Print version ISSN 0122-9761
bol. invemar vol.30 no.1 Santa Marta Jan./Dec. 2001
RECENT PLANKTONIC FORAMINIFERA FROM DEEP-SEA SEDIMENTS FROM THE EASTERN EQUATORIAL PACIFIC: PROXIES OF THE EQUATORIAL FRONT IN THE LATE QUATERNARY
FORAMINIFEROS PLANCTÓNICOS RECIENTES EN LOS SEDIMIENTOS DEL MAR PROFUNDO DEL PACÍFICO ORIENTAL ECUATORIAL: APROXIMACIÓN AL FRENTE ECUATORIAL EN EL CUATERNARIO TARDIO
Jose Ignacio Martinez and Geovanny Bedoya
Universidad EAFIT, Departamento de Geología, Grupo de Ciencias del Mar, A.A. 3300 Medellín, Colombia. E-mail firstname.lastname@example.org
Planktonic foraminifera recovered from 25 deep-sea sediment samples (core-tops) from the eastern Equatorial Pacific were analyzed for their geographic distribution and possible environmental controls. Samples collected deeper than the carbonate lysocline (~2800 m) show significant signs of dissolution, - when compared to sediment-trap samples -, resulting in the increase of the solution-resistant species Neogloboquadrina dutertrei, Neogloboquadrina pachyderma and Globorotalia cultrata and the reduction of the solution-susceptible species Globigerinita glutinata, Globigerinoides ruber and Globigerinoides sacculifer. Three bioprovinces were recognized by cluster analysis: (1) bioprovince I that occurs on the Cocos Ridge where G. cultrata and N. pachyderma are dominant, (2) bioprovince II that occurs on the Carnegie Ridge where N. dutertrei, N. pachyderma and Globorotalia inflata are dominant, and (3) bioprovince III that occurs in the Panama Basin where G. sacculifer and G. ruber are dominant. Bioprovinces I and II reflect a shallow thermocline induced by upwelling, although AOU, NO3 and PO4 and SiO2 are significantly higher in the latter region. Bioprovince III reflects a deep-mixed layer and low nutrient contents. Possible proxies of the Equatorial Front in the past are: (1) the Shannon diversity index, evenness and the number of species that show a latitudinal break at ~1.5oS and, (2) the G. cultrata / G. dutertrei ratio that decreases southward.
KEY WORDS: Planktonic foraminifera, Eastern Equatorial Pacific, Panama Basin, Deep-sea sediments, Micropaleontology.
Foraminíferos planctónicos recuperados de muestras del tope de 25 núcleos de aguas profundas del Oceano Pacifico Oriental son analizados respecto a las variables ambientales que controlan su distribución geográfica. Las muestras colectadas por debajo de la lisoclina de carbonatos (~2800m) muestran señales considerables de disolucion, - con respecto a las muestras de trampas de sedimentos -, dando como resultado el incremento en especies resistentes a la disolucion como Neogloboquadrina dutertrei, N. pachyderma y Globorotalia cultrata y el decrecimiento en especies suceptibles a la disolución como Globigerinita glutinata, Globigerinoides ruber y Globigerinoides sacculifer. Se reconocen tres bioprovincias, por análisis Cluster, así: (1) bioprovincia I que ocurre sobre la Dorsal de Cocos donde G. cultrata y N. pachyderma son dominantes, (2) bioprovincia II que ocurre sobre la Dorsal de Carnegie Ridge donde N. dutertrei, N. pachyderma y Globorotalia inflata son dominantes, y (3) bioprovincia III que ocurre en la Cuenca de Panama donde G. sacculifer y G. ruber son dominantes. Las bioprovincias I y II reflejan una termoclina somera inducida por surgencia oceanica (upwelling), aunque AOU, NO3, PO4 y SiO2 son mucho mayores en la segunda bioprovincia. La bioprovincia III refleja una capa de mezcla profunda y un contenido bajo de nutrientes en el agua. Como posibles indicadores ("proxies") de la posicion del Frente Ecuatorial en el pasado se sugieren: (1) el Indice de diversidad de Shannon, la equidad ("evenness") y el número de especies los cuales muestran un cambio latitudinal mayor a ~1.5oS y, (2) la relación G. cultrata / N. dutertrei que decrece hacia el sur.
PALABRAS CLAVE: Foraminíferos planctónicos, Oceano Pacifico Oriental, Cuenca de Panamá Sedimentos profundos, Micropaleontología.
The Panama Basin is a critical region for the understanding of Global Climate Change because it receives a huge volume of fresh water derived from the atmospheric transfer of moisture by the Trade Winds from the Atlantic to the Pacific Ocean. This transfer of moisture regulates the inter-oceanic difference in sea-surface salinity which ultimately drives the global circulation of the ocean, i.e. the so called "conveyor belt" (e.g. Broecker and Denton, 1989). The Panama Basin is also bathed by the cool nutrient-rich Peru Current and the warm and less fertile Equatorial Counter Current that meet along the Equatorial Front (e.g. Wooster, 1969; Pak and Zaneveld, 1974). This oceanographic setting is episodically disrupted by El Niño phenomenon. In the past this pattern would have differed considerably at the scale of glacial and interglacial cycles. Furthermore, the huge volume of palynological information on land makes the Panama Basin an ideal region for ocean-continent paleoclimate correlations following the recommendations of the international scientific community. Finally, sea-surface temperature (SST) reconstructions based on planktonic foraminiferal assemblages on the equatorial Pacific for the last glacial maximum (e.g. CLIMAP, 1976; Anderson et al., 1989) have remained in conflict with palynological evidences for the past 20 years despite the use of new proxies of paleotemperature, e.g. Sr/Ca in corals and alkenones (Uk37) extracted from deep-sea sediments. Therefore, more up-to-date SST reconstructions for the Panama Basin are needed in order to solve the conflicting results. Recently, SST reconstructions based on radiolarians have shown that cooling south of the Equatorial Front was significantly larger than suggested by CLIMAP (1976) for the last glacial maximum (LGM), i.e. ~5oC compared to <2oC (Pisias and Mix, 1997). Precise SST reconstructions are critical for the building of reliable climate and paleoclimate models; the predictive character of these models is, therefore, bly dependent on SST as a boundary condition.
Planktonic foraminifera are: (1) one of the most abundant Protista groups in the world ocean, (2) easily identifiable and, (3) have not evolved during the Late Quaternary. These characteristics make planktonic foraminifera an ideal proxy for the reconstruction of sea-surface environmental variables, e.g. temperature and salinity. However, the reconstruction of these environmental variables and the structure of the upper water column are highly dependent on a deep knowledge of the ecology of planktonic foraminifera in a region.
Planktonic foraminifera ecological and paleoecological studies in the Eastern Equatorial Pacific (EEP) include: (1) plankton-tow samples (e.g. Fairbanks et al., 1982; Thiede, 1983), (2) sediment trap moorings (e.g. Thunell et al., 1983; Thunell and Reynolds, 1984), and (3) down-core deep-sea sediments (e.g. Faul et al., 2000). None of these studies, however, have focused on the distribution of planktonic foraminifera at the sea floor, except for a localized study on the continental shelf in front of the Nariño Department (Cortes et al., 1990). On a different approach, other studies have considered the distribution of planktonic foraminifera on the entire Pacific basin, for the reconstruction of past sea-surface temperatures (e.g. Mix et al., 1999). Therefore, this paper aims to explore the ecological distribution of Recent planktonic foraminifera from a number of core-top samples widely distributed in the EEP region.
Even though planktonic foraminifera recovered from deep-sea sediments are excellent proxies of upper-water conditions (mainly SST and productivity), carbonate dissolution, dispersal and reworking of sediments might greatly blur the original sea-surface signal. For the Panama Basin foraminiferal dissolution begins at ~2800 m (Thunell et al., 1981), whereas significant sediment reworking and dispersal occur over the Cocos and Carnegie Ridges due to the action of deep-sea currents (e.g. Van Andel, 1973; Kowsmann, 1973; Lonsdale and Malfait, 1974; Yamashiro, 1975). Therefore, in order to understand the environmental variables that control the distribution of planktonic foraminifera, the effects of dissolution and reworking need to be assessed.
Physical Oceanography of the Eastern Equatorial Pacific (EEP)
The EEP is located in the eastern extreme of the equatorial current system in the Pacific Ocean (Fig. 1a). Surface currents in the region include the west-flowing North and South Equatorial Currents (NEC and SEC) and their corresponding, east-flowing countercurrents (NECC and SECC), plus the Peru, or Humbolt Current (e.g. Tomczak and Godfrey, 1994). Both, the NEC and SEC are driven by the Trade winds. Therefore, they are ber in the winter of their respective hemisphere, i.e. the NEC transports 45 Sv (Sverdrup = 106 m3s-1) of water with a speed of 0.3 ms-1 in February; whereas the SEC transports 27 Sv with a speed of 0.6 ms-1 in August (e.g. Tomczak and Godfrey, 1994). In February the ber influence of the northeast Trade winds also prevents the warm-water from the NECC to reach the Panama Bight. Conversely, in August, the northeast Trade winds are weaker (the southeast Trade winds ber) and the NECC reaches the Panama Bight. Due to the ber influence of the southeast Trade winds, upwelling is a common phenomenon in the EEP in August (e.g. Wyrkti, 1974). The NEC and SEC are fed by the south-flowing California Current and the north-flowing Peru Current respectively.
Upwelling caused by divergence (Ekman transport) is an important component of the SEC and affects the uppermost 200 m of the water column. Even though, the vertical speed of water is only 0.02 mh-1, -a rather low speed compared to other coastal upwelling regions-, the net transport of water reaches ~47 Sv (e.g. Wyrtki, 1981; Tomczak and Godfrey, 1994). Upwelling, -though weaker-, is also a common phenomenon in the center of the Panama Bight and the eastern side of the Gulf of Panama during February-March when it moves at a speed of 45x10-4cms-1 (Stevenson, 1970). Upwelling in the Panama Bight results in a primary production that ranges between 100 to 900 mgCm-2d-1 (Bishop and Marra, 1984; Bishop et al., 1986).
The NECC originates in the western Pacific Ocean and transport 45 Sv in the west to 10 Sv east of the Galapagos. The NECC ends in the Costa Rica Dome where the thermocline depth is minimum (e.g. Wyrtki, 1964). The SECC transport ~10 Sv of water at a speed of 0.3 ms-1 (e.g. Tomczak and Godfrey, 1994).
At the eastern extreme of the NECC (the Panama Bight), the north-flowing Colombia Current has a speed of 100cms-1 in August and 60cms-1 in February (e.g. Stevenson, 1970). The latter in response to negative effect of the ber northeast Trade winds by that time of the year. The Colombia Current has an average width of 108 km and follows a cyclonic path (e.g. Stevenson, 1970). The boundary between the cold (15o-19oC), saline (35 p.s.u., i.e. practical salinity units) west-flowing SEC and the warm (>25oC), fresher (<33.5 p.s.u.) east-flowing NECC constitutes a sharp boundary refer to as the Equatorial Front (e.g. Okuda et al., 1983; Fig. 1a).
The wing Equatorial undercurrent (EUC), or the Cromwell Current, runs along the equator at 200 m and 40 m water depth in the western and EEP respectively (Fg. 1b). The EUC is 400 km wide, 200 m thick, has a speed of 1.5 ms-1, and transports ~8 Sv of water of southern hemisphere origin (e.g. Toggweiler et al., 1989; Tomczak and Godfrey, 1994). The transport of water by the EUC increases towards the east where it reaches 35-40 Sv (e.g. Tomczak and Godfrey, 1994). The EUC can be identified by the deflection of isotherms and a salinity maximum. The EUC is ber during January-June and weaker during July-December (e.g. Tomczak and Godfrey, 1994).The Equatorial Intermediate Current (EIC) transport 7±4.8 Sv of water with speeds between 0.1 and 0.2 ms-1 and occurs at water depths between 300 and 900m (Fig. 1b). Closely related to the EIC, at 600 m in both hemispheres, occur the North and South Subsurface Countercurrents (NSCC and SSCC). The former countercurrent seems to play an important role in the formation of the Costa Rica Dome (e.g. Tomczak aTomczak and Godfrey, 1994).
The meridional shift of the Intertropical convergence zone (ITCZ), between 9oN in August and 1oS in February, causes changes in SST and salinity (SSS). Average SST in March reaches <28oC northwest of the Panama Bight and <24oC in front of Guayaquil (Levitus et al., 1994; Fig. 2a). Conversely, in September, the 28oC isotherm moves to the northeast, whereas a minimum of 21oC is found in the southwest (at 2oS; Fig. 2b). The maximum SST gradient is found at 4oN in September, thus reflecting the position of the Equatorial Front. Seasonal SST patterns for shorter time scales can differ markedly from the Levitus´s
et al. (1994) atlas, i.e. the 1970 to 1996 SST average from Colombian Navy cruises (Tchantsev and Cabrera, 1998). The mixed-layer depth, - defined as the depth where water temperature is 5oC lower than SST (Levitus et al., 1994)-, does not correspond to SST seasonal patterns (Fig. 2c, d). The mixed-layer is >10m deep in the northwest in March and >22m deep between 2 and 4oN and south of the Equatorial Front in September (Fig. 2c, d)
Average SSS shows a minimum of 31 p.s.u. in the eastern Panama Bight at 4oN, i.e. west of the San Juan delta, and maximum values (33.5 p.s.u.) in the northwest and southwest of the Bight during March (Fig. 2e; Levitus et al., 1994). This SSS pattern is maintained in September (Fig. 2f). However, extreme values reach 30 p.s.u. and 34.5 p.s.u. respectively (Levitus et al., 1994). The SSS delineation of the Equatorial Front is rather poor. This is not the case of annual dissolved oxygen and AOU (apparent oxygen utilization) that display a sharp contrast at 3oN reflecting the position of the Equatorial Front (Fig. 3a, c). Oxygen values are >4.7 mll-1 west of 86oW and <4.4 mll-1 at 1oN in the eastern Panama Bight in March, and <0.1 mll-1 north of 3oN with a maximum of >0.5 mll-1 at 2oS in September (e.g. Okuda et al., 1983; Levitus et al., 1994). Similarly, AOU is minimum in the northwest (-0.15) and maximum in the southeast at 1oN (0.25) for March, and <0.1 north of 3oN and exceeds 0.5 south of the equator for September (Levitus et al., 1994).
Nitrate (NO3), phosphate (PO4), silica (SiO2) and density (Fig. 3b, d, e, f) also display a b meridional contrast through the Equatorial Front, i.e. high values in the cold, saline, west-flowing SEC, and low values in the warm, fresher, east-flowing NECC (e.g. Okuda et al., 1983). Chorophyll and carbon fixation at 10m water depth and PO4 at 20m also display maximum values, i.e. 30 mgm-3d-1, 0.8 mgm-3, and 2 mll-1 respectively, south of the Equatorial Front in February (e.g. Forsbergh, 1969).
MATERIALS AND METHODS
A set of 3 cc twenty one core-top samples were obtained from the core repositories of the University of Rhode Island (cores TR),
Lamont Doherty Earth Observatory (cores VM), and the Ocean Drilling Program (cores ODP; Table 1). Supplemented information was added from six core-top samples (RC23-12, -20, -113, -138, RC6-69, and VM17-44) from Faul et al. (2000) study. Samples from Cortes et al. (1990) could not be compared due to taxonomic inconsistencies. Core-
top samples are assumed to represent the Present, though due to mixing by benthonic organisms they represent the last few hundred years. Core-top samples were recovered from 1700 and 3500 m water-depth. Samples collected below ~2800 m, i.e. the sedimentary lysocline (e.g. Thunell et al., 1981), are more affected by carbonate dissolution.
Samples were soaked in water and diluted hydrogen peroxide until reaction stopped. Wet sieving followed in the 63 and 150 mm size fractions, and drying of the sample at ~40oC. Counting of specimens was done on a sub-sample of ~300 specimens obtained with the aid of an Otto microsplitter in the >150 mm size fraction.
We follow in this paper a conservative approach to taxonomy thus grouping a number of species as variants of more well-known species (e.g. Parker, 1962, and Plate 1). This assumption might be imprecise as recent DNA sequencing studies have evidenced the existence of cryptic species of planktonic foraminifera undistinguishable on morphological basis (e.g. Pawlowski, 2000; Darling et al., 2000). Dissolution of planktonic foraminifera (percentage abundance) was evaluated by counting the number of fragments over the number of whole specimens.
Species assemblages were obtained by cluster analysis and diversity determined by the Shannon index (MVSP multivariate statistical package), whereas environmental variables were obtained from the World Ocean Atlas available on the Internet (Levitus et al., 1994)
Table 2 contains the percentage abundance of planktonic foraminifera, whereas Figure 4 shows their geographic distribution (percentage abundance) in the EEP Ocean. Neogloboquadrina dutertrei is the dominant species in the region showing higher percentage abundances south of the Equatorial Front. Globigerina bulloides and Globigerinita glutinata are more abundant toward the northwest, as it is Globorotalia cultrata, although the latter species is more abundant over the Cocos Ridge, i.e. close to the Costa Rica Dome. Globigerinoides ruber and Globigerinoides sacculifer show maximum
values between the equator and 6oN, though the former species concentrates over the Cocos Ridge away from the influence of the Costa Rica Dome. Globorotalia inflata is restricted to the region close to Ecuador (over the eastern Carnegie Ridge), whereas Neogloboquadrina pachyderma right-form is more abundant southeast of Galapagos Islands and over the Cocos Ridge between 4o and 6oN.
Fragmentation of planktonic foraminifera is significant in most of the core-top samples and is >70% between the equator and 2oN and in the northwest north of 5oN. This fragmentation distribution biases the percentage abundance of solution-susceptible and solution-resistant planktonic foraminiferal species, i.e. P. obliquiloculata and G. cultrata. Because fragmentation of planktonic foraminifera is linearly related to water depth, we should expect the Shannon diversity index, evenness, and the number of species to show a systematic relation with water depth. Shannon diversity index shows two populations that increase with water depth (Fig. 5a). These two tendencies appear to be related to latitude. However, a change in the number of species with water depth is less evident (Fig. 5b).
Shannon diversity index, evenness, and the number of species increase northwards (Figs. 5c, d and 6). This trend reflects a meridional environmental gradient that controls species distributions. However, there is a major break at ~1.5oS that roughly corresponds to the Equatorial Front (Figs. 5c, d).
Three bioprovinces are distinguished by Q-mode Cluster Analysis, unweighted pair group average (UPGMA) with an Euclidian distance coefficient (Fig. 7a). Bioprovince I includes samples along the Cocos Ridge; bioprovince II includes samples along the Carnegie Ridge, south of the Equatorial Front; and bioprovince III includes samples from the Panama Basin (Fig. 7b). Even though samples collected over the Cocos and Carnegie Ridges are shallower than those collected on the Panama Basin, there is not a systematic relationship of cluster bioprovinces with water depth, e.g. sample VM19-27 was collected at 1300 m water depth in the Panama Basin and is grouped in bioprovince III rather than II.
The distribution of planktonic foraminifera in deep-sea sediments of the Panama Basin greatly differs from their distribution in sediment-trap samples. Despite the different mesh sizes used in deep-sea sediment and sediment-trap studies, i.e. >150mm in this study and >330mm (Be et al., 1985) and >125mm (Thunell and Reynolds, 1984), it is evident that dissolution significantly alters planktonic foraminifera assemblages by eliminating the most fragile species. As already noted by Be et al. (1985) Globorotalia theyeri and G. ruber are dominant in sediment-trap samples collected at ~5oN-81oW from the uppermost 2000 m of the water column during July- August as compared to N. dutertrei that is dominant in deep-sea sediment samples. Nonetheless, sediment-trap samples collected at the same location do not show major differences in the flux of plantktonic foraminifera at 890, 2590, and 3560m (Thunell and Reynolds, 1984), thus suggesting that despite a foraminiferal lysocline located at ~2800m dissolution in the water column is ineffective in destroying planktonic foraminifera (Thunell and Reynolds, 1984). This contrasts with dissolution at the seafloor where the loss in carbonate is estimated to be ~84% (Thunell and Reynolds, 1984). This figure is in agreement with previous estimates of ~50% dissolution at ~1500 m, 80% at ~1750 m, and the carbonate compensation depth (CCD) at ~3400 m (Kowsmann, 1973). This shallower lysocline and CCD, when compared with more oceanic regions, is greatly due to an excess of organic matter rain to the seafloor that results in carbonic acid (H2CO3) in pore water. When comparing the annual average percentage of planktonic foraminifera from a sediment-trap (5.35oN, 81.88oW) collected by Thunell and Reynolds (1984) during a year period from December 1979 with ODP84 (5.75oN, 82.89oW) core-top sample reported here (Fig. 8), it appears that G. glutinata, G. ruber and G. bulloides are more abundant in the former than in the latter sample. Conversely, N. dutertrei, G. cultrata, and N. pachyderma are more abundant in the ODP84 core-top sample. As expected the most solution-susceptible species were eliminated from the sediment sample. Nonetheless, part of the difference could be due to other causes different to carbonate dissolution as follows: (1) for Figure 8 the 1979-1980 percentage average in sediment-trap samples was considered rather than the whole range of percentage variation; this is the case for N. dutertrei in sediment-trap samples that reaches a maximum value of 59.9% very close to the one determined for the ODP84 core-top sample, (2) the 1979-1980 period represents only a discrete time slice when compared to the centennial character of sediment samples and, (3) species like G. quinqueloba were not observed in the core-top sample due to its reduced size <150mm.
Rutherford et al. (1999) have found that diversity of planktonic foraminifera is related to the structure of the upper-water column (the thermocline depth) rather than to SST. Consequently, the largest diversity of planktonic foraminifera occurs at mid latitudes (central gyres) rather than the equatorial regions. This "hump-shape" pattern, - also reported for a number of animal and plant species -, suggesting that diversity is controlled by productivity (e.g. Rosenzweig and Abramsky, 1993). However, an excess of productivity results in low diversity. As for the present data set, south of the Equatorial Front productivity is in excess
and diversity therefore decreases thus representing the upper extreme of the "hump-shape" pattern. As with Rutherford et al.'s (1999) global map, the diversity map presented herein (Fig. 6) shows that diversity decreases when the thermoclina is shallow. However, different to Rutherford et al.'s (1999) results, dissolution prevents to establish a relationship between SST and diversity that might help to reconstruct paleo-SSTs from deep-sea cores. In the best case, the major change in diversity at ~1.5oS might help to reconstruct the past position of the Equatorial Front, i.e. the record of any abrupt down-core changes in the Shannon diversity index for cores located close to the equator.
The species G. bulloides, G. cultrata, G. glutinata and N. dutertrei have already been reported as associated to the equatorial "cold-tongue" in contrast to G. sacculifer, G. ruber and G. conglobatus that are associated to subtropical regions (e.g. Watkings et al., 1998). The former species are either symbiont barren or symbiont facultative (diatoms- or chrysophytes-bearing) and mostly herbivorous, whereas the latter species are dinoflagellate bearing and mostly carnivorous (e.g. Hemleben et al., 1989). Diatoms as a base of the food web are replaced by dinoflagellates during El Niño years (e.g. Rojas de Mendiola et al., 1985). This must be reflected in the presence of abundant G. sacculifer, G. ruber and G. conglobatus during El Niño years in the EEP.
The distribution of cluster bioprovinces I and II from the EEP corresponds to phytoplankton production levels in excess of 700 mgC/m2/day (Owen and Zeitzschel, 1970). These high productivity levels result from upwelling thus favoring the presence of G. cultrata and N. pachyderma in the north (around the Panama Bight and the Costa Rica Dome) and N. dutertrei, N. pachyderma and G. inflata south of the Equatorial Front. The difference is due to upwelling intensity that results in different food webs (e.g. Owen and Zeitzschel, 1970; Honjo, 1982).
The distribution of cluster bioprovince III from the Panama Basin coincides with a maximum abundance of benthonic foraminifera and bivalve larvae collected from filtered water samples (Thiede, 1983). Rosalina globularis is the most abundant benthonic foraminifera species. R. globularis develops an ephemeral floating chamber before sexual reproduction; therefore its abundance in the central Panama Basin is used as a proof of drifting from coastal regions as the species is epi-benthic and lives in shallow waters (Thiede, 1983). A similar reasoning applies to the bivalve larvae whose abundance increases shoreward (Thiede, 1983). As noted by Thiede (1983), these dispersal patterns could be related to the Colombian Current. Subsurface currents might be involved in their re-distribution either by supplying nutrients or by passively carrying benthonic foraminifera and bivalve larvae along their path, i.e. the Equatorial Underwater Current or the Cromwell Current whose path in the Panama Basin (Pineda, 1997) resembles the distribution of bivalve larvae.
Globorotalia inflata is a species dominant in transitional regions between the subtropics and the poles (20 to 60oS), lives in deep waters under the thermocline (>100 m), shows a wide range of tolerances to temperature and salinity and can facultatively host photosynthetic symbionts in their protoplasm (e.g. Hemleben et al., 1989; Hilbrecht, 1996). However, the species was found in TR163-38 core-top sample about 14o north of the nearest reported occurrence in core-top sample V19-41 (14.1oS; 96.1oW; Thompson, 1976). This might imply that episodic pulses of the Peru Current could carry G. inflata significantly north of its normal habitat.
South of the Equatorial Front, and co-occurring with sediment samples that provide abundant N. dutertrei, N. pachyderma and G. inflata, plankton tow collections provide abundant diatoms, coccolithophorids (mainly Emiliana huxleyi), copepods and euphausiids, among others (e.g. Jimenez and Bonilla, 1980; Bishop et al., 1986). N. dutertrei, N. pachyderma and G. inflata are symbiont-barren and live deep in the water column, thus suggesting that they should prey on diatoms and coccolithophorids and be consumed by copepods and euphausiids. Conversely, north of the Equatorial Front, dinoflagellates and chaetognaths co-occur with G. sacculifer, G. ruber, and G. glutinata. These species are symbiont-bearing or symbiont-facultative and preferentially should prey on dinoflagellates and are consumed by chaetognaths among other groups. At the Panama Bight diatoms and silicoflagellates are the dominant phytoplankton groups in the >53mm size-fraction whereas copepods, larvaceans, chaetognaths and ostracods are the dominant zooplankton in the >333mm size-fraction during July-August, i.e. after the upwelling event (Bishop et al., 1986). During July-August planktonic foraminifera in the >333mm size-fraction were more abundant than during November-December presumably due to the elevated supply of food and/or more predation on the smaller foraminifera (Bishop et al., 1986).
Even though, coastal and equatorial upwelling of sub-surface water have been suggested to be the main mechanisms responsible for vertical mixing and the supply of nutrients to the mixed layer of the EEP, vertical mixing could also result south of the Carnegie Ridge where a rough topography might prevent the free northward flow of deep-water currents. This mechanism, - documented elsewhere (Brazil abyssal plain) -, and has been suggested to account for the closing of global overturning circulation following the formation of deep-water in high latitudes and horizontal transport of ocean currents (Ledwell et al., 2000).
Despite the intense carbonate dissolution which significantly affects the preservation of planktonic foraminifera assemblages deposited on the seafloor in the EEP, there are a number of observations that can be used in paleoceanographic reconstruction as follows:
1) Because of carbonate dissolution, induced by the corrosiveness of deep water and intense organic matter rain to the seafloor, there is an anomalous increase of the solution-resistant species N. dutertrei, N. pachyderma and G. cultrata and a reduction of G. glutinata, G. ruber and G. sacculifer in deep-sea sediments with respect to sediment-trap samples.
2) Three bioprovinces were recognized on the EEP by cluster analysis: (a) bioprovince I that occurs on the Cocos Ridge where G. cultrata and N. pachyderma are dominant, (b) bioprovince II that occurs on the Carnegie Ridge where N. dutertrei, N. pachyderma and G. inflata are dominant, and (c) bioprovince III that occurs in the Panama Basin where G. sacculifer and G. ruber are dominant.
3) Bioprovinces I and II do reflect a shallow thermocline induced by upwelling (Costa Rica Dome - Panama Bight and equatorial divergence), although AOU, NO3 and PO4 and SiO2 are significantly higher in the latter region. Bioprovince III reflects a deep-mixed layer and low nutrient contents.
4) The occurrence of G. inflata in core-top sample TR163-38 suggests the episodic inflow of the Peru Current thus transporting this subtropical species into the Panama Basin.
5) Two possible proxies of the Equatorial Front in the past are suggested: (a) the Shannon diversity index, evenness and the number of species that show a latitudinal break at ~1.5oS and, (b) the G. cultrata / G. dutertrei ratios that decrease southward.
This work is part of the research project: "Late Quaternary paleoceanography of the Panama Basin, Colombian Pacific: Implications for Global Climate Change" funded by Universidad EAFIT - COLCIENCIAS (Programa Nacional de Medio Ambiente y Habitat). We thank Dr. John Firth (Ocean Drilling Program) and Dr. Steven Carey (University of Rhode Island, NSF grant OCE-9102410) for kindly providing the core-top samples and Julliet Betancur for diligently processing the samples. Maria Isabel Acevedo and Wilton Echavarria are acknowledged for their logistic support. Christina Ravelo kindly provided foraminifera countings from six core-top samples (Faul et al., 2000 study). We thank Georges Vernette for a careful review of the manuscript.
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For planktonic foraminifera raw data see Microfossil homepage (Data link) (http://www.geocities.com/CapeCanaveral/Launchpad/4680/).
Faunal reference list.
Globigerina bulloides d'Orbigny
Globigerina calida Parker
Globigerina digitata Brady
Globigerina falconensis Blow
Globigerinella siphonifera (d'Orbigny)
Globigerinoides conglobatus (Brady)
Globigerinoides elongatus (d'Orbigny)
Globigerinoides ruber (d'Orbigny)
Globigerinoides sacculifer (Brady)
Globoturborotalita rubescens (Hofker)
Globoturborotalita tenella (Parker)
Orbulina universa d'Orbigny
Sphaeroidinella dehiscens (Parker and Jones)
Globigerinita glutinata (Egger)
Globoquadrina conglomerata (Schwager)
Globorotalia crassaformis (Galloway and Follador)
Globorotalia crotonensis Conato and Follador
Globorotalia cultrata (d'Orbigny)
Globorotalia inflata (d'Orbigny)
Globorotalia scitula (Brady)
Globorotalia tumida (Brady)
Globorotalia ungulata Bermudez
Globorotaloides hexagonus (Natland)
Neogloboquadrina dutertrei (d'Orbigny)
Neogloboquadrina pachyderma (Ehrenberg)
Pulleniatina obliquiloculata (Parker and Jones)
DATE RECEIVED: 26/04/2001 DATE ACCEPTED: 10/09/2001