8th International Nannoplankton Association Conference


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Ines Wendler, Karin Zonneveld, Helmut Willems:
Calcareous dinoflagellates in the Arabian Sea: signals of production and diagenesis

Calcareous dinoflagellates are primary producers living in the photic zone of the oceans. During their life-cycle, they form calcareous cysts which can fossilise in the sediments and reflect the ecological conditions in the upper water-column at the time of deposition but are also subject to early diagenetic processes which may alter the primary signal. The study of surface sediment samples from the Arabian Sea has shown that the large spatial trends in abundance of the individual species can be related to the conditions in the upper 100m of the water-column, which are determined by the Asian monsoon system. On the basis of the cyst distributions, three areas could be separated: NW Arabian Sea off Somalia and Arabia influenced by seasonal upwelling of cold and nutrient-rich water, open ocean characterised by seasonal downwelling leading to a deep thermocline and lower nutrient concentrations in the mixed layer, and NE Arabian Sea off Pakistan with deep vertical mixing during winter and the influence of the Indus River discharge.

The most important controlling factors seem to be water temperature, degree of stratification and nutrient concentration or related competition with other plankton groups. A typical feature in the Arabian Sea is the presence of a permanent mid-water Oxygen Minimum Zone (OMZ) due to the high primary production and reduced ventilation. The OMZ has its greatest thickness and intensity in the NE Arabian Sea, where a clear relation between cyst accumulation rates and oxygen content of the bottom-water was observed: greater amounts of cysts were found in samples lying within the OMZ as compared to nearby samples from below the oxygen-depleted zone (but above the Calcite Compensation Depth). This points to a better preservation of the calcareous cysts within the OMZ, due to less corrosive pore-waters associated with (i) reduced organic matter degradation and lowered production of metabolic CO2, and (ii) enhanced sulphate reduction which increases alkalinity. Furthermore, there is indication of different dissolution sensitivity among the individual species.

We conclude that the recent distribution of calcareous dinoflagellate cysts in surface sediments of the Arabian Sea represents a combined signal of ecology and early diagenesis. The alteration of the primary signal is stronger in the NE of the working area. Despite the diagenetic overprinting, large trends in cyst distributions can be attributed to environmentally-induced differences in cyst production in the different areas.


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

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