台灣西南海域微量金屬沈積歷史

Abstract

台灣位於活動型大陸邊緣，板塊擠壓造山使其發展出眾多高山型小河，這些河流快速侵蝕地表挾帶大量陸源沈積物傳輸至海洋。自1970年代起，台灣工業迅速發展，1980年代開始，台灣重金屬汙染問題不斷浮現，甚至擴及沿岸地區，顯示人為活動已經對臺灣周遭環境造成一定程度的影響。在此地質環境和人為活動的交互作用下，這些流經高度工業發展和人口居住密集區域的河川，勢必會將這些經濟發展下對於環境造成負面影響的產物帶進海洋環境當中，此研究目的於探討這些人為汙染物於海洋沈積物中的堆積歷史紀錄，以了解其傳輸之控制機制、來源及其擴散影響的範圍。 高屏溪為台灣流域面積最大的一條河流，因流經人口居住密集及工業高度發展之地區，與其他世界大河相比，擁有顯著高濃度的顆粒態和溶解態金屬。高屏海底峽谷位於台灣西南海域，其峽谷頭部幾乎與高屏溪連接，為高屏溪帶來大量陸源物質往深海傳輸的主要通道。本研究主要利用鉛-210定年、粒徑分析、地化分析（分析元素包括Zn, Cr, Pb, Co, Ni, Cu, Cd, Fe, Mn, Al, Ti, Mg, K）等方法，分析在西南海域周邊採集之岩芯，並以其金屬對Al之比值與自然環境（平均上部地殼、平均臺灣沈積岩、揚子陸塊及平均頁岩）背景值相比，以分辨沈積物中微量金屬之來源。根據岩心採樣位置，可劃分為三大類：（I）高屏峽谷上段兩側陸坡站位；（II）高屏峽谷下段深海站位；（III）澎湖峽谷頭部異源站位。 本研究之結果發現在陸棚外之區域找不到顯著的人為汙染訊號，陸棚外大部分重金屬呈現出相對穩定的時間分佈，其和Al的比值皆接近或低於自然背景。然而，於峽谷上段陸坡站位的表層沈積物中記錄到輕微的Pb富集及其隨時間顯著增加的趨勢，但於深海站並未紀錄到此趨勢，顯示陸源的汙染訊號主要可以到達高屏陸坡。此外，雖與自然背景並無明顯差異，但在高屏陸坡上這些記錄到Pb隨某段時間劇烈增加的站位中，皆可清楚描繪出台灣工業發展開始的時期（1970年代）。除了微量金屬的人為輸入之外，也發現自然災害（地震，颱風等）亦會加速海洋環境中微量金屬的累積。在高屏陸坡和深海這兩個沈積環境中，兩者擁有相當的微量金屬累積質量和相對一致的Ti / Al莫耳比，顯示這些由高屏峽谷傳輸之沈積物可以越過陸棚，除了一部分堆積至陸坡，另外也運送相當大量的沈積物至深海中沈積。雖然沈積物於此研究區跨棚傳輸之特性，使深海成為陸源沈積物的重要匯區，但在本研究的深海站位並未發現微量金屬的污染記錄，顯示這些陸源之污染信號可以在進一步向遠洋傳輸的過程中被稀釋，揭示這些汙然源的微量金屬對於深海的影響是微不足道的。

Rapid economic and industrial development over the past five decades in Taiwan has caused the expense of environment. Heavy metals pollution issue has gradually emerged after the 1980s and the contaminated area has extended to the coastal environment. With the tectonic setting and climatic condition in Taiwan, a considerable amount of pollutants could be carried into the marine environment. The aim of this study is to investigate the distribution and transportation of heavy metals through sedimentary records offshore southwestern Taiwan. Gaoping River (GPR) is the largest river in southern Taiwan and stands out of other major world rivers for its high concentrations of dissolved and particulate metals. Gaoping Submarine Canyon (GPSC) has been proven to be the major pathway for the transportation of terrestrial materials discharged from GPR into the deep sea. In this study, 210Pb dating, grain size and geochemical analyses (Zn, Cr, Pb, Co, Ni, Cu, Cd, Fe, Mn, Al, Ti, Mg, K) were applied to the sediment cores sampled in three different sedimentary environments around the GPSC: (I) Gaoping Slope sites, (II) Deep sea sites at lower reach of GPSC, and (III) Penghu Submarine Canyon site. Since trace metals could be derived from natural or anthropogenic sources, reference background materials (UCC, ACST, UC-YC and Average Shale) are compared to distinguish the source of the trace metals. Compared to previous studies conducted in the nearshore regions, pollution signals are hardly to be found in our further seaward sites (Gaoping Slope & deep sea), most of the measured trace metals display a stable temporal distribution with a level near or under the natural background. However, slight enrichment of Pb and its sharp increase were still recorded within the surface sediments at the Gaoping Slope sites while the records are absent in the deep sea. Moreover, the Gaoping Slope cores which have conformably recorded the pollution of Pb can even clearly illustrate the onset of the industrial development in Taiwan despite their subtle difference from the natural background. Other than the anthropogenic input of the trace metals, natural hazards (earthquakes, typhoons, etc.) are also found to accelerate the accumulation of trace metals in the marine environment. The comparable amount of cumulative mass of the trace metals between Gaoping slope and the deep sea sites and the relatively consistent Ti/Al molar ratio between these two sedimentological regimes, all suggesting that the sediments discharged from GPR could cross the narrow shelf and made a considerable amount to transport and accumulate in the deep sea. Though deep sea can act as an important sink for the terrestrial materials due to the cross-shelf transport, pollution record was not found in the deep sea sites as the pollution signals can be largely diluted during the further seaward transport, implying the pollution in the deep sea is insignificant in the study area.