8th International Nannoplankton Association Conference


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Ulf Rogalla, Harald Andruleit, Jörg Mutterlose, Ulrich von Rad:
Coccolith assemblage variations in sediments of the oxygen minimum zone off Pakistan: alteration versus ecological changes

Hemipelagic sediments of the Oxygen Minimum Zone (OMZ) from the NE Arabian Sea off Pakistan were studied in terms of calcareous nannofossil assemblage variations during the past ~76ky.

This study is part of a BGR project, with the aim to reconstruct the palaeoclimatic conditions of the OMZ of the NE Arabian Sea in the Late Quaternary.

A feature of the Recent Arabian Sea is the existence of a stable OMZ between 200 and 1500m water-depth, related to the special oceanographic setting of high surface-productivity, related to summer and winter monsoons, and relatively sluggish advection of poorly-oxygenated mid-layer waters (Olson et al., 1993). Where this OMZ impinges on the shelf, dark, laminated and TOC-rich sediments accumulate (Schulz et al., 1996). These conditions persisted over most of the Late Quaternary (Schulz et al., 1998; von Rad et al., 1999), only interrupted by short breakdowns which can be related to N Atlantic Heinrich layers by absolute AMS 14C datings. These short-term events can be recognised easily by grey-values scans (e.g. Schulz et al., 1996).

In order to study the possible impact of biological surface-water conditions on the formation and destruction of the OMZ in the past geological history, we selected two profiles from within the OMZ, taken by the SONNE 130 cruise, for high-resolution calcareous nannofossil investigations: the 20.20m-long piston-core, SO 130 289KL, N of the submarine Indus Valley, with a maximum age of ~76ky, and the 17.86m-long piston-core, SO 130 261 KL, W of Karachi, for high-resolution investigations of the time-interval from Heinrich 1 to Heinrich 3 (~15-30ky). The age model of both profiles is based on absolute AMS 14C datings of already published parallel profiles of the SONNE 90 cruise (136 KL parallel 289 KL: (Schulz et al., 1998); and 137 KA parallel 261KL: von Rad et al., 1999). Additionally, about 20 AMS 14C samples of 261 KL were included.

One hundred and nine samples of 289KL and 105 samples of 261 KL were taken for nannofossil investigations, at an average sample interval of 20cm and 10cm, respectively. Besides, mean and fine-fraction carbonate samples were taken from 289 KL.

The calcareous nannoflora over the entire time-interval is dominated by placoliths, in particular Gephyrocapsa oceanica, Gephyrocapsa cf. G. ericsonii and Emiliania huxleyi. Additionally, Florisphaera profunda plays a major role in the assemblages. All four taxa together comprise at least 60% of the whole nannoflora.

Highest coccolith abundances and diversities were observed in pale-coloured, TOC-rich sections, especially in the Heinrich equivalent intervals, where mean carbonate content also reaches highest values. Very delicate coccoliths, e.g. holococcoliths, Algirosphaera robusta and Acanthoica spp. occur only in these sediments, or reach their highest absolute abundances, suggesting an enhanced dissolution component. Because the two profiles are sited well above the recent CCD of the N Indian Ocean (~4000m: Kolla et al., 1976), other reasons of alternating qualitative and quantitative coccolith preservation must be predicted. A probable cause for our observation results from an enhanced concentration in reduced sulphur species, e.g. H2S of TOC-rich sediments (von Rad et al., 1999). This leads to the assumption that changing bottom-water conditions play a major role in coccolith preservation. Besides this interpretation, nannoplankton associations from different sections with alternating bottom-water and sedimentation conditions need to be interpreted with attention.

However, as delicate nannofossils play a minor role in absolute and relative abundances in sediments of the OMZ of the Arabian Sea, dissolution has a negligible impact on palaeoclimatic reconstructions, especially when based on major groups with similar dissolution stability, e.g. placoliths (Schneidermann, 1977).


Kolla, V. et al. 1976. Calcium carbonate distribution in the surface sediments of the Indian Ocean. J. Geophys. Research, 81(15): 2605-2616.

Olson, D.B. et al. 1993. Maintenance of the low-oxygen layer in the central Arabian Sea. Deep-Sea Research II, 40(3): 673-685.

Rad, U. von et al. 1999. Multiple monsoon-controlled breakdowns of oxygen-minimum conditions during the past 30, 000 years documented in laminated sediments off Pakistan. Palaeo., Palaeo., Palaeo., 152: 129-161.

Schneidermann, N. 1977. Selective dissolution of recent coccoliths in the Atlantic Ocean. In: A.T.S. Ramsay (Ed.). Oceanic Micropaleontology, 2: 1009-1053.

Schulz, H. et al. 1996. Laminated sediments from the oxygen-minimum zone of the northeastern Arabian Sea. In: A.E.S. Kemp (Ed.). Palaeoclimatology and palaeoceanography from laminated sediments. Geol. Soc. Spec. Publication, 116: 185-207.

Schulz, H. et al. 1998. Correlation between Arabian Sea and Greenland climate oscillations of the past 110, 000 years. Nature, 393: 54-57.


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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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