Journal of Foraminiferal Research, zyxwvutsrqp v. 21, zyxwvutsrqpo no. 2, p. 85-92, April zyxwvutsrq 1997 RECENT BENTHIC FORAMINIFERA IN THE ABYSSAL NORTHEAST ATLANTIC OCEAN: RELATION TO PHYTODETRITAL INPUTS CHRISTOPHER w. SMART‘ AND ANDREW zyxw J. GOODAY* ABSTRACT Total (predominantly dead) foraminiferal assemblages have been studied at three abyssal sites in the northeast Atlantic Ocean: Porcupine Abyssal Plain (PAP, 49”N, 4,850 m), Madeira Abyssal Plain (MAP, 31”N, 4,950 m) and Cape Verde Abyssal Plain (CVAP, 21°N, 4,505 m). The assemblages are broadly similar at all three sites. Densities at the PAP site (mean 1,395 specimens per 25.5 cm2) are comparable to those at the CVAP site (mean 1,257125.5 cmz). However, density values are much lower at the MAP site (mean 283125.5 cm2), almost certainly because of the diluting effect of a Recent turbidite de- posit that is presumed to have been devoid of forami- nifera when emplaced an estimated 930 years ago. An- other important influence on total foraminiferal assem- blages is the fact that the PAP site (but not the southern MAP and CVAP sites) is subject to a seasonal pulse of food (phytodetritus), as revealed by time-lapse photog- raphy and multiple corer samples. This pulse is reflected in the fact that two species, zyxwvutsrq Epistominella exigua and Alabaminella weddellensis, are more abundant both ab- solutely and relatively at the PAP site than they are at either of the two southern sites. Both of these species are opportunists that exploit phytodetritus as a food source. Finally, only one calcareous species, Nuttallides umbon- iferus, is more abundant, both relatively and absolutely, at the two southern sites than at the PAP site. We spec- ulate that, in addition to other factors such as carbonate undersaturated bottom water, it may be adapted to a more oligotrophic food input regime that does not in- volve substantial pulses of phytodetritus. In general, our results support recent attempts to use certain species as proxy indicators of pulsed organic matter inputs in the paleoceanographic record. INTRODUCTION The influence of organic matter inputs on the abundance and distribution of species and populations of deep-sea ben- thic foraminifera is a topic of considerable current interest, as recently reviewed by Gooday (1994), Schnitker (1994) and Murray (1995). Along continental margins, there is good evidence that organic carbon fluxes, combined with oaygen penetration into the sediment (two closely linked variables) are decisive factors in controlling foraminiferal distributions (e.g., Lutze and Coulbom, 1984; Lutze and others, 1986; Corliss and Chen, 1988; Corliss and Emerson, 1990; Jorissen and others, 1992; Loubere and others, 1993; Mia0 and Thunell, 1993). There may also be a relationship between organic carbon fluxes and benthic foraminifera in 1 Department of Geological Sciences, University of Plymouth, Drake zyxwvut 2 Southampton Oceanography Centre, Empress Dock, Southampton, Circus, Plymouth, Devon, PL4 8AA, United Kingdom. Hampshire, SO14 3ZH, United Kingdom. central oceanic areas where oxygen depletion of sediment pore waters is not a major factor (Gooday, 1994). For ex- ample, Loubere (1991) demonstrated a correlation between surface primary productivity and benthic foraminiferal as- semblages along a transect in the eastern Pacific where pro- ductivity was the only significant variable. The present paper is one of three that address this theme as part of the Southampton Oceanography Centre’s (SOC) benthic biology project (DEEPSEAS). The project is based around three sites in the northeast Atlantic situated well away from the continental margin. In the first paper, Gooday (1993) discussed live populations of certain benthic fora- miniferal species (notably Epistominella exigua) that are associated with aggregates of organic matter (phytodetritus). The phytodetrital-dependentspecies were substantially more abundant at a northern site on the Porcupine Abyssal Plain (PAP, 48”N), which experiences a seasonal flux of phyto- detritus to the ocean floor (Rice and others, 1994), than at a southern site on the Madeira Abyssal Plain (MAP, 31”N), where this input does not occur. Gooday (1993) concluded that these species flourish at the northern site because they grow and reproduce opportunistically in response to their phytodetrital food source (Gooday, 1988; Gooday and Lambshead, 1989; Gooday and Turley, 1990). In the second paper, Gooday (1996) describes the density, taxonomic composition, diversity, and size structure of the entire foraminiferal community (soft-shelled as well as hard- shelled) in phytodetritus and surficial sediments (0-1 cm layer) at these two sites and at an additional site on the Cape Verde Abyssal Plain (CVAP, 21”N). Clear differences in population structure were shown to exist between the PAP and the two southern sites (MAP, CVAP). These studies (Gooday, 1993, 1996) dealt only with “live” (rose Bengal stained) foraminifera that were wet- sorted from sample residues. In the present paper, our ap- proach is micropaleontological rather than biological and is based on dried residues. Our main aim is to establish wheth- er a “phytodetrital signal” is also evident in the “total” (mainly dead) foraminiferal assemblage at the same three sites. The dried assemblages are numerically dominated by fairly robust species and contain none of the delicate and soft-shelled forms that are a major component of the “live” assemblages described by Gooday (1996). Hence, they rep- resent an interface between the highly diverse living faunas and the fossil assemblages that make up the paleoceano- graphic record. In a separate paper (Smart and others, 1994), we have discussed possible paleoceanographic applications of these observations, namely the use of E. exigua as a proxy indicator of phytodetritus deposition in ancient oceans. Relatively few micropaleontological investigations have considered Recent benthic foraminifera living on abyssal plains (>4,000 m depth) in the temperate northeast Atlantic Ocean. The most important studies are those of Phleger and 85