INA8

INA8
8^th International Nannoplankton Association Conference

ABSTRACTS

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Annemiek Vink, Carsten Rühlemann, Karin Zonneveld, Stefan Mulitza, Matthias Hüls, Helmut Willems:
Calcareous dinoflagellate cysts as sensitive indicators of rapid climate change: reflection of Heinrich Events in the western subtropical Atlantic Ocean
(Talk)

The use of calcareous dinoflagellate cysts in Late Quaternary palaeoenvironmental and palaeoceanographic reconstructions has developed significantly in the last couple of years but is still greatly hampered by the extremely limited amount of (palaeo-)ecological information available on individual species. Recent studies suggest that enhanced abundances of calcareous dinoflagellate cysts generally occur when the productivity is low, i.e. in oligotrophic, stratified surface-water conditions (e.g. Höll et al., 1999). To obtain a more detailed knowledge of the ecological preferences of each individual calcareous dinoflagellate species, surface-sediment samples from the generally oligotrophic western subtropical Atlantic Ocean were quantitatively analysed for their calcareous cyst content. The observed spatial variations in cyst association appear, for a large part, to be associated with the different water-masses (i.e. surface-water currents) characterising the area, and may thus be useable for reconstructing the movements of these water-masses throughout the Late Quaternary.

The western subtropical Atlantic Ocean plays a crucial role in Atlantic circulation, as most of the interhemispheric water-mass exchange occurs in this area. Surface-waters export heat and salt from equatorial regions, through the Caribbean into the N Atlantic, where it is essentially required for the formation of N Atlantic Deep Water (NADW). In return for this northward surface water transport, NADW flows back into the Southern Hemisphere and thus maintains the global thermohaline circulation and the thorough ventilation of the ocean basins (e.g. Broecker, 1991; Stramma & Schott, 1996). In order to enhance our understanding of past changes in the western subtropical Atlantic surface-current system, and its relation to changes in Atlantic thermohaline circulation on orbital, suborbital and millennial time-scales, calcareous dinoflagellate cyst accumulation rates were determined for a high-resolution sediment core, M35003-4 (water-depth 1299m), recovered from the Tobago Basin (Caribbean). This core provides an ideal high-sedimentation, well-dated record for detailed multiparameter analyses (e.g. delta¹⁸O, TOC, CaCO₃, SST) of the Caribbean-Atlantic water-mass exchange system during the last glacial/interglacial cycle (i.e. upto 58ky BP). Pronounced changes in cyst association, accumulation and organic carbon occur which are controlled by (i) significant southward shifts in the position of the N Equatorial Current during the last glacial period and the Younger Dryas cold interval; and (ii) rapid changes in local productivity in marine Isotopic Stage 3, which are associated with fluctuations in NE trade-wind intensity and coastal upwelling strength. Prominent cyst accumulation peaks clearly reflect the N Atlantic Heinrich Events, and represent extremely oligotrophic and stratified surface-water conditions. These palaeoenvironmental reconstructions provide important clues and insights into the causes and effects of interhemispheric climate variability and the temporal relation between low- and high-latitude (rapid) climate change during the last deglaciation, as well as during the Heinrich Events.

The calcareous dinoflagellate cyst distributions in core M35003-4 show that calcareous cysts react sensitively to rapid environmental/climate change and that they represent a new, powerful proxy with which to reconstruct changes in the relative positions of surface-water currents and/or local palaeoproductivity in the past.

References

Broecker, W.S. 1991. The great conveyor. Oceanography, 4(2): 79-89.

Höll, C., Karwath, B., Rühlemann, C., Zonneveld, K.A.F. & Willems, H. 1999. Palaeoenvironmental information gained from calcareous dinoflagellates: the late Quaternary eastern and western tropical Atlantic Ocean in comparison. Palaeogeogr., Palaeoclimatol., Palaeoecol., 146: 147-164.

Stramma, L. & Schott, F. 1996. Western equatorial circulation and interhemispheric exchange. In: W. Krauss (Ed.). The Warm Water Sphere of the North Atlantic Ocean. Gebrüder Bornträger, Berlin, Stuttgart: 195-227.

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

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Tania Hildebrand-Habel (hiha@micropal.uni-bremen.de)