早中全新期臺灣南部恆春地區之古環境：微孔珊瑚骨骼之穩定碳氧同位素組成及Sr/Ca比值紀錄

本研究分析鑽取自臺灣南部恆春頂潭仔地區之兩口微孔珊瑚（Porites；組成成分為霰石）材料岩芯的穩定碳氧同位素與Sr/Ca比值分析結果來重建早全新世晚期臺灣南部地區之古環境。 BH-3（長約25.3 cm；U-Th定年為8051±74~7837±31年前）及BH-2（長約122.4 cm；U-Th定年為7635±24~7436±28年前）兩口岩芯之珊瑚標本並非連續生長，根據X-Ray拍攝珊瑚標本用來辨認其最大生長軸方向，並以電鑽沿著生長軸方向以1mm等間距微取樣，再進行穩定碳氧同位素及Sr/Ca比值分析。BH-3氧同位素數值範圍為-4.86‰~-2.83‰（V-PDB；平均值為-3.87‰±0.53‰；1σ；N=188）；BH-2氧同位素數值範圍為-5.32‰~-2.52‰（平均值為-4.26‰±0.57‰；1σ；N=595）。BH-3岩芯碳同位素數值範圍為-3.02‰~1.40‰（平均值為-0.74‰±0.93‰）；BH-2岩芯碳同位素數值範圍為-3.85‰~2.05‰（平均值為-1.75‰±1.03‰）。BH-3與BH-2岩芯Sr/Ca比值範圍分別為8.99mmol/mol~10.25mmol/mol之間（平均值為9.64mmol/mol±0.23mmol/mol；N=188）和8.56mmol/mol~10.55mmol/mol之間（平均值為9.60mmol/mol±0.28mmol/mol；N=593）。根據微孔珊瑚之Sr/Ca比數值範圍共計分為18個區段（BH-3：5段，BH-2：13段），並以線性內差方式計算每一個區間之年代，由老至新這18個區段年代介於8051年前至7432年前之間，其Sr/Ca溫度最低溫和最高溫分別為15.6℃（N=6；7837年前）及28.2℃（N=3；7466年前），而此期間之Sr/Ca比值夏季與冬季溫度（夏季：23.6℃，冬季：17.8℃）則比現今的夏季與冬季溫度（夏季：29.2℃，冬季：23.3℃）低，顯示當時可能是一個較冷的環境。根據Sr/Ca比值方程式與海水氧同位素方程式（Shen, 1996）估算，8051~7432年前之海水氧同位素數值範圍為-2.66‰~2.29‰（VSMOW）之間，夏季平均值為0.11‰（N=96），冬季平均值為0.44‰（N=93），而估算之海水氧同位素數值則比現今南灣地區之數值來得大（夏季：-0.05‰，冬季：0.10‰）。假設當時之全球海水氧同位素數值為0.4‰，7635~7493年前（δ18Osw=-0.10‰，N=45）夏季降水量較多夏季季風較強，而在7466~7436年前（δ18Osw=0.58‰，N=100）則出現較高的蒸發效應。 BH-3珊瑚岩芯在8051~7837年前亦紀錄到較冷的事件（Sr/Ca年均溫：21.1℃），或許與北大西洋8.2 ka的降溫事件（持續時間約為150~200年左右）有關。此外，BH-2珊瑚岩芯紀錄中於7466~7436年前另有一段海水氧同位素較大之時期，可能反映了臺灣南部的另一起氣候轉變訊號。本研究珊瑚骨骼碳同位素紀錄在弱季風時期之數值較正常/較強季風時期來得大，可能反映出弱季風期間（降雨量較少），珊瑚共生藻之光合作用較正常/較強季風時期來得更加旺盛。

關鍵字

微孔珊瑚；穩定碳氧同位素；古環境；鍶/鈣比；早中全新期； 8.2 ka事件

並列摘要

Based on the stable carbon and oxygen isotope compositions and Sr/Ca ratio record of two Porites coral skeleton (aragonite) collected from the Dingtanzi area of Hengchun, southern Taiwan, the palaeoenvironment of southern Taiwan during the late Early Holocene is reconstructed. BH-3 (25.3 cm long; U-Th dated 8051±74 to 7837±31 yr BP) and BH-2 (122.4 cm long; U-Th dated 7635±24 to 7436±28 yr BP) two cores were drilled. Coral skeletons were photographed by X-Ray to identify their growth direction. Carbonate powders were micro-drilled using an electric drill at 1 mm intervals along the maximum growth direction for stable carbon and oxygen isotope and Sr/Ca ratio analyses. Oxygen isotope values of BH-3 and BH-2 cores range from -4.86‰ to -2.83‰ (V-PDB; average -3.87‰±0.53‰; 1; N=188) and range from -5.32‰ to -2.52‰ (average -4.26‰±0.57‰; N=595), respectively. Carbon isotope values range from -3.02‰ to 1.40‰ (average -0.74‰±0.93‰) for BH-3 core and range from -3.85‰ to 2.05‰ (average -1.75‰±1.03‰) for BH-2 core. Sr/Ca ratios of BH-3 and BH-2 cores are respectively between 8.99 mmol/mol and 10.25 mmol/mol (average 9.64 mmol/mol±0.23 mmol/mol; N=188) and between 8.56 mmol/mol to 10.55 mmol/mol (average 9.60 mmol/mol±0.28 mmol/mol; N=593). Based on the Sr/Ca ratios of Porites coral skeletons, 18 stages can be divided (BH-3: 5 stages, BH-2: 13 stages). Assuming constant growth rate of the Porites, in ascending order, age of these 18 stages are calculated from 8051 to 7432 yr BP. The minimum and maximum annual mean Sr/Ca temperatures of these 18 stages are 15.6℃ (N=6; 7837 yr BP) and 28.2℃ (N=3; 7466 yr BP), respectively. The summer and winter Sr/Ca temperatures (summer: 23.6°C, winter: 17.8°C) between 8051 and 7436 yr BP are lower than those of present (summer: 29.2°C, winter: 23.3°C), respectively. This may indicate that the overall early Holocene environment was cooler than that of present. Adopting oxygen isotope and Sr/Ca ratios equation for seawater oxygen isotope (Shen, 1996), the seawater oxygen isotope values range from -2.66‰ to 2.29‰ (VSMOW) from 8051 to 7432 yr BP, with average values of 0.11‰ for summer (N=98) and 0.44‰ for winter (N=93). The estimated seawater oxygen isotope values are greater than those of present in southern Taiwan (summer: -0.05‰, winter: 0.10‰). Assuming the global seawater oxygen isotope value was 0.4‰ around 8100 yr BP, more summer precipitation hence stronger summer monsoon was observed from 7635 to 7493 yr BP (18Osw = -0.10‰, N=45), whereas higher evaporation rate occurred from 7466 to 7436 yr BP (18Osw = 0.58‰, N=100). A cooler Sr/Ca temperature event was observed in BH-3 coral core from 8051 to 7837 yr BP (annual mean Sr/Ca temperature = 21.1°C). It may be an extension from the 8.2 ka cooling event in the North Atlantic Ocean (lasting about 150 to 200 years). The BH-2 coral core recorded an additional period of higher oxygen isotope values of seawater between 7466 and 7436 yr BP, which may indicate another climate change event in southern Taiwan. The carbon isotope values of coral skeletons during the weak monsoon period were greater than those during normal/stronger monsoon period. It may indicate that the photosynthesis of coral symbiotic algae during the weak monsoon period (when rainfall was less) was more active than that during normal/stronger monsoon period.