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Ulf Rogalla, Harald Andruleit, Jörg Mutterlose:
Calcareous nannofossil variations in sediments of the NE Arabian Sea over the past 200 ky
(Talk)

We present data of a high-resolution calcareous nannofossil investigation of hemipelagic sediments from the continental margin off Pakistan (22°29.31'N; 65°38.99'E) to reconstruct the palaeoclimatic variability in the NE Arabian Sea during the past 200ky. We chose the ~11m-long piston-core, SO90 94KL, sited at the upper Indus Fan in 2109m water-depth. The core location is above the CCD and below the, in Late Quaternary, unstable oxygen minimum zone (OMZ), thus avoiding selective dissolution effects. The age-model of this profile is provided by a high-resolution d18O record of the planktonic foraminifera, G. ruber (Schubert et al., 1998).

The nannoflora of SO90 94KL is dominated by placoliths, especially of the genus Gephyrocapsa. Gephyrocapsa oceanica, the dominating extant coccolithophore species of the NE Arabian Sea (Andruleit et al., 2000) is also the prevailing species in the Arabian Sea over most of the past 200ky. This species shows abundance peaks in advance, and after, each of the interglacial periods 7 and 5, at the beginning of Stage 4, and during Stage 3.

G. oceanica evolves to become the main contributor in the N Arabian Sea at the boundary between Isotope Stages 4 and 3, as a result of a marked decrease in abundances of Gephyrocapsa cf. G. ericsonii, leading to a general change in community structure. This event is time coherent with the rapid decrease in G. aperta abundance observed by Jordan et al. (1996) off NW Africa. Emiliania huxleyi, the third most abundant coccolith species, shows a remarkable shift to higher values at the boundary between Isotope Stages 5 and 4, indicating as well a compensation of G. cf. G. ericsonii. This species reaches maximum values in Isotope Stage 2, where it exceeds all other placoliths. Florisphaera profunda shows a remarkable distribution pattern over the entire section. This species, known as an indicator of nutrient-deficient surface-waters, shows only two significant abundance peaks which are related exclusively to temperature maxima at the beginning of Isotope Stages 1 and 5.53. This investigation suggests a short-term reduction, or possible breakdown, of the monsoon system at the glacial/interglacial transition, with reduced nutrient-enrichment in surface-waters, and deepening of the mixed-layer depth. G. flabellata, another deep-dweller related to oligotrophic conditions, shows abundance peaks more attributed to the beginning of Termination I and II, respectively. This leads to the assumption of an evolving short-term reduction in monsoon activity, coincident with oligotrophic conditions marked by deep-dwelling nannofossil species.

References

Andruleit, H.A. et al. 2000. Coccolithophore fluxes from sediment traps in the northeastern Arabian Sea off Pakistan. Marine Micropaleontology, 38: 285-308.

Jordan, R.W. et al. 1996. Coccolith and alkenone stratigraphy and palaeoceanography at an upwelling site off NW Africa (ODP 658C) during the last 130.000 years. In: A. Moguilevsky & R. Whatley (Eds). Microfossils and oceanic environments: 111-131.

Schubert, C.J. et al. 1998. Stable phytoplankton community structure in the Arabian Sea over the past 200,000 years. Nature, 394: 563-566.

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

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