Analysis of the abundance variations of the subspecies of Cycladophora davisiana

Abstract

The distribution and abundance of the three subspecies of the radiolarian Cycladophora davisiana were determined in 41 surface sediment samples, in 30 sediment samples representing conditions approximately 18,000 yr B.P., and at closely spaced intervals in four Atlantic and Pacific deep-sea cores each containing a marine sedimentary record of at least the last 150,000 years. Although all three variants are present in radiolarian-bearing sediments of most major ocean regions, only very rare occurrences of C. davisiana var. semeloides were recorded, with abundances never greater than 0.4% of the total fauna. The other minor variant, C. davisiana var. cornutoides, rarely exceeded 3% of the fauna, with greatest abundances in sediment samples from the northwest Pacific, Bering Sea, and Sea of Okhotsk. These two subspecies do not exhibit major variations in abundance and contribute only marginally to the much larger C. davisiana abundance variation pattern. The abundance of C. davisiana var. davisiana in surface sediment samples varied from less than 1.5% in most low-latitude cores to greater than 20% in samples from the Sea of Okhotsk. In the 18,000 yr B.P. samples from high northern and southern latitudes, this subspecies reached levels four to ten times greater than in corresponding surface sediment samples. The abundance changes of C. davisiana var. davisiana account for most of the high amplitude abundance variations in C. davisiana registered in Pleistocene sediments.