Abstract
Abstract. Live (Rose Bengal stained) benthic foraminifera from the Murray Ridge, within and below the northern Arabian Sea oxygen minimum zone (OMZ), were studied in order to determine the relationship between faunal composition, bottom water oxygenation (BWO), pore water chemistry and organic matter (organic carbon and phytopigment) distribution. A series of multicores were recovered from a ten-station oxygen (BWO: 2–78 μM) and bathymetric (885–3010 m depth) transect during the winter monsoon in January 2009. Foraminifera were investigated from three different size fractions (63–125 μm, 125–150 μm and >150 μm). The larger foraminifera (>125 μm) were strongly dominated by agglutinated species (e.g. Reophax spp.). In contrast, in the 63–125 μm fraction, calcareous taxa were more abundant, especially in the core of the OMZ. On the basis of a principal components analysis, three foraminiferal groups were identified and correlated to the environmental parameters by canonical correspondence analysis. The faunas from the shallowest stations, in the core of the OMZ (BWO: 2 μM), were composed of "low oxygen" species, typical of the Arabian Sea OMZ (e.g. Rotaliatinopsis semiinvoluta, Praeglobobulimina sp., Bulimina exilis, Uvigerina peregrina type parva). These taxa are adapted to the very low BWO conditions and to high phytodetritus supplies. The transitional group, typical for the lower part of the OMZ (BWO: 5–16 μM), is composed of species that are tolerant as well to low-oxygen concentrations, but may be less critical with respect to organic supplies (e.g. Globocassidulina subglobosa, Ehrenbergina trigona). Below the OMZ (BWO: 26–78 μM), where food availability is more limited and becomes increasingly restricted to surficial sediments, cosmopolitan calcareous taxa were present, such as Bulimina aculeata, Melonis barleeanus, Uvigerina peregrina and Epistominella exigua. Miliolids were uniquely observed in this last zone, reflecting the higher BWO and/or lower organic input. At these deeper sites, the faunas exhibit a clear succession of superficial, intermediate and deep infaunal microhabitats, which can be linked to the deeper oxygen and nitrate penetration into the sediment.
Highlights
Oxygen minimum zones (OMZs) are present at intermediate water depth (250–1500 m) along the continental margin of the eastern Pacific, off southwest and northwest Africa, in the Bay of Bengal and in the northern Indian Ocean (Arabian Sea)
Our 885 m (PA1) and 1013 m (PA2) stations are located in the core of the OMZ, stations between 1172 and 1379 m (PA3, PA4 and PA5) depths are in the lower part of the OMZ, the 1495 m (PA6) station is located in the dysoxic zone, and stations from 1791 to 3010 m (PA7, PA8, PA9 and PA10) are in the oxic zone (Fig. 2)
The organic carbon and chlorophyll a (Chl a) concentrations decreased with increasing water depth and bottom water oxygenation (BWO)
Summary
Oxygen minimum zones (OMZs) are present at intermediate water depth (250–1500 m) along the continental margin of the eastern Pacific, off southwest and northwest Africa, in the Bay of Bengal and in the northern Indian Ocean (Arabian Sea). OMZs are defined as areas where the oxygen concentration in the water column is < 22 μM (Levin, 2003; Helly and Levin, 2004). They are areas of high primary productivity where low-oxygen concentrations develop naturally and have persisted over geological timescales (Reichart et al, 1998) due to enhanced organic matter degradation. Jannink et al, 1998; Levin et al, 2000; Levin, 2003; Schumacher et al, 2007)
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