側向傳輸與海底熱泉對南沖繩海槽斜坡錳分佈之影響
中文摘要
摘要
本研究旨在探討南沖繩海槽錳及相關微量元素之分佈，以解釋南沖繩海槽溶解態錳( DMn )及顆粒態錳( PMn )於陸坡中層擴散舌之成因及海底火山對錳分佈的可能影響。研究過程乃利用大體積海水取樣方式分別於1998、1999及2000年的6-7月之間於沖繩海槽南端進行3個航次的水體採樣工作。除收集溫度及鹽度資料外，並測定溶解態錳( DMn)、顆粒態錳( PMn )、懸浮物濃度(TSM)及顆粒態有機碳( POC )、酸鹼度( pH )及二氧化碳(TCO2)等項目。實驗分析結果顯示在水深600~1000 m處在不同深度有不同濃度的DMn或PMn極大值，濃度介於5~10 nM 之間。本研究區域錳及相關金屬元素的分佈，除了受到元素本身化學的特性之影響外，亦可受到陸源側向輸送、底部沈積物釋放以及海底火山噴發的影響。以溶解態Mn的分佈為例，典型富氧情況下深水剖面中溶解態金屬Mn濃度應隨深度的增加而減少，但在研究區域確有顯著異常的現象。大部分測站的錳異常高值出現在400m、600m、800m、1200m及近底部區域。而造成此種高值分佈的原因可能來自陸坡側向傳輸及其他原因所造成。異常分佈程度常因地而異，大致由南（棉花峽谷）往北（北棉花峽谷附近）漸減。而且同一斷面中同一深度可能出現離坡度較遠處的濃度高於近坡處的濃度，而此種分佈並無法以側向輸送來解釋。由EK-500的影像顯示在高異常分佈的地區均有海底火山噴發的情況發生。而此噴發物質的擴散可往上高達水深200-800 m 不等。由此研判溶解態的Mn異常分佈主要還是受到南沖繩海槽底部火山噴發的影響。由研究區域內中顆粒態Mn的剖面分佈情形以及Mn的異常分佈係數(Kd=[Mn]p/[Mn]d)的變化顯示TSM，顆粒態Mn及Kd之異常值均相當吻合，因此推論火山噴發及側向傳輸均同時影響此研究區域 。雖然pH的分佈無法看出異常現象，但總鹼度(total alkalinity) 及總二氧化碳（Σ CO2 ）的分佈卻可觀察出異常的分佈情形。由以上PMn、DMn及其他相關參數時空上共同的異常分佈現象等可以推論南沖繩海槽海底火山以及側向傳輸為控制南沖繩海槽錳分佈的主要作用過程。
英文摘要
Abstract
This study elucidates the processes that may determine the distributions of Mn and other trace metals on the slope of the southern Okinawa Trough. In addition to salinity and temperature, large-volume seawater samples were collected for analyzing dissolved manganese (DMn), particulate manganese (PMn), suspended, particulate organic carbon (POC), pH and dissolved CO2 (TCO2 ) during June-July of 1998, 1999 and 2000. Concentrations of DMn and PMn in a transect decrease generally seaward from the shelf break. However, anomalous distributions of DMn and PMn occur usually between 600 m and 900 m in many deep profiles.
Such distribution patterns are probably influenced by the lateral transport of metals released from slope sediments or from the plumes of submarine venting systems. For example, dissolved manganese generally decreases with depth in the oxygenated condition, but in the study area unusual high DMn concentrations were observed commonly at depth 400 m, 600 m, 800 m and 1200 m. The unusual Mn signals become weak, in general, from the South Mien-Hua Canyon to the North Mien-Hua Canyon. The higher concentration of PMn sometimes was observed in deep stations than in shallow stations at the same depth implying that processes other than lateral transport may account for the distributions. Images obtained from EK-500 showed that submarine venting systems exist in anomalous Mn distribution areas. Anomalous distributions of PMn and Kd match DMn unusual distributions very well. Besides, the distribution of TSM is closely related to PMn distributions (R=0.433, p=0.01). Although the distribution of pH data appears to be normal, total alkalinity and total CO2 display anomalous distributions coincidently with those Mn unusual distributions. Meanwhile, the ratio (PMn/PAl) is higher for those unusual Mn distributions, and this is likely the effect of submarine venting.
