全新世早期微孔珊瑚骨骼穩定碳氧同位素組成與Sr/Ca比值反映之臺灣西南部古環境

Abstract

本研究分析採自臺灣西南部阿公店珊瑚礁岩芯（長約2.4公尺；U-Th定年8986±28～7870±29 yr BP）中微孔珊瑚（Porites；成分為霰石）生長較為連續之部分，進行穩定碳氧同位素以及Sr/Ca比值成分分析，以重建全新世早期臺灣西南部地區之古環境。 珊瑚標本以X射線照相以確定主要生長軸方向；之後沿生長軸以電鑽微取樣（間距1mm），進行穩定碳氧同位素以及Sr/Ca比值之分析。標本氧同位素數值範圍介於-3.36‰～-6.64‰ （V-PDB），平均值為-5.18‰±0.72‰（1；N=1634）。根據穩定氧同位素數值變化可觀察到106個震盪，其震幅約0.5~1‰。而Sr/Ca比值分析以每1cm為間距取樣，共分析169個標本，為追求數據的完整性，本研究額外挑選2點以每1mm為間距多做取樣（N=18），也以氧同位素震盪之峰值部分去做Sr/Ca比值分析（N=50）以了解季節之最高及最低溫度，其數值介於8.27mmol/mol～9.79mmol/mol之間，平均值為9.09mmol/mol±0.39mmol/mol（1；N=237），發現主要共可分為九個區段。而碳同位素數值範圍為0.79～-4.75‰，平均值為-1.40‰±0.82‰（1 ；N=1634），與氧同位素並無明顯的相關性。 年代由老至年輕，若根據微孔珊瑚之Sr/Ca比值溫度轉換公式，其溫度的震盪可分為九個階段：第一段（8700 yr BP），平均溫為32.3℃（N=8）；第二段（8640 yr BP），平均溫為22.3℃（N=22）；第三段（8595 yr BP），平均溫25℃（N=30）；第四段（8525 yr BP），平均溫為21.3（N=40）；第五段（8400 yr BP），平均溫為24.4℃（N=19）；第六段（8350 yr BP），平均溫為20.9℃（N=58）；第七段（8220 yr BP），平均溫為30℃（N=20）；第八段（8200 yr BP），平均溫21.4℃（N=17）；第九段（8090 yr BP），平均溫為29.6℃（N=23）。將所得溫度代入 Shen（1996）之海溫與海水氧同位素轉換公式，便可推得該時期之海水氧同位素數值範圍介於-2.0～2.0‰，夏季與冬季平均值分別為-0.3（N=71）及0.4‰ （N=44），比現今臺灣西南部實測之冬、夏海水氧同位素數值大。而若假設當時海水氧同位素值為0.5‰，則整體海水氧同位素反映出全新世早期臺灣西南部沿海地區當時夏季季風較現今強，降水量較現今為多的現象。 本研究之碳同位素與氧同位素數值並無明顯的相關性，但整體來看碳同位素數值自8700 yr BP至8090 yr BP有逐漸變大的趨勢，或許顯示出當時日照量逐漸變強，使珊瑚之光合作用更旺盛的現象。而整體珊瑚碳同位素的低值逐漸變大，振幅則逐漸變小，可能表示冬季日照量有增加的趨勢。For reconstructing paleoenvironment of southwestern Taiwan during Early Holocene, this study analyzed stable carbon and oxygen isotopes and Sr/Ca ratios of coral Porites reef core (2.4m long; U-Th dated 8986±28 to 7870±29 yr BP) collected in Agongdian area, southwestern Taiwan. After making sample clean and affirming that the sample was mainly composed of aragonite, X-Ray method was applied to determine the main growth direction of coral. Coral carbonate powder was micro-sampled along the main growth direction for stable isotope and Sr/Ca ratio analyses. For stable carbon and oxygen isotope analysis, the distance between adjacent sample points is one mm. 18O values of coral Porites range from -3.4‰ to -6.6‰ (V-PDB) with an average 18O value of -5.2±0.7‰ (1；N=1634). Based on the fluctuation of 18O values, One hundred and six oscillations with amplitudes between 0.5 and 1.0‰ were observed. For Sr/Ca ratio analysis, sample powder was picked every 1cm and a total of 169 sample points was collected. Extra 50-sample points between two peaks of 18O values are picked up to analyze Sr/Ca ratio with the purpose of finding out the maximum and minimum temperature. Sr/Ca ratios are between 8.3mmol/mol and 9.8mmol/mol and the mean Sr/Ca ratio is approximate 9.1±0.4mmol/mol (1；N=237). Nine stages of temperature can be divided bared on Sr/Ca records. 13C values of coral skeletons are from -4.9‰ to 0.8‰ and the average 13C value is about -1.4±0.8‰ (1；N=1634). There is no significant correlation between 13C and 18O values. In ascending order, temperature fluctuation can be divided into nine stages by temperature inferred from Sr/Ca ratio of coral Porites. The mean temperature in the first stage (8700 yr BP) was approximately 32.3℃ (N=8) and followed by the average temperature of 22.3℃ (N=22) in the second stage (8640 yr BP). It increased to 25℃ (N=30) in the third stage (8595 yr BP), then declined around 4℃ (N=40) in the fourth stage (8525 yr BP), and was followed by the increase to 24.4℃ (N=19) in the fifth stage (8400 yr BP). The mean temperature in the sixth stage (8350 yr BP) and seventh stage (8220 yr BP) was approximately 21℃ (N=58) and 30℃ (N=20), respectively. It decreased to 21℃ (N=17) in the eighth stage (8200 yr BP), then raised again to 30℃ (N=23) in the final stage (8090 yr BP). After that, calculated temperature is substituted into SST-18Oseawater transformation formula (Shen et al., 1996). The results show that calculated 18Oseawater values range from -2.0‰ to 2.0‰. The mean 18Oseawater values of summer and winter are -0.3‰ ( N=71) and 0.4‰ (N=44) respectively. In comparison with measured modern SW Taiwan 18Oseawater, the calculated 18O values of early Holocene seawater is greater than those of present. However, assuming that the mean 18Oseawater values was 0.5‰ in early Holocene, a strengthened summer monsoon and the high precipitation in Southwestern Taiwan during Early Holocene is inferred. In this study, there is no significant correlation between stable carbon and oxygen isotopes, and the consistency is quite different in different periods of time. However, in general, stable carbon isotope values increased gradually from 8700 to 8090 yr BP, indicating that sunlight was getting stronger and stronger during the time period studied. Moreover, the increasing of the lowest 13C values and the decreasing of amplitude between the lowest and highest 13C values during the period of time may indicate that the photosynthesis activities of symbiotic algae of coral was more and more stronger in corresponding to strengthening of sunlight in the winter.