南海東北坡在過去四萬年間氣候變遷沉積記錄與區域性變化

Abstract

南海是西太平洋最大的邊緣海，東北坡周遭環繞台灣、中國大陸、菲律賓及許多小島，承接大量經由河川帶來之陸源物質懸浮顆粒，其中以台灣懸浮顆利輸出量最為巨大。沉積物組成受到氣候、海平面升降和河川懸浮顆粒輸入通量改變，隨時序記錄不同沉積環境的特徵。本研究藉由沉積物中浮游性有孔蟲C14定年結果重建沉積年代，分析沉積物中有機物質碳氮比值、有機碳、碳酸鈣、生物矽濃度及粒徑大小分佈，探討此段時間內沉積物來源的變化及環境變遷影響機制。 研究結果發現，南海東北坡沉積物質受古氣候變遷影響，大致上可分成下列四種沉積環境狀態：冰期、過渡期、全新世與突發性事件(events)。全新世時期，深受台灣小河傳輸陸源物質所影響，全新世氣候較溫暖且降雨量大，河川懸浮顆粒輸出量大，大量陸源物質經由海底峽谷輸送至研究區域沉積。在過渡期期間，台灣端陸源輸出可能更為巨大，大量營養鹽輸入導致海洋基礎生產力在此時達到極大值，深水區的沉積物在此時沉積速率最快。冰期時，沉積物中陸源物質訊號比全新世時強，顯示此時陸源物質輸入不比全新世時少。全球性突發事件的發生，像是新仙女木事件、B/A暖期、D-O events與Henrichs events，均可在岩心中觀察到。另外，還發現許多強度不亞於全球性突發事件的區域性事件發生訊號，推測可能是颱風所造成。 在近台灣端之測站間冰期沉積速率為160 cm/ka，過渡期沉積速率更高達259 cm/ka。在距離台灣200公里遠的測站在間冰期有20~30 cm/ka的高沉積速率，過渡期沉積速率更高達30~74 cm/ka。造成此結果的原因除了氣候變遷改變河川輸入量外，亦受海底峽谷地形影響，導致台灣懸浮顆粒傳輸更遠。

South China Sea (SCS) is the largest marginal sea of the Western Pacific. It is a semi-enclosed basin. Rivers from China, Vietnam, Philipine and Taiwan are major sources of terrigenous material entring the Northeastern South China Sea. (Dadson, 2003; Lin et al., 2009; Liu. et al., 2003), in addition to as well as those produced in the water column. The objectives of this study are to understand spatial variations in sedimentation rates in the past 40000 year, to resolve temporal differences in sedimentation in the Northeastern South China Sea region, and to evaluate importance of small river particles to the marginal sea. A set of cores was collected in overlying water depth within 1600 – 3300 m for mutilsensor core logging of magnetic susceptibility, density, and porosity. Foraminifera (G. sacculifer, G. conglobatus, O. universa) were picked and AMS C14 analyses for age determination. Grain size, organic carbon, carbonate and biogenic silica content were measured. Sediment types were different during the LGM, transition period, Holocene and other events. We have found there were at least 5 types of events in the study region, i.e., YD, BA, DO, Henrichs and some unknown but larger in scale if not similar local events (most likely typhoon) that were alternating sedimentation in the study region. Superimposed on these events were the global scale climatic changes within the past 40k year, i.e., glaciation and deglaciation. Sedimentation rate and types of sediments were control primarily by the glaciation and deglaciation, which induce different scale of precipitation on land in delivering different amount/scale and types of sediments to the SCS. Events such as Henrichs, BA and DO seemed able to control SCS region in altering rate of sediment deliver to the SCS. Local events, most likely typhoon, were another type of mechanism in producing turbidite sedimentation to the region at various location at different time.