Late Cenozoic Paleoceanography of the western equatorial Pacific Ocean based on calcareous nannofossils-relation to stability of sea surfacenstratification-

摘要

Many outhors have explained that calcareous nannofossils have been widely distributed in the world???s oceans since the Miocene, and also as a good tool for paleoenvironmental analysis as well as for age determination, including paleoecologic and paleoclimatic condition. In this study, I describe in detail the Miocene to Quaternary paleoceanography of the western equatorial Pacific Ocean based on calcareous nannofossils of ODP, Hole 805B. This hole is located close to the equatorial line of Pacific Ocean, whereas including in Leg 30 Site 805, which have conducted in 1990 (latitude 1??13.68???N, longitude 160??31,76???E) at Ontong Java Plateau. Leg 130 are situated on the northeastern flank of the Ontong Java Plateau in the western equatorial Pacific Ocean, a region well known for its excellent pelagic records. Site 805 is located at a water depth of 3188 m. Hole 805B was drilled to 473, 3 mbsf, it was intended to occupy an intermediate position on the Neogene depth transect on the flank of the Ontong Java Plateau. udNannofossil ooze and chalk are the predominant sediment types recovered at Site 805, intervals of foraminifer nannofossil ooze and chalk are also present. This site having a higher foraminifer content and lacking intervals with high radiolarian abundances. The sedimentary section is stratigraphically complete, covering the Pleistocene to the upper Oligocene. The sequence of sediments were devided into two subunits (IA and IB) based on the degree of consolidation. The subunit IA from 805B-1H to 805B-30X is composed of 293,7 m of nannofossil ooze to foraminifer nannofossil ooze (Pleistocene to middle Miocene), and subunit IB from 805B-31X to 50X is composed of 317.3 m of nannofossil chalk, nannofossil chalk with foraminifers, and foraminifer nannofossil chalk.udA total of 317 samples were made samples preparation, around 0.0400 g each sample was pounded to be powder and was placed in breaker and 50 ml of water was added. 0.5 ml of suspension then withdrawn with a pippete and placed on the cover glass (18 mm x 18 mm). Next the cover glass was carefully dried on an electric hot plate set at 40??C, and placed on a slide glass with embedded medium to make slides. The slide was examined under Binocular Polarizing Microscope with an oil-immersion objective at lens 1500x magnification. Several thousands coccolith and Discoaster were identified for identify both last and first occurence datum of marker species. After that, 200 specimens were counted at random for analysis of nannofossil assemblages. In addition, number of coccolith and Discoaster were counted in the area of 18mm x 10??m for calculate the number/gram. Size distribution of Reticulofenestra spp. was also identified by measuring the 50 to 100 Reticulofenestra specimens in a sample.ud13 datums was determined in this hole, they are; the last occurence of Pseudoemiliania lacunosa (NN20/NN19 Zone); the last occurence of Discoaster brouweri (NN19/NN18 Zone); last occurence of Discoaster pentaradiatus (NN18/NN17 Zone); last occurence of Discoaster surculus (NN17/NN16 Zone); last occurence of Reticulofenestra pseudoumbilicus (NN16/NN15 Zone); first occurence of Ceratolithus rugosus (NN13/NN12 Zone); last occurence of Discoaster quinqueramus (NN12/NN11 Zone); first occurence of Discoaster berggrenii (NN11/NN10 Zone); first occurence of Catinaster coalitus (NN8/NN7 Zone); last occurence of Cyclicargolithus floridanus (NN7/NN6 Zone); last occurence of Sphenolithus heteromorphus (NN6/NN5 Zone); the LOC of Sphenolithus belemnos (NN4/NN3 Zone); and the first occurence of Sphenolithus belemnos (NN3/NN2 Zone). udCoccolith productivity shows that the fluctuated number, from NN2 Zone to upper part NN4-5 Zone is decreased, while Discoasters have the maximum number in this period. NN7/NN6 boundary coccolith increase again, and is decreased at upper NN8-10. After that coccolith number generally increasing and showing peaks in NN11 and NN12 Zone. on the other hand upper part of NN4-5 to NN 7 Zone Discoaster decreased abruptly, and then increased suddenly.After this period, Discoaster the number is decreased gradually until the lower Pleistocene. In addition, the increase of the maximum size is recognized in four intervals in NN2 to NN4-5 Zone, NN4-5 to NN8-10 Zone, NN11 to NN12 Zone, and NN13-15 Zone. Focusing to mode size, the mode of the Reticulofenestra size is characterized by positive correlation with the maximum sizeudThe relative abundance of Discoaster which lived in the lower photic zone under the stable sea with nutricline and thermocline, decreased step by step at NN5/NN6 and NN10/NN11 boundaries. Although the size of Reticulofenestra which is strongly influenced by nutrient, increased five times throughout the section, it drastically decreased in NN4-5 zone, NN10, NN12, and in NN15/NN16 boundary. Futhermore, this condition was correlated to the absence of a proto-warm pool (Interval 4: 8.6???6.5 Ma), which the thermocline cooled at the Ontong Java Plateau during this interval (Nathan & Leckie, 2009). As a result, the sea surface of western equatorial Pacific Ocean was changed from oligotrophic to eutrophic condition.udOn the basis of the relationship between Discoaster abundance and Reticulofenestra size change, the stability of the sea surface condition in the western equatorial Pacific Ocean collapsed step by step and also changed from oligotrophic to eutrophic from Miocene to the Quaternary.