| Summary: | 博士 === 國立臺灣大學 === 地質科學研究所 === 94 === A molecular and isotopic geochemical study on core sediments from San-liao-wan, Zai-kang and Yihju, southwestern coastal plain of Taiwan, and Wu-ku, Taipei Basin, was conducted in order to identify changes in the terrestrial vegetation over the past 50K years. Variation of the organic source in the sediments, as determined by the molecular study, correspond with stable delta-13C values of bulk organic matter and elemental C/N ratios. This evolution trend also coincides the general climatic changes defined by palynological studies. The delta-13C values of bulk organic carbon shows a shift from C3 terrestrial and aqatic sources (average -25‰) to a C4 dominated terrestrial source (up to -14.3‰) at Marine Isotopic Stage (MIS) 2. Thereafter decreased into to -25‰ during MIS 1. A comparison between the delta-13C values and the biomarker from vascular plants (CuO lignin oxidation products) indicates that the vegetation transition of the coastal area was from mainly C3 angiosperm in MIS 5 to a mixture of C3 and C4 in MIS 3-4. The C4 angiosperm increase in arid MIS 2 is marked by increasing lignin proxies C/V, S/V, Ad/Al(v), high bulk organic C/N ratio and delta-13C. Periods of high and various delta-15N in MIS 1 and MIS 7 represent different nutrient condition. In MIS 1, various delta-15N along with increasing TOC could be possibly the record of the unstable trend of aquatic productivity due to the dynamic mixing ratios of seawater and fresh water. In Taipei Basin, the 15N-enriched nitrate from denitrification might become the main source of nitrogen for the primary producer and led to heavy delta-15N in the sediment in MIS 7.
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