Abstract
Abstract. Rose-Bengal-stained foraminiferal assemblages (> 150 μm) were analysed along a five-station bathymetric transect across the core and the lower part of the oxygen minimum zone (OMZ) on the Indian margin of the Arabian Sea. Sediment cores were collected using the manned submersible Shinkai 6500 during the RV Yokosuka cruise YK08-11 in the post-monsoon season (October 2008) at water depths ranging from 535 to 2000 m, along a gradient from almost anoxic to well-oxygenated (0.3 to 108 μM) bottom waters. Stained benthic foraminifera were investigated from two different size fractions (150–300 μm and > 300 μm). Stained foraminiferal densities were very high in the core of the OMZ (at 535 and 649 m) and decreased at deeper sites. The faunas (> 150 μm) were dominated (40–80 %) by non-calcareous taxa at all stations. These were mainly species of Reophax and Lagenammina but also included delicate monothalamous taxa (organic-walled "allogromiids", agglutinated saccamminids, psammosphaerids and tubular forms). These new data from the Indian margin are compared to previous studies from the Murray Ridge, the Pakistan margin and the Oman margin. The fact that similar species were found at sites with comparable bottom-water oxygen concentrations but with very different surface water productivity suggests that, within the strongly developed Arabian Sea OMZ, bottom-water oxygen concentration, and not the organic flux to the sea floor, is the main factor controlling the species composition of the foraminiferal communities. Several foraminiferal species (e.g. Praeglobobulimina sp. 1, Ammodiscus sp. 1, Bolivina aff. dilatata) were confined to the core of the OMZ. These species are presently known only from the Arabian Sea. Because of their association with extremely low oxygen concentrations, these species may be good markers for very low oxygen concentrations, and could be used to reconstruct past OMZ variability in the Arabian Sea.
Highlights
The ocean floor hosts rich and diverse micro, meio- and macrofaunal communities
The present paper focuses on how foraminiferal faunas on the Indian margin respond to the combination of organic-matter fluxes that are likely to be moderate and extremely low oxygen concentration
Our results suggest that bottom-water dissolved oxygen concentrations (BWO) may play the major role in regulating benthic foraminiferal assemblages in the Arabian Sea
Summary
The ocean floor hosts rich and diverse micro-, meio- and macrofaunal communities. The distribution and abundances of these faunas are controlled by many environmental and physical parameters that change from the continental shelf to the deep ocean. Organic-matter fluxes and bottom-water oxygenation exhibit drastic changes between oceanic basins. Strong gradients in both parameters are found in oxygen minimum zones (OMZ; defined by permanent hypoxia, with dissolved oxygen concentrations < 22 μM; Levin, 2003). These mid-water features impinge on the continental slope at upper bathyal depths in the eastern Pacific, the northern Indian Ocean (Arabian Sea and Bay of Bengal), and to a lesser extent off southwest Africa Caulle et al.: Living (Rose-Bengal-stained) benthic foraminiferal faunas
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