RADIOLARIANS OF THE NORTH PHILIPPINE SEA: THEIR BIOSTRATIGRAPHY, PRESERVATION, AND PALEOECOLOGY.

摘要

Biostratigraphic preservational and paleoecologic studies were undertaken utilizing material from Leg 58 of the Deep Sea Drilling Project. These sediments are from five sites within the North Philippine Sea, three in the east Shikoku Basin and two in the Oki-Daito Ridge and Basin area. Radiolarians from the Shikoku Basin are characterized by being well preserved in the Pleistocene sediments, being absent from Pliocene sediments, and being moderately to well preserved in Miocene sediments. The oldest sediments recovered are from the lower middle Miocene Dorcadospyris alata Zone. Sediments from the Daito Ridge and Basin area are thick sequences of Pliocene terrigenous mud and clay which overlie Pliocene to Eocene pelagic clay, claystone, mudstone and resedimented shallow water sandstones. Radiolarian preservation is poor below the Pleistocene at Site 445 (the Pleistocene was not recovered at Site 446) except in isolated "preservational events". The oldest sediment recovered from the Daito Basin and Ridge area which can be dated using radiolarians is Eocene of the Podocyrtis chalara Zone. The radiolarian fauna of the North Philippine Sea reflects the existence of a strong western boundary current in this area. This northward flowing current, the Kuroshio, brings tropical forms into atypically high latitudes. This results in a mixed low and high latitude assemblage with the potential for tracing fluctuating paleoecological parameters. Poor preservation, wide sampling intervals and too few sites tempered this goal. As has been noted in other studies outside the tropics, the radiolarian zonation of Riedel and Sanfilippo can be used throughout the Paleogene and early to middle Miocene, but index species for their zonation are rare in higher latitudes after the middle upper to upper upper Miocene. This may be attributed to various causes, perhaps the most likely being the formation of the present water mass regime during this time with the isolation of Antarctica and closure of the Panama and Tethean seaways being important causal factors. The multiple factors influencing the preservation of siliceous microfossils were evaluated in these North Philippine Sea sediments. The very deep water (greater than 4000 m), highly productive western boundary current, high volcanogenic ash content, large benthic community and depressed silica corrosion zone due to upwelling leads to generally poor radiolarian preservation with short episodes of well preserved faunas.;Nassellarians, when observed in their commonly figured orientation, are physically and biologically misinterpreted. Their basal openings face down, away from food sources and from sunlight which is essential to the zooxanthallae the radiolarian might harbor. If nassellarians are depicted with their apical spine down and opening facing the water surface, the biology and functional morphology of many species becomes clearer. Many nassellarians show a decrease in the amount of silica from apical spine to feet with the distal chambers having large pores and spreading feet. Viewed as a functional whole, this orientation allows nassellarians to better utilize their food resources, aids in stability and positions zooxanthallae in a more favorable light.