南海北部海洋酸化與鉛汙染的珊瑚紀錄

Abstract

人為活動排放的過量二氧化碳已造成海水pH值下降，造成海洋酸化，同時也降低海水碳酸鈣飽和度(Ω)。Ω及海水中碳酸根離子濃度([CO32-])的下降對鈣化生物生長有極大的威脅。此外，工業活動排放大量污染物（如重金屬鉛）進入海洋環境，特別是邊緣海地區，鉛汙染的程度隨著近期經濟與工業活動興起而日益嚴重。南中國海地區因同時受自然（如：季風系統）與人類活動之影響，是研究大氣－海洋－人為活動交互作用及近期環境變遷重要且關鍵的區域。 本研究分析取自東沙環礁北礁台的多孔珊瑚(Porites spp.)，應用多重地球化學代用指標重建1968至2010年南海北部海洋環境之變遷記錄。珊瑚硼同位素以及硼鈣比分別為目前pH與[CO32-]最為可靠的指標，結合兩者可進一步計算完整的碳酸鹽化學參數(如：Ω，DIC，pCO2等)。研究結果顯示南海北部海水pH值自1968年以來有顯著的酸化趨勢（-0.0022 ± 0.0012 pH unit year-1），預期生活於南海的海洋鈣化生物將面臨更為艱困的環境。海水pCO2的上升則指示人為排放二氧化碳的影響。此外，在過去50年中珊瑚鈣化液pH值亦有顯著下降的趨勢（-0.0019 ± 0.0006 pH unit year-1），但鈣化液中的Ω與[CO32-]皆保持穩定，表明東沙的珊瑚可能受南海內波或自身調控機制而具有抵禦海洋酸化的能力。另一方面，珊瑚的鉛鈣比(Pb/Ca)與鉛同位素應用於重建南海北部鉛汙染史以及追蹤可能的鉛汙染源。Pb/Ca自1968年來呈現逐漸上升的趨勢，並在2000年後尤為明顯，可對應到中國與東南亞地區經濟快速發展的時段。珊瑚骨骼鉛同位素則指示出2000年後來自於中國地區燃油燃煤的貢獻增加，也可能提供關於南海中漏油事件的記錄。由於受陸源輸入影響較小，東沙提供一個更全面、具代表性的北南海海洋環境變遷記錄。

Increasing anthropogenic CO2 from fossil fuel combustion and land-use practices is reducing surface ocean pH (i.e. ocean acidification, OA), leading to the reduction of marine carbonate saturation state (Ω). The associated decline in Ω (and seawater carbonate ion concentration, [CO32-]) could substantially impact marine calcifying organisms, such as scleractinian corals, and potentially threaten coral calcification and the existence of these unique reef ecosystems. Marginal seas are particularly susceptible to the influence of natural variability and human activities and are thus critical areas for studying the response of coral reef ecosystems to environmental changes. Additionally, metal pollutant emitted from industrial activities, such as lead (Pb), has become severe due to rapid developments of industries and populations in Southeast Asia. The South China Sea (SCS) is under the influence of the East Asia monsoon system and can serve as an ideal location for investigating interactions between the atmosphere, ocean, and anthropogenic influences. Here we present multi-proxy coral records of Porites spp. collected from the Dongsha Atoll (DSA) in the northern SCS (NSCS) on a seasonal basis from 1968 to 2010 CE. Coralline δ11B and B/Ca are used as a proxy for calcifying fluid pH (pHcf) and [CO32-], respectively, to better constrain aragonite saturation state (Ωarag). The coralline δ11B-based surface ocean pH (pHsw) exhibits a secular trend of decreasing pH (-0.0022 ± 0.0012 pH unit year-1) since 1968 CE, implying a progressively difficult marine environment for marine calcifiers in the foreseeable future of the NSCS, and the rising pCO2 can be primarily attributed to the anthropogenic activity. The reconstructed pHcf also exhibits a rapid decline over the last 50 years (-0.0019 ± 0.0006 pH unit year-1). In contrast, the reconstructed Ωarag remains relatively constant, suggesting that Dongsha corals are resistant to OA by self-regulation on the carbonate system of the calcifying fluid. On the other hand, the coralline Pb/Ca and Pb isotope (206Pb/207Pb, 208Pb/206Pb) records are applied to reconstruct the pollution history of Pb and to trace the potential sources of Pb in the NSCS. Our results show a gradual increase in skeletal Pb/Ca since 1968 CE, indicating an elevated pollution of Pb, particularly after Year 2000, coincident with the period of rapid development of the economy in China and Southeast Asia. The coralline Pb isotopic composition clearly exhibits an increase in the contribution of Chinese petrol/coal after Year 2000. Compared to the other published SCS coral records that are influenced by terrestrial inputs, our record from DSA provides more representative and comprehensive information about the change in the ocean environment in the NSCS over the 50 years.