Elsevier

Marine Micropaleontology

Volume 62, Issue 1, 10 January 2007, Pages 45-73
Marine Micropaleontology

Live (Rose Bengal stained) and dead benthic foraminifera from the oxygen minimum zone of the Pakistan continental margin (Arabian Sea)

https://doi.org/10.1016/j.marmicro.2006.07.004Get rights and content

Abstract

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. Samples were taken from 136 m to 1870 m water depth during the intermonsoon season of 2003 (March–April). 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 (≈ 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 characterised by cosmopolitan faunas, containing also some taxa that in other areas have been described in deep infaunal microhabitats. The contrast between faunas typical for the upper part of the OMZ, and cosmopolitan faunas in the lower part of the OMZ, may be explained by a difference in the stability of dysoxic conditions over geological time periods. The core of the OMZ has been characterised by prolonged periods of stable, strongly dysoxic conditions. The lower part of the OMZ, on the contrary, has been much more variable over time-scales of 1000s and 10,000 years because of changes in surface productivity and a fluctuating intensity of NADW circulation. We suggest that, as a consequence, well-adapted, shallow infaunal taxa occupy the upper part of the OMZ, whereas in the lower part of the OMZ, cosmopolitan deep infaunal taxa have repeatedly colonised these more intermittent low oxygen environments.

Introduction

The surface circulation of the Arabian Sea is driven by southwest and northeast monsoonal winds, which cause a strong seasonality (Wyrtki, 1973, Burkill et al., 1993). During the SW monsoon (May to September), an anticyclonic surface circulation (Fig. 1, black arrows) causes an intense coastal upwelling off Somalia and Oman, and off the southwestern part of India. During the more gentle NE monsoon (November through March) a cyclonic surface circulation prevails (Fig. 1, gray arrows), and most of the upwelling phenomena disappear. Only off Pakistan, local and sporadical upwelling events are described during the NE monsoon (Wyrtki, 1973). The upwelling of nutrient-rich surface water leads to a high biological production during the SW monsoon (Ryther and Menzel, 1965, Qasim, 1977). A second maximum in biological productivity occurs during the NE monsoon (e.g. Caron and Dennett, 1999, Rixen et al., 2000). This second maximum is caused by convective mixing due to winter cooling and the injection of nutrients into the euphotic zone (Banse and McClain, 1986, Madhupratap et al., 1996). In contrast to the western Arabian Sea, where the organic matter fluxes are much higher during the SW monsoon than during the NE monsoon, the fluxes in the central and eastern Arabian Sea are nearly the same during both monsoon seasons (Rixen et al., 2000). According to satellite observations by Antoine et al. (1996), annual primary productivity in the NE Arabian Sea is very high, and amounts to 400 gC m 2 a 1.

In the Arabian Sea, the combination of high primary productivity and a pronounced thermohaline stratification leads to a stable and expanded mid-water (150–1400 m) oxygen minimum zone (OMZ) (You and Tomczak, 1993, Olson et al., 1993, Wyrtki, 1973, Helly and Levin, 2004), defined as the zone with oxygen concentrations below 0.5 ml/l (Levin, 2003a). The OMZ results from different factors including oxygen consumption by organic matter decay, supply of oxygen-depleted intermediate water-masses from the south and west (Swallow, 1984, Olson et al., 1993) and the semi-enclosed nature of the northern Arabian Sea (Wyrtki, 1973, Shetye et al., 1994).

Benthic foraminifera provide a valuable tool for reconstructing paleoceanographic parameters (Gooday, 2003), including paleo-oxygenation (Kaiho, 1994, Bernhard et al., 1997, Jorissen, 1999, Den Dulk et al., 2000, Gooday, 2003, Schmiedl et al., 2003) as well as paleoproductivity conditions (Herguera and Berger, 1991, Loubere, 1999, Loubere et al., 2003). The input of organic matter and the oxygenation of bottom- and sediment pore-water regulates faunal density, composition and vertical distribution in the sediment (Jorissen et al., 1995, Fontanier et al., 2002). High densities of benthic foraminifera in oxygen-deficient environments have been described by several authors (Phleger and Soutar, 1973, Bernhard, 1993, Sen Gupta and Machain-Castillo, 1993, Alve, 1994, Alve, 1995, Bernhard et al., 1997, Bernhard and Sen Gupta, 1999, Gooday et al., 2000). More particularly, high abundances of deep infaunal taxa such as Bolivina with a high length/width ratio, Chilostomella and Globobulimina are typical for most of these low oxygen settings (Mackensen and Douglas, 1989, Alve, 1994, Alve, 1995, Bernhard et al., 1997, Jorissen et al., 1998, Fontanier et al., 2002). However, shallow infaunal species also may be common (Hermelin and Shimmield, 1990, Jannink et al., 1998, Maas, 2000). In order to better constrain the correlation between faunal density and composition, and bottom water oxygenation in these dysoxic environments, more systematical investigations are necessary.

The composition of the benthic foraminiferal faunas of the Arabian Sea and Indian Ocean OMZ is still poorly known. Only few publications describe living (Rose Bengal stained) foraminiferal faunas from the OMZ. Gooday et al. (2000) and Hermelin and Shimmield (1990) described the faunal distribution at stations from the Northwest Arabian Sea from water depths below 400 m. In the Northeast Arabian Sea, Jannink et al. (1998) described two transects from 500 to 3000 m water depth. Until now, only one site in the upper part of the OMZ (233 m water depth) has been described in the Northeast Arabian Sea (Maas, 2000). Kurbjeweit et al. (2000) and Heinz and Hemleben (2003) provide an account of foraminiferal assemblages at greater water depths (> 1900 m) in more central regions of the Arabian Sea.

In this study we describe the hard-shelled foraminiferal assemblages sampled during the intermonsoon period (spring 2003) along a composite sampling profile (based on two transects) from 136 to 1870 m water depth. Our main aims are to improve knowledge of: (1) the faunal composition of live (Rose Bengal stained) and dead foraminiferal faunas, their bathymetrical distribution and microhabitats, (2) the relation between low oxygen environments and foraminiferal distribution and diversity indices and (3) the long-term adaptations that allow specific foraminiferal communities to survive in low oxygen environments. Preliminary observations on benthic foraminifera at the same sampling sites, based on ship-board sorting and including both hard- and soft-shelled taxa, were reported by Larkin et al. (2003).

Section snippets

Material and methods

During R.R.S. Charles Darwin Cruises 145 and 146 (12 March to 28 May 2003), 11 multicores were taken on the continental margin off Karachi, Pakistan (Fig. 1, Table 1). Two transects were sampled, constituting a composite bathymetric profile from 136 m (above the OMZ in spring 2003) down to 1870 m water depth (Fig. 2). 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

Foraminiferal densities

Total abundances of live (Rose Bengal stained) foraminifera are shown for each core in Fig. 3. Faunal densities in the > 150 μm fraction clearly decreased with increasing water depth. At 136 m, there were > 620 individuals/core (25.5 cm2 surface area), compared with only 13 per core at 1000 m and 50 per core below the OMZ. The densities of calcareous taxa showed the same trend with a strong decrease below 600 m water depth. Densities in the smaller size fraction showed rather constant values in

Discussion

The assemblages described above present a clear zonation across the Pakistan margin OMZ in samples collected during March/April 2003 (Fig. 15). The main features of this pattern are as follows.

  • 1.

    At 136 m, above the OMZ, where the bottom water was relatively well oxygenated (> 1.5 ml/l) during March/May 2003 (although not later in the year during the monsoonal season when this site becomes dysoxic), living foraminifera are present down to 3 cm depth in the sediment. Faunal densities and diversities

Conclusion

There is a clear zonation of benthic foraminiferal faunas in the Northeast Arabian Sea OMZ. Our composite profile from 136 to 1870 m water depth shows major faunal breaks at 300, 500 and 1000 m water depth. In the core of the OMZ and above the OMZ (136–500 m) a very specific fauna, unknown in other oceans, is found, dominated by shallow infaunal taxa such as Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata; deep infaunal taxa are poorly represented. In the lower part of the OMZ

Acknowledgements

We thank Greg Cowie for the chance to work on Arabian Sea material and for providing the opportunity for two of us (KEL and AJG) to sample on the Pakistan margin. We have special and kind thoughts for the crews and captains of the RRS Charles Darwin. We are grateful to members of the scientific parties on RRS Charles Darwin Cruises 145 and 146, particularly Rachel Jeffreys, Lisa Levin, Matt Schwartz, Christine Whitcraft and Clare Woulds, who helped in numerous ways. We are very grateful to

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