and the Antarctic Polar Front (APF) where flow of the Circumpolar Current is maximum. In this region, surface salinities range between 34.0%0 and 34.6%0 and surface temperatures fluctuate from 4 to 10°C (Deacon, 1937; Colborn, 1975). The Antarctic Polar Front (APF) marks the southern transition from warm to cold waters. This front is usually broad on the order of 50 to 100 km, and is characterized by high biological productivity to the south (Burckle, 1984). Antarctic surface waters exhibit a southward decrease in salinity from 34 to 33.8%o and a temperature decrease from about 4 to 2°C across the APF. The band of easterly winds near Antarctica and the irregular shape of the continent and the bathymetric effects, together produce several large cyclonic divergent cells characterized by upwelling of subsurface water: the Antarctic Divergence. Both the position of this divergence and its intensity are variable, being
Floral data
A total of 116 samples from the tops of piston cores and trigger cores have been studied. These samples are located in the Atlantic and West Indian sectors of the Southern Ocean, between latitude 29°S and 77.25°S and longitude 30°W and 70°E. Sources of the material include Marion Dufresne cores from Centre des Faibles Radioactivit~s and Universit~ de Bordeaux I, Vema and Conrad cores from the Lamont-Doherty Core Repository, Islas Orcadas cores from the FSU Antarctic Research Facility, and R. Gersonde (Alfred Wegener Institute for Polar Research) allowed us to do preliminary floral analyses of four samples from his own collection in the Weddell Sea. Cleaning and preparation of diatom slides followed the procedure of Riedel and Sanfilippo (1977) to extract radiolaria but sieving was replaced by successive decantations. In the siliceous deposits of the Southern Ocean, the diatoms range from 3 to 140#m. Some species e.g. Nitzschia curta or Nitzschia cylindrus which are lightly silicified and very small (15 #m) need a long decantation in the seven successive washings of distilled water. After
81
twelve hours of decantations of the frustules, some smaller species were at-ill floating. Therefore we added a step of seven minutes centrifugation after each washing at 1200 r.p.m. This allows preservation of all valves while eliminating the clay which stays in suspension. The diatom species population of each sample was quantified by counting at least 300 specimens encountered in random traverses. Specimens representing more than half of a valve were counted as one; for pennate diatoms, each pole was counted as one half.
1977-1978 constructed by the U.S. Navy from satellite imagery and conventional data. The summer and winter averaged sea-surface temperatures and the mean maximum and minimum extension of sea ice are shown in Figs.1 and 2. The chart constituting phosphate concentrations (inorganic dissolved phosphorus: Fig.3) is derived from Gorshkov (1978); the data for the Weddell Sea samples were drawn on the basis of investigations for February and March 1977 by E1-Sayed and Taguchi (1981).
Data analysis
Oceanographic data Phytoplankton assemblages Oceanographic parameters were derived by interpolation from various monthly or seasonal maps and documents. Sea-surface temperature values were taken from Wyrtki (1971), Gorshkov (1978) and different cruise reports for the summer temperatures (Antiprod I, 1978; Apsara I, 1982 and Apsara II, 1984). The present-day annual ice cover was averaged by area using Antarctic Ice Charts 1975-1976 and
90 °
6 0°
3 0"
One of the most difficult problems encountered was selecting samples truly representative of modern sediments, samples in which the thanatocoenosis may be associated to the present biocoenosis. From the 116 samples studied, 55 were rejected for several reasons: Seven of them contained extinct diatom species (Actinocyclus ingens, Hemidiscus karstenii), or
0°
30"
60"
90 °
40 °
Fig.1. M e a n s u m m e r sea-surface temperature and m i n i m u m sea-ice cover. The data for the Antarctic Polar Front are from Defant (1961) and Deacon (1983). Dots indicate position of surface sediment samples.
82
6 O"
90 °
WINTER o.r
.~
Cores used i n
90°
.
.
"'.".',"."I
0°
~
30'
'
60"
."".'"" 1~
;
90 °
,
/i T.: : /i Y:
.o
surf. . . . . diment
3 0°
6°°
.~: ~i".",'":'.'"'.".":":.".'"'.".' :':':":.".".'"':~ ~'..'.".:.'..'..'.'..'..'..'.'.."..""." ".,.'-.'.."" ~ . ~
'c
"~"
-~
0°
30"
"~:::~: ~g
60 °
90°
Fig.2. Mean winter sea-surface temperature and maximum sea-ice cover. The data for the Antarctic Polar F r o n t are from Defant (1961) and Deacon (1983). Dots indicate position of surface sediment samples.
~°
6 O"
3 0°
0°
30 °
60 °
90 °
,0 °
50 ~
Fig.3. Mean summer sea-surface phosphate concentrations (inorganic dissolved phosphates). Based on Gorshkov (1978). Dots indicate position of surface sediment samples.
83 TABLE I Positions and water depths of 28.~iston and trigger-top cores used in this study. Ship designations: MD = Marion Dufresne; RC = Robert Conrad; V = Vema; 1176 and 1277= Islas Orcadas; 1010 1223= Polarstern Core
Latitude
MD82 424 54°05.8'S MD82 436 6F'13.6'S MD80 301 54~00.0'S MD80 303 60~00.0'S MD80 304 5F04.0'S MD73 026 44~59.0'S MD24 KK63 5 F 5 6 . 0 ' S RCll-78 5ff 52.0'S RCll-80 46°45.0'S RC13-255 50°34.0'S RC13-263 53~'48.3'S RC13.273 55~04.5'S RC13- 274 53°09.0'S RC15.91 49°55.3'S
Longitude
Water depth (m)
Core
Latitude
Longitude
Water depth (m)
0°20.7'W 19°28.8'W 66°50.0'E 66°31.0'E 67°44.0'E 53°17.0'E 42°53.0'E 9°52.0'W 0°03.0'W 2°53.7'E 8°13.0'W 11°34.5'E 12°25.6'E 15°34.1'W
2350 3620 4540 4533 1950 3429 2550 3115 3656 3332 3389 4967 3372 3775
RC17-56 V16-60 V16-65 1176 82 1176 86 1176 88 1176 91 1277 2 1277 28 1277 41 1010 1192 1212 1223
65°24.0'S 49°59.5'S 45°00.0'S 49°31.2'S 48°02.6'S 46°57.8'S 44°56.7'S 45"02.1'S 61°28.0'S 69°59.9'S 77°20.0'S 77"25.0'S 75¢~30.0'S 76°00.0'S
37°43.0'E 36°45.5'E 45°46.0'E 13°11.5'E 13°49.0'E 14°18.2'E 15°02.9'E 22°28.2'E 9°ll.0'E 5°04.6'W 35°00.0'W 39°00.0'W 57~00.0'W 33°30.0'W
4794 4574 1618 4100 4338 5106 4649 4800 5322 1873 476 842 431 772
r a d i o l a r i a n species (Stylatractus universus). Others presented an a n o m a l o u s l y high perc e n t a g e of diatoms or r a d i o l a r i a clearly associated with the last glacial m a x i m u m (Eucampia antarctica - - B u r c k l e and Cooke, 1983; Cycladophora davisiana - - H a y s et al., 1976). At the opposite extreme, some o t h e r s had too m u c h c a l c i u m c a r b o n a t e for the latitude and w a t e r depth ' ( L o z a n o and Hays, 1976). We also rejected samples w h e r e the diatoms were largely dissolved or presented c o n s i d e r a b l e breakage. After f a c t o r analysis, we excluded the samples with low communalities. A total of 64 core-top samples were considered to r e p r e s e n t H o l o c e n e sedimentation. However, these samples were not homog e n e o u s l y d i s t r i b u t e d over the h y d r o g r a p h i c gradients: in p a r t i c u l a r , t h e s o u t h e r n limit of the p o l a r front was over-represented with 45 samples. To define an u n b i a s e d sample of the sediment p o p u l a t i o n as evenly d i s t r i b u t e d as possible, we kept only 28 samples (Table I) located a l o n g four b r o a d l y defined l a t i t u d i n a l t r a n s e c t s in the A t l a n t i c and West I n d i a n sectors of the S o u t h e r n O c e a n (Figs.l-3).
Statistical treatment The phytoplankton population of the S o u t h e r n Ocean is dominated by a few species, present in all the core tops, w h a t e v e r the surface w a t e r hydrology. A Q-mode factor analysis of the relative a b u n d a n c e of the different species in the 28 samples indicates the excessive role to the a b u n d a n c e of two domin a n t species: Nitzschia kerguelensis and Thalassiosira lentiginosa. Other species are associated in m u c h lower abundances. This ill-balance limits the possibility of i n t e r p r e t a t i o n of the floral changes in terms of surface w a t e r hydrology. To increase the weight of s e c o n d a r y species sensitive to local surface w a t e r hydrology, we have made several modifications to the data base: (1.) Of the 57 siliceous p h y t o p l a n k t o n species (55 diatom species and 2 silicoflagellate genera) identified in our set of samples, 24 have been eliminated because they are ubiquitous and in low relative abundance. (2) Several low relative a b u n d a n c e species are very specific to p a r t i c u l a r surface w a t e r hydrological conditions, for example: Hemidiscus cuneiformis,
84 TABLE II Conversion of each species percentage into abundance rank Species
Actinocyclus (charcotia) actinochilus Asteromphalus parvulus Azpeitia tabularis Chaetoceros sp. Cocconeis sp. Coscinodiscus furcatus Eucampia antarctica Hemidiscus cuneiformis Nitzschia angulata N. curia IV. cylindrus N. kerguelensis N. ritscherii IV. separanda N. sublineata Odontella weisflogii Porosira glacialis Rhizosolenia styliformis Roperia tesselata Schimperiella antarctica Thalassionema nitzschioides Thalassiosira antarctica T. decipiens T. delicatula T. eccentrica T. gracilis T. lentiginosa T. lineata T. oestrupii T. tumida Thalassiothrix sp.
Rank value 0
1
2
3
(in %)
(in %)
(in %)
(in %)
0
1
2-6
7-13
0
1
2
3
0 0 0
1 1 1
2 2 2-4
3-5 3 5-9
0
1
1
2-24
25-50
0 0
1 1
2
0 0
1 1
0 0
1 1
2
0 0 0 0
1 1 1 1
2-15 2-7 2-3 2
0 0
1 1
2 2
0 0
1 1
2-4 2-24
5 7 25-50
0 0 0
1 1 1
0
1
2-4 2-18
5 7 19-39
0 0 0
1 1 1
2
3
0
1
2
3
0 0
1 1
2 2
3 3
2-12 2-7 2-39
3
13-26 8-16 40-81 3
16-32 8-19 4 8 3-6 3 6 3
SILICOFLAGELLATES Dictyocha Distephanus
R o p e r i a t e s s a l a t a and the silicoflagellate genus D i c t y o c h a are only f o u n d n o r t h of the APF, and
a l w a y s in low p e r c e n t a g e s (Abbott, 1972; Defelice and Wise, 1981; Pichon, 1986). P r e s e n c e even in low a b u n d a n c e of these species is an i m p o r t a n t p a r a m e t e r . Therefore, we h a v e replaced the relative p e r c e n t a g e s of e a c h of these 33 species by a r a n k i n g in four a b u n d a n c e classes defined for e a c h species (Table II). We h a v e a r b i t r a r i l y s e p a r a t e d t o t a l absence ( r a n k
0), a n d t r a c e s or a b u n d a n c e less t h a n 2% ( r a n k 1), with the two last r a n k s c o v e r i n g the r e m a i n i n g r a n g e of the d i s t r i b u t i o n ( r a n k 2=2% - h a l f of the m a x i m u m a b u n d a n c e for e a c h species, r a n k 3 = m o r e t h a n h a l f of the m a x i m u m a b u n d a n c e for each species). This scale gives the same v a l u e for a species g o i n g from 5% relative a b u n d a n c e to its m a x i m u m of 7% (Thalassionema n i t z s c h i o i d e s ) as for a species t h a t goes from 40% to 81% ( N . k e r g u e l -
85
ensis). For the interpretation of the data we have to take into account that the resulting stochastic error within each rank is larger for the species of low relative abundance. The data base once modified was treated by the Q-mode factor analysis of Imbrie and Kipp (1971). Floral assemblages and their relationship to surface oceanography A factor analysis of 31 diatom species and 2 silieoflagellate genera from the 28 surface sediment samples produces three varimax factors which account for 86% of the distributional variance of the species data base. The varimax factor matrix (Table III) gives the TABLEIII V a r i m a x f a c t o r m a t r i x (28 s a m p l e s ) Communality
Assemblages 1
1 1277-2 2 RCl1-80 3 1176-91 4 MD73026 5 V16-65 6 1176-88 7 V16-60 8 RC15-91 9 1176-86 10 R C 1 3 - 2 5 5 11 1176-82 12 M D 8 0 3 0 4 13 R C l l - 7 8 14 M D 2 4 - K K 6 3 15 R C 1 3 - 2 7 4 16 M D 8 0 3 0 1 17 R C 1 3 - 2 7 3 18 R C 1 3 - 2 6 3 19 M D 8 2 4 2 4 20 M D 8 0 3 0 3 21 1277-28 22 R C 1 7 - 5 6 23 M D 8 2 4 3 6 24 1277-41 25 1010-W 26 1192-W 27 1212-W 28 1223-W
Variance
0.795 0.909 0.803 0.948 0.832 0.926 0.869 0.839 0.793 0.879 0.872 0.895 0.884 0.871 0.878 0.821 0.827 0.874 0.904 0.899 0.889 0.659 0.831 0.812 0.884 0.895 0.935 0.904
2
0.863 0.894 0.885 0.889 0.871 0.876 0.792 0.773 0.729 0.774 0.443 0.504 0.697 0.549 0.286 0.337 0.327 0.326 0.358 0.384 0.238 0.160 0.371 0.138 0.048 0.073 0.080 0.089
0.079 0.166 0.027 0.086 0.114 0.216 0.088 0.118 0.045 0.144 0.021 0.085 0.161 0.021 0.337 0.216 0.247 0.245 0.207 0.436 0.388 0.587 0.547 0.816 0.920 0.939 0.958 0.904
- 0.212 - 0.286 -0.139 - 0.387 -0.245 - 0.334 - 0.484 --0.478 - 0.510 -0.509 -0.821 - 0.796 - 0.593 - 0.755 - 0.826 - 0.813 - 0.812 - 0.841 - 0.857 - 0.749 - 0.826 - 0.537 - 0.628 - 0.358 - 0.074 - 0.092 - 0.104 - 0.281
32.788
20.062
33.148
composition of each sample in terms of the three varimax factors. The factor score matrix (Table IV) represents the relative contribution of each species to each factor. The communality, an index of how much of the compositional information in the original sample is accounted for by the varimax model is greater than 0.79 in all the samples, with the exception of two samples located near the Antarctic continent in the Western Indian sector. Interpretation of the analysis is facilitated because each of the three varimax factors is clearly associated with a specific hydrographic province. We have named the factors according to these zones.
Factor 1 (Subantarctic assemblage) This factor has an individual variance of 32.8%. It is dominated by Thalassionema nitzschioides with a varimax factor score of 0.50, Azpeitia tabularis (0.49), Thalassiosira lentiginosa (0.31) and the silicoflagellate genus Dictyocha (0.25). The dominant T. nitzschioides and A. tabularis species have been described as species living in the waters of the subantarctic region by Heiden and Kolbe (1928), Hustedt (1930), Hargraves (1968), Fenner et al. (1976). The abundance of A. tabularis remains very high in the plankton of the APF zone (Fenner et al., 1976). In sediments, Abbott (1972) finds these two species in his subtropical assemblage, Defelice and Wise (1981) also consider these species as characteristic of the sediments north of the APF. Burckle (1984) finds them in his factor 3 which overlaps the APF except in the Atlantic sector, in which T. lentiginosa is prevalent. In fact, this last species is found to be also present in antarctic and subantarctic waters where it is better represented (Hendey, 1937; Hustedt, 1958; Hargraves, 1968; Hasle, 1969). Among the associated species, H. cuneiformis and R. tesselata are present only in this assemblage; they have been described as tropical subtropical to transitional species by Hargraves (1968), Hasle (1969), Simonsen (1974), Fenner et al. (1976), Defelice and Wise (1981). Among the silicoflagellates, no Dictyocha specimen was found south of APF.
86 TABLE IV Varimax factor score matrix Species
Assemblages
1 Actinocyclus actinochilus Asteromphalus parvulus Azpeitia tabularis Chaetoceros sp. Cocconeis sp. Coscinodiscus furcatus Eucampia antarctica Hemidiscus cuneiformis Nitzschia angulata N. curta N. cylindrus N. kerguelensis N. ritscherii N. separanda N. sublineata Odontella weisflogii Porosira glacialis Rhizosolenia styliformis Roperia tesselata Schimperiella antarctica Thalassionema nitzschioides Thalassiosira antarctica T. decipiens T. delicatula T. eccentrica T. gracilis T. lentiginosa T. lineata T. oestrupii T. tumida Thalassiothrix sp. Dictyocha Distephanus
0.059 0.014 0.486 0.001 0.000 0.177 0.292 0.042 0.082 - 0.021 - 0.038 0.193 0.230 0.088 0.068 - 0.002 - 0.007 0.118 0.126 0.091 0.497 0.004 0.191 0.070 0.134 0.039 0.309 0.158 0.150 -0.014 - 0.031 0.251 0.148 -
-
-
-
2 0.468 - 0.021 - 0.055 0.003 0.000 0.004 0.503 0.002 0.057 0.378 0.329 - 0.031 - 0.025 - 0.066 0.494 0.032 0.077 0.005 0.006 - 0.001 - 0.013 0.064 - 0.007 0.002 0.004 - 0.073 0.054 0.008 0.004 0.018 - 0.064 0.022 0.010
3 -
0.067 0.017 0.010 0.058 0.000 0.081 - 0.016 0.027 - 0.141 0.055 0.017 - 0.507 0.041 - 0.165 - 0.121 0.006 0.011 - 0.050 0.081 - 0.148 0.187 0.026 0.069 0.025 0.072 - 0.373 -0.204 0.087 0.074 -0.039 - 0.530 0.151 - 0.232
T h i s a s s e m b l a g e p r e s e n t s it s m a x i m u m i n t h e a r e a n o r t h o f t h e A P F (Fig.4), w h e r e t h e temperatures of the surface waters range from
b l a g e is c h a r a c t e r i z e d b y five d i a t o m s p e c i e s :
12 t o 5°C i n s u m m e r , a n d f r o m 7 t o 2°C i n w i n t e r . T h e s a l i n i t i e s r a n g e f r o m 35%0 t o 34%0. T h e 0.75 f a c t o r l o a d i n g l i m i t c o r r e s p o n d s t o t h e actual position of the APF.
(0.32). A l l o f t h e s e s p e c i e s h a v e b e e n f o u n d t o be a s s o c i a t e d w i t h s e a - i c e ( V a n H e u r c k , 1909; H e n d e y , 1937; H a r t , 1942; K o z l o v a , 1964; B u n t a n d W o o d , 1963; B u y n i t s k y , 1977; K r e b s , 1977 a n d 1983; W h i t a k e r , 1977). B u r c k l e (1984) d e s c r i b e s t h i s a s s e m b l a g e n e a r t h e R o s s iceshelf, i n t h e P a c i f i c s e c t o r o f t h e S o u t h e r n O c e a n , and he a s s u m e s its o c c u r r e n c e in the Weddell Sea. Thus, this a s s e m b l a g e c a n be c o n n e c t e d w i t h
Factor 2 (South WeddeU assemblage) T h i s is l o c a t e d i n t h e s o u t h e r n study area, in the Weddell Sea A n t a r c t i c c o n t i n e n t (Fig.5). I t 20% of the total v a r i a n c e . T h e
portion of the and near the accounts for floral assem-
E. a n t a r c t i c a (0.50), N . s u b l i n e a t a (0.49), A . actin o c h i l u s (0.47), N . c u r t a (0.38), N . c y l i n d r u s
87 90 °
6
3 0°
O"
0°
30"
90°
60"
I
~
'1
1
(32.8 % )
> 0.75
0
°
imUiUI!!iiIEimm!iiiilli ii
P I:A(:TOR
4
60 °
#@
0.74-0.40 0.39-0.20 0.19-0.10
~
,o.,o =:~.E~':&:iii!iiiii~iiii!ii![i:fi 9o° ' ' . l ~ i i i ! i i iii i i i i ! i l L / ,
,,----o°
_
. 3o"
o 6o
~ 'm°
Fig.4. Distribution of subantarctic assemblage in surface sediments. Values plotted are those given in varimax matrix (Table III).
90 °
3 0 °
6 0 °
0°
30"
............................. !H::: • .i ' . ~ , ,i i i.i i i llmllll~
~ao,~.)
• " " " " ""-'-
.
. > 0.75 . .
. ~.
.
.
-:':"
:', ..............
. . . . . . . . . l. l.l.l.[.l .l .l l.l.[.l.J. l.l.l.].l .l . . . . . . . . . ., ..... ,,., ...... .,, llllllllll]lllJlll[ lllllllllll i[iii -~"
. i !.", .
60"
"-"
::
i~',:ii:
:: .l l.l.l.m l l ,l l, l,l ~ , _ , v . ~ . - . r..:,-:,.:, • ~i~ i: i i I [ ~ em%l i i '~*'="" "~" ""
:::: :': : ' : ' : ' : ' : ' : ' : ' : : ' V. . .v- .V: ."- .v- ..-". :v. :.-'. ". _ . : . : . o
I i i I iml ml41~iml$1 •
90 °
60o
;iiiii:iiiiiii:..
:.-:~i:,x~ ~'..:,:'"
I I • • • • ill e •
* * • ,,.,...,...**.
0.39-0.20
%',',%%
,lell%l /
0.,-0.,°
n,...o
ii ~:
' "~'~'~:i:
~'
0 °
30"
60 °
"~"
Fig.5. Distribution of South Weddell assemblage in surface sediments. Values plotted are those given in varimax matrix (Table III).
88 Hargraves, 1968; Hasle, 1969; Abbott, 1972; Fenner et al., 1976). Highest loadings for this factor are located between latitude 50°S and 55°S, and perfectly outline the well-preserved diatom ooze belt (Burckle, 1984). The geographical distribution of this assemblage is well defined between the APF north and the average limit of the maximum extension of the spring sea-ice, south (Fig.6). Factor 3 corresponds to the zone of medium salinity (33.80%0) and generally lower than 5°C sea-surface temperatures zones. It should be noted that the transition between the Subantarctic assemblage (factor 1) and the Open-ocean antarctic assemblage (factor 3) coincides with the main position of the APF, and that the South Weddell assemblage (factor 2) indicates the maximum retreat limit of sea-ice in the summer. Based upon the predominant variations between these three assemblages, these correlations may allow one to retrace the motions of the sea-ice, and the changes in the position of the APF.
a particular environment which, being ice-free during a very short period of the year (an average of 2 months), would only allow a quick development of some species in melting waters (E. antarctica), or within sea-ice (A. actinochilus, N. curta, N. cylindrus, N. sublineata). This South Weddell assemblage corresponds to a long annual sea ice cover.
Factor 3 (Open-ocean Antarctic assemblage) This has an individual variance of 33.1%. It is essentially determined by Thalassiothrix sp. (0.53) and N. kerguelensis (0.51) and to a lesser degree by Thalassiosira gracilis (0.37) and the silicoflagellate genus Distephanus (0.23). Among the accompanying species, the ones best represented are: Asteromphalus parvulus (0.17), Nitzschia separanda (0.16) and Schimperiella antarctica (0.14). The main species which define this assemblage are very common in waters and sediments of the subantarctic and antarctic zones (Heiden and Kolbe, 1928; Hendey, 1937; Hustedt, 1958; Kozlova, 1964;
~°
6 O"
3 0°
0°
_ _
.
33., .J. J
-
: <
i
.
.
90 °
.
,o°
.....~
, I J ....... :.: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ] l ~ n ~'~¢i~:::~:::::::::::::::[:[:~:~:;:[~::~:::::[:~z~:::~`~~
'
6#
• q~a" "m" • • • • • • m ' a w o • i l l lelelammlll '
( Iliilllllllliolll
I
.i.m m,'j'mlml.%'a-j'."u'~'j%~. ! , • • • • .
.
.
.
.
•
.
N
90°
.
h . • ===================================================================
0.75
o , , _ o.,o
.
60"
",:,:,', ::;:i i • ========================================================================================
0.74-0.40
~
30'
,~
'12. . . . .
P
ii[ii::: ..... ,;~
..s
. . . .
.
°°
3o"
60 °
~*
Fig.6. Distribution of "Open ocean" assemblage in surface sediments. Values plotted are those given in varimax matrix (Table III).
89
Surface hydrology
Relationship between floral assemblages and summer sea-surface temperatures
We h a v e tested the c o r r e l a t i o n s b e t w e e n the v a r i a t i o n s of six surface h y d r o l o g i c a l parameters: surface t e m p e r a t u r e s for summer, w i n t e r and spring, s u m m e r sea surface salinity, annual ice c o v e r and sea surface p h o s p h a t e c o n c e n t r a t i o n s for s u m m e r (Table V). The corr e l a t i o n coefficient r a n g e s from a high of 0.969 (summer versus w i n t e r surface t e m p e r a t u r e s ) to a low of - 0 . 0 0 2 (spring t e m p e r a t u r e s versus surface p h o s p h a t e c o n c e n t r a t i o n s ) . The surface t e m p e r a t u r e s form a g r o u p within w h i c h c o r r e l a t i o n s are high and related to the present h y d r o l o g y . The t r a n s f e r f u n c t i o n s t h e r e f o r e do not allow i n d e p e n d e n t e s t i m a t i o n s of summer, w i n t e r and spring t e m p e r a t u r e s . S u m m e r surface salinity, a n n u a l ice c o v e r and p h o s p h a t e c o n c e n t r a t i o n s show lower i n t e r c o r r e l a t i o n s . The d a t a base appears therefore specially well a d a p t e d for i n d e p e n d e n t e s t i m a t i o n s of summer t e m p e r a t u r e s , a n n u a l ice cover and phosphate concentrations.
Calibration of transfer function
The loadings of the t h r e e individual factors are plotted a g a i n s t observed s u m m e r seasurface t e m p e r a t u r e s (Fig.7). They exhibit an orderly succession of d o m i n a n c e between - 2 c C and 12°C: from " S o u t h Weddell" to " O p e n - o c e a n " and finally " S u b a n t a r c t i c " assemblages. The t r a n s f e r f u n c t i o n for the s u m m e r season presents a low s t a n d a r d e r r o r of estimate of __ 1.1~C and has a multiple c o r r e l a t i o n coefficient of 0.980 (Table VI). To m e a s u r e the robustness of the t r a n s f e r function, we h a v e plotted estimated a g a i n s t m e a s u r e d s u m m e r sea-surface t e m p e r a t u r e s for each of the 28 surface samples used for the regression. The dispersion of these v a r i o u s samples in the s c a t t e r d i a g r a m (Fig.8) is h o m o g e n e o u s . It implies t h a t the t r a n s f e r f u n c t i o n produces t e m p e r a t u r e estimates with similar reliability in all regions of the study area.
Relationship between floral assemblages and annual ice cover
We have used the multiple c u r v i l i n e a r regression m e t h o d of Imbrie and Kipp (1971) to q u a n t i f y the r e l a t i o n s h i p s between the dist r i b u t i o n s of the floral assemblages and s u m m e r t e m p e r a t u r e s , a n n u a l ice c o v e r and s u m m e r p h o s p h a t e c o n c e n t r a t i o n s of surface waters.
The plotted statistical weights a g a i n s t present day a n n u a l ice c o v e r in each sample show an obvious c o r r e l a t i o n for the S o u t h Weddell assemblage (Fig.9), a l t h o u g h samples are lacking for ice c o v e r b e t w e e n a few days and eight months/year.
TABLE V Correlation coefficients between all pairs of oceanographic parameters T-summer T-summer~ T-winter T-spring S-summerh Ice cover Phosphates
1.000 0.961 0.969 0.673 - 0.843 0.044
aT= Temperature. bS= Salinity.
T-winter 0.961 1.000 0.960 0.517 - 0.710 - 0.151
T-spring 0.969 0.960 1.000 0.630 - 0.825 - 0.002
S-summer 0.673 0.517 0.630 1.000 - 0.841 0.603
Ice cover -
0.843 0.710 0.825 0.841 1.000 - 0.469
Phosphates 0.044 - 0.151 - 0.002 0.603 - 0.469 1.000
90 100FACTOR I
z Z
80-
FACTOR 2 . / •
0
I
-2"
•
~ " ~ - - L _
1
•• "~ , , .
•
I
O"
FACTOR 3
r
I
i
2"
4"
,•
I
I
•
~•
I
I
6°
. J k
•
\
5
8"
,,
ZX ~
I
&
l
10"
12"
SUMMER SEA-SURFACE TEMPERATURE (°C)
Fig.7. General abundance trends for the three varimax assemblages related to summer sea-surface temperatures (dots = loadings for Factor 1, open triangles = loadings for Factor 2, black triangles = loadings for Factor 3).
TABLE VI Statistics of transfer function T-summer
Multiple correlation coefficienta Standard error of estimate Subantarctic South Weddell Open Ocean Subantarctic-South Weddell SubantarcticoOpen ocean South Weddell.Open ocean Subantarctic, squared South Weddell, squared Open ocean, squared Intercept
0.980 ± 1.1°C 31.288 38.620 - 38.819 - 31.220 31.175 36.782 2.904 - 21.873 - 11.529 - 19.186
Ice cover 0.945 ± 1.5 m./y. - 116.611 - 117.910 74.454 88.727 - 67.180 -82.077 52.206 64.669 18.576 65.565
PO4
0.943 ±0.2 mg-atP/1 17.889 17.160 - 11.880 - 13.323 9.544 9.574 - 8.590 - 8.101 - 2.819 - 8.938
'Adjusted for degrees of freedom.
The transfer function for this parameter s h o w s a s t a n d a r d e r r o r o f e s t i m a t e o f ___1.5 months/year and a multiple correlation coe f f i c i e n t o f 0.945 ( T a b l e VI). T h e e s t i m a t e d a g a i n s t m e a s u r e d a n n u a l s e a - i c e c o v e r is o n l y sensitive for total absence or a few days and, b e t w e e n 10 a n d 12 m o n t h s / y e a r of sea ice (Fig.10). The data base will be extended to increase this resolution.
Relationship between floral assemblages and sea-surface phosphate concentrations (January-March) F i g u r e 11 i l l u s t r a t e s t h e h i g h c o r r e l a t i o n s between the three varimax factors and the measured sea-surface phosphate concentrations for summer. The transfer function for this parameter has
9I STANDARD ERROR OF
STANDARD / ~
12 --
J
ERROR OF 12--
/ ESTIMATE
Q:
1CI
~J
uJ u
lO
8~
a: D LU
~
6
~
4
4° m
E
m
~- 2'
/ 2o
4o
6°
8o
10 °
12 °
-
o - - MEASURED SUMMER SEA-SURFACE
Fig.8. Estimated versus measured temperature diagram.
2
TEMPERATURES
summer
6
MEASURED ANNUAL S E A - I C E
sea.surface
]
I J ,
J
8
10
12
COVER ( ' m o H t h s / y e ~ ' )
Fig.10. Estimated versus measured annual sea-ice cover diagram (14 samples represent total absence of measured estimated sea-ice cover).
Validity of the transfer functions
a multiple correlation of 0.943 and a standard error of estimate of +_0.2 mg-atP/1, i.e. 15% of the observed phosphate concentration range. The estimated versus measured concentrations using this transfer function are plotted in Fig.12. This equation allows estimations of one of the nutrient limiting factor for productivity.
The precision of these equations may be judged in part by scanning the multiple correlation coefficients and the standard errors of estimate (Table VI). The geographical analysis of residuals (difference between observed
100 -
/
80z H
4
/
60-
.J FACTOR
2
40-
20-
0
I 0
t 2
t
I
I
4 ANNUAL
1
I
6 SEA
ICE
I
I
8 COVER
I
10
I
l
12
(MONTHS/YEAR)
Fig.9. A b u n d a n c e of Factor 2 (South Weddell assemblage) versus annual sea-ice cover for the 28 surface sediment samples.
92 100-
i
FACTOR 1
/
~
-
•
//,
~8'
STANDARD ERROR OF
ESn.ATE
...
i, •'
or
0.5
,
,
~,
,
,
1.0 1.5 SUMMER SEA-SURFACE PHOSPHATE CONCENTRATIONS
~ &
~,
2.0 (mg-atP/l)
Fig.if. General abundance trends for the three varimax assemblages related to summer sea-surface phosphate concentrations (dots=loadings for Factor 1, open triangles = loadings for Factor 2, black triangles = loadings for Factor 3).
2,00 H
/!iiii:ii
S
~_1,00
i
,/: 1,00
.
.
.
.
,
,
,
MEASURED SUMMER SEA-SURFACE TEMPERATURES
Fig.13. Estimated versus measured summer sea-surface temperature diagram for the Holocene samples not used in the transfer function.
of the paleohydrological equation. This result indicates the reliability of the calculated summer temperatures ( _ 1.1°C).
S u m m a r y and c o n c l u s i o n s
,/;/
:
%J../
2,00
MEASURED PHOSPHATE CONCENTRATIONS
Fig.12. Estimated versus measured sea-surface phosphate concentration diagram.
and estimated parameters in the different zones) may be used as a test of accuracy if applied to samples which have not been used to establish the transfer function. Therefore, we have estimated summer temperatures for 34 other Holocene samples, eliminated from the data set to obtain an even geographic coverage. The scatter diagram of observed versus estimated summer SST is shown plotted in Fig.13. The distribution of errors is random in relation to geographic location. All the estimations are within the standard error of estimate
(1) Use of specific semiquantitative (ranked abundance) data is proposed to derive transfer functions from the phytoplankton distribution (diatoms and silicoflagellates) in the Southern Ocean. (2) Three floral assemblages have been defined and described in terms of regional oceanographic conditions. They exhibit succession of dominance along a latitudinal temperature gradient. (3) Paleoecological transfer functions are derived and indicate high correlations with various oceanographic parameters. The most highly reliable estimates are for summer sea surface temperatures. (4) A comparison between estimated temperatures for 34 South Ocean core tops not part of the data set with observed temperatures shows that these equations may be used with an accuracy better than +I.I°C. Paleotemperature calculations from fossil diatom assemblages covering the last climatic cycle are
93 r e a s o n a b l e ( P i c h o n , 1985; L a b e y r i e e t al., 1986; L a b r a c h e r i e et al., 1987).
Acknowledgments This work was supported by Programme National d'Etude de la Dynamique du Climat, Mission Scientifique des Terres Australes et Antarctiques Franqaises, Centre National de l a R e c h e r c h e S c i e n t i f i q u e , U n i v e r s i t ~ d e Bordeaux I and Institut G~ologique du Bassin d'Aquitaine. We gratefully acknowledge the generous contribution of samples from the Lamont-Doherty Geological Observatory core c o l l e c t i o n s , R. G e r s o n d e ( W e g e n e r I n s t i t u t ) a n d D . S . C a s s i d y o f t h e F S U A n t a r c t i c Res e a r c h F a c i l i t y . W e t h a n k P r o f . J. M o y e s , Drs. L . H . B u r c k l e , J . C . D u p l e s s y , A. P u j o s a n d J.L. Turon for much stimulating discussion a n d c o n s t a n t h e l p , M. F o n t u g n e a s c h i e f s c i e n t i s t o f t h e c r u i s e A P S A R A 2, a n d t h e Comm a n d i n g Officer o f t h e N a v a l P o l a r O c e a n ography Center (Washington) for providing t h e w e e k l y A n t a r c t i c S e a i c e c o v e r a g e s . Dr. C. Sancetta improved the manuscript. T h i s is C . F . R . c o n t r i b u t i o n no. 832.
Appendix -- Floral references Actinocyclus actinochilus (Ehrenberg) Simonsen. Hustedt
(1958), pp. 122 126, figs.57-80. Asteromphalus parvulus Karsten. Hustedt (1958), p. 128,
fig.91. Azpeitia (Coscinodiscus) tabularis (Grunow) Fryxell and
Sims (1986). Hustedt (1958), p. 119, figs.48-56. Chaetoceros sp.: Some species such as Chaetoceros atlanticure var. bulbosum Hargraves and others were lumped
together under this taxon. Cocconeis sp.: Both species Cocconeis costata Gregory and Cocconeis fasciolata (Ehrenberg) Brown were lumped together under this taxon. Coscinodiscus furcatus Karsten. Hustedt (1930), pp. 396-398, figs.207-208. Eucampia antarctica (Castracane) Mangin. Hendey (1937), pp. 285-286, pl. XIII, figs.8 10. Hemidiscus cuneiformis Wallich. Simonsen (1972), p. 267,, figs.7-11. Nitzschia angulata (O'Meara) Hasle. Hasle (1965), pp. 24 26, pl. 1, fig.6; pl. 9, figs.1 6. Nitzschia curta (Van Heurck) Hasle. Hasle (1965), pp. 32 33, pl. 12, figs.2 5.
cylindrus (Grunow) Hasle. Hasle (1965), pp. 34 37, pl. 12, figs.6-12. Nitzschia kerguelensis (O'Meara) Hasle. Hasle (1965),, pp. 14 18, pl. 4, figs.ll-18. Nitzschia ritscherii (Hustedt) Hasle. Hasle (1965), pp. 20-21, pl. 1, fig.20; pl. 4, figs.1 7. Nitzschia separanda (Hustedt) Hasle. Hasle (1965), pp. 26-27, pl. 2, figs.23 29. Nitzschia sublineata Hasle (Fragilaria sublinearis Van Heurck, Fragilariopsis sublinearis (Van Heurck) Heiden and Kolbe). Hasle (1965), pp. 27 30, pl. 7, fig.l; pl. 11, figs.1 10; pl. 12, fig.1. Odontella weisflogii (Janisch) Grunow. Van Heurck (1909), p. 39, pl. 10, figs.136-137. Porosira glacialis (Grunow) Jorgensen (1905). Hasle (1973), pp. 6-10, pl. 1, figs.2, 4, 5; pl. 3, figs.13-17; pl. 14, figs.19-25; pl. 15, figs.26-29. Rhizosolenia styliformis Brightwell. Hustedt (1930), pp. 584 588, fig.333. Roperia tesselata (Roper) Grunow. Hustedt (1930), pp. 523-524, fig.297. Schimperiella antarctica Karsten. Hendey (1937), p. 256. Thalassionema nitzschioides Grunow in Van Heurck. Hustedt (1959), p. 244, fig.725. Thalassiosira antarctica Comber. Hargraves (1968), p. 96, pl. 96 97, figs.182 184. Thalassiosira decipiens (Grunow) Jorgensen. Hustedt (1930), pp. 322-323, fig.158. Thalassiosira delicatula Hustedt (1958), p. 110, figs.8-10. Thalassiosira eccentrica (Ehrenberg) Cleve. Hustedt (1930), p. 388, fig.201. Thalassiosira gracilis (Karsten) Hustedt. Hustedt (1958), pp. 109 110, figs.4 7. Thalassiosira lentiginosa (Janisch) Fryxell. Hendey (1937), p. 248. Thalassiosira lineata Jouse. Simonsen (1974), p. 9, pl. 1, fig.6-7. Thalassiosira oestrupii Ostenfeld Proshkina-Lavrenko. Hustedt (1930), p. 318, fig.155. Thalassiosira tumida (Janisch) Hasle. Hasle, Heimda and Fryxell (1971), pp. 316-318. Thalassiothrix sp.: The two species Thalassiothrix antarctica and Thalassiothrix longissima were lumped together under this taxon. Nitzschia
SILICOFLAGELLATES Genus Dictyocha Ehrenberg 1839. Defelice and Wise (1981), p. 60, pl. II, fig.20. Genus Distephanus Stohr 1880. Defelice and Wise (1981), p. 60, pl. II, fig.19.
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