mum ne rnI~RoPaLE km LOGY

ELSEVIER Marine Micropaleontology 28 (1996) 53-72

Planktic foraminifera from the southwestern Atlantic ( 30”~60’S) :

species-specific patterns in the upper 50 m

Esteban Boltovskoy a3c, Demetrio Boltovskoy b,c, Nancy Correa d, Frederic0 Brandini e

a Museo Argentina de Ciencius Nuturales “Bernardino Rivadavia” , Au. Angel Gallardo 470, 1405 Buenos Aires, Argentina

b Departamento de Ciencias Biolcigicas, Fucultad de Ciencias Exactas y Naturules, Universidad de Buenos Aires, 1428 Buenos Aires,

Argentina

’ Consejo National de lnvestigaciones Cientfjicas y Tkcnicas, Argentina

’ Servicio de Hidrograjia Naval, Au. Mantes de Oca 2124, 1271 Buenos Aires, Argentina

e Centro de EstudoJ do Mar, Univeklade Federal do Paran&, Au. Beira Mar s/n. Pontul do &I, PR 83255-000, Brazil

Received 17 April 1995; accepted 12 September 1995

Abstract

Planktic foraminifers were studied in 96 samples collected in the southwestern Atlantic (30”-6O”S, along 53”W) in November 1993, mainly from depths between 0 and 50 m. Very high proportions of juveniles (unidentified) were present throughout the area, especially north of 37‘S, where they accounted for up to 70-80% of all shells recorded. For most species no clear vertical specific stratification was detected in the O-50 m layer. Zoogeographic grouping of the 18 species identified allowed the defining of 5 distinct zones along the transect: Subtropical (north of 31”S, 80% warm water individuals); Warm-Transitional (34”-37”S, 35% warm water); Transitional (37”-49’S, 99% cold water); Subantarctic (49-55”s. 100% cold water); and Antarctic (south of 56”s. 100% cold water). Boundaries between foraminiferal assemblage zones are in good agreement with hydrological fronts described for the area. Comparison of the present data with planktic collections from the North Atlantic show large differences in the proportions of various taxa. In the 14-24°C range, G. bulfoides is much more abundant in the northern hemisphere than in the southern one, whereas G. quinqueloba, G. injZata

and G. rubescens show the opposite trend. On the other hand, temperature-related percentage contributions within the 14”-24°C range indicate that the preferred thermic regimes of the 9 species considered are remarkably similar in the North and South Atlantic collections compared. The southernmost planktic distributional ranges of selected warm water taxa are roughly coincident with those established previously on the basis of surface sediments, disagreements being chiefly attributable to selective dissolution on the bottom. In contrast, at the bottom percentages of cold water foraminifers (G. bulloides, G. pachyderma) are significantly enhanced with respect to their planktic populations, and their sedimentary northward limits extend well beyond their maximum ranges in the upper-layer plankton. It is suggested that submergence of these cold water species and northward displacement at subsurface depths is chiefly responsible for the mismatching patterns observed. Dissimilar species proportions in both hemispheres and plankton-sediment uncoupling can conceivably engender erroneous conclusions when distributional data are used for paleoecologic reconstructions with the aid of numerical analyses such as the Imbrie-Kipp transfer-function technique.

0377-8398/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved .SSDl 0377-8398(95)00076-3

1. Intr~u~~ion

Although planktic foraminifers have proved use- ful for biogeographic, hydrographic and paleoceano- graphic investigations, problems associated with the int~~retation of environmental info~ation con- veyed by species proportions and species-specific changes in relative abundance persist (see reviews in E. Boltovskoy, 1970; E. Boltovskoy and Wright, 1976; Hemleben et al., 1989). Vertical distribution patterns and seasonal changes in assemblage compo- sition have a strong bearing on the conclusions of biogeographic studies, as well as on analyses of currents and water masses based on biological indi- cators (e.g., E. Boltovskoy, 1970; Cifelli and Be”nier, 1976; Reynolds and Thunnell, 1985). Furthermore, both of the above, and interoceanic and interhemi- spheric differences in specific makeups at compara- ble ambient rbgimes, seriously restrict the wide ap- plicability of regional data for large-scale paleo- ceanographic studies (Be’, 1969; E. Boltovskoy, 1970; Berger, 1981; D. Boltovskoy, 1994). While sedimen- tary and sediment trap samples can provide much useful information, many of the gaps in our knowl- edge of the problems involved require analyses of planktic material. However, because plankton sam- ples are usually represented by a sequence of snap- shots which accounts for an insignificant proportion of the total time at which seasonal and interannual variability operate, accumulation of me~ingful in- formation is a slow process. This is probably one of the chief reasons why for most microfossil groups information on the living assemblages is much scarcer and fragment than that derived from their sedi- mentary deposits (D. Boltovskoy, 1994).

The present work presents the planktic distribu- tional patterns of foraminiferal assemblages recorded in the upper 50 m of the southwestern Atlantic, between the subtropics and the Antarctic. Compari- son with similar collections from the North Atlantic and with sedimentary data from the South Atlantic emphasizes some major problems in the application of microfossil data for pale~e~ographic surveys.

2. Material and methods

Ninety-six plankton samples were collected in November 1993, by means of O- 150 m stratified

;UBTROPlCAL

SUBANTARCTIC *13 .#”

**D..**

I ---- 700 600 50-J & 4

Fig. 1. Station map, main surface currents and fomm~ifem~ biogeographic zones (the latter according to E. Boltovskoy, 1970).

300

I350

400

.450

500

550

-600

vertical tows, between approx. 30” and 60’S, aiong a nominal longitude of 53”W (Fig. 11, from the Brazil- ian research and Antarctic support vessel Bar& de Tefl& (TABIA I cruise). Sampling depths typically covered the following fayers: 0, 5-15, 15-30 and 30-50 m, with some tows between 50 and 100 m, and a few ranging down to 150 m (Tables 1 and 2). Nets were 30 cm in diameter, provided with 30 ,um gauze and digital flowmeters (non-flowmetered, 64 or 72 pm nets for the surface tows). Material was preserved with 3-5% buffered formaldehyde. On average, 160 foraminiferal tests were hand-picked from each wet, unsieved sample (15,948 specimens in total), and identified under a compound micro- scope. Due to the lack of information on the volumes

E. Boltovskoy et al./ Marine Micropaleontology 28 (1996) 53-72 55

Table 1 Station data and numbers of fomminifers identified per sample

of water filtered in surface tows, and because of the separation technique used (hod-picking from unpro- cessed samples), which makes absolute qu~titative estimates marginally reliable, all counting data were transformed into percentages for analyses of species-specific distributional patterns. Unless other- wise indicated, percentage values used throughout the text and figures refer to identified shells only, excluding the unidentified fraction (see below) (per- centages indicated in Table 2 are based on total assemblages).

Water-temperature data are based on 117 surface me~urements and 45 XBT (O-200 to O-1800 m) launchings along the transect occupied. Chlorophyll a values were determined at I 13 sites at the surface and at 20 stations at 0, 5, 10, 25, 50, 75, 100, 1.50 and 200 m.

3. Environmental setting

Surface-layer currents and water-masses in the southwestern Atlantic (Fig. 1) were described on the basis of physico-chemical data (Emilsson, 196 1; Reid et al., 1977; E, Boltovskoy, 1971; Roden, 1986; Piola et al., 1987; Olson et al., 1988; Gordon, 1989; Peterson and Stramma, 199 I ; Peterson, 1992; Tsuchiya et al., 1994); and using biological indica- tors as current tracers and sensors of distinct envi- ronmental settings (E. Boltovskoy, 1970; Semenov and Berman, 1977; D. Boltovskoy, 1981).

South of the Polar Front (approx. 50” to 60’S, depending on longitude and season) the surface layer is occupied by Antarctic Surface Waters (-I 1.9” to 3.5”C), which sink under the less dense Subantarctic Surface waters at the Front and move north as Antarctic Inte~ediate Waters reaching at least 40”s and, according to some authors, as far north as 20”-25’S (as Antarctic or Subantarctic Intermediate Waters; Thomsen, 1962; Lenz, 1975; Tsuchiya et al., 1994). This southern area is characterized by east- ward and northeastward movement of the Antarctic Circumpolar Current (ACC, = West Wind Drift>. Part of the ACC branches off and moves north to northeast over the continental slope forming the Malvinas (= Falkland) Current (MC). Along the slope pure MC waters (typically 5”--12”C, up to 17°C at its no~emmost bounds) can reach, at the

56

Table 2

E. Boltouskoy rt 01. / Murine Micropuleontology 28 ( 1996) 53-72

Species counts for the entire collection surveyed (transformed into percentages)

I

,

1

1

, ,

,

I

,

b

,

1

1

1

,

1

,

,

1

:

,

1

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, , , 1

,

1

,

,

,

1

,

1

,

,

,

I

1

1

,

I

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: ,

,

8 ,

I

3

,

,

:

,

E. Boltouskoy et al./ Murine Micropuleontolo~y 28 (1996) 53-72 51

surface, as far north as 35%. North of this latitude Subtropical input, known as the Transition Zone the MC disappears from the surface, but Subantarctic (TZ, = Subtropical-Subantarctic Convergence Zone), waters are regularly present in subsurface layers as stretches between approx. 48” and 29% in the winter, far north as 2O”S, as evidenced by consistent records and between 49” and 34% in the summer; its western of typically Subantarctic and even Antarctic plank- boundary is located between ca. 56”W (at 46”-47%) ters in restricted upwelling cells off the Brazilian and 52”W (at 34”-35”s). The northernmost stations coast (E. Boltovskoy, 1970; Valentin et al., 1987; of the transect occupied (l-3, see Fig. 1) are located Brandini, 1990; see discussion below). To the east of in the Subtropical waters of the Brazil Current which ca. 55”W the MC is deflected toward the south and originates close to the equator upon branching of the the east entering a large anticyclonic gyre where Southequatorial Current. Subantarctic waters mix with Subtropical ones from Surface temperatures at the time of the cruise the Brazil Current (BC), and with waters from the varied between approx. 25” and 0°C. Vertical stratifi- central Atlantic. This large area of mixture and with cation was generally absent in the upper 50 rn, and isolated cells and tongues of pure Subantarctic and the north-south temperature gradient was smooth,

i

250

500

750

Stations 14 15 16 I:"19

1000 1-J-I 350 400 450 500 550

Latitude south

Fig. 2. Temperature fields along the transect occupied, as revealed by 45 XBT (O-200 to O-1800 m) launchings. Notice lack of vertical

stratification in the upper 50 m. Shaded sections denote areas with strong doming of thermoclines from depths > 500 m defining an

upwelling cell centered on 37-38”s.

with the exception of a conspicuous upwelling cen- tered on 37”-38”s (Fig. 2). Advection of deeper waters at these latitudes was also indicated by no- ticeable peaks in the concentrations of phosphates and nitrates: phosphates increased from a back- ground level of 0.4-0.6 to 1.6 pM and nitrates from 5-7 to 17 PM. Surface chlorophyll a, which was moderate throughout the transect (0.2-0.3 to 1.2-l .4 @g/l), also sho wed a dramatic response to this nutrient enrichment increasing to ca. 3.5 pg,/l at station 6 from the surface to about 25 m (Kocmur et al., 1993). Fig. 2 (lower panel) shows that doming of these colder waters is noticeable from depths over 500 m.

4. Results md discussion

4.1. General ouerview of the fauna

Almost all samples contained abundant foramini- fers, with the exception of a few which were either barren of them, or where calcareous shells were severely deteriorated, presumably due to the low pH values of the preservative (Tables 1 and 2). How- ever, in most samples the proportions of unidentifi- able specimens (mostly represented by early juve- niles) were very high (mean: 35%; see Table 2).

Lat . S

31 32 34 36 37 39 41 44 48 49 53 54 57 59

Samples collected with 30 pm nets had higher pro- portions of unidentifieds (36%), than those retrieved with 64 and 72 pm nets (30%), yet the two means did not differ significantly (P = 0.353, r-test with log-transformed data).

Proportions of juvenile (unidentified) foraminifers along the transect studied are illustrated in Fig. 3. Highest percentages were recorded between approx. 32” and 42”S, during the first 14- 15 days of the expedition. This interval coincides with the first phase of the position between a new moon and a full moon. Considering that for several foraminiferal species reproduction is closely coupled with the lu- nar cycle (Hemleben et al., 1989), one could assume that enhanced proportions of juveniles reflect foraminiferal reproductive cycles. However, relative abundances of unidentifieds were also closely (nega- tively) correlated with those of cold water forms (Pearson’s r: -0.836, P < 0.001); this could sug- gest: (I> that the juveniles of cold water taxa are more readily identifiable than those of the warm water ones, (2) that proportions of immature individ- uals are higher among the warm water species, and (3) both of the above. Unfo~unately, no measure- ments were taken during counts and identifications of the fauna, which precludes us from drawing firm conclusions, but a subjective general appreciation by the senior author of the identification difficulties

90

80

g 70

4 5 60

g 50

a 40

7 8 9 10 II $2 13 14 z 15 16 17 18 19 20 21

I? Date (Nov. 1993) ff

New Full moon moon

Fig. 3. Percentages of cold water and of unidentified (juvenile) foraminifers as a function of date.

E. Boltovskoy et al./Marine Micropaleontology 28 (1996) 53-72 59

encountered does not point at greater uncertainties with the warm water species, as compared with the cold water ones. However, because the northern reaches of the area surveyed are close to the distribu- tional boundaries of the Subtropical foraminifers en- countered (see below), it may be that many of their representatives develop atypical morphologies char- acteristic of marginally suitable living conditions (e.g., Hecht, 1974), thus defying identification. In- deed, highest proportions of unidentifieds roughly coincide with Subtropical-Subantarctic transitional waters (Figs. 3 and 51, where the Subtropical fauna is replaced by the Subantarctic one (see below).

In total, 18 foraminiferal species were recorded. Clearly warm water forms and those with higher affinity for warm than for cold waters were repre- sented by Globigerina falconensis, Globigerina rubescens, Globigerinella aequilateralis, Globigeri- noides conglobatus, Globigerinoides ruber, Glo- bigerinoides trilobus, Globoquadrina dutertrei, Globorotalia menardii, Hastigerina pelagica and Orbulina universa. Generally cold water foraminifers included the following: Globoquadrina pachyderma (left-coiling), Globoquadrina pachyderma (right- coiling), Globigerina quinqueloba, Globigerina bul- loides, Globigerinita uvula, Globorotalia truncatuli- noides and Globorotalia scitula. The degree of affin- ity for warm or cold waters is not alike for all the species within each group. The two categories indi- cate that in the area investigated the former are generally more abundant in waters associated with the Subtropical Brazil Current, whereas the latter preferably thrive in areas influenced by the Sub- antarctic Malvinas Current. These patterns indicate that their corresponding distributional centers, which in turn are defined by the ranges of reproductively active populations, are disjunct and located at oppo- site ends of the N-S axis. Degrees of mixing of faunas of different provenances are therefore depen- dent on hydrographic features, and on the environ- mental tolerance of the species involved.

As opposed to the above, Globorotalia injlata and Globigerinita glutinata do not seem to be associated with Subtropical or Subantarctic water masses (E. Boltovskoy, 1970, 1981a). Both are present in mod- erate abundances throughout the latitudinal range covered, but G. injlata usually peaks in mixed Brazil-Malvinas waters, and thus is defined as a

transitional species. G. glutinata, which spans the entire area without noticeable variations in abun- dance, is considered cosmopolitan.

The warm water assemblage was strongly domi- nated by G. ruber, G. trilobus and G. rubescens (Fig. 4), which accounted for 87% of all Subtropical foraminifers recorded. Most warm water species de- clined significantly south of 32’S, dropping to negli- gible proportions at 37”-38”S, yet traces of G. menardii, G. rubescens and G. aequilateralis were present as far south as 48”-49”s (Fig. 4). Species characteristic of Subantarctic and Antarctic waters were conspicuously more abundant in the collection (90%) than the warm water ones; their assemblages being dominated by G. pachydema (left), G. quin- queloba and G. bulloides (85% of the cold water specimens recorded; see Fig. 4). With the exception of G. scitula, G. truncatulinoides (both very rare) and G. pachyderma (left), cold water foraminifers were present throughout the entire transect covered (Fig. 4). A geographically isolated abundance peak of cold water taxa coincided with the strong up- welling at 37”-38”s (Fig. 2). Interestingly, highest proportions of Subantarctic fauna occur in the upper- most 10 m (see below, Fig. 51, where a strong increase in phytoplankton is indicated by sharp en- hancements of chlorophyll a values, reaching over 3 pg/l. South of approx. 41”-42’S cold water species account for over 50% and up to over 90% of all foraminifers (Fig. 4). On average, G. inflata made up 4% of the shells identified and was consistently present between 34” and 56”s; at 42”S, however, its contribution reached 33% (Fig. 4). G. glutinata, cosmopolitan, was generally scarce (mean contribu- tion: 2%), but appeared consistently at all latitudes (present at 22 of the 26 stations covered).

4.2. Vertical pattern in the upper 50 m

Fig. 5 illustrates the vertical profile of warm water and cold water foraminifers in the upper 50 m between 30” and 60”s (data for deeper layers are too scarce to yield meaningful results). Warm water assemblages dominated between 30” and 32”S, and, surprisingly, also yielded high proportions at 35”- 36”s below 30 m. Their overall contribution to the total of foraminifers, however, was generally below 50%, the remainder being largely represented by

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Sub

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E. Boltvuskoy et d/Marine Micropaleontology 28 (1996) 53-72 61

unidentified juveniles. However, the tight geographic

coupling between enhanced proportions of unidenti- fied specimens and those of warm water taxa might indicate that significant fractions of the former are

indeed Subtropical foraminifers (see above); thus, north of approx. 35”s the actual percentages of warm

water individuals are most likely higher than those

reported here. As far as the limited vertical resolution of our

data set suggests, no clear stratification of warm

water and cold water assemblages appears in the

upper 50 m. Although Subtropical forms accounted for higher percentages of the overall assemblages

between 31”-32”s and 36”-37”s below approx. 30 m than between 0 and 30 m, and subpolar

foraminifers also showed some peak values below 30 m south of 55”S, most of these discontinuities are

based on a few isolated data points and are therefore of limited significance. Furthermore, some blurring

and distortion is also due to the fact that, while actual data are based on towing intervals, the con- tours on Fig. 5 assume homogeneous specific com- positions between the bottom and the top of each tow, which most probably is not the case, especially in the deeper (and longer) tows. Data on some isolated cold water species, on the other hand, do suggest depth-related trends in association with lati- tude. For example, throughout the transect sampled the percentages of G. pachyderma were generally higher with increasing depth (Fig. 6B), suggesting that populations of this species dwell deeper as they are carried northward with the Malvinas Current (see below). The warm water G. ruber, on the other hand, was generally less abundant below the surface than at 0 m (Fig. 6A).

4.3. Fauna1 zones and hydrography

Faunistically distinct geographic areas along the transect, as revealed by a cluster analysis based on pooled data for all depths at each station, are illus-

trated in Fig. 7. Discrimination between hydrologi-

cally different sectors of the transect is very precise: with few exceptions the similarities within each bio-

geographic zone are very high when compared with the similarities between zones.

The northernmost (Subtropical) assemblage en-

compasses stations 1, 1A and lB, ending slightly north of the northern (summer) boundary of the

Transition Zone (as defined by E. Boltovskoy, 1970).

This result is consistent with the typical February position of the Transition Zone boundary (Fig. 71,

when Brazilian waters reach farthest south in the

southwestern Atlantic. At the time of the cruise (November), however, this boundary is located somewhat farther north, between stations 1B and 2

(Fig. 7). Warm water foraminifers dominated this area (73%), yet cold water ones were present in

sizable proportions as well (18%). The latter were

chiefly represented by G. quinqueloba and G. bul- loides (Fig. 4).

At stations 2-4 (Warm-Transitional in Fig. 7) the

proportions of cold water foraminifers exceeded those of the warm water ones, yet the latter were still present conspicuously (Fig. 4). The fauna of station

4, albeit generally similar to those of stations 2 and 3, was characterized by noticeably higher relative abundances of cold water forms (74%) than stations 2 and 3 (53 and 56%, respectively). Conversely, warm water foraminifers, which accounted for 30- 32% at stations 2 and 3, dropped to 14% at station 4.

The Transitional assemblage was characterized by an almost total dominance of cold water species,

with only traces of isolated warm water foraminifers (Figs. 4 and 7). The latter were usually not repre- sented by the species dominant farther north, but by foraminifers of moderate to low abundance in the

tropical assemblage, such as G. rubescens, G. aequi- lateralis, G. trilobus and G. menardii (Fig. 4). Sta- tion 9A differed from the rest of the area in its high numbers of G. inflata (33%; see Figs. 4 and ‘7). The above-mentioned upwelling of deep, cold waters at

37-38”s was also noticeable in the fauna1 ordination (Fig. 7). Indeed, th e composition of station 12A, the

southernmost of the Transitional group, was most

Fig. 4. Latitudinal distribution of the foraminiferal species recorded (pooled data for all depths), and surface temperature along the transect.

Curves for warm and cold water foraminifers exclude Globorotalia injZatu, Ghhigerinitu gfutinatu and the unidentifieds. Biogeographic

zones indicated are according to Fig. 7.

62 E. Boltovskoy er al./Marine Micropaleontology 28 (1996) 53-72

0

10

E 20 r

E ; 30

40

50

0

10

E 20

C

‘i Q1 30 0

40

50

0

IO

20

30

40

50

cl o-1 %

EZI l-10%

, 3O-50%

I 1 I 250%

Cold water foraminifers

/ o-1 0%

Fz3 1 O-40%

m 40-60%

Unidentified foraminifers

7 El O-20%

m 2040%

Latitude south Fig. 5. Vertical profiles of warm water, cold water and unidentified foraminifers along the transect. Contours assume homogeneous percentage con~bution to total taxocoenoses between bottom and top depth of each tow.

E. Boltovskoy et d/Marine Micropaleontology 28 (1996) 53-72 63

0

400 500 600 Latitude south

Fig. 6. Percentages of a warm water (G. ruber, A) and a cold

water (G. pachyderma. B) foraminifer throughout the transect

investigated, at different depths.

closely associated with those of stations 6 (37” 15’06” > and 7 (37”01’54”) (see Fig. 7).

Stations 13 through 16, located in Subantarctic

waters, were totally barren of subtropical species (with the exception of 1 individual of G. aequiluter-

alis at station 13). G. quinqueloba, G. pachyderma (left>, and G. bulloides accounted for 80% of the foraminifers in this area.

The Antarctic Convergence marked a conspicuous fauna1 limit (Fig. 7), characterized chiefly by a sharp increase in the dominance of G. pachyderma (left) (Figs. 4 and 7). This form averaged 73% of the

foraminifers recorded at stations 17-20A (range: 44-82%), followed by G. quinqueloba (mean: 20%).

Our results show very good agreement with the foraminiferal biogeographic zonation proposed by E. Boltovskoy (1970, 1981b), for the southern hemi-- sphere summer. As shown in Fig. 7, boundaries

between distinct assemblages coincide very precisely with the limits suggested in those publications, which also seem to be generally valid for several other zooplanktic groups, such as pteropods, chaetognaths and euphausiids (D. Boltovskoy, 1975; Van der Spoel

and Boltovskoy, 198 1; Dadon and Boltovskoy, 1982); as well as for the distribution of zooplanktic species richness and biomass (D. Boltovskoy, 1979, 1981). Breaks between distinct fauna1 zones, in turn, well reflect hydrographic fronts defining the distribution

of major waters in the South Atlantic. In February- April 1989 Tsuchiya et al. (19941, in a transect

performed to the east of our study area (25”W), identified 4 deep-reaching fronts: Polar (49.5”S), Subantarctic (45” and 48‘S), Subtropical (41”--42”S),

and Brazil-Current (35”s). With the exception of the

Polar Front, which at 25”W is located 5”-7” north of

its position at 55”s (Figs. 1 and 7). all others can be

recognized in the fauna1 zonation presented. Thus,

the Subantarctic Front overlaps our Transitional- Subantarctic boundary. The Subtropical Front, which

originates between the Brazil and the Malvinas Cur-

rents fronts in their confluence (Peterson and

Stramma, 1991; Tsuchiya et al., 19941, defines the

center of our Transition Zone. Finally, the Brazil Current Front (35”s) coincides with the northern

summer boundary of the Transition Zone, north of which Brazilian fauna becomes conspicuous in the

plankton (Fig. 7). As suggested earlier by E. Boltovskoy (1970, ,

1981a), temperature seems to be the chief driving force which defines the composition of-and the boundaries between-the assemblages defined.

Changes in salinity along the area transected are

moderate (approx. 34-36%0), and most of the Transi- tional and Subantarctic areas are characterized by

rather uniform values around 34.5%0 (cf. Gorshkov, 1977). Surface chlorophyll a levels varied around 0.5 pg/l throughout the cruise and did not show any defined latitudinal pattern, except for the peak of up to 3-4 pg/l centered on the upwelling cell at 37”- 38”s. Surface temperature, on the other hand, varied from ca. 25°C at station 1, to approx. 0°C at station 20A; mean values for the 5 domains defined differ- ing strongly (Subtropical: 22.7”C. Warm-Transi- tional: 18.4”C, Transitional: 13.9”C. Subantarctic: 6.1”C, and Antarctic: 1.6”C).

5. Interhemispheric comparison of foraminiferal thermic rdgimes

Given the wide application of foraminiferal re-

mains on the sea-floor for paleoceanographic inter- pretations, close coupling of compositional makeups with sea-surface temperatures represents an impor- tant advantage for tracing past climatic rCgimes. However, it has been noted that interoceanic and

64 E. Boltouskoy et d./Murine Mil .ropdeontology 28 (1996) 53-72

interhemispheric extrapolation of regional ecological data can bias distributional and paleoceanographic conclusions because of regional temperature-inde-

pendent differences in foraminiferal species propor-

tions (Be, 1969; E. Boltovskoy, 1970). Thus, data on interregional differences in the temperature-related

behavior of foraminiferal species is of interest for

Subtropical (60 : 20)

Warm Trans. (35 : 65)

5 ti Transitional (1 : 99)

s Subantarctic (0 : 100)

Antarctic 10 : 1001

critical assessments of potential biases involved in comparisons of geographically different settings.

The extended latitudinal range of our collection allows some restricted comparisons with north At-

lantic results based on similar material. We selected the collections reported by Be and Hamlin (19671,

and by Ottens (19911, which cover comparable lati-

700 60° 600 400

Correlation 1.00 0.50 0.00

I 1 I 1

Fig. 7. Foraminiferal assemblage zones alon g the transect surveyed as deftned by a cluster analysis based on pooled percentage data (unidentifieds excluded) for all depths at each station (ah species; Pearson’s r correlation coefficient and UPGMA clustering method, cf.

Sneath and Sokal, 1973; notice break in scale of levels of association at fop). Bar graphs on the left-hand side illustrate mean percentage

compositions of most abundant taxa within each sample-group defined. Numbers at top of bar graphs indicate mean warm water to cold

water ratios within each sample group.

E. Boltouskoy et al./ Marine Micropaleontology 28 (I 996) 53-72 65

tudinal ranges and were obtained in surface waters during the northern hemisphere summer (Fig. 8, map). Because total temperature ranges differed be- tween the 3 data sets, percentage data used were restricted to samples collected at water temperatures ranging between 14” and 24°C (thus, the mean tem- peratures for the 3 sets of stations compared ranged between 18” and 19°C).

Fig. 8 illustrates the percentage contributions of 9 selected foraminifers in the northern and southern Atlantic Ocean for the temperature offset of 4”-24°C. With the exception of G. ruber, which was much

more abundant in the collection of BC and Hamlin (1967) than in the material of Ottens (1991) the two North Atlantic data sets are very homogeneous. In contrast, data from the South Atlantic differ widely from both of the above insofar as at least 4 of the foraminifers considered have noticeably different rel- ative abundances at both sides of the equator. G. bulloides is almost twice as abundant in the North Atlantic (37-38%) as in the southern hemisphere (21%), while G. quinqueloba, G. injZura and G. rubescens are 1.5 to 14 times more abundant in our material than in either Ottens’ (1991) or in BC and

Dissimilar % in N and S hemispheres

Similar % in N and S hemispheres Variable %

Glob&ha quinqueloba Globigerinita glutinata

Globigerinella aequilateralis

oil a m

Globoquadrina dutertrei

a m

Globigerinoides ruber

n (1967)

This work

Fig. 8. Comparison of mean proportions of selected foraminiferal species in two collections from the North Atlantic (BC and Hamlin, 1967;

Ottens, 1991), with the present data; only samples collected at surface water temperatures ranging between 14 and 24°C are included.

Hamlin’s (1967) collections. G. ruber shows an erratic behavior, with highest relative contributions in the collection of Be and Hamlin (1967) (28%), intermediate values in the South Atlantic (13%), and lowest figures in Ottens’ (1991) data set (6%).

While, as shown, species proportions vary greatly between hemispheres, tem~ra~re-related percentage contributions within the 14”-24°C range indicate that the preferred thermic regimes are remarkably similar in all 3 collections. Fig. 9 shows that, for the 9 species considered, optimum temperatures usually differ by less than 2 degrees. The low value for G. dutertrei in Be and IIamlin (1967) is due to one single data point (17.5% at 14”C), and G. pachy- derma was eliminated from Ottens’ (1991) data be- cause it included only one (dubious?) record of this species at 24°C. It should be stressed, however, that intra- and interhemispheric concordance between the preferred therrnic regimes shown in Fig. 9 is proba- bly enhanced by the fact that the 14”-24°C tempera- ture offset used does not include the ranges of maximum abundance of several of the taxa consid- ered. Indeed, most of the cold water taxa have peak abundances below 14°C and at least some of the warm water ones do not seem to have reached their acme at 24°C (cf. Fig. 4). Nevertheless, we anticipate that the comparison presented shows generally valid

relationships, albeit probably somewhat strengthened by methodological artifacts.

Contrasting results (Figs. 8 and 9) suggest that, although the proportions of foraminiferal species recorded at a given sea surface temperature differ noticeably between hemispheres, when evaluated in- dividually the taxa in question thrive best at closely comparable tempera~res. Thus, while dissimil~ as- semblage makeups at comparable temperatures (Fig. 8) stress the importance of using local distributional data for deriving past sea surface temperatures from present-day specific compositions, interhemispheric similarities in the preferred thermic regimes of iso- lated species (Fig. 91 may substantiate geographic extrapolation of distributional data. Our results sug- gest that paleoecological techniques that rely heavily on species proportions, such as the transfer functions of Imbrie and Kipp f 19711, can be strongly biased if data on area1 distributions in one region are used for inferring past sea surface temperatures on the basis of downcore records from other regions. On the other hand, paleotemperature studies based on down- core relative abundance changes of selected foraminiferal species are less stringent in their re- quirements, and can probably benefit from species- specific environmental info~ation obtained in other regions.

23

+&- Ottens(1991) +- Bd&Hamiin(1967) + Thiswork

Fig. 9. Comparison of the preferred temperature rkgimes (mean weighted temperatures; i.e., C(ti + d,)/E(,(di), where li is temperature at station i and dj is percentage contribution at station i) of the foraminiferal species illustrated in Fig. 8.

E. Boltovskoy et al./Marine Micropaleontology 28 (1996) 53-72

Globigerinoides ruber Globigerinella aequila teralis

600

300

400

500

60” I///// I

Globigerina bulloides

600

67

n 0% or traces

1 <5%

M 5-10%

Fig. IO. Percentage contributions of selected foraminiferal species in the surface sediments (maps; data from Be, 1977). and in the plankton

(this work; pooled data for all depths at each station smoothed by a 3-point running mean; inset bars). Dotted lines for Globorotulia

rruncutulinoides and GIobigerino bulloides denote areas without data in Bt’s survey.

68 E. Boltouskny et cd./ Marine Micropuleontologyy 28 (1996) 53-72

5.1. Sedimentary imprint of planktic assemblages

For paleoceanographic purposes, another point of interest is the comparison of our results with surface-sediment data from the south Atlantic. Fig. 10 illustrates the percentage contributions of 6 se- lected foraminifers in our plankton tows and in the surface sediments from the same area investigated by BC (1977). Although this comparison is admittedly hindered by various procedural artifacts (mesh size, seasonal and interammal variability, vertical distribu- tion, etc.), it is useful insofar as it confirms some previous observations on plankton-sediment mis- matches that have an important bearing on paleo- ceanographic interpretations.

For G. ruber, a warm water foraminifer, and G. truncatulinoides, patterns in the plankton and in the surface sediments are very well coupled. Very low percentages of G. ruber extend slightly farther south on the bottom than in the near-surface waters; given the fact that this is a rather delicate species (e.g., Berger, 1968; E. Boltovskoy and Totah, 1992), this difference is probably not due to enhanced preserva- tion, but to random blurring of the planktic pattern upon settling on the seafloor.

G. aequilateralis, also warm water, has a notice- ably wider latitudinal range in the water than in the sediments. In all probability, the scarce output of this rare (mean abundance in the Subtropical domain: 3.32%, in the Transitional: 0.32%) and fragile species (Berger, 1968) gets readily dissolved, the few re- maining skeletons dropping to negligible proportions of the overall thanatocoenosis dominated by the more abundant and robust transitional and cold water taxa.

For G. inflata, plankton and sediments generally agree in the overall position of maximum propor- tions, but values are consistently higher on the bot- tom. According to Vincent and Berger (1981), G. inflata is among the most solution-resistant foraminifers, which seems to account for its en- hanced preservation in the fossil record.

As opposed to the above, the two cold water species, G. bulloides (Subantarctic), and especially G. pachyderma (Antarctic-Subantarctic), show dis- tinctly uncoupled plankton-sediment patterns. With the exception of some high planktic data for G. bulloides between approx. 52” and 57”s (lo-20%), in the sediments most boundaries for the relative

abundance ranges shown are shifted to the north of their corresponding planktic counterparts. This is especially conspicuous in the case of G. pachy- derma, where the bottom limits for > 50%, > 20% and 5-10% contour intervals are located approx. 6” to 13” north of their corresponding boundaries in the plankton. In the sediments of the southwestern At- lantic 0.1 to 4.9% values extend beyond the equator (Be, 1977), while in the surface plankton it practi- cally disappears at about 35”s (Figs. 4 and 10).

G. pachyderma is one of most solution-resistant foraminifers (Vincent and Berger, 198 l), which could partly account for its enhanced proportions in the sediments; but G. bulloides is among the most deli- cate (Vincent and Berger, 1981) and would there- fore not be expected to be better and/or more widely represented on the bottom than in the water- column.

While sedimentary distributions represent a multi-year average picture, our planktic data illus- trate spring (November) distributions, when the northward influence of the cold water Malvinas Cur- rent (and its associated fauna), has already shrunk with respect to the maximum winter (July-August) situation. Thus, a somewhat more southern planktic position of the peak proportions of these two species is expected. However, these seasonal variations by no means cover as large a latitudinal gradient as that observed in the plankton-sediment mismatches. For example, south-north displacement of isotherms be- tween August and November along 5O”W is only some 3-4” in latitude; significantly less than the 6 to over 30” shift in the foraminiferal proportions.

As suggested by D. Boltovskoy (1988), we con- tend that these equatorward ‘shadows’ of the plank- tic patterns are chiefly due to an interplay between the biological traits of the species involved and the hydrological features of the area. Cold water foraminifers which are subject to northward dis- placement by surface currents (Malvinas Current) are eventually carried outside of their thermic tolerance limits. Rather than perishing and sedimenting, these individuals can sink in the water-column until they reach waters cold enough to allow extended survival and thus withstand further northward displacement. This process can occur repeatedly in a step-wise manner, until finally the cell dies (from starvation?) and settles on the sea-floor in a much warmer area

E. Boltovskoy et al./Marine Micropaleontology 28 (1996) 53-72 69

than that which characterizes its near-surface distri- bution Our O-50 m data for G. p~c~y~e~~u furnish some evidence of this me~h~sm: Fig. 6B shows that at any given latitude the percentages of this cold water species are higher in subsurface waters than at the surface. At any rate, northward advection of subantarctic foraminifers is most probably especially significant at depths greater than those covered by our tows. Waters of Subantarctic origin are present at depths of ca. 300 m off the coasts of Brazil (Emils- son, 1961; Mesquita, 1983; Matsuura, 1986); and frequent records of Subantarctic and even Antarctic plankters in upwelling cells as far north as 23”N (E. Boltovskoy, 1970; Valentin et al., 1987; Brandini, 1990) strongly suggests that these organisms have been advected from higher latitudes at subsurface depths. Interestingly, according to our data, G. pachyderma (s.1.) thrives well at 6-9°C; between 30” and 40% at the time of the cruise this temperature range occurs at 300-600 m, these depths being the source of the cold waters of Subantarctic origin where Subantarctic plankters are found along the Brazilian slope. The isolated peak in the proportions of cold water fora~nifers in coincidence with the upwelling at 37”-38% is also probably, at least in part, due to this mechanism. G. quinq~elobu, G. bulloides, G. uuula and, to a lesser extent, G. pachy- derma (right) increase rather abruptly in this cell of colder waters (Figs. 4 and 5) connected at 400-500 m with the core of their Subantarctic populations farther south (Fig. 2). Although lack of a wider east-west coverage does not allow us to rule out the possibility that this colder water area was surficially connected with the main body of Subantarctic waters at either side of our transect, we suggest that this local peak can represent an isolated e~~cement of individuals which surfaced under favorable condi- tions after having sunk farther south and been trans- ported northwards by the Subantarctic Intermediate Waters.

Equatorward subsurface extensions of the ranges of cold water planktic organisms have been reported for many groups (pteropods, copepods, chaetognaths, etc.), and enhanced remains of colder water forms under warmer water areas attributed to the above-de- scribed mechanism were reviewed in detail by D. Boltovskoy (1988, 1994). Conversely, because sub- mergence does not offer a survival option to warm

water fossilizable forms, their sedimentary imprints are conspicuously better coupled with their planktic ranges.

Because the goal of paleoecological investigations is to identify the sedimentary signal that defines a geographic area characterized by particular ‘condi- tions (temperature, productivity) at the surface, rather than to assess the similarity between the plankton and the underlying thanatocoenosis, application of surface sediment-based data for paleoenvironmental reconstructions does not, in principle, require tight plankton-bottom matching (D. Boltovskoy, 1988, 1994). However, most current pale~~ological tech- niques, including the widely used method of Imbrie and Kipp ( 197 l), rely on the assumption that core top faunas are systematically related to the nature of the overlying surface waters, and that the relation- ship in question has not varied in the time-span under investigation. Yet conditions of sediment for- mation, including the intensity and strength of inter- mediate and deep currents carrying - toward the equator - cooled waters that sink at the poles have changed significantly over time (Kennett, 1982; Boyle and Keigwin, 1987; Herguera and Berger, 1991; Herguera et al., 199 11, thus conceivably affect- ing the con~ibution of cold water polar shells to lower-latitude areas.

Acknowledgements

This work was supported by grants from the University of Buenos Aires, Argentina (UBA EX- 0591, from the Consejo Nacio~al de Inves~gaciones C~ent~cas y Tkcnicas, Argentina (CONICET PID- BID 366), from the Servicio de Hidrugrufia Naval (Argentina), and from the Conselho National de Desenvolvimento Cienti$co e Tecnologico, Brazil (CNPq PROC No. 670001/93-7). Logistic and tech- nical assistance was provided by the Diretoria de Hidrografia e NauegaFao (Brazil), by crew members aboard BarZio de Teffk (Brazil), and by AI. Matsub- ara (Argentina). The senior author can be contacted at the following e-mail address: roo~biolo.bg.fcen. ubaar.

70 E. Boltouskoy et al./ Marine Micropaleontoh~y 28 (1996) 53-72

Appendix A. Systematic remarks

Although the present work is not aimed at survey- ing the systematics of the foraminiferal taxa recorded, given the widespread disagreements in the interpreta- tion of the taxonomy of Recent planktic foraminifers a list of the species encountered (in alphabetical order), and in some cases short remarks on the criteria used herein are furnished. Detailed accounts of the morphology and classification of South At- lantic fauna can be found in E. Boltovskoy (198 la).

Globigerina bulloides d’orbigny, 1826. Globigerina falconensis Blow, 1959. Globigerina quinqueloba Natland, 1938. As ex-

plained below, differentiation of this species from Globoquadrina pachyderma, forma superjiciaria presents serious difficulties. In material collected in the Drake Passage (E. Boltovskoy, 1971) the color of the protoplasm (brownish in G. quinqueloba, green- ish in G. pachyderma) was found to be a useful trait for their identification; unfortunately, in the present material protoplasm was either lacking or of unde- fined color in many specimens.

Globigerinella aequilateralis (Brady) = Globiger- ina aequilateralis Brady, 1884.

Globigerinita glutinata (Egger) = Globigerina glutinatu Egger, 1893. In the southwestern Atlantic, this cosmopolitan species has some preference to- wards warmer waters.

Globigerinita uvula (Ehrenberg) = Pylodexia uvula Ehrenberg, 186 1.

Globigerinoides conglobatus (Brady) = Globiger- ina conglobata Brady, 1884.

Globigerinoides tuber (d’orbigny) = Globigerina rubra d’orbigny, 1839.

Globigerinoides trilobus (Reuss) = Globigerina triloba Reuss, 1850.

Globoquadrina dutertrei (d’orbigny) = Globiger- ina dutertrei d’orbigny, 1839.

Globoquadrina pachyderma (Ehrenberg) = Aristerospiru pachyderma Ehrenberg, 1861, s.1. This typically cold water species has two formae: forma typica and forma superjkiariu. The latter most prob- ably represents juvenile specimens, which are usu- ally recorded in the uppermost layers. The forma typicu, on the other hand, dwells preferably in deeper waters, but at high latitudes is found at the surface as well. The two formae are linked by a continuous

range of intermediate specimens which are difficult to ascribe to either one of them, for which reason they were counted jointly in the present work, only differentiating left- from right-coiling shells. Forma superjkiaria is also difficult, and sometimes impos- sible, to differentiate from Globigerina quinqueloba; thus, the relative abundances given for these two species are approximate.

Globorotalia infzatu (d’orbigny) = Globigerina injktu d’orbigny, 1839. Generally peaking in the Transition Zone, but with some preference towards colder waters.

Globorotalia menardii (d’orbigny) = Rotalia menardii d’orbigny, 1826.

Globorotalia scitula (Brady) = Pulvinulina scitula Brady, 1882.

Globorotalia truncatulinoides (d’orbigny) = Ro- talina truncatulinoides d’orbigny, 1839. This species comprises two varieties: forma sinistralis (left-coil- ing) and forma dextrulis (right-coiling). In the South Atlantic the left-coiling form is more characteristic of colder waters than the right-coiling. Isolated and very small specimens of the former were recorded at several stations, and only one specimen of the forma dextralis.

Globigerina rubescens Hofker, 1956. Hustigerina pelagicu (d’orbigny) = Nonioniu

pelagica d’orbigny, 1839. Orbulina universa d’orbigny, 1839.

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