Production and preservation of calcareous dinoflagellate cysts in the modern Arabian Sea

Abstract

Subject of this thesis are the calcareous cysts of dinoflagellates which are unicellular, planktonic organisms. With the main aim of improving the applicability of calcareous dinoflagellate cysts as (palaeo-) environmental proxy, the major objectives of this thesis are (1) to examine the preservation potential of the individual cyst species, and (2) to contribute to the knowledge on the ecology of calcareous dinoflagellates. Surface sediment samples from different areas of the Arabian Sea and sediment trap material from the northern Somali Basin were quantitatively analysed for their calcareous dinoflagellate cyst content.The results demonstrate that (1) all of the studied species are affected by early diagenetic calcite dissolution with unequal preservation potential, resulting in alteration of the relative abundances, (2) calcite preservation is enhanced under low oxic bottom water conditions in the NE Arabian Sea, and (3) considerable calcite dissolution occurs above the lysocline in this area under oxic bottom water conditions, which is attributed to high rates of organic matter degradation. The basin-wide occurrence of the studied species points to their large environmental tolerance. Spatial differences in absolute and relative abundances of the individual species are discussed with respect to temperature, salinity, nutrient levels and stratification. The sediment trap data indicate that the combination of elevated nutrient supply and stratified surface waters is most favourable for high cyst production, whereas strongly stratified but nutrient depleted conditions result in very low cyst fluxes. Thus, calcareous dinoflagellate cysts can be applied as an indicator for stratified but not yet nutrient depleted surface waters, whereby the individual species may be used for the assessment of environmental conditions. However, the primary signal in the sediments can be strongly modified by post-depositional processes, especially in highly productive oceanic regions.