Analysis of Recent benthic foraminiferal assemblages from the oxygen minimum zone of the Pakistan continental margin

Live (Rose Bengal stained) and dead benthic foraminiferal communities (hard-shelled species only) from the Pakistan continental margin oxygen minimum zone (OMZ) have been studied in order to determine the relation between faunal composition and the oxygenation of bottom waters. During R.R.S. Charles Darwin Cruises 145 and 146 (12 March to May 28 2003), 11 multicores were taken on the continental margin off Karachi, Pakistan. Two transects were sampled, constituting a composite bathymetric profile from 136 m (above the OMZ in spring 2003) down to 1870 m water depth. Cores (surface area 25.5 cm2) were processed as follows: for stations situated above, and in the upper part of the OMZ, sediment slices were taken for the 0-0.5 and 0.5-1 cm intervals, and then in 1 cm intervals down to 10 cm. For the lower part of the OMZ, the second centimetre was also sliced in half-centimetre intervals. Each sample was stored in 10 % borax-buffered formalin for further processing. Onshore, the samples were wet sieved over 63 µm, 150 µm and 300 µm sieves and the residues were stained for one week in ethanol with Rose Bengal. After staining, the residue was washed again. The stained faunas were picked wet in three granulometric fractions (63-150 µm, 150-300 µm and >300 µm), down to 10 cm depth. To gain more insight into the population dynamics we investigated the dead (unstained) foraminifera in the 2-3 cm level for the fractions 150-300 µm and >300 µm. The fractions >300 µm and 150-300 µm show nearly the same faunal distribution and therefore the results are presented here for both fractions combined (i.e. the >150 µm fraction). Live foraminiferal densities show a clear maximum in the first half centimetre of the sediment; only few specimens are found down to 4 cm depth. The faunas exhibit a clear zonation across the Pakistan margin OMZ. Down to 500 m water depth, Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata dominate the assemblages. These taxa are largely restricted to the upper cm of the sediment. They are adapted to the very low bottom-water oxygen values (ab. 0.1 ml/l in the OMZ core) and the extremely high input of organic carbon on the upper continental slope. The lower part of the OMZ is characterized by cosmopolitan faunas, containing also some taxa that in other areas have been described in deep infaunal microhabitats.