ABSTRACT

The lower bathyal and abyssal environments of the northwestern Gulf of Mexico exhibit a persistently high benthic foraminiferal diversity. The distribution of individuals among species, given by the E index, is also relatively high for the deep Gulf, indicating an absence of strong dominance by a few species and a wide distribution of the foraminiferal biomass among species. There is no bathymetric variation in diversity. Absolute abundance of total foraminifera tends to decrease with increasing water depth. Several species are unrecorded at depths greater than 2,400 m, thus defining a depth separation within the foraminiferal assemblage. Relative abundances allow a more refined depth zonation: 1) a shallow, less than 2,000 m interval, 2) a mid-depth interval between 2,000-2,400 m and 3) a deep interval >2,700 m. The distribution of total organic carbon (TOC) in the lower bathyal and abyssal environments does not appear to be related to water depth, nor does it seem to influence species diversity or abundances except in the case of Nuttallides decorata which attains its highest relative abundances in areas of reduced TOC. C/N ratios indicate that the organic matter from most stations is derived from both marine and terrestrial sources. There is no relationship between organic matter source and water depth or species abundance, but the highest relative abundances of Epistominella exigua and Cassidulina subglobosa are found in association with a C/N ratio indicative of terrestrial organic matter.

INTRODUCTION

A relationship between water-mass boundaries and benthic foraminiferal associations has been documented in bathyal depths of the Gulf of Mexico (Denne and Sen Gupta, 1991, 1993). These investigations, however, have not clarified trends of bathymetric succession (if any) of benthic foraminiferal species in the lowermost bathyal and abyssal waters of this marginal sea. Waters at lower bathyal and abyssal depths belong to a single water mass, Gulf Basin Water (GBW), within which the variations in temperature, salinity, and oxygen are negligible. In this report, we present the first results of a study on benthic foraminiferal distributions from lower bathyal and abyssal environments, covering a bathymetric range of 1,538-3,850 m. This study of species diversity, abundance, and dominance trends lays the groundwork for an extensive survey of deep Gulf of Mexico Pleistocene-Holocene communities and paleocommunities of benthic foraminifera.

DEEP GULF OF MEXICO OCEANOGRAPHY

Gulf Basin Water (GBW), ubiquitous below a depth of about 1,500 m (Nowlin, 1971), is the deepest water mass in the Gulf. It forms at the Yucatan Straits as renewal waters from the Caribbean Sea enter the Gulf over the 1,900 m-deep sill. GBW consists of at least 50% Caribbean water as far as 350 km north of the Yucatan Straits (Carder et al., 1977), but a deep counter-current flowing southward from the Atlantic into the Gulf through the 800 m-deep Florida Straits may also contribute to GBW (Hurley and Fink, 1963; Newman and Ball, 1970; Poag, 1981, 1984). Water properties, except for dissolved silica, show insignificant vertical or lateral variation within GBW (Nowlin, 1971; Carder et al., 1977). At depths >1,500 m, GBW temperature is about 4.2°C, salinity is 34.95, and dissolved oxygen is 4.8-5 ml/L (Nowlin, 1971; Morrison et al., 1983). Hydrographic data from one of our stations (26°03N, 94°30W) yielded a temperature of 4.3°C, salinity of 34.94, and dissolved oxygen of 4.8-5 ml/L.

Knowledge of deep-water currents in the Gulf of Mexico is preliminary, and no simple models of regional bottom-water circulation exist, but it has been demonstrated that the Gulf of Mexico basin is not just a quiescent marginal sea (Lewis, 1992). In the western Gulf, upper-level circulation patterns are dominated by warm-core anticyclonic eddies which are shed from the Loop Current (Elliot, 1982; Vidal et al., 1992). Recent work reveals that deep-water currents exist to a depth of 3,000 m, and in some cases are related to Loop Current eddy shedding and propagation into the western Gulf (Hamilton, 1990, 1992). Hoffman and Worley (1986) suggest that deep-water circulation is counterclockwise and composed of several small cells as opposed to the large, clockwise, circulation gyres that drive upper-layer circulation. On the Mexican-Texan continental slope, warm-core eddies interact with bottom topography and generate secondary cyclonic and anticyclonic eddies at depth (Lewis et al., 1989). Another possible energy source for deep-water currents in the Gulf of Mexico is topographic Rossby waves. These low frequency motions have been documented in deep circulations of the northwest Atlantic continental rise (Hamilton, 1984) and recently have been recorded from parts of the eastern, central, and western Gulf (Hamilton, 1990).

The impact of deep-water currents and the eddies shed from the Loop Current on the biota of the deep Gulf is at present unknown. However, these anticyclonic, warm-core eddies have low nutrient levels, whereas the cyclonic eddies have much higher productivity (Biggs, 1992). This productivity variation suggests that a surface-ocean productivity gradient affecting organic carbon flux to the continental slope may exist (Loubere et al., 1993a, b).

SAMPLES AND PROCEDURE

This study uses core-top samples (top 5 cm) from 23 piston cores and four box cores; water depths of core stations range from 1,538 to 3,850 m (Table 1; Fig. 1). Samples were air dried, weighed, and washed over a 63 sieve. The sample residue was dried and reweighed to determine the proportions of the sand and silt/clay components. Each sample was split into a fraction containing approximately 300 benthic foraminifera, and the entire split was picked.

Species diversity was examined using three measures: 1) species richness (S), which is simply the number of species in a sample; 2) the Shannon-Wiener Information Function [H(S)], and 3) species equitability (E). Species richness is generally biased by sample size with larger samples tending to have a greater number of species. The diversity index H(S), given by H(S)=-p_iln(p_i,) minimizes sample size bias by considering the number of species in a sample and their relative abundances (Gibson and Buzas, 1973). Species equitability, given by E=H(S)/ln(S), measures the distribution of individuals among species within a sample; so E=1, if all species are evenly distributed (Buzas, 1972).

Analysis of total organic carbon (TOC) was performed on 13 core-top samples to examine the effect of organic matter abundance on the distribution and diversity of benthic foraminifera. These 13 samples were used because they were not previously processed for micropaleontological analysis, and the silt/clay fraction was available. A small portion of the dried, unwashed sample was removed, powdered with a mortar and pestle, and weighed. The remaining unwashed sediment was then processed for micropaleontologic analysis. The biogenic CaCO₃ was removed using 6 N HCl and the residue was rinsed three times with distilled, deionized water, dried,

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and re-weighed. The residue was analyzed using a CHN analyzer to obtain the organic carbon and nitrogen content of the sample.

RESULTS

Species Diversity

One-hundred sixty-six species of benthic foraminifera, 57 agglutinated and 109 calcareous, were identified from the 27 core-top samples. Values of species diversity were calculated using all 166 species. There is no distinct trend in species diversity with depth from lower bathyal to abyssal environments (Fig. 2). Diversity values are less variable for samples from waters deeper than 2,400 m, but this probably relates to the fact that there are fewer samples in this depth range than in the one above. For the entire sample set, S ranges from 20 to 61 with an average of 43 and standard deviation of 9.95, H(S) from 2.07 to 3.43 with a mean of 2.86 and a standard deviation of 0.36, and E from 0.61 to 0.91 with a mean of 0.77 and a standard deviation of 0.07 (Table 2).

As in the study of Gibson and Buzas (1973), we found no diversity increase with depth for the northwestern Gulf of Mexico. Our diversity values (Table 2), however, are considerably higher than those of Gibson and Buzas (1973), which range from 9 to 14 for S and 1.82 to 2.39 for H(S), contradicting their conclusion that the northwestern Gulf of Mexico has low benthic foraminiferal diversity. High species equitability values reported here are similar to those found by Gibson and Buzas (1973) and indicate the absence of strong dominance by a few species. These high E values, typical of the northwestern Gulf of Mexico, are greater than those of the northeastern Gulf and the delta regions within the Gulf (Gibson and Buzas, 1973).

Species Abundance Patterns

Absolute Abundance

In order to assess the effect of depth-related environmental variables on the size of foraminiferal populations, we determined the absolute abundances of the entire assemblage, as well as, those of the constituent species. This absolute abundance ("foraminiferal number"), expressed as the total number of benthic foraminifera g^-1 sed varies considerably, but overall, it decreases with depth (Fig. 3),

Figure 1. Map of the study area showing sample locations.

Table 1. Sample numbers and locations, water depth (m) and map numbers.

Table 2. Species diversity measures for each sample: S, number of species per sample, H(S), Shannon-Wiener Information Function, and E, species equitability.

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and all values higher than 200 g^-1 sed are confined to depths above 2,000 m. Most species that attain an absolute abundance greater than 10 g^-1 sed show the same trend (Figs. 4a-d). All have peak abundances at depths less than 2,400 m, and several, such as Bolivina lowmani, Bulimina marginata, Bulimina aculeata, Gyroidina orbicularis, Laticarinina pauperata, and Paumotua sp. cf. P. terebra, are unrecorded at depths greater than 2,400 m.

Relative Abundances

Several depth-related patterns of relative abundance reflected by variations in dominant taxa are recognized in the deep Gulf. Because relative abundance (percentages in our plots) depends on the abundances of all species in the sample, species with low absolute abundances in some samples (especially at depths below 2,400 m) may show high relative abundances in the same samples, and these are recognized as dominant taxa in these assemblages. Most species that constitute more than 10% of the assemblage in one or more samples show distinctly higher abundances in depths less than 2,400 m. Bulimina aculeata is common (>10%) at <2,400 m depths but absent in deeper waters, whereas Nuttallides decorata maintains a >10% abundance in the 1,500-2,400 m range and in the two deepest samples (3,632 and 3,850 m), but not in the intermediate (2,400-3,600 m) range (Fig. 5). Alabaminella turgida and Bolivina lowmani are the only species that maintain high relative abundances (>20%) at all depths (Fig. 6). Some minor species, however, such as Cibicides bradyi, Eggerella bradyi, and Trochammina globigeriniformis, are eurybathic within the depth range of our study (Fig. 7). Five agglutinated species, Glomospira charoides, Karrerulina apicularis, Saccammina sphaerica, Cribrostomides wiesneri, and Spiroplectella cylindroides, and three calcareous species Ioanella tumidulus, Gyroidinoides polius, and Stainforthi complanata, show a distinct trend of increasing relative abundance with increasing water depth, especially below 2,400 m (Figs. 8a-b). Finally, several species exhibit peak relative abundances in the 2,000-2,400 m range. These include Gavelinopsis translucens, Osangularia culter, O. rugosa, Cassidulina subglobosa, and Epistominella exigua (Fig. 9).

Total Organic Carbon

The results of TOC analysis of 13 samples (Table 3) indicate no distinct trend with water depth or species diversity (Figs. 10, 11). The TOC ranges from 0.62 to 1.01% with a mean of 0.89%. TOC values are consistent with those reported by Koons and Perry (1976) (Fig. 10) for the Gulf of Mexico and support their finding that the distribution of organic carbon is not depth related. Similarly, Dignes (1979) reported percent organic carbon values between 0.2 and 1.0 for water depths greater than 1,500 m and no trend with increasing water depth in the northwestern Gulf.

No relationship is discernible between TOC and the absolute abundance of the foraminiferal assemblage (Fig. 12) or that of the relative abundance of dominant species. One possible exception is with N. decorata (Fig. 13), which exhibits a steady increase in its abundance with decreasing TOC, and is most abundant in sediments with the lowest TOC.

The carbon/nitrogen (C/N) ratio of sedimentary organic carbon is related to organic matter source (Joyce et al., 1985). C/N values between 4-6 indicate marine organic matter (plankton) and values between 12-25 reflect a terrestrial plant source (Muller, 1977; Prahl et al., 1980). The C/N ratios at most stations fall between marine and terrestrial sources (Table 3), and show no relation to water depth (Fig. 14). Only three of our stations fall within the marine organic matter source range. Although the organic matter in most of our samples is apparently derived from both marine and terrestrial sources, the C/N ratio (13.3) of one sample indicates a terrestrial source. There is no clear relationship between the C/N ratio and the relative abundances of most dominant species, but two species (E. exigua and C. subglobosa) are most abundant at the station with the highest C/N ratio (Fig. 15).

Species diversity and abundances, both absolute and relative, seem to be little influenced by the amount or type of sedimentary

Figure 2. Plots of species diversity measures against water depth, showing persistently high species diversity, but no discernible trend with water depth. S is number of species (open diamonds), H(S) is the Shannon-Wiener Information Function (closed circles) and E is a measure of species equitability (open circles).

Figure 3. Plot of the number of foraminifera (per gram of sediment) against water depth, showing a decrease in absolute abundance of total foraminifera with increasing water depth.

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organic matter in the deep Gulf of Mexico. Thus, we are unable to confirm earlier reports on such correlations (E. exigua: Gooday, 1993 and Clark et al., 1994; B. albatrossi: Lagoe et al., 1994; O. culter and G. translucens: Denne, 1990). However, the associations between TOC and N. decorata and between C/N ratio and E. exigua and C. subglobosa need further exploration.

DISCUSSION

Despite a 2,300 m depth gradient in the study area, species diversity shows no distinct trend with depth. Diversity is persistently high, which reflects the large niche capacity of the lower bathyal and abyssal Gulf, although in most instances the amount of foraminiferal biomass being supported is quite small, and widely dispersed, as indicated by the very high species equitability values. Of the 166 species identified, only about 30 reach abundances high enough to be considered as being influenced by anything other than random variation. It is presently not possible to isolate the most significant factors responsible for the maintenance of the high foraminiferal

Figure 4. Plots of the number of individuals (per gram of sediment) of selected species against water depth, showing an overall trend of decreasing absolute abundance with increasing water depth. Note scale change.

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diversity (and its variability), but microhabitat heterogeneity, disturbance by macrobiota, and patchiness of food resources are likely candidates (Grassle and Maciolek, 1992).

The decrease in the foraminiferal density (number per unit weight) with increasing depth fits the general pattern for meiobenthos or macrobenthos density (number per unit area) in the 1,500-4,000 m depth interval in the Atlantic Ocean (Thiel, 1983; Rex, 1983). In the Gulf, even the dominant species exist in very small populations in depths below 2,400 m. As is known for the Atlantic macrofauna from comparable depths (Grassle and Maciolek, 1992), most dominant species do not constitute a very high proportion of the assemblage--usually not above 10%. Strong exceptions to this rule are provided by Alabaminella turgida, Bolivina lowmani, and Nuttallides decorata. Alabaminella turgida is abundant (maximum almost 25%) at all depths. Bolivina lowmani, whose distribution is more erratic, is probably meroplanktonic (Hueni et al., 1978). Nuttallides decorata, however, reaches an abundance peak (30-40%) at about the 1,600 m depth level. Following arguments on deep-sea macrofaunal abundance trends discussed by Grassle and Maciolek (1992), this may be conjectured as a disturbed zone (perhaps by currents or sediment movement). However, the high resolution (3.5 kHz) reflection profiles of these stations, IG-41-18 (26°59N, 93°07W) and IG-41-19 (27°00N, 93°07W), which are immediately south of Shepard Basin (Bouma and Bryant, 1994), show no evidence of large-scale sediment disturbance.

Despite the physico-chemical stability of the lower bathyal and abyssal environments in the Gulf of Mexico, species abundance patterns and dominance trends clearly change with depth. Two separate groups of species dominate the assemblage above and below the 2,400 m isobath. A third group is conspicuous at the boundary zone. This agrees with the general trend of change in marine communities with changing depth (Carney et al., 1983). Although the physico-chemical attributes of GBW remain nearly constant throughout its vertical extent, the hydrostatic pressure increases by about 230 atm within the depth range of our samples (Saunders and Fofonoff, 1976); this pressure gradient must have significant effect

Figure 5. Percentage plots of Nuttallides decorata and Bulimina aculeata against water depth, showing their highest relative abundances at depths less than 2000 m.

Figure 6. Percentage plots of Alabaminella turgida and Bolivina lowmani against water depth, showing their persistently high relative abundances at all depths.

Figure 7. Percentage plots of three minor species against water depth, showing their eurybathic nature.

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Figure 8. Percentage plots for eight species against water depth, showing a trend of increasing relative abundance with increasing depth. All of these species attain their highest abundances at water depths greater than 2700 m.

Figure 9. Percentage plots of five species against water depth, showing highest abundances between 2000-2400 m.

Figure 10. Plot of percent total organic carbon (TOC) against water depth, showing that the distribution of TOC is not related to water depth. Closed circles are data from this study, open circles are from Koons and Perry (1976).

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on the species composition of foraminiferal communities.

The deep bathyal and abyssal environments of the western Gulf of Mexico are very low in organic carbon and well-oxygenated (Poag, 1981, 1984; Loubere et al., 1993; Lagoe et al., 1994), but it is possible that intermittent input of organic material may influence some of the TOC variations seen. Recent studies such as those of Gooday (1988, 1993) and Smart et al. (1994) have documented benthic foraminiferal responses to sudden, sometimes seasonal influxes of phytodetritus. It appears that some species are able to take advantage of this fluctuating input of organic matter for varied time periods. Thus, episodic input of organic matter to the lower bathyal and abyssal environments may result in subtle abundance variations such as those reported here.

Figure 11. Plots of percent total organic carbon (TOC) against species diversity, showing no trend. S is the number of species and H(S) is the Shannon-Wiener Information Function.

Figure 12. Plot of percent total organic carbon (TOC) against absolute abundance of total benthic foraminifera, showing no trend.

Figure 13. Plot of total organic carbon (TOC) against percentage of Nuttallides decorata, showing a trend of increasing relative abundance of N. decorata with decreasing TOC.

Figure 14. Plot of C/N ratio vs. water depth, showing no trend.

Table 3. Percent total organic carbon (TOC) and C/N ratios.

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CONCLUSIONS

1. Lower bathyal and abyssal environments of the northwestern Gulf of Mexico have high species diversity. No distinct trend in species diversity with depth is evident. Species equitability (E) is relatively high, indicating an absence of strong dominance by a few species and widely dispersed foraminiferal biomass in the deep Gulf.
2. There is a distinct trend of decreasing absolute abundance with increasing water depth. Several species are not recorded at depths >2,400 m. Consequently, there is a discernible depth separation within the foraminiferal assemblage at 2,400 m based on the absolute abundances of Bolivina lowmani, Bulimina marginata, B. aculeata, Gyroidina orbicularis, Laticarinina pauperata, and Paumotua sp. cf. P. terebra.
3. Relative abundances of some species indicate three foraminiferal depth zones: (a) <2,000 m interval, (b) 2,000-2,400 m interval, and (c) > 2700 m. Only a few species reach relative abundances of >10%. Of these, N. decorata and B. aculeata attain their highest abundances at <2,000 m interval, together making up almost 40% of the assemblage at some localites. Thus, high abundances (>10%) of this pair of taxa are typical of depths less than 2,000 m. Gavelinopsis translucens, Osangularia culter, O. rugosa, Cassidulina subglobosa, and Epistominella exigua reach their highest abundances (most between 5-10%) between 2,000-2,400 m, thus defining a lowermost bathyal and uppermost abyssal zone. Finally, five agglutinated species and three calcareous species reach their highest abundances at depths greater than 2,700 m in a true abyssal zone.
4. TOC seems to have little, if any, effect on species diversity, absolute abundance or relative abundance. An exception is the case of N. decorata, which seems to reach high relative abundances in areas with relatively low TOC. C/N ratios indicate that the organic matter of most stations is of both marine and terrestrial origin, although four stations have a primarily marine source and one, a terrestrial source. There is no clear relationship between organic matter source and water depth or relative abundance in the deep Gulf. However, high abundances of Epistominella exigua and Cassidulina subglobosa seem to be associated with organic matter of terrestrial origin.