"Geological Context" of Emiliania huxleyi


Patrizia Ziveri
Geomarine Center
Vrije Universiteit Amsterdam
de Boelelaan, 1085
1081HV Amsterdam
The Netherlands
Email: zivp@geo.vu.nl


Emiliania huxleyi was first identified by Lohmann (1902) using a light microscope. The details and beauty of its structure were not visible however until the development of electron microscopes. The structure was then described from some of the earliest electron microscope observations of plankton - Braarud et al (1952), Deflandre & Fert (1954), Black & Barnes (1961). The phylogenetic origins of Ehux are well established from the fossil record (Perch-Nielsen, 1985; Romein, 1979; Gallagher, 1989; Young et al., 1992). The genus is descended from other coccolithophore types, some of which are still present in today's ocean. It is believed to have evolved from Toweius through Reticulofenestra and Gephyrocapsa (Young, 1989; Young et al., 1992).

Ehux is only seen in relatively young sediments, and must have evolved fairly recently. The first appearance datum (FAD) of Ehux in the sediment is used as a biostratigraphic marker. Depending on the different proxies used and the location, this entry has been dated at ~270,000 years ago using correlation with planktic foraminiferal delta-O18 records (Gartner and Emiliani, 1976), at ~268,000 years ago late in oxygen isotopic stage 8 (Thierstein et al., 1977) and at ~285,000 years ago (Ahagon et al., 1993). Using correlations based on magnetostratigraphy, Weaver (1993) dated it at ~242-268,000 years ago, while Hills and Thierstein (1989) estimated ~230,000 years ago.

In the living coccolithophorid flora, Ehux has the widest distribution and largely dominates the living assemblages world-wide with exception of a few oceanographic regimes. It also forms gigantic blooms readily visualised by satellite imagery. This blooming life-style started between ~80,000 and 50,000 years ago, marking the beginning of the so called Ehux Acme zone in the fossil record. This Acme zone starts later as one progresses towards the poles. The earliest onset time has been reported from the Tropics (McIntyre 1967, Thierstein et al., 1977, Jordan et al., 1996). In the Antarctic Ehux is absent and in the Arctic the acme zone is terminated at the start of the Holocene. In shallow marginal seas and inland seas, and in upwelling areas the Ehux Acme Zone began later than in other parts of the ocean or is absent. In these areas Gephyrocapsa spp., mainly G. oceanica, are generally more dominant (Okada and Honjo, 1975; Ziveri and Thunell, 1995). Ehux may have taken over an ecological niche formerly dominated by Gephyrocapsa spp.



Two SEM images showing Ehux coccoliths and whole coccospheres (some degraded) caught in a sediment trap (58 29'N, 20 29'W, 1044m depth) in the northeast Atlantic. The sediment trap interrupted the transit of these particles from the surface water photic zone (where the cells grew) to the sea floor (where they would have been incorporated into marine rocks, if they escaped dissolution). (Photos from Patrizia Ziveri and Saskia Kars).
[click on the small pictures to view them at full size]

Coccolithophores are a major contributor to the carbonate in deep sea sediments. In the Northeast Atlantic during the last glacial-interglacial cycle, coccoliths comprised 70-80% of the total carbonate during interglacial times, and a variable but lower percentage during glacial times (van Kreveld et al., 1996; Lotoskaya and Ziveri, 1995; Lotoskaya et al., in prep.). Discrepancies exist between the estimation of export production and the estimation of sedimentary preservation of calcium carbonate from coccoliths of Ehux. It is still unknown how much of the calcite produced during bloom events is stored in the sediment record, and how much is dissolved back into the water. The geological branch of the Global Emiliania Modelling (GEM) initiative is working in great detail on this problem. Using sediment trap techniques (capable of intercepting particle fluxes continuosly, in time fractionated sequences and over a long period of time) we are able to quantify the carbonate coccolithophore export production, and then compare it with the rate of accumulation of coccolithophore assemblages in surface and in deeper sediments. It has been shown that Ehux largely dominate the total coccolith flux in the northeast Atlantic (Samtleben and Bickert, 1990; Knappertsbusch and Brummer, 1995; Ziveri et al., 1996; Ziveri and Broerse, 1996), in the Northeast Pacific (Ziveri et al., 1995) and in the eastern Mediterranean (Ziveri et al., 1995). Unfortunately, only very few coccolithophore flux data and coccolith accumulation rates are available.

References

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