The biogeography of major diatom taxa in Southern Ocean sediments: 2. Open ocean related species

Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66 – 92 www.elsevier.com/locate/palaeo

The biogeography of major diatom taxa in Southern Ocean sediments: 2. Open ocean related species Xavier Crostaa,T, Oscar Romerob, Leanne K. Armandc,1, Jean-Jacques Pichona,F a

DGO, UMR-CNRS 5805 EPOC, Universite´ de Bordeaux I, Avenue des Faculte´s, 33405 Talence Cedex, France b Department of Geosciences, Bremen University, P.O. Box 33 04 40, 28334 Bremen, Germany c School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia Received 18 November 2004; received in revised form 23 March 2005; accepted 24 March 2005

Abstract Diatom assemblages from 228 core-top samples were investigated to determine the modern geographic distributions of 10 major open ocean species or species groups in the Atlantic and Indian sectors of the Southern Ocean. Our study gives a more comprehensive view of the relationships between diatom distribution and environmental pressures than previous studies, as our modern database covers a much wider area, and additionally highlights the relationships with sea ice cover and concentration. The 10 species or species categories can mainly be lumped into three groupings. First, a cool open ocean grouping composed of Rhizosolenia pointed group, Thalassiosira gracilis group and Trichotoxon reinboldii with maximum relative abundances occurring within the maximum winter sea-ice edge. Second, a pelagic open ocean grouping composed of Fragilariopsis kerguelensis, Thalassiosira lentiginosa, Thalassiosira oliverana and Thalassiothrix spp. group with maximum occurrences at the Antarctic Polar Front. Third, a warm open ocean grouping with maximum abundances observed within the Polar Front Zone and composed of the Rhizosolenia rounded group, the Thalassionema nitzschioides var. nitzschioides group and the Thalassionema nitzschioides var. lanceolata. Comparisons of the abovementioned 10 species or species groups with modern February sea-surface temperatures and sea-ice duration and concentration reveal species-specific sedimentary distributions regulated both by sea-surface temperatures and sea ice conditions that support the use of diatom remains to reconstruct past variations of these environmental parameters via qualitative and transfer function approaches. D 2005 Elsevier B.V. All rights reserved. Keywords: Diatom; Bacillariophyceae; Biogeography; POOZ; Southern Ocean; Sediments

T Corresponding author. E-mail addresses: [email protected] (X. Crosta), [email protected] (O. Romero), [email protected] (L.K. Armand). 1 Current address: Centre d’Oce´anologie de Marseille, Laboratoire d’Oce´anologie et de Bioge´ochimie, Campus de Luminy, case 901, F-13288 Marseille Cedex 09, France. F Deceased. 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.03.028

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1. Introduction Diatoms contribute up to 75% of the primary production of the Southern Ocean, thus playing an important role in global cycling of silicic acid and carbon (Tre´guer et al., 1995). The sedimentation and accumulation of diatom remains mainly occur between the winter sea ice edge and the Antarctic Polar Front (Lisitzin, 1960; Lozano and Hays, 1976) to create a more or less circumpolar bDiatom Ooze BeltQ (Burckle and Cirilli, 1987). The Antarctic continental shelf represents a second zone of high biogenic silica deposition but in a patchier pattern. Diatom oozes have been encountered in the Antarctic Peninsula region (Leventer et al., 2002), in the Ross Sea (Truesdale and Kellogg, 1979; Leventer et al., 1993), in the Prydz Bay (Taylor et al., 1997) and off Ade´lie Land (Leventer et al., 2001). The distribution of diatoms in sediments of the Southern Ocean is first dependent on the primary productivity and the phytoplankton community influenced by biotic and abiotic parameters. Nutrient availability and proportion (El-Sayed, 1970; Burckle et al., 1987), grazing pressure (Bodungen et al., 1985), sea-surface temperature (Neori and Holm-Hansen, 1982), sea ice presence (Abelmann and Gersonde, 1991) and stability of the water column (Leventer, 1991) are the most important primary processes driving distribution. Secondary processes during sedimentation and accumulation, such as aggregate formation (Smetacek, 1985), preferential dissolution of lightly silicified diatoms (Pichon et al., 1992b), and lateral and bottom transport (Burckle, 1981; Leventer, 1991) alter the original community resulting in a modified sedimentary assemblage (Burckle and Humphreys, 1986). Reconstructions of past sea surface temperatures (Burckle, 1984; Pichon et al., 1992a; Zielinski et al., 1998; Crosta et al., 2004) and past sea ice extent (Crosta et al., 1998a,b) are based on the assumption that fossil diatom assemblages are directly linked to surface water hydrology. Yet our understanding of the relationship of preserved diatom assemblages to surface water conditions remains patchy and influenced by early diatom occurrence reports (Kozlova, 1966; Donahue, 1973; Semina, 1979; DeFelice and Wise, 1981) in which some confusions existed (Fenner et al., 1976). Therefore, it is therefore necessary to improve our knowledge on the aforementioned relationships

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due to the evolving nature of diatom research, taxonomy and the integration of data sets. This paper represents the second paper in a series of three describing the diatom distribution in surface sediments from wide areas of the Southern Ocean (Armand et al., 2005; Romero et al., 2005). We here present distribution maps of 10 diatom species or species groups representative of the Permanent Open Ocean Zone of the Southern Ocean (Tre´guer and Jacques, 1992), and accounting for more than 2% of the total diatom assemblage in at least one of the 228 core-tops composing our database. This dataset is based on the original work of Pichon et al. (1992a) and subsequently improved for statistical-based reconstructions by Crosta et al. (1998a,b). Here we employ the relative abundances of the 10 diatom species or species groups from the 228 core-top samples and compare them to summer sea-surface temperatures, sea-ice duration and concentration in an effort to document how their sedimentary abundances reflect Southern Ocean surface water hydrology and their use in palaeo-environmental reconstructions.

2. Materials and methods Detailed methodology including database treatment, mapped surface sample locations and oceanographic boundaries, modern parameter sources and extraction for sea ice and sea surface temperature (SST) data are given in Armand et al. (2005, inclusive of figures 1–3). The reference dataset is composed of 228 core-top sediment samples derived from the Crosta et al. (1998a) dataset and unpublished counts from the South Tasman region (Armand, unpublished data) and the Subantarctic sector of the Atlantic Ocean (Romero, unpublished data). These samples are representative of modern to sub-modern climate conditions. The reference dataset will be hereafter referred to as the Diatom Database 228 (DD228). Taxonomic identification follows Hustedt (1958), Hasle (1965b, 1974), Fenner et al. (1976), Fryxell and Hasle (1976, 1980), Schrader (1976), Hargraves (1979), Simonsen (1982), Johansen and Fryxell (1985), Stockwell and Hargraves (1986), Hasle and Semina (1987), Hasle et al. (1988), Lee and Lee (1990), Priddle et al. (1990), Medlin and Sims (1993), Zielinski (1993), Moreno-Ruiz and Licea (1994),

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Armand (1997); and Armand and Zielinski (2001). We present in this paper the 10 species/taxa that are linked to open ocean conditions.

3. Results In the ensuing subsections each species or species group will be described in reference to their geographical and abundance distributions against February sea-surface temperature (SST), February and September sea-ice concentration and annual sea ice presence (SIP). Taxonomic issues specific to individual species or their groupings are addressed. Due to the geographical detail required, but not possible to deliver in this work, the geographic positioning of samples in the Ross Sea, Prydz Bay area and Antarctic Peninsula region (Armand et al., 2005, figures 1 and 2) have been summarized by regional boxes in all distribution maps. Previous studies on the sedimentary distribution of diatom species are numerous and widely distributed in the Southern Ocean. Yet, relative abundance data is more rarely published. Armand et al. (2005) schematically portrays such previous work in their figure 3. This figure is used as a basis for species’ discussion covered in this Section. Generalized reports on species distributions in the sediments were not included, nor was the phytoplankton distribution at the ocean surface covered with exception to some specific cases. Summary plots of a species or species group’s relative abundance against the physical parameters discussed through the remainder of the text is provided in Fig. 1a–d. 3.1. Fragilariopsis kerguelensis (O’Meara) Hustedt References used: Hustedt (1958), plate 10, figures 121–127; Hasle (1965b), plate 4, figures 11–18. In DD228, F. kerguelensis is the most dominant species preserved in surface sediments of our study. Maximum abundances between 70–83% are noted to fall in a zone constrained between the maximum summer sea-ice edge and the Polar Front (Fig. 2) where it is the main component of the bDiatom Ooze BeltQ defined by Burckle and Cirilli (1987). Occurrence in relation to modern SST reveals that F. kerguelensis occurs in greatest abundances under

February SST of 1–8 8C. Above 8 8C, the relative abundance of F. kerguelensis slowly decreases to less than 5% at 19 8C, whereas abundances fall sharply below 0 8C and above 20 8C (Fig. 1a). F. kerguelensis is found in sediment with up to 8 months per year of sea-ice cover overhead although there appears to be no preference between ice-free and ice covered regions. This is equally reflected in the plot of winter sea-ice concentrations against relative species abundance (Fig. 1d). In contrast, the majority of locations in which F. kerguelensis are observed experience icefree conditions through summer (b 20%, Fig. 1c). F. kerguelensis is considered endemic to Southern Ocean waters. Our geographic distribution clearly confirms from the sedimentary perspective earlier reports from the surface waters where F. kerguelensis dominates the assemblages of the open ocean zone south of the Polar Front (Froneman et al., 1995) and the northern boundary for its distribution is located approximately at the Subtropical Front (Hasle, 1976; Semina, 2003). Nonetheless, low abundances of F. kerguelensis have been observed in the phytoplankton up to 308 N in the southeast Atlantic (van der Spoel et al., 1973). Along the Antarctic coast, the lack of high abundances of F. kerguelensis has been observed by most workers (Jouse´ et al., 1962; Kozlova, 1966; Truesdale and Kellogg, 1979; Gersonde, 1984; Gersonde and Wefer, 1987; Kellogg and Kellogg, 1987; Stockwell et al., 1991; Leventer, 1992; Tanimura, 1992; Taylor et al., 1997; Zielinski and Gersonde, 1997; Cunningham and Leventer, 1998). The Russians (Jouse´ et al., 1962; Kozlova, 1966; Kozlova and Mukhina, 1967) made the first comments on the increased presence of F. kerguelensis in the sediments between 458 and 698 S where it can account for 80% of the total diatom assemblage. This statement still holds true in more recent studies (Abbott, 1973; Fenner et al., 1976; DeFelice and Wise, 1981; Zielinski and Gersonde, 1997; Crosta et al., 1998a; and this work). Zielinski and Gersonde (1997) review other factors in the distribution of F. kerguelensis, and note that its abundance in sediments, close to and north of the Subtropical Front, are reduced to 20% of the total diatom number preserved. High abundances in the Subtropical Zone sediments are considered related to dissolution processes that preferentially preserve strongly silicified valves of F. kerguelensis. Occurrences of F. kerguelensis outside

February Sea Ice Concentration (% of covered water)

Sea Ice Duration (Months per year)

February Sea Surface Temperatures (˚C)

-2 0 2 4 6 8 10 12 14 16 18 20 22 0 1 2 3 4 5 6 7 8 9 10 11 12 0 10

p. ol nb

ot si

5 0

ei

hr

ix

di i

sp

a an er

sa

liv

as

ou

tig in o

0 50

T. r

20 0

al

0 4

Th

5 0

T. o

0 5

T. le n

nd

ed gp T. va nit r. zs la ch nc io eo id la es ta T. ni tz sc hi oi de s T. gr ac ilis

ed nt oi

7 0

.r

0 100

69

0 4

4

a 1

Rh. poly Rh. curv

Rh. cras

b

c

20 30 40 50 60 70 80 90

100 0

September Sea Ice Concentration (% of covered water)

0

R

F. k

er

R .p

gu

el en

si s

gp

X. Crosta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66–92

10

d

20 30 40 50 60 70 80 90

100

Fig. 1. Relative abundances of the 10 major open ocean species and/or species groups in DD228 against February sea-surface temperatures (a), sea ice duration (b), February sea-ice concentration (c) and September sea-ice concentration (d).

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X. Crosta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66–92

17 4

2 1

11 4 2

6 1 4

F. kerguelensis 0 - 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20% 20 - 30% 30 - 50% 50 - 75% 75 - 100%

10 5 1

Fig. 2. Distribution of F. kerguelensis relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

the Southern Ocean were noted in modern sediments of the southeast Atlantic (Schuette and Schrader, 1981; Pokras and Molfino, 1986) and in glacial sediments of the Benguela upwelling system (Romero et al., 2003; Crosta, unpublished data). In the latter studies, sedimentary occurrences of F. kerguelensis are considered an allochthonous element introduced by northward Antarctic Bottom Water or Antarctic Intermediate Water movement. The results presented here for comparison with February SST provide a mirror to those observed in the South Atlantic sector, where only a slightly lower maximum abundance is found (83% versus 92%) (Zielinski and Gersonde, 1997). In relation to sea-ice cover, F. kerguelensis appears to be similarly at home in consolidated sea-ice conditions as in open-ocean

conditions during the maximum winter sea-ice extent. It is clear from our data, however, that highest abundances are observed in locations where summer open-ocean conditions exist and little, if any, sea ice occurs. 3.2. Rhizosolenia group Rhizosolenia specimens are usually broken in sediment assemblages and only the valve apices are generally preserved. Identification of fossil specimens is therefore based on the shape of the valve apex, on the shape of process (the needle-like extension protruding from the valve apex) and on the presence/absence and shape of otaria (the wing-like extensions between the valve and the process).

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Identifying Rhizosolenia specimens to species level is further complicated by the wide range of confusion and misinterpretation existing in the literature (for review see Armand and Zielinski, 2001). The diatom database DD228 was compiled before the comprehensive study of Armand and Zielinski (2001) and thus their species separations were not employed on the Rhizosolenia taxa observed in this work. Here, Rhizosolenia taxa are separated into the three following groups: (1) the Rhizosolenia pointed group, (2) the Rhizosolenia rounded group and (3) the Rhizosolenia otaria-less group, respectively. Only the first two groups are used in quantitative palaeoenvironmental reconstructions based on transfer function (Crosta et al., 1998a). Although this treatment represents a significant advance from that employed originally by Pichon et al. (1992a) who reported only two groups (with and without otaria), we continue to advocate considerable attention in Rhizosolenia genus taxonomy and ecology, and encourage Rhizosolenia taxa identification and separation as in Armand and Zielinski (2001). We intend to extend this discrimination throughout DD228 in the future. 3.2.1. Rhizosolenia pointed group References for Rhizosolenia antennata var. semispina: Sundstro¨m (1986), plate 17, figure 114 and 117; Armand and Zielinski (2001), figure 3D–E. References for Rhizosolenia styliformis: Sundstro¨m (1986), plate 9, figures 47–52; Armand and Zielinski (2001), figure 5A–L. Included in this group are specimens that present pointed otaria. These specimens mainly belong to R. antennata var. semispina with otaria extending far on the process and to its resting spore R. antennata var. antennata having no otaria but two easily recognizable processes (Sundstro¨m, 1986). Logically, specimens of R. styliformis possessing small pointed otaria that extend slightly onto the process (Armand and Zielinski, 2001) are also included into this group. We note that this latter species is very seldom observed in Southern Ocean waters and to date it has not been encountered in this data set. Rhizosolenia pointed group in DD228 is observed with maximum abundance within, and just north of, the winter sea ice edge in the Atlantic sector of the Southern Ocean (Fig. 3). A few occurrences are encountered in the Polar Front Zone of the Atlantic

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sector, and the R. pointed group is only found at trace levels north of the Subantarctic Front. Similarly, the species group rapidly vanishes from surface sediments in the south approaching the sea ice zone. When compared to modern summer SST, a maximum occurrence of 5% falls around 1–2 8C (Table 1), and the species presence sharply decreases towards both colder and warmer temperatures (Fig. 1a). Rhizosolenia pointed group is not encountered above 8 8C, and generally accumulates in sediments with low annual sea ice duration (Fig. 1b), unconsolidated sea ice during winter (Fig. 1d) and open ocean conditions during summer (Fig 1c). R. antennata var. semispina has often been confused with R. hebetata var. semispina. The latter taxon is restricted to northern polar waters (Sundstro¨m, 1986) and its records from Southern Ocean waters should represent R. antennata var. semispina. Most studies have depicted R. antennata var. semispina to dwell in Antarctic waters where it has been found to be a dominant species in the open ocean waters of late summer (Froneman et al., 1995) as part of the bshade floraQ. Similarly, Fenner et al. (1976) report R. antennata var. semispina as a cosmopolitan constituent from the sea ice zone to the Subantarctic Zone, but with a maximum occurrence in surface waters of the Antarctic Polar Front. This indicates a strong adaptability of the species although a clear preference for cool open ocean conditions is demonstrated (Ligowski, 1993). R. antennata var. semispina is one of the most prominent Rhizosolenia in modern sediments of the Southern Ocean where it reaches 4% at the winter sea ice edge (Fig. 3). This taxon also accounts for the biggest part of the R. pointed group as R. antennata var. antennata is seldom encountered and R. styliformis is completely absent from DD228. In other studies, low abundances of R. antennata var. semispina are found between the winter sea ice edge to the Subantarctic Zone with a maximum occurrence within the winter sea ice edge of the Atlantic sector of the Southern Ocean (Zielinski and Gersonde, 1997), hence reflecting its distribution in the phytoplankton. 3.2.2. Rhizosolenia rounded group References for Rhizosolenia polydactyla var. polydactyla: Sundstro¨m (1986), plate 12, figure 77; Armand and Zielinski (2001), figure 3F–I.

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17 1

27 9 1

17

R. pointed gp 0 - 1% 1 - 2% 2 - 5% 5 - 10%

Fig. 3. Distribution of the Rhizosolenia pointed group relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

References for Rhizosolenia curvata: Sundstro¨m (1986), plate 10, figures 57–61; Armand and Zielinski (2001), figure 3B–C. References for Rhizosolenia crassa: Sundstro¨m (1986), plate 20, figures 139–143; Armand and Zielinski (2001), figure 3J–K. References for Rhizosolenia sima var. sima: Sundstro¨m (1986), plate 21, figures 144–149; Armand and Zielinski (2001), figure 3L. Included in this group are specimens possessing rounded otaria as was frequently referred to in the literature (Hendey, 1937; Frenguelli, 1943; Abbott, 1973). Specimens belong to R. polydactyla var. polydactyla, R. curvata, and R. crassa. Following the earlier description, specimens of R. sima var. sima that thrive in the seasonal sea ice zone (Armand and

Zielinski, 2001) should also be included in this group. No specimens of the latter species were observed in this study. The distribution of the R. rounded group in modern sediments shows a maximum occurrence within the Polar Front Zone and the Subantarctic Zone of the Indian sector of the Southern Ocean (Fig. 4). The species group is almost absent from the Antarctic and the Subtropical zones. It is also less abundant in the Atlantic sector than the Indian sector of the Southern Ocean. When compared to modern summer SST, occurrence of about 1% is found between 2 and 19 8C, while maximum abundances fall between 9.5 and 14.5 8C (Table 1, Fig. 1a). The R. rounded group clearly has highest abundances located in the open ocean zone

X. Crosta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66–92 Table 1 Temperature ranges and maximum relative abundances of major open ocean diatom species and/or species groups in DD228 Taxa/taxa group

Fragilariopsis kerguelensis Rhizosolenia pointed group Rhizosolenia rounded group Thalassionema nitzschioides var lanceolata Thalassionema nitzschioides group Thalassiosira gracilis Thalassiosira lentiginosa Thalassiosira oliverana Thalassiothrix spp. group Trichotoxon reinboldii

Temp. range (8C)

Max. Temp. of max. occurrence occurrence (%) (8C)

1–22 80.1

1–7

1–19

5.5

1–1.5

0.5–19

4.2

9.5–14.5

2 –19

4.1

10.5–11.5

2.5–16

2.9

1.3–18 11.9 0.5–18 36.7 1.3–18 4.2 1.3–22 3.1 0.5–18

3.2

6.5–7 1–2 1–1.5 2.5–3.5 2–14 3–3.5

and is absent in the regions where sea-ice is present (Fig. 1b–d). In the literature, R. polydactyla var. polydactyla was found dwelling in very different environments, which indicates that the species is eurythermal or that it has been confused with other taxa. Hustedt (1930) and Manguin (1960) harvested R. polydactyla var. polydactyla in melted ice and in near-coastal environments. Hasle (1969) similarly restricted the taxon to cold Antarctic waters influenced by unconsolidated seasonal sea ice. Conversely, Fenner et al. (1976) encountered R. polydactyla var. polydactyla thriving from Antarctic waters to Subantarctic waters with a maximum occurrence in the PFZ. R. polydactyla var. polydactyla has been seldom recorded in sediments. Based on sediments recovered by Leg 177 drillings, Armand and Zielinski (2001) cite a continuous low presence of the taxon prior the Late Pliocene in sediments of the Antarctic Zone and the Polar Front Zone, and a more common occurrence in Subantarctic sediments during the Pleistocene. Additionally, around 15 new core-tops mainly from the Indian sector of the Southern Ocean were investigated for diatom assemblages and Rhizosolenia specimens were identified following Armand and Zielinski’s (2001) taxonomy. Specimens of R. polydactyla var. poly-

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dactyla were found most common in modern sediments of the Subantarctic Zone and the Polar Front Zone and were almost absent from sediments south of the Polar Front. A few specimens of R. polydactyla var. squamosa were encountered in Antarctic sediments overlain by open ocean waters (Crosta, unpublished data). These sparse sedimentary data then confirm abovementioned findings by Fenner et al. (1976) to the detriment of Hustedt (1930), Manguin (1960) and Hasle (1969). The latter studies may have confused R. polydactyla var. polydactyla with R. antennata var. semispina or R. styliformis emended (Sundstro¨m, 1986). Even less is known about the distribution of R. curvata in the phytoplankton. Hart (1937) and Hasle (1969) restrict the taxon to SAZ waters with decreasing occurrence toward the Polar Front. Rare presence of the species was noted in northern Antarctic waters (Hart, 1934; Hendey, 1937; Fenner et al., 1976). Few records of R. curvata in modern sediments exist. Sournia et al. (1979) found the taxon just south of the Antarctic Polar Front to the west of Kerguelen Islands, and Armand and Zielinski (2001) rightly note that records of R. styliformis in Subantarctic Zone surface sediment of the Indian Ocean by Crosta et al. (1998a) may be attributed to R. curvata. This was recently confirmed by diatom counts from new core-tops from the Indian sector in which R. curvata, identified following Armand and Zielinski (2001) taxonomy, was found in sediments of the Subantarctic Zone (Crosta, unpublished data). Similarly, little is known about the occurrence of R. crassa occurrence in phytoplankton. Hart (1934) and Hendey (1937) harvested the species in near-ice waters. Based on these rare phytoplankton occurrences Sundstro¨m (1986) attributed R. crassa to the Southern-cold region, whereas Armand and Zielinski (2001) tentatively ascribed the species to an under seasonal sea ice habitat. This is challenged by the study herein where no taxa with rounded otaria were encountered in modern sediments overlain by seasonal sea ice. Conversely, high occurrences up to 4% of a species attributed to R. crassa were noted in new core-tops from the Subantarctic Zone and the Subtropical Zone of the Indian Ocean (Crosta, unpublished data). Previous records of R. crassa in surface waters or surface sediments from the seasonal sea ice zone may have misinterpreted the taxon with R. sima

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24

27

10

16

R. rounded gp 0 - 1% 1 - 2% 2 - 5%

Fig. 4. Distribution of the Rhizosolenia rounded group relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

var. sima that also presents large rounded otaria and dwells in very cold waters (Sundstro¨m, 1986; Priddle et al., 1990; Ligowski, 1986, 1988; Ligowski and Kopczynska, 1991). A succession of the three taxa composing R. rounded group is clearly evident and is responsible for the large temperature range covered by the group (Fig. 1a). R. polydactyla var. polydactyla is found in Antarctic open ocean to Subantarctic waters from 2 to 7–8 8C with a maximum abundance within the PFZ. R. curvata is encountered from the APF to the SAZ from 5–6 to 13–14 8C. This taxon is certainly endemic to Subantarctic waters. Finally, a species attributed to R. crassa is noted in Subantarctic to Subtropical waters from 11–12 to 19–20 8C.

3.3. Thalassionema nitzschioides Grunow in Van Heurck Taxonomic studies by Moreno-Ruiz and Licea (1994) indicate that most previously described species of T. nitzschioides should remain at the variety level. Within the Southern Ocean several varieties have been described. These include: T. nitzschioides, a species with cosmopolitan distribution (Hasle and De Mendiola, 1967; Simonsen, 1974; Hallegraeff, 1986; Moreno-Ruiz and Licea, 1994); T. nitzschioides var. parva, a warm water taxa excluded from the Antarctic waters at the STF (Zielinski and Gersonde, 1997); T. nitzschioides var. lanceolata, an Antarctic species with noted decreasing presence south of the Polar Front (Zielinski and Gersonde, 1997); and T. nitz-

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Our T. nitzschioides var. lanceolata group is a combination of T. nitzschioides var. lanceolata and T. nitzschioides var. capitulata. These two varieties are difficult to separate when a continuum between lanceolated to capitated apiced forms are encountered in sediment samples. Maximum occurrences of T. nitzschioides var. lanceolata group in modern sedimentsfs are observed in the Subantarctic Zone of the Atlantic sector and in the Subantarctic and Subtropical zones of the Indian sector (Fig. 5). The region between 608 and 908 E has a perturbed hydrography due to the presence of many islands that divert the surface hydrologic fronts. The Subtropical Front here is separated into two branches, the southern branch being closely positioned to the Subantarctic Front, while the northern branch is

schioides var. capitulata, noted as an Antarctic water species (Hustedt, 1958; Moreno-Ruiz and Licea, 1994). In addition, a cold-water form, which appears geographically separated and strongly associated with sea-surface temperatures between 0 and 3 8C, T. nitzschioides forma 1, has been identified by Zielinski (1993) and Zielinski and Gersonde (1997). Below we describe our treatment of the various Thalassionema varieties in DD228. 3.3.1. Thalassionema nitzschioides var. lanceolata group References for T. nitzschioides var. lanceolata: Moreno-Ruiz and Licea (1994), figures 23 and 24. References for T. nitzschioides var. capitulata: Moreno-Ruiz and Licea (1994), figures 6 and 7.

24 26 10

15

T. nitzschioides var lanceolata 0 - 1% 1 - 2% 2 - 5%

Fig. 5. Distribution of the T. nitzschioides var. lanceolata group relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

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deviated to 328 S (Belkin and Gordon, 1996). Crossfrontal diffusion and the northward displacement of the northern branch of the Subantarctic Front may explain why high abundances of the T. nitzschioides var. lanceolata group are encountered as far north as 358 S in the Indian sector of the Southern Ocean. This northernmost distribution is reflected in the relationship of maximum core-top abundances of the group (3–4%) against modern SST values between 6 and 15 8C, with the warmest occurrence being located in the Indian Ocean sector (Fig. 1a). The T. nitzschioides var. lanceolata group has a very low tolerance to sea ice presence and is only encountered at trace levels when sea ice is present (Fig. 1b–d). The T. nitzschioides var. lanceolata group has been reported as a cosmopolitan species from the Norway Basin (Schrader and Fenner, 1976), the Gulf of Alaska (Schrader, 1973), the Gulf of California (Moreno-Ruiz and Licea, 1994), the Japanese region (Akiba, 1982) and the Austral Ocean (Hustedt, 1958; Simonsen, 1992). In Southern Ocean literature, all varieties have generally been lumped together but the presence of illustrations may help in re-identifying such occurrences correctly. In Fenner et al. (1976), the species identified as bT. nitzschioidesQ was found in small occurrence in the Antarctic Zone and in greater abundances towards the north. Illustrations 10 and 11 in Plate 14 (Fenner et al., 1976) indicate that the species is a combination of T. nitzschioides var. lanceolata and T. nitzschioides var. parva and confirms a most common presence of both species in Subantarctic waters. Similarly, Heiden and Kolbe (1928) and Hustedt (1958) report T. nitzschioides var. lanceolata group species from the SAZ. 3.3.2. Thalassionema nitzschioides group References for T. nitzschioides sensus stricto: Moreno-Ruiz and Licea (1994), figures 1–3. References for T. nitzschioides forma 1: Zielinki and Gersonde (1997), figure 3. Specimens of the T. nitzschioides group in DD228 present elongated and linear valves with rounded and isopolar apices. This group combines specimens of the species T. nitzschioides sensus stricto as described by Moreno-Ruiz and Licea (1994) and T. nitzschioides forma 1 informally described by Zielinski and Gersonde (1997).

Maximum abundances of 1–2% in modern sediments are observed in the Polar Front Zone. North and south of the Polar Front Zone, the T. nitzschioides group is present at trace levels (Fig. 6). Occurrences in relation to modern SST reveal a steady 2% presence between 7 and 14 8C, with maximum abundances occurring at 6–7 8C (Fig. 1a). One surface sample has an abundance of 3% at 16 8C, but this observation may have been confused with the elongated variations of T. nitzschioides var. parva, a warm water variety. Occurrences in relation to modern sea ice conditions confirm that the T. nitzschioides group is clearly dissociated with seaice conditions (Fig. 1b–d) and is purely an oceanic species in Southern Ocean waters. The T. nitzschioides group has been reported as a cosmopolitan species dwelling in temperate waters as well as austral waters. It is well represented in modern sediments of the Gulf of California (MorenoRuiz and Licea, 1994) and in the outer surface sediment samples off Portugal (Abrantes, 1988). All authors report the T. nitzschioides group as ubiquitous marine (Abrantes, 1988; Semina, 2003). In the Southern Ocean, maximum occurrences of the T. nitzschioides group from the phytoplankton are reported at the Polar Front, and its presence rapidly declining north and south of the Polar Front (Hasle, 1969). In modern sediments, abundances of the T. nitzschioides group have been found to attain 5% at the Polar Front position in the Atlantic sector of the Southern Ocean (Zielinski and Gersonde, 1997). Nevertheless, their results also indicate that a steady 2–3% presence of the species occurs up to the position of the Subtropical Convergence. This suggests that a combination of the T. nitzschioides group with warmer varieties, such as T. nitzschioides var. parva has occurred. The results obtained in our overall assessment indicate a succession of the different varieties of T. nitzschioides in modern peri-Antarctic sediments with the T. nitzschioides group restricted to Antarctic and Polar Front regions, the T. nitzschioides var. lanceolata group dominant in Subantarctic sediments and T. nitzschioides var. parva common in subtropical sediments (Romero et al., 2005). These findings highlight the necessity to separate each variety for quantitative reconstructions of SST using diatom-based transfer functions.

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24

27 10

17

T. nitzschioides 0 - 1% 1 - 2% 2 - 5%

Fig. 6. Distribution of the T. nitzschioides var. nitzschioides group relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

3.4. Thalassiosira gracilis group (Karsten) Hustedt References for T. gracilis var. gracilis: Hustedt (1958), plate 3, figures 4–7; Fryxell and Hasle (1979), figures 12–22. References for T. gracilis var. expecta: Hustedt (1958), plate 3, figures 8–10; Fryxell and Hasle (1979), figures 23–28. We combine T. gracilis var. expecta with T. gracilis var. gracilis occurrences to for the T. gracilis group because the gradational nature of the morphology of the two species continues to make rigorous differentiation impossible to implement. Additionally, Fryxell (1994) commented that the lack of taxonomic distinction between the two varieties may render their separation invalid, as they are both growth stages of the one species.

In DD228, the maximum abundance of the T. gracilis group, 11.9% (Table 1), is found in one coretop in the Antarctic Peninsula region. Aside from this deviant occurrence, increasing abundances of the T. gracilis group are found approaching the region of maximum winter sea-ice extent and the Antarctic coast (Fig. 7). North of the Polar Front, the T. gracilis group is found at trace levels. Highest abundances are related to February SST between 1 and 2 8C (Fig. 1a). A steady decrease from these high abundance peaks are noted to ~ 8 8C. With exception to the highest abundance point in the Antarctic Peninsula, distribution against sea-ice parameters reveals an increasing occurrence with increasing sea ice duration to a maximum of 8.5 months per year (Fig. 1b). Abundances then drop with continued sea ice presence.

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19 4 1

3 12 10 3

4 5

2 5 10

T. gracilis gp 0 - 1% 1 - 2% 2 - 5% 5 - 10% 10 - 20%

Fig. 7. Distribution of the T. gracilis group relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

During summer sea ice conditions, the species group appears most highly associated with an environment of open-ocean conditions diminishing in their abundance in regions of unconsolidated sea ice (b 40% concentration) (Fig. 1c). During winter, a uniform abundance is observed over open-ocean to consolidate sea-ice conditions (Fig. 1d). A slight increase in the group’s abundance is noted in the fully consolidated sea-ice locations. As noted in the geographical distribution presented in Fig. 7, the T. gracilis group has a wide distribution and is reported in the sediments by most workers. The most northerly record appears to have been made by Schuette and Schrader (1981) who found a trace occurrence in their samples off southwest Africa. Jouse´ et al. (1962), Kozlova (1966), Abbott (1973),

Donahue (1973); and DeFelice and Wise (1981) all report it in patchy or low numbers in sediments of the open-ocean region of the Antarctic Circumpolar Current, and on average with a maximum of 3%. Similarly, Fenner et al. (1976) encounter T. gracilis in greater abundance in Antarctic waters than Subantarctic waters. Along the Antarctic coast several reports of this species have been made (Truesdale and Kellogg, 1979; Prasad and Nienow, 1986; Gersonde, 1984; Gersonde and Wefer, 1987; Kellogg and Kellogg, 1987; Stockwell et al., 1991; Tanimura, 1992; Leventer, 1992; Taylor et al., 1997; Cunningham and Leventer, 1998). In almost all cases the abundances have been reported at less than 2%, with the greatest abundance being recorded in the Ross Sea (10% combined, Cunningham and Leventer, 1998).

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Distribution of the two varieties in Prydz Bay, the Ross Sea and along the George V Coast were noted with higher abundances of T. gracilis var. gracilis away from the coast, whereas a ubiquitous low distribution of the T. gracilis var. expecta occurs in these regions (Stockwell et al., 1991; Leventer, 1992; Taylor et al., 1997; Cunningham and Leventer, 1998). Zielinski and Gersonde (1997) report the grouped species as uniformly distributed through the South Atlantic but with highest abundances found in the Sea Ice Zone and Permanent Open Ocean Zone. As in this study, they had similar highest abundances in the Drake Passage/Antarctic Peninsula region. Zielinski and Gersonde (1997) believe the T. gracilis group has no relation to environmental parameters. This is not supported in this work since increased abundances do appear to increase in relation to sea-ice cover and cooler SST. On the whole, these occurrences are related to the heavily-silicified form T. gracilis var. gracilis, which is considered the winter form of the species (Fryxell, 1994). The decreasing abundance of T. gracilis var. gracilis in sediments away from the coast corresponds with the hypothesis that the variety is the winter form of the species and hence deserves the name as an ice-neritic form (Semina, 2003). 3.5. Thalassiosira lentiginosa (Janisch) Fryxell References for T. lentiginosa: Hustedt (1958), plate 4, figures 22–25; Johansen and Fryxell (1985), figures 49 and 50. Akin to the sedimentary distribution of F. kerguelensis, T. lentiginosa is found in almost all samples observed in the diatom database. Maximum abundances of T. lentiginosa are encountered from the maximum winter sea-ice edge to the SAF (Fig. 8). Occurrences then decrease in the Subantarctic Zone and the Subtropical Zone. In general, lowest abundances are observed on the Antarctic continental shelf, most notably in the Ross Sea and Antarctic Peninsula regions. The relationship between the relative abundances of T. lentiginosa in modern sediments and summer SST follows the same pattern as described for F. kerguelensis where a steady maximum abundance of 30% occurs between 1 and 8 8C (Fig. 1a). Relative abundances drop from 8 8C to arrive at trace levels near 18 8C (Table 1). Very low abundances occur

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below 1 8C. Relative abundances of T. lentiginosa show an inverse relationship with sea ice cover with high occurrence between 0 and 4 months of sea ice presence per year and a decline towards prolonged sea ice duration (Fig. 1b). During summer the species appears to be associated with open-ocean conditions to the sea-ice edge (Fig. 1c), whereas in winter the species occurs in regions that maintain open-ocean, to reasonably consolidated sea covered conditions most notably up to 70% concentration (Fig. 1d). T. lentiginosa is most frequently reported in highest abundances from sediments under the Permanent Open Ocean Zone to the Polar Front Zone (Jouse´ et al., 1962; Kozlova, 1966; Kozlova and Mukhina, 1967; Abbott, 1973; Donahue, 1973; Fenner et al., 1976; DeFelice and Wise, 1981; Zielinski and Gersonde, 1997; Crosta et al., 1998a; Burckle unpublished data, 1999). In the Indian Ocean sector, Kozlova (1966) reports maximum abundances between 10% and 41%, whereas in the South Atlantic the species maximum is recorded at 26% (Zielinski and Gersonde, 1997) and in the Pacific sector at 20% (Donahue, 1973). Northward of the PFZ, lower abundances are reported (Jouse´ et al., 1962; Kozlova, 1966; Abbott, 1973; Schuette and Schrader, 1981), averaging between 10% and 17% of the species total. Low Antarctic coastal abundance records of T. lentiginosa have been commented on or noted by several workers (Kozlova, 1966; Donahue, 1973; Gersonde and Wefer, 1987; Kellogg and Kellogg, 1987; Stockwell et al., 1991; Tanimura, 1992; Taylor et al., 1997; Zielinski and Gersonde, 1997; Cunningham and Leventer, 1998), except along the George V Coast where it reaches up to 12.4% of the diatom assemblage (Leventer, 1992). The species is an important contributor to the thanatocoenose in the sediments of the Southern Ocean. However, besides the species’ resistance to dissolution (Kozlova, 1966; Shemesh et al., 1989; Pichon et al., 1992b) and thus, increased presence in the sediments, its true primary signal of distribution remains obscure. The species is apparently not greatly influenced by sea-ice or SST, with exception to extreme warm or cold temperatures where T. lentiginosa abundances are reduced and only occasionally absent in the sediments. The species is in this sense a true pelagic (oceanic-inhabiting) species although Semina (2003) reports it as ice-neritic.

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23 1 13 6 8 10

T. lentiginosa 0 - 1% 1 - 2% 2 - 5% 5 - 10 % 10 - 20% 20 - 30% 30 - 50%

12 11 1

Fig. 8. Distribution of T. lentiginosa relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

3.6. Thalassiosira oliverana (O’Meara) Makarova et Nikolaev References for T. oliverana: Fenner et al. (1976), plate 14, figures 1–5; Akiba (1982), figures 1–5. The species T. oliverana has what appears to be a ubiquitous distribution in sediments of the Southern Ocean with exception of the Antarctic coast where abundances are reduced and occasionally rare (Fig. 9). Highest abundances are generally located between the maximum winter sea-ice edge and the Polar Front, akin to F. kerguelensis and T. lentiginosa. This generalized geographic distribution is expressed over a wide SST range (Fig. 1a), particularly where highest abundances (4–5%) are associated with February SST between 2 and 4 8C (Table 1). Some high occurrences in waters warmer than 8 8C make

us suspect bottom water transport as responsible for this presence. The occurrence of T. oliverana in relation with sea ice cover shows a general decrease from open ocean conditions toward prolonged sea ice cover in duration (Fig. 1b), which is equally observed against winter sea ice concentration (Fig. 1d). T. oliverana is clearly dominant in locations where open ocean conditions to sea-ice edge during summer occur (Fig. 1c). Previous accounts of T. oliverana maximum abundances in the sediments have always been contained to the open-ocean region but with decreased abundances or absence in sediments north of the STF (Jouse´ et al., 1962; Kozlova, 1966; Abbott, 1973; Donahue, 1973; Fenner et al., 1976; DeFelice and Wise, 1981; Zielinski and Gersonde, 1997; Crosta et al., 1998a). Maximum abundance of the species (9%)

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24 26 1 10

17

T. oliverana 0 - 1% 1 - 2% 2 - 5%

Fig. 9. Distribution of T. oliverana relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

is noted by Zielinski and Gersonde (1997) in the South Atlantic sector at temperatures ranging between 1.5 and 1 8C. Only Schuette and Schrader (1981) report T. oliverana to the north in one of their samples, off-shore southwest Africa. At the other extreme, several workers have tabulated rare occurrences of T. oliverana, b1.5%, in the sediments surrounding the Antarctic coast (Kozlova, 1966; Truesdale and Kellogg, 1979; Kellogg and Kellogg, 1987; Stockwell et al., 1991; Leventer, 1992; Tanimura, 1992; Cunningham and Leventer, 1998). From the distribution within the sediments, compared with SST and sea-ice parameters of this study, and the previous documentation of the species elsewhere in the Southern Ocean, T. oliverana appears inhibited by winter sea-ice distribution. Zielinski and Gersonde (1997) noted from their

distribution data that the species displayed strong links to the region between the Antarctic Zone and the Polar Front Zone with a temperature range of 2–5 8C. The results of DD228 support this description and suggest that this circumpolar relationship is furthermore linked to winter sea-ice formation even though there are few studies that report the species occurrence. 3.7. Thalassiothrix spp. group Schimper ex Karsten References for Thalassiothrix longissima: Hasle and Semina (1987), figures 1–25. References for Thalassiothrix antarctica: Hasle and Semina (1987), figures 26–59. Although there has been considerable discussion on the distribution of T. antarctica and T. longissima,

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Subantarctic Zone (Fig. 10). Conversely, very low occurrences are found in the seasonal sea ice zone and the Antarctic coastal regions. The distribution of the Thalassiothrix spp. group against modern summer SST shows a maximum abundance between 1 and 3 8C and a general drop toward warmer temperatures (Table 1). Nevertheless, a few points between 11 and 22 8C depart from the general decreasing trend (Fig. 1a). The relationship of the Thalassiothrix spp. group occurrence against sea ice parameters is more evident with a clear abundance decrease from open ocean conditions to areas of extended annual sea ice cover (Fig. 1b). Similarly, Thalassiothrix spp. abundances decrease in proximity to the summer sea ice edge as viewed in the February concentration plot (Fig. 1c). The species group distribution in comparison with

in particularly with respect to the latter species appearance in Antarctic waters (Hustedt, 1958; Semina, 1981; Hallegraeff, 1986; Hasle and Semina, 1987), the two species remain difficult to separate under the light microscope and are almost always counted as a combined group. As the Thalassiothrix spp. group specimens are very long and narrow, valves present in the sediments are generally broken into many parts. We, therefore, counted recognizable apices and divided this number by two to obtain the number of valves, following the Schrader and Gersonde (1978) methodology. Maximum occurrences of Thalassiothrix spp. group are encountered north of the wintertime sea ice limit and decrease north of the Subantarctic Front, although occasional high abundances are noted in the

24 28 1 10

17

Thalassiothrix spp. 0 - 1% 1 - 2% 2 - 5%

Fig. 10. Distribution of the Thalassiothrix spp. group relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

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September sea ice concentration reveals higher abundances in ice-free regions and much lower abundances in regions that are covered by more compact sea ice in winter (Fig. 1d). Maximum occurrences of the Thalassiothrix spp. group have been reported in surface waters of the South Atlantic, both south of the Polar Front in Antarctic waters and north of the Subantarctic Front in Subantarctic waters (Eynaud et al., 1999). In the same region, Froneman et al. (1995) conversely restricted this species group to the open ocean waters south of the Subantarctic Front. The same pattern is evident from the Pacific sector of the Southern Ocean (Hasle, 1969). In all studies encountered by us, the Thalassiothrix spp. group was not found in subtropical waters. The species group has been recorded in sediments from the South Atlantic (DeFelice and Wise, 1981; van Iperen et al., 1987; Zielinski, 1993; Crosta et al., 1998a) where a maximum abundance (9%) within the Polar Front Zone at summer SST between 2.5 and 6 8C is superimposed on a constant distribution covering the whole SST range (Zielinski and Gersonde, 1997). In the latter study, the northern expression of the group (3–5%) in the Subantarctic Zone was attributed to presence of T. longissima, (Zielinski and Gersonde, 1997) which is classified as tropical by Semina (2003). Our study indicates that where abundances between 1% and 4% range occur in waters warmer than 8 8C, these specimens should also be attributed to the presence of T. longissima. Although the Thalassiothrix spp. group is certainly typical of the Permanent Open Ocean Zone and Polar Front Zone, its distribution in modern sediments of the Southern Ocean is rather constant over a wide range of conditions. This ubiquitous distribution indicates preferential preservation of strongly silicified specimens, transport by currents, and/or reflect different ecological preferences of the taxa composing the species group. It is evident that the Thalassiothrix spp. group should be from now on discarded from transfer function investigations involving SST. 3.8. Trichotoxon reinboldii (Van Heurck) Reid et Round References for T. reinboldii: Hasle and Semina (1987), figures 67–70.

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T. reinboldii has generally been confused with Thalassiothrix spp. but has wider valves with a distinctive pole morphology that separates it from Thalassiothrix species (Hasle and Semina, 1987). In DD228, T. reinboldii is a poorly represented species in the global sedimentary assemblage, with generally low abundances less than 1%. The exception to this low representation is observed in a few core-tops in the Atlantic sector of the Southern Ocean near the winter sea ice edge (Fig. 11). When abundances are compared to modern summer SST, maximum occurrences are found at 1–2 8C (Table 1) superimposed against a steady low/trace abundances between 0 and 10 8C (Fig. 1a). T. reinboldii has not been encountered below a summer SST of 0 8C. The comparison of T. reinboldii with sea ice parameters does not show a clear pattern with a bi-modal distribution versus sea ice duration (Fig. 1b) and, although the species appears to be able to tolerate unconsolidated sea ice concentrations during summer, it clearly is abundant in sediments where overlying open ocean conditions occur (Fig. 1c). Distribution against winter sea ice concentration repeats the previous observation with other open ocean taxa reported in this work, where abundances are spread evenly over all locations of varying winter sea-ice concentration suggesting a potential preference for winter melt-back regions for summertime proliferation (Fig. 1d). Hasle (1969) assigned T. reinboldii with a very similar phytoplankton distribution to that of the Thalassiothrix spp. group, with common abundances between the winter sea ice edge and the Subantarctic Front, but with maximum abundances immediately south of the Polar Front in the Pacific sector of the Southern Ocean. Similarly, Hendey (1937) and Kozlova (1962) find maximum occurrence of this species in Antarctic waters, while Hustedt (1958) reports it in low abundances in northern subantarctic waters. Stockwell et al. (1991) noted T. reinboldii in the sediments of the Prydz Bay region with very low relative abundances of ~ 0.5%. Round et al. (1990) describe the species as abundant around Antarctica. Other records of this species in the sediments were not encountered and this is expected because of their resemblance to Thalassiothrix taxa. Although T. reinboldii is typical of the Permanent Open Ocean Zone, its low generally abundance in

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24

27 10

17

Trichotoxon reinboldii 0 - 1% 1 - 2%

Fig. 11. Distribution of T. reinboldii relative abundances in DD228. Location of the oceanic fronts from Orsi et al. (1995), and location of February and September maximum sea ice extent from Comiso (personal communication, 2002).

modern sediments and its sensitivity to transport by currents demonstrates that this species should not from now on be used in transfer function analyses involving SST.

4. Discussion The investigation of 228 surface sediment samples allows us to characterize the modern distribution of open ocean diatoms in the Atlantic and Indian sectors of the Southern Ocean. In addition to the biogeographic distribution, the abundance of each diatom species or species group from DD228 has been compared to both sea-ice conditions and summer sea-surface temperature data, thus providing indica-

tions of environmental pressures that may affect diatom distribution in the surface waters, which is later preserved in the sediments. It should be noted here that our study gives a more comprehensive view of the relationships between diatom distribution and environmental pressures than previous studies (DeFelice and Wise, 1981; Zielinski and Gersonde, 1997), as our modern database covers a much wider area, and additionally highlights the relationships with sea ice cover and concentration. The open ocean waters, especially in the Polar Frontal region, experience high spring blooms with elevated primary production and biomass (Stramski et al., 1999; Fischer et al., 2002) and constant summer primary production which can account for the same proportion of export as the earlier spring blooms (van

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der Loeff et al., 2002). The briefness of spring blooms complicates cruise-to-cruise comparison, but as a general picture, it has been shown that spring diatom communities are dominated by F. kerguelensis with subordinate contributions of Chaetoceros spp., Pseudo-nitzschia spp. and Thalassionema spp. (Burckle et al., 1987; Froneman et al., 1995; Fischer et al., 2002). The summer diatom community is more diverse with Thalassiothrix spp., Rhizosolenia spp., Corethron cryophilum and Pseudo-nitzschia lineola as main components and with lower contributions of F. kerguelensis (Froneman et al., 1995; Tremblay et al., 2002). A late summer cruise found F. kerguelensis to re-dominate the diatom community of the Polar Front region with Fragilariopsis grunowii, T. gracilis, Azpeitia tabularis and Fragilariopsis curta as accompanying species (Eynaud et al., 1999). Diatom sedimentation results in a patchy siliceous sediment band (Lisitzin, 1960; Lozano and Hays, 1976) called the bDiatom Ooze BeltQ (Burckle and Cirilli, 1987). The diatom ooze belt may result from a high preservation efficiency of the diatom frustules (Nelson et al., 1995), although this hypothesis was recently challenged (Pondaven et al., 2000). In the latter study, high opal content in Southern Ocean sediments derives from a strong export rather than a high preservation efficiency. Surface sediment communities are generally altered by different processes such as preferential dissolution during downward transport (Pichon et al., 1992a), grazing (El-Sayed, 1990), subsequent incorporation in fecal pellets (Smayda, 1970) and aggregate formation (Smetacek, 1985). Sedimentary assemblages are then different than from the original surface water communities (Burckle and Humphreys, 1986). Nevertheless, sedimentary assemblages maintain a blended signature of the original community produced during the spring– summer season (Truesdale and Kellogg, 1979; DeFelice and Wise, 1981; Burckle and Cirilli, 1987; Gersonde, 1988; Zielinski and Gersonde, 1997). The distribution of open ocean diatom species or species group from DD228 in modern sediments of the Southern Ocean agrees generally well with previous regional studies (Kozlova, 1966; Donahue, 1973; DeFelice and Wise, 1981; Burckle et al., 1987; Leventer, 1992; Tanimura, 1992; Zielinski and Gersonde, 1997). Only the Rhizosolenia group departs from previously depicted distributions described in

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the literature. This discrepancy is best explained by the taxonomic problems affecting both our DD228 and previous data sets, together with slide preparation and counting methods. All investigations were indeed done before the comprehensive study of Armand and Zielinski (2001) in which they clearly document earlier misinterpretations of Rhizosolenia species in the sediment record. In DD228, all specimens with rounded otaria were lumped together in the R. rounded group and all specimens with pointed otaria were combined in the R. pointed group. In a few coretops counted since 2001, following the Armand and Zielinski (2001) taxonomy, one of us (Crosta, personal observation) has been able to refine the relationships of Rhizosolenia species to summer SST and sea-ice parameters. A succession in taxa composing the R. rounded group discussed in this work is clearly observed. R. polydactyla var. polydactyla is encountered between 2 8C and 7 and 8 8C with greater abundances within the PFZ. R. curvata is found between 5 and 6 8C and 14 8C with greater occurrences in the Southern SAZ. R. crassa is unambiguously observed between 11 and 20 8C in Subantarctic to Subtropical areas (Fig. 1a). This is in contradiction to Armand and Zielinski (2001) who, based on Hart (1934), Hendey (1937), Sundstro¨m (1986) and on one core-top occurrence at 508 S and 908 E, placed R. crassa in ice-influenced waters. Investigation of more core-tops (Crosta, unpublished data) nevertheless indicates that this species has a warm water preference in agreement with the phytoplankton occurrences of (Hasle, 1969). We thus argue that cold occurrences of R. crassa may be due to misidentification with R. sima var. sima that has similarly large rounded otaria. Such continued observations on the distribution of the Rhizosolenia genus reinforce our objective of improving our DD228 database to accurately determine the distribution of each abovementioned species and their relationships to environmental parameters for which they appear discreetly linked. Other open ocean species presented here have a comparable distribution in modern sediments as described in earlier more regional studies. Rhizosolenia pointed group is mainly represented by R. antennata var. semispina with low participation of R. antennata var. antennata, the latter variety being the resting spore of R. antennata var. semispina (Sund-

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stro¨m, 1986). R. styliformis senso stricto, the third species of this group, has yet to be encountered in DD228. In the literature, R. antennata var. antennata, generally referred as to R. styliformis, has been encountered in surface sediments south of the Polar Front in the Pacific sector (Schrader, 1976) and in the Atlantic sector of the Southern Ocean (DeFelice and Wise, 1981; Zielinski and Gersonde, 1997). This is similar to the distribution evidenced in this study although R. pointed group achieves lower relative abundances in DD228. The agreement of all studies suggests that R. pointed group dwells in cold waters within the winter sea ice edge (Figs. 1a, 3) but prefers open ocean conditions throughout the growing season (Fig. 1b–d). This species also represents the main Rhizosolenia species found in Antarctic sediments. The T. nitzschioides var. lanceolata group is similar to the Thalassionema group of Zielinski and Gersonde (1997). Both groups show the same distribution (Fig. 5) and contribution to the total diatom assemblage with maximum relative abundances of 4% between 6 and 14 8C (Fig. 1a). The T. nitzschioides sensus stricto group is similar to the species identified as T. nitzschioides form 1 group of Zielinski and Gersonde (1997). With exception to one warm water occurrence (Fig. 1a), that may have been misinterpreted with T. nitzschioides var. parva, the T. nitzschioides group presents a comparable modern distribution to that of T. nitzschioides form 1 group although with lower relative abundances for the former species. Compared to Zielinski and Gersonde (1997) database, our DD228 lacks core-tops from the northern Weddell Sea where maximum abundances of T. nitzschioides form 1 are encountered. It is apparent that the Indian sector of the Southern Ocean is less favorable to the production and preservation of T. nitzschioides form 1 group. Thalassionema species are generally found in coastal upwelling or associated with nutrient-rich waters transported away from upwelling centers (Abrantes, 1988; Treppke et al., 1996). In Southern Ocean waters, it is possible that greater dust input and subsequent transport by the Weddell Gyre allow T. nitzschioides form 1 group to achieve higher numbers in the Atlantic sector than in the Indian which is more remote from micronutrient sources. Additionally, the Weddell Sea is a particular region where diatom production and particle fluxes are altered due to extended sea ice residence time and

to the presence of the Weddell Gyre (Gersonde and Zielinski, 2000). Inter-basinal discrepancies between diatom distribution and relative abundances are also described for cryophilic diatoms (Armand et al., 2005). It is obvious here that pelagic species are also affected by the particular conditions prevailing in the Weddell Sea. Comparison to additional literature is complicated because all Thalassionema taxa are lumped together in previous studies, hence damping the SST-linked succession of varieties observed here. F. kerguelensis represents the main diatom species of the Southern Ocean, reaching 80% of the total diatom assemblages of the sea-floor between the winter sea ice edge and the SAF (Fig. 2). It is the species that represents the main opal contributor to the bopal beltQ (Burckle and Cirilli, 1987) as previously originally observed in the Atlantic sector of the Southern Ocean by DeFelice and Wise (1981) and Zielinski and Gersonde (1997). T. lentiginosa and T. oliverana have the same distribution pattern as F. kerguelensis, displaying a maximum relative abundance between the winter sea ice edge and the SAF (Figs. 8 and 9). Specimens of these two species present heavily silicified valves that can significantly contribute to the opal belt. Thalassiothrix spp. has been shown to display maximum relative abundances in sediments of the PFZ in the Atlantic sector of the Southern Ocean (Zielinski and Gersonde, 1997). Thalassiothrix spp. are viewed as part of the shade flora reaching high phytoplankton abundances at low light depth (Laubscher et al., 1993; Kemp et al., 2000). The same distribution is observed in DD228 although the addition of core-top samples from the Indian sector of the Southern Ocean complicates the picture with occurrences in warm waters north of the SAF (Figs. 1a, 10). These northern occurrences may be linked to transportation of Thalassiothrix fragments by northward flowing bottom currents in the Crozet–Kerguelen region (Warren, 1978). Previous studies have assigned the T. gracilis group and T. reinboldii to Antarctic waters (Hendey, 1937; Kozlova, 1962; Donahue, 1973; DeFelice and Wise, 1981) although they have been detected in sediments of the Polar Front Zone (Zielinski and Gersonde, 1997). We believe that these northern occurrences are related to the presence of T. gracilis var. expecta that presents a more ubiquitous distribution and to the transport of fragments of the heavily silicified species

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T. reinboldii. In DD228, both the T. gracilis group and T. reinboldii are restricted to cold Antarctic waters (Fig. 1a) influenced by minimal sea ice coverage, especially during the diatom growth season (Fig. 1b– d). Both species groups are found at trace levels north of the Antarctic Polar Front (Figs. 7 and 11). In plotting the environmental data against diatom relative abundances in the DD228 it was illustrated that each species or group cited herein have strong affinities to SST and the presence/absence of sea-ice cover (Fig. 1). Each species or species group generally covers a limited range either of summer SST or sea ice conditions. Sea-ice parameters are more specifically important in determining the boundary of productivity to the south, such that summer sea-ice concentrations above 40% concentration provided an effective barrier to these open ocean species. Species abundances are generally weakly linked to winter sea-ice conditions, as seen by their evenly distributed abundances over a wide continuum of concentration, we interpret these distributions as illustrative of the preference of openocean species to melt back regions covered by sea ice for their spring proliferation. An exception to this is the generally clear avoidance of the open ocean species of the Rhizosolenia rounded group and of the Thalassionema genus to all sea ice conditions. We note a continuum in the diatom taxa with maximum abundances of a given species concomitant to a decrease or an increase in abundance of another species. The fact that peak abundances of different species are found at different SST and sea ice conditions is essential to validate the use of diatom remains to reconstruct past variations of these environmental parameters. In this vein, the Rhizosolenia pointed group, the T. gracilis group and T. reinboldii represent the coolest open ocean taxa of DD228. The cool taxa are followed by a group of pelagic open ocean species composed of F. kerguelensis, T. lentiginosa, T. oliverana and the Thalassiothrix spp. group. Warmer Antarctic taxa dominate when the abovementioned groups decrease. The warm, Antarctic group is represented in DD228 by the Rhizosolenia rounded group, the T. nitzschioides group and T. nitzschioides var. lanceolata. These open ocean species are replaced to the south by cryophilic diatoms (Armand et al., 2005) and to the north by subantarctic diatoms (Romero et al., 2005). The increase in the number of taxa to 25 species used in

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diatom transfer functions cover a summer SST range from 1–18 8C, a sea-ice duration range from 0 to 12 months per year and sea-ice concentrations from 15– 100%. The distribution of diatoms in modern sediments of the Southern Ocean highlights issues in taxonomic identification that require additional investigation. Indeed, misidentifications and species grouping clearly limit the sensitivity of any diatom transfer function and the resulting estimates. Mixing different species with different ecological preferences under one group blends the environmental signals of several species and hence, decreases the efficiency of the transfer function to give accurate estimations in downcore analyses. This study is based on the original database of Pichon et al. (1992a) in which several diatom species were misidentified (Armand, 1997). Although Pichon’s database has been recounted following an ameliorated taxonomy and successfully used to reconstruct sea-ice extent at the last glacial maximum (Crosta et al., 1998a,b) and over the last 200,000 years B.P. (Crosta et al., 2004), we show here that more work is still necessary on Rhizosolenia spp. and Thalassionema spp. due to the evolving nature of diatom research, advancing diatom taxonomy and the integration of data sets.

5. Summary and conclusions The study of 228 modern sediment samples from Atlantic and Indian sectors of the Southern Ocean allows us to determine the distribution of 10 major open ocean species or species group from the Subtropical Zone to the Antarctic Continental Shelf. The 10 species or species groups can be categorized into three groupings. First, a cool open ocean group composed of the Rhizosolenia pointed group, the T. gracilis group and T. reinboldii with an apparent tolerance to sea-ice conditions. Second, a pelagic open ocean group composed of F. kerguelensis, T. lentiginosa, T. oliverana and the Thalassiothrix spp. group. with maximum occurrences at the Antarctic Polar Front. Third, a warm open ocean grouping composed of the Rhizosolenia rounded group, the T. nitzschioides group and the T. nitzschioides var. lanceolata group with maximum abundances within the Polar Front Zone.

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Comparison with plankton studies (Hustedt, 1930, 1958; Hart, 1934; Hendey, 1937; Manguin, 1960; Hasle, 1969, 1976; Fenner et al., 1976; Burckle et al., 1987; Ligowski, 1993; Froneman et al., 1995; Eynaud et al., 1999; Fischer et al., 2002; Tremblay et al., 2002) indicates that sedimentary assemblages are different from the surface water communities due to the preferential dissolution and transport of thinly silicified diatoms. Sediments of the open ocean zone are subsequently enriched in the heavily silicified diatoms, F. kerguelensis and T. lentiginosa. Despite this enrichment/loss that mainly affects the proportion of the diatom species preserved in sediment, we show here that the pattern of diatom production in surface waters is conserved in modern sediments. This is a crucial step for the validation of diatom-based transfer functions to estimate past variations of sea-surface temperatures and sea-ice parameters. Diatoms have proved themselves to be one of the best tools to estimate past oceanographic changes in the Southern Ocean that are essential to constrain and/ or validate palaeo-climatic models. The quality of the reconstructions depends on our knowledge of diatom ecology and on our ability to accurately identify the species in fossil records. Because the distribution of open-ocean diatom species or species group from DD228 in modern sediments of the Southern Ocean agrees generally well with previous studies (Kozlova, 1966; Donahue, 1973; DeFelice and Wise, 1981; Burckle et al., 1987; Leventer, 1992; Tanimura, 1992; Zielinski and Gersonde, 1997) we conclude that the ecology of these species is no longer speculative and nearing pinpoint precision. Only the Rhizosolenia rounded group departs from previously depicted distributions and we believe that more work is needed on this genus. Although not perfect, the Rhizosolenia rounded group and the Rhizosolenia pointed group as determined here, represent a significant improvement since the Pichon et al. (1992a) database by discriminating cool taxa from warmer ones. The clear species-specific relationship of the remaining eight major species or species groups presented here against surface environmental parameters indicates that lumping several known species under one banner generally does not bias the ecological signal through its smoothing effect, thus giving consistency to diatombased sea-surface temperature (SST) and sea-ice duration (SIP) reconstructions. The Thalassiothrix

spp. group and T. reinboldii are more ubiquitous to SST and SIP, and may be considered as superfluous in future transfer function studies.

Acknowledgments Discussions with U. Zielinski, R. Gersonde, L. Burckle and A. Leventer during our research aided the development of this manuscript. We gratefully acknowledge L. Burckle (Lamont-Doherty Earth Observatory), D. Cassidy (FSU Antarctic Research Facility), and R. Gersonde (Alfred-Wegener-Institute), A. Leventer (Colgate University) and F. Taylor (IASOS, University of Tasmania) for their generous contribution of samples and/or percentage data. We thank Brian Harrold (Australian National University, Canberra) Rick Smith, Lisette Robertson and Nerida Bleakley (ACE CRC, Hobart) for help in drafting the maps. Two anonymous reviewers provided helpful comments on the manuscript. Xavier Crosta and JeanJacques Pichon’s research was funded by CNRS (Centre National de la Recherche Scientifique), PNEDC (Programme National d’Etude de la Dynamique du Climat), and Missions Scientifiques des Terres Australes et Antarctiques Franc¸aises (IFRTPTAAF). Oscar Romero’s research was funded by SFB26, University Bremen. Leanne Armand’s research was funded by an Australian Postgraduate Research Award (ANU, Canberra, Australia) and an Australian Research Council Postdoctoral Fellowship (F39800347) at the Institute of Antarctic and Southern Ocean Studies and the School of Earth Sciences at the University of Tasmania. This is UMR-CNRS 5805 EPOC contribution n81550.

References Abbott, W.H.J. (1973). Vertical and lateral patterns of diatomaceous ooze found between Australia and Antarctica. PhD Thesis, Department of Geology, University of South Carolina, USA. Abelmann, A., Gersonde, R., 1991. Biosiliceous particle flux in the Southern Ocean. Marine Chemistry 35, 503 – 536. Abrantes, F., 1988. Diatom assemblages as upwelling indicators in surface sediments off Portugal. Marine Geology 85, 15 – 39. Akiba, F., 1982. Late Quaternary diatom biostratigraphy of the Bellingshausen Sea, Antarctic Ocean. Report of the Technology Research Centre, J.N.O.C. 16, 31 – 74.

X. Crosta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66–92 Armand, L., (1997). The use of diatom transfer functions in estimating sea-surface temperature and sea-ice in cores from the southeast Indian Ocean. PhD Thesis—Australian National University, Canberra, Australia; p. 932, pl. 17. Armand, L.K., Zielinski, U., 2001. Diatom species of the genus Rhizosolenia from Southern Ocean sediments: distribution and taxonomic notes. Diatom Research 16 (2), 259 – 294. Armand, L.K., Crosta, X., Romero, O., Pichon, J.J., 2005. The biogeography of major diatom taxa in Southern Ocean sediments: 1. Sea ice related species. Palaeogeography, Palaeoclimatology, Palaeoecology. 223, 93–126 (this volume). Belkin, I.M., Gordon, A.L., 1996. Southern Ocean fronts from the Greenwich meridian to Tasmania. Journal of Geophysical Research 101 (C2), 3675 – 3696. Bodungen, B.V., Smetacek, V.S., Tilzer, M.M., Zeitzschel, B., 1985. Primary production and sedimentation during spring in the Antarctic Peninsula region. Deep Sea Research 33, 177 – 194. Burckle, L.H., 1981. Displaced Antarctic diatoms in the Amirante Passage. Marine Geology 39, 39 – 43. Burckle, L.H., 1984. Diatom distribution and paleoceanographic reconstruction in the Southern Ocean—Present and Last Glacial Maximum. Marine Micropaleontology 9, 241 – 261. Burckle, L.H., Humphreys, N.B., 1986. Diatom distribution in surface waters of the southern oceans: 1985–1986 austral summer cruise of U.S. Coast Guard Icebreaker Polar Star. Antarctic Journal of the U.S., 179 – 188. Burckle, L.H., Cirilli, J., 1987. Origin of the diatom ooze belt in the Southern Ocean: implications for the late Quaternary paleoceanography. Micropaleontology 33 (1), 82 – 86. Burckle, L.H., Jacob, S.S., McLaughlin, R.B., 1987. Late austral spring diatom distribution between New Zealand and the Ross Ice Shelf, Antarctica: hydrography and sediment correlations. Micropaleontology 33 (1), 74 – 81. Crosta, X., Pichon, J.J., Burckle, L.H., 1998a. Application of modern analog technique to marine Antarctic diatoms: reconstruction of the maximum sea ice extent at the last glacial maximum. Paleoceanography 13 (3), 284 – 297. Crosta, X., Pichon, J.J., Burckle, L.H., 1998b. Reapprisal of seasonal Antarctic sea-ice extent at the last glacial maximum. Geophysical Research Letters 25 (14), 2703 – 2706. Crosta, X., Sturm, A., Armand, L., Pichon, J.J., 2004. Late quaternary sea ice history in the Indian sector of the Southern Ocean as recorded by diatom assemblages. Marine Micropaleontology 50, 209 – 223. Cunningham, W., Leventer, A., 1998. Diatom assemblages in surface sediments of the Ross Sea: relationship to present oceanographic conditions. Antarctic Science 10 (2), 134 – 146. DeFelice, D.R., Wise, S.W., 1981. Surface lithofacies, biofaces, and diatom diversity patterns as models for delineation of climatic change in the Southeast Atlantic Ocean. Marine Micropaleontology 6, 29 – 70. Donahue, J.G., 1973. Distribution of planktonic diatoms in surface sediments of the Southern South Pacific. In: Goodel, H.G., et al., (Eds.), Marine Sediments of the Southern Oceans, Antarctic Map Folio Series, vol. 17. American Geophysical Society, p. 18 (pl. 9).

89

El-Sayed, S.Z., 1970. On the productivity of the Southern Ocean (Atlantic and Pacific sectors). In: Holdgate, M.W. (Ed.), Antarctic Ecology, pp. 119 – 135. El-Sayed, S.Z., 1990. Chapter 8: plankton. In: Glasby, G.P. (Ed.), Antarctic Sector of the Pacific. Elsevier, Amsterdam, pp. 95 – 125. Eynaud, F., Giraudeau, J., Pichon, J.-J., Pudsey, C.J., 1999. Sea surface distribution of coccolithophores, diatoms, silicoflagellates and dinoflagellates in the South Atlantic Ocean during the late austral summer 1995. Deep-Sea Research I, 451 – 482. Fenner, J., Schrader, H.J., Wienigk, H., 1976. Diatom phytoplankton studies in the Southern Pacific Ocean, composition and correlation to the Antarctic Convergence and its paleoecological significance. In: Hollister, C.D., Craddock, C., et al., (Eds.), Initial Reports of the Deep-Sea Drilling Project, vol. 35. U.S. Govt. Printing Office, Washington, D.C., pp. 757 – 813. Fischer, G., Gersonde, R., Wefer, G., 2002. Organic carbon, biogenic silica and diatom fluxes in the marginal winter seaice zone and in the Polar Front Region: interannual variations and differences in composition. Deep-Sea Research II 49, 1721 – 1745. Frenguelli, J., 1943. Diatomeas de las Orcadas del Sur. Extracto de la Revista del Museo de la Plata (Nueva serie). Seccion Botanica 5, 221 – 265. Froneman, P.W., Perissinotto, R., McQuaid, C.D., Laubscher, R.K., 1995. Summer distribution of net phytoplankton in the Atlantic sector of the Southern Ocean. Polar Biology 15, 77 – 84. Fryxell, G.A., 1994. Plaktonic marine diatom winter stages: Antarctic alternatives to resting spores (Reprinted). Kociolek, J.P. (Ed.), Memoirs of the California Academy of Sciences, Number 17, Proceedings of the 11th International Diatom Symposium, 12–17 August, 1990. California Academy of Sciences Publishers, San Francisco, pp. 437 – 448. Fryxell, G.A., Hasle, G.R., 1976. The genus Thalassiosira: some species with a modified ring of central strutted processes. Nova Hedwigia. Beiheft 54, 67 – 98. Fryxell, G.A., Hasle, G.R., 1979. The genus Thalassiosira: species with internal extensions of the strutted processes. Phycologia 18 (4), 378 – 393. Fryxell, G.A., Hasle, G.R., 1980. The marine diatom Thalassiosira oestrupii: structure, taxonomy and distribution. American Journal of Botany 67 (5), 804 – 814. Gersonde, R., 1984. Siliceous microorganisms in sea ice and their record in sediments in the southern Weddell Sea (Antarctica). Proceedings 8th Diatom Symposium Paris. Koeltz, Koenigstein, pp. 549 – 566. Gersonde, R., Wefer, G., 1987. Sedimentation of biogenic siliceous particles in Antarctic waters from the Atlantic sector. Marine Micropaleontology 11, 311 – 332. Gersonde, R., 1988. Siliceous microorganisms in sea ice and their record in sediment in the southern Weddell Sea (Antarctica). In: Round, F.E. (Ed.), Proceedings of the 9th Diatom-Symposium, Bristol 1986. S. Koeltz, Koenigstein, pp. 549 – 566. Gersonde, R., Zielinski, U., 2000. The reconstruction of late Quaternary Antarctic sea-ice distribution—the use of diatoms as a proxy for sea-ice. Palaeogeography, Palaeoclimatology, Palaeoecology 162, 263 – 286.

90

X. Crosta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66–92

Hallegraeff, G.M., 1986. Taxonomy and morphology of the marine plankton diatoms Thalassionema and Thalassiothrix. Diatom Research 1 (1), 57 – 80. Hargraves, P.E., 1979. Studies on marine plankton diatoms: IV. Morphology of Chaetoceros resting spores. Nova Hedwigia. Beiheft 64, 99 – 120. Hart, T.J., 1934. On the phytoplankton of the south-west Atlantic and the Bellinghausen Sea, 1929–1931. Discovery Reports VIII, 1 – 268. Hart, T.J., 1937. Rhizosolenia curvata Zacharias, an indicator species in the Southern Ocean. Discovery Reports XXI, 261 – 356. Hasle, G.R., 1965. Nitzschia and Fragilariopsis species studied in the light and electron microscope: III. The genus Fragilariopsis. Skrifter utgitt av det Norske Videnskaps-Akademi, Oslo 18, 1 – 45. 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 Skrifter, Scientific Results of Marine Biological Research. Universitetsforlaget, Oslo, p. 52. Hasle, G.R., 1974. Some marine plankton genera of the diatom family Thalassiosiraceae. Nova Hedwigia. Beiheft 45, 1 – 67. Hasle, G.R., 1976. The biogeography of some marine diatoms. Deep-Sea Research 23, 319 – 338. Hasle, G.R., De Mendiola, B.R.E., 1967. The fine structure of some Thalassionema and Thalassiothrix species. Phycologia 61 (2/3), 107 – 125. Hasle, G.R., Semina, H.J., 1987. The marine planktonic diatoms Thalassiothrix longissima and Thalassiothrix antarctica with comments on Thalassionema spp. and Synedra reinboldii. Diatom Research 2 (2), 175 – 192. Hasle, G.R., Sims, P.A., Syversten, E.E., 1988. Two recent Stellarima species: S. microtrias and S. stellaris (Bacillariophyceae). Botanica Marina 31, 195 – 206. Heiden, H., Kolbe, R.W., 1928. Die marinen diatomeen des Deutschen Sqdpolar Expedition 1901–1903. Deutschen Sqdpolar Expedition 8, 450 – 714. Hendey, N.I., 1937. The plankton diatoms of the Southern Seas. Discovery Reports XVI, 151 – 364. ¨ sterreichs und Hustedt, F., 1930. Die Kieselalgen Deutschlands, O der Schweiz. In: Rabenhorst, L. (Ed.), Kryptogamen-Flora ¨ sterreichs und der Schweiz, Reprint 1977. Otto Deutschlands, O Koeltz Science Publishers, West Germany, pp. 1 – 920. Hustedt, F., 1958. Diatomeen aus der Antarktis und dem Sqdatlaktik. Reprinted from bDeutsche Antarktishe Expedition 19838/1939Q Band II. Geographische-Kartographische Anstalt bMundusQ. Hamburg, p. 191. Johansen, J.R., Fryxell, G.A., 1985. The genus Thalassiosira (Bacillariophyceae): studies on species occurring south of the Antarctic Convergence. Phycologia 24 (2), 155 – 179. Jouse´, A.P., Koroleva, G.S., Nagaeva, G.A., 1962. Diatoms in the surface layer of sediment in the Indian sector of the Antarctic. Trudy Instituta Okeanologii Akademiya Nauk SSSR 61, 20 – 91.

Kellogg, D.E., Kellogg, T.B., 1987. Microfossil distributions in modern Amundsen Sea sediments. Marine Micropaleontology 12, 203 – 222. Kemp, A.E.S., Pike, J., Pearce, R.B., Lange, C.B., 2000. The bFall dumpQ—a new perspective on the role of a bshade floraQ in the annual cycle of diatom production and export flux. Deep-Sea Research II 47, 2129 – 2154. Kozlova, O.G., 1962. Specific composition of diatoms in the waters of the Indian sector of the Antarctic. Trudy Instituta Okeonologii Akademiya Nauk SSSR 61, 3 – 18. Kozlova, O.G., 1966. Diatoms of the Indian and Pacific Sectors of the Antarctic. National Science Foundation. Israel Program for Scientific Translations, Washington, D.C., p. 185 (pl. I–VI). Kozlova, O.G., Mukhina, V.V., 1967. Diatoms and silicoflagellates in suspension and floor sediments of the Pacific Ocean. International Geology Review 9 (10), 1322 – 1342. Laubscher, R.K., Perissinotto, R., McQuaid, C.D., 1993. Phytoplankton production and biomass at frontal zones in the Atlantic Sector of the Southern Ocean. Polar Biology 13, 471 – 481. Lee, J.H., Lee, J.Y., 1990. A light and scanning electron microscopic study on the marine diatom Roperia Tessalata (Roper) Grunow. Diatom Research 5 (2), 325 – 335. Leventer, A., 1991. Sediment trap diatom assemblages from the northern Antarctic Peninsula region. Deep Sea Research 38 (8), 1127 – 1143. Leventer, A., 1992. Modern distribution of diatoms in sediments from the Georges V coast Antarctica. Marine Micropaleontology 19, 315 – 332. Leventer, A., Dunbar, R.B., DeMaster, D.J., 1993. Diatom evidence for the Late Holocene climatic events in Granite Harbor, Antarctica. Paleoceanography 8 (3), 373 – 386. Leventer, A., et al., 2001. CHAOS (Coring Holocene Antarctic Ocean Sediments) cruise report, p. 160. Leventer, A., Domack, E., Barkoukis, A., McAndrews, B., Murray, J., 2002. Laminations from the Palmer Deep: a diatom-based interpretation. Paleoceanography 17 (2). Ligowski, R., 1986. Net phytoplankton of the Admiralty Bay (King George Island, South Shetland Islands) in 1983. Polish Polar Research 7 (1–2), 127 – 154. Ligowski, R., 1988. Distribution of net phytoplankton in the Scotia Front west of Elephant Island (BIOMASS III, October–November 1986). Polish Polar Research 9 (2–3), 243 – 264. Ligowski, R., 1993. Marine Diatoms (Bacillariophyceae) in the Antarctic Ecosystem and Their Importance as An Indicator of Food Source of Krill (Euphasia Superba Dana). Wydanwnictwo Uniwersytetu Lo´dzkiego, Lo´dz, Poland, p. 242. Ligowski, R., Kopczynska, R., 1991. Distribution of net phytoplankton in the sea-ice zone between Elephant Island and the South Orkney Islands (December 1988–January 1989). Polish Polar Research 12 (4), 529 – 546. Lisitzin, A.P., 1960. Bottom sediments of the eastern Antarctic and the Southern Indian Ocean. Deep-Sea Research 7, 89 – 99. Lozano, J.A., Hays, J.D., 1976. Relationship of radiolarian assemblages to sediment types and physical oceanography in the Atlantic and Western Indian Ocean Sectors of the Antarctic Ocean. In: Cline, R.M., Hays, J.D. (Eds.), Investigation of late

X. Crosta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66–92 Quaternary Paleoceanography and Paleoclimatology. Geological Society of America, pp. 303 – 336. Manguin, E., 1960. Les diatome´es de la Terre Ade´lie. Annales des Sciences Naturelles 1, 223 – 385. Medlin, L.K., Sims, P.A., 1993. The transfer of Pseudonotia doliolus to Fragilariopsis. Nova Hedwigia. Beiheft 106, 323 – 334. Moreno-Ruiz, J.L., Licea, S., 1994. Observations on the valve morphology of Thalassionema nitzschoides (Grunow) Hustedt. In: Marino, D., Montresov, M. (Eds.), Proceedings of the 13th Symposium on Living and Fossil Diatoms, 1st–7th Sept, 1994. Biopress Limited Publisher, Bristol, Maretea, Italy, pp. 393 – 413. Nelson, D.M., Treguer, P., Brzezinski, M.A., Leynaert, A., Que´guiner, B., 1995. Production and dissolution of biogenic silica in the ocean: revised global estimates, comparison with regional data and relationship to biogenic sedimentation. Global Biogeochemical Cycle 9 (3), 359 – 372. Neori, A., Holm-Hansen, O., 1982. Effect of temperature on rate of photosynthesis in Antarctic phytoplankton. Polar Biology 1, 33 – 38. Orsi, A.H., Whitworth, T., Nowlin, W.D., 1995. On the meridonal extent and fronts of the Antarctic Circumpolar Current. DeepSea Research 42 (5), 641 – 673. Pichon, J.J., Labeyrie, L., Bareille, G., Labracherie, M., Duprat, J., Jouzel, J., 1992a. Surface water temperature changes in the high latitudes of the Southern Hemisphere over the Last Glacial– Interglacial cycle. Paleoceanography 7 (3), 289 – 318. Pichon, J.J., Bareille, G., Labracherie, M., Labeyrie, L., Baudrimont, A., Turon, J.L., 1992b. Quantification of the biogenic silica dissolution in the Southern Ocean sediments. Quaternary Research 37, 361 – 378. Pokras, E.M., Molfino, B., 1986. Oceanographic control of diatom abundances and species distribution in surface sediment of the Tropical and Southeast Atlantic. Marine Micropaleontology 10, 165 – 188. Pondaven, P., Ragueneau, O., Tre´guer, P., Hauvespre, A., De´zileau, L., Reyss, J.L., 2000. Resolving the dopal paradoxT in the Southern Ocean. Nature 405, 168 – 172. Prasad, A.K.S.K., Nienow, J.A., 1986. Marine diatoms of sediments from Croft Bay, Antarctica. Antarctic Journal of the United States 21, 157 – 159. Priddle, J., Jordan, R.W., Medlin, L.K., 1990. Famille Rhizosoleniaceae. In: Medlin, L.K., Priddle, J. (Eds.), Polar Marine Diatoms. British Antarctic Survey, Natural Environment Research Council, Cambridge, pp. 115 – 127. Romero, O., Mollenhauer, G., Schneider, R.R., Wefer, G., 2003. Oscillation of the siliceous imprint in the central Benguela upwelling system from MIS 3 through to Early Holocene: the influence of the Southern Ocean. Journal of Quaternary Science 18 (8), 733 – 743. Romero, O., Armand, L.K., Crosta, X., Pichon, J.J., 2005. The biogeography of major diatom taxa in Southern Ocean sediments: 3. Tropical/Subtropical species. Palaeogeography, Palaeoclimatology, Palaeoecology. 223, 49–65 (this volume). Round, F.E., Crawford, R.M., Mann, D.G., 1990. The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, p. 747.

91

Schrader, H., 1973. Cenozoic diatoms from the northeast Pacific, Leg 18. Initial Reports Deep Sea Drilling Project 18, 673 – 797. Schrader, H.J., 1976. Cenozoic planktonic diatom biostratigraphy of the Southern Pacific Ocean. In: Hollister, C.D., Craddock, C., et al., (Eds.), Initial Reports of the Deep Sea Drilling Project, vol. 35. U.S. Government Printing Office, Washington, pp. 605 – 671. Schrader, H., Fenner, J., 1976. Norwegian Sea Cenozoic diatom biostratigraphy and taxonomy: Part I. Norwegian Sea Cenozoic diatom biostratigraphy: Part II. Diatoms at Leg 38, taxonomic references. Initial Reports Deep Sea Drilling Project 38, 921 – 1099. Schrader, H.J., Gersonde, R., 1978. Diatoms and silicoflagellates. In: Zachariasse, A., et al., (Eds.), Micropaleontological Counting Methods and Techniques—An Exercise on an Eight Meters Section of the Lower Pliocene of Capo Rossello, Utrecht Micropaleontological Bulletins, Silicy, pp. 129 – 176. Schuette, G., Schrader, H., 1981. Diatom taphocoenoses in the coastal upwelling area off south west Africa. Marine Micropalaeontology 6, 131 – 155. Semina, H.J., 1979. The geography of plankton diatoms of the Southern Ocean. Nova Hedwigia. Beiheft 64, 341 – 358. Semina, H.J., 1981. A morphological examination of the diatom Thalassiothrix longissima Cleve and Grunow. Bacillaria 4, 147 – 160. Semina, H.J., 2003. SEM-studied diatoms of different regions of the World Ocean. Iconographia Diatomologica 10, 1 – 362. Shemesh, A., Burckle, L.H., Froelich, P.N., 1989. Dissolution and preservation of Antarctic diatoms and the effect on sediment thanatocenoses. Quaternary Research 31, 288 – 308. Simonsen, R., 1974. The diatom plankton of the Indian Ocean expedition of R/V Meteor 1964/–1965. Meteor-Forschungsergebnisse. Reihe D., Bilologie 19, 1 – 107. Simonsen, R., 1982. Note on the diatom genus Charcotia M. Peragallo. Bacillaria 5, 101 – 116. Simonsen, R., 1992. The diatom types of Heinrich Heiden in Heiden and Kolbe 1928. Bibliotheca Diatomologica Band 24, 100 (pl. 1-86). Smayda, T.J., 1970. The suspension and sinking of phytoplankton in the sea. Oceanography and Marine Biology, A Review 8, 353 – 414. Smetacek, V.S., 1985. Role of sinking in diatom life–history cycles: ecological, evolutionary and geological significance. Marine Biology 84, 239 – 251. Sournia, A., Grall, J.R., Jacques, G., 1979. Diatome´es et dinoflagelle´s planctoniques d’une coupe me´ridienne dans le sud de l’Oce´an Indien (campagne Antipode I du Marion-Dufresne, mars 1977). Botanica Marina 22, 183 – 198. Stockwell, D.A., Hargraves, P.E., 1986. Morphological variability within resting spores of the marine diatom genus Chaeoceros Ehrenberg. In: Ricard, M. (Ed.), Proceedings of the 8th Symposium on Living and Fossil Diatoms. S. Koeltz, Koenigstein, pp. 81 – 95. Stockwell, D.A., Kang, S.H., Fryxell, G.A., 1991. Comparisons of the diatom biocenoses with Holocene sediment assemblages in Prydz Bay. In: Barron, J., Larsen, B., X, X. (Eds.), Proceedings

92

X. Crosta et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 66–92

of the Ocean Drilling Program. Scientific Results, vol. 119. Ocean Drilling Program, College Station, TX, pp. 599 – 609. Stramski, D., Reynolds, R.A., Kahru, M., Mitchell, B.G., 1999. Estimation of particulate organic carbon in the ocean from satellite remote sensing. Science 285, 239 – 241. Sundstro¨m, B.G.,1986. The marine diatom genus Rhizosolenia. A new approach to the taxonomy. PhD Thesis, Univ. Lund, p. 196. Tanimura, Y., 1992. Distribution of diatom species in the surface sediments of Lqtzow–Holm Bay, Antarctica. In: Ishizaki, K., Saito, T. (Eds.), Centanary of the Japanese Micropaleontology. Terra Scientific Publishing Company, Tokyo, pp. 399 – 411. Taylor, F., McMinn, A., Franklin, D., 1997. Distribution of diatoms in surface sediments of Prydz Bay, Antarctica. Marine Micropaleontology 32, 209 – 229. Tre´ guer, P., Jacques, G., 1992. Dynamics of nutrients and phytoplankton, and fluxes of carbon, nitrogen and silicon in the Antarctic Ocean. Polar Biology 12, 149 – 162. Tre´guer, P., Nelson, D.M., van Bennekom, A.J., DeMaster, D.J., Leynaert, A., Que´guiner, B., 1995. The silica balance in the world ocean: a reestimate. Science 268, 375 – 379. Tremblay, J.E., Lucas, M.I., Kattner, G., Pollard, R., Strass, V.H., Bathmann, U., Bracher, A., 2002. Significance of the polar frontal zone for large-sized diatoms and new production during summer in the Atlantic sector of the Southern Ocean. Deep Sea Research II 49, 3793 – 3811. Treppke, U.F., Lange, C.B., Wefer, G., 1996. Vertical fluxes of diatoms and silicoflagellates in the eastern equatorial Atlantic, and their contribution to the sedimentary record. Marine Micropaleontology 28, 73 – 96. Truesdale, R.S., Kellogg, T.B., 1979. Ross Sea diatoms: modern assemblage distributions and their relationship to ecologic,

oceanographic and sedimentary conditions. Marine Micropaleontology 4, 13 – 31. van der Loeff, M.M.R., Buesseler, K., Bathmann, U., Hense, I., Andrews, J., 2002. Comparison of carbon and opal export rates between summer and spring bloom periods in the region of the Antarctic Polar Front, SE Atlantic. Deep Sea Research II 49, 3849 – 3869. van der Spoel, S., Hallegraeff, G.M., Soest, R.W.M.v., 1973. Notes on variation of diatoms and silicoflagellates in the South Atlantic Ocean. Netherlands Journal of Sea Research 6 (4), 518 – 541. van Iperen, J.M., Weering, T.C.E.v., Jansen, J.H.F., van Bennekom, A.J., 1987. Diatoms in surface sediments of the Zaire deep-sea fan (SE Atlantic Ocean) and their relation to overlying water masses. Netherlands Journal of Sea Research 21 (3), 203 – 217. Warren, B.A., 1978. Bottom water transport through the Southwest Indian Ridge. Deep-Sea Research 25, 315 – 321. Zielinski, U., 1993. Quantitative estimation of palaeoenvironmental parameters of the Antarctic Surface Water in the Late Quaternary using transfer functions with diatoms. PhD Thesis, Alfred Wegener Institute for Polar and Marine Research, D27568 Bremerhaven, Germany. Zielinski, U., Gersonde, R., 1997. Diatom distribution in Southern Ocean surface sediments (Atlantic Sector): implications for paleoenvironmental reconstructions. Paleogeography, Paleoclimatology, Paleoceanography 129, 213 – 250. Zielinski, U., Gersonde, R., Sieger, R., Fqtterer, D., 1998. Quaternary surface water temperature estimations: calibration of a diatom transfer function for the Southern Ocean. Paleoceanography 13 (4), 365 – 384.