由珊瑚地化紀錄看一九七八年以來越南中部氣候及環境之變化

Abstract

座落於最大洋──太平洋、與最大陸──歐亞大陸之交界，環繞南中國海的雷州半島、中南半島、印尼、菲律賓以及台灣等地區，是全世界人口密度最高的區域之一。因此，此區域之氣候以及水文變化影響民生甚鉅，亟須詳盡的監控與解析。近期氣候與環境的研究，大多倚賴衛星觀測和實地的儀器測量；然而，礙於目前的科技限制，衛星資料的解析度難以提高，現地測量資料非但僅有約20年紀錄，且多有殘缺，若再考慮到受人類活動造成的環境衝擊，則又更難評估。 本研究在南海西側、位於越南中部的Son Tra島 (16°12'59.4', 108°1'57.1')採集一顆長寬各三十公分之現生微孔珊瑚，進行穩定碳、氧同位素、微量元素──包括：鎂鈣比值、鍶鈣比值以及鋇鈣比值……等等分析，佐以鈾釷不平衡絕對定年法與X光攝影，作為年代控制，期能由珊瑚骨骼的地化記錄來重建過去氣候和環境之變化。 由1978－2004二十六年之地化紀錄發現，由珊瑚骨骼的鍶鈣比值換算所得之溫度變化和越南中部峴港 (Da Nang) 氣象站實測之氣溫變化、以及衛星測量之區域1°×1° 平均海溫對比結果十分吻合；當冬季季風增強時，珊瑚記錄的水溫也同步下降，特別在季風盛行的年份，如1999/2000年及1993/1994年尤其顯著。珊瑚骨骼氧之同位素比值扣除溫度效應後，可推估海水水體氧同位素的變化，此1‰之年間差異亦表示此區在乾、濕季間海水鹽度有大約2-4 psu之改變。由季節性水體氧同位素值變化可知，此區降雨多集中在夏末到冬季，此觀察和當地實際觀測之氣象結果相同，而此季節性降水之變化同樣的也可從珊瑚鋇鈣比紀錄中發現。鋇鈣比由1978年以來年際變化有顯著的差異；自1992-1993年開始一路攀升，至2000-2004年上升達二到三倍，爾後才稍所下降。此微量元素變化現象指示雨水、河流帶來之陸源沉積物之變化，應受越戰結束以來越南之經濟發展造成的沿海水質下降的影響。 綜上所述，由越南中部之珊瑚地化資料可以得到高解析度的溫度、鹽度以及陸源風化物質的變化訊息，進一步了解季風年際的強弱變化、水文變化、甚或人類活動發展的情形。

Monthly-resolution geochemical proxies, such as 18 O, 13 C, Sr/Ca, and Ba/Ca, in a living Porites coral head, collected from Son Tra Island, central Vietnam (162'59.4', 108'57.1'), was used to quantita ?? tively reconstruct the local sea surface temperature (SST), the annual dynamics of the East Asian winter monsoon (EAWM), as well as the monthly change of regional hydrological conditions. By comparing the 1/4-century geochemical data, five features are exhibited. (1) The coral Sr/Ca-inferred summer SSTs correspond well with the 1x 1instrumental data to ?? demonstrate the validity of coral Sr thermometry. (2) The winter surface pressure difference, between the southern South China Sea (SCS) (0-10 癗 , 105-115 癊 ) and the northern SCS (22.5-32.5 癗 , 112-122 癊 ), matches coral winter SST data, indicating that the coral-inferred thermal extremes can be employed to retrieve the EAWM 18 O residual ( 18 O) suggests a 2-4-psu seasonal salinity change between dry and wet seasons. (4) The monthly 18 O data show that the regional rainy season is from late summer to winter which is consistent with the meteorological record. This phenomenon is also observed in seasonal variability of Ba/Ca record. (5) The high Ba/Ca background level of 10 mol/mol in 1992-2004, 2-3 times larger than the averaged value of 4 mol/mol in 1978-1992, suggests an enhanced terrestrial sediment discharge into the marginal sea over the past 10 years. Ba records reveal an economical impact on hydrological change since the Vietnam War. Our results show that the Son Tra coral geochemical records can be used as proxies of climate and environment changes and of human activity.