Recent benthic foraminifera in the Abyssal northeast Atlantic Ocean: Relation to phytodetrital inputs

Total (predominantly dead) foraminiferal assemblages have been studied at three abyssal sites in the northeast Atlantic Ocean: Porcupine Abyssal Plain (PAP, 49 degrees N, 4,850 m), Madeira Abyssal Plain (MAP, 31 degrees N, 4,950 m) and Cape Verde Abyssal Plain (CVAP, 21 degrees 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 cm(2)) are comparable to those at the CVAP site (mean 1,257/25.5 cm(2)). However, density values are much lower at the MAP site (mean 283/25.5 cm(2)), almost certainly because of the diluting effect of a Recent turbidite deposit that is presumed to have been devoid of foraminifera when emplaced an estimated 930 years ago. Another important influence on total foraminiferal assemblages 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 photography and multiple corer samples. This pulse is reflected in the fact that two species, Epistominella exigua and Alabaminella weddellensis, are more abundant both absolutely 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 umboniferus, is more abundant, both relatively and absolutely, at the two southern sites than at the PAP site. We speculate 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 involve 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.