由微孔珊瑚骨骼穩定碳氧同位素組成與鍶/鈣比分析探討 台灣西北部全新世中期之古氣候

Mid-Holocene paleoclimate of NW Taiwan inferred from d18O, d13C and Sr/Ca ratio of coral Porites skeleton

吳昭緯(Chou-Wei Wu)

指導教授 ： 米泓生 ； 王士偉

國立臺灣師範大學/理學院/地球科學系/碩士(2012年)

摘要

本研究挑選台灣西北部桃園海岸珊瑚礁岩芯(TY-05)，選取其中微孔珊瑚(Porites)生長較為連續之區段進行穩定碳氧同位素及Sr/Ca比值成分分析，並且同時分析本地區海水的氧同位素組成，以重建中全新世時期台灣西北部沿海地區之古環境。
珊瑚標本首先進行X射線繞射分析，以確認其組成為霰石且無受成岩作用影響；再進行X射線放射照相以確定主要生長軸方向；之後沿此生長軸以電鑽微取樣，以進行鈾釷定年、鍶鈣比值以及穩定碳氧同位素分析。
根據鈾釷定年結果，本研究所選取珊瑚之初始生長年代為5,845±55 yr BP；其氧同位素數值則顯示18年的週期震盪，其中第4-7年生長速率緩慢(平均3.2mm/yr)；因為生長緩慢的珊瑚骨骼之鍶/鈣比值與碳氧同位素組成，可能會受到動態分異效應變化的影響，而無法反映真實的環境訊號，因此分析數值須扣除此段生長速率過慢的部分。鍶/鈣比值的平均最高值和最低值分別為9.66 mmol/mol和8.59 mmol/mol (N=11)，且在第7年的夏季之後有數值明顯變大的現象。氧同位素數值最高值與平均最低值平均分別為-3.64‰和-5.15‰，而碳同位素數值最高值平均與最低值平均分別為-0.26‰和-2.82‰ (N=13)。
根據Porites珊瑚之Sr/Ca-SST轉換公式可得到5848 yr BP時平均冬夏季溫度分別為20.4°C與28.3°C。將所得溫度代入Abe at al.(1998)之18O-SST轉換公式，便可推得該時期之冬夏季平均水體氧同位素分別為0.15‰與0.10‰。其夏季海溫與現今實測值差異不大，冬季則較現今為高，冬夏溫差較現今為小，冬夏季平均海水氧同位素亦較現今為高。顯示在全新世中期台灣西北部沿海地區與整個東亞沿海地區相同，較現今為溫暖且蒸發作用較強。
第7年夏季開始SST與d18Osw同時發生變化，SST冬夏季均提高3°C，而18Osw冬季前6年約變化量為+0.08‰，夏季變化量為+0.46‰。其原因應為該時期夏季季風強度較強，造成黑潮支流入侵台灣海峽與傳輸量增加，將鹽度較高的黑潮水向北傳送而影響到此研究地區所造成。
將碳同位素對比於氧同位素週期與日照量週期，可發現碳同位素的極大值主要發生於春季，可能是受日照強度與共生藻營養鹽濃度變化的共同影響所致。另外可觀察到碳同位素極小值隨著時間逐漸變高，此現象為冬季變暖而使珊瑚共生藻較為活躍，使共生藻光合作用變強所造成。

關鍵字

珊瑚 ； 穩定碳氧同位素 ； 古氣候 ； 鍶/鈣比 ； 全新世中期

並列摘要

A drilling core (TY-05) was choosen from NW Taiwan and anaylzed the stable carbon and oxygen isotope composition and Sr/Ca ratio of the coral Porites inside. And analyzing the oxygen isotope composition of the seawater in the same study area, together this study will reconstruct the Mid-Holocene paleoclimate of NW Taiwan.
Coral samples were examined by XRD to make sure the coral is pristine. X-ray radiographs were taken to identify the main growth axis. Carbonate powders of coral were micro drilled for U-Th dating, stable carbon and oxygen isotope and Sr/Ca ratio analyses using an electric dental drill along the main growth axis.
Based on the U-Th dating result, our sample grew around 5,845±55 yr BP. δ18O data of the sample showed 18 year seasonal cycles. Annual growth rate between the 4th and the 7th years was abnormally low and less than 4mm/yr. Because the kinetic isotope effect may influence both δ13C and δ18O values of coral skeleton precipitated at < 4mm/yr growth rate (McConnaughey, 1989), the 4th -7th year records were eliminated.
The Average of the maximum (winter) and minimum (summer) Sr/Ca ratio values are 9.66mmol/mol and 8.59 mmol/mol (N= 11), respectively. An abrupt change of Sr/Ca ratio was found in the summer of the 7th year. The average of δ18O values winter and summer are-3.64‰ and -5.15‰ (N=13) , respectively. The d13C values of the maximum and minimum are -0.26‰ and -2.82‰ (N=13), respectively.
Sea surface temperature (SST) in 5845 yr BP inferred by Sr/Ca ratio are 20.4℃ and 28.3℃ in winter and summer, respectively. By coupling both SST and coral’s d18O record, the calculated d18Osw in 5845 yr BP, was 0.15‰ and 0.10‰ in winter and summer, respectively. A warmer and stronger evaporation condition was shown in this data in mid-Holocene, which consist with other record in East Asia.
The abrupt change of Sr/Ca ratio in the summer of the 7th year is truly a change in SST, which represent a 3℃ change in SST. The 18Osw also show a +0.46‰ change in summer, synchronously. It’s possible that the transport of Kuroshio Branch Water was increased in Taiwan Strait during summer, and may cause by a stronger East Asian Summer Monsoon.
Mean seasonal cycle of carbon and oxygen isotope values and the insolation at 30°N in 6000 yr BP are compared together. The maximum and minimum values of the oxygen isotope are defined as January and August, respectively. The maximum value of carbon isotope was found in spring, which preceded the seasonal insolation cycle. This result may due to the effect of both higher insolation and greater nutrient concentration. Winter stable carbon isotope show an increase trend was in this 11-year record. The increase trend in winter SST could cause a stronger photosynthesis that will increase the d13C data in coral skeletons.

並列關鍵字

coral ； stable isotope ； paleoclimate ； Sr/Ca ratio ； Mid-Holocene

參考文獻

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