INA8
8th International Nannoplankton Association Conference


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Elena Colmenero, José-Abel Flores, Ludvig Löwemark, Joachim Schönfeld, Francisco J. Sierro:
Paleoceanographic reconstruction of the Gulf of Cadiz during the last glacial cycle as revealed by coccolithophore assemblages
(Poster)


The Gulf of Cadiz is a key region in which to monitor Mediterranean and Atlantic water exchange dynamics and to achieve a complete understanding of the extension of the climatic events which happened in the N Atlantic during the Last Glacial Episode. Coccolithophore assemblages observed in samples from gravity core M39029-7 (Figure 1), recovered in the Gulf of Cadiz (362.5'N, 813.8'W; 1917m water-depth), are used here for a palaeoceanographic reconstruction of the Late Quaternary. A high-resolution sampling (averaged temporal resolution of 700 and, occasionally, 350 years) was performed on core M39029-7 in order to obtain coccolithophore fluxes and abundance through the core. The adopted age model is based on correlation of the core M39029-7 oxygen isotope curve with isotope curves obtained from other cores dated with 14C and situated in the area (Cacho et al., 1999; Cacho et al., subm.). Peaks of Ice Rafted Debris (IRD) have been observed and interpreted in our core as Heinrich Events 1, 2 and 4, in analogy with core SO75-26KL (Zahn et al., 1997). Core average sedimentation rate is about 14cm/ky.

The Holocene is characterised by an assemblage dominated by small Gephyrocapsa (G. aperta and G. ericsonii) and E. huxleyi (<3.5µm). These taxa are accompanied by minor proportions of Umbellosphaera spp., Calcidiscus leptoporus, Umbilicosphaera spp. and Neosphaera coccolithomorpha. This assemblage suggests warm waters and moderate productivity. During Marine Isotope Stages (MIS) 2 and 3, abundance of Umbellosphaera spp., Calcidiscus leptoporus, Umbilicosphaera spp. and Neosphaera coccolithomorpha decreases to less than 1%. Small Gephyrocapsa is still the dominant taxon, partially replaced by G. muellerae. Florisphaera profunda shows occasional peaks (that nearly reach 20%). At Termination 1, the proportion of E. huxleyi (<3.5µm) decreases abruptly and is replaced by E. huxleyi (>3.5µm). We interpret this large morphotype of E. huxleyi as a cold-water organism. Additionally, a biometric study carried out in this core and in cores MD95-2043 (Alboran Sea) and SU90-08 (N Atlantic Ocean) allows us to consider this event as a potential biostratigraphic datum (Figure 1).

Heinrich Events 1, 2 and 4 are observed in core M39029-7 by IRD peaks. The coccolithophore assemblage also reflects these cold episodes (Figure 1): F. profunda and E. huxleyi (>3.5µm) and, in minor proportions, G. muellerae show an increase in their abundances during Heinrich Events 1, 2 and 4. Reworked nannoliths from the Cretaceous also show maximum abundance peaks during these events. Another F. profunda peak associated with a decrease in the number of coccoliths per visual field and the correlation with oxygen isotope curve from core MD95-2043 (Cacho et al., 1999) allows us to identify Heinrich 3. Periodic variability in the coccolithophore assemblages is here interpreted as Dansgaard-Oeschger cycles.

References

Cacho, I., Grimalt, J.O., Canals, M., Sbaffi, L., Shackleton, N., Zahn, R. & Schönfeld, J. Variability of the western Mediterranean Sea surface temperature during the last 30, 000 years and its connections with the northern hemisphere climatic changes. Submitted to Paleoceanography.

Cacho, I., Grimalt, J.O., Pelejero, C., Canals, M., Sierro, F.J., Flores, J.A. & Shackleton, N. 1999. Dansgaard-Oeschger and Heinrich event imprints in Alboran Sea paleotemperatures. Paleoceanography, 14(6): 698-705.

Zahn, R., Schönfeld, J., Kudrass, H.R., Park, M.H., Erlenkeuser, H. & Grootes, P.M. 1997. Thermohaline instability in the North Atlantic during meltwater events: stable isotope and ice-rafted detritus records from core SO75-26KL, Portuguese margin. Paleoceanography, 12: 696-710.


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

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Copyright © 2000, most recent revision July 24, 2000

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