Living (Rose Bengal stained) benthic foraminifera from the Pakistan continental margin (northern Arabian Sea)

Abstract

The Arabian Sea is characterized by one of the world’s most pronounced oxygen minimum zones (OMZ) (<0.1 O 2 ml/l), which impinges on the seafloor at 200–1000 m depths. The OMZ in the Arabian Sea results from extremely high surface water productivity and moderate thermocline ventilation. Nine box cores were taken on two parallel down-slope transects covering depths from 500 to 2000 m. From these nine box cores living (Rose Bengal stained) benthic foraminifera were studied in detail. Within the upper part of the OMZ, Bolivina dilatata and Bulimina exilis are the most abundant species. In the lower part of the OMZ, Uvigerina peregrina and B. exilis are the most abundant. Just below the OMZ, at a water depth of about 1250 m, the assemblage is typically dominated by Rotaliatinopsis semiinvoluta and U. peregrina; in still deeper waters (1500–2000 m) Bulimina aculeata and Epistominella exigua are the most prominent species. Transect II was sampled three weeks after transect I; on the average, standing stocks were four times higher in transect II. However, down-slope species distributions are similar in the two transects, both in the 63 μm and the 150 μm size fractions. Also vertical (in-sediment) distributions are remarkably similar. This indicates that standing stock differences between the two transects can be ascribed either to the effect of patchiness or, more likely, to the nature and the amount of organic flux. We found unambiguous evidence that in and below the OMZ many benthic foraminifera persist in suboxic to anoxic microhabitats. This regards surface as well as subsurface habitats. Evidently, oxygen is not a limiting factor for a considerable number of species. The obvious relationship between species distribution and the OMZ might then be explained in terms of preferences for amount or type of organic flux. An alternative explanation involves favourable effects resulting from the absence of larger predators. The various models pertaining to the limiting balance between organic flux and oxygen are discussed.