博士 / 國立臺灣大學 / 地質科學研究所 / 89 / Based on deep-sea cores obtained during IMAGES III Cruise in June 1997, this study conducted multi-disciplinary approaches to examine the Late Pliocene-Pleistocene event stratigraphy in the South China Sea and the West Philippine Sea. The thesis composes four sections: 1) Late Pliocene-Pleistocene volcanic episodicity of the Philippine Arcs, 2) Estimates for the age, size and location of Australasian impact crater, 3) On the relationship between the Australasian impact and the magnetic reversal, and 4) The discovery of Youngest Toba Tuff in the South China Sea.
The Philippine arcs, a link of circum-Pacific volcanic belt, was formed by festoons of island arcs situated between the West Philippine Sea and the western Pacific marginal seas. The tephrochronology of Core MD972143 in the West Philippine Sea provide an excellent tool to elucidate volcanic episodicity of these arcs. A total of 25 major volcanic events were identified in the thick sequence of calcareous ooze of the core. Based upon oxygen isotope stratigraphy of benthic foraminifer Cibicidoides wuellerstorfi, a high-resolution astronomical chronostratigraphy over the past 2.2 Myr was constructed. Against this newly established time scale of core MD972143, the volcanic activities of the Philippine arcs are clustered in two periods: 0.4 Ma to the present (Phase I) and 2.1 to 1.4 Ma (Phase II). Between Phase I and II, there is a substantial cessation of major volcanic activity from 1.4 to 0.4 Ma, which was not documented in previous study on DSDP Site 292.
Australasian strewnfield, covering at least one-tenth of the Earth's surface, is the largest and the youngest of the four known tektite strewnfields. Although it is relative young and large, no crater for the Australasian strewnfield has been verified. We report two new sites with abundant Australasian microtektites from Core MD972142 in the South China Sea and Core MD972143 in the West Philippine Sea, respectively. Both records demonstrate that the Australasian microtektites were deposited slightly below the Brunhes/Matuyama (B/M) boundary. After correcting for the bioturbation effects and post-depositional remanence lock-in depth, our results indicate that the impact preceded the reversal by ~15 Kyr. Adopting a mean 39Ar/40Ar age of 778 ka for the B/M reversal, the Australasian impact was estimated to be 793 ka. The size of the Australasian source crater on the Indochina Peninsula is estimated to be 90-116 km according to the spatial variation of microtektite concentrations within the strewnfield.
Detailed paleomagnetic measurements from both cores confirmed the global existence of double decreases in paleointensity (DIP 1 and DIP 2) prior to and during the B/M reversal, separated in time by ~15 kyr. The upper paleointensity decrease (DIP2) centered over the B/M transition is associated with the polarity reversal at 778 ka. The mid-point of the pre-transition low (DIP1) accompanying excursional directions was correlated to the Australasian microtektite event at 793 ka. The association of the DIP1 with the Australasian impact and the temporal proximity to the B/M transition appears to support an efficient causal relationship between the huge impact and the geomagnetic excursion.
We identified for the first time the occurrence of the Youngest Toba Tuff from Core MD972151 in the southwestern South China Sea. Such result facilitates a re-assement of the volume of Toba fallout and re-examining the global impact of the mega-eruption. Our high-resolution planktonic oxygen isotopic record shows a significant positive excursion (>0.6 per mil) following the event, corresponding with the stadial between instadials (IS) 19 and IS 20. However, the U37K'-derived sea surface temperatures dropped by only 1 oC. The enrichment of heavy oxygen isotope indicates a substantial change in regional hydrography.
| Identifer | oai:union.ndltd.org:TW/089NTU00138002 |
| Date | January 2000 |
| Creators | Meng-Yang Lee, 李孟陽 |
| Contributors | Kuo-Yen Wei, 魏國彥 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 145 |
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