Planktic foraminifers and hydrography of the eastern and northern Caribbean Sea

Marine Micropaleontology 46 (2002) 387^403 www.elsevier.com/locate/marmicro

Planktic foraminifers and hydrography of the eastern and northern Caribbean Sea B. Schmuker a , R. Schiebel b; b

a Geological Institute of the ETH Zu«rich, Sonneggstrasse 5, 8092 Zu«rich, Switzerland Geological and Paleontological Institute and Museum, Sigwartstrasse 10, 72076 Tu«bingen, Germany

Received 23 July 2001; received in revised form 15 April 2002; accepted 16 April 2002

Abstract The distribution of living planktic foraminifers and their relation to the hydrography of the Caribbean Sea was investigated in plankton net tows and surface sediment samples taken along the Antilles island arc during April/May 1996. The planktic foraminiferal community was strongly influenced by spatial variations in salinity that were largely due to the influx of Orinoco River water into the southeastern Caribbean Sea and inflowing Sargasso Sea water in the north. Along the Antilles island arc, Globigerinoides ruber was the dominant species in the surface waters throughout. In the southeastern Caribbean Sea, where Orinoco River outflow influences the planktic community, standing stocks of planktic foraminifers ( s 100 Wm) between 4 and 50 specimens m33 were medium to low. The southeastern faunas between Tobago and Guadeloupe were characterized by increased proportions of Neogloboquadrina dutertrei. Highest standing stocks of 159 specimens m33 in the upper 20 m of the water column were recorded in the northeastern Caribbean Sea and the assemblages were characterized by high proportions of Globigerinita glutinata, associated with cyclonic eddies. In the Anegada Passage, where Sargasso Sea water flows into the Caribbean Sea, low standing stocks of 18 specimens m33 indicate oligotrophic conditions. Together with the oligotrophic surface waters, the Subtropical Underwater enters the Caribbean Sea through the Anegada Passage in water depths between 100 and 300 m. These waters are characterized by higher concentrations of Globorotalia truncatulinoides relative to the adjacent water masses. Comparison of the living planktic foraminiferal fauna with empty test assemblages from the water column and from surface sediments shows that differences in the faunal composition mostly correspond to the distribution of water masses and to the differential dissolution of species. In the vicinity of islands Globigerinoides ruber reaches higher relative frequencies than in the open ocean, pointing towards a higher tolerance of this species towards neritic conditions than in other species. 8 2002 Elsevier Science B.V. All rights reserved. Keywords: planktic foraminifers; population dynamics; ecology; hydrography; Caribbean Sea

1. Introduction

* Corresponding author. Present address: Geological Institute of the ETH Zu«rich, Sonneggstrasse 5, 8092 Zu«rich, Switzerland. Tel.: +41-1-6323676; Fax: +41-1-6321080. E-mail address: [email protected] (R. Schiebel).

Planktic foraminifers are unicellular open marine organisms that have often been used as water mass indicators in the Recent oceans and ancient sediments. The standing stock of planktic foraminifers observed at a speci¢c time and locality

0377-8398 / 02 / $ ^ see front matter 8 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 8 3 9 8 ( 0 2 ) 0 0 0 8 2 - 8

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Fig. 1. Location of the plankton net (circles) and sediment stations (squares) in the Caribbean Sea, along the Lesser Antilles (Virgin Islands^Tobago) and the Greater Antilles (Puerto Rico^Cuba). Station numbers with an ‘A’, ‘RC’ and ‘V’ as pre¢x refer to the Brown University Foraminiferal Database (BFD, ¢lled squares; Prell et al., 1999).

is the result of the interaction between biological (e.g. food supply) and physico-chemical factors (e.g. temperature, salinity, turbidity) (Be¤, 1977; Hemleben et al., 1989; Ortiz et al., 1995). Every species is adapted to a certain range of these factors. The temporal and spatial variability of these parameters results in a patchy distribution of planktic foraminiferal assemblages on various scales (Schiebel and Hemleben, 2000). The overall goal of the presented study is to investigate the distribution of living planktic foraminifers in relation to environmental parameters in the eastern and northern Caribbean Sea. Only few studies of living planktic foraminifers have been carried out in the Caribbean Sea so far. The relationship between planktic foraminiferal populations and water masses in the western Caribbean Sea and the Gulf of Mexico was investigated by Jones (1968). Miro' (1971) sampled the continental margin of Venezuela that is in£uenced by seasonal upwelling and therefore is not directly comparable to the southeastern Caribbean hydrographic setting investigated here. The distribution of live planktic foraminifers in the Sargasso Sea and northern Caribbean Sea, during January and February, was investigated by Be¤ (1971). His southernmost station was located close to eastern Puerto Rico. The temporal distribution of planktic foraminifers o¡ the southwestern coast of Puerto Rico was studied by Schmuker (2000a). The eastern Antilles arc is in£uenced by neritic

water masses that originate from the out£ow regions of the Orinoco and Amazon Rivers. These waters have higher nutrient contents, leading to increased primary productivity and higher turbidity, compared to the open oceanic waters of the western subtropical Atlantic (Mu«ller-Karger et al., 1989; Del Castillo et al., 1999). The in£uence of these neritic water masses on the planktic foraminiferal fauna in the eastern Caribbean Sea is discussed in the present work.

2. Location and hydrography The Caribbean Sea is a marginal ocean basin separated from the Atlantic by the Lesser and Greater Antilles island arc (Fig. 1). Most of the surface waters of the eastern Caribbean Sea originate from the tropical and subtropical Atlantic, and enter the Caribbean Sea through several passages between the islands of Grenada, Saint Vincent, Santa Lucia, Martinique, Dominica, Guadeloupe, the Virgin Islands (Anegada and Jungfern Passage), Mona, Hispaniola and Cuba. The Anegada and Jungfern Passages also allow for renewal of deep waters (Table 1: Antarctic Intermediate Water (AAIW) and North Atlantic Deep Water (NADW)), as their sill depth is below 1800 m water depth (Kinder et al., 1985; Fratantoni et al., 1997). The largest volume of water is transported into the Caribbean Sea through the Wind-

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ward Passage located between Cuba and Hispaniola (Kinder et al., 1985; Wilson and Johns, 1997). Out£ow from the Caribbean Sea takes place through the Strait of Yucatan into the Gulf of Mexico (Kinder et al., 1985). Intermediate and deep waters (Table 1) enter the Caribbean Sea also through these passages. The Caribbean hydrography exhibits a pronounced seasonality, and eddies are a frequent feature (Carton and Chao, 1999). From January until July, surface waters from the tropical Atlantic are mixed with fresh water from the Amazon River and are transported into the Caribbean Sea from the southeast through the passages in the Lesser Antilles (Moore and Todd, 1993). Between August and December, low salinity lenses, which originate from the Orinoco River out£ow (Mu«ller-Karger et al., 1989; Moore and Todd, 1993), are frequently observed in Caribbean surface waters (Froelich et al., 1978). Although evaporation generally exceeds precipitation in the Caribbean Sea (Yoo and Caron, 1990), the salinity of eastern Caribbean surface waters is lower than that of tropical Atlantic surface waters due to the in£ux of relatively fresh water from the Amazon and Orinoco Rivers. Along the Caribbean island arc, the highest sea surface temperature of more than 27‡C was recorded at the Pedro Bank (station 43) in the west and in the Tobago Basin (station 3) at the south of the transect (Fig. 2). Surface waters in the Anegada Passage (stations 9^16) of around 26‡C were slightly colder than in the central Caribbean Sea (26.5‡C). The thermocline was most pronounced in the Tobago Basin (station 3). Salinity increased from 35.5 in the southern to 36.5

389

in the northeastern Caribbean Sea (Fig. 2). The low salinity in the south is due to the admixture of fresh Amazon River out£ow water (Moore and Todd, 1993). O¡ Santa Lucia (station 4), a low salinity lens with a salinity of 34.5 was recorded in the upper 20 m, whose origin is not clear, but might be associated with the freshwater out£ow from the Orinoco River (Mu«ller-Karger et al., 1989).

3. Materials and methods The planktic foraminiferal fauna and hydrography of the eastern Caribbean was studied in April and May 1996 (RV Meteor cruise 35/1) between the Tobago Basin and Puerto Rico, with a single western station at the Pedro Bank south of Jamaica (Fig. 1). Temperature and salinity were measured with a conductivity, temperature, depth probe (Seabird SBE 19 Seacat Pro¢ler) in casts down to 300 m immediately before plankton sampling. Planktic foraminifers were sampled with a multiple opening^closing plankton net (0.5 mU0.5 m mouth opening, ¢ve cups, 100 Wm mesh size) between the sea surface and 2500 m water depth (Table 2). The upper 100 m were sampled in 20 m depth intervals. Below, the intervals ranged between 100, 200, 300, 500, 700, 1000, 1500, 2000, and 2500 m water depth. The sampled water volume was calculated by multiplying the area of the mouth opening ( = 0.25 m2 ) with the sampled depth interval. Samples were ¢xed on board in a 4% formaldehyde solution bu¡ered with hexamethyltetramine at pH 8.2. In the laboratory,

Table 1 Depth distribution of water masses of the Caribbean Sea Approximate depth range (m)

Water mass

Salinity

Abbreviation

Origin

0^100 100^300 300^600 600^1000 s 1000

mixed layer Subtropical Underwater Subantarctic Intermediate Water Antarctic Intermediate Water North Atlantic Deep Water

34^36.5 s 37 35.2^37.0 6 35 34.7^34.9

^ SUW SAAIW AAIW NADW

^ Sargasso Sea Antarctic Antarctic North Atlantic

The water masses were identi¢ed and their origin was attributed according to the studies of Wu«st (1964), Gordon (1967), Dietrich et al. (1975), Morrison and Nowlin (1982), and Kumar et al. (1991).

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Table 2 Geographical and temporal setting of the multinet casts and of the multicorer sediment surface samples Date

Lunar day

Location

Station No.

(N)

(W)

4/19

15

Tobago Basin

03

12‡05.3P

61‡14.6P

4/21

17

W of Santa Lucia

04

14‡24.7P

61‡37.7P

4/21 4/22

17 18

W of Dominica W of Guadeloupe

05 06

15‡27.2P 16‡25.1P

62‡13.9P 62‡27.2P

4/23

19

Anegada Passage

07

17‡28.0P

64‡13.9P

4/24

20

Anegada Passage

09

18‡57.0P

63‡43.9P

4/25

21

Anegada Passage

11

18‡49.5P

63‡58.9P

4/26

22

Anegada Passage

12

18‡18.1P

63‡38.0P

2/27

23

Anegada Passage

16

18‡03.1P

63‡38.5P

4/29

25

NW of Saint Croix

17

17‡55.1P

65‡02.1P

5/1

27

S of Puerto Rico

21

17‡40.4P

65‡26.1P

5/2

28

S of Puerto Rico

24

17‡02.4P

66‡00.0P

5/3 5/3 5/4 5/5 5/12

0 0 1 2 9

S of Puerto Rico S of Puerto Rico S of Puerto Rico Mona Passage S of Jamaica (Pedro Bank)

25 26 27 28 43

17‡45.2P 17‡30.4P 17‡39.0P 17‡56.7P 17‡38.7P

67‡00.5P 67‡02.8P 67‡10.1P 67‡29.4P 79‡09.1P

(1996)

Latitude

Longitude

MSN No.

Depth interval of MSN (m)

Water depth

1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1067 1068 1069 ^ ^ ^ 1070 1074 1075

0^100 100^700 0^100 100^700 700^2500 0^100 0^100 100^700 0^100 100^700 0^100 100^700 700^2500 0^100 100^700 0^100 100^700 0^100 100^700 0^100 100^500 0^100 100^700 0^100 100^700 700^2500 ^ ^ ^ 0^100 0^100 100^700

1301

(m)

2890

2300 885 3248 3326

1289 1121 1550 4465 1819 4701

1778 3815 1813 1266 975

Date is given as month/day. Lunar day ‘0’ corresponds to the full moon. Pre¢x of all station numbers is M350.

planktic foraminiferal tests were picked, dried, sieved into size classes of s 100, 125, 150, 200, 250, 315, and s 315 Wm, and counted on a species level, following the taxonomy of Be¤ (1967), Hemleben et al. (1989), and Brummer and Kroon (1988). We did not separately discuss the distribution of Globigerinella siphonifera Types I and II (Faber et al., 1988), which are, however, genetically separate species (Huber et al., 1997). A total of 32 species were identi¢ed (Table 4). Cytoplasmbearing tests (‘living specimens’) were counted separately from empty tests (‘dead specimens’). Data are available online at http://e-collection.

ethbib.ethz.ch/show?type = disspnr = 13559 (Appendix III, Table III-1, pp.155^164), and as Marine Micropaleontology Online Background Dataset1 . For comparison of the faunal composition of the plankton assemblages with the empty test and sediment assemblages, the relative abundance of species within the size-fraction s 150 Wm was determined. Sediment surface samples were obtained with a multicorer (Table 2), washed over

1

http://www.elsevier.com/locate/marmicro

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Fig. 2. Temperature (upper panels) and salinity (lower panels) during April/May 1996 in the upper 300 m of the water column. The triangles at the top of the panels indicate the position of the stations (Fig. 1). The left panels show the stations from the Pedro Bank (station 43) in the west to Puerto Rico (station 17) in the east. The panels on the right-hand side show the stations from the northern Anegada Passage (station 9) to the Tobago Basin (station 3). Updoming isotherms (upper left panel) indicate a cyclonic eddy (shaded) at stations 17^24. The Subtropical Underwater (SUW, shaded) is de¢ned by salinities s 37 (lower panels).

a 150 Wm screen, dried and split with an Otto microsplitter to obtain samples of at least 300 individuals to yield reproducible census data. Most of the plankton and sediment sites are lo-

cated at similar coordinates to enable direct comparison of the faunas (Fig. 1, Table 2). In addition, faunal abundance data from sediment samples (Brown University Foraminiferal Data-

Table 3 Comparison of planktic foraminiferal standing stocks reported from the Caribbean Sea Area

Author

Mesh size (Wm)

Standing stock (specimens m33 )

Equivalent standing stock

M35/1 (E’ Caribbean) PRIST (NE’ Caribbean) W’ Caribbean O¡ Venezuela Sargasso Sea

this study Schmuker (2000a) Jones (1968) Miro' (1971) Be¤ (1971)

100 150 200 74 202

4^78.6 0.1^17.7 2.4^5.1 26.5^42.4 0.5^11

^ 0.3^59.7 19.2^40.8 10.7^17.2 4.1^90.7

The equivalent standing stocks for a mesh size of 100 Wm are calculated to enable a comparison of the di¡erent studies, and refer to the formula of Berger (1969): Nf (equivalent) = Nf (actual)UM3a /1003 , with Nf (equivalent) = equivalent standing stock for a mesh size of 100 Wm, Nf (actual) = measured standing stock, Ma = used mesh size.

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base (BFD), Prell et al., 1999) and from the Puerto Rican island slope (Puerto Rico slope transect (PRIST), Schmuker, 2000b) were compared to our data (Figs. 1 and 8). Only samples with less than 20% fragmentation of planktic foraminiferal tests were included in our analysis in order to minimize dissolution e¡ects on the relative frequency of species.

4. Results 4.1. The living planktic foraminiferal fauna Along the Antilles arc, maximum numbers of 150 living specimens m33 ( s 100 Wm) were recorded in the upper 20 m at station 24 south of Puerto Rico (Fig. 3). Near Tobago (station 3),

Table 4 Live species ( s 100 Wm) sorted by maximum abundance (specimens m33 ) Species

Max. abundance

Globigerinoides ruber (d’Orbigny), 1839, white var. Globigerinita glutinata (Egger), 1893 Globigerinella siphonifera (d’Orbigny), 1839 synonym Globigerinella aequilateralis (Brady) Globigerinoides ruber (d’Orbigny), 1839, pink var. Globigerinoides sacculifer (Brady), 1877 Neogloboquadrina dutertrei (d’Orbigny), 1839 synonym Globigerina eggeri Rhumbler Globoturborotalita rubescens Hofker, 1956 Globorotalia menardii (Parker, Jones and Brady), 1865 synonym Globorotalia cultrata (d’Orbigny) Globigerinoides conglobatus (Brady), 1879 Globorotalia truncatulinoides (d’Orbigny), 1839 Turborotalita quinqueloba (Natland), 1938 Orbulina universa d’Orbigny, 1839 Globigerina falconensis Blow, 1959 Dentigloborotalia anfracta (Parker), 1962 Globigerinella calida (Parker), 1962 Globorotalia crassaformis (Galloway and Wissler), 1927 synonym Globorotalia crassula (Cushman et al.) Globorotalia hirsuta (d’Orbigny), 1839 Globigerina bulloides d’Orbigny, 1826 Tenuitella iota (Parker), 1962 Globoturborotalita tenella (Parker), 1962 Globorotalia scitula (Brady), 1882 Hastigerina pelagica (d’Orbigny), 1839 Tenuitella parkerae (Bro«nnimann and Resig), 1972 Turborotalita humilis (Brady), 1884 Globigerinita minuta (Natland), 1938 Neogloboquadrina incompta (Cifelli), 1961 Tenuitella compressa (Fordham), 1986 Candeina nitida d’Orbigny, 1839 Globigerinella digitata (Brady), 1879 Neogloboquadrina pachyderma (Ehrenberg), 1861 Pulleniatina obliquiloculata (Parker and Jones), 1865 Globorotalia in£ata (d’Orbigny), 1839 Streptochilus globigerus (Schwager), 1866

DomA (m)

Average depth (m)

Average T

Average S

72.4 35.0 18.6

0^20 0^20 0^20

37 39 51

26.55 26.47 26.46

36.24 36.17 36.30

18.2 17.2 14.2

0^20 0^20 0^20

34 40 39

26.54 26.26 26.27

36.18 36.24 36.12

3.6 3.4

0^20 0^20

54 52

25.96 26.32

36.42 36.13

3.2 2.6 2.4 2.0 1.6 1.2 1.2 1.1

^ 100^200 60^80 80^100 ^ 60^80 80^100 200^300

^ 161 53 59 ^ 66 58 230

^ 21.47 25.65 25.99 ^ 25.29 26.36 17.58

^ 36.75 36.30 36.43 ^ 36.18 36.34 36.26

1.1 1.0 1.0 0.8 0.7 0.6 0.6 0.6 0.4 0.4 0.4 0.2 0.2 0.2 0.2 6 0.1 6 0.1

200^300 0^20 40^60 ^ 200^300 80^100 20^40 40^60 ^ ^ ^ ^ ^ ^ ^ ^ ^

161 43 87 ^ 250 108 33 102 ^ ^ ^ ^ ^ ^ ^ ^ ^

19.12 26.21 24.67 ^ 16.81 24.85 26.55 24.32 ^ ^ ^ ^ ^ ^ ^ ^ ^

35.81 36.44 36.29 ^ 36.23 36.42 36.13 36.75 ^ ^ ^ ^ ^ ^ ^ ^ ^

(‡C)

The taxonomy follows Hemleben et al. (1989) and Brummer and Kroon (1988). Depth interval of maximum abundance (DomA) and weighted averages of species regarding to water depth, temperature (T) and salinity (S) are given for the most frequent species during M35/1. Rare species were not evaluated for their average distribution.

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Fig. 3. Standing stocks (specimens m33 ) of living planktic foraminifers ( s 100 Wm) along the Greater Antilles (left panels, vs. longitude) and the Lesser Antilles (right panels, vs. latitude). Note the di¡erent contour intervals in the upper to lower panels. The triangles at the top of the panels indicate the sampled stations (Fig. 1).

Dominica (station 5), and in the Mona Passage (station 28), standing stocks (living specimens m33 ) were about half those o¡ Puerto Rico. Lowest concentrations of living specimens in surface waters were found near Santa Lucia (station 4) and in the Anegada Passage (stations 9^16). Over the upper 100 m of the water column, average test concentrations of living planktic foraminifers ( s 100 Wm) ranged between 6 and 79 specimens m33 . Globigerinoides ruber (for white and pink variants see Fig. 4) dominated most assemblages along the Caribbean island arc (Fig. 4), and was most frequent at the stations 21 and 24 south of Puerto Rico (Fig. 5b). At both stations, the size spectrum of G. ruber (white) was dominated by medium test sizes of 150^200 Wm (Fig. 6). Other abundant species are Globigerinita glutinata (Fig. 5c), Globigerinella siphonifera (Fig. 5d), Globigerinoides sacculifer, Globorotalia menardii, Orbulina universa, and Neogloboquadrina dutertrei. Globigerinita glutinata made up 15^43% of the assemblages, and was rare at station 17. Neogloboqua-

drina dutertrei reached higher proportions at the southeastern stations 3^6 (Lesser Antilles) than at the northern stations (Figs. 1 and 4). Most of the species reached maximum absolute abundances in surface waters (0^40 m depth). The most frequent deep-dwelling species in the plankton nets was Globorotalia truncatulinoides, with a maximum abundance between 100 and 300 m (Fig. 5a). Other deep-dwelling species (Globorotalia hirsuta, Globorotalia crassaformis, and Globorotalia scitula) were rare. 4.2. The empty test and surface sediment assemblage The assemblage of empty tests in the water column was characterized by an increased portion of Globigerinoides ruber relative to the living fauna (Fig. 7). An apparent decrease of the small-sized species Globigerinita glutinata (on average 6 150 Wm) from the living fauna to the empty test assemblage results from the comparison of di¡erent test sizes, s 100 Wm and s 150 Wm, respectively.

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Fig. 4. Relative frequencies of the most abundant species (a) s 100 Wm in the upper 100 m of the water column (plankton net tows) and (b) s 150 Wm in surface sediments.

The foraminiferal assemblage in surface sediments of the eastern Caribbean Sea is uniformly dominated by Globigerinoides ruber, which comprises about half of the total assemblage. In general, the proportion of G. ruber in sediment assemblages decreases with increasing water depth

(Fig. 8). Other frequent species of the sediment assemblage are (in order of decreasing frequency): Globigerinoides sacculifer, Globigerinita glutinata, Neogloboquadrina dutertrei, Globigerinella siphonifera, Orbulina universa, and Globorotalia menardii (Fig. 7).

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Fig. 5. Standing stocks (specimens m33 ; s 100 Wm) of living (a) Globorotalia truncatulinoides, (b) Globigerinoides ruber (white), (c) Globigerinita glutinata, and (d) Globigerinella siphonifera in the upper 500 m (a) and 100 m (b,c,d) of the water column.

5. Discussion The planktic foraminiferal fauna of the Caribbean Sea comprises tropical and subtropical species (Be¤ and Hamlin, 1967; Jones, 1968; Be¤ and Tolderlund, 1971). The test concentrations of up to 79 living specimens m33 ( s 100 Wm) in the upper 100 m are similar to those reported previously from the western and southern Caribbean Sea (cf. Table 3) and to other low-productivity

regions of the world’s oceans (e.g. Be¤, 1971; Ufkes et al., 1998; Watkins et al., 1996). The overall faunal composition of the living assemblages along the Antilles is similar to previously described Caribbean faunas (Jones, 1968; Miro', 1971), and to the plankton assemblage in fall 1994 o¡ southwestern Puerto Rico (Schmuker, 2000a). The composition of the planktic foraminiferal assemblages along the Caribbean island arc was

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relatively homogeneous (Fig. 4). Di¡erences in the composition at the southeastern stations 3^6 compared to the northern Caribbean Sea stations could be related to higher primary productivity in this area. Coastal Zone Color Scanner images (Mu«ller-Karger et al., 1989) show that the southeastern Caribbean is in£uenced throughout the year by relatively nutrient-rich surface waters that originate at the Amazon River and Orinoco River out£ows. This results in elevated concentrations of phytoplankton (Bogdanov et al., 1968) in the southeastern, compared to the northeastern Caribbean. This is con¢rmed by increased numbers of Neogloboquadrina dutertrei, which is described as a thermocline dweller (Fairbanks et al., 1982; Ve¤nec-Peyre¤ et al., 1995), and is positively correlated to enhanced phytoplankton abundance (Watkins et al., 1996). In contrast, Globigerinella siphonifera and Orbulina universa seem to be more adapted to oligotrophic conditions, suggested by their elevated frequencies in the Puerto Rico region during March (Schmuker, 2000a). Most of the species reach maximum abundance in the upper 40 m of the water column (Table 4). Deep-dwelling species, Globorotalia crassaformis, Globorotalia hirsuta, Globorotalia scitula, and Globorotalia truncatulinoides, were rare and the latter seemed to be related to the distribution of Subtropical Underwater (SUW). The depth habitat of planktic foraminifers is probably related to temperature and salinity preferences and to speci¢c food sources (Be¤, 1977; Hemleben et al., 1989). Most planktic foraminifers feed on phytoplankton and/or calanoid copepods (Spindler et al., 1984; Anderson et al., 1979) that are abundant in the euphotic zone. The regional variability of standing stocks, the depth distribution in the water column, and the composition of the planktic foraminiferal fauna in the various areas will be discussed in the following sections. 5.1. Di¡erential reaction of Globigerinoides ruber (white) and Globigerinita glutinata to production across a cyclonic eddy Highest concentration of planktic foraminifers was recorded south of Puerto Rico, with Globige-

Fig. 6. Proportion of tests of Globigerinoides ruber (white) within six sieve size classes analyzed. Note that at both sites 21 (white bars) and 24 (black bars) the majority of G. ruber tests is found in the size class of 150^200 Wm, two days and one day before the full moon, respectively. According to Bijma et al. (1990) G. ruber tests s 150 Wm are considered adult.

rinoides ruber (white) as the dominating species (Fig. 5b). Globigerinoides ruber is a spinose, symbiont-bearing species that is frequent in tropical to subtropical waters (Be¤ and Hamlin, 1967; Be¤, 1971; Be¤ and Tolderlund, 1971), and feeds mainly on phytoplankton and calanoid copepods (Spindler et al., 1984). The high standing stocks of G. ruber at station 24 cannot readily be explained by temperature, salinity, or chlorophyll a concentration, which are similar at other Caribbean localities, where G. ruber had lower standing stocks (Fig. 2). In addition, the high numbers of G. ruber at stations 21 and 24 were not caused by reproduction alone, as shown by the test size distribution (Fig. 6) that was shifted towards mature tests and not an enhanced number of juveniles (cf. Bijma et al., 1990; Brummer et al., 1987). Instead, enhanced numbers of G. ruber at station 21 (Fig. 1) were associated to upwards bending isotherms (Fig. 2) that are indicative of a cyclonic eddy, causing entrainment of nutrients into the mixed layer and enhanced primary productivity in the central part of the eddy (cf. Fornshell et al., 1981; Kupfermann et al., 1987). Eddies can enhance the productivity of phyto- and zooplankton and, therefore, improve the food source of planktic foraminifers (Beckmann et al., 1987).

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Fig. 7. Relative frequencies of the six most abundant living species ( s 100 Wm) in plankton samples, compared to the species composition ( s 150 Wm) in the living, dead, and sediment assemblages. The plankton samples of the stations 9, 11, 12, and 16 (Fig. 1) were pooled to be compared to sediment station 12. The plankton stations 7, 17, 21, and 24 were pooled to be compared to the sediment station 24.

At station 24, in the periphery of an eddy, from the eddy exported primary producers or secondary producers such as copepods may have supported the diet of G. ruber and provided suitable thriving conditions. Globigerinita glutinata reached maximum abundance at station 21, in the central eddy (Fig. 5c). Within the center of a cyclonic eddy, the entrainment of nutrient-rich waters into the mixed layer from below will ¢rst be succeeded by new production of phytoplankton, particularly diatoms, close to the thermocline (cf. Sellmer et al., 1998). Dia-

toms are the predominant food source of G. glutinata (Spindler et al., 1984; Hemleben et al., 1989), and, as a surface- to subsurface-dwelling species (Ortiz et al., 1995; Watkins et al., 1996), G. glutinata is predestined to exploit this subsurface food reservoir. Regarding the trophic state and the occurrence of G. glutinata, similar scenarios have been described from the Panama Basin and the eastern North Atlantic (Thunell and Reynolds, 1984; Schiebel and Hemleben, 2000). In contrast to the eddy-related nutrient entrainment described here, the hydrographic situation may

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Sea water entering the Caribbean Sea through the Anegada Passage (Fig. 1), and by seasonal increase of G. siphonifera at the southwestern slope o¡ Puerto Rico during March, when oligotrophic conditions prevailed (Schmuker, 2000a). 5.3. Globorotalia truncatulinoides as a tracer of SUW advection

Fig. 8. Relative frequency of Globigerinoides ruber (white and pink) in sediments from the Caribbean Sea versus water depth. Di¡erent symbols represent samples from the PRIST area (¢lled triangles; Schmuker, 2000b), the BFD (¢lled squares; Prell et al., 1999) and the M35/1 cruise (open squares; this study). The line represents the least square ¢t based on an exponential decay equation (a+exp(3c(x3b)) with a correlation coe⁄cient of 0.7053. n = 36.

have been caused, e.g. by wind-driven mixing (Schiebel et al., 2001). 5.2. Globigerinella siphonifera as a tracer of oligothrophic waters Station 28 west of Puerto Rico in the Mona Passage is characterized by an elevated test concentration of Globigerinella siphonifera (Fig. 5d). This species hosts symbionts and is adapted to feed on zooplankton (Spindler et al., 1984). Be¤ and Tolderlund (1971) reported high relative frequencies ( s 20%) of G. siphonifera in the western Sargasso Sea, the Antilles Current, and the Gulf Stream. Consequently, the increased abundance of G. siphonifera in the Mona Passage (Fig. 5d) may indicate the in£ow of oligotrophic Sargasso Sea water (Michaels and Knap, 1996) from the north. This is con¢rmed by the high relative faunal portion of G. siphonifera within the Sargasso

In the Anegada and Jungfern Passage, including station 17 o¡ St. Croix, standing stocks in the upper 100 m were low. This may be caused by oligotrophic Sargasso Sea waters that £ow through these passages into the Caribbean Sea, where they mix with Caribbean surface waters (Wu«st, 1964). In the Anegada Passage, however, Globorotalia truncatulinoides (Fig. 5a) was frequent in waters below 100 m (Table 4), with highest numbers in the northern Anegada Passage (station 9). From the northern towards the southeastern and the western Anegada Passage, the test concentrations of G. truncatulinoides decreased. Globorotalia truncatulinoides is a non-spinose and asymbiotic foraminifer that predominantly feeds on phytoplankton prey (Spindler et al., 1984) and is usually described to dwell mainly in and below the thermocline in the tropics (Mulitza et al., 1997). However, reproduction and initial shell growth of G. truncatulinoides takes place within surface waters, and only subsequently the specimens descend to deeper waters during their ontogeny (e.g. Hemleben et al., 1985). Consequently, Lohmann and Schweitzer (1990) suggested that a strong thermocline, as it is prevalent in the tropics, prevents a successful reproduction by inhibition of vertical mixing. If reproduction is inhibited by strati¢cation in tropical waters, the individuals of G. truncatulinoides that occur in the eastern Caribbean must have originated from less strati¢ed (subtropical) sites. The horizontal and vertical distribution of G. truncatulinoides in the Caribbean Sea suggests that this species was carried from the Sargasso Sea (cf. Deuser and Ross, 1989) into the Caribbean Sea by the SUW (Fig. 2). This hypothesis is corroborated by the seasonal abundance of G. truncatulinoides in the plankton net samples from o¡ Puerto Rico (Schmuker, 2000a). Transport of planktic foraminifers within

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distinct water masses has been described by Berger (1971) and Schiebel et al. (2002). Therefore, G. truncatulinoides could be used as a tracer of the regional and temporal distribution of SUW within the Caribbean Sea. 5.4. The in£uence of low surface salinity on planktic foraminifers Low surface salinities o¡ Santa Lucia (Fig. 2 : 6 35.5) were characterized by extremely low surface standing stocks of planktic foraminifers (four specimens m33 in the upper 20 m of the water column). Below the low salinity lens, the planktic foraminiferal standing stocks reached higher values, similar to the average values observed in the Caribbean Sea. In contrast, low salinity conditions that prevail o¡ the Puerto Rican coast during fall (Schmuker, 2000a : 6 35.0) seem to increase the planktic foraminiferal standing stocks from an average of one specimen m33 (March) to eight specimens m33 (September). The di¡erent reactions of planktic foraminifers to low salinity events observed o¡ Santa Lucia (this study) and o¡ Puerto Rico (Schmuker, 2000a) were probably due to di¡erent hydrographic characteristics and the resulting di¡erences in the trophic structure : In contrast to the thin freshwater lens o¡ Santa Lucia (the upper 10 m of the water column), the low salinity lens o¡ Puerto Rico extended across the whole mixed layer (the upper 50^60 m). Moreover, the freshwater lens o¡ Santa Lucia was more proximal than that o¡ Puerto Rico, if the area of origin was the Orinoco River in both cases (Moore and Todd, 1993; Mu«ller-Karger et al., 1989). The low salinity lenses in the southern Caribbean Sea are characterized by high turbidity and low light penetration (Mu«ller-Karger et al., 1989). Consequently, these lenses have a negative e¡ect on shallow-dwelling planktic foraminiferal species (e.g. Globigerinoides ruber), which depend on symbionts and a phytoplankton diet (cf. Ortiz et al., 1995). These ¢ndings are consistent with other studies from the Gulf of Bengal (Guptha et al., 1997) and the Congo (Zaire) River out£ow (Ufkes et al., 1998). In contrast, species without symbionts, like Neogloboquadrina dutertrei, are re-

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ported to show a positive reaction to freshwater discharge from the Amazon River (cf. Maslin and Burns, 2000). 5.5. Comparison of living, dead, and sediment assemblages The overall composition of the living planktic foraminiferal fauna in the Caribbean Sea was quite di¡erent from the assemblage of empty tests in the water column, and more similar to the sediment assemblage (Fig. 7). The assemblages di¡er mostly in the proportions of Globigerinoides ruber and Globigerinoides sacculifer. The relative frequency of G. ruber in the living assemblages is lower than in the dead assemblages. This might be due to the short reproduction cycle of G. ruber compared to other planktic species. Globigerinoides ruber reproduces every fortnight (Bijma et al., 1990), while most other planktic species exhibit a lunar to annual reproduction cycle (e.g. Bijma et al., 1990; Hemleben et al., 1985; Erez et al., 1991; Schiebel et al., 1997). Therefore, G. ruber produces more empty tests than other species. Due to this strong increase of G. ruber in dead versus living assemblages, the proportion of other species decreases in the dead assemblage in the water column. However, the tests of G. ruber are less frequent in the sediment than in the empty test assemblages, which may be due to their low preservation potential and dissolution even in water depths above the lysocline (Dittert et al., 1999). In contrast, the proportion of G. sacculifer increases from the dead to the sediment assemblages, although the resistance to dissolution (not di¡erentiated for specimens with a gametogenic calcite crust) of G. sacculifer is only slightly higher than that of G. ruber (Coulbourn et al., 1980). Therefore, di¡erential dissolution is not a likely cause for the opposite trends of these two species between dead and sediment assemblages. Consequently, G. sacculifer must be more frequent in the Caribbean Sea during other times of the year than that covered by our sampling campaign (April/May). Seasonal di¡erences might also a¡ect the frequency of G. ruber in surface sediments and in the assemblage of empty tests that was sampled during the M35/1 cruise.

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In general, di¡erences between the living and dead assemblages in the water column and the test assemblage in surface sediments can be due to three processes: (1) Planktic foraminiferal faunas may show pronounced seasonal variability, out of which the M35/1 samples represent only a snapshot. In contrast, the sediment assemblage represents a much longer time-span and displays an average fauna over hundreds to thousands of years. (2) The di¡erential preservation potential of planktic foraminiferal tests in£uences the relative abundance of di¡erent species in sediment assemblages (Bonneau, 1978; Berger et al., 1982; Wu et al., 1990). Di¡erential settling velocity depends on test size and shape (Takahashi and Be¤, 1984; Schiebel and Hemleben, 2000), and causes di¡erential residence time of empty tests in the water column, and exposition of tests to dissolution. As a result, small specimens are preferentially removed from the settling assemblage (Adelseck and Berger, 1975). At the sea £oor, the tests are again subject to dissolution and winnowing, and small and thin-shelled tests are preferentially removed until they are covered by sediment. The AAIW (750^950 m water depth after Wu«st, 1964) is the water mass with the highest dissolution potential in the Caribbean Sea (Haddad and Droxler, 1996). However, we did not observe any dissolution e¡ects in the depth range of this water mass or the underlying NADW (Table 1) on the sediment assemblages investigated. (3) While settling, foraminiferal tests are transported by currents over several hundred kilometers, depending on water depth, current velocity, and di¡erential settling velocity of tests (Siegel and Deuser, 1997; von Gyldenfeldt et al., 2000). As a result, planktic foraminiferal tests may be spread over wide areas while settling through the water column, and the places of test production and burial are di¡erent, depending on test size and species. In addition, adjacent assemblages are mixed and homogenized while settling. Consequently, extraordinarily high or low proportions of species (e.g. Globigerinita glutinata and Globorotalia menardii in the living and dead assemblages of station 6, Fig. 4) are averaged out in the sediment assemblages.

5.6. Relative frequency of Globigerinoides ruber in sediments of the Caribbean Sea Similar to the living planktic foraminiferal assemblages in spring, also the sediment assemblages (Table 2) along the Antilles island arc are dominated by Globigerinoides ruber throughout. In addition to the data from the M35/1, samples from the PRIST (Schmuker, 2000b) and from the BFD (Prell et al., 1999) are included (Figs. 1 and 8). According to the combined dataset (M35/ 1, PRIST, and BFD), G. ruber is the most frequent species, followed by Globigerinoides sacculifer, Globigerinita glutinata, Neogloboquadrina dutertrei, Globigerinella siphonifera and Globorotalia menardii in varying proportions. In general, the BFD assemblages contain less than 45% G. ruber, while this species reaches usually s 45% in the M35/1 and PRIST assemblages. A plot of the relative frequency of G. ruber in the sediments versus water depth (Fig. 8) yields a good correlation between these two parameters. However, for example the samples M35024 and M35026 (Fig. 8) from water depths s 3500 m do not ¢t the general pattern of decreasing proportions of G. ruber with increasing water depth, which may be due to their location close to the shelf break of southwestern Puerto Rico. The distribution of other species, such as G. sacculifer, N. dutertrei, G. glutinata, G. menardii, and Orbulina universa does not indicate any relation to water depth or the distance from the shelf break. The decreasing proportion of Globigerinoides ruber with increasing water depth in the Caribbean Sea may be due to a combination of processes: G. ruber prefers light conditions that characterize fertile regions (‘green waters’) and is especially tolerant to varying salinities which are often observed in neritic realms (Bijma et al., 1992). Be¤ and Tolderlund (1971) describe peak abundances of G. ruber at salinities of either less than 34.5 or more than 36. Therefore, it might be better adapted to conditions that prevail in nearshore areas than other species. The high proportions of G. ruber in the average plankton assemblages in the PRIST area (Schmuker, 2000a) in fact indicate that this species is more successful in neritic habitats compared to other species. In addition,

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the decreasing trend in the relative frequency of G. ruber from east to west in the Caribbean Sea could also be due to a preference of G. ruber for neritic in£uences and fertile waters, as the in£uence of Orinoco and Amazon out£ow water decreases from east to west (Mu«ller-Karger et al., 1989). Dissolution, however, cannot be the main reason for the distribution pattern of Globigerinoides ruber, because only samples with low fragmentation ( 6 20%) were analyzed, and in most dissolution susceptibility rankings, G. ruber is directly followed by Globigerinoides sacculifer (Coulbourn et al., 1980), which does not show any correlation between relative frequency and water depth. The trend of increasing faunal portions of G. ruber is most pronounced in water depths 6 3000 m, that are unlikely to have a signi¢cant corrosive e¡ect on calcite. This pattern of increasing proportions of G. ruber with decreasing water depth does not apply to the pelagic regions of the ocean (e.g. Central Atlantic), where G. ruber reaches maximum frequencies also in deep-water sediments (e.g. Be¤, 1977).

6. Conclusions The planktic foraminiferal fauna of the eastern and northern Caribbean Sea has a relatively homogeneous composition and is similar to the southern and western faunas. Globigerinoides ruber is the most frequent species, followed by Globigerinoides sacculifer, Globigerinita glutinata, Neogloboquadrina dutertrei, Globigerinella siphonifera, and Globorotalia menardii. The relative frequency of G. ruber decreases with increasing water depth and distance from the shelf break. Highest frequencies of G. ruber occur SE o¡ Puerto Rico where updoming isotherms indicate a cyclonic eddy, and enhanced food supply can be assumed. Low numbers of G. ruber have been found along with a ‘freshwater lens’ (salinity 6 35.5) o¡ Santa Lucia (Lesser Antilles). In contrast, N. dutertrei was more frequent along the Lesser Antilles than to the north, and is positively related to freshwater input from the Amazon and Orinoco Rivers (cf. Mu«ller-Karger et al., 1989; Maslin and

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Burns, 2000). Globigerinita glutinata was most frequent SE o¡ Puerto Rico, where isotherms indicate the central part of a cyclonic eddy, and nutrients may be brought up into the photic zone and support primary production. Therefore, G. glutinata might prove useful for paleoceanographic reconstructions of eddy frequency and eddy intensity in the Caribbean Sea. The distribution of the deep-dwelling species Globorotalia truncatulinoides is related to the SUW, and may serve as a paleoceanographic proxy for the spreading of the SUW within the Caribbean Sea.

Acknowledgements Masters, crew, and principal scientists of the RV Meteor cruise 35/1 are gratefully acknowledged. We thank G. Schmiedl, S. Geiselhart, and K. Klose for taking the samples on M35/1, M. Bayer for processing the samples, and Ch. Hemleben for providing them. Thanks are extended to H. Hilbrecht for providing the M35/1 chlorophyll data. The manuscript bene¢ted significantly from discussions with Ch. Hemleben and H.R. Thierstein. We thankfully acknowledge the ¢nancial support from the Deutsche Forschungsgemeinschaft (project No. HE697/22-1), and the Swiss National Fonds (projects No. NF 20-4682896 and NF 2053-053676.98/1).

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