Upper Pliocene-Pleistocene calcareous nannofossil biostratigraphy of ODP1115B in the Solomon Sea, western equatorial Pacific.
| Autor: | Chih-Kai Chuang, 莊智凱 |
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| Rok vydání: | 2008 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 96 The Western Pacific Warm Pool (WPWP) is the largest source of heat and moisture on the Earth surface. For its importance the paleoceanographic studies of the WPWP have increased dramatically during the past decade. In an attempt to improve the resolution and accuracy of chronology of the stratigraphic records in the WPWP for the last 3 million years, the current study establishes calcareous nannofossil biostratigraphy of ODP site 1115B (9o11.382’S, 151 o 34.437’E, water depth 1149 m) in the Solomon Sea with a newly modified magnetostratigraphy of the Upper Pliocene-Pleistocene. The existing, unpublished oxygen isotope stratigraphy (Ke, 2008) provides a stratigraphic framework in which the chronological significances of the 14 recognized bioevents were evaluated against published results of other sites. Semi-quantitative estimations of relative abundances of environmentally-sensitive nannofossil taxa are used to infer paleoceanographic changes. The nannofossils in samples of the upper 216.43 mbsf of ODP1115B were identified under a light microscope at 1250X magnification augmented with additional scanning electronic microscopy when necessary. Among the 14 recognized bioevents, seven are of good correlation markers because of their global synchroniety. They are the first occurrence (FO) of Emiliania huxleyi, the last occurrence (LO) of Pseudoemiliania lacunosa, LO Reticulofenestra asanoi, LO large Gephyrocapsa, FO large Gephyrocapsa, LO Calcidiscus macintyrei and LO Discoaster brouweri. Seven of the bioevents are considered to be diachronous, including the reentrance (reen) of medium Gephyrocapsa, FO Reticulofenestra asanoi, LO Helicosphaera sellii, FO medium Gephyrocapsa, LO Discoaster pentaradiatus, LO D. surculus and LO D. tamalis. The reason of reen medium Gephyrocapsa being diachronous is that the medium-sized Gephyrocapsa omega (= G. parallela) favored warm water conditions and thereby shows a latitudinal heterogeneity of their first occurrence during marine oxygen isotope stages (MIS) 25-29. Changes in nannofossil assemblage composition signify several major climatic changes during the past 3 million years. The abundance of Discoaster spp. is used as an indicator of warm conditions. The major drops of Discoaster abundance during 2.8-2.6 Ma and 2.5-2.4 Ma are indicative of decrease of sea-surface temperature as resulted from the onset of northern Hemisphere glaciation and its subsequent major ice-sheet expansion. The relative abundance of Florisphaera profunda may indicate the strength of upwelling and therefore change in thermocline depth. The extinction of Reticulofenestra asanoi and the dramatic reduction of Gephyrocapsa omega at about 0.9 Ma related to the mid-Pleistocene revolution when the dominant periodicity of global climatic variation changes from 41 ka to 100 ka, as well as a major step-wise increase of global ice volume took place. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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