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Radiolaria: A key siliceous microfossil to unlock the history of the vertical and spatial ocean structure

le 15-06-2012 à 10:00

Salle Stendhal, DGO, Bâtiment B18

Noritoshi SUZUKI, Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Sendai City, 980-8578 JAPAN

A marine planktonic zooplankton, Radiolaria, is one of the useful microfossils to understand the oceans from the surface to 8,000-m water depths. The faunal compositions of radiolarians in sediments have been used to reconstruct the past oceanographic conditions based on the information on living radiolarians. The precision of reconstructed oceanographic conditions by using radiolarians becomes better than better by grace of corporative studies between marine biology and micropaleontology.
The biomass of radiolarians is roughly differentiated in water columns at the taxonomic order levels. Colonial radiolarians belonging to the Order Collodaria are dominated in the very near surface while the other radiolarians classifying into Spumellaria and Nassellaria are very abundant around the water depths of chlorophyll-a maximum water depths. Thus, the abundance of Collodaria in sediment is closely related with the near-surface water conditions whereas that of Spumellaria and Nassellaria is affected on the conditions of the water at the chlorophyll-a maximum water depths.
At the species level, several radiolarians have a habitat in different water depths. The deep-water species below the chlorophyll-a maximum water depths is generally very low abundance, but it is very important as a depth marker. In Kuril Trench, the northwest margin of the Pacific Ocean, a few species seems to be lived only in deeper than 4,000-m water depth. Deep-water dwellers in radiolarians have robust siliceous skeletons or extremely very fragile skeleton frames with very long thin rods.
The geographic distribution of a several hundred radiolarian species has been recently well mapped by Boltovskoy et al. (2010). This basic information will contribute to reconstruct the movement of the water masses from the surface to the deep in the past. What is controlled the distribution of radiolarian species? Symbiont-bearing species always have symbionts and the radiolarian species without symbiont always don’t have symbionts in my experience, and thus the survival possibility of not only the host radiolarians but also algal symbionts is apparently related the distribution of radiolarians.
The recent progress on living radiolarian studies will shed light on more complex and comprehensive paleoceanographic conditions.
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