ESTUDIO ZONA DE CRECIMIENTO ACTUAL (ZONA NORTE)

Quantitive is just poor qualitative. Number is Just one of an enormous variety of mathematical qualities that can help us understand and describe nature. We will never understand the growth o f a tree or the dunes in the desert if we try to reduce all of nature’s freedom to restrictive numerical schemes.

Ian Stewart

The interaction between individuals and their physical and chemical environment, and each other, define the objective of ecology. Benthic foraminifera are the single most abundant member of the benthos with hard parts that are found in deep-water environments. Though often considered to be largely cosmopolitan, they display significant patterns of endemism relative to individual oceanographic provinces. Deep-sea benthic foraminiferal assemblages, through the nature of their habitat are affected by oceanographic phenoma. For example, assemblages located above the Calcium Compensation Depth (CCD) will contain calcareous genera, those below the CCD will be dominated by agglutinating taxa.

Ecologie parameters influencing benthic foraminiferal assemblages include parameters such as water temperature, salinity, water energy, and substrate type. These may affect an assemblage singularly, or more likely in association with one another. In addition, biotic factors such as oxygen, food availability, predation, etc. play a vital role. Depth has long been considered a limiting factor to the foraminifera, however, modem thought suggests that it is the parameters associated with it (temperature, light, density, etc.) that have greater control (Murray 1991).

It is necessary to understand modern ecological dynamics before attempting to interpret fossil data. Neogene deep water systems are considered to be analogous to modern ocean floor environments (e.g.; Boltovskoy 1980; Berggren 1972). Recent investigations into modern slope and deep-sea settings (e.g.: Bernhard and Reimers 1991; Corliss 1985; Corliss and Chen 1988; Corliss and Silva 1993) has gone some way towards a greater understanding.^ossil systems beyond those of Bandy (1960). This permits relatively increased accuracy in the comparison of assemblages.

The modern deep water environment, while showing a certain degree of variability, shows nothing approaching the diversity of the modern shallow marine sector. However, the variables that do act upon the benthos show a profound affect upon assemblage composition therein.

Food can be considered one of the most pertinent influences upon deep-water benthos. In general, benthic biomass tends to be greatest near the continental margins and beneath upwelling regions. This decreases towards the central oceanic areas where oligotrophic conditions tend to persist. Upwelling systems, through the introduction of nutrient rich waters to the surface, facilitate phytoplankton blooms. This is essentially a function of upwelling rate and the nutrient concentration. Particulate organic matter (POM) descends through the water column, utilising oxygen as it is oxidised thereby expanding the Oxygen Minimum Zone (OMZ)

(Demaison and Moore 1980; Gooday 1988; Kamykowski and Zentara 1990; Southam e t al. 1982; Suess 1980). Hence, the benthos may be used as a proxy for oxygenation (BraccinI and Peypouqurt 1996; Brink, a t al. 1995; Chavez and Smith 1995; Hutchings, at. al. 1995; Josefson and Widbom 1988; Savarda and Bottjer 1991; Wignal and Myers 1988; amongst others). The response of a benthonic foraminiferal fauna to a relative oxygen content of the water mass is additionally examined (Almogi-Labin, at. al. 1990; Bernhard 1986, 1996; Kaiho 1991; Katz and Thunell 1984; Malmgren 1995; Moodey and Hess 1992; Perez-Cruz and Machain-Castillo 1990, Phleger and Soutar 1973; Sen Gupta and Machain-Castillo 1993; Tyszka 1994; amongst others). Sjoerdsma and Van der Zwann (1992) neatly summarise many of these ideas conceptually (Fig. 20). The ecology of diverse anoxic environments is comprehensively explored by Fenchel and Finlay (1994,1995).

Benthic-pelagic coupling of the deep-sea system is inherent in the understanding of flux studies (G raf 1989; Jiang and Watkins 1992; Reimers 1989; Staresinic, at. al. 1983; Walsh 1989; W efer 1989; amongst others). Benthic foraminifers have been proved to be a useful tool as palaeoproductivity proxies (Altenbach 1992; Altenbach and Samthein 1989; Berger and Diester- Haas 1988; Gooday 1988; Gooday, at. al. 1992; Herguera and Berger 1991, 1994; Mix 1989; Thomas 1996; Thomas and Gooday 1996; amongst others). Planktonic foraminifera are also utilised (Angel 1989; Thiede and Jünger 1992, amongst others), as are other members of the benthos (Jumars and Wheatcroft 1989). Other indicators such as opal and organic matter are strongly influenced by diagenesis complicating interpretations (see palaeoceanography section), but provide valuable additional information toward the oceanographic circumstance (Berger 1989; Berger and Herguera 1992; Berger and Wefer 1991, 1992; Berger, at. al. 1989, 1994; Billet, at. al. 1983; Bishop 1989; Emerson 1985; Jahnke 1990; Jahnke and Shimmield 1995; Legendre and Fèvre 1989; Sarnthein, at. al. 1992; Shimmield and Jahnke 1995; amongst others).

W hile benthic foraminifera abundance is largely subject to carbonate preservation, species composition remains a valuable instrument. However, basic species ranking systems oversimplify the oceanographic picture, account has to be taken of all data necessary. Variations in sedimentation rate, quality and quantity of food supply, oxygen content of the water mass, substrate properties, etc. are all contributory. Indices of diversity minimise the effects of sample size to give an indication of productivity. Organic rich environments trend toward a reduced diversity and high abundance (Berger, at. al. in press). Particular taxa are recognised as 'high productivity forms', i.e. Bolivina spp. and Bulimina spp. (Douglas and Woodruff 1981). Uvigerina spp. 6^ 'inked to low oxygen conditions in pelagic environments (Burke, at. al. 1993). Berger, at. al. (in press) propose high values of various Late Neogene Bulimina species in the Lower Congo Basin track a high productivity pulsing upwelling system and associated OMZ. Dominance of Bolivina spp. in the same area at the expense of Bulimina spp. are interpreted as low oxygen conditions at the sea floor (Phleger and Soutar 1973) where terrigenous silts dominate and productivity is moderate to high. Brun, at. al. (1983) recognised discrete associations of Bolivina along the W est African margin in relation to varying oxygen minima conditions. Increased

values of Uvigerina spp. are found to border the Angolan OMZ confirming the idea that this genus selects for a moderate productivity condition within an OMZ. Cibicidoides spp. are well adapted to a variety of environments. However, the genus (and associated Cibicides, Cibcorbis. Planulina, etc.) appear to avoid periods preferred by Bolivina spp. and other high productivity genera. Berger, e t al. (in press) note a pattern of alternating dominance between Cibicidoides spp. and Bulimina spp. offshore Lüderitz, suggesting a considerable fluctuation in productivity and supporting the notion of cyclic upwelling and mixing. In general, single taxon écologie parameters while a useful guide, remain ambiguous (Table 5).

Morphogroup analysis for ecological and palaeoecological interpretation is a useful tool. It is independent of species level taxonomy and is thus relatively elementary to translate from one worker to another. This approach permits comparison of assemblages of differing ages. The power of this relatively simple tool is demonstrated by the plethora of studies (for example; Severin 1983; Katz and Thunell 1984 Jones and Charnock 1985; Bernhard 1986; Corliss and Chen 1988; Nagy 1992; Tyszka 1994; amongst others). Successful work based upon wall composition and structure has also been conducted, for example Murray (1973).

Opportunism and morphogroup analysis has been extensively studied on modern foraminiferal faunas, however, these studies have concentrated mainly on calcareous forms. Hermelin and Shimmield (1990) and Hermelin (1992) isolated assemblages indicative of O M Z conditions in the Arabian Sea that were considered to be r-selected, while Sen Gupta and Machain-Castillo (1993) identified opportunistic species dominating low oxygen environments, these taxa were always present in low numbers until adverse conditions predominated when they essentially 'bloomed'. In particular, they noted the predominance of elongate morphotypes {Bolivina) in low oxygen conditions, but found no modern characteristic morphology indicative of poor oxygen levels. Similarly, Gooday (1993), identified modem abyssal calcareous opportunists that were characteristically small, trochospiral with thin, transparent tests. These calcareous species are interpreted as being opportunistically adapted to seasonal food pulses (Smart e t al. 1994). Infaunal taxa typify areas of intense upwelling off NW Africa where food flux is more constant through time. Linke and Lutze (1993) observed microhabitat preferences in living benthonic foraminifera showed a flexible approach towards food acquisition among specific faunas and were considered as opportunistic in life habit. Flexible approaches towards life habit are similarly outlined by Hilbrecht and Thierstein (1996), who record benthic behaviour in planktic foraminifera.

Kaminski e t al. (1988; 1995) noted Deep W ater Agglutinating Foraminifera (DW AF) in modem seasonally dysaerobic environments to be dominated by tapered, elongate morphotypes. These were considered as infaunal and opportunistic in life habit and correlate to morphogroup 'C of Charnock and Jones (1985). Similarly, Koutsoukos e t al. (1990) noted elongate, tapered agglutinates in the latest Cenomanian-earliest Turonian of north-east Brazil and the Anglo-Paris Basin to proportionally increase in numbers with decreasing oxygen levels in an OMZ. Tyszka (1994) subsequently described Jurassic calcareous benthonic foraminiferal assemblages that

decreased with interpreted falls in O^, while agglutinated, opportunistic faunas increased. Conversely, Preece ef. al. (in press) recorded a shift in microhabitat preference amongst a Neogene W est African assemblage of elongate agglutinated morphogroups utilising the model of Jorissen, ef. al. (1995), (Fig. 21).

Through applying morphologic and taxonomic parameters to benthonic foraminifera, Kaiho (1994) erected a dissolved oxygen index for palaeoenvironmental interpretation, although indexes have been developed using other aspects of the meiofauna, for example, by Wignall and Myers (1988), Allison, ef. al. (1995 [and references therein]).

Post-mortem alteration of assemblages must be born in mind when considering fossil benthic foraminifera. Pyrite infiliing of the test, a common phenomenon beneath an upwelling region, is not necessarily a reflection of anoxic conditions (Love and Murray 1963), but is more common to iower oxygen environments (Murray 1991). W ater mass characteristics may enhance or diminish certain aspects of the fauna. Alkaline-poor waters beneath OMZs can increase the preservation potential of calcareous taxa (Douglas and Heitman 1979), however, corrosive waters can lead to substantial dissolution (Schrader ef. al. 1983). Loubere, ef. al. (1993) highlight this issue through examining the taphonomic filtering processes that generate fossil assemblages, Bremer and Lohman (1982) find that the distribution of certain Atlantic benthonic foraminifera is most consistently correlated with the degree of CaCOa saturation.

Boltovskoy and Boltovskoy (1988) stated “the makeup and relative proportions of foraminiferal assemblages are much more influenced by geographic location and by ecological factors than by their evolutionary history". This statement was made in dismissal of benthic foraminferal use as a biostratigraphic tool. Clearly, benthic assemblages will reflect to a certain extent the characteristics of the dominant oceanographic system. It has long been recognised that palaeogeographic and / or palaeoclimatic events affect circulation and concomitantly foraminiferal biogeography throughout the Neogene. Berggren (1977) cited six major influences upon Atlantic circulation as the: Separation of eastern and western Tethys in the Burdigalian; Growth of the Antarctic Ice Cap during the mid-late Miocene; Gradual closure and separation of western Tethys and the Atlantic Ocean basins in the Late Miocene; Reconnection of the Atlantic and Mediterranean in the Early Pliocene; Severing of the marine connection between the Atlantic and Pacific through the shallowing of the Isthmus of Panama in the Early Pliocene; Initiation of polar glaciation in the Mid Pliocene. However, benthic bathyal to abyssal zonations have been attempted upon the basis of foraminifera as a supplement to planktonic biostratigraphy (Boltovskoy 1980; Berggren and Miller 1989). These schemes remain under-used as their basis, basic taxonomy, is continually revised (i.e.: Kaminski in press).

Depth zonation studies upon the basis of benthic forams have proved to be a popular approach. Early works such as Bandy and Arnal (1960) assumed that an isobathyal relationship existed between taxa. Subsequent work, for example that of Culver and Buzas (1980), identified water masses as a primary control upon distribution. Modern approaches adopt a more rigorous

perspective. Culver (1988), on the Gulf of Mexico, defined paiaeobathymetries upon the basis of generic identification (thereby gaining the advantages of the morphogroup approach outiined above). However, the results of Kurihara and Kennett (1988) have to be born in mind. They found certain species to significantly (-2 0 0 0 m) alter their water depth in the Pacific during Neogene time.

Through careful examination of the fauna and consideration of the mechanics of natural systems, inferences may be made toward palaeoceanographic settings and palaeoenvironmental controls. However, it is important to stress the necessity to become wary of circular reasoning in this approach. 'Controls' are impossible to establish and much data remain open to interpretation. Despite the recent plethora of studies conducted upon benthic foraminifera, the ecological factors controlling their distribution remain poorly understood.