宜蘭陸棚與南沖繩海槽沉降通量探討

Abstract

台灣東北部主要河川-蘭陽溪每年向東北海域輸出5~17 百萬噸懸浮顆粒經宜蘭陸棚輸入沖繩海槽西南端，顯示蘭陽溪可能為南沖繩海槽之最重要河川懸浮顆粒來源。但蘭陽溪河川輸出量與宜蘭陸棚及南沖繩海槽沉降量收支平衡關係尚未明瞭，故本研究採集宜蘭陸棚、宜蘭海脊與鄰近沖繩海槽陸坡區域沉積物，分析其顆粒大小組成、磁感率、孔隙率與沉積速率(210Pb)，期望能藉由宜蘭陸棚、坡與沖繩海槽西端沉積物之沉降通量與蘭陽溪懸浮顆粒量關係，進而了解蘭陽溪懸浮顆粒輸出之重要性。 研究結果顯示，沉積物物理性質特徵宜蘭海脊γ密度較高(低孔隙率)，且由宜蘭陸棚往沖繩海槽降低；沉積物磁感率區域變化受到陸源物質傳輸或火山作用影響，呈較不規則變化。沉積物顆粒粒徑呈由粗砂至黏土顆粒含量為主之分布，粗顆粒物質多分布在宜蘭海脊上，陸棚顆粒亦較粗(砂含量 0 ~ 50 %)，而陸坡與海槽區則以黏土顆粒為主。沉積速率以蘭陽溪外之陸棚區較高( >0.6 cm/yr)，並隨水深增加而快速遞減( <0.2 cm/yr)，但至水深約 1000 ~ 1500 m亦有高沉積速率且每年可達 1 公分之沉積中心。由210Pbex核種儲量空間分布顯示，宜蘭陸棚空間儲量較低，而陸坡與沖繩海槽區受到黑潮通過與橫向懸浮顆粒傳輸影響儲量增加。研究區域內總埋藏通量可達約每年 7.17百萬噸，此結果顯示蘭陽溪河川懸浮顆粒可供應研究區域內全部之沉降量。

LangYang River exports 5~17 million tons of suspended materials per year to the Ilan Shelf and later to the southwestern end of the Okinawa Trough. It is the most important suspended materials source of the South Okinawa Trough (SOT). However, mass balance disputes exist between the the source (the LangYang River) and fate of particle deposited in the SOT. This thesis investigates sedimentation rates and sediments properties in order to resolve the unbalance problem. Sediment samples were taken from the Ilan Shelf, Ilan Ridge, and Southern Okinawa Trough and determine the following parameters, grain size, density, porosity, magnetic susceptibility and sedimentation rates. The results show that Langyang River is indeed the major source of sediments in the region, from both physical properties and sediment budget. Sediment density decreased from shelf eastward to the Okinawa Trough. Magnetic susceptibility values vary irregularly due to inputs from volcanic and hydrothermal fluid of the Turtle Island. Coarse-grained sediments were found at the Ilan Ridge, and also on the Ilan Shelf with sand content reaching more than 50%. Grain size decreased rapidly away from the shelf with fine-grained clay became the predominant type of grain in the SOT. Sedimentation rates on the Ilan Shelf are high (>0.6cm/yr) and decrease northward and southward with increasing water depth away from the River mouth. However, another depocenter was found away from shelf at water depth about 1000 ~ 1500 m where higher sedimentation rate could reach >1 cm/yr. Based on sedimentation rate, the total sediment buried in this study region could reach 7.17 Mt per year. This value is very similar to that of the river particle annual export value of 6 Mt/year by Kao and Milliman (2005).