| Popis: |
The cause of high Cycladophora davisiana percentages, a deep-living (maximum abundance > 200 m) radiolarian species, in high-latitude (> 45°) late Pleistocene deep-sea sediments has been much debated. In this paper we show that in four subpolar North Atlantic cores these late Pleistocene high percentages (> 20%) result more from a reduction of other radiolarian flux, predominantly shallow-living species ( C. davisiana flux relative to Holocene flux. These findings are similar to those reported from the North Pacific (Hays et al., 2013). In both oceans, the late Pleistocene to Holocene shallow-living species' flux increase is larger than the concomitant decrease of C. davisiana flux. Similarly, high C. davisiana percentages (> 20%) found today in Okhotsk Sea Holocene sediments (Kruglikova, 1975; Robertson, 1975; Morley and Hays, 1983) result from low radiolarian concentrations in the overlying dicothermal layer (20–150 m) where year round temperatures are near 0 °C. It is likely that the high C. davisiana percentages in North Atlantic late Pleistocene sediments were caused by a similar overlying stratification that had important biological consequences for radiolarians and other plankton. A more significant time transgressive increase in Shallow and Others radiolarian Assemblage flux occurs initially in the southernmost core in mid-Holocene, ultimately recorded at the northernmost site during the late Holocene. Here flux of the Deep Assemblage and C. davisiana also increases. |
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