In an attempt to discriminate different sources of organic matter and reconstruct the paleo-environment of the Taipei Basin, northern Taiwan over the past 250,000 years, organic carbon and nitrogen content (C/N ratios) and isotope composition (δ 13 C and δ 15 N) are measured for sediment samples collected from core Wu-ku drilled at the depocenter in western Taipei Basin. The high δ 13 C values (up to -17.0‰) and high C/N ratio (up to 14.9 and 13.0, respectively) indicate that this area underwent a period of relative aridity during MIS 2 and mid MIS 6. Another two periods of high and variable δ 15 N values (up to 15.5 and 13.5‰, respectively) in MIS 1 and MIS 7 represent different nutrient condition in the basin. In MIS 1, high δ 15 N variability along with increasing TOC may reflect an unstable trend of aquatic productivity related to the dynamic mixing ratios of seawater and fresh water. The high δ15 N values observed in the sediments of MIS 7 could be explained by the employment of a 15 N-enriched nitrate from denitrification as the main source of nitrogen for primary producers.

ABSTRACT1 Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC * Corresponding author address: Prof. Yue-Gau Chen, Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC; E-mail: ygchen@ntu.edu.tw ) and high C/N ratios (average 9.3) with good correlation indicate that this area underwent a period of relative aridity from early Marine Isotopic Stage (MIS) 2 to the early MIS 1. In MIS 1, the δ 13 C and C/N ratio both vary initially before later steadying, indicating that the environment became progressively more stable, with C3 plants becoming dominant again. The Zai-kang core appears to have come from a shallow lagoon system that potentially preserved the record of δ 15 N from autochthonous primary producers. Regional organic source variation is revealed through the high δ 15 N values and constant C/N ratios in MIS 1 of the Zai-kang core. This may suggest that mangroves and macrophytes were the main aquatic organic matter source in the Zai-kang area during MIS 1.

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