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Journal of Foraminiferal Research, v. 29, no. 3, p. 209–221, July 1999

CHANGES IN CIRCULATION AND TROPHIC LEVELS IN THE PLIOCENE CARIBBEAN SEA: EVIDENCE FROM BENTHIC FORAMINIFER ACCUMULATION RATES LENNART BORNMALM, JOEN G. V. WIDMARK,

AND

BJO¨RN A. MALMGREN

Go¨teborg University, Earth Sciences Centre/Marine Geology, Box 460, SE-405 30 Go¨teborg, Sweden.

and 3.45, and at 3.4–3.35 Ma may be a result of strong but interrupted inputs of phyto-detritus into the Colombia Basin. Also in the upper part of the interval particularly between approximately 2.5 and 2.2 Ma the abundance of E. exigua exhibit increased values. At about 3.0 Ma N. umbonifera shows a drastic decrease and coeval recovery of C. wuellerstorfi, O. umbonatus, and Pyrgo murrhina. This faunal change could be attributable to (a) mixing between the base of nutrient-rich Antarctic Intermediate Water (AAIW) and the upper layer of Upper North Atlantic Deep Water (UNADW), and/or (b) nutrient-rich local river outflow (e.g. from the Rio Magdalena) together with, at least periodically, (c) increased bottom-water currents that favored the normally elevated and suspension feeding C. wuellerstorfi. The short term alternation in the benthic foraminifer abundance, i.e. the instant recovery of N. umbonifera in the lower part of the interval, may indicate an amelioration of deep-water conditions, which may have been associated with a slower inflow of bottom water into the Caribbean Sea. Moreover, the increased average benthic d13C value during the upper part of Interval 3 may also be a result of a better bottom-water ventilation in the Colombia Basin linked to the onset of the modern deep-water circulation, which most likely is related to periodically increased inflow of UNADW into the Caribbean Sea.

ABSTRACT

Changes in benthic foraminifer faunas throughout the late Neogene (about 5.8–1.8 Ma) were analyzed in DSDP Hole 502A (Caribbean Sea) to determine whether the development of the Isthmus of Panama and resulting changes in bottom-water circulation affected the benthic foraminifer community. Benthic foraminifer accumulation rates (BFAR) of the 11 most abundant and presumably also ecologically significant species revealed three intervals of distinct faunal developments: Interval 1 (prior to 4.65 Ma) exhibits a fluctuating pattern in the benthic foraminifer fauna with an increase of Epistominella exigua between 5.7 and 5.35 Ma, except at about 5.4 Ma. This variation of E. exigua may indicate a period of increased vertical flux of organic (phytodetritial) matter to the seafloor at the base of the sequence. Also towards the upper part of Interval 1, E. exigua shows periods of higher abundance, which could be related to higher vertical flux of phytodetritus to the seafloor. Interval 2 (4.65 to 3.9 Ma) is marked by a gradual increase of C. wuellerstorfi and Oridorsalis umbonatus, and decrease of Nuttallides umbonifera with periods of higher abundance of E. exigua. This faunal change can be related to alternations of sudden phyto-detritus inputs and increased circulation within the Caribbean Sea that resulted from the progressive emergence of the Panamanian landbridge changing the Caribbean Sea from a broad oceanic seaway into a marginal sea. The restricted surface-water flow over the Isthmus of Panama probably enhanced northward transport of warm, high-salinity waters into the high latitudes via the Gulf Stream and thus stimulated the total production of North Atlantic Deep Water (NADW) leading to an increased inflow of Upper North Atlantic Deep Water (UNADW) into the Caribbean Sea. The increased bottom-water activity in the Caribbean may have favored C. wuellerstorfi, which has been found to prefer an elevated suspension feeding position above the sediment surface. Intensified bottomwater circulation would allow more water to pass and thus provide more available food for this particular species. Interval 3 (about 3.9 to 1.8 Ma) began with a striking decrease of C. wuellerstorfi coeval with a rapid increase of N. umbonifera, which became the dominating species. This may have been a response to a declined velocity of the bottom-water currents in the Caribbean, probably caused by less inflow of bottom waters from the North Atlantic. The organic flux into the area may have been similar to Interval 2, but lower bottom-water current velocities may have favored the more oligotrophic species N. umbonifera relative to C. wuellerstorfi. The peak abundance of E. exigua between about 3.55

INTRODUCTION The Caribbean Sea is a marginal part of the subtropical Atlantic and an integral part of the equatorial circulation system, by which warm saline surface waters enter the Caribbean Sea by the Brazil Current. The gradual uplift of the Isthmus of Panama during late Neogene times restricted surface-water circulation into the Pacific and increased evaporation in Caribbean surface waters (Keigwin, 1982b, c). By the end of the Miocene three principal marine corridors connected Pacific and Caribbean waters: 1) through the Panama isthmian strait (Panama Canal Basin), 2) through a corridor between northern Costa Rica-southern Nicaragua (San Carlos Basin), and 3) through a corridor northwest of Colombia (Atrato Basin) (Savin and Douglas, 1985; Duque-Caro, 1990; Coates and Obando, 1996; Collins and others, 1996). Consequently, the Caribbean became a source region for heat and warm saline surface waters that, via the Gulf Stream, transport heat to the North Atlantic where it is cooled and becomes North Atlantic Deep Water (NADW). Therefore, the Caribbean influences the climate both on a regional (equatorial) Atlantic scale and on a hemispheric scale for the North Atlantic region and northwest Europe. The Colombia Basin in the Caribbean Sea is separated from

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FIGURE 1. Location and bathymetry (in km) of DSDP Leg 68 Site 502 (3,051 m) in the Caribbean Sea. The marked line illustrates the position of the cross section between the Windward Passage and Panama, which is given in Fig. 2.

the Atlantic by a sill at 1,650 m depth (Fig. 1), and its hydrography reflects not only bottom-water circulation in the Caribbean, but also the intermediate water circulation in the tropical Atlantic due to the inflow of Upper North Atlantic Deep Water (UNADW). Deep-sea benthic foraminifer faunas provide information about deep oceanic environments, particularly during times of significant change such as major reorganizations of deepand shallow-water circulation. In the late Miocene and Pliocene such changes took place in response to climatic changes caused by mountain building and ice build-up in northern and southern polar regions. The uplift of the Central American Isthmus is likely to have contributed to these environmental changes. The benthic foraminifer faunas in the Caribbean Sea provide a source of information that can be used to decipher the history of changes to a glacial world. Previously it was believed that major rapid changes in deep-water circulation have influenced benthic foraminifer faunas as a result of changes in physicochemical compositional changes in deep water masses. For example, Nuttallides umbonifera has earlier been associated with Antarctic Bottom Water (AABW) (e.g., Streeter, 1973; Streeter and Shackleton, 1979; Schnitker, 1974, 1979, 1980; Lohmann, 1978; Balsam, 1981; Murray and others, 1986; Caralp, 1987, 1988; Hermelin, 1986, 1989; Murray, 1991), which is highly corrosive and contains high concentrations of dissolved CO2 (Bremer and Lohmann, 1982; Mackensen and others, 1990), and/or indicative of old sluggish, oxygenpoor bottom waters (Miller, 1983). However, two decades ago Belanger and Streeter (1980) were sceptical that benthic foraminifer distributions are controlled primarily by physical and chemical deep-water properties. Although Norwegian-Greenland Sea bottom waters

have relatively uniform physical and chemical properties below 1,000 m, Belanger and Streeter (1980) found distinctly different benthic foraminifer assemblages at different water depths. Such results raised serious doubts about the use of benthic foraminifers as tracers of deep-water masses (e.g., Corliss, 1985; Corliss and others, 1986; Thomas and Vincent, 1987, 1988; Linke and Lutze, 1993; Gooday, 1988, 1993; Mackensen and others, 1993; Schnitker, 1994). Recent studies on living benthic foraminifers have focused more on their ecology particularly in terms of food availability and oxygen concentration of the environment. Results from these studies generally suggest that these parameters are the prime factors controlling both distribution and density of benthic foraminifers (Altenbach, 1985; Lutze and others, 1986; Altenbach and Sarnthein, 1989; Gooday, 1986, 1988, 1994; Corliss and Chen, 1988; Mackensen and Douglas, 1989; Corliss and Emerson, 1990; Hermelin and Shimmeld, 1990; Corliss, 1991; Barmawidjaja and others, 1992: Rosoff and Corliss, 1992; Sjoerdsma and Van der Zwaan, 1992; Jorissen and others, 1993, 1995; Rathburn and Corliss, 1994). Some species, which earlier have been used as deep-water tracers, have also been linked to certain food types. For example, high frequences of Epistominella exigua and Alabaminella weddellensis have been found associated with sudden high inputs of phytodetritus (Gooday, 1988, 1994; Gooday and Lambshead, 1989; Lambshead and Gooday, 1990; Thomas and Gooday, 1996). Recently, not only has food supply been implicated but also the seasonality of food availability been investigated (Pfannkuche, 1993; Rice and others, 1994). Thus, the distribution patterns of these species may be primarily linked to availability of phytodetritus, although this possibility needs to be substantiated by more data (Gooday, 1993; Schmiedl, 1995). In

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211

FIGURE 2. Cross section showing generalized water-mass stratification of the Colombia Basin between the Windward Passage and Panama (redrawn after Wu¨st, 1964) as indicated in Fig. 1.

addition, benthic foraminifer faunas with abundant phytodetritus-exploiting species, indicative of well-oxygenated, generally oligotrophic open-ocean regions with a seasonal phytodetritus pulse, can be distinguished from faunas with abundant Uvigerina, Bolivina, Bulimina, or Melonis species that, on the other hand, are found mostly where there is more continuous fluxes of organic matter to the sea floor, and perhaps reduced bottom-water oxygenation (Gooday, 1994; Schmiedl, 1995; Thomas and Gooday, 1996). Thus, the paleontological record may potentially be used to identify periods when seasonal phytodetritus deposition influenced benthic communities (Smart and others, 1994; Thomas and others, 1995) assuming food supply is indeed the primary controlling factor on benthic community structure. Furthermore, C. wuellerstorfi has been observed in elevated microhabitats which enables the species to profit from lateral food supply, a mode of suspension feeding, which makes it less dependent on fresh organic matter that settles directly on the sea floor (Linke and Lutze, 1993). This gives C. wuellerstorfi an advantage over other competing K-selected species such as N. umbonifera, which lack this ability. In a recent study by McDougall (1996), late Neogene benthic foraminifer distributions in the Caribbean Sea and the eastern equatorial Pacific Ocean were investigated to interpret the alternations of bottom-water masses and paleoceanographic changes in response to the closure of the Isthmus of Panama. Her work focused on faunal changes during a period between about 7 and 0.5 Myrs a time of major oceanic change in the late Miocene, the mid- and late Pliocene, and the late early Pleistocene. Her interpretations were based on the assumption that certain benthic foraminifer species are associated with particular deep-water masses bathing the Colombia Basin and independent of food supply (Fig. 1). In this study, we offer an alternative interpretation of environmental developments at DSDP Site 502 in the Carib-

bean Sea (Colombia Basin). Whereas McDougall (1996) used the benthic foraminifers as deep-water mass tracers, we base our interpretation on ecological grounds and place these within a framework of oceanic circulation change. Our objectives are: (1) to identify benthic foraminifer species/ assemblages usable for the interpretation of the paleoenvironmental evolution in the Caribbean Sea, and (2) to discuss how the transition from an open connection between the tropical Atlantic and Pacific to the closure of the Central American Seaway in the late Neogene may have affected the benthic foraminifer community. STUDY AREA AND OCEANOGRAPHIC SETTING The Colombia Basin is a large basin in the western Caribbean Sea with a depth exceeding 4,000 m (Fig. 1). It is connected with the North Atlantic Ocean through a sill at a depth of about 1,650 m at the Windward Passage between Cuba and Hispan˜ola (e.g., Sverdrup and others, 1942; Wu¨st, 1964). Today the intermediate waters east of the Caribbean Sea occur between about 850 and 1,800 m and contain approximately 85% Upper North Atlantic Deep Water (UNADW), 10% Upper Circumpolar Water (UCPW), and 5% Mediterranean Outflow Water (MOW), based on temperature and salinity data (Kawase and Sarmiento, 1986; deMenocal and others, 1992). Figure 2 depicts the present-day deep-water stratification in the Colombia Basin (western Caribbean Sea). Water below 50 m in the Colombia Basin is divided into four layers based on dissolved oxygen, temperature, and salinity characteristics (Wu¨st, 1964). The Subtropical Underwater (50–200 m) is associated with the oceanic warmwater sphere and is separated from the lower, cold-water sphere by a low oxygen layer (below 3.0 ml O2/l) at 400– 600 m. Antarctic Intermediate Water (AAIW) occurs between 700 and 850 m (temperature 5–78C; salinity 34.85 ‰) and between 1,800 and 2,500 m the UNADW is the

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domininating water mass (temperature 4.18C; salinity 34.98 ‰)(Fig. 2). The waters between these water masses (850– 1,800 m) constitute mainly mixtures of the water masses found above and below due to the normal vertical mixing caused by turbulent processes. The hydrographic data of the Atlantic Intermediate Water (AIW), which flows over the sill today, is mostly (85%) UNADW (temperature 3.88C; salinity 35.0‰), and the Colombia Basin is homogeneously filled with UNADW from its bottom (about 4,500–5,500 m) to about 1,800 m (Wu¨st, 1964; deMenocal and others, 1992). Core-top benthic foraminifer d13C values in the Colombia Basin are about 1.0 ‰, which is consistent with an UNADW source (Oppo and Fairbanks, 1987; deMenocal and others, 1992). The main outflow for deep Caribbean water is to the Gulf of Mexico via the Yucatan Passage (sill depth of about 2,000 m) (Metcalf, 1976). MATERIAL AND METHODS Benthic foraminifers were analyzed from 80 samples between 41.62 and 162.42 m in DSDP Hole 502A (Table 1). The hole was drilled in the Colombia Basin (11829.469N and 79822.749W, 3,051 m water depth) on a small fault block that rises above the basin floor (Prell and others, 1982) (Fig. 1 and 2). Sediments at Site 502 are composed mostly of nannofossil ooze, with foraminifers as a minor component, and are classified as foraminifer-bearing nannofossil marl. The entire sequence in Hole 502A ranges from the late Miocene to Recent (about 8 Myr) with good recovery and minor core deformation in most intervals (Prell and others, 1982). To establish a chronology, we used the paleomagnetic reversals summarized in Table 2 (Kent and Spariosu, 1982), the Magnetic Chron-6 carbon shift (about 6.1 Ma; Keigwin 1982c) as reference points, and calibrated the ages to the time scale of Berggren and others (1995). Previous studies of this site (e.g. McDougall, 1996; Bornmalm, 1997) were based on an older chronological model of Berggren and others (1985). Sedimentation rates were about 3.0 cm/k.y. throughout most of the sequence investigated. The section analyzed spans the interval between 1.81 and 5.77 Ma (Table 1) with a sampling resolution of ,10,000–200,000 kyr (about 50,000 kyr on average). Approximately 10 cm3 of each sample was prepared according to conventional procedures, including disaggregation in water and washing through a 63 mm sieve. After drying, the samples were sieved over a 125 mm screen, and benthic foraminifers were picked from the .125 mm fraction. In most cases, the entire sample was used for foraminifer analysis. Sample size (i.e., number of specimens per sample) ranged from 55 to 504 (Table 1). A dissolution index for Hole 502A was established by Bornmalm (1997) based on the degree of fragmentation of planktonic foraminifer tests. He computed the degree of fragmentation as the relative abundance (percentage) of fragments of planktonic foraminifers in relation to the sum of whole and fragmented tests of planktonic foraminifers; a fragment was identified if less than half of the test remained of the specimen. Although increased fragmentation may be produced during processing of samples in the laboratory,

the degree of fragmentation of planktonic foraminifer tests is generally considered to represent a reliable index of calcite dissolution (Thiede, 1973; Thunell, 1976; Malmgren, 1983). Le and Shackleton (1992) pointed out that this measure, in contrast to other measures of dissolution (e.g. ratios between dissolution-resistant and susceptible species), is essentially independent of ecologic influence. The taxonomic concepts follow those of Bornmalm (1997), who provided a detailed taxonomic account of the benthic foraminifer species encountered at Sites 502 and 503. About 150 taxa were identified at the generic or specific level (Bornmalm, 1997). The 30 taxa that occurred in at least 2% in any of the samples and with a mean relative abundance of at least 0.5% throughout the sequence were subjected to a Q-mode principal component analysis with varimax rotation of the initial orientations of the PC axes. These taxa constitute a major portion [83–99%] of the benthic foraminifer fauna in these samples. The first two PC’s, accounting for 85% of the total variability, are associated with variation in the abundance of N. umbonifera (first PC); and an inverse relationship between Cibicidoides wuellerstorfi and Oridorsalis umbonatus on the one hand, as Laticarinina pauperata, C. robertsonianus, C. kullenbergi, Sigmoilopsis schlumbergeri, C. mundulus, Pyrgo murrhina, Epistominella exigua, and Gyroidina neosoldanii on the other hand (second principal component). The remaining species do not contribute significantly to any of these principal components. Here benthic foraminifer accumulation rates (BFAR) of these 11 species were used as a measure of fluctuations in the major components of the benthic foraminifer faunas through this time interval. BFAR is strongly correlated to the flux of organic matter to the sea-floor, and as such, related to net productivity in surface waters (Herguera and Berger, 1991; Herguera, 1992; Berger and Herguera, 1992). In this study, BFAR data are employed to estimate paleofluxes of organic matter to the sea floor, and in turn, to investigate whether changes in this parameter could be related to the late Neogene shoaling of the Isthmus of Panama. Sediment accumulation rates (SAR) were established as sedimentation rates (cm/ka) multiplied with the mean drybulk density of the sediment, dwc (g/cm3) for a particular interval (Prell and others, 1982). Benthic foraminifer accumulation rates (BFAR) were established by multiplication of the SAR values with the number of benthic foraminifera per gram sediment (Table 2). The calcium carbonate data used here are from Gardner (1982). RESULTS Figure 3 illustrates the changes in BFAR with time of the 11 selected taxa from Site 502. Three intervals (1–3) of faunal development can be distinguished from visual inspection of their faunal compositional signatures. The boundaries between the intervals are placed at the major shifts in the BFAR of the four most abundant benthic foraminifer species (N. umbonifera, C. wuellerstorfi, O. umbonatus and E. exigua). These intervals are not based on rigorous quantitative analysis, but they should rather be seen as a temporal framework that simplifies the description of the successive faunal changes. The subdivision is supported

PLIOCENE CARIBBEAN SEA

by a robust variety of Hotelling’s T2-test (the multivariate analog of Student’s t-test), which shows that the differences in faunal compositions between all pairs of these intervals are statistically significant (Table 3; the BFAR values were log-transformed prior to the analyses in order to stabilize the variance). During Interval 1 N. umbonifera shows a fluctuating BFAR pattern followed by a decrease during Interval 2 to the lowest BFAR value of the whole interval studied. Interval 3 is characterized by high BFAR values of N. umbonifera, except at about 3.0 Ma, where there is a temporary decrease from about 90 to 0 N. umbonifera specimen/cm2/ kyr. Cibicidoides wuellerstorfi exhibits an inverse pattern compared to that of N. umbonifera, with higher BFAR values during Interval 2 and lower BFAR values during the lower part of Interval 3 (up to about 3.1 Ma), whereas they both show an increasing trend to its highest levels in the upper part of Interval 3 (Fig. 3). Also O. umbonatus shows higher BFAR values during Interval 2 and the upper part of Interval 3 compared to Interval 1 and the lower part of Interval 3 (Fig. 3). Cibicidoides kullenbergi, C. mundulus, C. robertsonianus, and L. pauperata exhibit higher BFAR values during Intervals 1 and 2 and low BFAR values during Interval 3 (Fig. 3). Both G. neosoldanii and S. schlumbergeri exhibit lower BFAR values during the lower part of Interval 3 than in the other intervals, whereas P. murrhina has a higher BFAR value in the upper part of Interval 3. Epistominella exigua shows a pattern with short episodes of high BFAR values distributed throughout the sequence (Fig. 3). DISCUSSION The late Neogene environmental history in the Caribbean Site 502 is reflected in changes of the composition of benthic foraminifer faunas. Four of these species, N. umbonifera, C. wuellerstorfi, O. umbonatus, and E. exigua, are the most abundant ones (Fig. 3) and thus the ones contributing the most to the variability among the samples. Hence, the following environmental interpretation is mainly based on these species. At the bottom of Interval 1 (5.77–5.35 Ma) there is an increase of E. exigua, which is succeeded by an increase in C. wuellerstorfi. This is followed by two temporary peaks of E. exigua at about 5.25 Ma and also in the upper part of Interval 1, whereas N. umbonifera shows a fluctuating pattern throughout this interval (Fig. 3). The increased abundance of E. exigua at the base of the sequence and at a few later levels in Interval 1 may indicate episodes of enhanced vertical flux of organic (phytodetritial) matter to the sea floor, since this species has been linked to high concentration of phytodetritus on the sea floor (Smart and others, 1994 and references therein). At about 5.25 Ma N. umbonifera, E. exigua, and O. umbonatus exhibit a short but strong BFAR increase. This could be related to an intensive but short input of organic fluxes. The succeeding increase of the Cibicidoides assemblage, dominated by C. wuellerstorfi, may indicate lower organic fluxes and/or higher bottom-water velocities and deep-water currents in the middle part of Interval 1. Such a scenario would be favorable for C. wuellerstorfi, which has been suggested to be able to profit from lateral food-supply and suspension feeding and

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thus making it less dependent on the deposition of fresh organic matter (Linke and Lutze, 1993) than, for example N. umbonifera and E. exigua that are mainly deposit feeders (e.g., Smart and others, 1994; Thomas and Gooday, 1996). In addition, C. wuellerstorfi often lives in an elevated position above the sediment and is attached to stones, shells, sponges, polychaete, and pogonophore tubes etc. (Linke and Lutze, 1993). This elevated life position enables the foraminifer to grab suspended and resuspended organic matter by the pseudopodes as the food passes by in the water column. This indicates that C. wuellerstorfi can sustain itself in more oligotrophic environments, where food is rather scarce and bottom-current velocities are enhanced. Within Interval 1, both C. wuellerstorfi and N. umbonifera exhibit a fluctuating pattern, where C. wuellerstorfi increases when N. umbonifera decreases and vice versa (Fig. 3). Thus, fluctuations in the proportions of these two species may reflect trophic-level changes in this relatively oligotrophic, deepwater environment, where N. umbonifera is more dependent on slightly higher trophic level than C. wuellerstorfi. Across the boundary between Intervals 1 and 2, C. wuellerstorfi decreases temporarily, whereas both N. umbonifera and E. exigua show a peak in BFAR values (Fig. 3), followed by continuously increasing BFAR values of C. wuellerstorfi throughout Interval 2 to one of its highest values in the sequence. Simultaneously, there is a decrease in N. umbonifera to its minimum values in the sequence (Fig. 3), clearly suggesting a significant environmental change that started in the Colombia Basin at about 4.65 Ma. This could be a result of increased bottom-water currents, which would favor a suspension feeder like C. wuellerstorfi (Linke and Lutze, 1993). However, E. exigua shows episodes of relatively high abundances during the middle of Interval 2, which may indicate that the deep-sea environment under discussion still experienced (at least periodically) significant inputs of phytodetritus. An even more conspicuous benthic foraminifer turnover took place at the beginning of Interval 3 (Fig. 3). Here a drastic decrease occurred in the Cibicidoides assemblage simultaneous with a rapid increase in N. umbonifera (Fig. 3), which became the dominant species during the lower part of Interval 3 (about 3.9 to 3.0 Ma). This may indicate a significant environmental event at about 3.9 Ma that probably is related to changes in deep-water circulation and/or the trophic regime. However, since the depth of the sill at the Windward Passage has been around 1,650 m throughout the latest Neogene (Driscoll and Haug, 1998), and AABW does not normally flow at depths less than 4,000 m in the world oceans today, an inflow of AABW over the Windward Passage into the Caribbean Sea during the late Neogene as suggested by McDougall (1996)(promoting the observed increase in N. umbonifera) must be regarded as quite unlikely. The environmental change at the beginning of Interval 3 could instead be a result of decreased velocity of the bottom-water currents and sustained organic input into the Caribbean Sea. This was probably caused by a decrease or slower inflow of bottom waters from the North Atlantic, which may have triggered an increase in the abundance of N. umbonifera. Our data show that E. exigua is occasionally common, particularly between about 3.8, 3.5–3.6 and 3.35 Ma within the lower part of Interval 3, where the dominance

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TABLE 1. Depths and estimated ages, total weights of the dry samples, numbers of specimens examined in the .125 mm fraction, and numbers of specimens per gram dry sediment in samples from Hole 502A.

Level

Core

Section

Interval (cm)

Depth (m)

Age (Ma)

Weight of sediments (gram)

Number of specimens examined

Foraminifera per gram sediment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

11 12 12 12 12 13 16 16 16 16 17 17 17 17 17 19 19 19 19 19 20 20 20 20 21 21 21 21 21 21 22 22 22 22 23 23 23 23 24 24 25 25 25 25 26 26 26 26 26 27 27 27 28 28 28 28 29 29 29 30 30 30 30 32

CC 1 2 3 3 1 1 1 2 3 1 2 2 3 CC 1 1 2 3 CC 1 2 3 3 1 2 2 3 3 CC 1 2 3 3 1 2 3 CC 1 2 1 2 2 3 1 1 2 3 3 1 2 2 1 1 2 3 1 2 3 1 2 2 3 1

11–13 96–99 47–50 0–3 88–90 12–15 4–6 100–103 51–53 0–3 51–53 0–3 100–102 51–53 0–3 0–3 98–100 51–53 0–3 0–3 97–99 55–57 0–3 97–99 55–57 0–3 101–104 37–39 50–53 0–3 103–105 43–45 0–3 101–104 46–48 105–107 52–54 2–4 102–104 51–53 52–54 1–3 105–108 50–53 15–17 115–117 57–59 1–3 102–104 50–53 1–3 104–106 21–23 118–120 68–70 18–20 96–98 45–48 0–3 51–54 0–3 105–107 0–3 5–8

41.62 46.87 47.76 48.80 49.67 50.43 63.55 64.51 65.51 66.51 68.42 69.43 70.42 71.42 72.39 76.72 77.69 78.45 79.47 80.46 81.91 83.03 83.99 84.96 86.06 86.98 88.02 88.84 88.97 89.58 90.94 91.85 92.91 94.48 94.77 96.82 97.80 98.81 99.73 100.73 103.63 104.54 105.58 106.53 107.66 108.66 109.51 110.47 111.48 112.42 113.42 114.45 116.52 117.79 118.33 119.33 120.27 121.26 122.32 123.32 124.29 125.35 125.80 128.36

1.81 1.93 1.96 2.00 2.03 2.06 2.52 2.56 2.59 2.63 2.70 2.73 2.77 2.80 2.84 2.99 3.02 3.05 3.09 3.12 3.17 3.20 3.24 3.28 3.33 3.37 3.41 3.45 3.45 3.48 3.54 3.58 3.62 3.67 3.68 3.76 3.79 3.83 3.86 3.90 4.00 4.03 4.07 4.10 4.14 4.18 4.21 4.24 4.28 4.31 4.35 4.39 4.46 4.51 4.53 4.57 4.61 4.66 4.70 4.74 4.78 4.82 4.84 4.95

20.70 15.58 17.96 13.40 14.52 19.19 4.43 14.54 14.71 17.56 16.23 7.01 13.18 13.03 15.23 11.91 13.85 14.94 12.83 14.66 9.51 12.64 12.55 12.81 8.96 12.10 18.80 13.91 15.89 16.14 6.72 13.95 15.05 13.58 14.55 6.3 15.06 6.26 7.16 13.70 12.97 17.20 14.89 1.52 5.36 14.93 15.61 15.83 15.85 15.41 15.96 14.95 15.67 12.58 15.36 15.07 12.78 14.80 13.00 12.96 13.55 14.56 16.56 15.74

358 433 504 317 299 281 298 431 292 357 159 306 227 301 448 197 292 423 327 342 380 305 314 305 180 326 307 217 141 481 254 305 294 240 346 298 289 305 226 392 124 292 273 89 125 272 199 219 207 307 193 157 112 107 193 121 79 105 327 173 125 208 125 115

17.29 27.86 28.06 23.66 20.59 14.64 67.27 29.64 19.92 20.33 9.80 43.67 17.22 23.10 26.92 16.54 21.08 28.31 25.49 23.33 39.96 24.05 24.30 23.81 20.09 26.94 16.33 15.60 8.87 29.80 37.80 21.86 19.53 17.60 24.26 47.30 19.19 48.72 31.56 28.61 9.56 16.98 17.46 58.55 50.56 13.33 8.01 13.77 13.06 20.31 12.09 10.50 7.02 8.66 12.57 8.03 6.18 6.96 25.15 13.35 9.23 14.22 7.67 7.31

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TABLE 1.

Continued.

Level

Core

Section

Interval (cm)

Depth (m)

Age (Ma)

Weight of sediments (gram)

Number of specimens examined

Foraminifera per gram sediment

65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

32 32 32 33 33 33 34 34 34 35 35 35 37 40 42 44

1 2 3 1 2 3 1 2 2 1 2 3 2 2 2 2

103–106 47–49 4–6 96–98 48–50 0–2 102–104 50–52 142–145 103–105 46–48 1–3 8–10 1–3 48–50 30–33

129.34 130.30 131.38 132.27 133.26 134.28 135.33 136.31 137.23 138.34 138.82 139.87 144.03 151.70 157.79 162.42

4.99 5.03 5.07 5.11 5.15 5.19 5.23 5.26 5.29 5.33 5.34 5.37 5.51 5.66 5.72 5.77

14.36 5.24 16.43 15.08 15.37 15.35 15.71 4.10 15.34 16.49 14.79 14.60 14.77 14.34 14.60 14.41

134 55 90 104 133 168 118 209 133 202 104 217 160 282 229 97

9.33 10.50 5.48 6.90 8.65 10.94 7.51 50.98 8.67 12.25 7.03 14.86 10.83 19.67 15.68 6.73

of N. umbonifera is sporadically interrupted (Fig. 3). This peak in E. exigua may suggest that N. umbonifera can be temporarily outcompeted, or numerically swamped, by fastgrowing opportunists, such as E. exigua, during episodes of enhanced phytodetritus deposition; with a higher temporal resolution (i.e., higher sedimentation rate and/or less bioturbation) such (geologically) short episodes would result in much more pronounced E. exigua peaks than what the geological record usually can provide. Another episode of environmental perturbations during Interval 3 occurred at about 3 Ma. This event is marked by a drastic decrease in N. umbonifera, followed by an instant recovery of the species (Fig. 3), whereas C. wuellerstorfi and P. murrhina dominate the fauna at this time period (Fig. TABLE 2. Ages and depths in sites of the magnetostratigraphic and biostratigraphic datum levels used here in the development of a chronology for Hole 502A. The magnetostratigraphy for Hole 502A is from Kent and Spariosu (1982). Depth of the first appearance datum (FAD) of the planktonic foraminifer species Globorotalia tumida is from Keigwin (1982a), and depth of the magnetic Chron-6 d13C shift is from Keigwin (1982b). Depths of the magnetostratigraphic boundaries, the FAD of G. tumida and the carbon isotope shift listed here are means of upper and lower depth limits given in these articles. Ages are calibrated to the time scale of Berggren et al. (1995).

Datum level

Olduvai Olduvai Matuyama/Gauss boundary Kaena Kaena Mammoth Mammoth Gauss/Gilbert boundary Cochiti Nunivak Nunivak Gilbert C2 Bottom FAD G. tumida Magnetic Chron-6 d13C shift

Top Bottom Top Bottom Top Bottom Top Top Bottom

Age (Ma)

Hole 502A depth (m)

Hole 502B depth (m)

1.77 1.93 2.47 3.04 3.11 3.22 3.33 3.58 4.18 4.48 4.62 4.89

39.70 46.85 — 78.13 80.05 83.55 — 91.90 — 117.10 120.40 135.25

— 42.45 — 73.79 75.54 79.15 — —

5.60

145.14

6.10

197

3). This rapid faunal change could be attributable to (a) mixing between the base of nutrient-rich Antarctic Intermediate Water (AAIW) and UNADW (Haddad and others, 1994), and/or (b) nutrient-rich local river outflow (e.g. from the Rio Magdalena) together with (c) increased bottom-water currents, which favor a normally elevated epibenthic species like C. wuellerstorfi. The short-term change in the benthic foraminifer composition at this transition may favor the third alternative, i.e. increased deep-water currents due to increased inflow of bottom water to the area. The instant recovery of N. umbonifera shortly after 3.0 Ma (Fig. 3) may indicate an amelioration of the deep-water conditions, which might be related to a variation in the inflow of bottom water into the Caribbean Sea. In summary, the faunal changes through the section are interpreted as follows: The environmental conditions at the bottom of the sequence (at about 5.77–5.35 Ma) are marked by rather moderate (meso-) trophic levels that slightly decreased between 5.2 and 4.9 Ma, followed by a small increase in the upper part of Interval 1. During the next period (Interval 2; 4.65–3.9 Ma) the environment is characterized by pulsating input of organic matter together with, particularly in the upper part of Interval 2, enhanced velocity of the bottom-water currents. This is probably due to increased inflow of deep water from the Atlantic. The most conspicuous faunal change occurred at the base of Interval 3 (3.9 Ma), under which true oligotrophic conditions (accompanied by sluggish bottom-water circulation) were reinstalled and prevailed to the end of the interval, except of the levels at about 3.35–3.5 Ma where the BFAR increases for E. exigua, presumably by sporadic inputs of organic matter to the deep-sea environment. At the beginning of the upper part of Interval 3 (about 3.0 Ma) the environmental development (from about 3.0–1.81 Ma) is marked by temporarily enhanced deep-water currents due to increased inflow of bottom water to the area. This was probably followed by an instantaneous deep-water flow through the rest of the sequence. Our interpretation of the environmental conditions at Site 502 is corroborated by several physico-chemical variables (i.e., degree of planktonic foraminifer fragmentations

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FIGURE 3. Changes with time in accumulation rates of 11 benthic foraminifer species that were interpreted as the most ecologically significant. Cibicidoides spp. is a summary of all the four Cibicidoides species (C. wuellerstorfi, C. robertsonianus, C. kullenbergi, and C. mundulus) included in the BFAR analysis.

CaCO3 preservation, and planktonic and benthic foraminifer stable isotope data; Fig. 4). During Interval 1 and the lower part of Interval 2 the degree of planktonic foraminifer fragmentation shows a pulse-like pattern with percentages generally .50%, indicating stronger dissolution and influence TABLE 3. Hotelling’s T2-tests of the differences in logarithmically transformed BFAR values of the 11 species between each pair of the three stratigraphic intervals identified here (interval 1: 162.42–121.26 m; interval 2: 120.27–100.73 m; interval 3: 99.73–41.62 m). Since the distributions of all samples deviate from multivariate log-normality, a robust variety of Hotelling’s T2-test was used following the principles described by Reyment (1991, pp. 286–288). A robust test (O’Brien, 1992) indicates that the covariance matrices are homogeneous, so the use of Hotelling’s T2-test, which requires homogeneous covariance matrices, is motivated. ‘‘d.f.’’ indicates the degrees of freedom for the Fdistribution, which is used to test the significance of T2. All T2 are statistically significant at least at the 5% level (significance levels are listed below the table). Intervals

T2

F

d.f.

1–2 1–3 2–3

43.67 94.92 218.31

3.36a 8.07b 18.26b

10, 30 10, 51 10, 46

a

p , 5%; b p , 0.1%.

of ‘‘older’’, less-ventilated deep waters (Fig. 4). There is also a strong variation in the CaCO3 content during Interval 1 to the base of Interval 2. Very low CaCO3 content in the middle part of Interval 1 and the lower part of Interval 2 may indicate reduced deep-water ventilation. Improved CaCO3 preservation, and lesser planktonic foraminifer fragmentation indicate that calcite dissolution decreased and calcite preservation improved towards the upper part of Interval 2 (Fig. 4). The timing of the amelioration in calcite preservation also roughly coincides with increased d18O values in planktonic foraminifers at about 4.2 Ma, which could be a result of the restricted surface-water communication between the Atlantic and Pacific oceans as a result of the emergence of the Panamanian Isthmus (Fig. 4; Keigwin, 1982b,c). Shortly thereafter, a benthic foraminifer change occurred, which could be related partly to the uplift of Site 502 above the lysocline (Gardner, 1982), and partly to better ventilation of the bottom waters due to increased bottomwater exchange between the Caribbean Sea and Atlantic Ocean. It is plausible that the overall trend of improved calcite preservation from about 4.0 Ma to the top of the sequence indicates enhanced inflow of bottom water to the Caribbean Sea at this time and that the bottom water mainly

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217

FIGURE 4. Fluctuations in degree of fragmentation of planktonic foraminifera (percentage of fragments relative to whole and fragmented tests), percentage of CaCO3, and d18O and d13C of benthic and planktonic foraminifera in DSDP Hole 502A. Isotopic measurements were made on the epibenthic species Cibicidoides wuellerstorfi (.125 mm) and the planktonic group Globigerinoides trilobus-G. quadrilobatus-G. sacculifera (.175 mm). The isotopic data of the planktonic foraminifera are from Keigwin (1982b). Absolute ages for Hole 502 are from the magnetostratigraphy developed by Kent and Spariosu (1982).

consisted of UNADW. The increasing benthic d13C values (Fig. 4) may also reflect enhanced production of NADW and the development of the modern deep-sea circulation. This was probably initially a result of increased northward transport of warm, high-salinity waters to high latitudes via the Gulf Stream, caused by the progressive emergence of the Panamanian landbridge (Driscoll and Haug, 1998; and references therein). The warm high-salinity water of the Gulf Stream may have stimulated the total production of NADW in the North Atlantic from about 4.0 Ma. However, recent studies indicate that the closing of the Panamanian isthmus began to have a major impact on intermediate and deep water circulation in the North Atlantic already at 4.6 Ma (e.g., Tiedemann and Franz, 1997; Haug and Tiedemann, 1998; Driscoll and Haug, 1998). This is also consistent with the increase in the formation of Upper North Atlantic Water (UNADW) and Laborador Sea Water at about 4.5 Ma (Mikolajewicz and Crowley, 1997). Therefore, the improved carbonate preservation and increased benthic d13C values at Site 502 at about 4.0 Ma may reflect a better deep water ventilation, as a result of increased inflow of UNADW into the Caribbean Sea. By studying benthic foraminifers, McDougall (1996) found four major changes in the faunal composition during the last 7 Myr in the Caribbean Sea and eastern equatorial Pacific Ocean, each of which she interpreted as alternations in different bottom-water masses bathing the Colombia Basin. Her interpretation was based on the idea that the distribution and abundance patterns in benthic foraminifers are controlled by physico-chemical properties, such as temperature, salinity, and oxygen content, of a certain deep-water mass. However, an alternative opinion presented here is that deep-sea benthic foraminifer changes are not necessarily de-

pendent on the alternation of different bottom-water masses influencing the environment where they live. Instead, distribution patterns and changes in benthic foraminifer faunas are more dependent on environmental gradients related to food supply and/or oxygenation of the environment, as already mentioned before and pointed out by Jorissen and others, (1995 and references therein). Our interpretation, therefore, differs from that of McDougall (1996). Figure 5 shows the ‘‘water-mass model’’ of McDougall (1996) together with the alternative ‘‘ecological model’’ for Site 502 presented here; in order to facilitate this comparison, both models are calibrated to the Berggren and others (1995) timescale. The most prominent feature of the watermass model concerns a significant middle-to-late Pliocene presence of AABW in the Colombia Basin. McDougall (1996) related this presence to high abundances of E. umbonifera (5N. umbonifera) and argued that AABW often represents cooler and less saline conditions. However, with regard to the physico-chemical variables (i.e. lesser degree of planktonic foraminifer fragmentation improved CaCO3 preservation, and stable isotope data in Fig. 4) we instead suggest that the deep-water environment of the Caribbean became quite well ventilated from about 4.1 Ma. From this time onwards the benthic oxygen and carbon isotope values increased together with decreased dissolution and improved calcite preservation (Fig. 4). This indicates that the deep water instead became gradually ‘younger’ and better oxygenated at Site 502 from about 4.1 Ma, rather than ‘older’ and less oxygenated as suggested by McDougall (1996). McDougall (1996) suggested that the alteration of high abundances of E. exigua and N. umbonifera in the mid Pliocene represents an alteration between AABW and modified AABW (‘old’ oxygen deficient bottom water) in the Colom-

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FIGURE 5. The ‘‘water-mass model’’ of McDougall (1996) and the alternative ‘‘ecological model’’ established in this study to explain alterations in the benthic foraminifer fauna at Site 502A.

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PLIOCENE CARIBBEAN SEA

bia Basin based on their preferences for different water masses. A similar mid-Pliocene alternation between the two species is also found in the present data, especially between 3.55 and 3.35 Ma (Fig. 3). However, this change can be explained from an ecological perspective as well. Although N. umbonifera and E. exigua can profit from a similar diet (Gooday, 1993), the non-opportunist N. umbonifera is able to survive, or even flourish, on a lower food supply than the opportunistic E. exigua (Loubere, 1991). Furthermore, Gooday (1993) argued that N. umbonifera could be outcompeted by the fast-growing opportunist E. exigua in areas with strong pulses of phytodetritus deposition. It has also been suggested that N. umbonifera is adapted to corrosive bottom waters (Bremer and Lohmann, 1982; Mackensen and others, 1990), which make the conditions less favorable for E. exigua and similar species. However, there is no evidence for increased calcite dissolution at 3.9 Ma at Site 502. In addition, according to Corliss (personal communication, 1997), N. umbonifera, C. wuellerstorfi, and O. umbonatus are deep-sea dwellers inhabiting environments marked by relatively low food supplies. Hence, the peak in E. exigua between 3.4 and 3.3 Ma and the high abundances of N. umbonifera in the mid Pliocene are probably more related to changes in the food supply than to a corrosive nature of the bottom water bathing the Colombia Basin. Finally, the deep-water connection between the Colombia Basin and the Atlantic is over a relatively shallow sill at a depth of 1,650 m at the Windward Passage. Although the sill depth may have been deeper and AABW could have flown at a shallower depth than today, it is not likely that such moderate modifications would have been sufficient to allow AABW (if present at these latitudes) to enter the Colombia Basin during Neogene times. We may also question whether one particular species can be used to ‘‘trace’’ a certain water mass for such long distances (i.e., from its source area in the Southern Ocean northwards across the equator), taking into account the alteration of hydrographic properties that takes place as it flows along its path. We do not, therefore, accept McDougall (1996) water mass model. SUMMARY AND CONCLUSIONS The last 5 million years of the geological history of the Caribbean Sea and eastern equatorial Pacific Ocean were marked by a number of distinct paleoceanographic events, which culminated in the final closure of the Central American Seaway between about 3.5 and 3.0 Ma (e.g. Keigwin 1982c; Coates and others, 1992). We studied changes in the benthic foraminifer community through the late Neogene (between about 5.8 and 1.8 Ma) from DSDP Hole 502A from the Caribbean Sea (water depth 3,100 m). Using benthic foraminifer accumulation rates (BFAR) of the 11 most ecologically significant species, we were able to identify three intervals (1–3) marked by specific faunal composition and development. 1. Relatively minor alternations in the benthic foraminifer fauna may indicate a rather stable, more or less oligotrophic deep-water environment in the Colombia Basin during the middle part of Interval 1 (5.3 to 4.9 Ma). The higher abundance of Epistominella exigua between 5.7 and 5.35 Ma, except at about 5.4 Ma, is probably a con-

sequence of increased vertical flux of phytodetritus to the seafloor during this period. This seems also to be the case in the upper part of Interval 1. 2. Gradual change in the benthic foraminifer fauna during Interval 2 (between 4.65 and 3.9 Ma) may have been caused by a combination of higher frequency of sudden inputs of phytodetritus and increased bottom-water circulation in the Caribbean Sea. The latter may be related to a more active tectonic setting of Site 502 as well as to the emergence of the Isthmus of Panama, which may have increased the bottom-water current activity by transforming the Caribbean Sea from a broad oceanic seaway into a marginal sea. 3. A major faunal turnover at about 3.9 Ma, at the beginning of the lower part of Interval 3 (3.9 to about 3.0 Ma) may indicate a significant environmental change, which probably was related to alternations in the deep-water circulation. The peak abundance of E. exigua between about 3.55 and 3.45, and at 3.4–3.35 Ma may be related to an increased input of phytodetritus into the Colombia Basin. 4. A rapid faunal change at about 3.0 Ma could be attributable to (a) mixing between the base of nutrient-rich Antarctic Intermediate Water (AAIW) and Upper North Atlantic Deep Water (UNADW), and/or (b) nutrient-rich local river outflow (e.g. from Rio Magdalena) together with, at least periodically, (c) increased bottom-water currents that may have favored a species like Cibicidoides wuellerstorfi, which has been found to utilize laterally suspended organic matter. The deep water in the Caribbean Sea became gradually cooler and younger during this interval, which may reflect, at least during shorter periods, increased inflow of UNADW and the development of the modern psychrospheric deep-sea circulation. ACKNOWLEDGMENTS We wish to thank Ellen Thomas, Robert Speijer, Michal Kucera, Frans Jorissen, and Andre´e Schaaf for reviewing an early version of the manuscript, and Otto Hermelin, Department of Geology, University of Stockholm, for generously providing the material from DSDP Site 502. We also thank Lowell Stott and an anonymous individual for their constructive reviews. REFERENCES ALTENBACH, A., 1985, Die Biomasse der benthischen Foraminiferen. Auswertung von ‘‘Meteor’’-Expeditionen in o¨stlichen Nordatlantik: Doctoral Thesis, University of Kiel, Germany. , and SARNTHEIN, M., 1989, Productivity Record in Benthic Foraminifera, in Berger, W. H., Smetacek, V. S., and Wefer, G. (eds.), Productivity of the Ocean: Present and Past, John Wiley, New York, p. 255–269. BALSAM, W., 1981, Late Quaternary sedimentation in the western North Atlantic: Stratigraphy and paleoceanography: Palaeogeography Palaeoclimatology Palaeoecology, v. 91, p. 13–20. BARMAWIDJAJA, D. M., JORISSEN, F. J., PUSKARIC, S., and VAN DER ZWAAN, G. J., 1992, Microhabitat selection by benthic foraminifera in the northern Adriatic Sea: Journal of Foraminiferal Research, v. 22, p. 297–317. BELANGER, P. E., and STREETER, S. S., 1980, Distribution and ecology of benthic foraminifera in the Norwegian-Greenland Sea: Marine Micropaleontololy, v. 5, p. 401–428. BERGER, W. H., and HERGUERA, J. C., 1992. Reading the sedimentary

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Received 31 March 1998 Accepted 8 February 1999